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ramblin_jack
08-15-2004, 04:11 PM
Take the bounce out of your landings


One of the most scrutinized portions of any flight is the landing. To the unknowing passenger, a pilot can literally be lost 3 hours of a 4 hour flight, but if he makes a smooth landing he is considered one hell of a pilot. It is only natural that the entire flight be evaluated on the last few seconds when the airplane, once again, comes back to earth. Even as we land in our flight simulators, every pilot that landed before us becomes a critic, counting the number of bounces before we finally roll out of the landing. As the old saying goes, ďany landing you can walk away from is a good oneĒ.

When flying a land based airplane, there are two types of landing gear configurations. Conventional landing gear consists of the main landing gear and a tail wheel. Tricycle landing gear consists of the main landing gear and a nose wheel. The P-40, for example, being the former and the P-39 the latter. By taking a look at the mechanics of the landing we will be able to understand just what it is that makes us bounce down the runway.

In general terms, the landing is a stall just inches above the ground resulting in a touch down hardly noticeable to the occupants of the airplane. Aircraft designers attempt to, by means of flaps and slats, design an aircraft that lands at the slowest possible forward speed. There are two main reasons to design for the slowest possible landing speed. First is structural consideration, the faster the landing speed, the heavier the structure must be therefore increasing the weight of the airplane and taking away from the payload. Second is runway length, the landing run of an airplane increases ďas the square of the speed,Ē that is, if you double the landing speed you make the landing run four times as long.

With tricycle landing gear the center of gravity must be located in front of the main landing gear in order to keep the nose wheel on the ground. The opposite is true regarding conventional landing gear with the center of gravity located behind the main gear in order to keep the tail wheel on the ground. With both airplanes sitting on the ground, you will notice the difference in the angle-of-attack of the wing on each airplane. The tricycle gear airplane will have little if any positive angle-of-attack, while the tail wheel airplane will have considerable positive angle-of-attack. Keeping that observation in mind, think of the lift that would be produced if either aircraft had forward motion.

Every good landing is preceded by a good approach. Proper speed and decent control are essential to making a good landing. The proper approach speed for any aircraft is generally 1.3 times VSO. (VSO means the stalling speed or the minimum steady flight speed in the landing configuration). VSO in both the P-40 and the P-39 is approximately 80 mph. Therefore the approach speed should be approximately 105 mph. To establish the proper approach speed and descent rate you must remember that your speed is controlled by your pitch attitude and your decent is controlled by your power setting. That might seem backwards, but for any given pitch attitude, an increase or decrease in power will cause the airplane to climb or descend and for any given power setting, and change in pitch attitude will cause the airspeed to increase or decrease. By making incremental adjustments to both power and pitch you can maintain the appropriate approach speed and rate of descent.

Landing the tricycle gear airplane:

On the approach to the runway you should have the proper pitch and power set that will give you an approach speed of 105 mph and a descent rate of around 500 fpm. As you approach the runway and begin the landing flare, reduce the power to idle and slowly and smoothly increase back pressure just enough to keep the airplane from climbing while holding the main gear off the runway as long as possible. The flare will result in a increasing nose high attitude that will bleed off excess airspeed and the touch down will be occur just above the stall on the main landing gear. After touch down, continue increasing back pressure, the airplane will slow and you will no longer be able to hold the nose gear off the runway. As the nose gear settles to the ground hold full aft back pressure. Maintain directional control with the rudder and brake as necessary to come to a stop or taxi off the runway. While rolling out after landing, or during taxi, continue to hold full aft back pressure.

http://www.frontiernet.net/~starship/p39_main_gear.jpg

In the above scenario, we essentially have the perfect landing. However, not all landings are perfect. Letís look at what happens when we have other than the perfect landing in a tricycle gear airplane. If your rate of descent is too high or your timing of the flare is too late, you will hit the runway harder than normal. Because the center of gravity is located in front of the main landing gear, the nose of the airplane will pitch down decreasing the angle-of-attack on the wing and eliminate any lift being produces. This will, in effect, transfer the entire weight of the airplane to the landing gear and the airplane will stay on the ground. If your approach speed was at or above the recommended speed, applying back pressure after touch down would raise the nose of the airplane thus increasing the angle-of-attack causing the airplane to become airborne. Thereís your bounce.

As you can see, with tricycle landing gear we can get away with being sloppy and still land the airplane with little or no bounce.

