Questions of interest part 2:
Aerodynamic sideforce created by the sideways airflow as a hang glider sideslips

August 25 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: Relative to a conventional airplane or sailplane, and relative to the mass of the aircraft (plus pilot), how much aerodynamic sideforce does a hang glider generate in a situation where it really does slip sideways through the airmass?

 

A: Very little, but enough to detect with a slip-skid bubble (although we should note that the bubble will respond to component of the drag vector that acts sideways in the aircraft's reference frame, as well as to aerodynamic sideforce.)

 

Q: Are these aerodynamic sideforces strong enough to be very significant to the way the pilot perceives the "control feel" of the glider?  For example, are there situations (other than perhaps during tow which is a whole different ballgame) where a pilot's body naturally tends to ride far enough to one side of the centerline of the control bar to be noticeable to the pilot?  To give a more detailed example, does it ever happen that a pilot is using his muscles to exert a significant roll torque in one direction (let's say in the direction that would tend to make the glider roll toward the left), but his body is actually riding on the other side (to the right of the centerline) of the control bar?  If this happened we could view the pilot's muscle inputs as acting in opposition to the aerodynamic sideforces and preventing him from swinging further to the right of the centerline of the control bar, and we could view these muscle inputs as being equivalent to the (stabilizing) "pendulum" roll torque that would arise whenever a heavy mass was rigidly fixed to the centerline of the base bar and a sideways airflow acted on the wing to create a create a strong aerodynamic sideforce, acting on the sail.

 

A: Whenever I've used my muscles to create a significant roll torque in one direction (for example, in the direction that would tend to make the glider roll toward the left), the force I exerted with my muscles always caused my body to be positioned on that side of the control bar.  I've never felt that I had to exert a significant roll torque just to stay centered on the control bar.  I feel that the aerodynamic sideforces generated as a hang glider slips sideways through the air are small enough that they have little effect on the "control feel" of the glider.  I feel that if a pilot decided to exert whatever roll torque was needed to hold his body rigidly at the centerline of the control bar, the roll stability dynamics of the glider would not be very much different than if he allowed himself to swing freely from side to side.  (It's a different story in pitch, as we'll see below.)  However I've only started thinking about this problem in these particular terms fairly recently and will be making more in-flight observations on this point in the future.

 

Q: In flight on a given glider with a given pilot, are there large variations in the position where a pilot's body tends to hang in relation in the control bar in the fore-and-aft sense?

 

A: Most definitely. The "neutral point" where the pilot's body tends to hang in relation to the control bar, in the pitch dimension and in the roll dimension both, is entirely dependent upon the direction of the G-loading acting on the glider, which is equivalent to the net aerodynamic force that is being created by the glider.  The part of the net aerodynamic force vector that would determine the pilot's "neutral position" in the roll axis is the aerodynamic sideforce created by a sideways airflow around the aircraft.  The part of the net aerodynamic force vector that would determine the pilot's "neutral position" in the pitch axis is the magnitude of the drag force, i.e. the L/D ratio, which is strongly dependent on the wing's angle-of-attack.  If a pilot wanted to hold himself in a fixed position on the control bar in the fore-and-aft sense, there would be many instances (for example when the glider was in a steep turn or when the glider had entered a steep dive) that he would have to exert a strong force with his muscles to remain in that fixed position.  We can view the pilot's muscle inputs as preventing him from swinging further forward (or in a few cases, from swinging further aft) in relation to the control bar, and we could view these muscle inputs as being exactly equivalent to the "pendulum" pitch torque that would arise whenever a heavy mass was rigidly fixed to the centerline of the base bar and the drag vector increased or decreased, causing a change in the direction of the net aerodynamic force that the glider was creating, i.e. causing a change in the direction of the G-loading, including a change in the magnitude of the drag force acting on the wing.  (In linear unaccelerated flight, this change in net aerodynamic force (G-loading) would be equivalent to a change in the glider's pitch attitude in relation to the horizon--clearly the "pendulum effect" created by a heavy mass rigidly attached to the base bar would create a stabilizing pitch torque to return the glider to its original pitch attitude if the glider's pitch attitude changed but the glider somehow continued on in linear unaccelerated flight.  A good example of a case that has little to do with linear unaccelerated flight, but where the pilot's "neutral position" on the control bar is still determined by the glider's L/D ratio, is when the glider is inverted at the top of a loop.)

 

Q: Apart from its effect on roll rate (see part 3 of this series), does adverse yaw produce enough aerodynamic sideforce to significantly slow a hang glider's turn rate (rate of curvature of the flight path) in relation to the bank angle at any given moment as the glider enters a turn? 

 

A:  As described above, the aerodynamic sideforce produced by a sideslip in a hang glider seems to be quite small.  This conclusion is based on two groups of in-flight experiments, as well as on the observations regarding "control feel" that are given above.  In one group of experiments, as I rolled a hang glider from wings-level into a bank, and as the glider adverse-yawed, I estimated the aerodynamic sideforce created by the sideways airflow over the glider by watching slip-skid bubbles.  In another group of experiments I noted the direction and angle of bank (which is entirely separate from the direction and magnitude of roll torque) that was required to create a straight-line, non-turning flight path when I used a rudder or a small wingtip-mounted drogue chute (don't try this at home without contacting me first!) to yaw the nose of a hang glider out of alignment with the actual direction of the flight path and relative wind.  Since the aerodynamic sideforces that I observed in these experiments were quite small, I don't feel that the adverse-yaw motions arising from a hang glider pilot's roll control inputs typically create enough aerodynamic sideforce to slow the turn rate (rate of curvature of the flight path) very much, for any given bank angle.

 

Q: Would a controllable rudder be a beneficial addition to an off-the-shelf modern hang glider?

 

A: If the aerodynamic sideforces resulting from a sideslip are small, using a rudder to eliminate sideslip would produce only a small increase in turn rate for any given bank angle.  On the other hand, the wing would likely be more efficient whenever there is no sideways component in the airflow.  But if the sideways airflow (sideslip) that is created by adverse yaw as a glider rolls into a turn really creates a significant performance penalty, then one would expect most hang glider pilots to install fixed vertical fins on the keels of their gliders to reduce adverse yaw.  Most hang glider pilots prefer to fly without fixed vertical fins on their gliders.

 

Also, a controllable rudder used in the "normal" way would have a very adverse impact on a glider's responsiveness in the roll axis, at least in some corners of the flight envelope.  See part 3 for more.

 

 

For more, 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 3: Roll torque created by the sideways airflow component as a hang glider sideslips

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