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Does distributed propulsion create unwanted lift at cruise speeds?

The great thing about slats and slotted flaps is that you can "turn them on" when you need high lift at takeoff, and "turn them off" when you don't need high lift during cruise.

So what about distributed propulsion by using 4 engines with props covering the majority of the wingspan? You can't turn off the extra lift off if you're trying to cruise as fast as possible.

I understand distributed propulsion adds a clmax of about 3.

Typically, you only need a Cl of 0.5 during cruise, but most GA planes cruise at about 75% power, almost at full power as on takeoff, so wouldn't a cl total of say 1-2 be generated with distributed propulsion? That's 200 to 400% higher lift than needed.

To get rid of this extra lift and drag, wouldn't you need to lower the nose until the wings produce a " negative induced cl" of about -.5 to -1.5, adding the "distributed propulsion cl" of 1-2, for a total desired cL of about 0.5?

Is this correct, at least conceptually? Or does lift due to distributed propulsion trend to zero at a zero AOA? i.e. it's just additive if the wing is producing lift above a zero AOA

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  • $\begingroup$ Note that it's clmax, not cl you're talking about. $\endgroup$ – Sanchises Oct 28 '19 at 18:14
  • $\begingroup$ Why all the references to clmax instead of cl? Something is wrong with this question. $\endgroup$ – quiet flyer Oct 28 '19 at 18:29
  • $\begingroup$ @Fred you are correct in questioning the effects of prop blast on total lift, but need to consider magnitude. For a very light model with a high lift wing and excess power, it might be quite dramatic. Also, consider mounting props ahead or behind the wings (or elsewhere). Remember, prop wash is going to be turbulent (even if counter rotated) or, as was the case with the B-36, the wing interferes with the prop airstream, which will affect thrust production (and gave the plane an audible pulse that was heard from many miles away). Some prefer to keep the wing "clean". $\endgroup$ – Robert DiGiovanni Oct 29 '19 at 8:14
  • $\begingroup$ @Fred just another light bulb about (perhaps the lost art) of propwash: it may actually keep the local AOA of the airstream more constant (from its flow) and control lift simply by adding or removing throttle. This may help stall proof the wing to a greater degree. You may wish to look into the Navy "flying flapjack", they also used propwash to counteract wing tip vorticies. $\endgroup$ – Robert DiGiovanni Oct 29 '19 at 22:39
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The fundamental way distributed propulsion adds extra lift is by blowing additional air over the wings. This is a direct thrust-to-lift conversion.

If you produce less thrust, you produce less lift with this arrangement.

So, no, you don't go to a negative AoA to counteract the excess lift produced by running all of your motors at full power. You never produce that excess lift in the first place. This involves running your wing propulsors at reduced power, and if you have lift-assist propulsors, such as blown flaps, turning them off altogether.

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  • $\begingroup$ I'm talking about just using 4 normal airplane engine as a "propulsor". At a full speed cruise, you are producing almost maximum thrust with your props along the entire wingspan, so they would be producing almost maximum distributed lift, no? $\endgroup$ – Fred Oct 28 '19 at 20:31
  • $\begingroup$ @Fred Not quite. Cruise thrust tends to be in the range of 15% to 30% of takeoff thrust. $\endgroup$ – Therac Oct 28 '19 at 20:36
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    $\begingroup$ You don't cruise at 75% of full thrust. You may cruise at 75% of currently available power, but that produces much less than 75% of takeoff thrust. $\endgroup$ – Therac Oct 28 '19 at 20:48
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    $\begingroup$ @Fred, prop efficiency drops off as density decreases. Less air molecules to get traction on. $\endgroup$ – Michael Hall Oct 28 '19 at 21:08
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    $\begingroup$ @Fred It's not. The faster you go, the less thrust your prop produces. For propeller aircraft, it decreases from 3.5-5 lbf/hp at standstill to 1-3 lbf/hp at cruise. $\endgroup$ – Therac Oct 28 '19 at 21:40

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