How does an airplane fly? Explain Bernoulli’s Principle. What is downwash?
An airplane flies because its wings generate lift by creating pressure differences and by deflecting air downward.
According to Bernoulli’s Principle, as the velocity of airflow increases, its pressure decreases. Air flows faster over the curved upper surface of the wing, creating lower pressure above the wing compared to the higher pressure beneath it. This pressure difference contributes to lift.
Downwash is the downward deflection of airflow behind the wing. The wing accelerates air downward, and by Newton’s Third Law, the reaction force acts upward on the wing, contributing to lift.
What are the four forces of an aircraft?
Lift: aerodynamic force acting upward
Weight: gravitational force acting downward
Thrust: force that propels the aircraft forward
Drag: aerodynamic resistance opposing motion
What makes an aircraft climb?
Excess thrust
What is lift? Explain the lift formula. As a pilot, what factors can you control in the formula?
Lift is the aerodynamic force perpendicular to the relative wind that opposes weight.
Lift equation:
L = ½ ρ V² S CL
Pilot-controlled factors:
Airspeed (V)
Angle of attack (affects CL)
Configuration (flaps affecting S and CL)
Explain parasite and induced drag. How does the drag curve look for a piston aircraft and jet aircraft?
Parasite drag is unrelated to lift and includes form, skin friction, and interference drag. It increases with airspeed.
Induced drag results from lift production and wingtip vortices. It increases with angle of attack and is greatest at low speed.
Total drag forms a U-shaped curve. Induced drag dominates at low speed; parasite drag dominates at high speed. The shape is similar for piston and jet aircraft.
Slightly pushed up for commercial jet because of wing diameter.
Does the A350 have higher induced drag than the PA28? Why?
For the same lift requirement, the A350 has lower induced drag due to its higher aspect ratio wings and more efficient aerodynamic design.
What is a Boundary Layer? What is the velocity of the Boundary Layer relative to the freestream air? What are the two flows of Boundary Layer?
The boundary layer is the thin layer of air adjacent to the wing surface affected by friction.
Velocity increases from 0% at the surface to about 99% of freestream velocity.
The two types are:
Laminar flow
Turbulent flow
What is the angle of attack? What is the angle of incidence? What is the pitch angle? What is the chord line? What is washout? What is the camber line? Why is washout used?
Angle of attack: angle between chord line and relative wind
Angle of incidence: fixed angle between wing chord line and aircraft longitudinal axis
Pitch angle: angle between aircraft longitudinal axis and the horizon
Chord line: straight line from leading edge to trailing edge
Washout: wing twist where the tip has a lower angle of incidence than the root
Washout reduces the angle of incidence at the wingtips so the root stalls first, keeping aileron control during a stall.
Camber line: line midway between the upper and lower surfaces of an airfoil, showing its curvature
What other part than wings can be considered airfoil?
Stabilator, propellor
What is dihidral & anhedral?
Dihidral: upper inclination of the wing
Improves roll stability
Used on commercial jets and training aircrafts
Anhidral: lower inclination of wing
Improves roll responsiveness & maneuverability
Common on military aircrafts
What is a stall? Is stall speed always the same? What factors affect it? How do we recover from a stall?
A stall occurs when the critical angle of attack is exceeded, causing airflow separation and loss of lift.
Stall speed is not constant. It varies with:
Weight
Load factor
Bank angle
Configuration
Maneuvering
Recovery:
Reduce angle of attack
APply power as needed
Regain controlled flight
What devices in a modern jet could help prevent a stall?
Leading-edge devices (slats / slots)
Trailing-edge flaps
Stick shaker
Stick pusher
Stall warning and protection systems
What is a deep stall and how is it formed?
A deep stall occurs when disturbed airflow blankets the horizontal stabilizer, preventing pitch-down recovery. It is associated with T-tail aircraft at high angles of attack.
Do we have T-tail aircraft for training?
Diamond DA40
Diamond DA42
Piper Seminole
What is a spin? Explain the spin recovery procedure.
A spin is an aggravated stall with autorotation caused by yaw.
Recovery (PARE):
Power idle
Ailerons neutral
Rudder full opposite rotation
Elevator forward to reduce AOA
What are the wing designs?
Rectangular
Tapered
Elliptical
Swept
Delta
Why do large jets have swept-back wings? What devices reduce induced drag?
Swept wings delay the onset of compressibility effects at high speed.
Devices that reduce induced drag:
Winglets
Increased wingspan
High aspect ratio wings
What is a V-g diagram? What can you interpret from it?
A V-g diagram shows the relationship between airspeed and load factor.
It defines:
Structural limits
Stall boundaries
Maneuvering speed (VA)
Explain the concept of stability (longitudinal, lateral, directional) (Static & Dynamic).
Longitudinal: pitch stability
Lateral: roll stability
Directional: yaw stability
Static stability refers to initial response after disturbance.
Dynamic stability refers to motion over time after disturbance.
What is the definition of CG? Do you prefer CG forward or aft? Can CG be changed in flight? How does CG affect stall speed, controllability, maneuverability, fuel consumption?
CG is the point where aircraft weight is concentrated.
A slightly forward CG is preferred for safety and stability.
Forward CG:
Higher stall speed
Greater stability
Reduced maneuverability
Aft CG:
Lower stall speed
Reduced stability
Better maneuverability
CG can change in flight due to fuel burn.
For commercial jets what CG is preferred?
Slightly aft.
Aft CG reduces the forces on the aircraft specifically when manoeuvring. With aft CG, the forces will be less.
What is the definition of CP?
The center of pressure is the point along the wing chord where the total aerodynamic force acts.
It moves forward with increased AOA and aft with decreased AOA.
What is the Mach Number? What is the Critical Mach Number? What is Mach tuck?
Mach number is the ratio of aircraft speed to speed of sound.
Critical Mach is the lowest Mach at which airflow over any part of the aircraft reaches Mach 1.
Mach tuck is a nose-down pitching tendency caused by shockwave movement over the wing.
Which part of the aircraft has its Mach1 number exceed first?
Upper part of the wings
