Aerodynamics And Dangerous Situations Flashcards
1
Q
Transverse flow effect
A
- As aircraft accelerates into forward flight, front portion of MR disc is more efficient due to clean air (difference in lift between the fwd and aft portions)
- As the aircraft moves forward, the air flowing through the MR disc becomes more horizontal then vertical. The horizontal airflow increases the lift in the front of the MR disc. This gain is felt 90 degrees later in the plane of rotation (gyroscopic procession) which cause a slight right roll
- Occurs at 10-20 KIAS (recognized by right roll)
- Less induced flow in forward disc area- increases AOA= more lift
- More induced flow in aft disc area- decreased AOA= less lift
- gyroscopic procession: a force applied to a rotating body will occur 90 degrees later in the plane of rotation
- Due to gyroscopic procession, the increased lift of the forward disc, takes effect 90 degrees later
- results in right roll
- nose pitch up
2
Q
Transitional lift
A
- Improved rotor efficiency resulting from directional flight
- Horizontal airflow reduces vortices and induced flow
- forward movement or wind - As airspeed increases, rotor efficiency increases
- Tail rotor efficiency also increases
3
Q
Effective translational lift (ETL)
A
- Occurs between 16-24 KIAS (when transitioning int fwd flight)
- Rotor is no longer effected by vortices and utilizes relatively undisturbed air (clean air)
- Induced flow is effectively minimized (reducing induced drag):
- same AOA will produce more lift - When passing through ETL, the nose will rise and yaw left, requiring additional forward cyclic and right pedal
4
Q
Dissymmetry of lift
A
- Unequal lift across the rotor in forward flight:
- the unequal lift between the advancing and retreating blade of the MR due to unequal RW velocities. - Velocity of the relative wind changes throughout the plane of rotation
Advancing blade: (produces more lift)
- flaps up
- AOA decreases
- angle between the cord line and RRW decreases, decreasing the AOA
Retreating Blade: (less lift)
- flaps down
- AOA increases
- angle between chord line and RRW increases, increasing the AOA - Blade flapping occurs to offset the effects of this and thereby creates a condition of equal lift on both sides of the rotor disc
5
Q
LTE/high wind pedal turns
3 types of LTE
A
- A sudden loss of tail rotor efficiency due to the wind striking the helicopter at certain sectors resulting in an unintended yaw to the left
1. Main rotor disc interference (285-315 degrees)
2. Weathercock stability (120-240 degrees)
3. Tail rotor vortex ring state (210-330 degrees)
6
Q
LTE (main rotor disc interference)
A
- 285-315 degrees
1. A left quartering headwind causes MR vortex to be blown into the tail rotor by the relative wind
2. Creates a turbulent environment for the TR to operate in
3. During hard right hand turn: - the TR must reduce thrust
- once the MR vortex comes in contact with TR, it initially increases the AOA of the TR blades increasing the TR thrust
- the increase in the AOA requires right pedal pressure to maintain rate of turn
- as the MR vortex passes the TR, the TR AOA is reduced, resulting in a reduction of thrust, and right yaw acceleration begins
- can develop into an uncontrolled rapid rotation about the mast as the TR is continually pushing through its own vortices
7
Q
LTE (weathercock)
A
- 120-240 degrees
- Occurs when hovering in a tail wind
- The aircraft attempts to weathervane its nose into the wind
- Starts a slow, uncommanded turn either to the right or left (depending on wind direction) if a resisting pedal input isn’t made
- If the pilot allows a right yaw to develop, and the tail of the helicopter moves into this region, the right yaw can accelerate quickly
8
Q
LTE (tail rotor vortex ring state)
A
- 210-330 degrees
- A left crosswind
- Wind blows TR vortices back into itself
- Results in a non-uniform, unsteady flow into the TR
- Vortex ring state causes:
- variations in TR thrust
- oscillation in TR thrust
- rapid and continuous pedal movements are necessary to compensate
9
Q
Translating tendency
A
Tendency of aircraft to move in the direction of the TR thrust
Counteracted by:
- transmission and mast are tilted slightly
- cyclic is rigged so rotor is tilted slightly when cyclic is centered
- pilot compensates with left cyclic
10
Q
Retreating blade stall
A
- A stall that occurs on the retreating blade of the MR disc because of the high AOA required to compensate for dissymmetry of lift
- Occurs at high airspeed (over VNE)
- As speed increases, flapping increases
- Retreating blade flaps down far enough to exceed critical AOA (zero lift, all drag)
- High weight, low rotor RPM, high DA, turbulence, and steep turns can cause it.
RECOGNITION:
- increased vibrations
- nose pitch up stall occurs at 270, 90 degrees later max effect is 180 degrees (due to gyroscopic precession)
- left roll
RECOVERY:
- aft cyclic
- lower collective
- slow down
PREVENTION:
- never exceed VNE
- avoid flight in turbulence at high speeds
11
Q
Settling with power
A
- A condition where helicopter is settling into its own downwash
- Occurs on a steep approach and a tail wind OGE hover
- Helicopter remains stationary by propelling s large mass of air down through its MR
- Some of the air is recirculated near the tips of the blades, curling up from the bottom of the rotor system and rejoining the air entering the rotor on top
- as long as those wing tip vortices are small, there effect is a small loss in rotor efficiency - If the aircraft starts to descend vertically, it’s settles into its own downwash, which enlarges its wing tip vortices
- As a result of the AC descending, the airflow of the inner blade sections is upward, producing a secondary vortex ring on the inner portion of the MR disc (2 sets of vortices)
- Rotor efficiency is lost even though full power is applied
CONDITIONS:
- 300’ p/min or greater rate of descent
- A/S less then ETL
- 20% of power applied
RECOGNITION:
- vibrations
- drastic increase in descent rate
- decrease in cyclic control
RECOVERY:
- lower collective
- apply forward cyclic
- pull to climb power when positive airspeed
12
Q
Low RPM recognition and recovery
A
1. When rotor RPM drops, blades try to maintain the same amount of lift by increasing pitch. Increasing pitch angle increases drag, which requires more power to keep the blades turning at the proper RPM. When there isn't any more power, there's no more lift and the aircraft begins to descend. CAUSES: - engine failure - over pitching the blades with the collective (asking for power it doesn't have) - overriding the governor - death gripping the throttle - rolling the throttle the wrong way - governor malfunction - high density altitude RECOGNITION: - engine/rotor decrease noise - VSI will indicate a decent - RPM's will decrease - Vibrations - low RPM warning horn and light RECOVERY: - simultaneously roll on throttle and lower collective - slight aft cyclic 2. Robinson blades stall at 80% plus 1% per every 1000' in altitude (no recovery)
13
Q
Low G recognition and recovery
A
CAUSES: - a pushover - abrupt forward cyclic - turbulence - following terrain 1. When one of these maneuvers is performed, the AOA and thrust of the MR is reduced, causing a low G or weightless flight condition. Lateral cyclic has zero effect. Since the TR is above the center of gravity, the TR thrust causes heli to rapidly roll to the right. If an attempt to stop rolling with cyclic inputs, the rotor can exceed flapping limits and cause mast bumping. The MR hits the mast. RECOGNIZED BY: - felling of weightlessness - rapid right roll (TR above CG) RECOVERY: - gentle aft cyclic (to reload the rotor which produces thrust) - then correct for right roll with left cyclic DANGERS: - mast bumping
14
Q
Dynamic rollover
A
- 3 conditions
- pivot point other than CG
- rolling moment
- thrust greater then weight - If helicopter is allowed to pass 15 degrees, lateral cyclic control is lost
- Recovery: lower collective
