Flight Performance
Left Turning Forces
There are four basic forces that will cause an aircraft to turn left while in flight when the pilot intends for the airplane to fly straight. These are:
Torque
Asymmetric Thrust
Precession
Slipstream
Torque
Torque is a force created by the spinning propeller. In Newton's Third Law we are told that every action has an equal and opposite reaction. Torque is the reaction to the spinning propeller. The propeller rotates clockwise (or to the right), as seen by the pilot in the cockpit. The airplane reacts to this rotating by attempting to rotate counter-clockwise (or to the left). This causes the aircraft to yaw left.
Asymmetric Thrust
Asymmetric Thrust, also known as "P Factor", has an effect at high angles of attack and at high power setting. At these times the descending blade of the propeller (which is on the pilot's right side) has a greater angle of attack than the ascending blade. Due to the higher angle of attack on the right side of the airplane, more lift is created on the right side of the airplane, causing the aircraft to yaw to the left.
Precession
The concept of precession has to do with gyroscopes, and the properties of a gyroscope. A gyroscope is basically an instrument that is constructed so that three discs spin on each plane (like the three axis) and are able to remain spinning in the exact same way. One property of a gyroscope is something called "rigidity in space", which means that a gyroscope does not like to change the way that it is rotating.
The propeller can act like a very simple gyroscope, because it is spinning. If an airplane changes very suddenly from nose up to nose down, the effect of precession will cause the aircraft to yaw left. This is because of the propeller acting like a gyroscope.
Slipstream
The propeller pushes air backwards in a spiral or "corkscrew" motion. This spiral causes a high pressure area on the left side of the tail and a low pressure area on the right side of the tail. Because of the different pressure areas, the tail is pushed from the high pressure area towards the low pressure area (from the left to the right). When the tail moves right, the nose yaws left.
Climbing
The elevators cause the airplane to pitch up and down, and by doing this divide the energy of the engine into speed and altitude. The pilot can cause an airplane to move only forward, and travel at a very high speed, or the pilot can cause the aircraft to climb very steeply, but this slows the forward movement of the airplane.
The best rate of climb is the angle of climb that will allow the airplane to gain the most altitude in the least amount of time. This angle is normally used on take off. You may see it abbreviated as "Vy".
The best angle of climb is the angle of climb which will gain the most altitude in the least distance over the ground. It is used for climbing out of airports that have obstacles in the way. You may see it abbreviated as "Vx".
Gliding
Gliding is flying without an engine. When an aircraft glides, the force of thrust is no longer a factor in the forces equation. Equilibrium is maintained by lift, weight and drag only. The "angle of attack", or gliding angle determines how fast the aircraft will be moving.
Turns
For an aircraft to turn, several things must happen. First, the wings are rolled using both aileron and rudder so that the aircraft is tilted over at an angle. When this happens, the imaginary line that lift works through is also tilted away from vertical. Since lift is no longer acting vertically upwards, the amount of lift used has to be increased to keep the aircraft from descending. (see diagram)
When the angle of bank is increased in a turn, the properties of the turn change. The steeper the angle of bank: the greater the rate of turn, the smaller the radius of the turn, the higher the stalling speed, and the greater the loading on the aircraft. Increasing the airspeed in a turn also has an effect on the properties of a turn. The higher the airspeed: the slower the rate of turn, and the larger the radius of the turn.
When an aircraft is flying straight and level, the load factor, as you will remember from section 2.4, is equal to 1. In a banked condition, centripetal and centrifugal turning forces act on the airplane and increase the loading. The steeper the angle of bank, the greater the loading. At an angle of 60? the load factor is 2, which means that the aircraft has "weight" of twice what it weighs on the ground. To find out the weight of the aircraft at a specific angle of back, use the load factor chart.
Stalls
"Stalling" in an aircraft is referring to the amount of lift that the wings are producing, and not to the engine stopping. A stall happens when the wing can’t produce enough lift to balance the weight of the airplane.
In normal flight air flows smoothly over the wings of the aircraft, producing lift. If the angle of attack is increased, it becomes difficult for the air to flow smoothly over the wings of the aircraft. Near the leading edge of the wing the air will continue to flow smoothly over the surface of the wing, but as it approaches the trailing edge the airflow becomes turbulent or "burbles". This is called the "separation point". As the angle of attack increases, the separation point moves forward, towards the leading edge of the wing. At a certain angle of attack, the lift being produced by the wings is no longer enough to keep the airplane flying, and the airplane stalls.
