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Is there a correlation between the lift coefficient and the proximity between the wing and the ground? I know that the lift coefficient is dependent on angle of attack; However, based on experiments that I have conducted, the results disproved the claim somehow, at the same angle of attack (0 degrees), the lift generated is higher when in ground effect than out of ground effect. Based on the experiment, given that wind velocity, wing span and density (according to the ISA) is the same, does this meant that ground effect (proximity when in ground effect) can bring a change in lift coefficient? Moreover, is there a specific correlation,is it linear or exponential?

It is really weird how I can find literally anything but the correlation between proximity and lift (or lift coefficient), there are essay on how ground effect affects forward swept wing aircraft, and yet there's not a single essay about proximity and lift coefficient :(

enter image description here

Graph from https://www.faatest.com/books/FLT/Chapter17/GroundEffect.htm

enter image description here My collected data

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  • $\begingroup$ According to Youtube, the lift is exponential to proximity when in ground effect $\endgroup$
    – JVAV_master
    Jul 17, 2021 at 12:29
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    $\begingroup$ What is the source of your graphs? ANA? Should include citation in question. $\endgroup$
    – quiet flyer
    Jul 17, 2021 at 13:42
  • $\begingroup$ I've learned, that a wing in ground effect behaves like having a higher aspect ratio. So i would guess this increase in cl-slope comes from higher virtual aspect ratio. But it is just a guess. $\endgroup$
    – Artur
    Jul 17, 2021 at 14:17
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    $\begingroup$ @JVAV_master what type of airfoil did you use? What do your results "disprove"? $\endgroup$
    – Robert DiGiovanni
    Jul 17, 2021 at 22:48
  • $\begingroup$ "the lift generated is higher when in ground effect than out of ground effect" is pretty much the definition of ground effect. $\endgroup$
    – Guy Inchbald
    Jul 18, 2021 at 12:34

3 Answers 3

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Setting the "experiment" aside for now, we can see that ground effect does indeed increase Lift. The first picture of the excellent reference Chapter 17 Ground Effect tells much. As one our most prominent writers often tells us, the vortex rolls off the entire trailing edge, not just at the wing tips.

Further reading compares ground effect (on induced drag) at 1 span length, 1/4 span length and 1/10th span length. 1/10th span length lowered induced drag by over 40%!

Reasons for this may lie beyond "coefficient of lift", which deals with airfoil type and Angle of Attack.

From the first picture, we can see downwash is reduced. From the graphs, stall (of the upper wing) occurs at lower AoA. This implies weakened circulation over the top of the wing, resulting in earlier flow separation. This would seem to make ground effect bad but ...

Greater lift is generated by increased air density underneath the wing. This is not unlike the phenomena of "water skiing" where the (much) greater density of water can hold up the entire weight of a person at less than 20 knots. Water skis are not know for their top lift.

There is also the thought that ground effect extends wingtip vortices away from the wingtip.

In summary, increased air density under the wing allows the plane to maintain lift at a lower AOA. Combined with reduced wing tip vortex drag, induced drag decreases as ground proximity increases.

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  • $\begingroup$ So is the increased air density under the wing a factor of the change in lift coefficient? Since I was thinking of using the ISA density at sea level to calculate the lift coefficient of the experiment. $\endgroup$
    – JVAV_master
    Jul 18, 2021 at 3:11
  • $\begingroup$ JVAV_master well, technically, density Rho is considered separately in the Lift Equation, but you're on the right track. $\endgroup$
    – Robert DiGiovanni
    Jul 18, 2021 at 3:23
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    $\begingroup$ Robert, at clearly subsonic speeds density changes very little, it is pressure that is increased on the lower side. $\endgroup$
    – Peter Kämpf
    Jul 18, 2021 at 3:41
  • $\begingroup$ From PV=nRT one can express higher pressure from higher density. Certainly not comparable in strength to a shockwave, but over the entire bottom of the wing. $\endgroup$
    – Robert DiGiovanni
    Jul 18, 2021 at 4:11
  • $\begingroup$ well with the experiment the wind speed is roughly around 3.4m/s, which is pretty slow but still works for a smaller model. $\endgroup$
    – JVAV_master
    Jul 18, 2021 at 13:39
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enter image description here

As mentioned in this excellent answer, ground effect of aeroplanes can be modelled as extra lift from the mirror image of the plane. The image above depicts the effect on the tail plane, main wing is identical.

