Since the wiki doesn't have as much documentation on using Arducopter with traditional helicopter setups, I'm wondering if anyone can give me a nudge in the right direction.
I'm planning to automate takeoff, landing and basic flight using Arducopter on a traditional helicopter, but I'm not sure how to program throttle modulation to account for ground effect during both takeoff and landing.
Does anybody know how to mathematically model the ground effect into a function that I can use to dynamically adjust the throttle of the craft?
I know the easiest way is probably to set the throttle to a very high value until it reaches the altitude wherein ground effect becomes negligible but I'm looking for a more efficient method of doing this, wondering if anybody has any idea.
If making an accurate mathematical representation is too strenuous, I'm wondering if there's a better way of programming arducopter to dynamically alter the throttle based on the strength of ground effect(maybe by measuring the pressure in the area under the rotors by measuring its proximity to the ground relative to the length of its rotors).
Any help would be greatly appreciated, thank you in advance!
Replies
I am also interested in automating takeoff and landing of traditional helicopters. I am familiar with flying them manually (RC ones I mean), and I can say that the ground effect is very unpredictable and depends a lot on wind conditions and what surface you are near.
The question "does anybody know how to model the ground effect" is the right one to ask, as in "does anybody on the planet know". It is really, really complicated. Flight simulators based on fluid dynamics do a decent job of it, but none of that can really be applied to a real-life control situation because we can't measure 3D air flow that way. When a system is too chaotic to model, you have to rely on reactive control systems using the parameters that you can actually measure.
I just found this 1971 NASA paper on the subject that may provide some insight: http://archive.org/details/nasa_techdoc_19720006337 (public domain). They conclude several interesting things that are applicable in our situation. The first is that the profile of lift versus height varies dramatically with pitch angle. Your idea of adding pressure sensors under the rotor is interesting but I don't think it will tell the whole story, since the main variation is the difference between the pressure at the center and the tips of the blades.
The second conclusion is that the "slipperly" sideways behavior while hovering is due to a periodic pattern of up- and down-wash vectors that tilt the effective thrust vector in a pretty much chaotic pattern. This happens in a sub-second time scale and I don't think could be adequately measured by pressure sensors either.
Neither effect is one that I would want to compensate for analytically; in both cases a high-bandwidth control loop will be required to compensate for whatever disturbance they cause. I was looking at the PX4 optical flow sensor, and it seems like a promising way to hover in the presence of ground effect. It would probably require a very different set of gains or control laws, but I don't think we could land a traditional heli without it. GPS would be way too slow and coarse to hold position accurately enough to land without tipping over. A very good altitude measurement is also required (and maybe a switch on the landing gear), because the most important part of landing is slamming the collective pitch to negative once you touch the ground.
If I don't find a suitable solution to this problem, I may end up making it my Master's research thesis. We have an excellent rotorcraft group in the Aerospace engineering department at my university, so I may be able to find someone there who already knows the answer or how to approach it.