Making a buck doesn’t have anything to do with the OP. The question was purely hypothetical, and I assumed your average bugsmasher.
Let’s assume a single-engine airplane of 160 horsepower that climbs at 400 feet per minute. It would take about 25 minutes to get to 10,000 feet. Fuel burn is nominally 9 gallons per hour, but varies with altitude. Cruise speed is 120 knots, and best rate of climb is 70 knots. Max throttle at 10,000 yields 75% power. Lets make the trip length 300 nautical miles. There is no wind, and standard conditions prevail. Course is over a flat sea, exceptionally large dry lakebed, or whatever; as long as it’s flat.
Given that wings are more efficient in ground-effect (requiring less power to maintain a given altitude and airspeed, but burning more fuel because of the low altitude) and that engines burn less fuel at higher altitudes (burning less fuel in cruise, but requiring more to reach the cruising altitude), which profile will burn less fuel: A climb to 10,000 at Vy, cruising there to the destination; or an in-ground-effect flight?
Okay, Johnny L.A., that’s a good question. And I confess that I’m not positive that a wing-in-ground-effect vehicle really is more fuel efficient, only that it is more efficient in theory. The Caspian Sea Monsters could have been super fuel hogs, for all I know. Of course, with military stuff, fuel is a minor consideration; I’m sure the Soviets were thinking more about flying under enemy radar than efficiency.
The general answer, I think, is that, higher altitudes are more efficient for longer hauls. Transatmospheric planes can theoritcally make interncontinental travel quite fast; I am sure they are much more efficent than an intercontinental flight of similar speed at a normal altitude, if that is possible. I mean, you can average Mach 3.3 without leaving the atmosphere, but I wouldn’t call the SR-71 exactly a fuel efficient plane. Of course, nobody really wants to pay to develop a “space plane”; one design I saw had four sets of engines, for different speeds: turbojets, ramjets, scramjets, and rockets. Yipe!
For short hauls at ordinary speeds, ground effect must be much more efficient. I mean, compare a hovercraft to a Cessna. That doesn’t really answer your question; but it’s the best I can do as a non-pilot, non-engineer. Perhaps there’s a fuel-usage-by-altitude website, at least for popular lightplanes. “How to get from Reno to Denver in a Cessna 172 with the least fuel possible” or something.
Thank you for the concern, manhattan! Now I’ll be watching out for you after the next NYC gathering, to make sure you stagger home safely.
I really wanted to go with Johnny L.A., but my fiancée was sick, and so I stayed home to take care of her. (She was feeling sick Saturday night, better Sunday morning, and then sick again Sunday at 10:00 AM. I wish she would make up her mind! :))