# What is the effect of uneven ground on ground effect vehicles?

I understand ground effect as far as Wikipedia explains it. There are two ways to ask my question;

• A ground effect vehicle flies along and encounters an obstacle (say a big wave) that's maybe about half the flying height. What will this feel to the craft? My assumption would be a short moment of more lift, which would feel bumpy at best and could damage the craft at worst.

• How choppy may the ground be? From the articles about Ekranoplans and similar concepts I read all these vehicles seemed to be designed for inner seas like the Caspian Sea or the Wattenmeer, not for the Pacific. This may be a coincidence or simply wrong. But can we quantify somewhat how seaworthy such a craft would be, or what were the most extreme conditions in which a ground effect vehicle has operated?

• When I try to answer the question myself, the one 'killer' condition I can come up with is when the ground rises faster than the craft can gain height (duh). Is this correct, are there others?

I've put this in a list so you can see where my misconceptions might be when addressing the underlying question: the ground in ground effect....

The ability of a ground effect (air)craft to able to fly with (small) perturbations in height from the ground is called height stability. In general, height stability is achieved if the derivative of the lift coefficient with increasing height is negative. In this case, a decrease in height (due to waves etc.) causes an increase in lift. As a result, the aircraft is returned to its original height due to the increased lift (with the opposite happening when height is increased).

As the ground effect increases as the height decreases, designing the ground effect aircraft with height stability is not very difficult and most of the ground effect aircraft designed and operated have height stability. A good example is the Lun Ekranoplan, which returned to its height after the height was altered by approximately 0.5m (due to missile launch).

"Lun Ekranoplan" by Soviet Navy - Stock Archives of Soviet Navy. Via Wikipedia.

Also, the Sea state limitations are a scale phenomenon — the larger the ground effect aircraft, rougher the seas it can handle. The Russians (Soviets) have the most experience in operating ground effect vehicles and according to them, sea state limit for a 500-tonne aircraft is around 2.5 m and the safe operating heights in terms of mean wave heights is given by,

$h = \frac{1.54 H_{\frac{1}{3}}}{2} + 0.1 c$

where,

$h$ is the vertical height measured from the mean wave height,

$c$ is the chord,

$H_{\frac{1}{3}}$ is the average of the 1/3 highest wave.

When this height exceeds the ground effect height the craft operates as an aircraft at a lower efficiency.

The main problems in case of operating a ground effect aircraft in choppy waters (as you noted, they are mostly operated in 'calm' seas) are twofold:

• Takeoff and landing are dangerous in rough seas, due the impact loads on the hull and wings. This is the most important limitation for operating in rough seas.

• Stability — The wing is already unstable, and near the ground in high lift region, it is more so; this is the reason most of the ground effect vehicles have proportionally larger tail surfaces compared to the conventional aircraft. A good example is the Soviet -90 'Orlyonko', which had tail surface nearly 50% of that of the wing.

"A-90 Orlyonok" by Kaboldy - Own work. Licensed under CC BY-SA 3.0 via Commons.

The main problem with waves (or in general, variations in height) is that the pitch stability changes with height. This results in the requirement of a large amount of control power in order to maintain trim — another reason for large tail control surfaces. Generally, the aircraft is stable in roll (the dipping wing generates more lift and the aircraft corrects itself).

Another thing to note is that the maneuverability of the ground effect aircraft is pretty limited if it's unable to fly in OGE (Out of Ground Effect). Soviet GE aircraft could fly in OGE for clearing obstacles.

References: Wing in Ground Effect Craft Review by Michael Halloran and Sean O'Meara

• for clarification: the height h is the lower safe height, the craft operates in a band between h and the ground effect height that's roughly the wingspan? (so when h > wingspan, it's an inefficient plane)? What does "the wing is already intsable" mean? What does OGE mean? This answer looks great, I just want to understand a few more details!
– mart
Nov 9 '15 at 6:56
• @mart When h>wingspan, the aircraft operates just like any other conventional aircraft. The advantages due to ground effect are lost. Compared to ground effect aircraft, they are inefficient. Wing alone configuration is unstable. That is the reason for having a horizontal stabilizer- to make the aircraft stable. OGE means Out of Ground Effect. Nov 10 '15 at 3:39