Hey all,

Has anyone used Variable pitch propellors on their quadcopter?? I'm thinking about using them on my new project and was wondering if anyone had any experience with them or could let me know if its a dead end or if its even done??

Any help would be greatly appreciated :)

Thankyou,

Harry 

Tags: large, pitch, propllors, quadcopter, variable

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It adds quite a bit of additional mechanical complexity, and ultimately the only material advantage is a higher forward speed capability (the ability to increase the blade section AoA at higher rates of inflow).

Here's a paper from some MIT guys who did some research, but it was focused on control response.  If you have a mission profile where ultimate transient handling response is your goal, i.e. precision flying in a very turbulent aerodynamic environment, it might make sense to pursue.  If you don't find this paper to be daunting, and don't mind treading down your own technical path, then go for it. 

The electric multicopter's primary advantage is mechanical simplicity.  If you really think being able to change the pitch of the blade in flight suits your goals better, and if you don't want to embark on your own course of invention (with all the trials and tribulations that entails), then trying a conventional single rotor helicopter might be a better idea.

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Yeah, I actually just had a mechanical failure on my heli yesterday.  The tail pitch slider broke, and I lost rudder control.  Thanks to the excellent APM stabilization, I was able to mostly save it from doom.

This is the same part you'd use in a VP Quad.  I can't imagine having 4 of these things on a quad, waiting to fail...

That's basically what we're up against.  A VP quad increases the number of failure points maybe 10X a basic quad.  So you better have a really good reason for wanting to do it.

Inverted flight and all the 3D fun it brings. ;-)

I worked on this project under Mark Cutler, I was his undergraduate hardware guy. If you have any specific questions about the hardware setup, let me know. I spent my summer working on that quad. Ultimately we found that your performance is going to be limited by vibration. The variable pitch mechanisms we used all had a certain speed where they would essentially vibrate themselves to death and thus bottlenecked the performance. Look at page 7 of the paper that Brad posted above.

Just to note, they tried using helicopter tail rotors before I joined the project. Apparently it shook itself to death. 

Thanks mike, 

Do have a few questions, what did you use for the flight planner/code? i cant find out if the arducopter flight planner and code of the ardupilot Mega works with variable pitch or how to make it work with variable pitch?? Would you know anything about that or know someone who does?

ANd im using main blade rotor heads but yeah i think no matter what you use there will be a vibration problem :s

your help would be greatly appreciated :)

Harry

Harry, 

In terms of flight planner and code, I believe that the quad was using this board with 'custom' code on it. One thing you have to remember is that it was flown indoors using a VICON system for position and velocity data. There was definitely some MATLAB interface, and the actual flying was done with a python script. Iver used the APM stuff for a few months now, but I don't think it is setup for variable pitch. You may have to modify the arduino's code yourself to integrate that--Im sure someone on the forum will know how to do that--I'm not a programming guru unfortunately. I think that the APM definitely has the potential though. 

From a control-law standpoint, I remember that variable pitch alone was very difficult to use to stabilize the quad even with all the VICON gear and simulations to tune the gains properly. Ultimately they used pitch for large attitude corrections and motor RPM for small corrections-- I havent read through the full paper above, but it probably talks about that. I would actually recommend trying to fly it using just motor control first (fixing the pitch of the blades) before you step into the variable pitch world. 

If you look back on the MIT ACL youtube channel, some of the other 'uberquad' videos showed some of the old mechanics and old footage of flying. 

Mike

Vibration is the unfortunate bane of every lifting airscrew.  I know some engineers invest virtually their entire careers in this field on full-scale single-rotor choppers.

Which brings me to my next point about variable vs. fixed pitch - making the rotors rigid (like an airplane propeller) drives the fundamental resonances and harmonics up the frequency spectrum where they are easier to dampen.  That said, on my 46" diameter rotors, with some blades made out of carbon fiber and some fashioned from polystyrene over aluminum (7075 alloy and yes, it sounds strange but it works), the center hub is a solid 6061 aluminum bar.  Yes, it does flex a little, but the point is that some of the yield pieces had to be thrown out because they exhibited excessive vibration for reasons that are beyond my ability to discern.

The lesson was to balance and test every component individually before attempting to fly.  The variables are too many to account for them all.

Mike,

The project looks very interesting. What was the frame based off of? I wanted to try and replicate this project for myself and it would be nice to use all the same parts. My intent is to do something slightly different though and use only one motor to run all props. Anyway, any help could be useful. Thanks in advance.

The frame was a custom cut carbon fiber foam core. 

The variable pitch system I believe is posted somewhere in this forum but I can find it if you need it. 

The motor in the middle is difficult. They tried that paired with trex 450 tail blade assemblies before my time and were unsuccessful. I think they ran into vibration issues and i dont think it actually flew because it didnt have enough power. It was one of the early prototypes. 

I can get into more details if you would like, email me at mklinker@mit.edu

Mike,

It is funny that I just read your comment here since I started a project back in December to build a variable pitch hex copter using 450 tail assemblies. (single motor) I just finished putting the assembly together and have begun configuration.

One of the first problems that I have encountered is the servo behavior when connected to the APM2.5. The basic response is as expected, but they are very twitchy/jumpy as if there is significant electronic noise on top of the signal. I have tried connecting the servos directly to the receiver and the behavior is normal.

I have started with ArduCopter (latest build) and have tried changing the default ESC UPDATE SPEED (RC_SPEED) to 125 Hz with no improvement.

Are you aware of any reason the output from the APM2.5 would be different than that of the receiver with respect to the ESC/Servo signal?

(Yes, I am hoping that the stability logic in ArduPilot is sufficient with careful tweaking of the PID control parameters, but if that fails, I will have to dive into the code. But first things first.. need to get the servos a clean signal.)

Todd

Yeah  this is my main concern as well :S

I need it to be quick and also need it to eventually carry a load, i Thought that if it was variable pitch then when a load was applied it could be able to increase the RPM and blade angle to lift the load, making it theoretically possible to be more efficient then if i had to design it with a fixed pitch props. Im going to start off with fixed pitch and as i gain experience and practice and knowledge will convert to variable pitch as it is designed to be developed this way.

Thankyou :)

Harry

Thanks so much for that Brad, going to have a good look through it now, really appreciate it :) But yeah for what i need it to do i think i will eventually need it to be variable pitch but thats a way off yet, just going one step at a time :)

Harry

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