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.: http://ardupilot.org/copter/docs/heliquads.html

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:


(Brad Wilkinson) #106

@bnsgeyer - Bill - I found that the ESC I had was bad. At least it has gone bad since I got it. So I think I have a solution in hand.

When I first flew the V383 out of the box with the stock FC, it performed just fine - although my piloting technique/skill/experience left a great deal to be improved on (having never done this before in my life). It has a stabilize mode, and a 3D mode - where the experienced heli pilots can do all of the loops and rolls and flips and such that these are capable of out of the box - but not me!

I kept practicing my flying, and meanwhile I ordered the new Pixhawk4 kit, Taranis radio set, and some other things to upgrade to Ardupilot control via Copter 3.6.0, following Tridge’s lead.

During the practicing, I noticed that the V383 started hopping, for lack of a better description. I was practicing mainly at about 5-10 feet altitude, just to keep it close and learn the moves. At first, I thought the hopping was from wind turbulence causing barometer fluctuation, or may ground effect, or what ever. But I also found that the collective/throttle control on the stock setup was extremely sensitive, and I was having a very difficult time stabilizing a flight. I had ventured to higher altitudes to try to rule out ground effect and turbulence, but it wasn’t helping. With the stick at 50% and hands off, the craft would rise and descend constantly on its own. I was during this that I crashed it for good, bending the tubes. So I made new aluminum tubes, and when I received the new electronics, went down the path of setting everything up new.

While trying to figure out all of this control problem on the bench (with your helpful input), I decided to go back to the original radio and FC to see what I got out of the motor. So I just hooked up the ESC/motor to the original FC/RX and operated it with the throttle stick on the TX. Well, I found that at any collective/throttle stick position, low speed or high speed, the motor surged. It would go to full commanded speed and then cut to a freewheeling spin-down, and as it reached the bottom it would go back up. I cut the heat-shrink off of the ESC and it appears that some of the solder has melted, and maybe a chip has let the smoke out. It’s difficult to say for sure. Needless to say, I have ordered a new ESC!

Thought you may find this interesting, after all the head-scratching I did, doing the basic test highlighted the problem. I haven’t gotten everything back together yet, hopefully any day now, so I will post when I have a first flight result.


(Daro) #107

@brad.wilkinson I am very sorry for the late reply. I am currently away from my lab and will be back in 04 of September.

Thank you so much for your support. It means a lot to me. I’ll try it, and will let you know

Much appreciated


(Bill Geyer) #108

Brad,
Glad you were able to figure this out. Looking forward to your first flight results.

Regards,
Bill


(Brad Wilkinson) #109

Hello Bill,

Well, unfortunately I’ve developed more problems now, and still don’t have a successful flight to report.

I finally got the motor working, with the new 30A ESC. But with all of the added weight, the pitch requirement was too much, and it was overloading the ESC (now I think it was too small anyhow, should’ve been 45A at least). The overload was causing the BEC voltage to drop, and then servos would go crazy. So I went with a separate BEC, but there still wasn’t enough current to drive the motor to produce the required lift. So now I have upgraded the motor and ESC for more power (about double), and that seems to be enough.

But now I have a crazy roll oscillation going on - seems to be at about 2-4 Hz, and 10-15 degrees of roll. I thought at first it was ground resonance, since the landing gear on this V383 is crazy wobbly, and the side to side rocking is pretty bad. I was able to stabilize it to some extent by control inputs, but can’t really tell. I actually got it to hover a bit on my patio, and it seemed stable so I thought I would try to fly it.

On my first flight at the field, the wobbly roll started again as I ramped up the throttle, and I thought I would go ahead and get it off the ground to stabilize it. But that didn’t work, it actually got worse once it was airborne, and resulted in a crash landing as I cut the throttle. I tried again and succeeded in fully crashing and breaking a servo control horn and the rod to the collective bell-crank. It seems to have “moments of clarity” where it will stabilize, and then off it goes again.

Anyway, I need some help reading the logs. I don’t know what most of the parameters mean, at least I don’t know what the abbreviations or acronyms stand for. Is there a reference somewhere for the interpreting the logs? I think I can post one somewhere if that will help, but need to know which one is best.

Any ideas?

Thanks,
Brad


(Brad Wilkinson) #110

@bnsgeyer, I have succeeded in getting this thing off the ground.

I turned the rates down to start:
ATC_RAT_RLL_P, and ATC_RAT_PIT_P = 0.08
ATC_RAT_RLL_D, and ATC_RAT_PIT_D = 0.0007
ATC_RAT_RLL_I, and ATC_RAT_P_I = 0.125

This seems to have successfully dampened the oscillations. There is still something there, I can see it on the ATT.Roll and IMU.GyrX logs. But this doesn’t seem to be in correlation with the vibrations, since it happens even when the motors are off. Look at this chart:

In the middle, I crashed into a potted plant on my patio (not the ideal test flight environment, surely), then I shut down the motor, picked the machine up and set it down in the middle of the patio. You can see the oscillation on the Roll and Gyro data, even with the motors off. The landing gear on this machine are severely springy and allow wobble in roll just sitting on the ground. So it appears that maybe that has something to do with it. I’m sure there are other factors and some residual vibration in the frame. The rotor blades (plastic) are seriously imbalanced, especially after multiple ground and vegetation contacts, and need improvement - so there is a lot of harmonic vibration as the rotors speed up and slow down, but at higher speed they are fairly smooth still.

