Aerodynamics Flashcards
Why do jet aircraft climb at constant IAS and then transition to constant Mach
number?
At lower levels jet aircraft is limited with the Vno.
Climbing at constant IAS = increase in TAS & MN.
At changeover level (often between FL260 and 280) climb at
constant MN (CAS and TAS will reduce in the climb).
This is because at higher level the aircraft is limited with a Mmo
What is the purpose of winglets?
Winglets are aerodynamic efficient surfaces located at the wing tips.
Designed to reduce induced drag & increase the range.
Reduce the strength of the tip vortex
Dispense (distribue) the spanwise airflow from the upper and lower surface
=> preventing the intermixing of these airflows that otherwise would create induced drag vortices.
Wing tip stall is also delayed…
Describe a wing of a jet?
The wing of a jet is designed thin, with minimal camber, and swept wings.
How do swept wings work?
The swept wing increases its Mcrit speed.
Distance covered by the airflow travelling from the leading to the trailing edge is increased.
=> Its acceleration is more gentle as it rises over the thickest part of the wing.
=> The airflow will become sonic later
What are the advantages/disadvantages of a swept wing?
+ High mach cruise speeds (higher Mcrit)
+ Stability in turbulence (due to poor lift qualities, it is more stable in turbulence, the swept wing is less responsive to updraughts)
- Poor lift qualities
- Higher stall speeds
- Speed instability (poor lift at lower speeds, below VMD and above VMC / VMD = Induced Drag is equal to Parasite Drag)
- A wing-tip stalling tendency is particular for a swept wing aircraf
What is a superstall?
= Deep stall. Aircraft unable to recover. Reduction or loss of elevetor authority (T-tail)
What are the advantages/disadvantages of a T-Tail?
+ Increase elevator efficiency.
High elevator effectiveness is required for high lift wings often mounted above the fuselage, thus the tail has to be out of the downwash
- Possibility of deep stall
Why some aircraft doesn’t need a dihedral
Because the CG is located beneath the wing
Explain manoeuvring speed?
This is the highest speed at which full flight controls deflection can be made without exceeding the limit load factor.
Up to Va if the load factor is exceeded the aircraft will stall first.
At speeds above Va, high load factors could lead to a structural damage.
Va varies with aircraft weight.
What do spoiler do?
Spoilers consist of opening panels that extend from the upper surface of the wing and spoil the airflow over the wing => reducing lift.
Roll control: spoilers are raised on one wing only to create an imbalance of lift that produces a rolling moment => More efficient roll control than ailerons.
As speed brakes: they are raised symmetrically on both wings to a flight detent position, which cause a large increase in drag.
As ground lift dumpers: they are raised symmetrically resulting in a large increase in drag and causing the aircraft to sink to the ground.
What types of high lift devices are there?
- Trailing edge flaps (Fowler flaps) increase lift at lower level of deflection.
- Leading edge falps (Krueger flaps) and slats increase lift by creating a longer wing chord line, camber and area.
- Slots delay the separation of the airflow and therefore produce an increase of CLmax.
Leading flaps: increase stalling angle
Trailing edge flaps: decrease stalling angle
How do slats work?
Slats are aerodynamic surfaces on the leading edge of the wings
Allow the wing to operate at a higher angle of attack. This create a longer wing chord line, area and give a higher lift coefficient.
Slats re-energise the boundary layer and the aircraft can fly at slower
speeds, take-off and land in shorter distances.
How do leading edge flaps work?
Leading edge flaps increases the leading edge camber resulting in an increase of Clmax and delay separation.
Krueger flaps is present on some Boeing aircraft, to promote root stall on a
swept wing
Krueger flaps are used on the inboard.
What is DutchRoll?
Dutch Roll is an oscillatory instability associated with swept wing aircraft. It is a combination of rolling and yawing motions.
When the aircraft yaw is introduced, the strong “dihedral effect” will roll
the aircraft due to the lift increase on the wing into wind.
The increased induced drag on the rising wing will yaw the aircraft in the opposite direction.
Dutch Roll occurs when the lateral stability is predominant over directional stability
What is a Yaw Dumper?
Yaw Dumper is a system introduced to prevent Dutch Roll and coordinate turns.
This is a gyro system that is sensitive to changes in yaw and it feels the signal into the rudder
Applies opposite rudder to the yaw before the roll occurs
