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Old Feb 22, 2013, 10:30 PM
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Edmonton, Alberta, Canada
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Rotating a spinning propeller around it's tip

Hi, everyone. This is a monocopter:

The scenario I am talking about is for a monocopter:
AUVSI11: Lockheed Martin Samari monocopter UAV (1 min 37 sec)


I am wondering about the effects of a spinning propeller that is itself being spun producing an extreme form of p-effect. Imagine a 16" propeller spinning around a motor bolted to one end of a 8" long stick and the stick is pinned at the other end so that the propeller drags itself in a circle.

As the propeller spins around the motor, the tip speed on the outside is much much greater than the tip speed on the inside of the circular path being traced out causing a severe P-effect. In this case, very severe since the propeller radius is a significant fraction (100%!) of the radius of the circlular path it is tracing out.

I presume this is very stressful on the blades and but do they risk breaking? I realize nothing solid can be given on this...I am talking about a 16x12" APCE prop being spun at 6000RPM tracing out a circular path on an arm that is 12" long. So that the propeller radius is 66% of the diameter of the circular path being traced out.

Would you expect any vibration or weird airflow problems?

An increase or decrease in thrust vs straight line flight?

An increase or decrease in airspeed vs straight line flight?
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Old Feb 23, 2013, 01:23 AM
Have you seen my Cobra?
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Thousand Oaks, CA
Joined Mar 2004
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This is essentially the problem faced by helicopters. The advancing and retreating blades operate at vastly different airspeeds. A combination of cyclic input and flapping/teetering prevents a lift imbalance and associated blade bending. In your case the outboard blade will be operating at a lower angle of attack due to the higher airspeed and thus producing less thrust. This imbalance will manifest as a vibration as well as a significant pitch moment (90 degree gyroscopic precession). There's also a purely gyroscopic effect from the rotating propeller mass being rotated on another axis. This will produce a pitch moment in the opposite direction of the aero moment.

A 16" prop at 6000RPM is pretty serious already, spinning it around would certainly be worthy of concern for structural failure. Some things can help:
- A folding prop will emulate the flapping hinge of a helicopter and prevent blade bending stresses and associated vibration.
- A folding prop that is on a complete U-joint such that it folds in any direction (up/down as well as fore/aft) will allow the above benefit to occur without changing the rotational inertia of the prop and thus imparting a torque shock to the motor.
- A 3-bladed prop will eliminate the mass-induced vibration since the moment of inertia is consistent about any axis on the prop plane. This is one reason why most aerobatic planes use 3-blades. At the very least, I would suggest starting with a 3-bladed folding prop. Maybe even carbon F5B blades like those made by RFM.
- A delta-3 teetering hinge will eliminate thrust-induced vibration. This is the method used on helicopter tail rotors since cyclic control is not available. This is a mechanical coupling of the flapping and cyclic modes that allows the rotor to passively balance thrust asymmetry that would normally occur in forward flight.
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Old Feb 23, 2013, 09:51 AM
Grad student in aeronautics
United States, GA, Atlanta
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Maybe just use a 24" prop.
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Old Feb 23, 2013, 11:27 AM
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Don't forget precession effects. As it is, the driving propeller is already trying to twist the monocopter blade as it spins. If you allow the propeller blades to flap the propeller disk would probably just reach the flap limits and remain stuck there.
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Old Feb 23, 2013, 11:40 AM
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Palmdale, CA
Joined Oct 2000
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This is one more in the endless ways to make simple things very complex.
Does it, can it, do anything much less complicated machines don't already do?
The monocopter would attract attention to itself merely by the way it operates.
As for the desire to make one, large things rotating rapidly with minimum or no control are dangerous!
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Old Feb 23, 2013, 11:50 AM
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The 'Wack, BC, Canada
Joined Oct 2002
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I'm not sure if calling this a P effect is accurate. Although the effect you're describing certainly does occur.

The classic description of the P effect is where the aircraft is operating at an extreme angle of attack. For example a P-51 during takeoff. It's where the downward traveling prop sees a large increase in angle of attack and produces more forward lift than the upward traveling side. The result being a strong yawing force.

In this monocopter's case the effect is there but it's produced by the yawing rotation of the wing itself. And since the whole point of the motor is to generate a yaw like rotation of the wing the effect is actually beneficial.

As for the prop and motor shaft bearings there is no doubt that there is going to be some cyclical loading as the prop spins and the blades see changes in lift loading during the rotation. But any sort of model prop and motor bearings are going to be able to withstand these loads for an almost indefinite time. Although I suspect the motor bearings will wear out in 1/3 to 1/2 the time compared to using it in a normal mode where side loads are not as high.

Another way to tell is sound. Motor/prop vibrations induced by this mode of operation would likely show up as an increase in noise. You tube videos of the Samari show that there is some high frequency buzz but it's hard to say if it's worse during flight than it is when simply running in the hand.
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Old Feb 23, 2013, 12:03 PM
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ISTR Cox had a single-bladed "rotocopter" with an .020 at one end of a wing.
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Old Feb 23, 2013, 12:59 PM
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Edmonton, Alberta, Canada
Joined Oct 2008
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Quote:
Originally Posted by Sparky Paul View Post
This is one more in the endless ways to make simple things very complex.
Does it, can it, do anything much less complicated machines don't already do?
The monocopter would attract attention to itself merely by the way it operates.
As for the desire to make one, large things rotating rapidly with minimum or no control are dangerous!
The only thing alternatives are tri/quadcopters and helicopters. Quadcopters are horribly inefficient while helicopters are among the most, if not the most complicated flying machine out there. A monocopter would allow me make a vertical take off vehicle with simpler mechanics and greater efficiency (because of lower RPM, greater wing chord, and larger rotor diameter). It counts for a lot because I am also an EE rather than a MecE so am uncomfortable with higher energy liquid fuel engines and complicated rotor mechanics, but ma more comfortable with complicated control circuits

Also, monocopters are supposed to be inherently stable while (flybarless) helicopters are not so. Helicopters have much of their control complexity offloaded into more complicated mechanics instead of electronics, yet they are still unstable. So although in principle the control electronics of a helicopter might be simpler, they also have smaller margins for tuning error while still retaining more complex mechanics. They also crash more catastrophically.

