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Old Mar 03, 2013, 04:52 AM
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Australia, New South Wales, Sydney
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Effect of prop diameter on airframe roll rate

If the goal is to maximize roll rate for an aerobatic/3D airplane, is increasing the prop diameter always going to be counter-productive, or is the effect more subtle and dependent on airframe design specifics?

For some reason which I'm struggling to explain even to myself, my guess is that as the airspeed approaches pitch speed, swinging a larger prop simply causes a reduction in the roll rate. Airflow over the ailerons is as fast as it's going to get (in level flight), and the additional rotating mass and inertia of a larger prop would presumably add to the roll resistance, thus reducing the maximum roll rate.

To put it another way, I believe at maximum airspeed the prop is a hindrance to rolling - the plane would roll even faster if the prop could magically be made to disappear - and thus the smaller the prop the faster the max roll rate.

Is that assumption correct?

Where it perhaps gets more interesting is at low airspeeds. There is now significant wash from the prop over the ailerons, and that would contribute to faster rolling. At zero airspeed (a hovering 3D plane), a larger prop might turn more of the aileron length into a force-generating surface, thus potentially increasing the roll rate.

Or is that reasoning way off?

Thanks!
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Old Mar 03, 2013, 06:18 AM
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Germany, BW, Stuttgart
Joined Mar 2012
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I think your reasoning is mostly correct.

I think it's fair to say that under almost all conditions, the propeller shaft exerts a torque on the airplane that is opposite the dircetion of the propeller rotation (I suppose the direction of the torque could change if you had a very slow, constant-speed prop and you initiated a roll rate that was comparable to the prop RPM).

As long as the torque always acts in one direction (and you neglect the effects of the dreaded "spiral slipstream"), the propeller is always helping your roll one direction and hindering your roll in the other direction. This is apparently a pretty small effect because you don't normally see any significant configuration asymmetry to compensate for the torque due to the propeller (even on airplanes with big props like F-4U's and P-51's).

Does the hindering effect in one direction get bigger as the prop diameter goes up? I think the answer is "yes" if you have a constant speed prop and "not really" if your motor is instead providing constant torque to the prop.
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Old Mar 03, 2013, 11:21 AM
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Palmdale, CA
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The early planes with the rotary engines required the pilot to hold against the torque created roll... trimming the surface having not been invented yet.
The all-fly comma shaped verticals on the Fokkers and Nieuports demanded the pilot hold the rudder centered with his feet on the rudder bar, otherwise the plane would swap ends. Those old timey pilots had to be strong!
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Old Mar 04, 2013, 02:03 PM
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The Buchon (a post-war BF109 re-engined with a Rolls Royce Merlin) is apparently a handful to fly because it has an huge prop and engine torque with a relatively small wing and a narrow landing gear, and I believe several were damaged in failed takeoffs. Even the later mark Spitfires re-engined with the Griffon had to use counter-rotating props because otherwise they would be almost impossible to handle.
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Old Mar 10, 2013, 11:26 PM
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Malaysia, Selangor, Kajang
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I believe your reasoning is a bit off but it depends on what kind of roll you have in mind.

For regular aerobatic or F3A style axial rolls the roll rate is dominated primarily by the inertia of the wings and the speed of the aircraft. Higher speed = higher roll rate simply because the higher kinetic energy of the airflow imparts a higher force on the ailerons. Lighter, shorter wings = higher roll rate because there's less inertia to oppose the roll. The prop does have an indirect effect to this but it's more an issue of pitch rather than size. To get really high speeds you need higher pitch, preferably in the 1:1 range. That is because lower pitch props stall at high speeds (technically not stall but rather it's effective AOA approaches its zero lift angle, still most people refer to this phenomena as stalling). At high enough speed and with short enough wings you can get roll rates so high that would make your plane look like a flying drill bit. At such high roll rate you won't have a fast enough reflex to apply elevator or rudder corrections. Just nose up a little and apply a tiny touch of up elevator to neutralize elevator trim and watch the plane arc to the end of the field.

For a proper, more controlled roll or harrier roll the prop does have a fairly significant effect on the roll rate. But it depends on the direction of the roll. Assuming your prop rotate clockwise (instead of the Russian way) a larger prop will increase roll rate if you roll left and decrease roll rate if you roll right. This has been my experience to date and I believe that this is just the result of the torque amplifying or dampening the roll. For harrier rolls specifically, especially if it's a really slow harrier, a larger prop may also increase roll rate because the larger prop wash affects more aileron surface.
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Old Mar 16, 2013, 08:42 AM
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Reduce aspect ratio on the model - this will do more for rioll rate than any other trick.
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Old Apr 09, 2013, 11:45 PM
Lear 35, GII, GIII pilot
Hazlehurst GA
Joined Jan 2001
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Quote:
Originally Posted by richard hanson View Post
Reduce aspect ratio on the model - this will do more for rioll rate than any other trick.
What he said. Also, what he is saying is, the shorter the span, the quicker the roll rate.(while keeping the wing area constant.) I have found this to be true. Another factor is keeping the mass as close to the center axis as possible. Servos closer to the fuselage, no heavy devices on the wingtips, etc. All of that keeps the roll inertia low, meaning the roll rate starts and stops quicker, which is what I think you want also. I think these design factors have more effect on roll rate than prop size.
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