Webmaster

BRAZOS BUZZARDS R/C CLUB

GRANBURY, TEXAS

SOME VERY BASIC AERODYNAMICS

CHARLES PEACOCK

There are four forces that effect an airplane in flight, and they act in pairs. Lift opposes weight, and thrust opposes drag. To get your aircraft to behave like you want it to, you've got to manage these four forces. Understanding them makes it easier.

Lift

Lift is the upward force produced by a wing as it moves through the air. It's the force that counteracts the gravity of an aircrafts weight. Ask engineers how a wing produces lift and they'll go on about circulation theory, the shape of the wing, and "Bernoulli's Theorem." They'll talk your ear off but the most useful information is reduced to this. A wing keeps an airplane up by producing lift due to airfoil and angle of attack. The angle at which a wing meets the air is called the angle of attack or AOA. This is not the angle between the airplane's nose and the horizon. It's the difference between where your wing is pointing and where it's actually going. You can control the amount of lift a wing generates by adjusting two things: speed and AOA. To produce a certain amount of lift at low speeds, a large angle of attack is required. To produce the same amount of lift at high speeds, less angle of attack is required. If the speed is very low, the angle of attack you'll need to maintain lift will be so large that at a certain point (critical angle of attack) the air cannot flow smoothly over the wing, and the wing will stall. You can also add lift by extending flaps. With the flaps extended they deflect air down To force the air down, an equal upward force is produced on the flap increasing lift.. Flaps also cause an increase in drag.

Weight

Weight opposes lift  For your ship to fly, the wings must develop enough lift to counteract it's weight. The "real" weight of your aircraft changes only as fuel is used up. But changes in "apparent weight"  are caused by maneuvering. For example, a level turn with a 60-degree bank puts a 2-G load on the plane.. Both seem to weigh twice as much as they do when in straight-and-level flight - and in a way they do - because of the increase in "apparent gravity." During maneuvers, you have to adjust the amount of lift to compensate for the changes in weight caused by G-forces. To stay level during a steeply banked turn, for example, you'll need to raise the nose slightly (increase the AOA) and add more power (thrust) to produce more lift to balance you out.

Thrust

Thrust is the forward force provided by an airplanes propeller, and is opposed by drag (the resistance of the air as the airplane moves through it).An airplanes propeller creates thrust in the same way it's wings create lift: air is deflected backward, so the propeller (and the aircraft) move forward. The more powerful the engine (and bigger the propeller), the greater the thrust, and the faster the airplane can fly. Thrust is also the most important factor in determining a planes ability to climb.

Drag

Drag is the rearward-pulling force that opposes thrust, and has two components: "parasitic drag" and "induced drag". Parasitic drag is caused by friction between the air and an airplane's structure. The more things there are sticking out into the airflow, the higher the parasitic drag. Your plane is designed to have as little parasitic drag as possible, but the faster you go, the more there will be. Parasitic drag increases with speed. As the angle of attack increases, lift pulls an airplane upward and backward. The upward component of lift is called "effective lift"; the backward component is called "induced drag". Effective lift counteracts weight to keep the airplane flying. Induced drag counteracts thrust and slows the airplane down. The slower you go (the bigger the angle of attack), the greater the induced drag. Eventually you'll need to add more power to generate the lift necessary to remain aloft.

What causes left yaw during takeoff (as well as all other times)?

Four phenomena:

P factor - Each propeller blade produces a certain amount of thrust. When a tail dragger airplane is on the takeoff roll, as long as the tail is down, the downward-moving propeller blade  has a higher angle  of attack and produces more thrust than the upward-moving blade. The result is "P factor" - asymmetric propeller loading that creates a yawing motion to the left for clockwise (from the cockpit) rotation of the prop. This effect disappears once the tail lifts and the aircraft is level still on the takeoff roll. This requires you to reduce right rudder when the tail lifts.  Tricycle gear aircraft are easier to control on takeoff as this factor is not present This is the major factor of the four. 

Torque - When the engine turns the propeller in one direction, there is an "equal and opposite force" that makes the plane roll in the other direction. When your throttles high but your airspeeds low (as during takeoff), the plane will try to roll in a direction opposite to the rotation of the prop resulting in a yaw to that direction. This effect is most pronounced during acceleration as the torque increases with engine rpm faster than the airspeed increases..

Spiraling Slipstream - A propellers spiraling slipstream one side of the tail and causes the nose of the plane to yaw in the same direction the reactive force causes it to roll.

Gyroscopic Precession - Because it's big and spins rapidly, your planes propeller behaves like a gyroscope. This makes it subject to the effects of "gyroscopic precession". When a force acts on a gyroscope, the gyroscope behaves as if the force were applied at a point 90 degrees to the direction of rotation. If your planes propeller turns clockwise (viewed from cockpit), then when the tail comes up on the take-off run the nose goes down - and the gyroscopic precession makes the plane swerve to the left