General Questions Flashcards
Wave drag is
• The separation of the airflow behind the shock wave
A shock stall is the result of
• The boundary layer behind the shock wave becoming turbulent and separating, spilling rearwards and striking the tail plane, creating buffet and rearward CP movement. Rear movement of the CP causes nose pitch down
Wing design for high speed flight takes into consideration
- Minimal camber to delay shockwaves
- Maneuverability
- Low thickness chord ratio
The primary purpose of sweep back is to
• Increase the value of the critical Mach No.
How does sweep back work
• By sweeping the wing the freestream air that travels along the effective chord is less therefore less acceleration is achieved resulting in a lower speed over the wing and a higher achievable aircraft speed before Mcrit is reached.
Advantages of a swept wing?
- MCRIT increased.
- Higher economical cruise speed
- Increased lateral (roll) stability
Disadvantages of sweepback
- Lower Cl
- Extensive use of high lift devices
- High drag @ high AOA
- Use of vortex generators, wing fences to reduce wingtip ‘pooling’
A swept wing aircraft pitches up / down at a stall
• A nose pitch up results from the wing tips stalling first moving the CP inwards and forwards (wash out is used to try and prevent tip stalls)
Wash out is
• A decrease in incidence from root to tip – to prevent wing tip stall
What devices are used to prevent wing tip stall (spanwise flow)
- Wing fence
- Saw tooth leading edge
- Vortex generators
What happens to the center of pressure in a stall in a swept wing aircraft
• Tips will stall first so CP moves inward and forwards & nose tends to pitch up
During a turn what happens to the CP of a swept wing Aircraft
• As the wing gets higher in a turn the outer portion become higher than the inner portion which creates its own form of washout resulting in a lower AOA and causing the CP to move inwards and pitch the nose upwards.
Dihedral is
• The upward inclination of the wing to the lateral axis to provide lateral (roll) stability
Anhedral is
• Negative dihedral – usually with high mounted swept wings to combat dynamic instability (dutch roll)
Area Rule
This is a design function to ‘blend’ areas where wings, tail, join the fuselage to minimise the increasing and decreasing cross section, minimizing the amount of drag formed by shockwaves
Aspect ratio
• = span (width) ÷ chord (length)
High Aspect ratio (subsonic speeds)
- Better lift
- Better lift/drag ratio
- Less induced drag due reduced wing tip vortices
Mach tuck
• Is when the aircraft is accelerated through the transonic range causing the CP to move rearwards and increasing the lift generated by the tail plane due to modified airflow from the wing causing a nose pitch down.
If Mach tuck is not corrected the result will be
• The nose pitch down causes further speed increase which causes further movement rearwards of the CP which causes further nose pitch down…..etc
Oscillatory instability is
• A combined yawing and rolling movement
Dutch roll is
• Oscillatory instability when the rolling motion is predominant
Dutch roll is a result of
• A yaw to the left or right which makes the outside wing speed up producing more lift resulting in a roll, after which because of the greater exposed area of the faster wing it has a higher drag component therefore causes a yaw in the opposite direction, resulting in a roll in the direction of the yaw
A stabilising device to prevent Dutch roll is
• A yaw damper (gyroscopically operated stabilisers)
