Constant velocity or constant angular rate ?

[Mark wood]

I have a friend who does a single entendre.  I didn’t see a single entendre in the catalog.  Is that a humpty bump variant?

I know, crazy right?  What's entertaining is to fly an unknown sequence made out the catalog figures without ever having flown it before.

[Igor Burger]

That is doable. A variable pitch propeller is necessary to achieve it.

Looks like I missed something here, but since question was answered in 3rd comment and I am not going to mess with C++ problems and sphere geometry, I will put only few comments.

So - As Paul mentioned, the light prop was developed especially for easier RPM controll. As Brett mentioned its (the prop) and motor mass inertia moment limits bandwidth, it is very imprtant for controll. That is also reason why I stick with small diameter motors (2826 - like AXI). We tested also MVVS which has stator diameter 35mm and difference was obviouse.

BUT there is also another problem, thrust controlled by RPM is asymetric, means prop can better pull than brake. Reasons are 2 - the first is chambered airfoil, that was another object of optimization for new props, and second is lower RPM during braking also means lower reaction to model speed changes (the same prop at higher RPM will change the thrust as function of speed difference more that the same prop at lower RPM).

That means that pitch controlled prop will not only react quicker (with quick servo - I do not think it will be so quick with normal servo - reaction time of classic servos is not much higher that reaction of my power train, I cen easily show that frequency of of RPM change input can be higher than standar R/C servo - we will use probably small light servo which can make run from one side to the other loaded at say 0.2s what is visible lag; but high speed servos working at 400Hz inputs will be probably enough).

However I am not sure if I want have such prop on contest model. Fixed prop is fixed prop and I am sure it will never dismuont during official flight, I can be sure that I can easily replace if we fly on typical WCh potato fields etc 

But who knows, may be Mark will push it to reliable solution then I will think about it.

[Mark wood]

However I am not sure if I want have such prop on contest model. Fixed prop is fixed prop and I am sure it will never dismuont during official flight, I can be sure that I can easily replace if we fly on typical WCh potato fields etc 

But who knows, may be Mark will push it to reliable solution then I will think about it.

I fully agree with you on the motor and fixed pitch braking part. That is the whole reason I launched on the variable pitch propeller effort in the first place. The reliability part is why I put a hinge in the propeller hub in the rotation plane to reduce / limit the bending fatigue. We'll see where that goes.

[Howard Rush]

Igor, what is the moment of inertia of your 12” flat-back propeller?

[Brett Buck]

That means that pitch controlled prop will not only react quicker (with quick servo - I do not think it will be so quick with normal servo - reaction time of classic servos is not much higher that reaction of my power train, I cen easily show that frequency of of RPM change input can be higher than standar R/C servo - we will use probably small light servo which can make run from one side to the other loaded at say 0.2s what is visible lag; but high speed servos working at 400Hz inputs will be probably enough).

However I am not sure if I want have such prop on contest model. Fixed prop is fixed prop and I am sure it will never dismuont during official flight, I can be sure that I can easily replace if we fly on typical WCh potato fields etc 

   I would not think it out of the question that we might have to make our own servo, and it may not look like a typical RC servo. But I did a bit of scribbling and convinced myself that you can manage it with a very fast conventional servo, because throw of any reasonable pitch-changing mechanism will be tiny compared to a typical RC output arm, so at least you won't be rate-limited any more. Getting the slop out of it, or what such a fast system will do with slop/hysteresis, that's another story.

   As you note, the thing that has put me off the variable-pitch system is the potential for winding up with a blade sticking out of my forehead (or someone else's forehead). Paul working on it gives me some hope, because he is a professional at this sort of problem.

   And, as always, having an "ideal" system, while a neat idea, is probably not necessary. The existing systems work well enough that they are no longer the determining factor, so maybe it doesn't need to be any better. Simply defining what it is we are trying to do in clearly-stated engineering parameters seems to have eluded us to date.

