Would plain and slotted flaps increase lift substantially in a distributed propulsion application for a slow moving aircraft (15mph)?

I understand that:

  1. A classic NACA foil without flaps such as the NACA 650-18, as used on Zenith 701 STOL aircraft produces a maximum lift coefficient (Clmax) of approx. 1.5.

  2. Add a plain non-slotted flap, and the Clmax increases to roughly 2.2.

  3. Add slotted flaps, and the Clmax increases to roughly 2.8.

  4. The maximum Clmax of distributed propulsion is 12.

  5. Given a GA aircraft such as the Zenith 701, with an engine of 100HP, with a stall speed of 45mph, means the entire wing is bathed in air travelling at 45mph when flying at 45mph.

  6. Assume the same given aircraft now has multiple propellers installed along it's entire leading edge in a distributed propulsion arrangement, with multiple engines totalling the same total Hp, flying at roughly 15mph and with an propeller exit speed of 45mph. This also means the entire wing is bathed in air travelling at 45mph. ( assume 100% efficiency, no swirl angle, etc.).

See graph below. (Solid black graph - Source: Aviationchief.com. Added graphics estimated by the OP.)

  1. As distributed propulsion is just "extra air", and flaps move the lift slope curve up and to the left, I would think the wing would see substantial increase in Clmax if plain flaps or slotted flaps added.

  2. I understand that in distributed propulsion, a wing with a Clmax of 6, on a slow flying aircraft, (15mph), but prop exit speed 300% faster at 45mph, the local AoA would always be under 10 deg.

  3. I would think there would be a substantial increase in lift when using a plain flap and even more when using a slotted flap, but almost no increase in Clmax above 10 deg AOA.

Is this correct thinking?

Effects of adding flaps in distributed propulsion


1 Answer 1


No, that thinking is incorrect.

The air involved in lift creation is roughly the amount that flows through a disk with a diameter equalling wing span, and your distributed propulsion concept will involve a far smaller amount of air.

The maximum Clmax of distributed propulsion is 12.

In theory maybe. Practical values are half that, at most. Now consider that this lift should ideally be independent of engine health and power settings, or you will plummet from the sky when power fails or you need to throttle back. Distributed propulsion will not save you from empty fuel tanks or batteries, so this scenario must be considered. Besides, the lift coefficients you cite in your question are valid for 2D flow; in reality the outer wing will contribute much less to provide some margin for control and stall behavior.

  • $\begingroup$ @ Peter Kamp: Good to know. I think the Nasa's X57 boosts power from about 270 Hp to about 350Hp. What about adding plain or slotted flaps? Would there be a substantial increase in lift (as shown in the graph above) - a higher increase than without distributed propulsion, since flaps offset the lift slope curve up and to the left? $\endgroup$
    – Fred
    Commented Jul 6, 2020 at 17:29
  • $\begingroup$ @Fred: In order to create more lift, you need to accelerate more air (higher span, higher speed) by a bigger amount (slats, flaps). Every bit helps. Adding props along the leading edge helps (but not by the amount you assumed) as does adding flaps. And slotted flaps work better than plain flaps. Double-slotted flaps with chord increase are even better. But also heavier, so more lift on the wing is needed for the same minimum speed. After adding enough gizmos you will run into diminishing returns. $\endgroup$ Commented Jul 6, 2020 at 17:54
  • $\begingroup$ Following your wing span example above, does that mean a wing with the same area, but half the span, and double the chord, needs 1/2 the air/mass flow to create the similar lift? $\endgroup$
    – Fred
    Commented Jul 9, 2020 at 14:19
  • 3
    $\begingroup$ @Fred: No, it means it uses only a quarter of the air mass and will accelerate that by four times the amount that a wing of twice the span would need to accelerate the air mass it can influence. The area of the circle is proportional to span squared! This also means that induced drag is four times higher for the more narrow wing! $\endgroup$ Commented Jul 9, 2020 at 15:56

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