When I was coding for the open source FlightGear flight simulator, I had a chance to discuss this issue with some hard-core aeronautical engineers, and I also did a bit of reading, trying to get ground effect right. While nobody is absolutely certain yet, the best explanation seems to be that ground effect actually cuts drag (by interfering with wingtip vortices) rather than increasing lift. You can find more information at Wikipedia.

Flight Instructor Duncan MacDougall hollered from the front seat of the E2 Cub, “Do a 360 degree turn to the right.” I added full power, smoothly banked into the turn and watched the horizon pass around in front of us until the entry point coming up alerted me to level the wings and ease stick back pressure. Just then I felt a shake and a gentle rocking of the wings. To my question, Duncan answered, “You hit your prop wash.”

Back on the ground my instructor explained that by hitting our own prop wash it meant that the turn was perfectly level. I appreciated the compliment but could not believe that the backwash of that little prop passing by the fuselage after over a minute could still be strong enough to rock the airplane with two people in it.

Years later I found out that it was wing tip vortices we had hit and it did not mean I had made a level turn; it meant we had descended slightly.

Ever since those early $2-a-lesson days, I keep hearing about “ground effect” and how it causes an airplane to float over the runway or how the airplane gets “horsed” off the ground but can’t climb out of ground effect. Accidents are blamed on it. Even the astronauts reported entering ground effect on their first return in the space shuttle.

Like that “prop wash”, I think the prevailing spin on ground effect is “hog wash.”

Granted that air closest to the ground is most conductive to flight because it is denser than air one wingspan higher and air near the ground can’t be a downdraft because the ground is in the way. Maybe Piper should advertise better takeoff performance than Cessna because their wings are closer to the ground.

If ground effect were all it’s cracked up to be, wouldn’t beginning flight students be scattered all over the ends of runways where they ran out of ground effect?

The Federal Aviation Administration refers to ground effect in certifying aircraft and they claim, “The airplane is considered to be out of ground effect when it reaches a height equal to its wing span.”

In FAA’s Basic Helicopter Handbook there is an illustration with the caption “Ground effect results from the cushion of denser air built up between the ground and helicopter by the air displaced downward by the rotor.”

Why not prove if the above is true with a simple test? Mount a pressure instrument such as a spare altimeter on a helicopter skid and watch the reading on it while the helicopter takes off. Will it show a higher pressure at the instant before liftoff? It will not if what I learned in high school physics still holds true– that air cannot be compressed unless it is confined.

Ground cushion vehicles do confine air under them by use of “skirts” that cause the machine to rise a bit off the surface so air can escape under the skirt. Any airplane or helicopter flown by us men does not have a skirt.

What causes airplanes and rotorcraft to fly is to have their lifting surfaces push down masses of air equal to or greater than their own weights. This holds true at any altitude or closeness to the ground. Besides, no wing or rotor blade knows how far down below the ground is.

As an old bold pilot still enjoying flight adventures, I have never experienced floating over a runway except when I held too much airspeed and I have never lifted off and found it difficult to climb unless I was holding too high an angle of attack for the available thrust.

How about you?

Glenn Gibbons ATP 790358 1725 SW 52nd Street, Cape Coral, FL 33914 239-945-7075 ggibbons@swfla.rr.com AOPA #00026550

My biggest problem with a short-field landing on an obstructed field is giving too much clearance over the trees — I think you have to almost set up for a landing on the treetops so that you will clear them as tightly as possible.

We’re starting to get a good community of aviation blogs. I had trouble finding any at first (and most were student-pilot blogs), but now I can barely keep up. I’ll look forward to reading yours.

On the other hand, if the pilot is carrying in *too much* airspeed on final, the Pipers may well float more than Cessnas, because then parasite drag plays a larger part than induced drag, and you’re not quite so close to stall on the L/D chart…

Good blog, I’ll put you on my blogroll. If you’re interested, read Blogging at FL250, wherein I chronical my life of ultra-junior regional pilot and sometime freight dog. It’s at fl250.blogspot.com.

Take Care,

Sam

Unless you’re already at stall speed on short final over the fence (a scary thought), you’re going to need some kind of a flare to transition to a slower touchdown speed. With a short-field approach, you have a slower approach speed (and thus, a shorter flare), but it’s still there, and the flare still ends when the plane nears stall speed — if a plane floats longer in ground effect, it will take longer to decelerate to its minimum speed.