Critiques and notes: the myth of the slipping turn in
hang gliding and "conventional" aviation
August 2006 edition
Steve Seibel
www.aeroexperiments.org
steve at aeroexperiments.org
Preface: This article offers a detailed look at the way some authors have
presented the subject of slips, skids, and "coordinated" flight.
Authors and works discussed here at this time include Peter Cheney's "Hang Gliding for Beginner Pilots" (1997), Dennis Pagen's
"Hang Gliding Training Manual" (1995) and "Performance Flying" (1993), Wolfgang Langewiesche's "Stick and Rudder" (1944), Neil Van Sickle's "Modern Airmanship" (1966), John F. Welch, Lewis Bjork, and
Linda Bjork's edition of "Van Sickle’s Modern Airmanship"
(1999), the FAA's "Instrument Flying Handbook"
(1980), the Jeppesen company's "Instrument Commercial
Training Manual" (1998 and 2002), and Charles L. Robertson and Jackie
Spanitz's "Instrument Rating 2003 Test Prep"
(2002). In the future this article will be expanded to offer brief notes on the
treatment of slips, skids, and "coordinated" flight in many more
flight training manuals and other books, from the past and from the present,
written for pilots of airplanes, sailplanes, ultralights, and hang gliders.
The main purpose of this website is to communicate some interesting
theoretical ideas and experimental methods and results, and not to focus too
much on the ideas of others. Nonetheless by highlighting the broad spectrum of
views in the aviation community, this article may help the reader to understand
some of the motivation for some of the aerial experiments described on this
website. Also, this article should provide some interesting food for thought
for readers who want to examine some other people's ideas on the topics I've
been discussing on this website, and draw their own conclusions.
CRITIQUES AND NOTES from the world of hang
gliding and ultralight piloting:
(Click here to advance to the "conventional"
airplane and sailplane section)
I want to emphasize that I think the vast majority of the work done by our
leading instructors and authors in the world of hang gliding, is truly
excellent. In fact all the works discussed in this section initially caught my
attention precisely because the authors have done such a good job of conveying
their ideas in a concise and complete way, and also because the ideas contained
in these works are representative of ideas that are common throughout the hang
gliding community and also appear to some degree in the larger aviation
community. Despite the excellent quality of the training materials currently
available to new hang glider pilots, I do feel that there are a few areas where
our paradigms about the way our wings work can be improved. I'm well aware that
ideas change over time--as my own have--and a book can only be as
"snapshot": the works discussed here may or may not represent the
current views of these authors. This section of this article is a work in
progress; I don't mean to single these authors out in particular and in the
future this section will be expanded to include a much wider scope. We'll
eventually present a more complete commentary that examines many different
training manuals going back to the earliest days of modern hang gliding and
ultralight piloting, including some notes from early gurus of the hang gliding
and ultralight movements, like Jack Lambie and Michael Markowski. The idea that
"pulling in the bar, or failing to push out the bar to give an adequate
nose-up pitch input, will make the glider slip earthwards toward the low
wing", or that "pushing the control stick forward, or failing to
apply enough back pressure on the stick, will make the ultralight slip toward
the low wing" will be a common thread through nearly all of these many
books by many different authors. For the time being, our focus will be mainly
on books originating in the U.S.--it's possible that we'll find that this idea
is not quite as widespread when we expand our scope to examine training manuals
and other books from other parts of the world.
For now, here are notes on a few modern sources from the American hang gliding scene:
**Peter Cheney, "Hang
gliding for beginner pilots", 3rd edition, 1997. Published by Matt Taber.
This is the "Official Flight Training Manual" of the U.S. Hang
Gliding Association. A readable, colorful, comprehensive book and a very useful
training aid. On p. 68 we read that "In theory, every winged aircraft
turns the same way", and that "Pitch control is used to control the
airspeed and keep the glider from diving through the turn." So far, this
is excellent. However on p. 70 we are given a large diagram of a hang glider
slipping sideways through the sky, along with the comment that "A slipping
turn is caused by the pilot not applying enough pitch control in the turn, so
that the glider dives through the turn instead of carving around at a constant
airspeed. In a slipping turn, the airspeed will increase, and the glider will
'slip' sideways toward the ground."
See the theory sections of this aeroexperiments web site for details on a
different view--during careful in-flight tests, I've found that a hang glider
sideslips only briefly, due to adverse yaw and yaw rotational inertia, when a
pilot banks the glider into a turn. I've found that this brief slip is not
affected by the pilot's pitch inputs: an inadequate nose-up pitch control input
does not make the glider slip sideways through the air. After the bank angle is stabilized, a hang glider will typically show only a very slight amount of sideslip, due to the "airflow curvature" effect examined in more detail in the "theory" section of this website. Like the more pronounced sideslip due to adverse yaw and other related effects as the bank angle changes, the slight sideslip that we see in a steady, stabilized turn is not minimized in any way by a pilot's pitch "coordination" inputs.
**Dennis Pagen, "Hang
Gliding Training Manual", 1995.
All of Pagen's books are very well done, and are an asset to any student or
accomplished pilot. But, Dennis also does a good of illustrating the prevailing
widely held beliefs about slipping turns in hang gliders. On pp. 128-129 we are
given diagrams
illustrating the idea that if the pilot applies inadequate nose-up pitch
pressure, the glider will not "carve" through the turn, and the turn
rate will be low, and the resulting lack of "centrifugal force" will
make the glider slide sideways toward the earth. Some of the text reads as
follows: "Merely banking the glider does not turn it, but creates a slip
to the side. Thus we can say: 'banking allows a turn but does not cause
it'". "Without a nose-up pitch control we can only manage a slip to
the side when we bank". "This glider is slipping or diving due to not
enough pushout."
Again, see the theory sections of this aeroexperiments web site for details
on a different view--during careful in-flight tests, I've found that a hang
glider sideslips only briefly, due to adverse yaw and yaw rotational inertia,
when a pilot banks the glider into a turn. I've found that this brief slip is
not affected by the pilot's pitch inputs: an inadequate nose-up pitch control
input does not make the glider slide sideways through the air.
Pagen then goes into a bobsled analogy, the detailed critique of which will
be left as in exercise for the reader. (Hint--what forces are exerted against
the sled by the icy surface? Is there a centripetal (horizontal, and
perpendicular to the sled's path) component to these forces, in relation to the
path of the sled at any given moment? Is it really true that the sled does not
turn at all--i.e. that there is no curvature in the sled's track over the ice--
if the track is not curved in the pitch sense? Is it really true that the
flight path of a banked aircraft will have no curvature in the pitch sense, if
the pilot allows the wing to remain at the original angle-of-attack, so that
the total G-loading initially remains at 1-G? What are some of the differences
between the physics of a sled, or car, running on a fixed track, and the
physics of an aircraft which is not constrained to run on a fixed track?)
Pagen does do a very good job of keeping the contrast between min. sink vs.
high-speed turns, separate from the contrast between "slipping" vs.
"non-slipping turns". He does not repeat the fallacy held by some
hang glider pilots that every sustained, high-speed turn will inherently
involve a lot of sideslip because of the high descent rate. He sees sideslip
(or lack of sideslip) as being driven by the quality of the pilot's control
inputs as he enters the turn, not by the fact that glider eventually ends up at
a high airspeed or a low airspeed. Pagen conveys the idea that even a
high-speed turn can be entered with a minimum of sideslip, if the pilot first
pulls in for airspeed and then makes the appropriate pitch
"coordination" inputs as he banks the glider into the turn. Pagen
also conveys the idea that any type of turn entry will eventually lead to a
coordinated (non-slipping) turn, though it's not always clear whether he views
the glider's inherent yaw stability, or the glider's inherent pitch stability,
or both, as being the dominant factor that eventually "coordinates"
the turn.
