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What is the best prop location for distributed propulsion - pusher, tractor or mid top wing ( blown flap) for an ultralight, max speed about 60mph, for MINIMUM takeoff distance ( 20-40') and about 15mph stall speed?

I would assume Blown flap, as the wing and prop would see clean air, and the flap sees the highest air speed increase due to exit speed of the prop, assuming 100% coverage.

My assumptions are:

  1. Tractor configuration: entire wings sees prop exit speed but air is turbulent, so less lift, prop is in clean air, so max thrust.

  2. Blown flap (prop on top of wing): wing is in clean air, so max lift, flap sees highest increase in air speed in this configuration, so max lift, prop is in clean air, so max thrust.

  3. Pusher configuration: wing and flap in clean air, so good lift, prop in dirty air, so less thrust

Is this correct?

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If your looking for max performance in short takeoff distance, the best solution is to blow the wing with your engines. Either you chose propeller or mini distributed electrical jet fans will only change the position you will set them and both idea might actually work. In tractor configuration the prop exit speed is actually doing most of the work to create lift. You can watch some good stall competitions videos where the plane may takes of with brakes locked. For example this video from Steve Henry

That's definitely the configuration giving you the most lift at low speed. Some other well known solution on larger airplanes including NASA and USAF experiments have the engine closer to the wing:

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The Breguet BR941 had a full blown wing with 4 huge 4m diameter propellers and with flap going down to 95° allowing for less than 200m take off and 120m landing. enter image description here

The NASA QSRA (Quiet Stall Research Aircraft) has again 4 engine blowing the full wing at landing to allow for more lift and allowed it to land on the USS Kitty Hawk.

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Both YC-14 and YC-15 where project to reduce landing distance in the USAF that finally led to the development of the C-17 globemaster.

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You are not wrong to go with tractor config.

In my opinion the best location of the propeller is pusher. The reason is that there is no airframe behind to interact with a heavily turbulent propeller wash. The thrust is developed on propeller blades alone and the airflow on the wing is the result of wing being thrusted forward in stationary air. It has nothing to do with propeller wash. The wing benefits from the most undisturbed air possible but the propeller gets the wash off the wing and airframe. For ultralight it may not be a big deal because of good thrust to weight ratio.

The tractor solution puts the engine ahead of the wing and is more common. The propeller gets the soft air and the wing gets the propeller wash. Maximum thrust, but not the best lift or drag.

Distributed propulsion means you have more contributors to propulsion.

15mph stall is similar to paramotor. You may go even lower if you’ll consider a gyro plane.

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  • $\begingroup$ Why not the blown flap (prop on top of mid wing)? Prop and wing in clean air and the flap sees the highest increase in air speed. $\endgroup$
    – Fred
    Jan 3, 2020 at 15:40
  • $\begingroup$ I’m not used to that solution. Usually float planes worry for salt spray and place the propeller up above, for no other apparent reason. $\endgroup$
    – WindSoul
    Jan 4, 2020 at 4:22

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