Friday, February 12, 2010

More Aeroydnamics!

As part of my continuing study of the ground school materials for my private pilot's license, I have been reading through two separate references. The primary is the ASA textbook for private pilot ground school; the secondary is "The Proficient Pilot" by Barry Schiff.

I have not gotten too far into the text. My last series of notes on aerodynamics is continued here, mostly to reinforce it in my own head but also for your reading if you're into this sort of thing.

Continuing on the concept of drag, it is important to understand that parasite and induced drag will vary in their proportional influence depending in your airspeed. As you accelerate to higher airspeed, parasite drag increases exponentially; a doubling of airspeed quadruples parasite drag. Induced drag, on the other hand, is a result of lift development and varies with angle of attack. At slower airspeeds induced drag becomes a major concern.

This talk of induced versus parasite drag brings us to the concept of the region of reversed command. The term "reversed command" refers to the phenomenon that occurs wherein more power is required to maintain less airspeed. Barry Schiff uses the example of a pilot approaching a mountain airstrip: as the pilot approaches the strip, he notices that he is a little low on altitude, and so he raises the nose to climb. Unfortunately for the pilot, rather than climbing, his airplane slows down and sinks. The pilot reflexively raises the nose again and adds power, but it is too little too late, and he smashes to earth, banging his airplane up and wondering what the deuce happened.

What happened was: the pilot was in the region of reversed command. Flying low and slow, he made the error of pitching up without adding power, which caused him to slow down and fall from the sky, not to climb as he expected. In slow flight, approaches, or other maneuvers requiring lowered airspeed, it is necessary to add power to stay aloft even though your airspeed is lower. Not what you might think at first, but it's important to understand the concept.

The practical upshot of all this: control your altitude with power, your airspeed with pitch. Pitching to altitude will cause you to enter mushing flight and lose the altitude you seek to gain. Especially in low and slow flying, remember this concept.

Continuing on, we come to control and control surfaces. Control surfaces are important because they enable you to, well, control the airplane. A plane is controlled along three different axes:
  • The LATERAL or PITCH axis, which extends from wingtip to wingtip and is controlled with the ELEVATOR;
  • The LONGITUDINAL or ROLL axis, which extends from nose to tail and is controlled by the AILERONS; and
  • The VERTICAL or YAW axis, which extends from the central floor to ceiling of the aircraft and is controlled by the RUDDER.

Control surfaces work by redirecting airflow in a specific manner. When the plane is controlled on the ROLL axis by the AILERON, it will enter a bank attitude with one wing higher and one wing lower than the other. When the RUDDER is activated, airflow is directed around it and the plane YAWS. And when the ELEVATOR is used, airflow will push or pull the tail and PITCH the airplane one way or another.

Pitch control:

Pitch control is achieved by using the elevator, which is the movable control at the backside of the horizontal stabilizer. Control around the Lateral (pitch) axis is also achieved to some extent with the throttle. Increasing the throttle in a low-tailed prop plane will "blow the tail down" by blasting air back and over the top surface of the elevator. A reduction in power will also reduce the pressure on the elevator, which will pitch the nose down. This effect of the propellor on the elevator helps the plane to maintain a stable angle of attack and thereby maintain a stable airspeed.

Roll control:
Roll is controlled by ailerons, which are located on the outer trailing edge of the wing. When the ailerons are deflected, the airplane will "roll" by banking. When one aileron is lowered, the camber and the angle of attack increase the lift of that wing and raise it. At the same time, the aileron on the opposite side is raised, the change in camber and angle of attack will lower the lift of that wing and sink it.

Also contributing to bank and turn is adverse aileron drag. This occurs when the wing of the down aileron is dragged backwards, slowing the turn. Adverse aileron drag is compensated for by the rudder, though some aircraft are designed to overcome this imbalance by equalizing aileron drag.

Yaw control:
The rudder is a movable surface attached to the back of the tailfin that is used for yaw control. Like the ailerons, it alters camber and angle of attack on the tailfin, varying the forces on either side of the tail. It is used to offset forces moving the nose of the aircraft from side to side, such as P-forces from the turning propeller.

Since the rudder is located at the far back of the airplane, there is a long lever arm between the rudder and the nose. Small rudder inputs often translate to large movements of the nose. Some planes have shorter tails or a shorter "lever arm" and require greater rudder input. These planes are known as "short coupled" aircraft.

That's about it for control axes. Next up: control and stability!

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