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Isn't ground effect a big factor in STOL operations? Don't low wings produce greater ground effect than high wings?

Or is it a practical concern, that most real-world STOL operations are in unimproved or slightly-improved locations, and high wings are more likely to clear "runway" obstacles?

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  • $\begingroup$ Most STOL aircraft are small, and most small aircraft are high-wing. $\endgroup$ – Sean Oct 6 at 3:28
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You're correct that the low wings produce more ground effect compared to their high winged counterparts. However, during takeoff and landing, it can result in undesirable handling qualities- the aircraft can 'float' the runway; on contrary, the high wing design is more stable. There are other reasons for STOL aircrafts to have high wing designs, most of which has to do with the location of their operation, as you noted:

  • Most of them are operated from austere airfields. The high wing location gives some protection from the debris (and obstacles).

  • For the same reason, engines mounted on the high wings have better protection compared to the low wing mounted ones from FOD ingestion.

  • Most STOL aircraft use large TE flaps. In case a low wing is adopted (and also if propeller is used, like Twin Otter, for example), the required ground clearance will require long and consequently heavy landing gear. This can be avoided in case of high wing design.

  • The visibility is better compared to low and mid wings.

Historically, STOL aircraft have been high wing. Also, some of the STOL aircraft are (military) cargo aircraft- the high wing means there is no wing spar across the cargo compartment; plus, the low fuselage means easier loading/unloading of cargo.

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    $\begingroup$ Long landing gear on an unimproved field are also at a greater risk of being damaged, thus would have to be made even heavier to take the additional punishment. $\endgroup$ – FreeMan Jul 5 '16 at 12:50
  • $\begingroup$ Also, ground effect would not offer any benefit anyway because in most short-field operations the aircraft needs to pass over obstacles that are higher than where the ground effect has any effect. $\endgroup$ – Jan Hudec Jul 5 '16 at 18:37
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I suspect part of it is due to historical design: High-wing STOL worked in the past so why change it now?

That said, there are numerous reasons why high-wings are good for STOL:

  • Better visibility, no wings blocking the view below. Helps the pilot see/avoid obstacles on the ground.
  • As you say, high wings also improve clearance over rough ground. This is particularly important on grass strips.
  • The aircraft is more aerodynamically stable. This can be of assistance when operating in difficult conditions and can, in some circumstances, mean that the aircraft is more resistant to crosswinds than it would be for a low-wing design.
  • You mention the ground effect: This is not necessarily a good thing. High-wing designs are not as susceptible to the ground effect and therefore may have better landing performance (less float).

-edit- Oh, and one other thing (that's more of an opinion, though): High-wing aircraft are, in my experience, easier from a practical perspective. STOL aircraft often operate in difficult environments and often carry cargo. With high wings it's easier to get stuff into/out of the cabin.

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  • $\begingroup$ The third point is wrong. Aircraft are not stable in roll independent of whether high or low wing and while high wing provides stronger sideslip-roll coupling, it can be adjusted as necessary by dihedral or anhedral. $\endgroup$ – Jan Hudec Feb 23 at 21:30
  • $\begingroup$ I said "more stable" rather than just "stable". $\endgroup$ – os1 Feb 25 at 9:25
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High wing airplanes have their center of lift above the lateral center of rotation. This means the majority of the mass of the aircraft is below the lifting surface. This adds to longitudinal as well as lateral stability--especially at low speeds. Sort of a pendulum effect. Ground effect happens at less than one wingspan. In small aircraft that's 25 to 40 feet. This would also be reduced at the higher angle of climb that STOL aircraft exhibit. The only way you could improve the stability of a low winged aircraft in that regard, one would have to drastically increase the wing's dihedral. This would raise the stall speed slightly, reducing its lifting capacity. Even though this effect would be very slight, the object of a STOL is to perform short takeoffs and landings.

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  • $\begingroup$ The vertical position of the centre of lift is totally irrelevant to the stability. Suspended object is stable, because the suspension resists lateral forces and therefore creates rolling moment when the object is rotated. But lift remains parallel to the CoG–CoL line and does not create any moment when the aircraft is banked, so which is higher does not matter. Also while high wing alone is indeed more stable (for aerodynamic reasons; there is an explanation somewhere on this site), stability can be adjusted by other design features, so it is not factor for choosing high wing. $\endgroup$ – Jan Hudec Jan 3 '16 at 19:09
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I believe that the high wing with extended slotted flaps divides MORE prop blast UNDER the wing creating "blown flap" downwash & a slightly less turbulated flow over the top of the wing (from prop axis being below the wing). I think this would keep a laminar flow on MORE of the inboard top TE section of the wing.. benefiting in MORE IMMEDIATE CONTROL of lift with power rather than lag of airspeed transition/top-blown. I believe that the low wing with prop blast above the wing also creates plenty of lift effect on its own, but the underside of the flaps will receive less lift from lesser percentage of prop blast. In the end the high wing is out of the way&offers unique handling & control benefits, while the low wing is less capable of "dumping" lift immediately when throttle is cut in ground effect. Low wings may be beneficial with their intrinsic "float" deeper into full stall on SOFT fields where rollout distance is not a concern when landing, & possibly would be sooner to break ground on takeoff rollout, reducing ground drag sooner. It really matters in the end what the primary purpose of the aircraft is to be.. a "burrow" that will get you in&out of anywhere, a Rolls Royce "highway traveler" or the addition of a STOL kit to an aircraft to ENHANCE (either for effect or safety. or both) its performance.

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    $\begingroup$ If you could rewrite the answer (use the edit button), remove the CAPS, and break it into 2 paragraphs or more, it would be a lot more coherent and readable. $\endgroup$ – ymb1 Mar 8 at 4:18
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Looking @ some nose propped planes, the high wing ones seem to direct 2/3 or more prop blast into the high pressure area under the wing, forcing even higher air volume & density under the wing when in ground effect, giving more control over lift. Low wing nose propped planes seem to direct 2/3 of prop blast OVER the wing, & the 1/3 that would pass under MAY (?) be "deflected" off the ground laterally some & "split" over the wing, reducing the "blown" flap effect when at lowest points of ground effect prior to touchdown. All said, high wings with flaps make for a good "solid-short arrival" & low wings with flaps make for some floaty- squeaky landings on that denser "air cushion"!

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    $\begingroup$ Rather than create a new answer, it would be better to revise your existing answer. $\endgroup$ – selectstriker2 Mar 8 at 14:30
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High wings allow for BIG balloon tundra tires to be supported by conventional gear (taildragger) readily with minimum modification, tricycle MAIN gear may allow the same, but NOSE gear tire size may be limited by origional equipment or available modifications

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