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In this awesome video, shot at the "Mach Loop" in Wales, F-15Cs fly down the valley, then cross a ridge very low to the ground. While it's difficult to tell from the video just how low they really are, they seem to be fairly close. How much effect on flight characteristics do the pilots feel as the ground comes up to meet them, then falls away?

To clarify a couple of points from the comments, I'm interested in any effects on flight control that are caused by flying fairly low over that ridge and through the valley (to grandmother's house we go).

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    $\begingroup$ I'm not quite sure that "impact" is the correct choice of word there... $\endgroup$ – Greg Hewgill Mar 3 '16 at 1:13
  • $\begingroup$ Yeah, I thought about that, but not too hard... :) Feel free to modify it if/when you come up with something better. $\endgroup$ – FreeMan Mar 3 '16 at 1:27
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    $\begingroup$ When flying that high the ground has no effect on flight except on the pilot's mental process. $\endgroup$ – J Walters Mar 3 '16 at 1:57
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    $\begingroup$ Yup, that's a better word! Thanks @JonathanWalters. $\endgroup$ – FreeMan Mar 3 '16 at 2:08
  • $\begingroup$ @JonathanWalters: Try saying that again with a straight face after some ridge flying in a glider!!! $\endgroup$ – Peter Kämpf Mar 3 '16 at 23:29
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At the high speed of the F-15s in the video ground effect is almost negligible. Ground effect is about the restriction of downwash by the ground, and at high speed and air density downwash is rather small. Also, the duration of close proximity to the ground is too short for ground effect to fully develop.

The biggest effect will be air turbulence. Since air will neither flow into the ground nor out of it, the vertical speed of atmospheric turbulence is greatly reduced when flying close to the ground. Also, the scale of turbulence will get smaller with proximity to the ground. While gusts may rock your wings when flying high, at very low altitude (less than one wingspan) the turbulence will average out over the wing.

When flying over a ridge, any wind speed component perpendicular to the ridge line will mean updrafts on the windward side of the ridge and downdrafts on the leeward side. This will be the strongest effect in this particular case of F-15s flying over a ridge at Mach 0.7 to 0.9.

Now back to that ground effect meme which seems to find wide agreement here: Ground effect is a stationary process which is strongest at low speed, when the downwash behind the wing is strong. In the case shown here, the aircraft is only momentarily in close proximity to the ground. The pressure field around the aircraft will never reflect that proximity because it is gone before the aerodynamics could start to react. This is completely instationary aerodynamics, and air, having mass, will take some time to respond. However, it is never given this time.

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Low is of course a relative term but at a sufficiently low altitude a plane will physically experience ground effect. This generally occurs with in one wing spans length from the ground and manifests its self as reduced drag on the airframe, lower stalling speed and a general floating feeling.

In this specific case the planes appear to be flying low in a relatively mountainous area. The pilots may experience certain mountain related wind conditions while flying this low as well.

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  • $\begingroup$ Why would there be a floating feeling. If you are in straight and level flight in ground effect, the G forces and vestibular sensations will be roughly the same as in straight and level flight at altitude. You would not feel lighter. $\endgroup$ – user13148 Mar 3 '16 at 14:34
  • $\begingroup$ The floating feeling is in reference to the fact that it feels like the plane is floating on a cushion of air (not you floating within the airframe). Since the wings become more effective the plane has a tendency to not want to continue to decent as it enters ground effect. $\endgroup$ – Dave Mar 3 '16 at 14:53
  • $\begingroup$ Oh, upon entry into ground effect... I agree then. I thought you meant during flight in ground effect. $\endgroup$ – user13148 Mar 3 '16 at 15:20
  • $\begingroup$ Although, I have a hard time distinguishing the floating feeling that is due to my intentional rotation as I enter ground effect (so that I don't continue my decent into the ground) and any floating feeling that would have been due to ground effect on its own. Have you ever set yourself up in a stable decent and just observed by how much ground effect slows that decent with no input from you? I'm not sure it would be enough to impart a "feeling". $\endgroup$ – user13148 Mar 3 '16 at 15:35
  • $\begingroup$ Its worth it to do some low passes over the runway with a bit of power added (maybe 15% over idle) and try and hold the plane 10ft off the ground. You should be able to see the effect during that maneuver. Another things that shows it is an intentional ballon on your round out, the plane will seem to gently bounce up on the cushion of air. $\endgroup$ – Dave Mar 3 '16 at 15:37
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Ground effect is defined:

...the increased lift (force) and decreased aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface.

Source: Wikipedia

The effect is that the aerodynamic drag is decreased and subsequently lift is increased giving the ability for the aircraft to fly at a lower speed than normally it could. It is also a reason why overloaded planes can get off the ground but then stall as soon as they try to get some altitude.

Ground effect happens when the aircraft is at an altitude that is less than the wingspan of the aircraft. For an F-15, this means that they would need to be within 43' of the ground. Having watched the video its difficult to determine the exact altitude but I'd hazard to say that they are more than 50' off the turf.

But the other part of your question is:

How much effect on flight characteristics do the pilots feel as the ground comes up to meet them, then falls away?

It can be significant, especially in low-wing aircraft. Speaking from a GA perspective, the controls become lighter and the airplane floats or doesn't want to touch the ground. However the composition of the surface has a large effect on this. Soft earth or water has a lesser effect than a hard surface like pavement. Passing over peaks at 250+ mph is probably going to have little effect whereas coming in to land will be much more noticeable.

There are a few fighter pilots that hang around here that may be able to tell you what it feels like in a fly-by-wire aircraft, but in a cable and pulley system its definitely noticeable. So much so that it can make an otherwise un-flyable aircraft fly. There are aircraft designed to operate solely in ground effect and has been a factor in aircraft crashes.

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    $\begingroup$ I'd also say that (at least in my experience) it's hard to separate the psychological effects from the actual aerodynamic ones. Though my experience is mostly Piper Cherokee over dry lakes/sagebrush. $\endgroup$ – jamesqf Mar 3 '16 at 5:24
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    $\begingroup$ Ground effect is pretty non-existent at a mountain peak as it requires a flat surface or almost flat and occurs at roughly 0.5 to 1.5 times the wingspan with the wings level. But the questions is not about ground effect, but about any noticable effect on the 'flight-control' and other than wind effects there shouldn't be any $\endgroup$ – Chris V Mar 3 '16 at 8:26

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