# Could an airliner be as efficient lower and slower?

Could an airliner ever be as efficient at 15 or so meters using ground effect as it is at 50,000 km even if it has to go slower and be shaped differently?

• I fail to understand if you ask for lower speeds (see Why are jet aircraft never designed with a slower cruise speed? for lower altitude (see Can ekranoplans (GEV) be more efficient than traditional airliners?) – Manu H Feb 14 '20 at 8:11
• I guess there's a typo at the higher mentioned altitude, 50,000km is kinda high for an airliner, that's beyond geosynchronous orbit 😉 – Jpe61 Feb 14 '20 at 9:27
• @Jpe61 -- My understanding is that the altitude at which orbital flight is possible varies with velocity. "Geosynchronous" simply means that the velocity required to maintain orbit exactly coincides with the velocity required to maintain a fixed position over the earth's surface. This velocity tends to be rather high. If we dropped an airliner moving at, say, 500 mph into a an elevation of 50,000 km over the earth's surface, it would certainly not be in any sort of orbit. Rather, it would plummet like a rock. So, there is no reason to assume that "50,000 km" is necessarily a typo. – quiet flyer Feb 14 '20 at 18:00
• Pretty obvious to me he means 50000 ft, or maybe, and a bit more mistakenly, metres. – John K Feb 14 '20 at 18:38
• @quietflyer Regardless of the speed of the... ummm... spaceliner(?), if it was at an altitude of 50,000 km it would be above geosynchronous orbit. It might not be in one, that's for sure. Regarding the speed you mentioned, would it be groundspeed, it certainly can't be airspeed at that altitude, so we can't reference that 🤔 At a groundspeed of 500 mph the spaceliner would be distancing itself from earth. And furthermore, since the question does mention "airliners" and "50,000km", surely there is a typo somewhere. – Jpe61 Feb 14 '20 at 18:39

It's important to note that an airliner cruising at 50000 ft (I assume that's what you meant, and that's actually business aircraft territory; airliners are generally below the low 40s). Is going pretty slow from an indicated airspeed perspective. At 50000 ft an airplane is going roughly twice its indicated airspeed (it's actual speed through 3D space is twice the dynamic air pressure acting on it - if you stick your hand out, you'll feel the force of a 200 kts acting on your hand, even though you're actually going 400 kt). So, such an airplane is actually optimized to only fly at 240 kts anyway, if it's cruising at a true airspeed of 480 kts.

What it means is, however you want to optimize for sea level flight, the higher you go the faster you can go in reality for a given fuel burn, whatever the configuration, burning far less fuel per unit of distance in the process.

It depend whether the airliner is propeller driven or jet powered.

Propellers are most efficient at low altitudes, but air resistance decreases with pressure as you go higher, enabling the same thrust to push the plane faster. Propellers are at their most economical (very roughly) around 10-20,000 ft.

Jets are most efficient at moderately high altitudes, where the air is considerably less dense so the same thrust makes the plane go faster still. But fly too high and the air gets so thin the plane begins to struggle. So jet airliners are at their most economical (equally roughly) around 30-40,000 ft.

Could an airliner ever be as efficient at 15 or so meters using ground effect as it is at 50,000 km even if it has to go slower and be shaped differently?

Yes it can. The most efficient shape for an airliner is no shape at all, so that the passengers have to get out and walk. (On 15-meter-tall stilts of course.)

Alternatively, consider attaching electric motors to the wheels, so that the airliner could taxi to its destination along established roadways. 15-meter-tall landing gear could be used to maintain the prescribed altitude. "Stub" wings could be retained, so that the aircraft could be claimed to be "using ground effect" to some infinitesmal degree. This mode of transportation would surely be more efficient, in terms of energy expended per unit mass transported per mile, than climbing to 50,000 km, at least for any total distance equal to less than one half of a complete orbit of the planet.

Unless, of course, the airliner is assumed to be magically transported to 50,000 km before we begin the calculations to establish efficiency. If this is the case, the airliner can follow a ballistic trajectory toward its destination and then, upon entering the atmosphere, transition to gliding flight. In this particular case, flying between the same two geographic points while maintaining an altitude of 15 meters AGL would likely represent a greater expenditure of energy than following the ballistic/ gliding profile described above.

• 🤭 your chance for "peer pressure" -badge is nearing. – Jpe61 Feb 14 '20 at 17:03
• Cool! Just remember, its not kosher to change a question in a way that invalidates an existing answer. Anyway-- this whole answer is meant to be a gentle suggestion that the original question was insufficiently constrained, and also probably contained a typo, which had not yet been fixed at the time the answer was posted. – quiet flyer Feb 14 '20 at 17:25
• To glide, there must be air. – user253751 Feb 14 '20 at 17:35
• I upvoted you... – Muze Feb 14 '20 at 20:47
• How should it read? I really like your answer and if I can reask this as a new question and leave this one as is then you can use your answer there? – Muze Feb 14 '20 at 20:48