Relationship of Speed to Angle of Attack

Question 1

Coefficient of lift

Answer 1

● Maximum coefficient of lift for an airfoil is constant!
● It has nothing to do with the density of the air, weight, or anything else.
● Our max coefficient of lift always happens at a very specific angle of attack, and the angle of attack that gives us this value will not change for that specific airfoil

Question 2

AoA: Constant Altitude

Answer 2

● Assuming our airplane will keep a constant altitude, we find that with a decrease in airspeed the following things will happen:
○ Lift will initially decrease.
○ The angle of attack must be increased to maintain the altitude and to maintain the same amount of lift.
○ Induced drag increases.
■ (More downwash and greater vortex generation!)
○ The slower we fly, the greater our induced drag will become

● Assuming that we attempt to maintain a constant altitude, with an increase in airspeed we will find the following:
○ Lift initially increases.
○ The Angle of Attack must be decreased to prevent a climb.
○ Induced drag decreases with the decrease in angle of attack.
■ (However, parasite drag will increase!)
○ The faster we fly, the greater our parasite drag will be

Question 3

Best Glide

Answer 3

○ The average airspeed for best lift/drag ratio for the aircraft.

○ With a strong headwind, increasing airspeed slightly beyond best glide speed will improve our ground speed and although our sink rate increases – glide range improves.
○ The opposite holds true for a strong tailwind; Reducing airspeed slightly below best glide speed, will decrease our sink rate, and take advantage of the tailwind for a longer period of time – improving range

Question 4

Sensors

Answer 4

● An aircraft stalls at a constant angle of attack.
○ For most aircraft, this will be in the range of 16-18 degrees AoA.
● Types of Alpha Sensors:
○ Vane Sensor
○ Pressure Sensor

Alpha Sensors: Vane Sensor
● Measures angle of attack.
● These are typically found on one of the sides of the aircraft’s nose.
● It sends angle of attack information to the flight computer

Pressure Sensor
● Pressure sensitive holes are installed both above and below the leading edge of a wing.
● Another method is to install two pressure ports into a pitot or pitot-like tube.
● This difference in pressure can be converted into angle of attack information

Question 5

Force Couples

Answer 5

● Force couples are two forces equal in magnitude but acting in opposite directions to each other.
● Lift and weight are force couples in unaccelerated flight.
○ Lift force acts through the Centre of Pressure.
○ Weight force acts through the Centre of Gravity.
● Thrust and drag are also force couples in unaccelerated flight.
○ Thrust acts through the propulsion system.
○ Drag acts through the Centre of Pressure parallel to the relative airflow

-centre of pressure is average point of pressure

Question 6

Horizontal Stabilizer

Answer 6

● The horizontal stabilizer is typically a symmetrical aerofoil that is installed in the path of the downwash from the wings.
● Provides a downward force to keep the nose up.
○ The CofP behind CofG wants to push the nose down and the horizontal stabilizer offsets this.
○ Downwash creates an upside down angle of attack that pushes the tail down and keep the nose up

Question 7

Speed Versus AoA

Answer 7

● To reduce our speed–or as the speed decreases:
○ Centre of pressure moves forward.
○ Less downward force is required on the horizontal stabilizer.
○ Angle of attack must be increased for level flight