This Prandtl Lifting Line Theory can be explained as extra lift from the mirror image of the wing. Torenbeek Appendix G.7 contains some expansion of the ground effect with flaps extended, with three diagrams on its effect on airfoil lift, below.

enter image description here

Graph b. shows the two lift slopes of a wing in and out of ground effect. For a given angle-of-attack $\alpha$, there is indeed a higher $C_L$ in ground effect.

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  • $\begingroup$ I submit there is higher Lift for a given AoA. Coefficient of lift is AoA (and airfoil type). Could your "mirror image" also be the lower wing of a biplane? It is said most of the lift is from the upper wing but ... $\endgroup$
    – Robert DiGiovanni
    Jul 18, 2021 at 11:19
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Ground effect is complicated. That is because it actually consists of at least two effects, subsumed under a single term:

  1. The wake cannot continue downwards past the wing, so it is deformed when the ground blocks it, and
  2. Air flow on the lower side of the wing is partially blocked.

The first effect depends on the height of the wing relative to wingspan and the second one on the height of the trailing edge relative to wing chord. Therefore, plotting all effects over one single parameter will only be valid for a narrow range of conditions, and omitting those makes the graph worthless.

Take angle of attack (AoA): At low AoA, the lift curve slope is higher than in free flow while at high angle of attack it becomes lower. Hing AoA means also high induced angles, so here the blocking effect of the wake by the ground becomes more pronounced and reduces induced effects. To say that "ground effect increases lift curve slope" is only valid for low AoA.

Next, AoA is based on the definition of the wing reference chord. It does not include the lift provided by camber, neither does it make clear how twist, changes in airfoils, flap deflections and washout change the local lift over span. Only when that information is included can ground effect be described properly.

Or look at how height is measured. Is that the height of the leading edge, or the quarter chord line or the trailing edge? Some authors even omit that distinction and seem to think it doesn't matter how height relative to the ground is defined. This also makes their graphs effectively worthless.

And then there are authors who present their results without labels or legends. Your question is testament to you being part of this distinct group. I am sorry to say that this makes your graph worthless as well.

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  • $\begingroup$ Can you give me adivse what to do now? Unfortunately I found out to late my diagram is not airfoil section of wing it is CDGE(chord-dominated ground effect) test for airfoil in wind tunnel , so all my question doesnt make sense, because airfoil dont have 3D effects. So can I use this "2D" pressure distribution for wing or it is completly different? aviation.stackexchange.com/questions/96059/… $\endgroup$
    – user628075
    Nov 23, 2022 at 19:01
  • $\begingroup$ @JurgenM Depends on the aspect ratio of the wing. The diagram explains well what happens away from the wingtip, and the higher the aspect ratio, the more of the wing is "away from the wingtip". I find the diagram very helpful and illustrative. It also shows how important the distance between trailing edge and ground is. Or is h a different parameter? You should explain better what the symbols and axes show, then the question does make sense and is useful. $\endgroup$
    – Peter Kämpf
    Nov 23, 2022 at 19:11
  • $\begingroup$ OK thanks for help. Yes h is changing and c is fixed. I will edit this in text. But I think is not correct ask members "if decrease in downwash increase eff. AoA.." if diagram show airfoil(2D), because downwash/eff. AoA is related to wing,3D effects..The goal of question is find out is wing really feel effective AoA in reality caused by downwash or this just mathematical manipulation how explain change in lift with numbers.. $\endgroup$
    – user628075
    Nov 23, 2022 at 19:51
  • $\begingroup$ @JurgenM You get high pressure below the wing if the gap between trailing edge and ground is small. Increasing AoA doesn't change this much beyond a few degrees. The high pressure forces more air to flow over the wing, so at low AoA the local flow angle at the nose if higher, but doesn't increase much with increasing AoA - the pressure will not rise linearly, after all. So at low AoA lift is higher but at high AoA it grows only slowly, compared to free flow. It's actually quite simple - just look at how the molecules will behave and forget the wake. $\endgroup$
    – Peter Kämpf
    Nov 23, 2022 at 22:41
  • $\begingroup$ "The diagram explains well what happens away from the wingtip, and the higher the aspect ratio, the more of the wing is "away from the wingtip" higher the aspect ratio of airfoil in wind tunnel=more show 2D airfoil and less show real 3d wing pressure distribution? $\endgroup$
    – user628075
    Nov 24, 2022 at 19:17

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