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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

nodapic
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member2017
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    "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. – DeepSpace Aug 30 '20 at 14:21
  • @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 – member2017 Aug 30 '20 at 14:25
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    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. – Harper - Reinstate Monica Aug 30 '20 at 20:42
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    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. – slebetman Aug 31 '20 at 13:44

1 Answers1

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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.

John K
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  • 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? – member2017 Aug 30 '20 at 13:35
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    22kts stall speed,this plane can fly 22kts without engine and head wind??or this include propeller wash? – member2017 Aug 30 '20 at 14:18
  • 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. – John K Aug 30 '20 at 15:47
  • 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! – member2017 Aug 30 '20 at 15:59
  • @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. – John K Aug 30 '20 at 16:00
  • @member2017 at 20-25 degrees the vertical component is not massive but it's not nothing either. If you go find a vector calculator online and assume the engine makes about 300 lbs of thrust you could work it out. Maybe 50-75 lbs of lift is coming from the engine (a guess). Not nothing. – John K Aug 30 '20 at 16:03
  • So stall speeds is allways reffer with power ON for every plane? – member2017 Aug 30 '20 at 16:13
  • Published stall speeds are normally power off, and based on a math formula. Power on stall speeds are normally a bit slower due to the effects I described. On a normal airplane, maybe 3kt; on a STOL ultralight, maybe a bit more, say 4 or 5 kt. A slatted STOL aircraft won't really have what you can call a stall break anyway; it just mushes down faster, but there will be some minimum controllable speed you could call the stall speed. It's always slower with power on. – John K Aug 30 '20 at 16:22
  • @John K: Even in a more normal airplane, e.g. a Piper Cherokee, you can use engine power (and nose-up attitude) to fly at a speed which would be a stall with power off. – jamesqf Aug 30 '20 at 18:31
  • Just to put the enormous effect of headwind into proportion: tailwind (due to wind shear) has crashed a lot of planes of all sizes. From Wikipedia: "Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries." Source: https://en.wikipedia.org/wiki/Wind_shear#Impact_on_passenger_aircraft – Klaws Sep 01 '20 at 08:45