Ground Effect
Ground effect was first fully analyzed during
The closer to the ground a flying airplane is, the greater the benefits of ground effect. Ground effect dampens downwash vortices as well as wing tip vortices. This means that while in ground effect an aircraft has less induced drag. Lower (by half) induced drag allows a reduced angle of attack which further reduces induced drag. The airplane 'magically' acquires the performance possible only with a longer wing. Longitudinal stability is likewise greater such that the elevator control forces required to raise the nose may be up to fifteen degrees greater than that required at altitude. The forward C.G. limit is determined by the ability of the elevator to lift the nose while in ground effect.
The benefits of ground effect return to the pilot during the flare phase of a landing. The extent of the benefit is commensurate with airspeed, ground proximity, and wing length. Low-wing aircraft have a ground effect advantage over high-wing aircraft which inhibits landing skill transition from low-wing to high-wing but not so much from high-wing to low-wing. It is apparently easier to adapt to the presence of ground effect than to its non-presence.
Ground effect is created when the wing vortices are changed by proximity to the ground. In this change the ground acts so that downwash is reduced and the resultant induced drag is reduced. This makes possible a lower angle of attack that will keep the plane aloft albeit close to the ground. A much higher angle of attack would be required out of ground effect. A high wing aircraft is incapable of getting the same effect as a low-wing aircraft. For this reason many low-wing aircraft tend to land flatter than high-wing aircraft. You don't see many nose high landings in low-wing aircraft.
Ground effect becomes a potential hazard when poorly understood and utilized. Low-wing aircraft with a short wing span has a critical approach and flare speed below which an aircraft will 'fall through' any available ground effect and make 'firm' runway contact. The same aircraft with a long wing and excess airspeed will 'float' off the end of the runway before excess airspeed can be dissipated in the increased efficiency of ground effect. The first hot day of spring (density altitude again) is an occasion for numerous hard landings where the pilots approach and flare is based on the ground effect available on winter days. The airspeed, flare level, airport, and aircraft may be constant but variations in temperature will change the ground effect. Even experienced pilots need an excuse for an occasional poor landing. On the same hot day a takeoff may allow the aircraft to become airborne and remain so in ground effect and not have the capability of climbing out of ground effect. In density altitude situations this increased performance in ground effect can deceive the pilot into thinking that it will continue to be available during climb. Takeoff accidents arise every year or so at Lake Tahoe because aircraft are unable to climb out of ground effect. I once flew over two miles in ground effect before acquiring the speed needed to climb.
Round-out
Let us assume that in a no wind condition, a constant airspeed, full flaps, and power at 1500 RPM the runway is made. Where you look is important. As an automobile driver you have already developed a skill useful in landing. You should look down the runway the same way you would in a car going 70 mph. If you look out too far you could fly into the runway or roundout too low and balloon. If you look too close to the airplane, like out the side window, you will probably roundout too high. Do what comes naturally from your driving to level the aircraft somewhere between hip and shoulder high above the runway. This requires you to exercise your best judgment as to height. The rest of the landing is energy management; the trading of speed for altitude lost at an ever-decreasing rate. You should touchdown within the first 1/3 of the runway.
We are now down to the last hundred feet of the approach. You can now safely ignore the airspeed indicator and rely on visual and kinesthetic senses. You are entering the round-out. This is the transition from 100' to about 3 or 4 feet above the runway. You look down the runway much as you would in an automobile going 60-70 miles per hour. You already have the required visual skills. Landing is just a new application. Once you have leveled off, the round-out becomes the flare. At most, use only two fingers on the yoke.
There is a football skill that does transfer readily to the flare for landing flying skill. I don't wish to imply that football players make better pilots but I do believe that they can transfer a visual skill to flying to make the better landings sooner. Football players, as a group, exercise their visual skills to be able to see straight ahead while being aware of what is occurring at the outer edges of their peripheral vision.
As a pilot you need a similar ability to see at a distance the end of the runway until the nose rises to cut off the view and at the same time you must have sensed the position of the visual horizon. As you move the yoke back and up, you are attempting to rock the aircraft on its pitch axis in such a way that the visual horizon remains constant. If the horizon seems to rise, it means that the aircraft is sinking. If the horizon seems to fall, it means that the aircraft is rising. Either of these movements can be controlled, to a degree, by moving the yoke back or holding it still. You need this peripheral vision skill because you are in the process of covering the runway with the nose of the aircraft.