Quote:
Originally Posted by vespa View Post
This is essentially the problem faced by helicopters. The advancing and retreating blades operate at vastly different airspeeds. A combination of cyclic input and flapping/teetering prevents a lift imbalance and associated blade bending. In your case the outboard blade will be operating at a lower angle of attack due to the higher airspeed and thus producing less thrust. This imbalance will manifest as a vibration as well as a significant pitch moment (90 degree gyroscopic precession). There's also a purely gyroscopic effect from the rotating propeller mass being rotated on another axis. This will produce a pitch moment in the opposite direction of the aero moment.

A 16" prop at 6000RPM is pretty serious already, spinning it around would certainly be worthy of concern for structural failure. Some things can help:
- A folding prop will emulate the flapping hinge of a helicopter and prevent blade bending stresses and associated vibration.
- A folding prop that is on a complete U-joint such that it folds in any direction (up/down as well as fore/aft) will allow the above benefit to occur without changing the rotational inertia of the prop and thus imparting a torque shock to the motor.
- A 3-bladed prop will eliminate the mass-induced vibration since the moment of inertia is consistent about any axis on the prop plane. This is one reason why most aerobatic planes use 3-blades. At the very least, I would suggest starting with a 3-bladed folding prop. Maybe even carbon F5B blades like those made by RFM.
- A delta-3 teetering hinge will eliminate thrust-induced vibration. This is the method used on helicopter tail rotors since cyclic control is not available. This is a mechanical coupling of the flapping and cyclic modes that allows the rotor to passively balance thrust asymmetry that would normally occur in forward flight.
Last night I just found that 13.4x13.5 3-blade glow propellers and 13x13 4-blade propellers are available from APC- they just don't show up on their home website propeller list for some reason. That will absorb the same amount of power from the motor I have as the 16x12 APCE propeller and is also stronger and has a smaller radius for better ground clearance and will experience something closer to straight-line flight as it spins around the monocopter.

I'd totally go for a carbon or glass equivalent of the propeller though. Unfortunately I cannot find any.

I am not comfortable with the idea of using folding propellers because it seems to me they might throw a blade more easily, but at the same time allow for some flapping. There isn't some kind of teetering gimbal I can place on both sides of the prop hub that will allow a fixed propeller to teeter to do the same thing as the prop adapter clamps down on it is there? Perhaps just some very tough elastomer washers similar to what they use in rigid-rotor full-sized helicopters and all model helicopters to allow flapping?

Quote:
Originally Posted by Brandano View Post
Don't forget precession effects. As it is, the driving propeller is already trying to twist the monocopter blade as it spins. If you allow the propeller blades to flap the propeller disk would probably just reach the flap limits and remain stuck there.
I would think the centripetal force of the spinning propeller would be great enough to overcome that...but who knows? It's another reason I am not comfortable with a folding propeller.

Quote:
Originally Posted by BMatthews View Post
I'm not sure if calling this a P effect is accurate. Although the effect you're describing certainly does occur.

The classic description of the P effect is where the aircraft is operating at an extreme angle of attack. For example a P-51 during takeoff. It's where the downward traveling prop sees a large increase in angle of attack and produces more forward lift than the upward traveling side. The result being a strong yawing force.

In this monocopter's case the effect is there but it's produced by the yawing rotation of the wing itself. And since the whole point of the motor is to generate a yaw like rotation of the wing the effect is actually beneficial.

As for the prop and motor shaft bearings there is no doubt that there is going to be some cyclical loading as the prop spins and the blades see changes in lift loading during the rotation. But any sort of model prop and motor bearings are going to be able to withstand these loads for an almost indefinite time. Although I suspect the motor bearings will wear out in 1/3 to 1/2 the time compared to using it in a normal mode where side loads are not as high.

Another way to tell is sound. Motor/prop vibrations induced by this mode of operation would likely show up as an increase in noise. You tube videos of the Samari show that there is some high frequency buzz but it's hard to say if it's worse during flight than it is when simply running in the hand.
I agree. Ther term P-effect is not accurate. It would not be the traditional P-effect which is caused by asymetrical AOAs on the advancing and retreating blades of the propeller, but rather caused by assymetrical speed of airflow entering each side of the propeller.

I just P-effect it to sum things up in case my description of the scenario did not get across as it was the closest commonplace term I could come up with that was somewhat related to the phenomena I was talking about. My original post contained a more detailed description but it somewhat defeated my attempt to make things readable so I removed it.

The sound is a good point at least when starting it up for the first time- slowly ramp up the RPM and pay attention to whether the sound starts changing in an overly unpleasant way and kill it and change to ever smaller and smaller propellers.

Quote:
Originally Posted by Sparky Paul View Post
ISTR Cox had a single-bladed "rotocopter" with an .020 at one end of a wing.
I was considering this route, but I came to the guesstimated conclusion that a fuel engine could not run at the end of the wing with the all the forces on it?!! Do you have more info on what it is called?
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