Aerodynamic Factors

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Aerodynamic Factors
2013-11-26 17:13:30

Aerodynamic Factors
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  1. Loss of Tail Rotor Effectiveness (LTE)

    Add'l Considerations
    TH 67 i m s b o i's o n t m G t, t p i c o c t p a d c l.
    M h i t w, p h c, a a r d t a l a a l a.
    LTE is the occurrence of an uncommanded and rapid right yaw rate which does not subside of its own accord and if not quickly reacted to, can result in loss of aircraft control.

    • Add'l
    • TH 67 is more susceptible because of it's operation near the maximum GW thus, the pilot is consistently operating closer to power and directional control limits.
    • Maintain heading into the wind, precise heading control, and avoid right downwind turns at low altitude and low airspeeds.
  2. Loss of Tail Rotor Effectiveness (LTE)
    High GW/DA
    -I i e w i p r t h, t a i i t a o l p a a t d i l p a f r.
    Low IAS
    -A a E o t w o t t, t m l p i a f r.
    Power Droop
    -R p a m c t p d, a d i m r R c a c d i t r R a t.
    • High Gross Wt/DA
    • -Increase in either will increase power required to hover, thus an increase in the amount of left pedal applied and the decrease in left pedal available for recovery.
    • Low IAS
    • -Airspeeds above ETL offload the workload of the tailrotor, thus more left pedal is available for recovery.
    • Power Droop
    • -Rapid power application may cause transient power droop, any decrease in main rotor RPM causes a corresponding decrease in tail rotor RPM and thrust.
  3. Three Wind Regions for LTE
    W f t r w a t w v t a i t r w.
    A w m s L & R u y.
    T r y c d i L a r i c a b t p.
    • 120-240
    • Winds from this region will attempt to weather vane the aircraft into the relative wind.
    • Acft will make slow Left & Right uncommanded yaws.
    • The right yaw can develop into LTE and requires immediate corrective action by the pilot.
  4. Three Wind Regions for LTE
    Vortex Ring State
    W w t r w c a v r s t d a t T/R w c T/R t v.
    T h w m u p, r, a y e.
    P w l w b h a m f o n a r y r t b.
    • 210-330
    • Winds within this region will cause a vortex ring state to develop around the T/R which causes T/R thrust variations.
    • The helicopter will make uncommanded pitch, roll, and yaw excursions.
    • Pilot work load will be high and must focus on not allowing right yaw rate to build.
  5. Three Wind Regions for LTE
    Disc Vortex State
    W xx t xx k w c m r t v t b d i t T/R.
    T T/R w o i a e t e.
    T h w e t t t m a s u r y, w t p m a t n f s l p.
    • 280-330
    • Winds 10 to 30 knots will cause main rotor tip vortices to be directed into the T/R.
    • The T/R will operate in an extremely turbulent environment.
    • The helicopter will exhibit the tendency to make a sudden uncommanded right yaw, which the pilot must anticipate the need for sudden left pedal.
  6. Transverse Flow Effect
    T F i a c t r f t i d-f o a i t r h o t r s.
    T i d-f c a g i f, a l a o a; t r l, t t f p o t r d.
    R i a r r a v m n d t o a d l b xx t xx k.
    • Transverse Flow is a condition that results from the increased down-flow of air into the rear half of the rotor system.
    • The increase down-flow causes a greater induced flow, a lower angle of attack; thus reduced lift, than the forward portion of the rotor disc.
    • Results in a right roll and vibrations most noticeable during take off and during landing between 10 to 20 knots.
  7. Dissymmetry of Lift
    I t d i l c b t a a r b d n-s f.
    T s o t r w d f i a t t r r w o t a b (r i m l) a s f t r r w o t r b (r i l l).
    T c, t a b f u (i i f) a t r b f d (d i f) w a l t e.
    • Is the difference in lift created by the advancing and retreating blades during non-stationary flight.
    • The speed of the relative wind during flight is added to the rotational relative wind of the advancing blade (resulting in more lift) and subtracted from the rot rel wind of the retreating blade (resulting in less lift). 
    • To compensate, the adv blade flaps up (increasing induced flow) and the retreating blade flaps down (decreasing induced flow) which allows lift to equalize.
  8. Effective Translational Lift
    O b xx-xx k a i t r l b i r t v a h a m h a f, t r i f, i a o a, a t i l.
    Occurs between 16-24 knots and is the rotor leaving behind its rotor tip vortices and has a more horizontal air flow, this reduces induced flow, increases angle of attack, and thereby increases lift.
  9. Settling with Power
    A c o p f w t h s i i o d w.
    I i a l a, d v >xx f, a u xx%-xx% o a p w i r t s t d; u i w o t i f v a t h o t r s.
    I a t c b n q i c (u n e p a), t d c a a d c, t v r s w d a t m r.