  One thing I do know, we aren't going to figure it out entirely as a paper exercise - while that might be very helpful and useful, at some point we are going to have to build something and see.

     Brett

 

[Mark wood]

   I would not think it out of the question that we might have to make our own servo, and it may not look like a typical RC servo. But I did a bit of scribbling and convinced myself that you can manage it with a very fast conventional servo, because throw of any reasonable pitch-changing mechanism will be tiny compared to a typical RC output arm, so at least you won't be rate-limited any more. Getting the slop out of it, or what such a fast system will do with slop/hysteresis, that's another story.

   As you note, the thing that has put me off the variable-pitch system is the potential for winding up with a blade sticking out of my forehead (or someone else's forehead). Paul working on it gives me some hope, because he is a professional at this sort of problem.

   And, as always, having an "ideal" system, while a neat idea, is probably not necessary. The existing systems work well enough that they are no longer the determining factor, so maybe it doesn't need to be any better. Simply defining what it is we are trying to do in clearly-stated engineering parameters seems to have eluded us to date.

  One thing I do know, we aren't going to figure it out entirely as a paper exercise - while that might be very helpful and useful, at some point we are going to have to build something and see.

     Brett

I wouldn't use a servo for more than just the positioning aspect reason. Directly driven the pitch mechanism has to carry the propeller hinge moment continuously and a servo would have to carry that. This would lead to high current drain and potential oscillation.  Blades separating is a very real concern, catastrophic to the airplane and hazardous to people. My propeller design is past mere paper and is close to a stage it can be sent to a machine. I'm honestly holding for more test time on the hinge propeller before proceeding. Part of that test time involves the flight data package I'm working on.

[Igor Burger]

Igor, what is the moment of inertia of your 12” flat-back propeller?

If I remember well, it was somewhere at 70 to 75 ukg.m^2. But I am sure it was less than 11" prop as it has wider blades (it is more than 1g heavier). And aproximately 2/3 or full carbon prop from the same mold.

[Igor Burger]

   I would not think it out of the question that we might have to make our own servo, and it may not look like a typical RC servo. But I did a bit of scribbling and convinced myself that you can manage it with a very fast conventional servo, because throw of any reasonable pitch-changing mechanism will be tiny compared to a typical RC output arm, so at least you won't be rate-limited any more. Getting the slop out of it, or what such a fast system will do with slop/hysteresis, that's another story.

...

One thing I do know, we aren't going to figure it out entirely as a paper exercise - while that might be very helpful and useful, at some point we are going to have to build something and see.

There is one problem with them, they have dead band, so if you want some kind of precision, and AoA range for our application is narrow, then it is necessary to make some gear/arm ratio that servo runs in its full range anyway and we are back at that speed problem. However as I said before there are high speed digital types with little narrower dead band (but still present), but beside price there is weight and size penalty.

It is solvable problem, I just wanted to say it is not so trivial and "automatical" as it looks for first look.

But I can only agree with last sentence, it looks the solution is easy until you try it in real  ... then come those "yes but"s and that simple solution needs bugfixes and improvements until you find that it was completaly wrong idea and with gained experience finally come usefull solution ... 3 or 4 years later 

[Mark wood]

There is one problem with them, they have dead band, so if you want some kind of precision, and AoA range for our application is narrow, then it is necessary to make some gear/arm ratio that servo runs in its full range anyway and we are back at that speed problem. However as I said before there are high speed digital types with little narrower dead band (but still present), but beside price there is weight and size penalty.

It is solvable problem, I just wanted to say it is not so trivial and "automatical" as it looks for first look.

But I can only agree with last sentence, it looks the solution is easy until you try it in real  ... then come those "yes but"s and that simple solution needs bugfixes and improvements until you find that it was completaly wrong idea and with gained experience finally come usefull solution ... 3 or 4 years later 

Isn't that the truth. I'm at least a year in to my variable pitch efforts and am still working on getting good data. Of course I've spent some time working on airplanes too which hasn't helped the effort.