I believe that the comments on "slipping" vs.
"non-slipping" turns in the Hang Gliding Training Manual are not
accurate as written, but would become very accurate if the idea of a
"sideslip" were replaced with the idea of a "nose-low, accelerating,
diving turn" and the idea of a "coordinated turn" were replaced
with the idea of a "stabilized, constant-speed turn". The dynamics
that Pagen is describing when he talks about "sideslip" are really
pitch-axis dynamics, not yaw-axis dynamics. With the proper pitch input, any
kind of turn--at any bank angle and airspeed--can be entered in a smooth way
with no abrupt, sharp drop of the nose or sudden, rapid increase in airspeed.
Without the proper pitch input, any kind of turn--at any bank angle airspeed--can
be entered in a way that involves an inadequate initial G-loading and a marked
drop of the nose and a rapid increase in airspeed. Again, during a long series
of careful in-flight experiments, I've found that these pitch-axis dynamics do
not make a hang glider slip sideways through the air.
**Dennis Pagen,
"Performance Flying", 1993.
Many of the comments I've offered on the "Hang Gliding Training
Manual" also apply to "Performance Flying". Of special interest
is the detailed diagram
on p.45 comparing a "slipping turn" with a "coordinated
turn". The diagram implies that the wing's lift vector, acting
"square" to the wingspan, is the only real aerodynamic force at play,
in both the coordinated turn and in the slipping turn. No mention is made of a
sideways aerodynamic force vector during the slipping turn. According to the
diagram, the only forces actually acting on the pilot during are
"weight" and "apparent centrifugal force", with the net
force acting on the pilot being the vector sum of these two forces. We aren't
told precisely what generates the "apparent centrifugal force" or how
to calculate its magnitude starting from the actual, real, aerodynamic forces
that the glider is generating. But we are told that the right amount of pitch
control will "carve" the turn and increase the "apparent
centrifugal force". The diagram illustrates the idea that too little pitch
control will result in an inadequate "apparent centrifugal force"
vector. The inadequate "apparent centrifugal force" vector then is
said to allow gravity to pull the pilot toward the low side of the control bar.
No mention is made of the key role that the real, aerodynamic sideforce vector,
created by the sideways airflow over the aircraft, plays in creating the
"sideways" force that a pilot experiences during a slip or a skid in
any type of aircraft.
In fact that particular shortcoming, in a nutshell, is also the flaw in the
vast majority of diagrams that purport to illustrate the forces during slips or
skids both in the world of hang gliding and ultralight piloting and in the
world of general aviation: the sideways aerodynamic force vector generated by
the sideways airflow is omitted, and "centrifugal force" and gravity
are enlisted to take its place.
See the theory section of this aeroexperiments website for a more accurate
way to draw these vector diagrams, and for an
explanation of why the weight or gravity vector and the "apparent
centrifugal force" vector should both be omitted from a diagram of the
forces that act to move the pilot toward the high side or the low side of the
aircraft during a skid or a slip. If we are looking at the tendency of the
pilot's body to shift toward the high side or the low side of the turn, then
the only relevant force vector is the real, tangible, sideways aerodynamic
force vector created by the sideways motion of the aircraft through the air;
i.e. by the fact that the nose of the aircraft is not pointing directly into
the airflow or relative wind.
Also, as we've already noted, careful in-flight observations will show that
this sideways airflow over the aircraft is a result of the aircraft's dynamics
in the yaw axis and is basically unaffected by the pilot's pitch inputs; see
the theory section of this website for more on this. I've found that a hang
glider does slip briefly due to adverse yaw as the pilot makes a roll input,
but that an inadequate nose-up pitch control input does not play a role in
making the glider slide sideways through the air. I've also found that the
sideways aerodynamic forces that are created when a hang glider slips sideways
through the air are quite small, due to the lack of cross-sectional area of the
glider as seen in a side view. Even when the angle of sideways airflow over the
glider is quite large--for example immediately after the pilot has made a large
roll input and adverse yaw has swung the nose of the glider toward the outside
or high side of the turn--the real sideways aerodynamic forces generated by the
sideways airflow over the glider, and the apparent sideways "force"
in the opposite direction on the pilot's body (due to his inertia), are quite
small. The experimental observations behind these comments are presented in
more detail in the theory section of this website.
See also the interesting notes on p. 35 and 53-54 of "Performance
Flying". These notes and diagrams
illustrate the idea that if a pilot pulls in the bar while rolling into a steep
turn, the glider will first sideslip and accelerate, with a steep glide angle,
and then as the G-load increases, the glider will transition to a sustained,
high-speed, non-slipping turn with a somewhat flatter glide angle. The comments
on the airspeed and G-loading changes are detailed and entirely accurate, in my
opinion. But I don't agree with the idea that the glider is slipping during the
whole time that the G-loading is low and the flight path is inclined steeply
downward and the airspeed and G-loading are rising. During experimental tests
of this maneuver, I've found that the glider is not slipping any more than if
it were performing the same change in bank angle--i.e. rolling from wings-level
into a turn--with the normal nose-up pitch "coordination" input to
keep the nose from dropping and the airspeed from rising. I've also found that
the sideslip that is associated with the change in bank angle--in other words
the sideslip due to adverse yaw and yaw rotational inertia---is quite brief,
and ends long before the G-loading has risen substantially and long before the
glider has started to pull out of the initial steep accelerating dive. In
other words the glider's yaw stability dynamics are basically independent of
the glider's pitch stability dynamics, and operate on a shorter timescale.
Also, as noted above, I believe that whatever sideslip does take place does not
increase the glider's drag very much, because a flying-wing aircraft has so
little cross-sectional surface area (as seen in a side view). Also, I believe
that the point where the glider is bleeding off energy most rapidly occurs at
the moment in time where the airspeed and the G-load (and the lift vector, and
drag vector) reach their peak values, not when the G-loading is low and the
flight path is inclined the most steeply downward, as Pagen suggests. Drag is
the key to energy loss. The G-loading is very roughly equivalent to the lift
vector. (More precisely, the G-loading is the same thing as the net aerodynamic
force vector created by the airflow over the glider, which is very roughly
equal to the lift vector). For any given angle-of-attack, lift and drag are in
a fixed ratio (the L/D ratio), and both vary in proportion to the square of the
airspeed. So the glider will lose energy the fastest whenever the G-loading is
high, especially if the pilot has the control bar well pulled in, which keeps the angle-of-attack low and the L/D ratio low and the D/L ratio high. High G-loadings are generally caused by high airspeeds (or they also may be caused by rapid nose-up pitch inputs that increase the angle-of-attack faster than the aircraft can bleed off airspeed). Combining a low D/L ratio with a high G-loading is the best way to bleed energy from the glider. Pagen suggests that the glider is bleeding
off energy most rapidly at the point in time where the G-load (lift force) is
still low and the flight path is still inclined steeply the most downward,
because he believes that this is where we see the most "sideslip". In
my opinion, at this point in time the glider is simply converting potential
energy into kinetic energy. At the point in time where the flight path is
inclined the most steeply downward, the airspeed is still rather low, and drag
is still rather low, so this energy conversion is relatively efficient: little
energy is being lost to drag. As the airspeed continues to rise and the
G-forces started to build, so does the drag force, and the glider starts to
bleed off some of its total energy (kinetic + potential). At the same time, the
increased G-loading starts to pull the flight path up into a flatter
trajectory. When the airspeed hits it peak value the trajectory is typically
fairly flat but nonetheless this is the point where the glider is losing energy
the fastest, because the drag vector is now large. Another way to look at this:
at the point where the airspeed hits its peak value, the trajectory is often
typically flat but nonetheless this is the point where the glider is losing
energy the fastest, because none of the altitude that the glider is losing at
this point is being converted into kinetic energy (increased airspeed).