Here is the parameter file:
BW_V383_HeliQuad_20180904_1105.param (14.2 KB)

The log is slightly to big to upload so I will have to figure out how to work that out.

Anyhow - I am a lot happier now that I actually have accomplished a low stable hover!

Brad


(Bill Geyer) #111

@brad.wilkinson
Glad to hear you got it to hover successfully! Sorry I haven’t been more responsive to your posts. I did look over your parameters and had some suggestions.

First you should try tuning it using the helicopter tuning method in the tradheli section of the copter wiki. I see that you are not using any ATC_RAT_PIT_VFF and ATC_RAT_RLL_VFF for pitch and roll axes. I would suggest that you incrementally add VFF and check the logs to see if the desired and actual rates are matching as described in the tuning wiki. this will do two things for you. first it will ensure that the controller has the necessary control power to stabilize the aircraft and two it will allow you to reduce the P gain in order to stay away from the roll oscillations.

Next, I see that you are using mode 3 for rotor speed control but your throttle curve is a straight line from 0 to 1000. Why do you have it set this way. I can’t imagine this provides constant rotor speed. typically the curve is pretty low gradient through 50% to 75% and then ramps up to 100%. Before you adjust that, what do you have your collective pitch at for H_COL_MIN and H_COL_MAX.

Lastly be careful with your H_LAND_COL_MIN. the LAND_COL_MIN should be 0 to 200 (where 1000 is full collective) which should be about -2 to -1 deg collective pitch on the blades, probably no lower than -2 collective pitch. 500 seems a little high unless your collective pitch range is -10 to 10 deg.

recommend PSC_ACCZ_P should be 0.3 instead of 0.5.


(Brad Wilkinson) #112

@bnsgeyer, thanks for taking a look at that, I will look into those things as soon as I can. I am on the way to Interdrone in Vegas at the moment, so won’t get a chance to do more testing until this coming weekend.

The parameters are pretty much straight out of Tridge’s file, with the only changes I have made being the ones around the ESC and motor control stuff from before, and the ATT ones I mentioned above. The throttle curve is dead straight so that I could get it to work, and because what Tridge had in the file was a bit soft for the extra weight. My intention is to adjust that so that I get hover at 50%. Now that the new ESC has the governor feature, I may go back to a headspeed set point, but all of this is TBD. And this thing actually has more than +/- 10 degrees of pitch, more like 30-40 degrees. But I don’t think most of that will ever be used, and I really don’t intend to try inverted flight, at least not until my piloting skills increase significantly,

I had read about the Position Control thing in one of the other posts, so I will definitely adjust that. Once I get to the position hold mode of flight testing, I can tweak stuff more as needed then. Right now I’m just working my way through the Stabilize mode and steady hovering, so keep your fingers crossed.

Thanks,
Brad


(Kyle) #113

@brad.wilkinson @tridge @bnsgeyer

Got my setup hovering (for brief moments at a time 1m from the ground)!

Unfortunately one of the belts looks like it’s going to snap soon, (not sure why). Any idea where I can buy the belts for the Stingray online, shipped to Canada? I’ve scoured the net, this is what I found.

Part numbers:
nd-ys5-as4005
B222MXL 4.5

https://www.droneshelis.com/Drones/Stingray/Parts/nd-ys5-as4005.html
hk looked like they have some close stuff but not quite,

Anybody found a place to get parts on these old aircraft?

Also curious if anybody knows where to buy new collective pitch quads?


(Brad Wilkinson) #114

Kyle, congrats on the hover, that is more difficult to achieve with these than most realize I think. I’m just getting there myself.

Here is a site that has the parts for the WLtoys V383, although quite expensive.
https://www.wl-toys.com/Wltoys-V383-Parts-Quadcopter-Brushless-motor-6-channel-UFO-PARTS-Battery-62-221-1149-1.htm

The V383 is pretty much a copy of the Stingray. It may actually be the same one, just rebranded by the Chinese manufacturer. I got mine through Banggood for $200.


There are other sites who sell this model, too. And prices and shipping vary a good bit. Parts are a little more difficult to find, but I think the rotors and hub are essentially 450 class helicopter tail rotors, probably Align-Trex or Hobbyking.

The belts are standard synchronous timing belts, and aren’t all that expensive. In the US, that’s pretty much Gates, and although there are others Gates is sort of industry standard stuff so it’s available everywhere. Most industrial supply houses have these.

The Gates 178MXL019 will be the best fit for these machines as far as I can tell. The key parameters are a tooth pitch of 2mm (0.08"), width of 4.8mm (0.19", the 4.5 on the stock belt means 4.5mm width, but those are harder to find, 5mm may work as well), and 222 teeth in the belt. This corresponds to about a 444mm pitch length, based on 2mm pitch x 222 teeth. But the pitch length is actually the center of the belt backbone, not the teeth, so the belt length is slightly more - in this case 17.76" or 450mm. You can just measure the outside of the belt, or if one breaks you can lay it flat and measure it. Just realize that it could have stretched a little in the process of failing.

Here is one source, about $3 each!

Hope this helps.