Shades of the airplane treadmill - the "blown" wing

I just read this article https://spectrum.ieee.org/aerospace/aviation/nasa-remakes-the-airplane-wing-with-electric-propulsion-and-a-whole-lot-of-propellers about “blown” wings - an aircraft design that uses extra propellers to blow air across a wing so that the airspeed past the wing is significantly higher than the aircraft’s actual airspeed.

Quote:
"The blown wing provides a solution to this conundrum. During takeoff and landing, air can be blown over the wing at higher speeds, providing additional lift without sacrificing cruise performance. Although a few aircraft have been developed in the past with blown wings, the use of combustion engines for propulsion limited how far their designers could go. They had to use relatively few, large propellers, which aren’t well suited to pushing air at high speed.

It would be more effective to distribute a large number of small propellers across the span of the wing, but for most of aviation history, that arrangement has been impractical."

I can’t help being reminded of all the discussions about airplanes on treadmills…

The effect is greatly increased by shaping the wing like a channel, like in this demonstration: https://www.youtube.com/watch?v=oMFUWVj1_DQ

It’s certainly counterintuitive, but the demonstration is rather convincing. And, well, the whole airfoil thing is complicated.

Wouldn’t it give more lift, then, to mount jet engines on top of the wings instead of hanging them below?

A blown wing can give lift to a slow-moving or even stationary plane. But the purpose of jet engines (on airliners at least) is not to give lift to a slow-moving plane, it’s to make the plane go places, and to go to those places as efficiently as possible. Since you want the plane moving rapidly forwards through the air, it’s not clear to me why a blown wing would be desirable for an airliner unless it has some non-obvious gain in efficiency (or if runway length is a constraint).

The gain in efficiency is to have a smaller wing for a given size plane, let’s assume for a given runway/takeoff length. The required size (area) of an airliner’s wing is determined by takeoff/landing condition. At cruise it could have a much smaller wing and cruise drag be reduced if it did. You can make the wing smaller using devices or arrangements which increase lift only at low speed, as long as they don’t create too much more drag at high speed, or weigh too much.

This has long been the case with the complicated arrangement of leading edge slats and trailing edge slotted flaps on jet airliners which are only extended at low speed. You could do without those, if you made the wing bigger, but therefore heavier, and creating more drag at cruise speed.

Distributed propulsion arrangements (more propulsors along the wing ‘blowing’ over it) are just an extension of the same idea. You produce more lift from a given area at low speed, allowing a smaller wing with lower weight and lower drag at cruise speed, and avoid offsetting this too much with the extra weight and drag of the lift increasing system itself. The key to making the ‘blown wing’ concept more attractive lately and in the near future (though it doesn’t absolutely depend on this) is distributed electrical propulsion.

The NASA X-57 illustrates the idea just looking at the picture


It has an obviously smaller wing than normal by blowing across the span with distributed electrically driven props. In cruise this small wing creates less drag as well as weighing less than a conventionally sized one, and you avoid adding too much to drag in cruise by feathering the little props and driving the plane with the bigger wingtip props only. This particular arrangement of basically straight wing and unducted propeller is suitable for small short range airliners. But you could extend and adapt the concept potentially to jet speed and big airliners.