Experimental results: yaw experiments with a controllable rudder and wingtip-mounted drogue chutes on flex-wing hang gliders

Experimental results: yaw experiments with a controllable rudder and wingtip-mounted drogue chutes on flex-wing hang gliders

August 6, 2007 edition
Steve Seibel
www.aeroexperiments.org

General:

Beginning in approximately 2002, I've been carrying out some experiments with yaw control devices on flex-wing hang gliders. Some of these experiments involved a controllable rudder mounted on the keel. Others of these experiments involved small drogue chutes deployed in flight from the wingtips.

The controllable rudder was flown on a Wills Wing Spectrum, a Wills Wing Skyhawk, and an Airborne Blade. For some photos, see www.aeroexperiments.org/galleryexperiments.

The small drogue chutes were deployed from the wingtips of a Wills Wing Spectrum, a Wills Wing Skyhawk, a Wills Wing Raven, a Wills Wing Falcon, an Airborne Blade, a Icaro Laminar R-12, and an Aeros Stealth KPL. For some photos, see the link given above.

The small drogue chutes, and the rudder, all had the effect of yawing the nose of the glider to point to one side in relation to the actual direction of the flight path and relative wind and any given moment. For example, when I deflected the rudder to the left, or deployed a drogue chute from the left wingtip, the nose yawed visibly to the left and the yaw strings streamed to the left, showing that the nose was pointing to the left of the actual direction of the flight path and relative wind, so that the airflow over the glider had a right-to-left component. The yaw strings continued to stream to the left until the rudder was brought back to center or the drogue chute was jetisoned, regardless of any subsequent changes in the bank angle or the direction of the flight path.

Roll torque due to yaw or sideslip:

In general, the rudder created a "wrong-way" roll torque. In other words, the roll torque created by the rudder acted in the opposite direction from the roll torque that we would see when deflecting a rudder in most "conventional" airplanes or sailplanes with dihedral. In general, when I deflected the rudder to the left and locked it in that position, this created a continual right roll torque that made the glider try to roll (bank) toward the right. If I did nothing to counteract this roll torque, the glider would end up in a right turn with an increasing bank angle, even though the yaw strings demonstrated that the nose of the glider was pointing to the left of the actual direction of the flight path and relative wind at any given moment. To neutralize this right roll torque and make the glider fly in a straight line, I had to keep my body shifted to the left of the centerline of the control frame.

In general, a wingtip-mounted drogue chute also created a "wrong-way" roll torque. In other words, the roll torque created by a wingtip-mounted drogue chute acted in the opposite direction from the roll torque that we would see when making a yaw input in most "conventional" airplanes or sailplanes with dihedral. In general, when I deployed a drogue chute from the left wingtip, this created a continual right roll torque that made the glider try to roll (bank) toward the right. If I did nothing to counteract this roll torque, the glider would end up in a right turn with an increasing bank angle, even though the yaw strings demonstrated that the nose of the glider was pointing to the left of the actual direction of the flight path and relative wind at any given moment. To neutralize this right roll torque and make the glider fly in a straight line, I had to keep my body shifted to the left of the centerline of the control frame.

In the experiments with the wingtip drogue chutes as well as the experiments with the rudder, this "wrong-way roll torque" effect was always much stronger at low angles-of-attack (high airspeed) than at high angles-of-attack (low airspeed).

In the experiments with the wingtip drogue chutes as well as the experiments with the rudder, the "wrong-way roll torque" effect was always much stronger with the VG loose or absent than with the VG tight. With the VG fully tight, at the min. sink airspeed--i.e. in the high-angle-of-attack (low airspeed) part of the flight envelope--some of the gliders actually showed a dihedral-like response to a yaw input rather than an anhedral-like response. As the bar was pulled the bar in to decrease the angle-of-attack, the coupling between yaw and roll progressively changed from positive, to neutral, to negative-- i.e. the glider changed from showing a dihedral-like roll response to a yaw input, to showing no response to a yaw input, to showing an anhedral-like response to a yaw input.

The Airborne Blade was the only glider with a VG that was flown with both a rudder and a wingtip drogue chute (in separate flights.) Flying the glider with VG tight and noting the "neutral" airspeed where a left rudder deflection created neither a left nor right roll torque, and also noting the "neutral" airspeed where a drogue chute deployed from the left wingtip created neither a left nor a right roll torque, gave a way to compare the overall dynamics at play in these two experiments. The "neutral airspeed" with the drogue chute was only slightly higher than the "neutral airspeed" with the deflected rudder. This suggests that the two types of experiments were roughly equivalent. This suggests that in the rudder experiments, only a small part of the "wrong-way roll torque" was due to the fact that the rudder shifted the keel to one side in relation to the rest of the airframe. Nonetheless I still feel that the experiments with the wingtip drogue chutes offer a more valid representation of how a flex-wing hang glider responds to a sideways component in the relative wind, in normal flight.

Aerodynamic sideforce:

Note: the aerodynamic sideforce effects we'll discuss below are rather insignificant in comparison to the roll torque effects we discussed above. The aerodynamic sideforce effects are theoretically interesting, but play only a very minor roll in the glider's flight dynamics.

In addition to looking at the direction and magnitude of the roll torque that arose when I deflected a keel-mounted rudder or deployed a drogue chute from a wingtip of a flex-wing hang glider, I also used a slip-skid bubble to measure the direction and approximate magnitude of the aerodynamic sideforce that existed when the gliders flew in yawed attitude with the rudder deflected or with a drogue chute deployed from a wingtip. This sideforce was quite small.

Interestingly, in the experiments where a keel-mounted rudder was deflected to the left, causing the nose of the glider to point to the left of the actual direction of the flight path and relative wind, so that a yaw string streamed to the left, the slip-skid bubble actually deflected very slightly to the right. A conventional slip-skid ball would have deflected very slightly to the left. This is the opposite deflection of what we see when we deflect the rudder to the left in a "conventional" airplane or sailplane with a fuselage and fixed vertical tail. This indicates that the aerodynamic sideforce created by the rudder itself was actually stronger than the aerodynamic sideforce created by the relative wind striking the various surfaces of the hang glider in a sideways manner.

The above results indicate that the when the keel-mounted rudder was deflected to the left, the hang glider had to be banked slightly to the left to yield a straight-line flight path.

In the experiments where a drogue chute was deployed from the left wingtip of a flex wing hang glider, causing the nose of the glider to point to the left of the actual direction of the flight path and relative wind, so that a yaw string streamed to the left, the slip-skid bubble deflected very slightly to the left. A conventional slip-skid ball would have deflected very slightly to the right. This deflection is in the same direction as we see when we deflect the rudder to the left in a "conventional" airplane or sailplane with a fuselage and fixed vertical tail. The very small magnitude of this deflection shows that the impact of the sideways component in the relative wind against the various surfaces of the hang glider created only a very small aerodynamic sideforce.

The above results indicate that when a drogue chute was deployed from the left wingtip, the hang glider had to be banked slightly to the right to yield a straight-line flight path.

Click here for an expanded version of this article with more interpretation of the results.

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