Ground effect can be your worst nightmare on a check ride. You begin your flare just above the runway and... You float, and float, and float. Suddenly, your Piper Warrior feels like a glider - why doesn't it glide like this on an emergency approach to landing?!?
Nearly every pilot knows what ground effect does - but why does it happen? Hint: it has nothing to do with a cushion of air. Unless you're flying one of these:
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If you're looking for two words to explain ground effect, they're "wingtip vortices." These things seem to be the root of all evil in aviation - they cause induced drag, they create wake turbulence, and - when they're disrupted by the ground - they cause ground effect.
Ground Effect: When The Ground Limits Your Wingtip Vortices
Take a look at this SR22 - flying close to the ground and flying high. Assume that it's flying at the same airspeed and angle of attack in both cases. Notice how the wingtip vortices are larger when it's up high?
When you're flying very close to the ground, the ground limits your wingtip vortices - they can't get as big. So, they cause less downwash. Take a look at this downwash example:
Flying low to the ground limits your vortices, making them smaller. This means that the air flowing off the back of your wing has less downwash. But why does that reduce drag?
Less Downwash = More Vertical Lift And Less Drag
Take a look at the downwash example again. Lift always points perpendicular to the relative wind. Downwash angles the relative wind downward, pointing lift backward.
When you have less downwash, your lift vector isn't tilted back as far, so more of it points up (opposing weight) and less of it points back (acting as drag).
And, it takes energy to create downwash and vortices - and that loss of energy creates drag. So, when you're in ground effect:
- You have more vertical lift, opposing weight,
- You have less rearward lift, reducing drag
- You have smaller vortices and less downwash, reducing drag
You Need To Be Really Close To The Ground For Ground Effect
Why does a Piper Warrior seem to float more than a Cessna 172? Because the Warrior is a low wing aircraft - and that means your wing is much closer to the ground on landing.
Ground effect only begins to show up when you're within one wingspan of the ground. But, it really reduces your drag when you're within 20% of your wingspan to the ground. At that height, your wing only generates 60% of its normal induced drag. What's that mean?
A Cessna 172 has a 36 foot wingspan - so 20% of that is roughly 7 feet. Since a 172's wing is roughly 7 feet high (at the wingtip) when parked, it's hard to get into that 20% sweet-spot.
A Piper Warrior has a 35 foot wingspan - so 20% of that is roughly 7 feet. And, its wing sits about 3.5 feet high (at the wingtip) when parked. That's why a Piper Warrior floats more than a Cessna 172 - its wing gets much closer to the ground - almost down to 10% of its span. That means you'll have only 40% of your induced drag!
Ground Effect Has Its Advantages - The Soft Field Takeoff
Of course, ground effect does have its advantages. It helps you lift off of a soft or contaminated field, and using it effectively is a major part of a soft field takeoff technique. And - it's also helped during low altitude cruise!
B-29s: Using Ground Effect To Extend Your Range
During World War II, B-29 aircraft in the Pacific flew extremely long routes (1,500 miles each way) from airbases in the Mariana Islands to Japan. Engine failures were common, and aircraft often lost multiple engines on a trip. That generates a lot of drag, cutting range and forcing a bail-out over the vast Pacific.
With multiple engines out, many crews used ground effect to their advantage. They flew their aircraft low over the water, boosting range and getting them back to base.
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