By Dennis Pagen
These guidelines are in no way intended to replace your owner's manual as a source of correct information or a training program under the guidance of a certified instructor. They are presented to describe some of the unique attributes of rigid wings that a safe pilot must understand and to point out precisely why you need to heed your owner's manual and instructor.
Rigid Wing Background
Rigid wings have been a part of hang gliding since the very beginning of flight. Indeed the wings of Otto Lillienthal and the gliders of the Wright Brothers were rigid wing hang gliders. The development of foot-launched rigid wings has paralleled that of flex wings, although for more than a decade beginning in about 1980, new rigid wing designs were rarely marketed. Currently, however, a host of new rigid wings have found sudden popularity. It is due to this resurgence of rigid wings that we offer this timely guideline.
Rigid wings differ from flex-wings in that the former have a solid construction so that the wing surfaces remain more or less fixed throughout the entire flying regime. Contrasting to this arrangement is the wing of a flex-wing glider which is simply a sail held in an airfoil shape by battens but otherwise free to shift from side to side and change in twist. Rigid wings maintain the wing's shape with the airframe structure, while with a flex-wing the wing shape is maintained by the structure and tension in the sail itself.
It is the flexibility of a flex-wing's sail that allows it to be controlled by weight shift. The rigidity of a rigid wing normally requires aerodynamic control systems (spoilers, tip rudders, elevons, etc). This factor is what determines one of the main differences between a rigid wing and a flex wing. Some rigid wings, however, control pitch (airspeed) with weight shift and roll (banking) with aerodynamic controls that are connected to a movable control bar which thus simulates weight shift. Other rigid wings have full 3-axis controls activated by a stick like a sailplane or airplane.
The other great difference between rigid and flex-wing gliders is the lower twist (washout) in the rigid wing. The twist in a rigid wing is controlled with the airframe -- either a leading edge D tube or a second spar at the rear of the wing -- while on a flex wing it is held by sail tension. The result is that rigid wings can utilize greater spans without suffering from increased twist in the wings as do flex wings. Greater spans and reduced twist allow better performance in general.
The drawback to lower twist or rigidly held twist is that spins are more likely. It is a fact that rigid wings spin more readily than a flex wing. The reason is that twist in a wing is a gradual lowering of the angle of attack from the center (root) of the wing to the tip. The lower angle of attack at a wing's tip helps prevent it from stalling in a turn, which is the normal cause of a spin. Flex wings allow their twist to increase on the inside wing as a turn is initiated. Rigid wings (including sailplanes and airplanes) do not. For this reason we issue to following warning:
Rigid Wing Flight Warning
All rigid wings can spin. Inadvertent spins are entered by turning with low to medium bank angles at slow speeds, then either slowing further, encountering turbulence or making an abrupt roll control. Two hundred feet or more can be lost quickly before recovery. The recovery itself may overstress the wing if the pilot does not exercise extreme caution.
The fact that rigid wing hang gliders are used frequently to soar close to a ridge or hill and thermal in light or turbulent conditions make them potentially vulnerable to spins since slow flight and shallow turns are required for success.
The possibility of overstressing and breaking a rigid wing in flight is of greater concern than with a flex wing since the latter is more elastic and relieves its load as the G forces increase. All aircraft have two important limitation speeds: the first is the maximum safe speed for maneuvering (Va) and the second is the maximum safe speed for straight flight in smooth air (Vne). Almost all rigid wings can be made to exceed their safe operating speeds (Va and Vne) quite readily. In this sense they have more in common with sailplanes and airplanes than class 1 hang gliders.
Weight-Shift Rigid Wings
Weight-shift rigid hang gliders simulate the control feeling of a flex wing, but use aerodynamic controls (usually spoilers or tip rudders) for roll. Some of these designs may be resistant to spins because they incorporate more twist and wider tip areas than others do. However, all pilots of these designs should know the spin characteristics of their craft.
Learning to fly these hang gliders with no previous hang gliding experience must be approached as carefully as when learning to fly any aircraft. We recommend learning to fly in a flex-wing hang glider, then transitioning to a higher performance flex wing and finally to the rigid wing. Dual training in the rigid wing is desirable. We also recommend towing under a very experienced instructor as the preferred training method so takeoffs can be performed from a cart with wheels. The greater weight of rigid wings compared to flex-wings makes them undesirable for beginning foot-launch training.
The transition to a "weight-shift" rigid wing from a flex-wing is quite easy because they basically launch and land the same. However, a different feeling and slightly different technique on launch may be experienced. Furthermore, roll rates are usually different. Finally, the potential for spinning and exceeding the maximum safe airspeed must be considered. We recommend spin training in a sailplane with awareness of V speeds for pilots transitioning to weight-shift rigid wings.
3-Axis Rigid Wings
Rigid wings with full 3-axis control have all the benefits of the weight-shift rigid wings, as well as all the spin and speed limitations. Furthermore, since they are frequently operated from wheels, have pilot enclosures and may be heavier, flex-wing experience has less value when transitioning to 3-axis rigid wings.
In either case -- starting from no experience or transitioning -- we highly recommend sailplane lessons before learning to fly 3-axis controlled rigid wings. There are not many two-place 3-axis rigid wing hang gliders, so dual sailplane training is desirable. This training should include stall recognition, spin entry and the necessity to obey the airspeed limits.
Of a bit lesser value is training in powered ultralights or airplanes covering the matters listed above. These aircraft tend to fly with heavier wing loadings so airspeeds and stick feel may be different from what you will encounter on the hang glider.
Flying wing hang gliders and even those with conventional tails tend to have very different flying characteristics compared to sailplanes and airplanes. For example, while conventional aircraft strive for balanced controls, that is, the same force and reaction in pitch and roll, this isn't possible with hang gliders. To do so would cost too much in complexity and weight. Thus, on many 3-axis hang gliders pitch control may be very sensitive while roll control is much less so. The important point is to be aware of this possibility (read the owner's manual and listen to experienced pilots' reports) when you transition from other aircraft to a 3-axis rigid wing hang glider.
Finally, if a rigid wing is equipped with flaps as most are, you should be aware of the effects of these devices. Flaps greatly alter the airfoil shape and thus its flying characteristics. On most hang gliders, the effects of flaps are benign, but they may affect behavior in takeoff, landing and stall as well as the feeling of roll, pitch and trim. Use flaps sparingly in smooth air with plenty of ground clearance until you learn their effects.
Flying rigid wings can be a safe and rewarding way to take to the air. They are not necessarily more difficult or complex to fly than flex wings, they are just different. Do not assume you can readily adapt to them without a careful, gradual approach. Proper instruction can save you expense and help prevent injury. Remember to learn the glider's limits carefully with plenty of altitude in reserve.