Back to Pagen's view of this maneuver: Pagen goes on to suggest a whole
series of reversing turns as a way to create a high descent rate, for example
to escape strong, widespread lift. Each time the pilot rolls the glider from
wings-level into the next turn, Pagen instructs him to pull in the bar to
"unload" the wing and make the flight path curve sharply downward,
due to the inadequate G-loading. Again, Pagen views this process as a
"sideslip". Then when the G-load and airspeed start to build
substantially, Pagen instructs the pilot to ease out the bar and roll toward
wings-level to begin the next reversal, since the high G-load is taken as a sign
that the glider is "no longer slipping". Then as the glider passes
through wings-level into the next bank in the opposite direction, the pilot is
supposed to pull in the bar again to "unload" the wing and make the
flight path curve downward again. By keeping the glider "slipping" as
much as possible in this manner, the pilot is supposed to be able to maximize
the glider's descent rate with respect to the airmass.
In my opinion, the main advantage of this technique is not the fact that the
glider is slipping. The glider will in fact be skidding and then slipping to
some degree, due to adverse yaw and yaw rotational inertia, each time it rolls
toward wings-level and then on into the next turn. (The nose will be yawed
slightly away from the direction that the glider is rolling; technically this
is called a "skid" when the glider is rolling toward wings-level and
a "slip" when the glider has passed wings-level and is continuing to
roll into the next turn.) But as we've noted above, these slips and skids will
be basically unrelated to what is going on in the pitch axis and the pilot's
pitch control inputs and the changes in G-loading. Also, as we've noted above,
these slips and skids will do little to increase the drag of an all-wing
aircraft with no fuselage.
In my opinion the main advantage of the reversing-turns technique is the
fact by pulling in the bar to decrease the angle-of-attack and G-load while
rolling quickly into a turn, the pilot throws the glider way out of
equilibrium--the vertical component of the G-load or lift vector at this
instant is very much smaller than the glider's actual weight. As a result the
flight path will curve sharply downward and the glider will accelerate rapidly
and will eventually momentarily reach a peak airspeed that is quite a bit higher
than what the pilot could sustain in a stabilized, fully pulled-in turn at the
same bank angle. When this peak airspeed is reached, the G-loading will also
have risen to quite a high value, and a high G-loading implies that all the
aerodynamic forces, including the drag vector, are quite large. At this moment
in time the glider is losing energy rapidly. Drag scales according to airspeed
squared, so if the pilot can gain a few extra mph by this technique, he'll
increase the rate of energy loss from the glider, at least briefly. Based on
theory and on in-flight experiments, I agree that this reversing-turns method
can produce some very high momentary sink rate values, and also some very high
momentary airspeed values, and also can momentarily bleed energy from the
glider at very high rate. I don't agree with Pagen's assessment of exactly what
point in the whole process the glider really bleeding off energy the fastest,
nor do I agree with the idea that sideslip is playing an important role in the
whole process.
While testing this method in flight, I've found that one problem with the
reversing-turns method is that each time the pilot rolls through wings-level,
the glider's descent rate decreases dramatically--in fact if the glider is
carrying a lot of airspeed as the pilot starts to level the wings, it can be
difficult to prevent the glider's flight path from arcing up above the
horizontal into a zoom climb. Based on my in-flight experiments, I'm skeptical
that the reversing-turns method would ever produce a higher average sink rate
over the long run--for a given peak G-loading--then the pilot would achieve by
holding the bar fully-pulled-in while flying at whatever bank angle produced
that same G-loading in a sustained, constant-bank, constant-speed maneuver.
(It's difficult to say this with absolute certainty, because it's difficult to
fly a hang glider consistently enough to repeatedly hit a particular G-loading
target, even when flying with a G-meter). It's possible that a pilot using the
reversing-turns method would not realize how many G's he was really
pulling--sometimes a briefly-held G-load is not immediately sensed by the
body--so he might mistakenly think he was achieving a relatively average sink
rate without stressing the glider very much. But structurally, it's no worse to
hold a given G-load for a long time than it is to apply the same G-load
repeatedly for short intervals.
In a personal communication, and also in an article in Hang Gliding
magazine, Pagen has pointed out that a pilot who holds a steeply banked turn
for a prolonged period may suffer from severe vertigo, and that the
reversing-turns method helps to avoid this. This is undoubtedly the case, but a
pilot could address this issue by occasionally reversing the turn direction,
without going through all the complicated pitch inputs.
We'll now take leave of our discussion of the reversing-turns maneuver and
address one other point that Pagen makes in "Performance Flying":
It's interesting to read what Pagen has to say about skids. If we believe
that pulling in the bar while banked produces a slip, then shouldn't we also
believe that pushing out the bar while banked, to produce a momentary zoom
climb, produce a skid? Due perhaps to "excess centrifugal force", or
perhaps an "excess vertical component of lift", etc.? But we read on
p.54 of Performance Flying: "While a slip is a falling to the inside of a
turn, a skid is a sliding to the outside of a turn. Skids on a hang glider
occur only during yawing, are short lived and thus of little importance. We
mention them here to inform those pilots familiar with other forms of aviation
that they can ignore skids when flying by tug and struggle (weight
shift)."
I would say that precisely the same is true of slips, if we mean real
slips--caused by adverse yaw and yaw rotational inertia and other related
effects that yaw the nose toward the outside or high side of the turn--rather
than "slips" that are caused by inadequate G-loading and are
signified by a drop of the nose, a downward curve in the flight path, and a
rising airspeed. These "slips" are not slips at all, they are really
just dives. The real slips are yaw phenomena, and occur mainly while the hang
glider is rolling to an increased bank angle, and--just like skids--are short
lived and of relatively little importance in the grand scheme of things, though
they do have a definite effect on a hang glider's roll rate and on the pilot's
roll control authority. (More on this topic elsewhere on this website).
Peter Cheney and Dennis Pagen are both in very in good company--note the
strong similarity between some of the ideas described in "Hang Gliding for
Beginner Pilots" and "Hang Gliding Training Manual" and
"Performance Flying", and the views described in Wolfgang Langewiesche's
classic physics-for-pilots text, "Stick and Rudder".
For more on theory and practice of slips and skids in hang gliders and trikes, see these related articles on the Aeroexperiments website:
Questions of interest part 1: Relationship between pitch inputs and sideslips in hang gliders and other aircraft
Questions of interest part 2: Aerodynamic sideforce created by the sideways airflow as a hang glider sideslips
"What makes an aircraft turn?"
You can't
"feel" gravity!
