Ground Effect:
The First and Last Few Seconds

By Hank Canterbury

 

I often wondered why many pilots flying Bonanzas and Barons seem to touchdown three-point.  The three-point touchdown is defined by the FAA as “a simultaneous touchdown of the main and nose wheel with excessive speed, followed by application of forward pressure on the elevator control.”  Unfortunately, that is not a very good technique, as it may damage the nose gear assembly, which isn’t designed to endure such impact forces.  The three-point landing is also  indicative of being too fast, which results in much longer-than-necessary rollouts,  as well as wheelbarrowing.

 

My hypothesis is that three-point touchdowns may be a result of one or all of the following reasons:

• First, it is unnatural and inconvenient to land when the nice full view on final out the windshield is being reduced as the glare shield rises higher during the flare. Pilots subconsciously stop increasing backpressure to raise the nose sufficiently to achieve the desired touchdown attitude and airspeed (approximately 10 degrees).
• Second, there are two aerodynamic effects that alter the performance of our planes when our wings are close to the ground. To better understand the issue, let’s look at those usually unnoticed aerodynamic effects in play during both takeoffs and landings.  Understanding the “what” and “how” of ground effect should lead to better landings and safer takeoffs.

Ground effect appears to be a mysterious phenomenon that few pilots understand – and most pilots don’t adjust for its effects on performance.  Within ground effect, aircraft performance is altered by drag and the changing effectiveness of the elevator.  Pilots encounter these effects both at liftoff and descent when landing.  The landing condition effects may not be as serious as takeoff when high density altitude and very low excess power may exist.  More on this point later.

Generally, the effects are considered to come into play when the wing is within one wing span of the surface; that’s somewhere around 33 to 36 feet above the ground for our Beechcraft.  The closer to the surface the wing is flying, the more powerful the effect on drag and elevator effectiveness.

At a full wing span above the surface, there is little impact. However,  as the wing gets closer — say 10% of its span above the surface — a noticeable drag reduction occurs, whereby up to 50% of induced drag is eliminated.  This can cause the plane to “float” significantly longer than expected before touching down, especially if more airspeed than necessary is carried into the flare. Simultaneously, the elevator loses negative – or downward – lift, which leads to the nose dropping, unless compensated for by additional aft yoke.

Guess what the altitude is where such a large drag reduction occurs?  That’s right: the height very close to when the plane is about to touch down and where it would be if resting on its landing gear.

There are two other situations where ground effect can have an adverse outcome on landings.

Forced landings, either practice or for real, are usually flown at increased airspeed on final than normal.  I frequently observe pilots who do not plan on a longer float to dissipate this excess airspeed before the plane is ready to touchdown (at the correct attitude and speed), and they “force” the plane on too soon usually resulting in a bounce back into the air or touchdown on the nose wheel.  For twins, landing with one engine feathered or simulated feathered, there will also be significantly less drag than normal from the single wind-milling propeller when the throttle is closed than normal with two engines turning.

Usually, pilots under stress maintain more speed than necessary into the flare, sometimes at “blue line” or above, after the runway is assured, rather than slowing to the normal two engine final approach speed.  When in ground effect, couple that with a natural tendency to carry far greater airspeed on final while under stress, and very long landings  usually result. Worse yet, the plane is forced onto the runway far too soon, often causing damage.

Take a look at the following diagram showing this relationship.

Let’s look at why the aerodynamic interactions occur when in ground effect.  When in free air well away from the surface, a wing (airplane) creates a ‘downwash’ flow of air behind the wing that bends the air flow downward. This causes the air flow to descend at an angle from the free-stream relative wind that is approximately equal to the wing total Angle of Attack [AOA] (see figure below).

This action is created when the relatively higher pressure air below the wing flows upward around the tips toward the lower pressure on top of the wing, thereby inducing vortices that roll inward. In turn,  this shifts the wind flow direction over the wing down and toward the wing root.  Furthermore, this downwash creates an Angle of Attack on the horizontal tail, which is a different angle than that of the main wing.

The horizontal surface and elevator usually produce a downward or negative lift to balance the wing’s natural desire (moment) to pitch downward.  But when a plane enters ground effect, the surface blocks the downwash from descending as steeply behind the wing. This significantly reduces the downwash angle the closer to the surface the wing flies.

Because the horizontal tail/elevator doesn’t have as much AoA as before, the lift on the tail (down force), is reduced, causing the pitch attitude to drop unless more up elevator is applied to compensate for the loss of downward lift.

