Which prop creates more lift? Leading edge or blown flap?

In distributed propulsion there seem to be 3 prop locations: leading edge (NASA X57, blackfly), blown flap (Lillium), and pusher prop (Kittyhawk).

Which would create more lift, leading edge or blown flap?

  1. Leading edge: The entire wing chord sees about 50% of prop exit speed (turbulent though), the prop pushing 50% of the air under the wing (although it would create lift by hitting the flap (assume 45-90 deg flap position)

  2. Blown Flap: wing sees 100% of prop exit speed but only on about 45% of the of the chord ( assuming 50% chord flap), but the first 45% of the wing sees non turbulent air. Blown flap has the advantage of providing vertical thrust.

See pictures below.

Lillium - blown flap ( assume props, not the actual electric turbines it uses)

Illium blown flap

Kittyhawk - pusher prop

Kittyhawk with pusher props

Nasa X57 - leading edge/tractor prop

Nasa X57 with tractor props

Below are 3 Cp plots from xflr.

First plot has no flap, but has a roughly 30 deg angle in (-) pressure drop.

Naca 2415 Cp plot 2

Adding a 50% chord flap, increases the suction at 0% chord???

Was surprised to a spike in negative pressure at 0% chord from a 50% chord, 45 deg flap!!! Would have thought there would be a negative pressure spike at 50-60% chord....

Naca 2415 Cp plot 1

What would the shape of the pressure plot be when distributed propulsion is added to the flap? See plot below.

Naca 2415 Cp plot with blown flap

  • 1
    $\begingroup$ Your airfoil's upper side seems to have separated flow from 40% downstream. The 50% flap stalls the airfoil. $\endgroup$ Commented Mar 9, 2020 at 17:21
  • $\begingroup$ Why doesn't NASA just build that cute sporty with 2 large wing tip props and slats and flaps??? $\endgroup$ Commented Mar 10, 2020 at 12:58
  • $\begingroup$ @RobertDiGiovanni: Because they first need FFA approval. With empty batteries it will be incapable of 61 kts minimum speed. If you just want the cute plane (without tip props), get the original from Tecnam. $\endgroup$ Commented Mar 11, 2020 at 17:06
  • $\begingroup$ @Peter Kampf well, it is an electric design, but may be capable of lower glide speed with slats and flaps. Also, I would look for ways to avoid empty battery scenario, such as a backup generator. I agree 2 large props would produce a lot of drag...unless they were feathered! Good link to the original Tecnam. $\endgroup$ Commented Mar 11, 2020 at 20:15
  • $\begingroup$ Why is an empty battery an issue? Isn't that just like running out of fuel? Wouldn't a reserve quantity like in in gas engines just solve the problem? $\endgroup$
    – Fred
    Commented Mar 11, 2020 at 20:37

1 Answer 1


The X-57 should show the highest gain in lift from the props. They will wrap the full wing in accelerated flow. XFLR is not capable of capturing this effect, since the pressure distribution should be similar to the case without props, only the reference dynamic pressure is higher than that of the prop-less wing. Of course, swirl effects will add a spanwise pressure variation that is missing without the props.

Next best is the pusher arrangement of Kitty Hawk. Here the props can swivel, so maximum lift is less of a concern. The prop arrangement has been chosen to minimize noise from the prop blades cutting through the wing wake; that should explain the considerable distance between props and wing. At that distance the suction of the prop will affect the flow over the wing much less that what the X-57 should be capable of. In a pressure plot the main effect should be a delayed flow separation: The wing will tolerate a slightly higher angle of attack.

Worst of all is Lilium: Here the inefficient, small-diameter props sit in ducts which all have their own boundary layer and losses. Next, any accelerated flow only affects the upper side of the wing, so half of the effect will be lost. This arrangement, however, will make the plane sound much louder and powerful than Kitty Hawk's. Judge for yourself whether that is a worthwhile achievement.

  • 1
    $\begingroup$ Lilium is showing a much higher power requirement to lift and cruise 5 people. Helicopter data could be in this study (which features one very large (more efficient) rotor). $\endgroup$ Commented Mar 9, 2020 at 18:50
  • $\begingroup$ What is "half the effect will be lost"? Is it the prop wash hitting the flaps, generating a reaction force upwards? $\endgroup$
    – Fred
    Commented Mar 11, 2020 at 16:22
  • 1
    $\begingroup$ @Fred: The accelerated flow only works on the upper wing of Lilium. Compare with the two other concepts where it affects both sides. $\endgroup$ Commented Mar 11, 2020 at 17:04
  • $\begingroup$ I guess I don't understand how accelerated flow on the bottom of a wing creates upwards lift except for the reaction force of the upward swirl hitting the bottom of the wing or air hitting the flaps and deflecting downwards. I thought it is mostly the camber of the upper surface that creates lift, no? $\endgroup$
    – Fred
    Commented Mar 11, 2020 at 18:12
  • 1
    $\begingroup$ @Fred: Lift can be explained in several ways. It is the reaction force to the downward acceleration of air flowing over the wing, which in turn needs a pressure difference to work. When more air is involved, the forces grow. If only one side has that benefit, only one side (or half the possible airflow) benefits. $\endgroup$ Commented Mar 11, 2020 at 20:07

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