Friday, September 28, 2012

Power Control and Stabilty

Plane Stability - Plane Performance and Throttle Settings

What is your plane stability after you trim for a particular speed? A plane, with perfect stability, will maintain that speed whether the power if off or the power is increased. Right? 

Try this in actual flight at cruising speed in straight and level flight and you will discover what actually happens when you change the power setting. Cut the power and the typical plane will descend in a glide at a speed well above your cruising speed. The glide will steepen into a dive. Push the throttle wide open and the plane will settle on a different, slower speed as it climbs.

Just why does a plane speed up when you close the throttle and slow down when you open up the throttle? Seems contradictory to most pilots. 

Plane design and propeller blast is one reason. The tail surfaces ride in a place, on a plane, where the propeller blast hits them. In cruise mode, at a specific speed, the tail surfaces receive an extra wind in power on flight causing an extra down force on the tail surfaces. With power off that wind and extra force disappear. 

In designing a plane there is something called a thrust-line location involved in a planes development. It concerns the location of the propeller in relation to the rest of the airplanes design. Consider a Cessna, with a high wing, with its engine mounted low to provide good vision for a pilot. This design arrangement will cause a nosing up tendency when the pull of the propeller coupled with the backward pull of the high wings drag with power on. This effect disappears when the power is off.

A change in relative wind created by the wings at cruise speed with the power on and how it changes with power off. Remember that the tail surfaces reside in the downwash created by the surfaces of the wing while in flight. With power on, the propeller blast hits certain parts of the wing. These parts work in an extra sharp relative wind that sends an extra sharp downwash to the tail surfaces. This produces an extra down force on the tail surfaces. When the power is off this effect disappears too.

The effect becomes more pronounced the heavier and more powerful the plane. To a pilot the effect is important because under some conditions an airplane may want to stall it self. 

From a safety point of view this may occur during a go-around as you attempt to land when a plane attempting to take off the runway suddenly appears in front of your plane. 

If your plane, that cruises at 100 mph in straight and level flight, stalls at 45 mph and your approach speed is 70 mph after trimming properly you could stall the aircraft in a go-around. You open the throttle wide to climb away and re-enter the airport flight pattern. If you don't simultaneously re-trim or apply strong forward pressure on the stick the plane wants to climb like mad. It will climb into a power stall because you think the plane will climb at 70 mph. (no change in airspeed).  Instead, the plane will want to fly at 45 mph or less to quickly gain altitude. 

You must respond by applying strong forward pressure on the stick and/or re-trimming to lower the nose of your plane to a safe climbing attitude and the speed up to a safe 70 mph.

Each plane, in review, has a built in tendency to keep its own   angle of attack constant and to keep its speed constant regardless of how the the amount of power delivered by the plane may change. This is basic. This basic tendency is overcome by the effects of power on and off discussed. You, the pilot must be aware of these contrary tendencies and act accordingly.