Ground effect!

[Kamikazi]

Ground Effect!

When a landing aircraft descends to within one wingspan-length of the ground, induced drag is heavily reduced as the air beneath the plane compresses with the ground. This reduction of induced drag will cause the plane to stop its downward descent- without that extra drag, the plane's lift becomes more effective.

However, inherent dangers exist in ground effect. Once you enter ground effect, the induced drag disappears, as well as multiple other lift-creating devices (can't think of them off the top of my head). In ground effect, nearly 90% of your lift is being created by your angle of attack. Your instructor (I hope) teaches you to *never* release any back pressure on the flare, no matter what. This is the reason why: once you release back pressure and decrease your angle of attack (no matter HOW small), ALL of that lift you were creating with your AOA is now GONE. The plane will drop like a stone.

I have first hand experience with this one. I was riding observer with my flight partner, and was watching our sink rate on final out the side window. We flared a bit high, so my partner let a SLIGHT amount of back pressure off- the aircraft DROPPED, and we hit the ground HARD, bouncing back into the air. Lesson learned: once you apply backpressure on the flare, LEAVE IT THERE. Mininum Controllable Airspeed in Landing configuration for us is about 33 knots, and our approach speed is 65. With that much extra airspeed, you can afford to hold the nose up, let the speed come off, and let the plane gently settle down. If necessary, apply throttle to soften descent rate, do NOT pitch up.

The other side to ground effect is on takeoff. While in ground effect, like I said before, almost ALL of your lift is in your AOA. Your AOA on takeoff is dependent heavily upon your flaps (if you use them on takeoff, we don't). If still in ground effect, and you pull your flaps up, you're likely to drop like a rock. At this point, you're still dependent upon AOA and ground effect to fly. By pulling up your flaps, you reduce your AOA, and your wings are not yet creating all of the lift they need to safely climb.


Just wrote it for a buddy at Frugals, thought someone might like to read it here

-Capt. Jeff "Kamikazi" Stanek
Reserve, 301st FW

[Chowda]

You're pretty close, just a couple thoughts:

Induced drag is decreased by the change in mean local flow (MLF) as the downwash is deflected (not compressed, nor cushioned), by the surface. This surface interference also disturbs the production of wake vortices and alters upwash (local relative wind is closer to remote relative wind).

As for pitching moments entering ground effect, here's something to look for next time you're observing. Watch for the nose to pitch down when entering ground effect (high wing aircarft). Apon entering grond effect, downwash is decreased, the negative AoA is reduced (tail), and thus downward lift on the tail is reduced causing the nose down pitch.

When leaving ground effect (takeoff, go-around), the increased downwash hits the horizontal tail and causes a down load on it and a nose-up pitching moment. This pitch up, coupled with a sharp increase in drag when leaving ground effect could put you in a tough situation, if no corective action were to be taken.

The third thing to watch for is the airspeed indicator. Maybe Brian can chime in with low altitude, high speed passes over the runway in the L-39 on this one
There is a small, but measurable, increase in static pressure below the wings as an aircraft enters ground effect. The airspeed indicator will read lower than normal when in ground effect. (The not-so highspeed pass) :lol:
The opposite effect occurs leaving ground effect as the airspeed indicator will read higher (actually correctly).

The AoA discussion will have to show up on another post. My brain is smokin'

(All smarty smart aerodynamic talk courtesy of Mr. Charles E. Dole, Flight Theory for Pilots, 3rd edition)

[Kamikazi]

Good read Chowda!