What height over a fixed plain (such as water) can a plane begin to take advantage of the ground effect or when air is compressed between the wing and ground near cruizing speed? Could an airliner type plane fly above the water for an added lift for range in an emergency where a plane may lack the power to climb?
Ground effect only comes into play within about 1 wingspan’s height above a surface and it is largely unnoticeable until approx 1/4 to 1/10 of a wingspan above the surface.
While it is true that ground effect greatly increases the lift to drag ratio of the wings, it does not reduce parasite drag nor does it account for the fact that it is more efficient to fly at higher altitudes where the atmosphere is much thinner for greater speeds per unit of fuel consumed. Flying low to the ground is dangerous as there is less margin for error if something goes wrong and would limits operations to day VFR conditions.
Transport airplanes have been test flown and certain flight profiles have already been determined to offer the greatest range. These occur at altitude and on reduced power settings.
Heavy sea transports which to use wing in ground effect for lift have been proposed in the past, but been found to be in efficient compared to existing jetliner designs. WIG craft have been used for short transport missions like island hopping, where that kind of a craft would be faster than a ship, and more efficient than a light airplane.
As noted in the answer linked in comments, the practical limit for ground effect is about half the wingspan over a relatively flat, impermeable surface (you'll get better effect from smooth water than from heavy swell, for instance). Flying in ground effect for either economy (reducing power needed to fly) or in an emergency (engine out prevented climbing, but allowed ground effect flight for considerable range) has been done many times, and there have been vehicles designed to fly almost entirely in ground effect (ekranoplan was the Russian name for these).
What height over a fixed plain can a plane begin to take advantage of the ground effect or when air is compressed between the wing and ground?
It varies, but, generally speaking, ground effect usually becomes noticeable somewhere around half the plane's wingspan above the ground. So, for a plane with a 50-foot wingspan, ground effect would only be viable below 25 feet.
Could an airliner type plane fly above the water for an added lift for range in an emergency?
Theoretically, yes, but it's unlikely to be very helpful.
- Flying that low is dangerous in and of itself. Even out over the water, there are boats, buoys, rogue waves, breaching whales, etc., etc., etc. Not to mention, you have to be very careful when you turn not to dip your wing in the water, and even a slight downdraft will kill you before you have a chance to react.
- Fly high enough, and the reduced air density (and subsequent reduced drag) can increase your range more than what ground effect would give you.
- Ground effect is highest over a smooth surface, so if there are waves, you won't get as much drag reduction as you might be expecting.
That all said, it is possible, and has been done several times.