Available horsepower and adding drag to an airplane both create extra lift? How is that possible?

  1. Available Horsepower

    According to the Zenith, available horsepower not used to overcome form drag and induced drag contributes to rate of climb.

  2. Adding drag

    NASA's report on the X57, states that drag might need to be added to get rid of excess thrust to create more lift.

    From the conclusion of that NASA report on span-wise propellers generating lift:

    A potential constraint on the design of the high-lift propellers system is the total thrust. If large amounts of lift augmentation are required from the propellers, then high thrust values from the high-lift propellers are likely. If these propellers produce excess thrust, then the aircraft will be unable to sustain flight at the desired speed. This implies that drag producing devices may need to be added to the aircraft or the primary propellers operated as windmills to produce additional drag.

    (High-Lift Propeller System Configuration Selection for NASA's SCEPTOR Distributed Electric Propulsion Flight Demonstrator, NASA)

    This concept seems to be exactly opposite of the first concept!!

  3. Rumours: Ultralights reach a max speed, any extra power is then converted to lift. Is this true?

How do these concepts work?

  • $\begingroup$ Do you have a direct quote from the NASA report to support #2? Because you are right, it makes no sense. Context is needed to determine if you misinterpreted it somehow. $\endgroup$ – Michael Hall Jan 17 at 16:43
  • $\begingroup$ Source and direct quote from NASA report added above. $\endgroup$ – Fred Jan 17 at 16:51
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    $\begingroup$ @Fred, based on the quote you provided your interpretation that adding drag will create lift is incorrect. Basically what it is saying is that if you rely heavily on propellers to create lift, you may end up with a lot more thrust than you need for normal forward flight. That's really all it's saying. Adding drag to compensate would be silly. As far as # 1 & 3, if you are overpowered you can either speed up, pull back the throttle to slow down, or climb. What specifically about the concept of excess power do you not understand? $\endgroup$ – Michael Hall Jan 17 at 17:47
  • $\begingroup$ You are mixing a lot of concepts together. First you don't need a lot of thrust to climb. To climb fast you need fly steeper which means your engine needs to fight gravity harder in order to not slow down. You actually need less lift in a climb than flying level. Lift argumentation is a totally different concept that I has nothing to do with 99% of production airplanes. In this case it's only meaningful within the context of that paper and that plane. $\endgroup$ – user3528438 Jan 17 at 17:47
  • $\begingroup$ @Michael Hall: I think (based just on the quote) that the problem is that it can't fly slowly enough. If the propellors produce enough lift, they also produce thrust which makes the plane go fast. It'd be something like trying to land your high-performance sailplane without spoilers or speed brakes. $\endgroup$ – jamesqf Jan 17 at 17:47

Equating rate of climb with more lift is wrong. In order to climb the aircraft needs more thrust but a bit less lift. Therefore, the first statement is wrong: Adding more power does not create more lift. The only case where this happens is at high speed and the maximum sustainable lift coefficient: More power allows to fly at a higher load factor, but depending on the flight envelope, might result in a crash. The explanation does not make that connection, so it is wrong.

The second explanation is special for the X-57 because the concept uses the propeller slipstream to create blown lift. At low speed, the thrust needed to maintain flight speed is low and might not be sufficient to create enough lift on the small wing. Adding some drag allows to fly at higher thrust without climbing away. But only in this special case when most of the wing is in the propeller slipstream. So the second statement is correct but only for a single case.

Artist concept of the NASA X-57 Maxwell

Artist concept of the NASA X-57 Maxwell (image source).

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  • $\begingroup$ If I increase throttle, it adds horsepower, which allows an increases in engine rpm, which increases thrust, which increases speed, which causes me to climb because the wings are producing more lift, no? $\endgroup$ – Fred Jan 18 at 15:03
  • $\begingroup$ How does distributed propulsion (DP) work in a pusher configuration, if the props don't add airflow to the wings? $\endgroup$ – Fred Jan 18 at 15:29
  • $\begingroup$ Re my comment above, I'm talking about climbing in straight and level flight, not pitching the aircraft up. eg. economy cruise climb at 100ft/min $\endgroup$ – Fred Jan 18 at 15:32
  • $\begingroup$ @Fred: If you add power you increase the flight path angle. Angle of attack will be unaffected, details depending on the power impact on the pitch equilibrium. Climb has noting to do with more lift, only the initial acceleration to a higher climb speed needs a bit of extra lift. There is no "straight and level" climb. Either you climb or you fly level. Please read the linked answer!!! $\endgroup$ – Peter Kämpf Jan 18 at 20:08
  • $\begingroup$ @Fred Even pusher props will add a bit of extra flow speed over the wing, but a lot less than tractor props. Basically, distributed propulsion with pushers is possible but has little impact on lift. $\endgroup$ – Peter Kämpf Jan 18 at 20:10

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