Module 4 Transonic Flight Flashcards
Why can shockwaves form on control surfaces?
The local airflow speed can increase when a surface deflects, so normal shockwaves can form on control surfaces of a transonic aircraft
Give an example of a shockwave formation on a control surface
The shockwave would form at the hinge area of a control surface first, and then the shockwave will move rearward on the control surface with an increase of Mfs. The pressure will be increased behind the shockwave, which makes the controls feel heavy, aerodynamic force experienced on the control surface may be to strong for the pilot to control.
What are some of the effects of shockwaves over control surfaces?
The change of pressure distribution causes ineffectiveness of the control surface or can even produce the opposite effect from initial control intention. (adverse stick force)
What is a longitudinal control issue when a shockwave forms over a wing of an aircraft?
Aircraft may experience a nose down pitch, the nose becomes heavy due to the shockwave formed on the wing. Also called mach tuck. When the Mfs increases and shockwave moves rearward the CP moves rearward away from the COG this helps to intensify the nose down pitching moment.
What are the further consequences of mach tuck?
the nose down motion of mach tuck causes a further acceleration of the aircraft increase Mfs and further worsening the control difficulties.
What are some design options for transonic elevators that can be used to deal with control difficulties?
Thin tail plane
- Design to increase Mcrit of elevator and delaying formation of shockwave.
Use of Mach trim system
- Provides control with aerodynamic forces in other direction to correct for adverse stick force
All movable slabs of elevator
- Can provide pilots with more choices of actions in longitudinal control to reduce effect of shockwaves
An adjustable and power operated tail plane
- To overcome extra forces on the elevator
What are some lateral control issues in transonic flight?
Shockwaves forming on the ailerons:
Alerions cannot operate effectively, since the pressure increases behind the shockwave
Can cause flutter and vibrations
Aircraft can go into roll condition due to the varying lift from the shockwaves.
Lift in the opposite direction, “alerion reversal”
what causes alerion reversal?
When greater lift is produced by the deflecting alerion. The lift will twist the wing about its lateral axis, if the wing is not rigid enough it will change the aoa in the opposite direction, therefore lead to roll in the opposite direction.
What are design features associated with a wing and its alerions to deal with lateral control difficulties?
Vortex Generators: Delay the formation of shockwaves, and re energize the airflow over the control surfaces.
Small outboard alerions: requires less lift to produce roll moment and banking, which will reduce deflecting angle.
Inboard Spoilers: Which disrupt air flow to reduce the lift on the roll downward wing
What is the directional transonic control issues?
When shockwave forms on the hinge of the rudder in a transonic flight, directional control is effected.
What can be experienced when a shockwave forms on the rudder?
Ineffective rudder
Rudder feels heavy to move as the shockwave moves rearward
Can cause oscillation yawing disturbance like Dutch roll if directional stability is weak.
What are design features to assist the directional stability control in a transonic flight:?
Yaw dampeners: to reduce directional oscillation and eliminate Dutch roll
Conventional fin and rudder with combination of power
All moveable slabs, fin/rudder on the vertical stabilizer
What is the high speed limit?
Airspeed where the aircraft starts to mach stall, caused by strong normal shockwaves and separation point behind the shockwave. Formation of a shockwave increases the locoal pressure and causes boundary layer separation of the turbulent wake this can cause a complete loss of lift and significant increase in drag.
What is the low speed limit?
Aircraft is at low speed and stalling aoa, caused by boundary layer separation over the aero foil and air pressure distribution, which produces lift over the aerofoil is interrupted and destroyed.
What is coffin corner?
The point were high speed limit and low speed limit intercept at a certain altitude.
