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Some ground effect theory explain increase in lift with reduction in downwash,which cause increase in effective AoA.

One theory mention only increase in static pressure under the wing "cushion effect", because wing partialy block airlfow.

When comapre wing in ground effect(h/c=2.5%) and wing out of ground effect(OGE),both at geometric AoA = 6°, at this diagram we can see how biggiest change in Cp is done at bottom side of wing, change in Cp at top side of wing is very small,almost negligible.That mean bottom side is "key", side that make a difference.

It is well known when wing increase AoA, change in Cp is allways higher on top side,on bottom side change in Cp is small.

Is effective AoA increased at wing in ground effect, or if effective AoA is increased why change in Cp is not higher at top side compare to change in Cp at bottom side?

In other words, I dont see connection with "downwash/increased eff. AoA theory" with this diagram. But maybe I am wrong?

Unfortunately diagram show airfoil(wing with isolated end effects) test in wind tunnel, we can assume that same pressure distribution is at wing.

Cp= pressure coefficents, x/C= percent of chord lenght, h= height from ground to trailing edge, c= chord line length

h/c= smaller number,closer to ground

Diagram source enter image description here

enter image description here

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  • $\begingroup$ @Pilothead I added it. $\endgroup$
    – Jurgen M
    2 days ago
  • $\begingroup$ Interesting read, thanks. $\endgroup$
    – Pilothead
    2 days ago

3 Answers 3

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Unfortunately, the text or the diagram never explain what the height h is. I suspect it is the height of the trailing edge over the ground, measured in percent of the wing chord. Unfortunately, the lack of a clear definition devalues the data.

What is clear is that a smaller h translates into more blocking of the flow between wing and ground. Pressure is higher than in free flight, so more of the oncoming air is forced above the wing. This means that the flow around the leading edge at moderate angle of attack in ground effect looks like the flow at a higher angle of attack in free flight. This has four consequences:

  1. The leading edge experiences more suction, so drag is reduced.
  2. More suction at the upper front and more pressure at the bottom mean more lift at the same angle of attack compared to flight out of ground effect.
  3. The flow around the leading edge will separate earlier when the angle of attack is increased, so maximum lift will be reduced.
  4. When angle of attack is increased further, the pressure at the lower side will not grow linearly, having reached a high value already at low angle of attack.

In ground effect it is no longer possible to talk of the lift curve slope as if this is a constant. It is steep at low and shallow at high angle of attack. So yes, the effective angle of attack is increased in ground effect at low positive angles of attack but is reduced at higher angles.

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  • $\begingroup$ Why wouldnt pressure at lower side rise more at higher AoA, when wing increase AoA it "blocks" more air under the wing and I think reduce airflow speed more and more, going towards stagnation pressure? Where I am wrong? $\endgroup$
    – Jurgen M
    Nov 24 at 22:08
  • $\begingroup$ @PeterKampf 2. More suction (from higher local AoA) at leading edge means ... more thrust? All this talk about wing tip vorticies and downwash at the trailing edge. Airline pilots have disconnected flaps and lowered slats only to reduce (net) drag. If only people would read their own data, eh? That, combined with higher coefficient of lift (near the ground), seems to be "ground effect". $\endgroup$ 2 days ago
  • $\begingroup$ @JurgenM The maximum pressure possible is limited. More than stagnation pressure is physically impossible. And when pressure is already high (air is decelerated substantially) at moderate AoA, there is little left to increase pressure further when AoA is increased. $\endgroup$ 2 days ago
  • $\begingroup$ From the reference your assumption of h meaning is correct. $\endgroup$
    – Pilothead
    2 days ago
  • $\begingroup$ @Pilothead Why assumption? You can see at picture h is distance from ground to trailing edge. $\endgroup$
    – Jurgen M
    2 days ago
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That plot (source) shows that the closer to the ground the airfoil is, the bigger $C_p$ becomes i.e. the lower (due to Bernoulli) the speed on the bottom of the airfoil becomes.

And this is the main effect of the vicinity with the ground i.e. reducing the speed beneath the airfoil increasing the pressure ($C_p$ closer to 1). The rest is only "equal transit time" explanation.

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  • $\begingroup$ "The rest is only "equal transit time" explanation" That is what I want to say, all this story about 3D wing, downwash that change effective AoA is just mathematical manipulation of reality ,to explain with numbers change in lift. Do you agree? $\endgroup$
    – Jurgen M
    Nov 23 at 12:48
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    $\begingroup$ I agree but! by a practical point of view how do you use these results? You increase the slope of the $C_L$ vs. $\alpha$ plot or viceversa you decrease the $C_{D_i}$, just like if AoA were higher. $\endgroup$
    – sophit
    Nov 23 at 13:09
  • $\begingroup$ Problem is I didnt find anywhere in this site,that downwash/eff.AoA theory for 3D wing is only math concept. I didnt find text where they say that eff. AoA actualy dont exist in reality.. $\endgroup$
    – Jurgen M
    Nov 23 at 13:30
  • $\begingroup$ So you claim that wing in ground effect and out of ground of effect "feel" same AoA? So there is no eff.AoA in reality, only in math? $\endgroup$
    – Jurgen M
    Nov 23 at 14:20
  • $\begingroup$ I don't claim anything, I just say this $\endgroup$
    – sophit
    Nov 23 at 14:26
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Is effective AoA increased in ground effect?

Yes, because there is less downwash.

Why not change in Cp higher on top side?

Because there is less downwash.

Downwash in front of the wing reduces effective AoA. Downwash behind the wing helps drive upper wing circulation.

Airfoils in ground effect stall at a lower AoA, but have a higher coefficient of lift due to the "cushion" effect.

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  • $\begingroup$ @RobertDigivanni "Because there is less downwash." If downwash is less,then eff. AoA is higer,so change in Cp must be higher at top side.. end I think downwash dont exist in front of wing,it is just mathematical concept in theory. $\endgroup$
    – Jurgen M
    Nov 23 at 12:49
  • $\begingroup$ Well, go back to your picture (and wind tunnel studies). Just like the bow of a ship at sea turned sideways , the airfoil pushes air up and down. But I do agree with "top" Cp and "bottom" Cp being studied separately. Remember, it is the airflow, not the AoA, that determines Cp. AoA is just a measurement of pitch relative to the freestream. "Effective AoA" really talks about the actual airflow, which can vary locally. $\endgroup$ Nov 23 at 13:41
  • $\begingroup$ @Roberto "Effective AoA" really talks about the actual airflow, which can vary locally." Yes but it turn out that effective AoA exist only in math theory not in reality. ? $\endgroup$
    – Jurgen M
    Nov 23 at 13:50

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