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Author Topic: Help - Torque roll & prop design  (Read 493 times)
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ghcrash
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« on: February 05, 2017, 12:02:08 PM »

What can I do to the prop to reduce torque roll?

I made a Mini-stick to use as a test bed for learning about prop design.  It flies well enough on about half power (turns) but torque rolls into the ground if I try for max turns on the rubber.   I know that I can do things to the airframe (washout, wing offset and such) to minimize the tendency to roll under high torque launches.  What I’m curious about is if, and what, can be done to the prop design to reduce the torque felt by the airframe.

 Will reducing the prop’s diameter, pitch or blade area have any effect on the tendency to torque roll?   Which would have the most effect, altering the diameter, pitch  or blade area?

I will take any and all advice.
     George
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Olbill
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« Reply #1 on: February 05, 2017, 12:06:41 PM »

I don't fly ministick but all of them will torque roll. I think your best bet is to start with one of the successful designs. The only ones I'm familiar with are the Poonker and Nick Rays's mini which also flys very well.
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ghcrash
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« Reply #2 on: February 05, 2017, 12:55:57 PM »

I don't fly ministick but all of them will torque roll. I think your best bet is to start with one of the successful designs. The only ones I'm familiar with are the Poonker and Nick Rays's mini which also flys very well.

While this is very true if I was trying to come up with a 'winning' plane.    Again, the intent here is to learn something about rubber prop design .
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frash
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« Reply #3 on: February 05, 2017, 01:18:28 PM »

Two possible hints:

If you can reduce rubber thickness, try to do so.

An expert that I timed for in Ministick, suggested warping the inside (left) wing down about 1/4 in or so 2 or 3 times just before launching. It looks like none of the warp stays in, and you don't want it to stay in for much of the flight, just for the first half minute or less to get you past the troublesome torque roll.

Fred Rash
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dslusarc
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« Reply #4 on: February 05, 2017, 01:35:43 PM »

The way I have reduced the rolling is by model design. Tall wing posts,polyhedral wing and stab, wing offset and stab offset. The model will lean left. Only in high ceilings will I get one or two rolls after launch. I do think higher pitch props , over 2:1 P/D roll less. The reason being at launch I think the lower pitch props have more excess thrust so the excess thrust makes the model climb so steeply the nose comes up then the model starts to stall then rolls on about a 45 degree axis. The higher pitch props at launch seem to have less excess thrust and climb slower so they do not nose up so much so can pull through. When my model does roll it is more a horizontal axis. The model starts to bank left after launch and after about half a circle it may bank more then finally it will complete the roll when it gets close to knife edge, then start to climb out. By then the torque has fallen off a little then climb out is normal. Most of the time it rolls left and stays banked and never rolls. I attribute that to the tall wing posts so the motorstick and rubber have a pendulum effect counteracting the torque.  

Don
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Hepcat
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« Reply #5 on: April 12, 2017, 09:20:01 PM »

George,
Your opening post said what could you do to the prop to reduce torque roll, and also you wanted to use the mini stick to learn about propellers.  I think you might not be going the easiest way to achieve your objectives. Smiley  As Bill has said everyone has had trouble with mini sticks rolling and I think the first model to overcome it was the K777 with a very high mounted wing and large tip plates.  Coming now to the propeller aspect. The propeller torque formula says Torque= torque coefficient x air density x n squared x D to the fifth power. The torque coefficient is like the lift coefficient on a wing, as the wing depends on the wing section and wing area for lift so the prop depends on blade angle and blade area for thrust. Air density is pretty constant. Wing lift depends on flying speed squared, prop thrust depends on rotational speed n squared. There is also a certain analagy between the span of a wing and the diameter of a propeller in that a high aspect ratio of a wing and a large diameter on a propeller both lead to high efficiency. I am sorry but that is rather a long way round to say the propeller must be like that to absorb the torque in the motor because that is what flies the aeroplane.  Now I have said all that I realize that Fred Rash has aready given an answer.  Use a longer thinner motor taking more turns and then the propeller can be smaller.
I must say I enjoyed Don Lusarki's excellent analysis.  Do you think Don that the high pitch propellers could also have stalled early in the flight giving the slow hover before the climb away d a stalled prop would absorb a lot of torque.
John
 


John
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ghcrash
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« Reply #6 on: April 13, 2017, 09:02:39 AM »


I'm always amazed at what I learn, about a lot of things, when I ask what I think is a simple question.  Thanks to everyone for your replies.  I'm not sure that I understand everything that has been said but I definitely learned quite a bit, about plane design as well as props.

As a couple of you pointed out, a Ministick is probably not the best airplane for learning about props.  I agree.  It's advantage is that It can be flown in a small area. 

I realized about half way through the responses that I may have used the term torque roll in a different way than what it means to other people.  It seems that several people use the term to describe when a plane pitches nose up then rolls to the side.  To me this is just a stall with prop torque causing the plane to rotate once it loses air speed.  The problem I was (am) having is that the plane, from lever flight rolls left past 90 degrees then goes nose down into the floor.  This happens within the first few seconds after launch and there is no nose high pitch up at any time.  I found early on that the problem didn't occur with a smaller rubber but the plane wouldn't climb.  The problem also didn't occur if I used a smaller prop with the big rubber (smaller= .019", bigger= .021") but the flight times dropped. 

Again thanks to all.   I've learned quite a bit.

   George
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