Complete analysis of forces: fully balanced turn, turn with inadequate lift or G-load, slipping turn, non-turning slip, and skidding turn
Notes for new hang glider and trike pilots--on sideslips
Looking for the "slipping" turn while hang gliding--overview
Causes of adverse yaw in hang gliders and "conventional" aircraft--with notes on yaw strings, slip-skid balls, rudder usage, yaw rotational inertia, aerodynamic "damping" in the roll axis, and flex-wing billow shift
CRITIQUES AND NOTES from
the world of "conventional" airplanes and sailplanes
The majority of modern training manuals for "conventional"
airplanes and sailplanes emphasize the idea that the rudder is the only control
surface that can prevent a slip or a skid, and do not support the idea that the
pilot's pitch control inputs will have very much influence whether the aircraft
will sideslip or skid. However, a careful search through older works from the
training literature for sailplanes and airplanes will yield some books that do
claim that that a pilot's pitch inputs do play a large role in
"coordinating" a turn and preventing a sideslip. These include
Wolfgang Langewiesche's "Stick and Rudder" (1944)
and Neil Van Sickle's "Modern Airmanship" (1966).
We'll address these two classics below.
Modern books written for fledgling "conventional" airplane pilots
typically offer an explanation of how to use the rudder (and only the rudder)
to prevent a slip or skid, but then go on to give an abysmal explanation of the
actual forces at play. As we've said earlier in this article, in a nutshell the
problem with the "explanations" of slips and skids that are offered
in general aviation training manuals is this: typically the sideways
aerodynamic force vector generated by the sideways airflow over the aircraft is
omitted, and "centrifugal force" and gravity are enlisted to take its
place. Much as hang gliding training manuals typically suggest that the pilot
can control the "centrifugal force" vector via his pitch inputs,
general aviation training manuals typically suggest that the pilot can control
the "centrifugal force" vector with his rudder inputs. In most cases
this is not incorrect, technically speaking--we can indeed define the apparent
"centrifugal force" vector in such a way that it is influenced by the
pilot's rudder inputs--but since this apparent "centrifugal force"
vector does not really exist as a real physical force, this is a very confusing
way of looking at things and invariably leads to errors later in the analysis.
We'll address some examples of these errors below. The inclusion of the
apparent "centrifugal force" vector invariably overshadows the
important role of the real, actual, aerodynamic sideforce that is created when
an aircraft moves sideways through the air in a skid or a slip. It is much
better to explicitly include the real, aerodynamic sideforce that is created by
a slip or a skid, and leave the apparent "centrifugal force" idea out
of the picture entirely. It is important to convey the idea that the rudder
acts to keep the nose of the aircraft aligned with the airflow, or to yaw the
nose toward the inside or outside of the turn. It is also important to convey
the idea that if the nose is yawed toward the inside or outside of the turn,
this will generate a real, aerodynamic sideforce, created by the airflow
impacting the side of the fuselage and other parts of the aircraft. This real,
aerodynamic sideforce is what throws the pilot toward the high side or the low
side of the aircraft. Once we understand the importance of this real
aerodynamic sideforce, we do not need to invoke the vague idea that the
apparent "centrifugal force" vector will move the pilot toward the
high side or the low side of the turn in a skid or a slip. Also, explanations
of the rudder's effect on slip or skid that involve the idea of
"centrifugal force" will break down completely in certain situations,
such as in sustained knife-edge flight, when there's really no way to express
the vertical, upward, aerodynamic sideforce vector, created by the pilot's
heavy application of top rudder, as an apparent "centrifugal force"
vector.
In a very fundamental sense, we "put the cart before the horse" when we imagine that the "wrong rudder input" creates the "wrong turn rate for the bank angle", which then creates the "wrong amount of centrifugal force for the bank angle", which then creates an apparent side load in the reference frame of the aircraft, pilot, and slip-skid ball. The true causal chain is as follows: when we get lazy with the rudder (or when we over-apply the rudder) and allow the nose of the aircraft to point in a slightly different direction than the aircraft is actually moving through the airmass, this creates a sideways component in the airflow or relative wind, which impacts against the side of the aircraft and creates a real, tangible, aerodynamic sideforce in the reference frame of the aircraft, pilot, and slip-skid ball. This real, tangible aerodynamic sideforce is what displaces the pilot and the slip-skid ball to one side of the cockpit. A secondary result of this real, tangible aerodynamic sideforce is that the turn rate becomes higher (in a skid) or lower (in a slip) than would normally be associated with the bank angle.
That's how to explain slips and skids without invoking "centrifugal force"!
Besides deleting the "centrifugal force" idea from our diagrams
and explanations, if we're looking specifically at the forces "felt"
by the pilot and by the slip-skid ball, which are the forces that act to move the
pilot and slip-skid ball toward the high side or low side of the aircraft, then
we should also leave the weight or gravity vector out of the picture. Gravity accelerates every molecule of the aircraft and contents together as a unit and cannot
push the pilot or the slip-skid ball toward the high side or the low side of the cockpit. (If this idea seems counterintuitive, see the article in the theory section of this website entitled "You can't feel gravity",
and also the vector
diagrams for slipping, skidding, and coordinated flight in the theory
section of this website.)
In a normal, "coordinated" turn, the reason that the pilot and the slip-skid ball do not feel a side-load in their reference frame has nothing to do with the fact that the downward pull of gravity is being "balanced" by some other force. The situation is actually much simpler than that. The wing's lift vector acts squarely "upwards" in the reference frame of the aircraft and pilot and slip-skid ball. In a normal, "coordinated" turn no other significant aerodynamic forces are present (except for thrust and drag). Therefore there are no aerodynamic sideforces in the reference frame of the aircraft and pilot and slip-skid ball. That's why the slip-skid ball stays centered. In a slip or a skid, the sideways component in the airflow impacts the fuselage and other parts of the aircraft, and this creates a real, tangible aerodynamic sideforce in the reference frame of the aircraft and pilot and slip-skid ball. That's why the slip-skid ball does not stay centered. In all 3 cases--"coordinated" flight, slipping flight, and skidding flight--gravity is present, but is not "felt" in any way by the pilot or the slip-skid ball. Of course the gravitational force vector must be included if we want to analyze the actual acceleration of the aircraft, rather than the tangible force "felt" by the pilot and the slip-skid ball. For example, in the case of a normal, "coordinated" turn, even though the net aerodynamic force "felt" by the pilot acts "straight up" in the aircraft's reference frame, when we add the aircraft's weight vector into the picture, we get the true net force vector, which is purely horizontal in the reference frame of the external world.
The idea that "gravity" and "centrifugal force" play an
important role in the dynamics of sideslips and skids is nothing but excess
baggage. All we really need to know is this--if the nose of the aircraft is
pointing toward the high side of the turn--i.e. if the aircraft is
slipping--then the airflow or relative wind will strike the low side of the
fuselage in a way that creates a real, aerodynamic sideforce toward the high side
of the turn. The pilot will "feel" this aerodynamic side force in a
very tangible way--it will be obvious to him that the aircraft's G-loading
vector or net aerodynamic force vector now includes a side load, and is no
longer pushing up squarely through the bottom of the seat. If we want more
details about the way that the pilot's body and the slip-skid ball will be
displaced to the side, then we need to know this as well: the detailed
mechanics of the way that this aerodynamic sideload is transmitted to the
pilot's body (through the seat and seat belts) and to the slip-skid ball
(through the curving floor of the track that the ball runs in) are such that
the pilot and the slip-skid ball will not experience the full extent of this
aerodynamic sideload until they have been displaced toward the low side of the
aircraft, at which point they will again be in equilibrium in relation to the
aircraft, because they will be experiencing the full force of the sideload from
the sideways airflow component. If the nose of the aircraft is pointing toward
the inside of the turn, then airflow will strike the high side of the fuselage in a
way that creates a real, aerodynamic sideforce toward the low side of the turn.
