Ground Effect

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Wingtip vortices have caused lots of problems for aircraft, but there is another side to them. AVweb's Linda Pendleton explains how wingtip vortices affect your own plane.

Airmanship

Wingtip vortices are formed whenever a wing produces lift -- we discussed this in my last article (Wake Turbulence). Wingtip vortices can affect aircraft following you, but did you know they have an effect on the aircraft you are flying?

A Review of Wingtip Vortices

Letís take another look at how wingtip vortices are formed. The airflow over the top of the wing is at a lower pressure than the airflow under the wing. This is part of the production of lift. Because the air above the wing is at a lower pressure, the higher-pressure air from below curls around the wing at the tip. The higher pressure air from below the wing causes the lower pressure air above the wing to partially separate from the wing. The pilot sees this as drag -- lift-induced drag -- and itís the drag that increases with an increase in lift. As the airflow leaves the trailing edge of the wing, this curling of the high-pressure air around the tip of the wing forms a horizontal tornado. This is the wingtip vortex. The vortex on the left wing curls clockwise as seen from behind the aircraft, and the right side vortex curls counter-clockwise. (There are also vortices behind the flaps, but the main vortices that affect both following aircraft and the one youíre flying are those from the wingtip.) The vortices tend to sink until they are about a thousand feet below the aircraft that formed them, and they tend to move toward the aircraft centerline.

Higher pressure from beneath the wingtip swirls around the tip to the area of lower pressure above the wing. These vortices continue to swirl behind the aircraft.

All airplanes produce wingtip vortices. Remember when you first did steep turns and you felt a jolt as you turned through 360 degrees, and your flight instructor told you that the jolt indicated you did a good job and crossed through your own wake? Yup, that was an example of wingtip vortices. The vortices produced by general aviation aircraft are usually not dangerous to other GA aircraft -- and certainly not to airliners -- but the opposite is not true. The vortices behind an airliner -- especially a heavy or a 757 -- can be disastrous as we saw in the last article on this subject. But, how -- besides that jolt on steep turns -- do wingtip vortices affect the aircraft that generated them?

Can You Get Rid of Vortices?

If you could figure out a way to eliminate the induced-drag vortices, youíd make Bill Gates and Warren Buffet look like paupers. The world (at least the aviation contingent) would beat a path to your door. It would seem that if the high-pressure air below the wing could be kept from curling around the wingtip and separating the lower pressure air from the wing, that induced drag could be decreased. One way to accomplish that reduction of air separation is with a wing of infinite span. Since an infinite-span wing is impossible, aerodynamicists came up with a compromise that does reduce some of the induced drag. The winglets you see on the wingtips of some airplanes reduce the induced drag by breaking up the vortices before they can curl around the wing and displace the lower pressure air above the wing. The pilot sees this as a reduction in drag.

Anything that keeps the high-pressure air beneath the wing from trying to displace the air layer above the wing will reduce drag. When the airplane is less than a wingspan above the ground, the ground will break up the vortices and keep them from curling around the wing -- and thatís part of what we call ground effect. When the vortex is prevented from curling around the wing, the induced drag from lift is reduced.

The pressure pattern of the air around the aircraft is in the shape of a cylinder when the aircraft is more than a wingspan above the ground.

However, the vortices are not the total story when it comes to induced drag. If you imagine standing behind the left wingtip and looking at the vortex, you would notice that the vortex from the left wingtip curls clockwise and would be pushing air downwards on top of your head. Now, move over to the right wing. The vortex here curls counterclockwise and again, pushes air downwards on top of your head. Youíve just experienced downwash. The downwash causes the lift vector of the wing to be inclined slightly backwards from the vertical. This can be broken down into the vertical component of lift and a horizontal component. The horizontal component is the drag component: the greater the downwash angle, the greater the drag.

The pressure pattern of air around an aircraft flying out of ground effect is almost cylindrical with positive pressure below the wing and negative pressure above the wing. Pressure differentials are felt quite a distance from the airframe, and the cylinder of affected air has a diameter close to that of the wingspan of the aircraft.

When the aircraft is within a wingspan of the surface, the pressure pattern of air around the aircraft becomes flattened on the bottom by interaction with the surface. This lengthens the effective wingspan of the aircraft.

