No, have not done any FFT on any heli’s. And yes, the engine is the primary source of vibration, both from mechanical vibration and noise. The rotor vibration frequencies are not detectable. Bill had me try some experimental software some time back that was supposed to detect the rotor rpm using the vibration. It did not work.
However, the rotors have been balanced and tracked on my helicopters using a RPX DynaTrack with the strobe. So it’s not going to be able to detect anything at that frequency because it doesn’t exist.
I had to snap this photo to show something interesting. This is a piston 800-class machine with a V3x in it
That silver material over the flight control - what is it? It is a foil-backed thin foam insulation material that comes from those accordion things that you can fold out to put in the windshield of your car to keep the sun out. My wife wondered what happened to hers. I told her I noticed it was old and I need it and she needs a new one. She said it she just got it two weeks ago. Ooops.
But that stuff is handy. With that stuff over the control vs not being there it reduces the vibration by about 25%. Vibes in all three axes run 4-6 on this heli.
Since it is impossible for it to do anything for mechanical vibration I attribute the reduction in vibration to a reduction in noise with that thing over the controller. You can see where the outlet of the muffler is and that’s probably about 120-130 dB right at the exhaust tip.
Interesting that you weren’t even able to pick up rotor frequencies, I would’ve thought maybe even with great balance and tracking you’d be able to find traces of say rotor wash impacts on the fuselage/tail boom.
Also, great ingenuity with the foam insulation, very impressive you’re able to get such low vibes on a gas heli.
When Bill had me try that custom firmware build it would pick it up during runup, but then it lost it. The rotor was balanced and tracked at 1,530 rpm.
Those survey helicopters have had considerable work done to the engines for doing camera work. It’s not in the flight control or mount. They are 2mm stroker engines. Just like the rotor, they have some vibration during runup. But once they reach the speed they were designed to run at they are as smooth as a turbine.
My guess is that it shields the unit from the mass of fast moving air coming from the rotor. That air could induce vibrations, expecially if it hits cables and make them vibrate, the pretty long you have in the back of the unit i mean.
Just a guess.
It is certainly possible. However, the downwash velocity is not all that high at that location in hover. In forward flight it could be. The inflow to the main rotor in forward flight is from the front, then downwards and back underneath the rotor. It flows directly over the tail rotor, which increases the tail rotor efficiency in forward flight.
This occurs at the translational lift speed of 16-24 KIAS in all helicopters when the nose comes up all by itself and they start to fly like a fixed-wing instead of using brute power to move air mass equal to the weight of the machine. It is called “blowback” by helicopter pilots but the aerodynamic reason for it is much more complex.
I have noted the reduction in the measured vibration with that cover on there at all air speeds and flight profiles.
Turns out I did end up receiving the plastic version of the Pixhawk 3 Pro so I sent it back without unboxing or testing. Then I noticed that on the Drotek website they have announced the end of life cycle for the 3 Pro. However, Paul at Drotek did let me know that they are working on a flight controller based on the fmuv5 hardware spec, with a similar form factor, damping etc to the 3 Pro so I’ll keep an eye out for that. They don’t expect to release until 2021 though…
Dropix is EOL. The Pixhawk 3 Pro is a different unit. It has been around for at least three years so I’m sure it will eventually be discontinued. But before it is I’ll buy their remaining stock so I got a supply of decent controllers
I did a similar test with the car audio. I put the flight controller on the film of the audio and pasted it with double-sided tape. However, I found that the stickiness and location of the double-sided tape would seriously affect the consistency of the test. USB or other external devices generally do not lead to the consistency of test results.
You’re right- the mounting can have an impact on the amount of vibration imparted on to the flight controller with this setup. In order to make sure that the conditions are as close to identical between the tests, we would need to mount them all to the same plate at the same time.
I decided to run the tests again, but this time I mounted 2 flight controllers at a time to the speaker and compared the two (see pic). I bonded the flight controllers together to ensure there was no relative motion between them, so I think this is as close as possible I will be able to get to a direct comparison between flight controllers.
The tests I did were: Pixhack V3 vs. Pixhack V5+; Pixhack V3 vs. CubeBlack; Pixhack V3 vs CubeOrange. I compared all to the Pixhack V3 because a) it’s the best at removing low frequency vibrations and b) it was the easiest to mount to.
These are the results of those tests:
These results are largely in line with the original results I gathered and the experiences people have had with these flight controllers. As expected, the Pixhack V3 is again the clear winner. The Pixhack V5+ isn’t too bad either. The CubeBlack has a bad resonance at ~90Hz and the CubeOrange has a bad resonance somewhere between 60Hz and 90Hz. It’s worth noting that at higher frequencies the Cubes do much better.
I wonder if it would make much difference on the Cubes (any others that have it) to use the header extension USB with thin wires to the carrier rather than the direct USB with the chunky cable.
In an actual aircraft where the vibration can be transferred to the control via the wiring it might make a difference. But it appears in this case the vibration is being directly applied to the control itself so I think the stiff USB cable has minimal effect for the test.
I think it is pretty well known that the V3/V3x will handle darn near anything on piston helicopters. I was running this one tonight, breaking in a new engine. That V3x is mounted on a homemade aluminum mount above the tail box, stuck to the aluminum plate with two strips of body mounting tape (the stuff they use to stick body moldings on cars and you have to use a prybar to get it off). It never clips the IMU’s even running it without blades on the ground, and zero drift on the attitude solution. Doing that will blow most other flight controls all to hell and the swashplate will be tipping back and forth like it’s nuts from the vibration. Engine is running at 10,000 rpm in this shot.
IMO this is the best test platform. I’m not even sure a speaker can simulate the vibrations of a real aircraft. Because the real aircraft has components all turning at different speeds and simultaneous different frequencies. You can pick the worst one and notch it and try to attenuate it in software. But that helicopter has five different shafts, all turning at different speeds in the transmission and driveline. From 160 some Hz from the engine to very high frequency vibrations from high-speed gear teeth mesh. In the end the flight control is either good and it handles it, or it don’t. You can’t “fix it” in software.
Have you noticed that when pixhack V3 is compared with V5 +, it has a value of 20 instead of 27 at 120Hz? This is the question I met when testing. If the test is standard, should it be 20 or 27? If the sensor is normal, it indicates that the force applied to the flight control during the test is not consistent. This makes me quite confused because I want to improve the shock absorption mode of IMU. If there is no definite standard, it has no reference significance. In addition, if you want more accurate results, my suggestion is to perform FFT on its log and observe its resonance frequency. Sometimes, the resonance frequency of IMU is higher than that of aircraft. Although it has certain damping effect, it is not a good thing. The correct damping method should be that the resonance frequency of IMU needs to be lower than that of aircraft.
Have you ever FFT the log? Through FFT, you can know the frequency of vibration on the aircraft and analyze the damping effect of different flight controllers
Interesting testing and discussion.
It seams like I did every possible mistake in building my Heli.
I used a clone Trex 700 (Hobbyking Assault 700 DFC) and replaced most of it to original Align part due to vibrations (bent shaft, poor parts quality, etc.)
Now I can see that the PH2.1 is the worse FC for helicopters.
So, now I’m working on a new Heli (I have the Gaui X7 frame which is a very solid frame) and would like to choose a FC.
The V3 cost around 350 USD (with Aluminium case) and the V3x is about 200 USD for all pack. Is it worth the extra 150 USD ?
Is there other good options for FC?