Again the pilot and the slip-skid ball will feel this aerodynamic sideforce in
a very tangible way--the G-loading or net aerodynamic force vector will no
longer be pushing squarely up through the bottom of the seat, but rather will
be tilted toward the low side of the turn. And again the detailed mechanics are
such that this tilted force vector will not be fully transmitted to the pilot's
body, and to the slip-skid ball, until they have been displaced toward one side
of the aircraft--in this case toward the high side or outside of the turn. If
the nose of the aircraft is not pointing toward either the high side or the low
side of the turn, but is directly aligned with the flight path and airflow
(relative wind), then there will be no aerodynamic sideforce created by a
sideways airflow striking the fuselage, and the pilot and slip-skid ball will
not "feel" any sideways acceleration and will not be displaced toward
either the high side or the low side of the aircraft. In this case the key
point is not that the "right amount of centrifugal force" is present.
Rather, in this case the key point is that the fuselage is directly aligned
with the airflow, so no aerodynamic sideforce is being generated. Again, see
the vector diagrams
contained in the "theory" section of this website for more on this.
The pilot's rudder inputs will control whether the nose of the aircraft is
pointing toward the high side or the low side of the turn, or is correctly
aligned with the aircraft's path of travel so that there is no sideways
component in the airflow.
The better training manuals for airplane and sailplane pilots do in fact
emphasize the idea that a slip or skid involves a sideways airflow over the
aircraft, which generates a sideways aerodynamic force component, which pushes
the slip-skid ball and the pilot toward the low side or the high side of the
turn. These manuals go on to emphasize that the rudder is the control which
determines whether or not there will be a sideways airflow over the aircraft:
if the nose is allowed to point toward the high side of the turn, then there
will be a sideways or spanwise airflow striking the low side of the fuselage,
and if the nose is allowed to point toward the low side of the turn, then there
will be a sideways or spanwise airflow component striking the high side of the
fuselage. The best presentation of these ideas tends to come from the sailplane
community, probably because sailplane pilots almost always fly with a yaw
string or telltale rather than a slip-skid ball as the primary instrument for
detecting slip or skid, so they are more likely to think about sideslip and
skid in terms of the direction of the airflow (relative wind) and the resulting
aerodynamic forces, and less likely to invoke the idea of some kind of
"imbalance" between lift, "centrifugal" force and gravity.
Some excellent training manuals from the sailplane community include Richard
Wolter's "Art and Technique of Soaring", and all of Tom Knauff's
books. These books deal with the forces of slipping and skidding flight in an
accurate, understandable way. Some of these excellent books will be examined in
future editions of this article.
For now, in addition to Wolfgang Langewiesche's "Stick
and Rudder" (1944) and Neil Van Sickle's "Modern
Airmanship" (1966), we'll also look at some material related to the
FAA's instrument written exam.
**Wolfgang Langewiesche.
"Stick and Rudder", 1944 and 1972.
This is THE classic physics-for-pilots book. It is very well-written and easy
to understand and accurate and comprehensive. Langewiesche devotes a whole
chapter to the detailed physics of turning flight, delving deep into nuances
such as the airflow curvature effect, which makes an aircraft tend to require a
touch of outside ailerons and inside rudder when turning at a low airspeed,
especially if the wingspan and the tail moment-arm are long (p. 221-222). All
of this is very highly recommended reading. The only idea that I find suspect,
in the entire book, is Langewiesche’s suggestion that inadequate back stick
pressure will make the airplane sideslip. As detailed elsewhere on this
website, I’ve found not any evidence in support of this idea, during careful
trials in airplanes, sailplanes, and hang gliders. Also it's not at all clear
to me why Langewiesche believes that an inadequate nose-up pitch input will
create a slip, but does not believe that an excessive nose-up pitch input will
create a skid. The two situations should be mirror-images of each other. But
here’s what Langewiesche has to say on the subject, with a minimum of
additional commentary:
From p.205: "The Turn: Errors and Corrections"
"It may now be worth while to discuss certain errors that the pilot is
likely to make in a turn and certain corrections he can make.
"If he applies too much back pressure, what happens? The wing force is
then excessive, and it will carry the airplane around briskly and at the same
time lift it up. The ship will carry the nose rather high and will gain
altitude. But there will be no slip or skid. The ship will simply execute a
perfectly flown climbing turn. This can also be stated the other way ‘round: If
your altimeter creeps up during a turn the reason is that you are holding too
much back pressure, and the remedy is to relax your back pressure slightly.
"If the pilot applies too little back pressure to the stick, what
happens? The airplane slips slightly, at least for a while, until the speed has
built up. It carries its nose low and keeps losing altitude. A pilot who has
turn-by-rudder ideas is then tempted to use bottom rudder in an attempt to get
more turn and stop the slip. But the remedy is simply to increase the back
pressure slightly. This can also be expressed the other way ‘round: If during
any part of the turn the airplane slips, the reason is almost certainly lack of
wing force, that is, lack of sufficient back pressure. The rudder should be
left alone until an increase in back pressure has been tried. If an airplane
loses altitude during a turn, the reason is again lack of sufficient wing
force, and the remedy is to increase the back pressure. This is so in a
correctly flown turn. If the ship is losing altitude because it is stalled,
then that is another matter.
"The secret of keeping a turn strictly level lies therefore entirely in
the proportion of back pressure to bank. For a given bank, too much back
pressure gains altitude, and too little back pressure loses altitude and also
temporarily causes slipping."
From pp. 219-220: "...just put on the rudder on the side toward which
you are being thrown. For instance, in a turn to the left, you may have the
feeling that you are sliding toward the left in your seat; put on slight left
rudder and that feeling will disappear. A little later you may have the feeling
that you are being thrown against the right-hand side of the seat; put on some
right rudder; never mind that it is a left turn; put on right rudder till the
feeling disappears. This is true only provided that you are flying a basically
correct turn; particularly, provided that you are carrying the correct amount
of back pressure on the stick. If you don’t, if you just bank to the right
without putting on back pressure, you will get a slip to the right, and to
correct that slip by rudder would lead to undesirable results—as we have
seen."
The "undesirable results" mentioned here were explained a few
pages previously (see pp. 193-196)—excessive inside or bottom rudder tends to
yaw the nose earthwards which then creates a rise in airspeed which leads the
pilot to pull aft on the control stick, while also adding outside aileron to
prevent the rolling action induced by the bottom rudder, assuming that the
aircraft has some dihedral effect which creates a positive coupling between yaw
and roll. So by holding too much inside or bottom rudder, the pilot is also
forcing himself to hold too much aft stick and too much inside or top aileron,
all of which are an invitation to a stall or spin. This is certainly a good
critique of what happens when a pilot applies too much inside rudder, so that
the nose yaws toward the low side of the turn and the aircraft skids, but I
believe that the claimed relationship between inadequate back pressure and
sideslip is a "red herring" in Langewiesche's otherwise excellent
analysis.
From pp. 223-226: "What should a pilot do when a turn goes sour? At
first glance it will seem that there can’t be any one answer. [...] Only one
thing will always work—stick forward. Leighton Collin’s investigation of flying
accidents suggests that this is probably the most important single rule in all
of flying; when anything goes wrong during a turn get the stick forward. [...]
