Author Topic: Counterpaw - design, build, fly.  (Read 5415 times)

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #165 on: 02, May 2017, 08:53:49 PM »
This counter rotating propeller business is more complicated than I thought. I've been doing some more research and found -

NA F82 Twin Mustang - props turning outwards at top, reversed to inwards at top to cure excess drag on take off.
Lockheed P38 Lightning - props turning outwards at top.
DH 103 Sea Hornet - Props turning inwards at top

Other aircraft such as the DH Mosquito and P61 Black Widow, which you might expect to have used CR's, didn't, both having the props turning anticlockwise looking from the front. Not a problem in the P61 apparently, with tricycle undercarriage, but the Mossie was notorious for it's swing on take off and landing.

Turning inwards at the top makes more sense in the case of an engine failure. If the left engine stops, the good engine on the right will yaw the aircraft to the left, but it's torque reaction, turning clockwise looking from the front, will roll the aircraft to the right. Depending on other characteristics of the design, dihedral etc. there will at least be some tendency for the two to cancel out. If I'd used inwards at the top rotation for Counterpaw's first flight, would it have flown successfully on the one good motor? Either way I'm considering reversing the rotation for the next flight attempt.

Offline Ashley Hoyland

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Re: Counterpaw - design, build, fly.
« Reply #166 on: 03, May 2017, 08:34:55 AM »

Just to follow my theory (and it is just my theory as far as I know) on prop rotation direction. If you leave the rotation with the top turning away from the fuselage I believe to establish directional stability an alternative solution to changing the direction of the propellers would be to position the rudder and fin upside down, (much more area below the fuselage centre line than above it) to keep a positive spiral flow on the largest part of the vertical stabiliser.

Could it be that the P38 'gets away with it' because it has twin fins and rudders in line with the propellers.

« Last Edit: 03, May 2017, 08:39:33 AM by Ashley Hoyland »