Servers by jDrones

Collective Pitch Quad support

(Bill Geyer) #101

OK so I may have been hasty with my point. thanks for keeping me honest. cyclic is used to tilt the tip path plane to overcome the rotor response to forward airspeed (i.e. dissymetry of lift) and keep the thrust vector tilted forward. Allowing the rotor to flap can also be use to balance dissymetry of lift which is done in the case of a gyro copter which lives in a state of autorotation. Athough they are being pushed through the air by a propeller.

So the challenge becomes keeping the nose down attitude to maintain forward flight with the rotor in a blow back state. So collective pitch can be increased on the aft rotor(s) and decreased on the forward rotor(s). As long as the rotors are interconnected as one increases torque due to increased collective pitch, that load will be shared and rotor speed will be maintained at some equivalent state between the rotors.

This is just a design consideration. Keep the arms long enough to overcome this issue.

Again if they are interconnected how will the rear stall. it is using the front rotors to power it.

So I always appreciate your insight. You bring up many good points for design consideration for the CP quad. I’m not sure, but maybe you do, have the data to show that what I’m suggesting won’t work. My point is that autorototation is the state of using airflow through the rotor disk to maintain rotor speed. autogyros show that longitudinal cyclic is not required to maintain the rotor in a state of autorotation but flapping is required to account for the dissymetry of lift in forward flight. I think your point is that a successful autorotation in practice requires some forward airspeed to allow for the transition from descent to landing and in conventional helis that means longitudinal cyclic. But I don’t think it is out of the realm of possible for CP quads to perform autos to touchdown. Key design considerations include
Interconnected rotors
higher inertia blades
keeping the blades away from the fuselage

Great discussion!

(Brad Wilkinson) #102

I think you need to use Copter 3.6.0 or later for the heli-quad. That’s what is mentioned on the web doc, right under the first video.:

This capability was only added in October 2017, but I don’t know the dates on the revisions so I don’t know for sure how old 3.6 is.

I’ve had other problems with my motor esc calibration and such, but everything else seems to be working fine. Can’t fly yet because of esc problem. I’m using the Pixhawk 4, but I think the 3.6 version works fine on the other Pixhawk, too.

(Chris Olson) #103

I think the issue here is driveline losses. There is not a lot of extra power to work with in autorotation. If you use the more efficient rotor (front one) to drive an inefficient one (rear) you increase the descent rate, which is exactly what happens to a CH-47.

Increasing the descent rate results in having to have more stored energy (that isn’t there) to arrest it in the flare. Which results in a crash.

higher inertia blades

This can improve the autorotation capability of any helicopter. The Cabri G2, for instance, has steel weights in the blade tips specifically for that purpose.

keeping the blades away from the fuselage

Look at an induced flow diagram for autorotation profile. Not only shrouding by the fuselage, but tip vortices, turbulent air, and energy already robbed from the air by rotors in front hinder performance of rotors in the rear. On a quadcopter, in an attempted autorotation profile, that induced flow thru the front rotors goes dead thru the rear no matter how far away you have them. In a powered flight profile they are more like a Chinook design. That is the serious issue with effectiveness of the rear rotors - they become a drag source and do not contribute to autorotation profile.

Again, this problem is apparent in the CH-47. It flies on the front rotor, “real stable all the way to the crash site”. And is the reason it is not certified by Boeing, or even mentioned in the US Army training manual - it is considered impractical to even try it.

FWIW, the V-22 cannot be autorotated either.

And for reference sake, the largest and most powerful helicopter on the planet - the Mil Mi-26 Halo - autorotates beautifully. There is two autorotation landings recorded with civilian Mi-26’s, both done with approximately 20 tons of payload onboard. Considering a Mi-26 has 23,000 shaft horsepower, is the size of an Airbus A320, and can carry a payload equal to a C-130 Hercules, the fact that it was designed to be autorotated with both engines out is pretty impressive.

(Chris Olson) #104

For folks that want to practice autorotation with ArduPilot - helicopter or CP quad, it can be done. ArduPilot was never designed for it - but with our new throttle curve in 3.6 a lot of practice can be done safely to learn the basics.

I recommend starting from about 6 feet with your high headspeed setting. You’re normally get way more headspeed in the auto flare than you use in normal powered flight. So to simulate the flare use the high headspeed setting.

Simply shut down the engine. Your initial response will be to over-pitch it - you’ll use “panic collective”, it will flare up, stall the rotor and just drop like a rock. Practice collective management until you can set that helicopter down soft as a feather from 6 feet.

Now, switch to the new throttle curve and set the curve 0 point to a setting that leaves the engine running but the rotor is free-wheeling. Set your pitch with collective down to -2.5. Fly your helicopter up to about 100 feet and chop the power by pulling the collective down. This will teach you very quickly that you can’t just stand there and watch it. It will fall like a stone. So you learn how to set up an autorotation glide flying in a circling descent, or whatever. You learn how much cyclic is required to make it happen and you can easily bail out by just pulling pitch. You can practice your flares with this setup at a safe altitude and easily bail out.

In this second stage of practice you’ll learn quickly that you’re actually in the feathered or positive pitch range during autorotation - you can actually control the headspeed and descent rate just a full-size does. A 500 will come down pretty quick. You can fly a 800-class 300-400 meters in autorotation pretty easy.

Now you’re ready for your first full-down auto. Set it back to a normal throttle curve, fly up to 100 feet or so, and cut the power with throttle hold. Make sure you set your H_RSC_RAMP to 1 in case you want to bail out - get caught in the wind, whatever. You already know how to flare, know you can flare fairly high starting at about 10 feet or so, and you know the collective management it takes to set it down gentle. Make sure you nose the heli into the wind before touchdown - the wind helps a LOT with translational lift.

Your first couple will likely be a bit rough because you’re combining everything you learned from several hours of practice in the “stages”. And autorotation in the flare and managing collective is pure “seat of the pants” - there is no real science to it. It’s a learned skill. With the smaller helicopters you probably won’t do it on a regular basis because they’re a little bit harder to handle. But with 700’s and 800’s once you get the “hang” of it you’ll find yourself practicing it after auto missions or fun flying just to do it. The bigger heli’s have pretty good hang time even at full UAV weight.

For the CP quad guys, let us know how far you successfully get in practicing the “stages”. Since you’re using the same throttle control as the Big Boys you have access to the throttle curve and more advanced helicopter power managment et al that normal quadcopters don’t.

If you want to take RC heli autorotation to the next stage, learn how to do it on FPV. That is REALLY fun! And use a fixed FPV camera on the heli that has a shot of the nose or something in one side - a left or right front skid mount works really good. Having the frame attitude reference in the FPV against earth reference, and using like FatShark goggles is so realistic that you’ll be “hooked”. :grinning:

(Kyle) #105

thanks!, that fixed all that param stuff :smiley: