Questions from the Cockpit: A good grounding

Vicki, a student pilot from Colorado, writes: My instructor says that on soft field takeoffs, we can rotate into ground effect before the plane is ready to fly. Does this mean that ground effect lowers stall speed?

The simple answer is yes. Or at least, in effect, yes. But it’s not as simple a question as it appears. For one thing, “stall speed” isn’t a real thing at all. But let’s leave that for the en route segment of today’s column, and start at our departure airport: KGER, which stands for Ground Effect Review.

Every pilot knows that airplanes, particularly the ones with their wings on the bottom where the good Lord intended them to be — not that I’m biased or anything — suddenly fly better when they are about to kiss the concrete. This can be vexing when all you want to do is land after a too-long day, and you find your plane skimming down the rapidly diminishing runway like a hockey puck. But at other times we can deploy this close-quarters phenomenon of improved performance — this ground effect — to our advantage, as in the case of your soft field takeoff.

When I was learning to fly nearly 40 years ago, ground effect was described to me as a “cushion of air.” My flight instructor, who had been a World War II Marine Corsair pilot, flew us out over the eastern plains of Colorado and then took us down to the deck. As prairie dog mounds zipped by on either side, he extolled the virtues of ground effect.

“Just try to crash the plane,” he chortled with glee, as he urged me to push the yoke forward to feel the protective cushion, “you can’t! Ground effect protects you.” 

I recall thinking we’d likely crash just fine, even without me trying.

But is there really a cushion? Well, yes, because the plane compresses the air under it as it approaches the ground, and the wing’s downwash gets trapped between the wing and the runway, but that’s only a small part of the picture.

So small, in fact, that the latest edition of the Pilot’s Handbook of Aeronautical Knowledge describes the cushion of air as “imaginary.”

OK, so if there isn’t a cushion of air, then what’s going on? Why does the wing that was losing lift just fine on short final suddenly have a surge of lift at the flare? And why can we lift off of the runway in this ground effect on takeoff long before we have proper flying airspeed?

Well, ground effect is actually made up of many aerodynamic elements, but the lion’s share of what SKYbrary calls the “lift bonus” of ground effect comes from reduced induced drag.

Here’s how it works: Picture yourself flying a banner-towing plane. Who wouldn’t want to do that? But that’s one banner-crop of drag behind the airplane, right? Well, here’s the thing: No matter what you fly, you’re pulling an invisible banner behind you, in the form of your wingtip vortices. That’s where the lion’s share of the infamous induced drag that’s a by-product of lift comes from.

But close to the ground, rather than streaming out behind you, the vortices strike the earth’s surface — effectively creating a shorter banner to tow. Ah-ha! Less drag! And thanks to the magic mathematics of aerodynamics, less drag means more lift.

The drag banner starts shortening about a wing span’s height off the runway, and the reduction in drag gets more and more impressive the lower you get. At a quarter wing span’s height off the runway, induced drag is reduced, compared to its inflight force, by about a quarter. At a one-tenth span — about 3 feet for most GA aircraft — it’s reduced by a whopping 50%!

And this is why low-wing planes feel ground effect more than high-wing planes. The only way to get a Cessna 172’s wing 3 feet off the runway is to land inverted.

Meanwhile, for takeoffs, this reduction in drag near the surface means that you can generate enough lift to rise off the ground at an airspeed too low for proper flight, your instructor’s “before the plane is ready to fly.”

Of course, off the ground isn’t the same as actually flying. Once you rise out of ground effect, your tow banner is back, and if you aren’t up to flying speed, you’ll fall right back into ground effect, or perhaps all the way through it to the ground itself.

But does all of this mean the stall speed is less in ground effect? Well…

OK, here’s the thing about stall speed. Like my cushion of air over the prairie dogs, it’s imaginary. You won’t find “stall speed” in any of the glossaries in aviation training handbooks, and the Airplane Flying Handbook goes so far as to say the term is “misleading.”

This is because airplanes — well, more correctly, airfoils — stall when their critical angle of attack is exceeded. It has nothing whatsoever to do with speed. A plane can stall at any speed. 

But we talk about stall speed all the time, so what’s up with that?

The problem is that, prior to recent technological advances, we had no realistic way to determine our angle of attack in flight, so for decades we used speed as a rough proxy for visualizing angle of attack. Said another way, we use speed to predict when a stall will likely occur for a particular configuration or condition.

For instance, the Vso, indicated by the beginning of the white arc on your airspeed indicator, is the “stall speed” for the airplane in landing configuration, which usually means gear down and flaps dirty. But it assumes power fully off, props at low thrust, cowl flaps closed, CG at forward most point within the envelope, and loaded to max gross — which of course is impossible, at least legally. If you took off at max gross, you’d burn off at least some weight even going around the pattern a single time.

The point?

Well, it’s good for reference, and for comparing one plane to another, but it’s not pure. The airplane may or may not stall at this stall speed.

So tying all of this together, and getting ready to land back at your question: Thanks to the reduction in the drag acting on the wing close to the ground, lift is increased and therefore the stalling angle of attack is reduced. In fact, it’s reduced by quite a bit, anywhere between 2°-4°. This means that we reach the critical angle of attack at a slower speed once in ground effect than out of ground effect, everything thing else being equal. 

That being said, then, is the “stall speed” decreased in ground effect?

Well, bearing in mind that stall speed is a figment of our imaginations, but using the tradition of speed as a predictor of angle of attack, and thus when a stall might happen, I think it’s safe to say that, yes, ground effect lowers “stall speed.”