- The primary purpose of sweep back is to..
o Increase the value of the critical mach number
- How does sweep back work?
o 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?
o Mcrit increased
o Higher economical cruise speed (delays onset of compressibility effects)
o Increased lateral stability (roll)
o ‘Softens’ the onset of the force divergence number
- Disadvantages of sweepback?
o Lower CL max (flatter curve giving higher stall speed)
o Extensive use of high lift devices
o High drag at high AOA (CL max obtained at high AoA, overcome by slots & flaps)
o Use of vertex generators, wing fences to reduce wingtip ‘pooling’
o Prone to wing tip stalling (tips are aft part of the wing and separation point is closer to the leading edge.)
o Taper adds to tip stalling but reduces induced drag.
o T tail makes stalling worse as it provides little or no buffet warning (use stick shaker)
o Sweepback increases drag at low speeds as it gives more spanwise flow than high speeds (due to time).
- A swept wing aircraft pitches up or down at a stall?
o 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…
o A decrease in incidence from root to tip – to prevent wing tip stall
- What devices are used to prevent wing tip stall (spanwise flow)?
o Wing fence
o Saw tooth leading edge (generate a vortex to re-energise the flow)
o Vortex generators (re-energise the flow)
- What happens to the center of pressure in a stall in a swept wing aircraft?
o 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.
Downgoing wing has a relative airflow more from below, upgoing wing has a relative airflow more from above.
- Dihedral is…
o 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
o 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. The area of cross section should increase gradually to a maximum, then decrease gradually. Giving more streamline shape. Max cross-section area should be approx half-way.
- Formula for Aspect Ratio is…
o AR = Span (width) / Chord (length)
- High Aspect ratio (subsonic speeds) gives…
o Better lift
o Better lift/drag ratio
o Less induced drag due reduced wing tip vortices
- What is Mach tuck?
o 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.
- What happens if Mach tuck is not corrected?
o 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…
o A combined yawing and rolling movement
- What is dutch roll and what causes it?
o It is oscillatory instability when the rolling motion is predominant and,
o It is 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.
o A yaw damper is a gyro system sensitive to changes in yaw which feeds a signal into the rudder which then applies rudder to oppose the yaw. With this device, a Dutch roll will not develop because the yaw which triggers it all off is not allowed to develop. It applies the rudder in the correct direction and in the correct amount, thus preventing the slip starting or building up and stopping all rolling tendency Apart from the swept wing, the basic cause of Dutch rolling tendency is lack of effective fin and rudder area.
- What is a stabilising device to prevent dutch roll?
o Yaw Damper
- What is snaking?
o Oscillatory instability when the yawing motion is predominant
- Oscillatory instability is worse when…
At high altitudes
On sweepback aircraft at low IAS
High altitude = High angle of attack = High dihedral effect = High chance of dutch roll.
Approach at slow speeds = High angle of attack = High dihedral effect = High chance of dutch roll.
- The aircrafts aerodynamic ceiling is…
o The point at which the high airspeed mach buffet and low speed stall buffet merge, called coffin corner.
- The manoeuvre envelope must be…
o At an altitude and airspeed sufficient to avoid stalling and slow enough to avoid structural damage
- Gliding angle depends on…
o Ratio of lift to drag ratio – greater weight does not effect gliding angle or range, does effect speed.
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