Folding prop and Precession

[frank williams]

Ever consider a folding prop for a stunt ship?

So ..... I've flown this on an electric several years back, and don't remember too much about the flight other than I thought that during maneuvers I could see the prop plane tilting, but wasn't sure.

I got to thinking that maybe with a folding prop the gyro processional moment might be reduced.  The gyro force on the blades would just push the blade one way or the other and not react back to the shaft to produce a moment  Does this make sense or an I missing some parts.

[Steve Helmick]

Seems like harsh treatment for your Dremel!!!  Howard should be along soon to post something....ciphering or the NYE Address. Weather wasn't too bad here today, so he may still be flying at Stonehedge. Might have left his lines and handle box out on the circle. He does that, you know...  Steve 

[Chris McMillin]

Ingenious. Neat idea, Frank. Happy New Year.
Chris...

[Brett Buck]

Ever consider a folding prop for a stunt ship?

So ..... I've flown this on an electric several years back, and don't remember too much about the flight other than I thought that during maneuvers I could see the prop plane tilting, but wasn't sure.

I got to thinking that maybe with a folding prop the gyro processional moment might be reduced.  The gyro force on the blades would just push the blade one way or the other and not react back to the shaft to produce a moment  Does this make sense or an I missing some parts.

    The H vector is horizontal when you start, and vertical when you finish.

     Brett

[frank williams]

..... but what happens in-between ...

I should add ... the dremel tool setup with the prop is not for the faint of heart ...

[Brett Buck]

..... but what happens in-between ...

I should add ... the dremel tool setup with the prop is not for the faint of heart ...

   Torque must be applied to re-orient the H vector, due to conservation of angular momentum.  It works for helicopters (and is effectively mandatory for helicopters) because the rotor disc stays horizontal, or is precessed by the force of the blades due to the cyclic, not by the shaft.

      Brett

[RandySmith]

..... but what happens in-between ...

I should add ... the dremel tool setup with the prop is not for the faint of heart ...

Its  better  than using a  1 hp  router !

Randy

[frank williams]

Yes ... the router was exciting .... a router motor and a 12x6 prop will fly .... quite well I might add

Brett .... so ... for a stunt ship with folding blades .... the prop disk will re-orient and the procession will be less?

[Brett Buck]

Yes ... the router was exciting .... a router motor and a 12x6 prop will fly .... quite well I might add

Brett .... so ... for a stunt ship with folding blades .... the prop disk will re-orient and the procession will be less?

     It had better re-orient itself, if not, you are going to have another, far more interesting problem. Ultimately the integral of the torque over time will be the same as it was before, the peak of the precessional torque maybe be less, but thats countered by taking longer and oscillating at some frequency determined by the mass properties of the blades.

     Brett

p.s.    BTW, I don't recommend anyone try the 12-6 on a router test, particularly if you plan on rapidly reorienting it to simulate a corner. 25,000 rpm on a Rev-Up 12-6 is probably pushing the limits of wisdom just from loads alone, if you start trying to yank it around to simulate a corner, you both make it almost certain that you will come in line with the blades, all that precessional torque is being applied by the blades trying to flap up and down 417 times a second. You might also melt the router, and you are really very likely to melt your Dremel tool.

[frank williams]

I just had to try one more experiment ... the idea that the folding blade shouldn't transmit a moment back to the prop shaft is still intriguing .  I this "rigid body dynamics"?

The folding prop pin isn't totally friction free.   Suppose I had a totally free hinge point.  So .....  I made up a set of "numchucks" out of motor mount stock.  I attached them to the center section of the assembly and attached the "blades" with string.  totally no moment transfer at the joints.

The feeling of the assembly when running is interesting.  The plane of the rotation changes and floats around in response to a hard pitch or yaw acceleration.  The feeling of the hard precessional moment is minimal.  I still think the folding prop should have a reduced gyro moment than that of a solid prop.

[Joe Gilbert]

Would you lose some drive in corner while the prop repositioned is self?

[frank williams]

not necessarily .... the hope would be to reduce yaw during corners ...

[pmackenzie]

FWIW, when AXIs first came out they suggested to use only folding props to reduce loads on the mounts. Current instructions "strongly recommend" it.

Quote from the AXI motor manual

The AXI motor design with its rotating case significantly increases
the need of a robust motor mount. We strongly recommend the
use of a folding propeller even with aerobatic models, even the
world F5A champion uses a folding propeller on his motor of a
similar design. If you are using your motor at the upper end of it's
power range, or if you must use a fixed propeller, please make
sure that your motor really is securely mounted.

