Flight Mech Conceptual questions

What determines the forward CG limit?

The elevator - must have enough aerodynamic authority to raise nose (epecially during landing)

Which parameters affect the location of forward CG limit?

Wing flaps - Deploying flaps increases nose down pitching moment, requires nose up control authority

Nose gear load - CG should be further back depending on how heavy the nose is

What is the difference between "stick free" and "stick fixed"?

Stick fixed - PIlot is in control of the elevators

Stick Free - Pilot is not in control. Elevators align with the flow of oncoming air

Is stick free or stick fixed more restrictive to the CG limit?

Stick Free - Elevator being free to move with the wind, reduces stabilizing capability

What are five conditions/scenarios that a vertical tail must overcome?

1 - Engine Failure: VT must overcome asymmetrical thrust

2 - Crosswind landing: VT must manage heavy sidewind to align with runway

3 - Lateral gusts/Heavy side wind

4 - Spin recovery: VT must provide enough yaw to break spin

5 - Adverse yaw: VT must withstand pilot inputs to the aileron to roll aircraft

Given the following two planes, one with upper wing and the other is with a lower wing. Which one should have a larger dihedral angle to ensure static roll stability?

Lower wing - fuselage creates a destabilizing rolling moment. A high wing is inherently stable bc of high pressure zone under the wing

Explain the difference between downwash and sidewash. On which components of the airplane each one of them affects?

Downwash - affects downward angle of wing. primarily affects HT. Generally caused by winglift generation

Sidewash - affects the side angle. Primarily affects VT. Generally caused by fuselage interference

Define the static of margin, S.M.? Which of the three static margins is suitable for a commercial airplane: 1%, 10%, 50%?

Point between CG and NP. Expressed as a percentage of the chord.

10% is ideal for commercial

1% more so for fighters. Unstable with high maneuverability

50% not ideal for any plane

Suppose we want to change the angle of attack from a1, which is an equilibrium state, to a higher angle a2 at a new equilibrium state. Show schematically the old and new stability curves.

Pilots should deflect the elevator upward.

Explain how roll creates yaw

Through adverse yaw

Aileron input - when aileron goes down to raise wing, the other goes up to counteract

Drag imbalance - raising wind creates lift the dragging wing creates drag

What is the Kutta condition? What is its physical meaning?

a boundary condition applied in potential flow theory to calculate the lift of an airfoil

air flowing over the top and bottom surfaces of a sharp-trailed airfoil must leave the trailing edge smoothly

What is a centrogram? Plot a centrogram of an airplane that is statically stable in pitch

a design chart used in aircraft weight and balance engineering to track how CG shifts as the aircraft's weight changes during a flight

What is the difference between static and dynamic stability?

Static - initial tendency of the aircraft to return to its original equilibrium state immediately after being disturbed

Dynamic - The behavior over time of the aircraft's motion

What are the advantages of a canard?

Stall safety - Canard is designed to stall before main wing, naturally drops the nose

Lift efficiency - extra lift from the canard

What are the disadvantages of a canard?

Higher landing speeds - The nose down pitch is too much

Directional instability - a far forward wing requires a heavy VT to compensate

What is C_NB and when is it most stable

How well the plane resists yaw

The bigger the tail, the more positive the number (C-17, Antonov, B777)

What is C_LB and when is it most stable

How strongly an aircraft wants to roll when experiencing side slip

The more dihedral or swept back, the more negative the number (B737, A350)

Low wings are unstable, positive number