Landing the tail wheel airplane:

On the approach to the runway, as with the tricycle gear airplane, you should have the proper pitch and power set that will give you an approach speed of 105 mph and a descent rate of around 500 fpm. As you begin the landing flare, reduce the power to idle and slowly and smoothly increase back pressure just enough to keep the airplane from climbing while holding the main gear off the runway as long as possible. This flare will produce a nose high attitude that will bleed off excess airspeed and the touch down will be made just above the stall speed on the main landing gear and the tail wheel. Continue to increase back pressure pinning the tail wheel to the ground. Once the tail wheel and the main landing gear are firmly on the runway the angle-of-attack will not change and the airplane will remain firmly on the ground. Maintain directional control with the rudder and brake as necessary to come to a stop or taxi off the runway. As you slow down, continue to hold full aft back pressure, this will firmly pin the tail wheel on the ground for maximum directional control.

http://www.frontiernet.net/~starship/p40_three_point.jpg

In terms of the perfect landing, thereís not much difference in landing either the tail wheel airplane or the nose wheel airplane. Both landings occur in a nose high attitude and touch down is just above the stalling speed. The difference being the nose gear slowly settling to the runway as the tricycle gear airplane rolls out. Now letís look at what happens to the tail wheel airplane when we land with an excessive rate of descent. As the main gear comes in contact with the runway, because the center of gravity is located behind the main gear, the tail of the airplane drops. As the tail of the airplane drops, the angle-of-attack on the wing is increased and additional lift is produced. This additional lift brings the airplane up off the runway causing the first bounce. The pilots normal reaction is to push forward on the stick to counter the bounce. Once again the main gear contacts the runway, the tail drops, and additional lift is produced causing the airplane once again to become airborne. Depending on just how much excess speed you have the bounce and re-bounce will be repeated until you run out of airspeed or lose control and crash. This is referred to as Pilot Induced Oscillations (PIO) and has sent many an airplane careening out of control.

Thereís a third type of landing that is applicable to tail wheel airplanes. Itís called a wheel landing. The wheel landing in generally flown at a speed slightly higher than the normal 1.3 times VSO and is intended for use in crosswind and/or gusty wind conditions. It also improves forward visibility on the initial portions of the landing and roll-out. While the wheel landing requires a little more finesse and additional runway it too should be mastered.

The Wheel Landing:

Approach the runway with airspeed slightly above normal and touch down on the main wheels in a level flight attitude. At the time of touch down your rate of descent should be less than 50 fpm. As soon as the main gear makes contact with the runway apply a slight forward pressure on the stick. Your objective is to not let the tail of the airplane drop. Doing so would increase the angle-of-attack, increasing lift, causing the airplane to lift off the runway and then you have your bounce. Continue to apply whatever forward pressure is needed to keep the tail off the ground while you reduce the power to idle. As the airplane slows you will have to apply more and more forward pressure until full forward. At that time the tail wheel will settle to the ground, but because of your low airspeed the increased angle-of-attack will not produce enough lift to bring the airplane back into the air. You can then bring the stick full aft and pin the tail wheel on the runway for additional effectiveness with directional control. If, after landing, you just jam the stick forward you run the risk of nosing the airplane over and angering you crew chief.

http://www.frontiernet.net/~starship/p40_wheel_landing.jpg

When taking runway length into consideration the three-point landing, with its slower approach speed, will use the least amount of runway.

Spend some time in the traffic pattern mastering these landings. Your practice will payoff.

Rambliní Jack

352nd Persecutor
08-15-2004, 04:36 PM
Terrific information, RJ. I sure hope Husker reads it. Probably not enough pictures for him, though!! :lol:

clayp
08-18-2004, 03:53 PM
At last a forum just for me.... :lol:

352nd Oscar
08-19-2004, 09:26 AM
Excellent Information!!

Given this *#$% game is meter based, I did a little conversion:

Approach Speed of 105 miles per hour is 168.98112 (167) kilometers per hour
Landing Speed of 80 miles per hour is 128.74752 (129) kilometers per hour

The one I couln't convert was the 500/50 feet per second references. Anybody got that one?

Oscar

352nd_Deacon
08-19-2004, 10:11 AM
Sorry, Oscar, but my guages don't go that slow. Maybe that's why I'm always landing "hot" and having to have my ground crew steal, er, borrow other members' tires and an occasional landing gear.

352nd Oscar
08-19-2004, 11:13 AM
Understand completely - although, my landings of late have been somewhat improved. I understand from my crew chief that there is a ready supply of fuselage and wing parts, but landing struts are in extremely short supply. This situation has not escaped the eye of 8th AF Operations and the situation is being investigated. The IG has already sent out notifications to Squadrons that "creative" workarounds will be delt with harshly.

I want to update my conversion information with the following:

DISCLAIMER

If you attempt to land your aircraft using these conversions, you do so at your own risk. Please consult the manufacturers flight manual for your particular aircraft for the "proper" and "specified" takeoff and landing speeds. Failure to follow prescribed flight envelop specifications could be hazardous. Management will not be held liable for accidents or loss of life resulting from information provided.

Oscar

352nd_Husker
08-19-2004, 11:15 AM
Yes Pers ... Husker read this, over and over and over again ... let's hope it helps LOL!!!! :lol:

ramblin_jack
08-20-2004, 06:54 AM
Oscar,

It would do no good to convert feet per minute into meters per minute, because you do not have a gauge to show that information. At least not in the P-40. I haven't researched the none american aircraft yet.

RJ