Questions of interest part 1:
Relationship between pitch inputs and sideslips in hang gliders and other aircraft

August 26 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.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 in. 

 

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 altitude quickly.

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

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

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