Preventing spins with rigid wing hang gliders

During the past season several cases of spinning have been reported by pilots of rigid wing hang gliders. It is time to reconsider how and why this phenomenon occurs. Marcus Hoffman-Guben offers some valuable advice.

About the author:
Dipl.-Math. Marcus Hoffmann-Guben
German rigid wing hang glider champion 2001
Rigid wing hang glider test pilot for A.I.R. (2 years): Atos testing including world championships in 1999 & 2000.
Rigid wing hang glider test pilot for Flight Design (2 years): Exxtacy 135, Exxtacy Bi, Ghostbuster, Axxess, Axxess+ testing including world championships 2001.

Translated: WildArt! Creations 2002

Not just a beginners problem

Several experienced pilots have reported entering spins completely unexpectedly under "normal flying conditions". The physical explanation for this is usually that the pilots in question do not recognise the point when air-flow across the wing breaks off partially and the wing goes into a stall, until it is too late. However, there are some constructional trimming steps which can be taken to reduce the danger of spinning.

Goals
In order to develop effective strategies for the prevention of spinning it is initially necessary to recognise when, how and why spinning occurs.
The following points have been put together with the aim of sharpening the awareness of rigid wing pilots - firstly to help them analyse their instinctive and conditioned responses learnt from flying flexible hang gliders, and secondly to help them change or adapt their reactions and responses accordingly. Additionally, specific constructional details are highlighted which can result in an increased susceptibility to spinning in rigid wing hang gliders.

To emphasize: neither it is wished to stir up fears of spinning a rigid wing hang glider, nor should real existing problems be ignored or relativized.

The spin
Spinning a hang glider is a three-part process:

Entering
Spinning
Recovering

1. Entering a spin
A spin on a rigid wing hang glider is induced when the pilot causes air-flow on one wingtip to break off and hence stalling it, by pushing the control bar too much while trying to tighten a curve.

Aerodynamically viewed, provoking a spin is not a question of air-speed but only dependant on the angle of attack of the wing !

However, as a reduction in airspeed is achieved through changing the gliders angle of attack, spinning rigid wing hang gliders is often associated with slow flying in practice.

2. Spinning
While in a spin, the rotation speed, flight-path diameter and bank angle of the glider can not be controlled through normal steerage impulses, they are far more dependant on the mass distribution within the wing:

While spinning, gyro-forces are dominant and result in non-controllable, non-steerable flight !

This makes spinning with rigid wing hang gliders particularly dangerous !

3. Recovering
In most cases it is possible to recover from a spin, as long as the height reserve is sufficient.

Factors which influence the susceptibility to spinning

Three of the main factors which influence the susceptibility of rigid wing hang gliders to enter spinning are: 1. wing twist ; 2. control bar position ; 3. centre of gravity (trimming). Other factors such as the distribution of lift across the wing, the wing profile distribution, the wing twist distribution and the wing taper are not considered here, but certainly influence the spinning characteristics of a rigid wing in a very complex manner.

1. Wing twist
Wing twist is necessary in swept-back rigid wing designs to enable comfortable handling of the hang glider during curved flight. The wingtips are generally twisted up (negatively) by 5 to 7 degrees. If the critical angle of attack is exceeded then the twist at the wingtips helps to prevent the tips from stalling first and therefore (in curved flight) inducing a spin.

Wing twist helps to prevent tip-stalling and reduces spinning susceptibility in rigid wing hang gliders !

The negative influence of increased twist at the wing tips (especially when flying at highspeed final glides) is usually the reason that competition pilots try to tune their gliders to actively reduce the twist. But too less twist can dangerously influence the stall characteristics of the glider.

For safety reasons never reduce the wing twist under the limits set in the gliders certification !

2. Control bar position
When surprised by unexpected turbulence in the air, pilots generally respond automatically by adopting an ergonomic "alert position" - pushing the bar backward to a particular position where they feel ready to react quickly. Control bars which are mounted further forward increase the static margin (longitudinal stability) of the glider when pilots go into this "alert position" and reduce the danger of stalling a tip and entering a spin in turbulent air.

If the designer of the glider has mounted the bar sufficiently far forward then unwanted stalls and spins will be almost impossible due to restricted pilots arm length !

3. Centre of gravity and trimming effects
Flexible hang gliders are usually trimmed to fly too slow, to improve comfort and handling when thermalling. Although this is not too critical on a flexible hang glider, this is not the case with a rigid wing hang glider - trimming habits should not be inherited!

Should a rigid wing hang glider be trimmed too far back, entering a thermal and inducing a turn at the same time can then lead to a loss of control, unexpectedly stalling one side of the wing and resulting in a spin. That aside, a slowly trimmed rigid wing hang glider requires much less pressure on the bar to stall it than a faster trimmed one which also increases the risk of unexpected stalling and spinning.

Should a lightweight pilot borrow a heavier pilots glider then it is important to re-adjust the centre of gravity by bringing the hang loop point further forward! Read the guidelines in the gliders manual !

Spoilers and their effects

When a spoiler works it doesn’t only create the required roll and yaw forces, but also induces an unwanted pitch moment nosing up.

When a turn is started it is important that speed is maintained by pulling on the control bar to compensate for spoiler pitch-up (during the turn and not just before the turn) !

The main danger from the induced spoiler pitch is when thermalling slowly (just above the stall point of the wing), and then trying to core the thermal better by activating the spoiler. This can then abruptly stall the inner wing and cause a spin !