When the aircraft is close to the surface -- in ground effect -- coming into contact with the surface modifies the almost cylindrical vortex-induced circulation around the wing. This flattens the cylindrical circulation pattern and reduces the downwash angle of the air behind the wing. This flattening of the cylindrical circulation spreads the pattern outwards below the wing and increases the effective span of the wing. The aerodynamic aspect ratio of the wing is also increased. (The aerodynamic aspect ratio of the wing is measured between the cores of the vortices, which occur at about 80% of the geometric wingspan outside of ground effect. This aerodynamic aspect ratio has a strong inverse effect on lift-induced drag.)

When the aircraft flies close enough to the ground that the sag of the vortices trailing the wingtips is restricted by coming in contact with the ground, the backward-tilting angle of the total lift vector is reduced, thereby reducing its horizontal component and reducing induced drag.

The combination of the reduction in the downwash angle of the air behind the wing and the increases of both effective wingspan and aerodynamic aspect ratio of the wing occur when the wing is close to the surface. These increases in aerodynamic efficiency of the wing are what we call ground effect.

Is Ground Effect Good or Bad?

Well, that depends. You can use ground effect to accelerate an airplane after lift-off on a short- or soft-field takeoff -- or you can float halfway down the runway by being too fast on final. Therefore, whether ground effect is good or bad depends -- like most things in aviation -- on your understanding and use or misuse of the phenomenon.

Short- and soft-field takeoffs have at least one element in common -- they both use ground effect. Youíll remember that in each takeoff technique, the aircraft is lifted off the runway and then immediately leveled to remain in ground effect for acceleration. The aircraft will accelerate much faster without the rolling friction of the tires on the runway/takeoff surface and will accelerate much faster in the reduced drag of ground effect. Care must be taken to remain in the ground effect until a safe airspeed for continued climb has been achieved. Itís possible to fly in ground effect at speeds that would not be possible when out of the drag-reducing effect.

Just as it can be said that ground effect reduces the lift-induced drag, climbing out of ground effect will have the opposite effect. There will be a dramatic increase in the drag, which may be seen by the pilot as a loss of thrust. At any rate, the increase can be quite dramatic when an aircraft that was flying just a second ago quits and settles back to the runway. This has happened many times to those who thought they were getting away with grossly overloading an aircraft. Just as they are thinking, "I knew the charts were too conservative. Just look how my aircraft is performing ..." the aircraft climbs -- slowly -- out of ground effect and refuses to fly further. Some unlucky folks have been beyond the end of the paved runway when this occurred and were required to have their airplane hauled out of the field of a farmer who was less than impressed with their "test pilot" antics. Some did not live to attend their NTSB hearing.

Because the elevator of most aircraft is affected to some extent -- depending upon its mounting in relation to the wing -- by the downwash from the wing, the elevator can be much less effective in ground effect. That can cause interesting problems on both takeoff and landing. The test pilot we just talked about may find that not only does the drag on his airplane increase dramatically when he climbs out of ground effect, but the elevator all of a sudden becomes more effective. And, before he knows it, that large amount of nose-up elevator he was using to haul his overloaded beast from the runway suddenly puts him into an extreme nose-up pitch and the aircraft stalls while falling back to mother earth.

On the opposite end of the flight, a heavily loaded aircraft with a CG forward of the forward limit may not have enough elevator authority in ground effect to keep the nose wheel from being the first gear to touch down on landing. This does not make for graceful landings. In fact, the effectiveness of the elevator in ground effect has a great influence on the setting of the fore and aft limits of the CG.

When an aircraft enters ground effect during the landing flare, the aircraft may tend to float because the lift-induced drag is reduced quite dramatically as the aircraft descends below one wingspan distance from the ground. Any excess speed at all -- you know, the 10 knots for Ma and the kids -- will cause this float to become excessive. This can cause an inexperienced pilot to grope for the ground and possibly induce pitch oscillations. At any rate, the bent nose gear and the excess wear on the tires and the brakes are of no use to anyone except the folks who sell spare parts.

As you can see, wingtip vortices affect not only the aircraft in trail, but they have a great effect on the aircraft you are flying. Like anything else in aviation, once you understand the phenomenon you can use it to your own benefit and not be surprised by that lovely float at the end of an otherwise perfect approach. Use ground effect to your benefit to facilitate short and soft field takeoffs. Just remember to stay in the ground effect until you have obtained a safe climb speed. Be ready for the decreased drag encountered when descending into ground effect on landing by controlling your speed on final and maintaining the recommended speeds. Donít be lulled into a false sense of security by those "extra few knots" that will cause you to float an amazing distance down the runway.