An aircraft will always stall at a certain angle of attack. It is difficult to judge the angle of attack, and most small aircraft don't have an angle of attack indicator. To help a pilot recognise when an aircraft is approaching a stall, the manufactures of airplanes publish the airspeeds that match the angles that the aircraft stall at. Pilots are also taught to recognise the symptoms of an approaching stall.
As an airplane approaches a stall, a light shaking or buffeting can be felt as the separation point moves closer and closer to the leading edge of the wing. As the angle of attack continues to increase, the ailerons stop being effective and lateral control is lost because the air no longer flows smoothly enough over the ailerons for them to function. Finally, the wings stop producing lift, the aircraft stalls, and the nose drops.
To recover from a stall, the nose must be lowered to increase airspeed and allow for air to flow smoothly over the wings.
An airplane that is properly loaded (does not have too much cargo) will stall at an indicated airspeed very near the speed published by the manufactures. This airspeed does not change, even with altitude.
An airplane will stall if the critical angle of attack
is exceeded, at any airspeed, and at any attitude.
Factors Affecting Stall
Turns:
As the angle of bank increases, the amount of lift required to keep flying is increased because the load factor increases. To increase the amount of lift that the wings are producing, the pilot must increase the angle of attack. The airplane, moving at a higher airspeed, will stall earlier in a turn than in straight and level flight. To find the speed that an aircraft will stall at in a turn, multiply the normal stalling speed by the square root of the load factor in that turn. See the chart below.
Degree of bank Load Factor Square root
15 degrees 1.04 1.02
30 degrees 1.15 1.07
45 degrees 1.41 1.19
60 degrees 2.00 1.41
75 degrees 3.86 1.96
Example: The stalling speed of a glider in level flight is 31 miles per hour. In a 60degree banked turn:
31mph x 1.41 = 43.71 So the glider will stall at an indicated airspeed of 43.71 mph in a 60 degree banked turn.
Centre of Gravity
Depending on where the weight inside of the aircraft is, the centre of gravity will move. If the centre of gravity is moved as far forward as possible, the stall speed will increase. If the centre of gravity is as far backwards as possible, the stall speed will decrease.
Weight
Adding weight to the aircraft forces the aircraft to produce more lift to maintain flight. To do this the angle of attack must be increased, and so the airplane will stall at a higher airspeed.
Turbulence
Turbulent air can change the way that the aircraft is moving through the air, and may also temporarily change which direction the relative airflow is from. This may cause the angle of attack to be greater than the critical angle, and the airplane to stall.
Flaps
Flaps increase the lifting effect of the wings and lower the stall speed.
Snow, Frost, Ice, Dirt
Any substance clinging to the wings will cause the airflow over the wings to be less smooth, and less lift to be produced. This will cause the stall speed to be higher.
Spinning
If a stalled airplane has one wing that is more stalled than the other, a wing may drop. Turbulence may also cause one wing to drop. This will cause a spin, also called autorotation. Spins may also be purposely entered by using rudder during a stall, in the direction that the pilot wishes the aircraft to spin.
In a spin, the airplane follows a corkscrew path towards the ground. Speed will increase to a certain point, and then stabilise. The aircraft is normally pitched down at a steep angle. The load factor is 1. A spin has three stages: the incipient stage, the developed stage, and the recovery. In the incipient stage, the aircraft stalls and rotation begins. In the developed stage the aircraft is moving almost straight downwards and the airspeed is stable. In the recovery the pilot follows the steps to return to normal flight.
In a spin, both wings are stalled, so any use of ailerons to try and recover will only make the spin worse. For most recoveries, power should be brought to idle (if in a powered aircraft), and ailerons held at neutral. Rudder in the opposite direction of the spin is held until the spinning motion stops. Then the control column is moved forward to lower the nose and unstall the aircraft, the wings levelled, and the aircraft returned to normal flight.
Spiral Dives
A spiral dive is much different from a spin. A spiral dive is a very steep descending turn. Airspeed and loading increase rapidly, and the angle of back is very steep. In a spin it is important to recognise and recover smoothly to avoid doing damage to the aircraft. The throttle should be closed (if in a powered aircraft) and the wings levelled smoothly. The aircraft is probably in a dive at this point, and the dive should be smoothly recovered to normal flight.