Flaps

Pulling on positive flap (i.e. angling them down) increases the lift of the wing at constant airspeed. Through this the trim-speed and minimum speed of the wing are consequently reduced. This effect is measurable on all current rigid wing hang gliders (see the test reports in "Fly and Glide": Exxtacy (5/1998 p. 38), Atos (7/1999 p. 33), ESC (8/2000 p. 48), Atos S (11/2000 p. 37 ff.), Exxtacy Bi (9/2001 p. 44) and Star (7/2001 p. 56, 58)).

Pulling on flap a rigid wing hang glider can have either a pitch-up or pitch-down effect, depending upon the particular model. Pitch-down (i.e. the bar moves back on its own accord) is noted on the E7 (see test report in "Fly and Glide" 7/1999 p. 32), Atos, Atos S ("Fly and Glide" 11/2000 p.32) and Star ("Fly and Glide" 7/2001 p. 56, 58). On the Exxtacy and Ghostbuster models, pulling on the flaps creates a pitch-up moment (i.e. the bar moves forward).

Through flaps which induce a pitch-down moment, because the bar moves back, the pilot has a greater range to push-out forwards from the trim position. This means it is then easier to stall the wing and this in turn increases the danger of unexpectedly entering a spin while in a turn. (see for example the test report on the Atos S for small pilots with short arms in "Fly an Glide" 11/2000 p. 38).

While thermalling the flaps should not be set at more than 15 degrees !

The advantage of flaps which induce pitch-up moments is clear - here the danger of pushing out too far and stalling is reduced, which is also positive should the pilot go into the "alert position" in turbulent air.

Moving A-frame

As the moving A-frame of a rigid wing hang glider is used to control the spoiler cables and is not tensioned in place by cables as with a flexible hang glider, during a spin the pilot and A-frame will be thrown towards the outer wing by gyro-forces. If the pilot continues to hold the bar then this creates an unwanted and uncontrolled spoiler reaction on the inner wing. This in turn makes it more difficult for the air flow across the inner wing to recover, which is necessary to exit from the spin. Accordingly, pulling in on the bar must be increased to help recover from the spin.

Because of the partly chaotic behaviour of a rigid wing hang glider in a spin, large gyro-forces result especially during the recovery phase. These forces, often caused by the unwanted spoiler reactions, can stress the glider over its structural limits and in some cases can lead to its destruction (see for example the report from the experienced US competition pilot Bo Hagewood who invoked a spin on a large Atos and subsequently broke it ("The Oz report", under www.davisstraub.com/OZ/)!

Spin susceptibility of rigid wings compared to flexible hang gliders

The empirical results of an opinion poll which I have conducted under many experienced rigid wing hang glider pilots and the evaluation of my own year-long experience as a test pilot for different manufacturers lead me to the following conclusions:

Even though certified rigid wing hang gliders are constructed with much less twist when compared with flexible hang gliders, they have no general tendency to be more susceptible to unexpected spinning !

This would appear to contradict the current public opinion of the hang gliding community. However, depending on the particular model, even without the VG full on, it is often easier to start a flexible hang glider spinning than a rigid wing hang glider in its certification configuration. This last sentence is important:

Only fly rigid wing hang gliders in configurations that have been certified !

Although rigid wing hang gliders are not generally more susceptible to spinning than flexible hang gliders and therefore not generally more dangerous, things are a lot different when are actually in a spin! Due to the problems mentioned above which are caused by the A-frame mounting and often result in chaotic states during uncontrollable spins, the danger associated with spinning rigid wing hang gliders is significant greater than that of a flexible hang glider!

Experimental spinning with rigid wing hang gliders should definitely be avoided !

Remedies

Being conscious of your own reflex reactions and trying to consciously change them

By flying flexible hang gliders certain reflexes are learnt, and with time become instinctive reactions. Such conditioned reflexes must be consciously controlled to prevent them from influencing rigid wing flight negatively. Some of the most commonly observed unconscious reflex reactions which can provoke critical situations if they are applied to rigid wing hang gliders are the following:

Landing out of a slow turn
Flying too slowly and too close to obstacles during landing approach
While thermalling, flying on the stall-point and trying to squeeze into the core by impulsive diagonal steer movements.

In order to fly a rigid wing glider safely, it is important to be consciously aware of the air-flow over the wing at all times !

Warning signals at the stall-point

Before a rigid wing hang glider goes into a stall, the wing sends certain "warning signals", which can be of great help to a receptive pilot. As the bar is pushed forward from the trim position:

the bar pressure increases
the sink rate increases
the directional stability is reduced (turn to one side)
the steering control is reduced (the spoilers do not work as effectively as at lower airspeeds)

It is important to recognise these "warning signals" immediately. In turbulent air this is often difficult especially for beginners.

Thermalling at the stall point and then "squeezing" into the core as is often performed with flexible hang gliders, can lead to a spin if this technique is applied on a rigid wing hang glider !

Outlook

The advances made in sail-planes over the past few years reveal a lot of interesting developments which in my opinion are very relevant for rigid wing hang glider designs: instead of merely trying to improve the glide-angle the emphasis is on making handling characteristics more easy while maintaining the same performance.

Developing rigid wing hang gliders along these guidelines would lead to changes which include modifications to the profile depth distribution, increased twist at the wingtips and control bar positioned far enough forward.

Note: Further information and in-depth aerodynamic details for a better understanding of some of the problems mentioned here can be found on the DHV homepage under "News" in an informative article on spinning.