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1.4 The "Four Forces"

In flight, there are four aerodynamic forces that act on the airplane

• Lift – the upward-acting force
• Weight – the downward-acting force
• Thrust – the forward-acting force
• Drag – the rearward-acting force

These forces are in an equilibrium only when the airplane is in unaccelerated flight. In other words, the sum of all the upward forces (Lift) = the sum of all downward forces (Weight). And, Thrust = Drag.

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1.5 Angle of Attack

The angle that the wing is actually striking the oncoming air mass is referred to as the angle of attack.

The angle of attack can be visualized by the angle between the wing chord line and the direction of the relative wind (the on-coming air).

The wing chord line is an imaginary straight line from the leading edge to the trailing edge of the wing.

The relative wind is the direction of airflow with respect to the wing.

Additionally, Bernoulli’s Principle states that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases. This principle explains what happens to air passing over the curved top of the airplane wing. Since air is a moving gas, one can begin to see how and why an airplane wing develops lift; as the wing moves through the air, the flow of air across the curved top surface increases in velocity creating a low-pressure area above the wing, thus creating lift.

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1.6 Inherent Stability of Airplanes

Most manufacturers design airplanes to be inherently stable. An inherently stableairplane requires less effort to control, because it tends to return to its original condition (position and attitude) after being disturbed by some force, such as turbulence or by the pilot knocking or altering the yoke accidentally.

Longitudinal stability is the quality that makes an airplane stable about its lateral axis. The location of the center of gravity (CG) with respect to the center of lift on the wing determines the longitudinal stability of an airplane.

Airplanes, except for T-tail airplanes, will normally pitch down when power is reduced and the no other control inputs are made. This is because the downwash from the propeller slipstream on the elevators reduced when the power is reduced. As a result, the elevator effectiveness is reduced also. This is what causes the airplane to pitch down.

The center of gravity of the aircraft plays crucial role in determining the stability of the aircraft. When the center of gravity (CG) of an airplane is located at, or rear of, the aft CG limit, an airplane can become unable to recover from stall conditions. Additionally, with an aft, or rear-of-aft CG, an airplane becomes less stable and at all airspeeds.

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1.7 About Stalls and Spins

A stall is the rapid decrease in lift that is caused by the separation of the smooth airflow over the wing. A stall is what occurs whenever the critical angle of attack is exceeded during flight.

An airplane can be stalled at any airspeed, and in any flight attitude in reference to the horizon.

An airplane in a will stall at the same indicated airspeed regardless of altitude, in a specific configuration (for example, with the flaps fully extended). This is because the airspeed indicator is directly related to air density. Also, the angle of attack at which a wing stalls will remain constant regardless of the weight, the way the airplane is loaded, etc.

Spins - A spin is an aggravated stall that results in an airplane descending in a spiral-corkscrew path. In order for an airplane to enter a spin, the airplane's wings must be stalled first. Then, an airplane begins to spin when one wing is "less" stalled than the other wing.

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1.8 Flight in Ground Effect

Ground effect- The result of the interference of the ground (or water) surface with the airflow patterns about an airplane.

An airplane is affected by ground effect when it is within about the length of the airplane's wingspan above the ground, but the effect is most often recognized by pilots when the airplane is less than half the wingspan's length above the ground.

The vertical component of the airflow around the wing is restricted by the ground surface. This alters the wing's upwash, downwash, and wingtip vortices. The reduction of the wingtip vortices due to ground effect alters the spanwise lift distribution, and reduces the induced angle of attack and induced drag. Therefore, the wing will require a lower angle of attack in ground effect to produce the same lift coefficient or, if a constant angle of attack is maintained, an increase in the lift coefficient will result.

Ground effect may cause an airplane to float on landings, or permit it to become airborne with insufficient airspeed to sustain level flight at altitudes above the area of ground effect.
Also, an airplane may settle back to the ground abruptly after flying through the ground effect area if the pilot has not attained recommended takeoff airspeed.

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1.9 How Airplanes Turn

For an airplane to turn, a portion of the total lift must be transferred into a horizontal component of lift.

The horizontal component of lift makes an airplane turn.

A pilot uses coordinated aileron, rudder, and elevator to bank the aircraft and attain this horizontal component of lift.

The rudder on an airplane controls the yaw (the rotation about the vertical axis), the rudder does not cause the airplane to turn.

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1.10 About P-Factor and Torque

P-factor- A tendency for an airplane to yaw to the left due to the descending propeller blade on the right (as seen from the rear) producing more thrust than the ascending blade on the left.

P-factor (asymmetric propeller loading) causes the airplane to yaw to the left when at high angles of attack. P-factor is greatest at low airspeeds, high angles of attack, and high power settings; it is quite noticeable during takeoff.

The torque effect (another left-turning tendency of airplanes) is also greatest at low airspeed, high angles of attack, and high power.

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1.11 Load Factor Explained

Load factor- During turns, the additional weight that must be supported by the aircraft structure and wings due to the airplane's weight PLUS the centrifugal force created.

The amount of excess load that can be imposed on an airplane's wings varies directly with the airplane's speed and the excess lift available. An increased load factor will result in an airplane stalling at a higher airspeed.

1. At low speeds, very little excess lift is available, so very little excess load can be imposed.
2. At high speeds, the wings' lifting capacity is so great that the load factor can quickly exceed safety limits.

As bank angle increases, the load factor increases.
The wings not only have to carry the airplane's weight, but the centrifugal force as well.

NOTE: During your exam, the chart above is given with the bank angle in degrees on the horizontal axis (along the bottom of the graph), and the load factor in G units on the vertical axis (up the left side of the graph). Additionally, notice that there is a table that provides the load factor corresponding to specific bank angles which can be found on the left side of the chart. Use this table to answer the load factor questions given.

To compute the load factor, multiply the airplane's weight by the load factor which corresponds to the given angle of bank.

EXAMPLE:
If an airplane weighs 2,000 lb., what weight would the airplane's wings be required to support in a 60° bank?

To solve:
First, find the 60° angle of bank of the table. Note that for a 60° angle of bank a load factor of 2.000 applies.
Then, simply multiple the 2,000 lbs. by the load factor of 2.000. So, (2000 lbs. x 2.000 = 4000 lbs.).

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Lesson 1 - Aerodynamics of Flight eFlash Cards

Lesson 1 - Aerodynamics of Flight Study Quiz