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Tail Bouncing Issue

Hello Bill,

I’ve changed NS_LOG_BAT_MASK to 1 and fly.
Regarding changing INS_NOTCH_ENABLE, I can change it to 1 but I could’t find how to change the depth & width, Please advise.
Thank you very much for your help.

Rotem

Rotems,
after enable the NOTCH filter with -1- you write it to the FC. After that you must REFRESH the Parameter List and you will see the additional options.
Or now after the restart of the FC you will see everything you need.

Rotem,
It looks like you need to balance your blades better and there could also be an issue of blade track. here is a snapshot of your vibrations in flight

So the frequency is on the bottom and the first peak in both plots corresponds to your rotor speed or rotor imbalance. To give you an idea of a well balance rotor system will be for this first peak. Mine was around 0.5 to 0.7 for the Y acceleration plot and around 0.3 for the X gyro plot. So your vibrations are about 10 times more than mine. You can use this batch logging feature to dynamically balance your rotor system. you can find how to use Mission planner to do this analysis here. http://ardupilot.org/copter/docs/common-imu-batchsampling.html Be aware that I manually pulled out the section of data from your log to look at just the flight. This feature basically uses all of the samples in the log and averages them. So if you use the whole log then you will see much higher values because it includes the data on the ground too and this is where we see a lot higher vibration. So you can still do the blade balancing using the whole log but you will have to look at what it is initially and then see how low you can get it. Then you can pass me a log and I can pull out just the hover and see how it compares to mine. The way I tackled doing the balancing was to use a small piece of tape 4 cm by 0.5 cm and put it at different points along one blade. You will have to restart the controller each time so you get a separate log in order to conduct individual analyses using this tool. look at the first peak and see if there is any improvement. keep on adding weight or move to the other blade if the vibrations just keep getting worse as the tape is moved further out on the one blade. I hope this makes sense.

The peak at twice the first peak is the 2/rev which I believe is due to lead-lag of the rotor blade and tracking but I’m not positive. Look at your blade tracking as the heli is in flight. If the blades don’t look like a knife edge (meaning each blade is passing right in the same location that the previous one did) then you may have to adjust your blade track.
The only other thing that might affect the magnitude of your vibrations is the tightness of the blade bolt which if you made it really tight would put more vibrations in the heli. However, I have not played with this to see if that has any affect.

The last thing I will point out is these vibrations are also dependent on the flight controller location and mounting which could have a big effect. So try the dynamic balancing of the blades to see if you can significantly lower that first peak. If you can get it down into the range of my readings then your flight controller mounting may be fine and it was just the rotor blade balancing.

@rotems. Disregard the method I discussed above to determine if your rotor blades are dynamically balanced. I’ve learned a little bit more about the FFT tool in mission planner. It will not be able to predict this. Sorry about the misinformation.

Hello Bill,

I appreciate your help in this issue.

I think the blades are dynamically balanced, my balancing method is to lay the blade on an knife edge and mark the balance point and add tape as needed until both blades balancing point is match.

I will try to work it out, maybe replace the heli head (as I mention before, it’s clone trex 700) or in the worse case I will buy a new frame and start all over (considering Protos700X).

Thank you
Rotem

Rotem,
This is not the best way to statically balance and rotor blades. This just ensures the CG of each blade is the same. you would also have to ensure that the blades weigh the same for this to work. The best way I found two statically balance rotor blades is to remove the shaft and hub from the helicopter and attach the blades to the hub so that they are straight out from the blade grips and tight. Then I use a prop balancer that I mount to my bench so as to not fall over due to the weight of the rotor. I start with the blades parallel to the ground and see which blade falls. I put tape on the blade that doesn’t fall. And then check to see that the rotor blades stay parallel to the ground when I let go of them.

Hello Bill,
I will try this balancing method and will try to fly again.

Regarding the tape that you are adding, it will change the CG of the blade, no?

So you can balance the blades using your method to get the CG the same. Then mount them on the rotor hub and see if they balance on the hub. If they don’t then add the tape to the CG of the blade. That way it will change the weight of the blade but won’t change the CG of the blade.

Hello Bill,

I replaced the Heli head and during disassemble the original head I’ve found that one half of thrust bearing in one of the blade holder is missing!

I performed a flight with the new head hoping that the vibration will disappear but I think it’s almost the same (I’ve attached the flight log file).

