In iterating over the landing gear design for my flying wing, I have realized that I will need to use a "fly off" type configuration, where takeoff is accomplished by simply increasing speed with all three wheels on the ground until air born, rather than rotating at a lower speed. This is because pitch authority is limited; it's enough for rotation, but it doesn't leave a lot of slack, and it makes main gear location really, really critical. And With the main gear correctly placed on this pusher, there's no way at all it's going to stay on all three gear when the pilot gets out... and that violates a requirement for me.
However, in my analysis, the following issue came up. Assuming I want to "fly off" at about 1.1*Vs in takeoff configuration (20° flaps, for various reasons), this corresponds to an 8° AoA (referenced to the cruise AoA). Unfortunately, this is greater than the maximum angle of attack (at 0.9*Vs) in the landing configuration (40° flaps). In other words, using an 8° gear angle would make it impossible to land main-gear first. As a secondary issue, the 8° ground angle, combined with relatively long gear (pusher prop clearance), would make getting in and out uncomfortable. Lowering the gear angle enough to make normal landings possible would make take-off speed uncomfortably high (>1.25*Vs).
Reasoning independently, the gear angle as determined by landing configuration should be such that (a) the main gear touches first; (b) at max touchdown speed, no tendency to take back off in ground effect when all three wheels are on the ground; and (c) a no-rotation go-around is possible. (a and b are actually equivalent here.) These requirements can be satisfied with a gear angle of about 2° (depending on takeoff speed for go-around).
Finally, for ground operations, lower angle is better. Visibility is not a big issue (15°+ relative to cruise AoA), but getting in and out is. 2° would be fine for this; -2° would be better, if it's free (no additional design).
One way I envision doing this is via a retractable nose-gear strong enough to support full operation in any position (my understanding is that the eznoselift gear has this property). From back to front, the positions would be:
1) Full retracted;
2) Forward enough to give a -2° angle, used only for ground operations and parking
3) Perpendicular to the ground, giving an 8° angle used for takeoff
4) Forward about 15° beyond this position, giving a 2° angle used for landing
Getting in is easy; you're parked at -2°. Ground maneuvering is optimal. At run-up, angle is changed to 8°; mechanism is strong enough to lift the nose with pilot in it, of course. All operations are possible in the position, although landings must be done with partial or no flaps. In the air, rear retraction is straightforward. On approach, flaps to 20°, gear to take-off position (moving forward). Flaps to 40°, gear to landing position. If full flaps are never applied, gear stays in take-off position to allow easier go-around. As with the eznoselift, manual system will have to be a crank rather than just gravity.
Thoughts? Am I nuts?
However, in my analysis, the following issue came up. Assuming I want to "fly off" at about 1.1*Vs in takeoff configuration (20° flaps, for various reasons), this corresponds to an 8° AoA (referenced to the cruise AoA). Unfortunately, this is greater than the maximum angle of attack (at 0.9*Vs) in the landing configuration (40° flaps). In other words, using an 8° gear angle would make it impossible to land main-gear first. As a secondary issue, the 8° ground angle, combined with relatively long gear (pusher prop clearance), would make getting in and out uncomfortable. Lowering the gear angle enough to make normal landings possible would make take-off speed uncomfortably high (>1.25*Vs).
Reasoning independently, the gear angle as determined by landing configuration should be such that (a) the main gear touches first; (b) at max touchdown speed, no tendency to take back off in ground effect when all three wheels are on the ground; and (c) a no-rotation go-around is possible. (a and b are actually equivalent here.) These requirements can be satisfied with a gear angle of about 2° (depending on takeoff speed for go-around).
Finally, for ground operations, lower angle is better. Visibility is not a big issue (15°+ relative to cruise AoA), but getting in and out is. 2° would be fine for this; -2° would be better, if it's free (no additional design).
One way I envision doing this is via a retractable nose-gear strong enough to support full operation in any position (my understanding is that the eznoselift gear has this property). From back to front, the positions would be:
1) Full retracted;
2) Forward enough to give a -2° angle, used only for ground operations and parking
3) Perpendicular to the ground, giving an 8° angle used for takeoff
4) Forward about 15° beyond this position, giving a 2° angle used for landing
Getting in is easy; you're parked at -2°. Ground maneuvering is optimal. At run-up, angle is changed to 8°; mechanism is strong enough to lift the nose with pilot in it, of course. All operations are possible in the position, although landings must be done with partial or no flaps. In the air, rear retraction is straightforward. On approach, flaps to 20°, gear to take-off position (moving forward). Flaps to 40°, gear to landing position. If full flaps are never applied, gear stays in take-off position to allow easier go-around. As with the eznoselift, manual system will have to be a crank rather than just gravity.
Thoughts? Am I nuts?