What you really do is reduce Angle of Attack. [...] "But if you let the
stick come forward in a steep turn," someone will object, "you stop
the turn. The airplane will then just simply hang there, ‘way over on it’s
side, and a violent slip will result." This is perfectly true, but it does
no harm, and it is necessary. [...] "But what about the resulting
sideslip"? It is true that if you let the stick forward during a steep
turn, you will suddenly hang there on your side, and slip off. But sideslip has
never done anybody any harm. It is an extremely safe maneuver. Loss of control
is just about impossible, and even if the pilot should fail to stop the
sideslip the airplane’s stability would stop it anyway; the dihedral will tend
to level the wing, while the vertical tail area will tend to nose the aircraft
down and around, and the ship will turn into a steep fast spiraling glide.
Moreover, once the back pressure is released, once the airplane is at a low
Angle of Attack, you can make unhurried use of your ailerons to level your
wings."
Langewiesche gives very valuable advice when he suggests that pilot relax
the back pressure on the control stick and "unload" the wing if
things begin to "go wrong". And it certainly is true that a slip is
generally a very safe maneuver, unlike a skid which is often an invitation to a
spin. But once again, as detailed elsewhere on this website, during careful
trials in airplanes, sailplanes, and hang gliders I’ve found no evidence that
an "inadequate" pitch input will make an aircraft slip sideways
through the air, even during semi-aerobatic trials where I "unloaded"
the wing's lift vector all the way to 0 G's.
** Neil Van Sickle, Major
General USAF, editor. "Modern Airmanship" 3rd edition, 1966.
This is a comprehensive training manual oriented both towards introductory
military flight training, and towards the civilian aviation world. Lots of
interesting reading if you like the technical stuff. Here is what we read about
turn coordination:
"By increasing the back stick pressure in a bank, you increase the
angle of attack, thus increasing the total lifting ability of the wing. With
exactly the right amount of back stick pressure, at any airspeed and bank
angle, the "vertical" component of lift will be just enough to keep
the altitude constant. Any further back stick pressure will result in a climb;
any less will result in a descent. By maintaining altitude in a turn, the
elevator also prevents a slip that would otherwise develop. Many pilots do not
recognize this function of the elevator. Ask 10 pilots what causes an aircraft
to slip (slide to the inside) during a turn and possibly nine would reply
"too little rudder". The rudder is necessary while the wing is being
moved into the bank, when it counteracts the aileron drag created by the top
aileron, but then it is centered. Releasing back stick pressure during a turn
of constant bank angle when the rudders are no longer being used will result in
a slip. You can counteract the slipping tendency with inside (bottom) rudder;
but the combination of lost lift and bottom rudder will result in a descending
spiral. The reverse of a slip to the inside is a skid to the outside, it is
simply the result of too little bank allowing centrifugal force to slide the
aircraft outward." (pp. 308-309).
Again, during careful trials in airplanes, sailplanes, and hang gliders I’ve
found no evidence in support of the idea that pitch inputs affect whether an
aircraft will sideslip or skid. Also, to be consistent, it seems that Van
Sickle should also argue that too much back stick pressure should create a
skid. It's not clear whether he is making this claim or not. Van Sickle's
comment about a skid being caused by "too little bank" invites a lot
of questions--"too little bank" in proportion to what? To airspeed?
To the amount of rudder input? To the amount of back stick pressure? All of
these claims have been made at different times by different authors (and none
are correct); it's not clear which of these ideas Van Sickle has in mind.
**John F. Welch, Lewis
Bjork, Linda Bjork, editors. "Van Sickle’s Modern Airmanship", 8th
edition, 1999.
This new version of the classic text contains a good section on dihedral
effects and yaw and roll stability. In general the book has been updated and
aimed more at the world of civilian aviation. Here's what we read about turn
coordination:
"By increasing the back stick pressure in a bank, one increases the
angle of attack, thus increasing the total lifting ability of the wing. With
exactly the right amount of back stick pressure, at any airspeed and bank
angle, the "vertical" component of lift will be just enough to keep
the altitude constant. Any further back stick pressure will result in a climb;
any less will result in a descent." (P. 441).
This is almost verbatim from the earlier edition, and is entirely accurate,
at least if the aircraft is on the "front side" of the power curve or
sink rate curve. If the airspeed is so low that aircraft is on the "back
side" of the power curve or sink rate curve, then an increase in back
pressure will temporarily make the aircraft climb but after a few seconds the
sink rate will increase as the aircraft settles back into equilibrium.
All the comments about the elevator acting to prevent a slip have been
deleted from the newer edition. Also deleted were the comments about a skid
being caused by "too little bank" and by "centrifugal
force". In my opinion this is a vast improvement.
**Instrument Flying Handbook
AC 61-27C", US Dept. of Transportation, Federal Aviation Administration,
revised 1980.
This is the final authority on instrument flying, as far as the FAA is
concerned. The test questions on the FAA written test for the instrument rating
are chosen in accordance with the content in this book, and the various study
guides for this written test, which private companies have produced, make
repeated reference to the pages in this government document.
Diagrams
on page 45:
Diagram at top of page shows three aircraft, in a left turn, all at the same
bank angle.
The first is labeled "1. Normal turn: centrifugal force equals
horizontal lift". There is a vector labeled "lift", acting
"square" to the wingspan or perpendicular to the wingspan of the
banked aircraft. The "lift" vector is broken down into components
labeled "horizontal lift" and "vertical lift". There is
also a vector labeled "load", which is equal and opposite the "lift"
vector. The "load" vector is broken down into a vertical component
labeled "weight" and a horizontal component labeled "centrifugal
force"; the implication is that the "load" vector will always be
the vector sum of "weight" and "centrifugal force".
Commentary: so far so good--perhaps. In reality, the "lift" vector
and the "weight" vector are the only real vectors at work here,
everything else is fluff.
The total, net, aerodynamic load on the aircraft is simply the vector sum of
the real aerodynamic forces generated by the aircraft, which in this case works
out to be equal to the "lift" vector, if we assume that thrust and
drag are equal and cancel each other out. The "weight" or
"gravity" vector makes no contribution to the real aerodynamic forces
generated by the aircraft. The real aerodynamic forces generated by the
aircraft are the only forces that put a stress on the aircraft structure, or on
the pilot's body, or are '"felt" by the pilot, or act to move the
pilot or the slip-skid ball toward the high side or the low side of the turn as
explained above. If we want to see the "apparent load" that is
generated by the inertial response of objects (like the pilot, the slip-skid
ball, etc) to this real aerodynamic load, we simply flip the real aerodynamic
force vector or G-loading vector around to point in the opposite direction.
There's no need to muddy the picture by talking about "centrifugal
force". In the case of a properly "coordinated" turn, since the
total, net aerodynamic load acts squarely "upward" in the aircraft's
reference frame, we find that that the "apparent load" that is
"felt" by objects in the airplane--such as the pilot, the slip-skid
ball, etc--acts squarely "downwards" in the airplane's reference
frame. This is true even though the aircraft is generating a net, horizontal,
centripetal, turning force, due to the vector sum of the aircraft's net
aerodynamic force vector plus gravity, as seen by an observer in the outside
world. See the vector
diagrams for coordinated flight and slipping flight in the theory section
of this website for more on this. We don't need to invoke "the right
amount of centrifugal force" to explain why the pilot doesn't
"feel" this net horizontal force as an unbalanced load on his body in
a properly coordinated turn. We simply need to recognize that the pilot can't
"feel" the gravity vector, and that the net aerodynamic forces being
generated by the aircraft are acting squarely "upward" in the
aircraft's reference frame, with no sideways component, because the rudder is
being used as needed to keep the nose of the aircraft pointing squarely into
the airflow or relative wind.
