The title is my question. Does the ground effect change as the temperature of the ground changes? In cold weather, a more cushion effect can be expected, and in hot weather vice versa?
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2 Answers
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No. There is nothing in the physics of ground effect that depends on temperature.
On a hot day, you will have higher density altitude. Your true approach speed will be greater (for the same equivalent airspeed). You may perceive a difference in the way the landing feels because you are moving faster across the ground, but there is no difference in ground effect.
Also, on a hot day, thermals can form over dark areas of ground. This convective activity can cause an aircraft on final to float.
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I will assume:
- You are asking about the temperature of the air near the ground, not the temperature of the ground itself.
- The ground effect is the pressure caused by compression of air between the wing and the ground.
- The ambient pressure does not depend on the temperature and the ground effect is isentropic.
The temperature of the air near the ground has a quite small relationship to the ground effect. The ground effect can be simplistically described with the isentropic compressibility, which is inversely related to the square of the speed of sound. As temperature increases, the speed of sound increases slightly. This would cause an increase in ground effect. The difference would be too small to be relevant to pilots or aircraft designers.
If you accounted for turbulence and phase changes (such as water condensing) then you may see other differences due to deviation from the ideal gas law. These may be slightly less tiny near the melting point of water.
answered Sep 22 at 19:08
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$\begingroup$ So much wrong here. Ground effect has nothing to do with compressibility -- isentropic or otherwise. Ground effect can be observed with potential flow assumptions (inviscid, incompressible). Potential flow of an aircraft has boundary conditions that die off far away. The ground plane inserts a plane of symmetry. This can be replaced by placing a mirror of the flow across the symmetry plane. So, if the wing was a horseshoe vortex, in ground effect, it would be the same as a mirror pair of horseshoe vortices in free air. Temperature does not appear in potential flow. $\endgroup$ Sep 22 at 23:34
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$\begingroup$ Wind tunnels do have climate control. Many closed circuit wind tunnels include coolers to keep the flows' temperature from constantly rising during long runs. Some wind tunnels are sealed and pressurized. This way, they run at higher than ambient pressure, helping them to match a desired flight Reynolds number. The NTF - National Transonic Facility at NASA Langley is a pressure tunnel that injects liquid Nitrogen to cool the flow (and keep it cold). It does this to try to simultaneously match Reynolds and Mach number. $\endgroup$ Sep 22 at 23:39
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$\begingroup$ Some wind tunnel models are heated - and observed with an IR camera. The differences in surface temperature can reveal where the flow transitions from laminar to turbulent. These instruments are only sensitive for a narrow range of temperatures - this kind of experiment will only work with a tunnel operating over a certain range of temps. $\endgroup$ Sep 22 at 23:40
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$\begingroup$ 1. Planes cannot fly in inviscid fluids. 2. Just because something can be observed under some assumptions does not mean the assumptions always apply when the thing is observed. 3. You may be right about wind tunnels - I look forward to more answers to the other question. $\endgroup$ Sep 23 at 1:41
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