    Conditions conducive:
    D a; S a (>xx*); F f; M/U; O; H a m c.
    • A condition of powered flight where the helicopter settles in its own down wash. 
    • If in a low airspeed, descending vertically > 300 fpm, and using 20% to 100% of available power with insufficient remaining to stop the descent; upward inflow will overtake the induced flow velocity at the hub of the rotor system. 
    • If allowed to continue by not quickly increasing collective (unless not enough power available), then decreasing collective and applying directional cyclic, then vortex ring state will develop around the main rotor.

    • Conditions conducive:
    • Downwind approach; Steep app (>30*); Formation flight; Masking/Unmasking; OGE; Hovering above max ceiling.
  10. Dynamic Rollover
    W a p o t a o s a a a p p a a r m c t a t e t c a.

    Prevent b k t h a p f a m s a m c i.
    R b m s a m c r.

    Human factors:
    I; i; d c a; i c a; l o v r.

    Physical factors:
    M/t r t; C; C; g s; s l a; L f c c C t m f.
    When any part of the acft or skid acts as a pivot point and a rolling motion causes the acft to exceed the critical angle.

    • Prevent by knowing the human and physical factors and making smooth and moderate collective inputs.
    • Recovery by making smooth and moderate collective reduction.

    • Human factors:
    • Inattentive; inexperience; delayed corrective action; improper corrective action; loss of visual reference.

    • Physical factors:
    • Main/tail rotor thrust; CG; Crosswind; ground surface; sloped landing area; Low fuel condition causing CG to move forward.
  11. Airflow during a hover (OGE/IGE)
    A i i c p t i a o a a g t l n t h.
    H I a t g t d t r t v a r t i f, r t b p r t h.
    H O l t g e a r a h p f a g a o a.  T h p i i d a t p r.
    An increase in collective pitch that increases angle of attack and generates the lift necessary to hover.

    Hovering IGE allows the ground to disrupt the rotor tip vortices and reduce the induced flow, reducing the blade pitch required to hover.

    Hovering OGE loses the ground efficiency and requires a higher pitch for a greater angle of attack.  The higher pitch increases induced drag and the power required.
  12. Retreating Blade Stall
    E a c c t s o t r w t m o e t s o t r r w.
    T c t s r: R F; N S; a N L, t e t t r t s r.
    T r p l p o t r b i n e t m t l g b t a b, c t a t p u a r r.

    H B L; H D; H G-m; L R R; T a

    R C; R a; R s o m; D t l a (i p); I r R t n l;
    • Excessive airspeeds can cause the speed of the relative wind to meet or exceed the speeds of the rotational relative wind.
    • This causes three stall regions: Reverse Flow; Negative Stall; and Negative Lift, to extend toward the rotor tip stall region. 
    • The remaining positive lift portion of the retreating blade is not enough to match the lift generated by the adv blade, causing the aircraft to pitch up and roll right.

    • Factors:
    • High Blade Loading; High DA; High G-maneuvers; Low Rotor RPM; Turbulent air

    • Recovery:
    • Reduce Collective; Reduce airspeed; Reduce severity of maneuver; Descend to lower alt (if possible); Increase rotor RPM to normal limits;
  13. Relative Wind
    Relative wind moves in an opposite but parallel direction in relation to the airfoil.
  14. Total Aerodynamic Force
    • Pressure differential b/t the upper and lower surfaces of the air foil combined with air resistance to the passage of the air foil.
    • TAF acts at the center of pressure on the airfoil and is normally inclined up and rearward.
    • May be divided into two parts, lift and drag.
  15. Translating tendency
    • During hovering flight, the single rotor counter-clockwise rotating helicopter has a tendency to drift to the right.  This is caused by the rightward thrust of the tail rotor.
    • One or more of the following factors are normally implemented to compensate:
    • -Controls are rigged so main rotor is tilted slightly left when cyclic is centered.
    • -Transmission may be mounted so mast is slightly left when fuselage is laterally level.
    • -The collective pitch control may be designed so the main rotor tilts left as cyclic is increased.
  16. Compressibility
  17. Autorotation
    • Driven Region
    • ~30% of blade disk radius.  Nearest to tip of rotor blade.

    • Driving Region
    • 25 to 70% of blade disk radius.  Between Driven and Stall region.

    • Stall Region
    • Inboard 25% of blade radius.  Operates above stall AOA and causes drag which slows blade rotation.