Usually this is not done intentionally, but rather as a result of the pilot forgetting to "hook in". The weaker the wind and the less lifting force the glider applies to the pilot, the more the hang strap must be raked forward, while the direction of the net aerodynamic force created by the glider is fixed by the glider's L/D ratio so long as angle-of-attack remains constant.īut perhaps it is sufficient to observe that many hang gliders have been seen to fly rather well, at least in terms of basic pitch stability, when launched off a hill with no person attached.
Since the forward force exerted by the pilot's feet on the glider must be equal to the drag of the glider, it follows that net aerodynamic force created by the glider must be closer to vertical, than the line of the hang strap is. The net aerodynamic force vector created by the glider has a ratio of vertical : horizontal equal to (weight of glider + upward lifting force exerted on pilot) : (drag of glider). The hang strap's ratio of vertical : horizontal must be equal to (the upward lifting force exerted on the pilot) : (forward force exerted by pilot's feet on glider). Note that the hang strap and the net aerodynamic force vector from the glider are not exactly parallel to each other, as the pilot stands on flat ground in the wind with the hang strap tight. If anything, there may be some slight tendency for the glider to pitch up to a higher angle-of-attack when the wind is light, which may suggest that the glider's CG is slightly behind the point of connection with the hang strap.
My experience is that, standing on flat ground with the hang strap tight as described above, scaling the wing force up or down doesn't significantly change the glider's trim angle-of-attack, suggesting that the hang strap and the wing's net aerodynamic force vector are both approximately in line with the CG of the glider. Isn't scaling the wind force up and down exactly the same as scaling the glider's weight up and down while keeping the wind force the same, assuming that the glider isn't changing shape under load significantly? So if the trim angle-of-attack stays the same as the wind speed changes, doesn't this suggest that the hang strap, and the wing's net aerodynamic force vector, are both directly in line with the CG of the glider itself? In other words, does scaling the wind force up and down change the glider's trim angle-of-attack? If the pilot exerts no "push" or "pull" on the control bar with his hands-if he lets go of the control bar entirely- does the glider tend to trim to the same angle-of-attack when the wind is just barely blowing hard enough to keep the hang strap tight, so that the glider is barely lifting more than it's own weight, as when the wind is blowing so strong that the pilot has very little weight on his feet? One way to think about this is to consider the case where the pilot is standing on flat ground with the wind blowing, with the glider producing enough lift to lift all of it's own weight and some of the pilot's weight, so that the hang strap is tight. In this case the hang strap can be considered to be the only connection between the glider and the pilot, which means the pilot's weight can be assumed to act as if it is located at the point where the flexible hang strap connects to the glider.Īnd where is the CG of the glider itself, in relation to the point of connection between the hang strap and the glider? If we are interested in how the glider flies " at trim", then we want to know what happens when the pilot exerts zero force with his arms. In the latter case, we don't constrain the pilot to be in a fixed position relative to the glider, but instead are interested in the force that the pilot must exert on the control bar with his arms to get the desired result, and the resulting torque exerted on the glider by the pilot. We can view a hang glider (with attached pilot) either as a single rigid system- in which case we must assume that the pilot is exerting whatever force is necessary to hold himself in a fixed position with his arms- or as two independent bodies connected by a flexible strap and also by the pilot's arms.
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