Please advise what do you think and how to proceed from here.

Thank you very much
Rotem

Hi Rotem,
Yeah it doesn’t look much better. Please do the following
set INS_NOTCH_ENABLE to 1. Then restart the flight controller and make the following settings
INS_NOTCH_FREQ 24
INS_NOTCH_BW 10
INS_NOTCH_ATT 30

Also
INS_GYRO_FILTER 10
INS_ACCEL_FILTER 10
ATC_RAT_PIT_FILT 10
ATC_RAT_RLL_FILT 10
PSC_ACCZ_FILT 10

After making all of these changes, be sure to restart your flight controller. do a short flight in stabilize and we can look at how the filters did. We’ll see if we can get the vibrations knocked down some. then we can look at how well your heli is tuned.

Hello Bill,

I’ve change the parameters and performed a short flight.

I can see any differences in vibration from the last flight.

Please see attachedParam 290919.param (14.6 KB)
the flight log and param file.

Thank you very much
Rotem

@rotems have you tried increasing your headspeed? I think I read earlier that you are running ~1,300 rpm? That is only ~350 fps blade tip speed, which is very, very low for a 700-class machine.

I have never had good results from running low headspeed. Your helicopter will likely run smoother at higher headspeed, it will produce more lift and provide better control response. You don’t have to run 3D speed. But for the various sizes of heli for UAV I like to see:

  • 600-class, 350-400 fps
  • 700-class, 425-450 fps
  • 800-class, 450-500 fps

This equates to approx rotor speed in the various sizes:

  • 600-class, 1600-1650 rpm
  • 700-class, 1550-1600 rpm
  • 800-class, 1500-1600 rpm (higher rpm for heavy-lift 800’s over 1lb/ft^2 disc loading)

Helicopters will exhibit dynamic imbalance at various speeds. It’s like balancing a tire for your car. You can static balance the tire with a bubble balancer. But put that statically balanced tire on a dynamic balancer and the tire might wobble at say 38 mph, but it runs smooth at 55 mph. That wobble at 38 mph is due to a dynamic imbalance at that speed.

It is very difficult to dynamically balance helicopter rotors without the aid of an electronic balancer like a RPX DynaVibe. But it is easy to identify the dynamic imbalance speeds by looking at vibration. So if you find a speed that exhibits high vibration, run it either faster or slower than that speed to see if it clears up. You should be able to find a “sweet spot” within the acceptable rpm performance range where it runs smoothest. And that is the speed to run it at.

This is how I have determined what to set the governor at in all my heli’s. I run it at the highest possible speed within the acceptable tip speed range, at which it runs smooth. And then track it with the RPX DynaVibe to get rid of the 2/rev “beat” of a two-blade rotor in static hover. You’re still going to get the 2/rev “beat” with a two-blade in forward flight. But it can be tuned out in forward flight too, in full-size, in trade for having a little higher vibration in static hover.

At any rate, I would suggest trying a higher headspeed. It won’t make much difference to your flight time with your electric heli. And I’m afraid you might be beating your head against the wall running it at 1,300 rpm and trying to solve vibration problems.

Hi Rotem,
Here is a comparison of you roll rate signal before and after the filter settings
Before

and after

So we are getting there and I think you could increase INS_NOTCH_ATT to 40 and see if it reduces the vibrations more.

@ChrisOlson from the vibration data it appears he is running his head speed just shy of 1500 RPM. But you have a good suggestion, a different headspeed may give better vibration results. It is alway better to fix vibrations mechanically if possible but it looks like our filters are doing a good job.
Edit: thinking about chris’ suggestion more, that may be good to do. Reason being is that the rotor speed you are using currently is resonating with the aircraft structure and exacerbating the amplitude. If you move away from this frequency then you move away from the natural excitation frequency of the structure and your vibrations should reduce. To do a good comparison, you want to do this with the same filter settings so you can see when it gets better.

If it’s 1500, then try 1520 or 1540. It usually only takes 20 rpm or so to move away from a resonant frequency that causes vibration to reach the controller. With pistons I’ve seen it where you can even see a frame vibration in the tail, move it up by 20 rpm, the tail quits vibrating and the vibration to the controller cleans up. When I see that I usually move it up by another 20 rpm and see if stays clean. If it does run it as far away from that resonant frequency as you can, without over-speeding it.