We can't emphasize this point enough: the actual gravity or weight vector
has no direct influence on the "apparent load" that is experienced by
the pilot, the slip-skid ball, and the aircraft structural members such as the
wing-fuselage connective structure, etc.. To see this clearly, just consider
what happens when the pilot shoves the control stick forward to "unload"
the wing to the zero-lift angle-of-attack to create a 0-g trajectory. Gravity
does not go away, but the load on the aircraft goes to zero, because the net
aerodynamic force has gone to zero. The weight vector represents a real force,
and affects the aircraft's acceleration through space, but does not directly
contribute to the structural or aerodynamic load on the aircraft, nor does it
act to shift the pilot, or the slip-skid ball, toward the high or low side of
the aircraft. In other words the pilot, and the aircraft structure, and the
slip-skid ball, can't "feel" gravity. See our commentary earlier in
this article, and the article in the theory section of the website entitled
"You can't feel
gravity", for more on this.
Nor can we emphasize enough that we should delete the vector labeled
"centrifugal force" from our diagrams--this is not a real force. The
horizontal forces on a turning aircraft cannot be in balance, or there would be
no turn. A turn is driven by a net centripetal force, and haphazard,
ill-considered references to "centrifugal forces" lead to confusion
and errors.
Continuing with the FAA's diagram:
The second airplane is labeled "2. Slipping turn: centrifugal force
less than horizontal lift". The vectors labeled "lift",
"vertical component of lift", and "horizontal component of
lift" appear to be identical to those in the previous diagram. So is the
vector labeled "weight". However the vector labeled "centrifugal
force" is shorter than in the previous diagram, so the resulting
"load vector", which is depicted as the vector sum of
"weight"+"centrifugal force", is no longer quite
"square" to the wingspan. Instead, it is angled to point slightly
toward the low side of the aircraft, in the aircraft's reference frame.
Commentary: now we see the Achilles heel of this sort of approach.
"Centrifugal force" is not a real force. It is only an apparent force
generated by the inertial response of the pilot, slip-skid ball, and other
aircraft contents to the real, aerodynamic forces being generated by the
aircraft. Therefore the apparent "centrifugal force" vector can only
be a mirror image in of the horizontal component of the real, net aerodynamic
force vector being generated by the aircraft as it moves through the air. The
FAA's diagram leaves us with the impression that nothing has changed in the
real, aerodynamic forces being generated by the aircraft. If nothing has changed
in the real, aerodynamic forces being generated by the aircraft, then why has
the apparent "centrifugal force" vector changed? Why are we not shown
the real aerodynamic sideforce vector that is generated by the fuselage and
other parts of the aircraft as they move sideways through the airmass?
In a very narrow technical sense, this diagram is not grossly incorrect, if
we assume that the shortened "centrifugal force" vector is meant to
reflect the fact that the aircraft is generating a real, aerodynamic sideforce,
which has not been included in the diagram, toward the outside or high side of
the turn as the fuselage and other aircraft components move sideways through
the air. But this is an extraordinarily convoluted and unenlightened way of
looking at things--why not omit the weight and "centrifugal force"
vectors entirely, and why not include the real, aerodynamic sideforce vector,
which is the real, direct cause of the "unbalanced" sensation
experienced by the pilot (and the slip-skid ball) during a sideslip?
And why in a manual generated by the FAA--an agency whose central concern is
safety--why do we not see any recognition that the true net "load"
generated by the aircraft is simply the net aerodynamic force created by the
wing and other aircraft surfaces, rather than some "balance" between
non-existent and non-tangible forces (i.e. "centrifugal force" and
gravitational force, respectively)? No aircraft has ever suffered structural
failure as the direct, immediate result of the pull of gravity, or as a result
of "centrifugal force".
The third airplane in the FAA's diagram
is labeled "3. Skidding turn: centrifugal force greater than horizontal
lift". The vectors labeled "lift", "vertical component of
lift", and "horizontal component of lift" appear to be identical
to those in the previous diagram. So is the vector labeled "weight".
However the vector labeled "centrifugal force" vector is longer than
in the previous two diagrams, so the resulting "load vector", which
is depicted as the vector sum of "weight"+"centrifugal
force", is no longer quite "square" to the wingspan. Instead, it
is angled to act slightly toward the high side of the aircraft, in the
aircraft's reference frame.
Commentary: again we see the Achilles heel of this sort of approach. See all
the comments in the corresponding paragraphs above in relation to slips. Now
the diagram as drawn leaves us with the impression that apparent "centrifugal
force" vector is for some reason larger than the net, aerodynamic,
horizontal force generated by the aircraft. In reality, the apparent
"centrifugal force" vector can only be a mirror image of the
horizontal component of the real, net, aerodynamic force generated by the
aircraft. And why, in the reference frame of the aircraft and pilot, does there
to be a complete absence of any real, sideways, aerodynamic force components in
the diagram as drawn? And again why are we misled as to the true cause of the
"load" upon the aircraft, by the faulty inclusion of the weight
vector?
While we're on the subject of loads--it is certainly true that if all the
vertical forces are in balance, at some given bank angle, then the net force on
the aircraft has to be greatest in the skid, and less in coordinated flight,
and least in the slip. See the diagrams of a slip,
a skid, and a "coordinated" turn in the theory section of this
website for more on this. After all, we know that if all the vertical forces
are in balance, then the vector sum of all the vertical aerodynamic force
components equals the aircraft weight regardless of whether the aircraft is
slipping or skidding, and we also know that the net horizontal force component
(and the turn rate) is greatest in the skid and least in the slip, due to the
aerodynamic sideloads from the slip or skid. However type of analysis only
considers the net load on the aircraft structure, not all the individual loads
at play. It is very misleading to only think about the net vector sum of the
various aerodynamic loads. After all, an aircraft could disintegrate in
straight and level 1-G flight, due to catastrophic drag loads and thrust loads,
if enough thrust were applied to the airframe, even though the net vector sum
of the aerodynamic and thrust forces would be only be 1 G. In both a slip and a slid,
drag will be high, and the fuselage and vertical tail will be experiencing a
high sideload. This could be just as problematic in a slip as in a skid, even
if the total, net aerodynamic force is relatively low in the slip. One of the
questions on the FAA instrument exam test seems to imply that a skid is more
threatening to the aircraft structure than a slip because the total, net aerodynamic
load on the aircraft is greater in a skid; in my opinion this chain of logic is
extremely incomplete.
It is true that if the bank angle is very high, then the wing's lift vector
will be noticeably smaller in the slip than in the skid, if the vertical force
components add up to zero. (The FAA's diagrams fail to illustrate this point
because the FAA has drawn the lift vector exactly the same way in a slip, in a
skid, and in coordinated flight; see the diagrams of a slip,
a skid, and a "coordinated" turn in the theory section of this
website for a more accurate depiction). But in the case of a recovery from an
unusual attitude in instrument flight, we can't assume that the vertical forces
acting on the aircraft add up to zero. Instead, we have to calculate the wing's
lift vector based on the airspeed and on the control stick position, i.e. the
wing's angle-of-attack. Together these will tell us the number of
"G's" that the pilot is "pulling".
