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This plane's ground speed looks slower than a person's walking speed, maybe 10km/h!? It can not be possible that an airflow of 10km/h around wings keep plane in the air.

What is the key for such a slow flight?

a) head wind

b) blast from propeller make additional lift at wings

or

c) maybe propeller upward thrust component help to lift plane in the air

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    $\begingroup$ "This plane ground speed looks like slower than man walk, maybe 10km/h!? It can not be possible that an airflow of 10km/h around wings keep plane in the air." An aircraft's ground speed can tell nothing about the speed of the airflow around the wings. $\endgroup$
    – DeepSpace
    Aug 30, 2020 at 14:21
  • $\begingroup$ @DeepSpace,Yes I know that is reason why I ask where is the trick! From my intuition airflow speed at wings must be at least 50km/h to keep 350kg in the air $\endgroup$
    – member2017
    Aug 30, 2020 at 14:25
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    $\begingroup$ A headwind nearly sufficient to fly the plane on its own, plus prop wash. For a Dash-7 sized cargo liner that can do that, see an Antonov-2. $\endgroup$ Aug 30, 2020 at 20:42
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    $\begingroup$ Yes to everything but there is also an additional factor you missed. You are right, the lift generated as such slow speeds in such conditions is small. Therefore in order to take off the weight of the entire plane has to be smaller than that! That is the reason for removing significant parts of the fuselage and modifying everything on that plane to get it as light as possible. $\endgroup$
    – slebetman
    Aug 31, 2020 at 13:44

1 Answer 1

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Yes, yes and yes:

  1. At that event the winds were 12-15kts.
  2. There is some lift increase from propeller wash impinging on the inboard wing and flap. But most airplanes benefit from this (my own PL-2 has flaps that go all the way under the fuselage and the sink rate difference power on and power on is a lot).
  3. Propeller thrust is contributing to the total lift as soon as the thrust line is tilted above horizontal, as a vertical component is added to the thrust vector. With a slatted wing allowing the wing to operate at up to 25 degrees AOA with the associated deck angle, engine power is starting to contribute a significant part of holding the plane up (if a 300lb thrust force is inclined 25 deg, the vertical thrust or "lift" component is about 130lbs), which is why it plummets the instant the power is removed (to better imagine the effect, just imagine you cankeep pitching the plane's deck angle to 90 degrees, where ALL of the lifting force is coming from the prop).

So, an ultralight like that should have no problem slowing to 21 or 22 kt in that configuration (a Zenith 701 can maintain control down to about that speed although its published stall speed is 30 mph or 26 kts), dropping another knot or two as it gets into ground effect, add in the head wind that was present, and you're down to a ground speed of a person running.

The headwind is kind of "cheating" in terms of absolute performance, but keep in mind that it's a competition between aircraft in those conditions on that day, so the headwind isn't a factor in the relative performance of the competitors, except to the extent it's varying at the time and small variations in wind speed are probably enough to give one plane a victory over others with more or less identical STOL performance.

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  • $\begingroup$ Still looks imposible to me ,because to have strong propeller wash over wings then plane must accelerate forward,but plane is slow as human walk!How much thrust it has before landing 0:16sec at video?Is max thrust force greater than plane weight? $\endgroup$
    – member2017
    Aug 30, 2020 at 13:35
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    $\begingroup$ 22kts stall speed,this plane can fly 22kts without engine and head wind??or this include propeller wash? $\endgroup$
    – member2017
    Aug 30, 2020 at 14:18
  • $\begingroup$ No power off it's minimum flying speed will be quite a lot higher, on the order of 26-27 kts, where you will run out of tail power to hold the nose up. Slatted wings on STOL airplanes can't normally be brought to a conventional stall break, but will rather just develop higher and higher sink rates. I know a guy who was able to tease an actual stall break from his Pegazair which has slats, through some extreme maneuvers, and the reaction was quite violent. Anyway, when you do STOL approaches like that you are totally dependent on power and if the engine quits you are coming down hard. $\endgroup$
    – John K
    Aug 30, 2020 at 15:47
  • $\begingroup$ Thrust line is not tilted so much above horizontal,so it seems that most of lift comes from propeller wash over wings. Maybe they have organize STOL-POWER OFF COMPETITON! $\endgroup$
    – member2017
    Aug 30, 2020 at 15:59
  • $\begingroup$ @member2017 it's travelling over the ground at a walking speed but actually flying about twice as fast through the air. The winds were 12-15kt as I said. If you were in a paramotor on that sort of day, which takes off at about 12 kt, you could hover in place if you held it just above the stall speed. $\endgroup$
    – John K
    Aug 30, 2020 at 16:00

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