You can many times see the effect of these dynamic imbalances when spooling down. A helicopter that runs smooth otherwise may rock on the skids or shake the tail at a certain speed as it slows down.

Just about all of them will exhibit this to a certain extent, even in full size where you sometimes get noticeable stick shake during spool up or spool down. And then there’s some, like Robinson’s that shake the stick all the time no matter what speed it’s at and you can tell by the frequency of the stick shake if the headspeed is right or not.

Hello Chris, Bill,

I didn’t try to increase the head speed, I will try it but I would like the Heli to run as low as possible in order to get max flight time (I see 700 heli running as low as 1000rpm).
I’m thinking maybe the FC location is wrong (half on the frame and half on the tail boom).
Chris, can you please explain what fps calculation is? why it’s rising with the frame size?

Bill,

I can see the big different after the filter activation but why I cant see any differences in the VIBE graph?

thank you very much for your support!

Rotem

Rotem,
So the filters have no effect on the VIBE log data. This is showing the raw vibration data. I think it does this because the EKF does not use the filtered signals. The filters are designed to be used for the attitude and rate controllers only. For some reason the EKF can’t use the filtered IMU data. It has to be unfiltered and so it is still important to keep the vibrations low so it doesn’t effect your EKF solution.

It is pi * diameter of rotor * rpm / 60. It is the main reason RC helicopters can’t go as fast, can’t carry as much, and aren’t as stable as full-size manned helicopters. In full-size, anything over 2.5 lbs/ft^2 disc loading will be running at 700+ fps. Light utility class and trainers <2.5 lbs/ft^2 will be from 550-650 fps.

Since RC is typically not more than 1 lb/ft^2 you can get by with 450 fps. But it’s not ideal. The amount of lift from the rotor is related to the square of the speed. Sacrifice lift for low speed to get more flight time, expect that the helicopter will be sluggish, unstable, will have poor payload capacity, and poor forward flight performance.

Obviously, 800-class and larger machines have a larger rotor, it is more practical to get to similar performance and stability of light utility class full-size. 700 and smaller will never make it to that performance level because their rotors are too small and it is not practical to run them at high enough rpm to get there. So they will be performance-limited and fly at lower disc loading than a larger machine. Sure, you can run a 700 at 1,000 rpm but that doesn’t mean it flies decent or is stable unless it is very, very light and carrying only a battery for a payload.

So in practical terms a Bell 206B-3 Jet Ranger with a 37 ft main rotor runs at 394 rpm, blade tip speed is 763 fps. It can take off at 3,200 lbs, which is ~3 lbs/ft^2 disc loading. Can your 700 helicopter take off at 61 lbs gross weight to match that disc loading of the Jet Ranger? No, it can’t. That’s because you’re running the rotor too slow. You’re lucky to get a 700 to take off at 20 lbs, which is 1 lb/ft^2, and it certainly isn’t going to do it at 1,000 rpm.

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Hello Chris,

Thank you for the detailed explanation!
I’m probably missing something in the fps calculation:
3.141.51000/60 =78, what I’m doing wrong?

So, how to calculate the “sweet spot” of RPM, power consumption and payload calculation?
Is a 3 blades heli is better \ efficient at low head speed?

Thank you
Rotem

Thank you Bill for your answer.

Maybe you are using meters/sec instead of ft/sec.

A three-blade rotor of the same diameter has higher disc solidity, will produce more thrust than a two blade, requires more power than a two-blade, and is less power efficient due to introducing more drag vs the amount of thrust produced (called lift to drag, or L/D, ratio).

In full size helicopters the primary reason to add more blades to the rotor is to create more lift from a given rotor size at the expense of fuel efficiency. Two-blade rotors are the most fuel efficient, generally speaking. But if you compare two common trainers, the Robinson R-22 with a two-blade and Guimbal Cabri G2 with a three-blade, the G2 has a smaller rotor but the blade tip speed is the same at 650 fps because the G2’s rotor turns at 530 rpm. The G2 is also a heavier helicopter but it burns more fuel than the R-22. The G2’s heavier rotor gives it better autorotation characteristics.

One of the main advantages of a three-blade is that they are fully-articulated soft-in-plane, where the two-blade is a teetering semi-rigid type, subject to mast-bumping. RC, however, has quite crude rotorhead designs compared to full-size so you will likely not find a fully-articulated RC three-blade head.

So there is pros and cons, but there is no free lunch.

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