Slip (yaw) roll coupling in flex-wing hang gliders

Interpreting in-flight observations: roll torque created by the combined effects of anhedral and sweep in flex-wing hang gliders, VG off versus VG on, high airspeed versus low airspeed

Steve Seibel
www.aeroexperiments.org

October 13, 2006 edition

 

In "Interesting experiments: adding a controllable rudder and other yaw devices to 4 flex-wing hang gliders", we explored what happened when we made experimental yaw inputs while flying flex-wing hang gliders.

Here's how we can interpret these results in terms of the "downwind" or "upwind" roll torque, or the positive or negative "coupling between yaw (slip) and roll", created by the combined effects of anhedral and sweep.

In "Competing effects of sweep and anhedral", we found that if a wing has both sweep and anhedral, then for a given yaw (slip) angle, the dihedral-like effect of sweep will tend to dominate when the wing as a whole is flying at a high angle-of-attack (low airspeed), and the anhedral will tend to dominate when the wing as a whole is flying at a low angle-of-attack (high airspeed). Depending on how much sweep or anhedral are present, one or the other of these effects may end up dominating across most of the flight envelope.

The results of the experiments with the controllable rudder and the wingtip drag devices strongly suggest that many flex-wing hang gliders with no VG, and many flex-wing hang gliders with the VG full off, have enough anhedral to create a net "upwind" roll torque or net "negative coupling between yaw (slip) and roll" at all angles-of-attack, except perhaps in some cases at angles-of-attack very close to the stall. Because of the way that the competing effect of sweep is most pronounced at high angles-of-attack, this net "upwind" roll torque or net "negative coupling between yaw (slip) and roll" is much stronger at low angles of attack than at high angles-of-attack. In other words, this net "upwind" roll torque or net "negative coupling between yaw (slip) and roll" is much stronger at high airspeed than at low airspeed. In other words, the flight characteristics arising from the wing's anhedral geometry are most pronounced at high airspeeds, but dominate across most or all of the flight envelope when the VG is off or absent.

This net "upwind" roll torque or net "negative coupling between yaw (slip) and roll" allows a flex-wing hang glider to harness the sideways airflow arising from adverse yaw, to create a helpful roll torque. This effect will be most pronounced at low angles-of-attack (high airspeed). This will cause the glider to respond more quickly to the pilot's roll inputs than would otherwise be the case. In other words, the peak roll rate that can be sustained with a given weight-shift roll input will be higher than would otherwise be the case.

The results expressed in "Interesting experiments: adding a controllable rudder and other yaw devices to 4 flex-wing hang gliders" strongly suggest that most flex-wing hang gliders have more anhedral with the VG off than with the VG on. After all, the Airborne Blade exhibited a net anhedral-like "upwind" roll torque or a net "negative coupling between yaw (slip) and roll" even in the low-speed portion of the flight envelope with the VG fully off, but exhibited a net dihedral-like "downwind" roll torque or net "positive coupling between yaw (slip) and roll" in the low-speed portion of the flight envelope with the VG fully on. This is consistent with our visual observations of the geometry of flex-wing hang gliders, as described in "Looking at anhedral in flex-wing hang gliders: VG off versus VG on".

This relationship--the decrease in anhedral that occurs as the VG is tightened-- is undoubtedly of the reasons why flex-wing hang gliders respond more slowly to pilot roll inputs when the VG is fully on than when the VG is fully off. In the low-speed, high angle-of-attack portion of the flight envelope with the VG fully on, when the pilot begins to roll the glider from wings-level into a turn and the glider adverse-yaws and this creates a sideways airflow over the aircraft, the sideways airflow will create a net "downwind" or unfavorable roll torque that will slow the glider's roll response to the pilot's roll control inputs. (Of course, the reduction in billow-shift that occurs when the VG is tight is another reason why flex-wing hang gliders respond more slowly to pilot roll inputs when the VG is fully on than when the VG is fully off.)

Note that our test glider--an Airborne Blade--had a conventional pulley VG system. In a glider with a cam VG system, the leading edge tubes will stay "in plane" with the keel tube, rather than dropping downward with respect to the keel tube, as the VG is system tensioned. This means that when the VG system is tensioned, a glider with a "cam" VG system will see an even greater decrease in the anhedral geometry of the outboard parts of the wings than will a glider with a conventional pulley VG system. In light of this, it seems valid to conclude that most flex-wing gliders with VG systems of any kind will have more anhedral in the outboard parts of the wings when the VG is loose than when the VG is tight. By the same logic, at any given airspeed, most flex-wing gliders with VG systems of any kind will show a weaker "upwind" roll torque or "negative coupling between yaw (slip) and roll", or a stronger dihedral-like "downwind" roll torque or net "positive coupling between yaw (slip) and roll", when the VG is tight than when the VG is loose. At the high-speed end of the flight envelope there will likely be an "upwind" roll torque or "negative coupling between yaw (slip) and roll" regardless of the VG type and the VG setting. The gliders where we are most likely to see the strongest dihedral-like "downwind" roll torque or net "positive coupling between yaw (slip) and roll" at the low-speed end of the flight envelope with the VG tight are the gliders with cam VG systems.

 

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