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I saw this video that NASA posted, which presents the idea of putting several small propellers on a wing:

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They claim that by doing this you can force air to blow over the wing more quickly. This would make the wings act as if the aircraft was flying faster than it really was, resulting in shorter takeoffs and more control on landings.

However while watching the video I kept thinking, wouldn't the turbulence and slipstreams caused by the propellers actually make the wings perform worse? Or at least neutralize the proposed added lift and control?

Having no fresh airflow over the wings seems like a problem. Have any aircraft ever tried this concept before with successful results?

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It would provide pretty good STOL performance at low speed because there is a significant increase in lift with the wing in the slipstream of a propeller, especially with flaps down. One of the biggest differences between a jet and a turboprop while flying a landing approach is that power settings have a direct effect on sink rate with a turboprop where a quarter or a third of the wing span is in the propeller slipstream; not so much on a jet.

Then when you speed up, the inefficiency of all those draggy nacelles and relatively inefficient smaller propellers, the weight, the maintenance nightmare, the fuel burn. Yikes. That's why it's not done.

There's a reason that airplanes are all built to a few near-universal configurations. They are the arrangements that hit a sweet spot, in the balance of compromises game, for the airplane's mission, and they are timeless until some major new paradigm comes along. That's not one of them.

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    $\begingroup$ Brilliant answer thank you! That was something along the lines of what I expected. So would it be correct to assume that all these propellers having a wing in their slip stream would cause additional thrust lost to exchange for lift in an already fuel thirsty design. $\endgroup$ – YAHsaves Jul 31 '18 at 2:21
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    $\begingroup$ Well if you had a 2000 HP engine on the wing replaced with 6 333 HP engines, the airplane could probably fly quite a bit slower, with power on, than with one engine on the wing, especially if there was a full span flap, but just about every other parameter would be much worse. The other thing is there would be a huge difference between minimum flying speed power on and power off. You'd slow to some speed with approach power and if you went to idle it would plummet. Regular turboprops already do that to some degree. $\endgroup$ – John K Jul 31 '18 at 3:51
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    $\begingroup$ Re "There's a reason..." isn't a big part of the reason that it's much easier to build and maintain (and control) a few large IC engines than a multitude of smaller ones? So if you have workable electric engines, you can use many of them, as with NASA's X-57: nasa.gov/centers/armstrong/news/FactSheets/FS-109.html $\endgroup$ – jamesqf Jul 31 '18 at 17:24
  • $\begingroup$ This design is not intended for combustion engines like you assume, but for electric aircraft, and indeed this has been done 25 year ago. The many smaller motors reduce the need for spanwise load carrying and provide yaw control without a rudder. Another factor is increased redundancy for UAVs that remain flying for months at a time. $\endgroup$ – user71659 Jul 31 '18 at 17:57
  • $\begingroup$ Even if they are electric motors, you have somewhat the same problem. The thrust per watt would be low compared to a single large propeller on each wing, because of many small inefficient propellers, so there is a benefit but it comes with a host of negatives. If I was building an electric transport aircraft I would still want the fewest engines with the largest propellers I could get. For a UAV, u have a good point but there is still a huge price for that redundancy, although for a specialized use like that it could be worth paying. Depends on the objectives. $\endgroup$ – John K Aug 1 '18 at 0:16
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The F-4 Phantom (early models) had a system called Boundary Layer Control (BLC). that, when the flaps were down (like for landing), release high pressure high velocity air diverted from the last stage of the engine compressor, out of multiple small holes in the leading edge of the wing. It basically "reenergized the boundary layer air flow over the top of the wing, and effectively did exactly what your question postulates. As a result, landing speeds were reduced by 10-15 knots. Prop wash does the same thing. Problem with your idea is that prop wash is localized to just behind the prop, not spread out over the entire wing. Multiple propellers require multiple engines, or some mechanical system to distribute the power from one engine to multiple propellers (weight!).

A perhaps better solution, (I'd have to do the engineering!), would be to use multiple small discharge holes in the leading edge of the wing, as the F-4 did, hooked up to a BLC system using high pressure, high velocity air from an engine driven compressor.

But I suspect that the benefits (a slight reduction in landing speeds) might not be sufficient to mitigate the additional weight, complexity, cost and problems of such a system. Indeed, the BLC system in the F-4 was eventually ripped out due to the problems associated with system failures.

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