It is also true that when the pilot finds the aircraft in a steeply banked
"unusual attitude", hitting "top rudder" to slip the
aircraft might help to support some of the aircraft's weight, slowing the
increase in airspeed, and therefore reducing the risk of exceeding the redline
airspeed, and also reducing the risk that the pilot may be tempted to pull too
many "G's". If the sideloads from the slip are not a concern, then
applying heavy "top rudder" in an unusual attitude scenario might be
a good policy. It's not clear whether the small size of the "load"
vector in the FAA diagram illustrating a "slip", along with the test
question that makes the point that the "load" is smaller in
"slip" than in a "skip", is intended advocate this course
of action, or not. If so, a much better approach would be an explicit
instruction to "step on the sky" by applying heavy top rudder during
recovery from an unusual attitude; this recommendation is a feature of at least
one modern unusual attitude recovery training program. The assumption in the
FAA's diagrams that the total vertical force component will be zero is
completely out of place if the diagrams are intended to have any relevance to a
recovery from an unusual attitude, where the pilot will likely be trying to pull
the aircraft out of a steeply banked diving spiral.
We've covered a lot of ground and addressed some rather subtle issues here.
But the central point of our analysis remains this: regardless of exactly what
the FAA's diagrams are intended to illustrate or convey, the real, aerodynamic
sideforce created by the sideways movement of the aircraft through the airmass
plays a key role in the stresses and accelerations experienced by an aircraft
during a slip or skid, and is the sole cause of the sideways forces
"felt" by the pilot and the slip-skid ball. This real, aerodynamic
sideforce is completely absent from the FAA diagrams.
**Jeppesen Sanderson Company
authors and editors. "Instrument Commercial Training Manual", 1998
and 2002.
This weighty tome was bright and colorful but a bit unenlightening at
times--in particular the study questions at the end of each chapter seemed to
be written by committee (which they probably were). As far as slips and skids
go, the authors basically repeated a simplified version of what may be found in
the FAA's official "Instrument Flying Handbook AC 61-27C", along with
their own nice, colorful, misleading diagram:
P. 2-10:
Figures:
1. Aircraft in right turn—"slipping"—"centrifugal force"
vector pointing horizontally to left is shorter than "horizontal component
of lift" vector pointing horizontally to right. No other vectors are
shown.
2. Aircraft in right turn—"skidding"—"centrifugal force"
vector pointing horizontally to left is longer than "horizontal component
of lift" vector pointing horizontally to right. No other vectors are
shown.
3. Aircraft in right turn—"coordinated"—"centrifugal
force" vector pointing horizontally to left is equal in length to
"horizontal component of lift" vector pointing horizontally to right.
No other vectors are shown.
Detailed comments: again one gets the impression that the
"horizontal component of lift" vector is supposed to represent the
net, horizontal, aerodynamic force that the aircraft is creating. Since the
apparent "centrifugal force" vector is always just a mirror-image of
the true, horizontal, aerodynamic force that the aircraft is generating, we are
left wondering how the "centrifugal force" vector can ever be longer
or shorter than the horizontal component of the real, aerodynamic force vector.
Also we are left wondering why the aircraft is not generating a real,
aerodynamic sideforce component as it slips or skids through the airmass. It's
hard to draw too many conclusions from a diagram that only includes horizontal
force components--and at least the authors left gravity out of the picture--but
it seems that the overall message is that the rudder somehow changes the turn
rate, which changes "centrifugal force", which makes the ball or the
pilot tip to one side or the other. In fact the accompanying text states
exactly that. For example, we read the following: "Slip. Because of
insufficient right rudder pressure, the airplane is not turning fast enough for
this angle of bank. The horizontal component of lift exceeds the centrifugal
force which opposes the turn. As a result, the ball falls to the inside of the
turn, and passengers fall against the right side of the aircraft. To balance
the forces and coordinate the turn, increase the amount of right rudder
pressure and/or decrease the amount of bank." So now we are left with the
vague impression that the rudder, not the bank, may play a key role in turning
the aircraft! It's true that the rudder does influence the aircraft's turn rate
(via the aerodynamic sideforces that are generated if the fuselage moves
sideways through the airflow) so in a very narrow technical sense the comments
here are not grossly incorrect--except for the comment about "decreasing
the amount of bank" which is totally out of place. Again, what is lacking
is recognition of the idea that in a "balanced" turn, the key point
is not that the "right" amount of centrifugal force exists. Rather,
in a "balanced" turn the key point is that the rudder is used as
needed to ensure that the fuselage is not allowed to move sideways through the
airflow, so no aerodynamic sideforce is generated. The authors should have
deleted the "centrifugal force" vector, and included the real,
aerodynamic sideforce vector created by the sideways-moving aircraft in the
slip or the skid. Then it would become clear that the function of the rudder is
simply to keep the nose pointing straight into the airflow, so that no
aerodynamic sideforces are generated. We'd still need a full 2-dimensional
diagram--not just a peek at the horizontal force components--to really
understand why there ends up being a net aerodynamic sideforce component, in
the aircraft's own reference frame, in the case of a slip and skid, but not in
the case of "coordinated" flight, even though the aircraft is
generating a net horizontal force component in all 3 cases. Just picking out
the horizontal components might seem to simplify things, but in reality it only
obfuscates. For a better presentation of the forces at play in slips or skids,
see the diagram
in the "theory" section of this website.
**Charles L. Robertson and
Jackie Spanitz. "Instrument Rating 2003 Test Prep", by Aviation
Supplies and Academics Inc., 2002.
Like the source cited immediately above--and naturally enough for a test
prep book--this book repeats (and further compounds) the errors and ambiguities which are found
in the FAA's official "Instrument Flying Handbook AC 61-27C". This
time we're treated to the full two-dimensional diagram
of the forces in a slip or a skid. See page 3-20. The diagrams have been
further mangled in their migration from the FAA's book to this one. Even in the figure illustrating a "normal" turn,
the vector labeled "vertical lift" is noticeably longer
than the actual vertical component of the vector labeled "lift", and the vector labeled "horizontal lift" is noticeably shorter than the actual horizontal component of the vector labeled "lift". Also, the vector labeled "weight" is significantly longer than the actual vertical component of the vector labeled "load". In the figure labeled "slipping turn", the authors have chosen illustrate the apparent sideways load on the aircraft by modifying the "centrifugal force" vector and nothing else. In the figure labeled "skidding turn" the authors have chosen to illustrate the apparent sideways load on the aircraft by modifying the vector labeled "horizontal lift" and nothing else. No reason is given for this inconsistency. No effort has been made to ensure that the vector labeled "load" continues to at least vaguely reflect the vector sum of "weight" and "centrifugal force" in the diagram labeled "slipping turn". No effort has been made to ensure that the vector labeled "lift" continues to at least vaguely reflect the vector sum of "horizontal lift" and "vertical lift" in the diagram labeled "skidding turn". And of course, there is no hint anywhere of the real, aerodynamic sideforce that will be
generated as the aircraft slips or skids sideways through the airmass.
Basically, it is impossible to make sense out of this diagram. No offense
toward the authors--and this book is in fact a very effective study tool for
the FAA written instrument exam--but this sort of confusion is exactly what one
would expect from a work whose main purpose is to boil down rote procedures and
legalistic rules into tidbits of "knowledge" that match the answers
on a multiple choice exam.
For more on the theory and practice of slips and skids in sailplanes and airplanes, see these related articles on the Aeroexperiments website:
Questions of interest part 1: Relationship between pitch inputs and sideslips in hang gliders and other aircraft
"What makes an aircraft turn?"
You can't
"feel" gravity!
Complete analysis of forces: fully balanced turn, turn with inadequate lift or G-load, slipping turn, non-turning slip, and skidding turn
Looking for a connection between pitch inputs and sideslips in sailplanes and airplanes--overview
Causes of adverse yaw in hang gliders and "conventional" aircraft--with notes on yaw strings, slip-skid balls, rudder usage, yaw rotational inertia, aerodynamic "damping" in the roll axis, and flex-wing billow shift
