Aerodynamics Flashcards
Settling with Power
Not aerodynamic
Insufficient power to stop a descent
Too hot and high, wind shear
Vortex Ring State
Requirements:
- Powered flight
- High rate of descent (>500 fpm)
- Low airspeed (<20 mph indicated)
Purely aerodynamic condition. Helicopter descends into it’s own vortex which is “recirculated” as the helicopter descends faster than the vortex descends and results in the recirculated air being drawn up, then almost vertically down into the rotor.
Causes a critical increase in the angle of attack and corresponding loss of lift –> helicopter descends up to rates of 6000 ft/min.
Retreating Blade Stall
Angle of Attack of retreating blade is higher to compensate for lower blade tip speed (against wind). Above VNE this can cause the tip of the blade to stall.
- Vibration, nose pitches and rolls to direction of retreating blade
Jack Stall
Servo transparency
Flight controls stiffen from abrupt manoeuvres or low G manoeuvres.
Dynamic Rollover
Rolling of helicopter around a pivot point. Must be anchored to the ground in some way and light on the skids.
Caught on a rock, tie down, lateral cyclic during takeoff
Ground Resonance
Frequency of blades matches natural frequency of aircraft on landing and can cause destruction. Lift off and land again. Only with 3 bladed systems.
Over-pitching
Pilot rapidly increases collective considerably and the engine cannot produce enough power to overcome the large, swift increase in drag on the rotor system –> rotor system quickly slows down and loses efficiency causing the helicopter instantly to sink
LTE
Loss of tail rotor effectiveness.
Critical, low-speed aerodynamic flight characteristic –> can result in an uncommanded rapid yaw rate which, if not corrected, can result in the loss of aircraft control
Can be caused by wind from:
- 285-315 > main rotor disc vortex interference (vorticies that interfere with tail rotor)
- 210-330 > tail rotor vortex ring state
- 120-240 > weathercock stability
–> unanticipated yaw occurs near the pedal peaks when wind is coming from left or right. (If wind is from the left, note that pedals will not be close to a stop). If not enough left pedal is applied fast enough, an endless right yaw might develop.
Mast Bumping
Occurs when blades flap excessively and come in contact with the tail boom (i.e. in severe turbulence, often in startup or shutdown).
Can also occur in flight with low G manoeuvres (i.e. in a mountain wave where aircraft descends faster than gravity, and weight of fuselage is “disconnected” from blades)
Critical Wind Azimuth
In hover:
Wind is blowing on nose: somewhat less power pedal will be needed –> increased aerodynamic efficiency of the main rotor requires less tail rotor thrust.
- Wind from left: even less power pedal because wind complements tail rotor thrust.
- Wind from right/rear (50-210 degrees): CRITICAL WIND AZIMUTH — reduces efficiency of tail rotor and requires more left pedal than the no-wind condition
* will run out of left pedal to maintain hover at this heading in 40kt wind
