4
$\begingroup$

People talk about wing tip vortices and say things like the ground acts as an aerodynamic mirror. What (if any) difference would there be for a wing flying underneath a 2D plane? As in the picture below:

Ground effect

The uppermost plane is flying at an altitude well outside ground effect. The middle one is flying in ground effect, and the bottom one in the hypothetical situation with a plane on top of the plane (pun intended).

Source of the picture, edited to suit the question: http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml

$\endgroup$
4
  • $\begingroup$ You don't really need an infinite plane, just one that is large compared to your wingspan. $\endgroup$ – Sanchises Sep 30 '20 at 17:38
  • $\begingroup$ @Sanchises - I think the OP meant continuous instead of infinite. One long enough in the longitudinal axis to be present throughout the entire flight. I doubt there would be a ground effect advantage from the reduction of wing-tip vortices. But, would there be any other type of advantage like changes in pressure above the wing? And, how close would the aircraft have to fly in order to take advantage of this benefit? $\endgroup$ – Dean F. Sep 30 '20 at 17:52
  • $\begingroup$ Correct. Size/dimensions not exactly relevant. I'm just talking above vs below or ground vs ceiling. $\endgroup$ – Turkeyphant Sep 30 '20 at 18:37
  • 2
    $\begingroup$ I added a picture diplaying the situation as I imagine the question means it. If I am mistaken, please remove my edit to the question. $\endgroup$ – Jpe61 Sep 30 '20 at 19:58
4
$\begingroup$

In computational fluid dynamics (CFD), ground effect can be simulated by flying two airplanes / wings in mirror formation (the lower one inverted).

That mirrored wing is only a mathematical trick. By distorting the flowfield in an identical, albeit inverted, way, the effects of the lower wing cancel out those of the upper wing in the plane of symmetry, and vice versa. This way, the flowfield on both sides looks exactly like one where the plane of symmetry cannot be crossed by the flow, just as the ground cannot inhale or expel air.

Now, with the more precise description: Yes, there is also a ground effect for the plane flying below a hypothetical ceiling. Of course, it would help if the vertical tail would not stick up, for minimum distance and maximum effect. Just as the ground stops the downward movement of the wake, a ceiling will prevent air from filling the space left by the downward movement of the wake, so the wake will stick to the ceiling. Call it the ceiling effect, if you will. In both cases the induced drag drops and the center of pressure moves a bit backwards. I wager to predict that the drag reduction over distance relative to wing chord is the same as in "regular" ground effect.

Proof: Fly a model helicopter indoors and get it up to the ceiling. You will notice that it will get stuck there and needs much less power to stay airborne. The rotor blades are like wings, and due to the rotor position on top of the craft, the ground effect is very noticeable. In order to get the helicopter unstuck you must reduce power a lot, such that the helicopter will drop like a stone.

In CFD, this can readily be proven by using the trick described above, only now with both wings / airplanes flying inverted to their original orientation for the ground effect calculation.

$\endgroup$
7
  • 2
    $\begingroup$ The way I read the question, he's talking about flying in free(ish) air under a continuous solid ceiling. I have to think there would be some effect, I'm just not sure what that would be... $\endgroup$ – Zeiss Ikon Sep 30 '20 at 17:54
  • $\begingroup$ Yes as above, I'm talking about a 2D plane above the wing not below it. That's why I said plane instead of solid. $\endgroup$ – Turkeyphant Sep 30 '20 at 18:36
  • 1
    $\begingroup$ This is an interesting thing to explore. If lift is the product of moving a package of air down, and a large part of that package is above the wing (I read somewhere long ago that the displaced air extends about half a span above the wing), you might think that lift is drastically reduced once the wing gets close enough to a solid surface above and as it gets close, at some point it would stop climbing if held at a constant AOA. So, what happens when you try to fly a model, or hover a drone, right against the ceiling I wonder... $\endgroup$ – John K Sep 30 '20 at 19:55
  • $\begingroup$ @ZeissIkon. Thank you for explaining this. I thought immediately of the old CFD trick for ground effect simulation. But the same trick can of course be done with both airplanes inverted. The results are identical: Less drag and a backward shift of the center of pressure. $\endgroup$ – Peter Kämpf Sep 30 '20 at 20:47
  • 2
    $\begingroup$ I remember as a kid flying prop-copter toys -- pull a string and a propeller with a ring on the tips would fly off the handle. Flown indoors, and sufficiently near level, these would "stick" to the ceiling for up to several seconds. I was fascinated, and used to pester my mother to stick the thing to the ceiling again and again. Seems like loss of lift close to an overhead surface isn't a huge issue... $\endgroup$ – Zeiss Ikon Oct 1 '20 at 11:16
1
$\begingroup$

The situation with the ground above your aircraft is very similar to F1 cars front wing. And even if the airflow around it is much more complicated than the airflow around and airplane wing, ground effect seems to have a great impact.

The front wing is inarguably the most important part of a Formula 1 car, despite only providing between 20 and 30% of total downforce it is responsible for the quality of airflow reaching the rest of the car; generate too much downforce from the front wing and rear downforce will be reduced. Too much upwash in the front wing wake can stall the suspension members, or even cause the flow to separate at the front edge of the sidepod, which can adversely affect the rear wing and diffuser.

While the front wing profiles are shaped like a conventional wing section, a cambered upper and lower surface producing downforce, its force characteristic are dominated by ground effect, namely the Venturi effect in the convergent-divergent path under the wing, such that the air flowing on the lower surface of the wing is multiple times faster than would be the case is the wing were suspended high in the air.

https://www.f1technical.net/features/21667

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.