Questions of interest part 1:
Relationship between pitch
inputs and sideslips in hang gliders and other aircraft
August 26 2005 edition
steve at aeroexperiments.org
Here are some of the questions that I'm interested in, many
of which I've explored though in-flight experiments. Brief answers are also given--many of these topics are explored
in much more detail elsewhere in the "experiments" and
"theory" sections of this website.
This article was meant to serve as a concise yet comprehensive
introduction to these ideas, and in most cases I've tried to keep the theory
down to the bare minimum that was necessary to avoid ambiguities in meaning.
Your comments, questions, and related in-flight observations
are most welcome!
Q: What is a slip or sideslip?
A: In the most general terms, an aircraft is said to be
slipping whenever the nose of the aircraft has been yawed to point in a
different direction than the aircraft is actually moving with respect to the
airmass, so that the nose is no longer aligned with the actual direction of the
flight path and relative wind. A yaw
string or telltale will blow out of alignment with the centerline of the
aircraft, illustrating the sideways component in the airflow over the aircraft.
More specifically, if the nose is has been yawed to point
toward the "outside" or "high side" of the turn we call the
situation a slip or a sideslip, and if the nose has been yawed to point toward
the "inside" or "low side" of the turn we call the
situation a skid. A yaw string or
telltale blows toward the outside or high side of the turn in a slip, and
toward the inside or low side of the turn in a skid.
Q: If a hang glider
or trike pilot fails to move the control bar forward to increase the lift
vector (G-loading) and hold the airspeed roughly constant as he rolls a
flex-wing hang glider into a turn, does the glider or trike slip sideways
toward the low wing as it dives and accelerates?
A: Generally, no.
Sideslips are caused primarily by adverse yaw, as a pilot rolls a glider or trike from wings-level into a turn. Similarly, adverse yaw will cause a skid--where the nose yaws to point toward the inside of the turn--as a pilot rolls a glider or trike from a banked attitude back to wings-level.
Q: If a hang glider or trike pilot quickly pulls the control
bar aft while the glider is in a banked attitude, does the glider or trike slip
sideways toward the low wing as it dives and accelerates?
A: Generally, no.
Q: For the above two questions, what about in the particular
case of hang gliders that have reputation for being strongly prone to
sideslipping, such as the Moyes CSX?
A: I don't know. I
haven't seen this behavior in my Airborne Blade, either with the VG on or
off. Perhaps what is actually being
described is a high degree of spiral instability at low angles-of-attack, or
something similar. If you have access to this particular glider (or another with strong reputation for "slipping"), and you are interested in doing some simple experiments with a yaw string mounted on a dowel extending forward from the centerline of the base bar, please get in touch with me, and I'll send you the minimal hardware that is required!
Q: If a sailplane or airplane pilot fails to move the
control stick or yoke aft to increase the lift vector (G-loading) and hold the
airspeed roughly constant as he rolls the aircraft into a turn, does the
aircraft slip sideways toward the low wing as it dives and accelerates?
A: Generally, no.
Q: If a sailplane or airplane pilot quickly pulls the
control bar aft while the glider is in a banked attitude, does the aircraft
slip sideways toward the low wing as it dives and accelerates?
A: Generally no, though I have seen some tendency to
sideslip (the slip-skid ball fell toward the low wing) near the top of a
wingover where the bank angle was vertical or near vertical and the airspeed
was allowed to get very low and the G-loading was also very low as the aircraft
"floated" over the top of the maneuver. Due to the low airspeed, one would expect an aircraft's inherent
yaw stability mechanisms to be relatively ineffective in such a situation, so
yaw rotational inertia could become more important than usual. The three-dimensional dynamics of this
situation, and the attending yaw-rotational-inertia-related effects, are quite
different than during a case where an aircraft is flying in a constant-banked
turn or is rolling from wings-level into a bank.
Q: So what is meant when a hang glider pilot talks about
"slipping" to increase his descent angle for landing?
A: I don't usually
use this type of landing approach, and I would say that this situation isn't
really a "slip". By pulling
in the control bar and decreasing the wing's angle-of-attack, the pilot causes
the flight path to curve downward and the descent rate to rise. To a large extent though, this is just
trading altitude for kinetic energy.
The point at which the aircraft is bleeding off energy the most
effectively is not when the flight path begins curving sharply downward,
because here the airspeed--and the drag force--are still low. Creating a high drag force is always the key
to subtracting energy from a glider.
Once the airspeed and the G-loading (sum of all aerodynamic forces) and
the lift and drag forces start to rise, the aircraft will be bleeding off
energy much more rapidly, especially if the angle-of-attack is kept low, so
that the L/D ratio is kept low (or more descriptively, so that the D/L ratio is
kept high). Eventually of course the
aircraft will come back into equilibrium, but abruptly pulling in the bar can
lead to a temporary airspeed spike, and temporary spike in drag force, that is
higher than the pilot could obtain by leaving the bar all the way pulled
I'm not a proponent of the idea that a series of repeated
maneuvers involving repeated pitch inputs (pulling in the bar, and then letting
it back out again, so that it can be pulled in again) in combination with
repeated rolling maneuvers, so that the glider enters a series of diving,
accelerating turns in alternating directions, is the fastest way to lose
For more, see these related articles on the Aeroexperiments website:
Questions of interest part 2: Aerodynamic sideforce created by the sideways airflow as a hang glider sideslips
Questions of interest part 3: Roll torque created by the sideways airflow component as a hang glider sideslips
"What makes an aircraft turn?"
Notes for new hang glider and trike pilots--on sideslips
Looking for the "slipping" turn while hang gliding--overview
Looking for a connection between pitch inputs and sideslips in sailplanes and airplanes--overview
Complete analysis of forces: fully balanced turn, turn with inadequate lift or G-load, slipping turn, non-turning slip, and skidding turn
Causes of adverse yaw in hang gliders and "conventional" aircraft--with notes on slips, skids, yaw strings, slip-skid balls, rudder usage, yaw rotational inertia, "airflow curvature", aerodynamic "damping" in the roll axis, and flex-wing billow shift
The myth of the slipping turn in hang gliding and "conventional" aviation