During the flare for landing, ground effect has two primary effects on the plane:

• First it reduces the angle of attack of the elevator, which, in turn, reduces the downward lift causing the pitch attitude to drop. This results in a “nose wheel first” or
  three-point touchdown.

To counter that undesired outcome, additional backpressure must be exerted on the yoke to simply maintain whatever pitch you had just before touchdown.  This subtle drop in pitch usually goes unnoticed or is not compensated sufficiently by the pilot, which results  in 3-point arrivals.

• Second, and more significantly, ground effect actually changes the true lift vector angle by rotating it forward, which is why the total drag on the plane becomes less. How? The true lift vector is perpendicular to the average relative wing about the wing.  (See the diagram below by a terrific professor of mine during graduate school, Mr. H. H. Hurt, Jr.)

Thus, the decreased downwash angle lowers the average relative wind angle of air circulation about the wing.  This forward shift of the wing’s lift vector is perpendicular to the average relative wind of the wing. It is also inclined more aft when at higher angles of attack before entering ground effect – and that is what reduces the drag component.

The horizontal portion of the true lift vector that is parallel to the free stream relative wind is known as “induced drag”.  As the average AoA of the wing is reduced, the true lift vector rotates forward.  Thus, the portion or component acting parallel to and opposite to the forward motion of the plane is also decreased!  Essentially, the forward rotation of the true lift vector in ground effect lowers the average angle of attack of the wing, thereby reducing stall speed and drag.

The net effect of these two forces appears to the pilot as a nose-down pitch change and ‘floating’ since there is significantly less overall drag on the plane.  To correct for this effect in the flare requires continuously more aft yoke to either hold the nose up or raise it a bit farther. It also requires patience while waiting for the plane to slow down.  In smooth air, these effects are more noticeable, although they occur, regardless.  Extra airspeed carried into the flare aggravates these effects.

Figure 130, p 67, Aerodynamics for Naval Aviators, Jan 1965

 

It is important to remember that takeoffs have just the opposite effect.

Landing mistakes very near the surface usually don’t present life-threatening problems, but takeoffs can quickly result in the big sink even at full power.  As opposed to the landing phase, in a climb, induced drag increases  by the square of the angle of attack [AoA]. At normal liftoff speeds, induced drag quickly increases as the plane leaves the ground, already at a high angle of attack, and immediately begins to climb out of ground effect.

Now think of a high gross weight and/or density altitude takeoff situation, which already produces less excess thrust horsepower available to accelerate and climb.  You now have a very tense and serious situation developing.  The plane may have just enough power to lift off and fly while in ground effect where reduced drag is present, but when it struggles higher reaching maybe 40 to 50 feet above ground level, downwash angle increases, lift vector rotates aft, drag goes up and, bada-bing, airspeed begins to decrease.

Total drag is now higher than when you just lifted off in ground effect.  In a vain attempt to stop the sink and keep climbing, pilots invariably raise the nose even higher, which further increases drag.  At this point, because the aircraft is already at full power, there is nothing a pilot can do to make up for the increased drag other than to descend to gain speed — and altitude is now selling at a very high premium!  So, as the airspeed bleeds off, the nose goes higher (denial) and the plane sinks back to the ground.  The outcome is usually not good!

How to prevent it?  Recognize during takeoffs that the plane can become airborne in the reduced drag environment close the ground; however, it may not be able to continue climbing as the drag increases quickly when flying out of ground effect.

Think about the technique for a soft field takeoff.  You become airborne sooner at low airspeed but must level off temporarily just above the runway in order to accelerate before establishing a climb attitude.  Don’t attempt to climb until airspeed has increased sufficiently to provide the lift and additional excess thrust to overcome the increased drag so you can continue climbing.

One more caution: remember climb angle will also be substantially reduced in these conditions, and the distance to climb even a modest couple of hundred feet will be much greater than you imagined.

Do conservative takeoff performance calculations, add some incremental percentage to your results, and if performance appears marginal, change some conditions!  Offload passengers or bags, take on less fuel, go when temperatures are lower.  Add a margin of safety to your calculations and use those for your decision.  “Hope” is not a plan, a strategy or technique that works.

To wrap up this discussion, pilots should be aware that:

• Carrying excess airspeed into the flare will cause an abnormally long float before touchdown
• The pitch (nose) will drop unless steady, additional backpressure is applied during the float
• Takeoffs for high density conditions where only marginal excess horsepower / thrust is available can quickly create a situation where  it is impossible to climb out of ground effect until more airspeed is attained.  Oh, how sweet an afterburner would be right then!

Fly Often! – Train Regularly! – Practice More.

Hank Canterbury
ATP, CFII SEL & MEI
FlyF33@aol.com