11
$\begingroup$

Ekranoplans or Ground Effect Vehicles (GEV) fly low enough to the ground that their wings stay in the ground effect. This increases lift and efficiency.

But GEV fly in the lower atmosphere (literally ground level) which greatly increases drag compared to flying in the stratosphere.

Is it possible that a GEV designed using contemporary composite materials and high bypass engines could be more efficient per cargo weight-mile (or passenger mile) than a tradition airliner? Or are GEV inherently less efficient due to the altitude they fly at?

This question is specific to fuel efficiency over distance. Feasibility, safety, navigation, time, and cost of airframe are not part of the equation.

$\endgroup$
  • $\begingroup$ This nasa study suggests GEV could be more efficient but it’s light on details: nari.arc.nasa.gov/sites/default/files/attachments/… $\endgroup$ – RoboKaren Nov 20 '19 at 15:41
  • $\begingroup$ .. but it’s the reason I ask the question because at least my received wisdom is that GEV were inherently inefficient except for very short hops such as crossing the Caspian Sea. $\endgroup$ – RoboKaren Nov 20 '19 at 15:42
  • 1
    $\begingroup$ I always thought that the extremely low aspect ratio negated most of the ground effect benefit from an efficiency perspective. $\endgroup$ – John K Nov 20 '19 at 17:20
  • 3
    $\begingroup$ @JohnK, no. Much of the ground effect per se manifests itself in reduction of induced drag - which is exactly the reason to have long wingspan on 'normal' airplanes. GEV simply don't need long wingspan: they achieve low induced drag by other means. Furthermore, ground effects generally scale with wing chord, so GEV benefit from wider chord. $\endgroup$ – Zeus Nov 21 '19 at 1:02
  • $\begingroup$ You need to specify the trip length, as it takes time to climb to altitude. A GEV would be more efficient for trips where there isn't time to reach altitude. $\endgroup$ – Robin Bennett Nov 21 '19 at 9:35
2
$\begingroup$

As some of the comments above (which are really on the money) suggest it's a complicated question to answer because it is a case of "horses for courses" and that is even if we focus purely on straight-forward fuel efficiency. But given the programs that have been attempted (or are still ongoing) and also various studies it seems that ekranoplans seem to work best in two very distinct roles. Either as small, short range puddle-jumpers or as very large trans-oceanic monsters. For the later see also this which may well come to pass.

I guess at some level such utterly different roles is a problem for ekranoplan development. But, yes, in principle, there would certainly appear to be certain, very different niches, for which the ekranoplan is extremely competitive in terms of fuel consumption. Having said that the second niche is arguably also competing against conventional surface sea-freight which of course complicates matters but offers interesting possiblities. Aircraft like the Boeing Pelican (which didn't get past concept stage) was designed not for standard air-cargo pallets but for ISO containers. Now that capability is a real game changer as long as the cost per kg/km fuel economy is as good or better than a conventional air-freighter which I suspect is very doable. At least in principle.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ The answer doesn't address the core question, "are GEV inherently less efficient due to the altitude they fly at?" If any of the links you've provided address that, it is better to include the essential parts of the answer here. - From Review $\endgroup$ – ymb1 Feb 10 at 4:44
2
$\begingroup$

Fuel efficiency of a given type can vary considerably over different distances. The conditions necessary for optimal efficiency also vary widely between types. The jet is best at high speeds and high altitudes, so acceleration/climb and deceleration/descent are important considerations on short journeys. The piston engine is more efficient at low altitudes, but its propellers make for slower airspeeds and this makes long journeys unattractive.

A jet-powered GEV is definitely less efficient than a high-altitude jet, but a piston-engined GEV is in its element and can also make use of ground effect to increase lift and improve aerodynamic efficiency. Theoretical arguments one way or the other are one thing and opinions differ, but whether or not it can prove more efficient for such routes than a conventional airplane must wait for practical demonstration.

But the ekranoplan suffers a curious limitation in that its maximum speed over water turns out to depend on its wing chord, or fore-aft measurement. Wave interference effects mean that the longer the chord, the faster it can go. Above its maximum interference-free speed, its efficiency is among the performance criteria which fall sharply. Thus, the very large but relatively short-range ekranoplan is potentially the most efficient. This was the niche at which the Caspian Sea monster was aimed.

Of course, finding an obstruction-free route for any distance over land is next to impossible.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.