[Tim Wescott]

   Torque must be applied to re-orient the H vector, due to conservation of angular momentum.  It works for helicopters (and is effectively mandatory for helicopters) because the rotor disc stays horizontal, or is precessed by the force of the blades due to the cyclic, not by the shaft.

      Brett

I'm really not good with the angular momentum stuff (tensors just make me tenser -- yes, it's lame, but you got to use 'ullage' in a grammatically and technically correct sentence recently, so it's still points to you).

But, I think the angular momentum may well be absorbed by the air, due to differential lift of the blades -- and there ought to be some pretty strong damping, or the whole folding prop thing wouldn't fly (as it were) on sailplanes.

That's not to say that having the effective thrust angle of the prop pointing any which way as you do your maneuvers won't drag the airplane around even worse than with a rigid prop -- but I could see the actual momentum transfer being to the air, and not the motor shaft.

[Paul Walker]

Frank,
Have you tried one of Igor's hollow props?
The standard prop is 26 ish g, and the hollow prop is 16 ish g. It makes a significant difference in the yaw you are looking to eliminate.

If you try one, be sure to put an exact copy of that prop, but solid on before the hollow prop so you can feel the difference. I first tried it the day before a competition and there was so much difference I put the heavy prop back on so as not to distract me during the competition. As soon as I returned home, the hollow prop went back on and I have not used a solid prop since. It is as close to a no yaw effect as I have ever felt.

[frank williams]

FWIW, when AXIs first came out they suggested to use only folding props to reduce loads on the mounts. Current instructions "strongly recommend" it.

Quote from the AXI motor manual

Pat ... thats very enlightening .... the first front mount motor electric I made was a retrofit of a gaser and in the first hard corner the brand new Plentenburg was thrown high into the air after it ripped itself out of the front end.  Great respect earned for GP .... the Plettenburg missed the soft ground by about 12 inches and splattered on the concrete.

Paul .... I have one of the light weight Igors, but I haven't had a chance to fly it yet .... I know it'll make a big difference.... I normally fly a Yatsenko wood pusher that weighs about 23 grams, but is a 13.4 inch diameter, however still a good prop.

[dale gleason]

I'm assuming it's unsafe to run a foam-filled "hollow" carbon prop on an IC engine .  The same as running a plastic (APC) electric prop on an IC engine.  Knocking that much weight off a prop sure sounds inviting, but, dangerous.

dg

[frank williams]

There is the Belko ic prop that is foam filled, but its not that much lighter than a regular carbon.  The Belko has thicker percentage sections that are foam cored, with a neat sweptback planform.  Its a good bigbore prop.

I'be got to add that just running the "numchuck" contraption on the dremel is really interesting.  Maybe dangerous, but I don't run it flat out, just enough to hold the "chucks" out.  The disk of rotation really floats around.  After the initial pitchup of the dremel, the disk swings to point right and then responds with the disk moving around to the left and then stabilizing.  I'll try to make a video of it in action.

[Igor Burger]

Yes I did :-)) On small model, so I did not affraid of large folding prop, however it survived.

And yes it limits precession peaks, but remember that those forces are converted to forces perpendicular to the screws. It means that hub of the prop is loaded similar to IC pulsation. And those tiny roots of blades are probably not made to withstand it permanently, at least not those plastic, probably carbon blades will survive easier. And who know what will do that alu H part under such vibration. I affarid it must break sooner or later.

[Igor Burger]

FWIW, when AXIs first came out they suggested to use only folding props to reduce loads on the mounts. Current instructions "strongly recommend" it.

I did not see it :- ))) ... funny, they probably put it there when they saw how we fly 1" larger props and 3 blade, with battery of 1 cell more than they recommend and it tight turns :- ))) ... and especially when the guy saw motor is hot after flight, he just told with calm face, that is OK, that bearing has no chance anyway in those conditions :- P ... so you will change it soon anyway :- ))))))))

I do not know if you all know, but factory for AXI motors is few hundreds of meters from Hradec Kralove C/l stadium - on opposite side of the street (someone will remember), so they saw what we do with them and they were scared :- )))))))))))

[Igor Burger]

I'm assuming it's unsafe to run a foam-filled "hollow" carbon prop on an IC engine .

Our props have empty hubs, so it is absolutely not usefull on IC engines. Heat, high pressure of nut and oscillations will make it danger and damage soon.