I flew over to Lockhart (50R) today to visit a friend. This morning's temp was 31, but was probably 55 or so as I turned final. I lined up with the grass between the runway and parallel taxiway. My touchdown point was midfield, just past the midfield access to the runway, where the grass started again. Like most airports I've been to the grassy areas are slightly below the surface of the taxiways and runways, perhaps 2 inches near the pavement to maybe a foot in the center. It was a beautiful sunny day. 10 mph slight crosswind/headwind. The stall horn was just on, power close to idle, grass under me. As I approached the midfield taxiway wheels were about two feet above the grass to account for the slightly elevated (couple of inches) pavement. My intention was to be wheels down on the grass just on the other side of the taxiway.

Then an interesting thing happened. I expected the plane to continue the final foot of decent as I crossed the taxiway, but it didn't. Instead it was lifted back into the air what felt like ten feet, but was really probably about four. Naturally I missed my intended touchdown point by perhaps sixty or seventy feet. And this wasn't a sudden gust, it felt just like someone put their hand under the plane and gently, but quickly lifted her into the air.

I've made lots of similar midfield grass landings and lots of different airports, but never experienced this where there wasn't a sudden gust of wind. So what caused it? Perhaps the morning hours sun heating that thin piece of pavement. Or perhaps the difference in height between the grass and the pavement creating a ground effect ramp. Or maybe a combination of both. But I've done this type of landing hundreds of times before, some at this very airport, without this happening. I really don't know the exact cause.

But it did put a smile on my face. It's one of the things I really love about flying. Just when you think you know what to expect, you find out you don't.

In low ground effect, like spraying at one to three feet, gusts, up and down drafts, etc. don't cause climb or descent. They take you sideways a bit. Flying very close to a ridge to get oregraphic, ridge lift and going parallel to a pass or deep saddle in that ridge or mesa will cause a climb like an updraft does at altitude. I think it is the venturi effect of the pass or saddle. Are there buildings or terrain near the airport that could give you this venturi effect?

contactflying wrote:In low ground effect, like spraying at one to three feet, gusts, up and down drafts, etc. don't cause climb or descent. They take you sideways a bit. Flying very close to a ridge to get oregraphic, ridge lift and going parallel to a pass or deep saddle in that ridge or mesa will cause a climb like an updraft does at altitude. I think it is the venturi effect of the pass or saddle. Are there buildings or terrain near the airport that could give you this venturi effect?

Ridgelift is a good description of what it felt like. And there was no sideways movement.

Where the spaces between the buildings allow the wind to exit straight through rather than having to go over (venturi) you got the extra lift. Going over obstructions causes extra (ridge) lift too, but venturi lift is usually compact and strong.

I'll bet you it was the higher asphalt vs. grass that caused it. The closer to the ground the more efficient the airfoil. Super cool that you felt the induced drag difference like that!

Dang ol' double post got me

Talking about ground effect, I flew with a very experienced missionary bush pilot who has flown more than 25,000hrs in Alaska and Africa.
The way he trained missionary pilots for landing in sand bars was going to a very long runway and have the pilot fly the whole runway full flaps and good amount of power at 3ft off the ground.

That way the pilot learns the plane and how it reacts to wind changes etc so he corrects with power, then when getting close to the end he will reduce all the power and the plane will stop very quickly.

Im going to try this one day in a long runway

I had several great flights in ground effect this year, on the skis and just inches over various frozen lakes, for miles at a time. About 1500 rpm at the prop, one notch of flaps, and 55 mph plus or minus. Sometimes grazing the snow so about as low as you can get. At one lake with some open water I got to see how the snow skis water skied, very well indeed.

Didn't the Russians do some research on full size transports that were designed to STAY in ground effect? Not hover craft, but some kind of aircraft that would take advantage of the effect for much greater efficiency. Nothing seems to have come of that.

I hadn't heard of the Russian one, but another fairly recent one. Found this info:
http://en.wikipedia.org/wiki/Ground_effect_vehicle

Motoadve-I teach the same thing as your missionary pilot. I call it hover taxi. Ground effect is good free energy that can be used on takeoff for faster acceleration and on landing to have very slow and controlled movement to the desired touchdown point.

I'd suggest that it was a gust on the nose.

Ever watch a line of pelicans flying parallel to the beach just inches over a breaking wave? A perfect example of ground effect and ridge lift. They can go for miles it seems and never flap a wing. I think the rising wave pushes a cushion of air up and they just surf on that rising cushion.