In the round out as the aircraft loses speed you will learn to sense a kinesthetic sinking as in a very slow elevator. It is the kinesthetic sense that lets you sense rise or fall in small amounts. The peripheral movement of the horizon give you a dual reference as to when and how much to move the yoke back and up. Concurrent with this up/down potential we have a deceleration occurring. As the aircraft slows it requires ever-greater deflection of the elevator to give the same effect. We are endeavoring to rock the aircraft on its pitch axis with the elevator until it stalls without any fall in the visual horizon out the sides of cockpit.
At the very first sinking sensation you should begin and continue the yoke movement as practiced previously. If you observe the horizon falling, it means that the aircraft is rising. A rising horizon means the aircraft is sinking. What you are trying to do is to maintain the horizon in a stable position as you gradually increase the pitch attitude. Any abrupt or instinctive movement of yoke or power will be counter productive.
Where you look in the flare makes a great difference in the landing to follow. Look down at the ground and you will probably round out too high and give the spring gear a chance to do its thing. Focus too far and you could fail to round out. Looking too far down the runway can result in flying into the ground. When that happens you do a kangaroo with wings imitation until the nosewheel collapses.
Move the yoke too soon or too fast and the desired rocking motion will become a far less desirable balloon. A balloon is a rapid increase in altitude along with a loss of airspeed. The balloon is the onset of a corresponding rapid fall. The fall, because of the reduced airspeed, will go right through the previous altitude all the way to the ground with a THUMP, If we are slow enough there will be more of a SQUISH. of the landing gear absorbing the shock. Just a bit more speed can send us back into the air nose high the nose will drop first and SMASH the nose wheel and strut on the ground along with the propeller tips. It will not rise again and will probably slide only a few feet.
No damage or injury inside the cockpit. Enough outside, however.
You have already acquired the sighting skill needed for the round out and flare. It is the same as the one you use while driving at the equivalent speed. Use it until the rising nose causes the runway to disappear. This is going to be very near level flight. The roundout should be planned to bring the wheels to about a hip high height. Any higher will reduce the effectiveness of ground effect. Too low will likely result in a yoke reaction accentuated by the ground effect to give a balloon.
Do not change your sighting point until the previously selected approach point has passed. At the POH gross weight recommended approach speed we can expect to have several hundred feet of float in the flare before touchdown. Later on, we will use a slower speed to reduce the float and improve landing accuracy. However we are not through, just being patient.
Flare as It Should Be
Unfortunately, the flare cannot be simulated as to the runway rising up into the windshield on short final only to disappear during the flare. The flare is the transition from descending flight to level decelerating flight just prior to the "hold off" landing finale. Most students give up too soon on the flare by failing to continue the back and up yoke movement. The student should be aware that the geometry of the arm-wrist-yoke will cause some twisting that will result in one wing being low. Use your rudder to help hold that wing up and keep the nose straight with the runway. Unless required for a crosswind the wings should be level.
The round out and flare with full flaps requires that control movement and power decrease be so applied as to minimize float or a rapid sink rate. The flare dissipates energy by stopping the descent prior to touchdown. Landing practice is working toward repeatable results down to the flare. We are learning the anticipation, judgment patience and skills needed in applying control pressures. Use of a consistent configuration, airspeed, and patterns helps the pilot to attain repeatable results. Once the basic consistencies have been mastered, we learn the variations required by variable conditions.
We are trying to level out high enough over the runway so as to avoid the "bounce" caused by ground effect but yet no so high as to not have the benefits of that effect during the final moments of landing. Where you flare and how you flare will vary with climatic conditions. You want to level off before hitting the runway with the wheels about belt high with power on. As deceleration takes place the back and up yoke movement increases in both speed and amount until at the stop. One impediment to good landings is keeping your eyes on the runway just in front of the plane. By looking farther down the runway you will be able to level (flare) the plane appropriately.
The flare (C-150) should be entered at 60-knots. Any effort to slow up sooner will reduce the ground effect benefits needed later in the landing. You should avoid moving the yoke forward during the flare. The apparent mistake you are trying to prevent will only reappear in another larger form. In the one-second it takes you to react with the yoke it is already too late. Go-around. If the situation seems to require forward yoke, just hold it still for a few moments and the aircraft will resume its sink. Reducing the power another hundred will have the same effect. Using the forefinger on the throttle makes power reduction in 100-rpm increments easier. Once the sinking resumes continue the back/up yoke pressure.
The ideal, at this time, is that the aircraft has reached its stall angle of attack on the pitch axis. The stall warner lets you know of this occurrence. The elevator is to the backstop. The aircraft makes a slight drop to the ground and is through flying. The pilot continues to hold the yoke full back and up. The flaps are removed while the nose wheel remains clear of the runway. If no power is added, as for takeoff, the nose wheel touches and the strut compresses to allow steering with the rudder pedals. With power the takeoff can proceed without the nose wheel ever touching.
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