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An ekranoplan takes advantage of ground effect to make it "fly" very economically.

Is it possible for an eraknoplan to fly at the speed of sound, how does ground effect work at such high speeds?

How a sonic boom works

I'm thinking theoretically; assuming that the wind won't push it in any way, and that there is a huge open expanse for it to go through.

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    $\begingroup$ A car has broken the speed of sound, so it's certainly possible. Hitting a wave at supersonic speeds would be pretty catastrophic. $\endgroup$
    – GdD
    Commented Jul 25, 2022 at 15:04
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    $\begingroup$ Not to mention how few places there are with long enough stretches of calm waters to justify flying that fast. Monster lakes like Superior or Baikal tend to be almost as rough as large bays. Rivers aren't straight enough for transsonic speeds. And the ocean, of course, can have waves several meters high even in locally calm conditions, due to distant storms. $\endgroup$
    – Zeiss Ikon
    Commented Jul 25, 2022 at 15:46
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    $\begingroup$ The problem are reflected shockwaves which will upset longitudinal trim. $\endgroup$ Commented Jul 25, 2022 at 19:40
  • $\begingroup$ @PeterKämpf the frequency of the reflected shockwaves vs the damping of longitudinal trim would make a great study (with scale models first). $\endgroup$ Commented Jul 25, 2022 at 19:50
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    $\begingroup$ @PeterKämpf your comment is begging to be expounded into an answer 👍 $\endgroup$
    – Jpe61
    Commented Jul 26, 2022 at 19:15

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No, supersonic flow will still show lower drag in ground effect. However, wave drag will become so high that the performance advantage of flying in ground effect would become small. Just fly a little slower and performance will improve more than can be achieved with flying lower. Also, pressure loads on the structure will go up as height is reduced.

From "Transonic and supersonic ground effect aerodynamics" by G. Doig, published in Progress in Aerospace Sciences 69, August 2014, pages 1-28:

L/D of an RAE2822 in ground effect

it can be seen that approaching Mach 1 will severely reduce L/D. Unfortunately, this measurement does not extend into pure supersonic flow, but the trend should already be obvious. However, flying close to the ground still gives a performance advantage at equal Mach numbers.

NASA-TM-X-611 has now been declassified and shows pressure profiles along the length of both an F-104 and an F-105D in ground effect. From the linked paper:

The tests were primarily concerned with shockwave potential to damage structures and break windows, etc., but provide almost certainly the first comprehensive measurements of strong shock/ground interaction from a low-altitude supersonic overflight.

Peak pressure over height for an F-105D in ground effect

Even at moderate height, the peak pressures start to go up as height is reduced. This means that supersonic flight in ground effect requires a strong structure on the lower side! Altitudes were still outside of those which cause shocks to be reflected back to the airplane, and the test pilots did not report any control problems. However, when flying very close to the ground, shocks will be reflected by it and hit the lower side of the airplane, causing changes in pitch:

Schlieren pictures of projectiles in ground effect

Studies of NATO 5.56 mm projectiles at Mach 2.4 in ground effect show this. Since the projectiles were spinning, the changed aerodynamic forces only produced a lateral precession. Interestingly, when the reflected bow shock is reflected into the near wake at heights between 1 and 1.3 times diameter, it lowered drag by raising back pressure.

The projectile was discovered to experience a marked pitching moment, initially nose-downwards at the high end of type C (h/d = 0.75) reflection interactions, then more strongly nose-up with continued decreasing ground clearance, as the pressure distribution over the rear of the projectile was increasiingly distorted by the build up of high pressure behind the series of shock/ground interactions.

Wake and vortex ring structure behind a Mach 2.4 projectile for various ground clearances

Wake and vortex ring structure behind a Mach 2.4 projectile for various ground clearances. All figures and citations copied from the cited article. For the original sources, please refer to the references section in that paper.

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No, supersonic speed would not disable ground effect.

Ground effect is caused by air that is deflected downwards by the wing reflecting its energy back up to the bottom of the wing.

evidence that ground effect interacts with downwash is shown by the fact that ground effect lowers stall AoA.

Higher speeds only increase the energy of air being deflected downwards, and at supersonic speeds, a well defined shockwave also develops. Designers of the XB-70 took advantage of this by adding a highly Ekranoplane-like anhedral to the movable delta wingtips in order to "ride" the higher air pressure underneath at supersonic speeds, generating additional lift.

There would be considerable safety concerns attempting to develop this concept, as unknown obstacles would be almost impossible to avoid at supersonic speeds. Low level sonic boom would also be a concern.

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