Aerodynamics Flashcards
Describe flapping, feathering, and hunting
Flapping: overcomes dissymetry of lift
Feathering: compensates for dissymetry of lift
Hunting: relieves the stresses that build up due to the coriolis effect on blades
drag is always parallel to the relative wind. Produced by the resistance of the air to an object passing through it. Types:
parasite: airflow around the aircraft (increases when A/S increases)
profile: airflow through lift producing components (remains relatively flat)
induced: results from producing lift (decreases when A/S increases)
Relative wind def
Types
def: air in motion relative to an airfoil, equal to and opposite the flight path velocity of the airfoil
Types: rotational (horizontal flow of air, constantly changes direction during rotation), and resultant (relative wind modified by induced flow)
Total aerodynamic force def
Factors affecting TAF
def: sum of all forces acting upon an airfoil. Always perpendicular to the tip path plane
Factors affecting TAF: air density, viscosity, airfoil SA, shape, AoA, airflow velocity across airfoil.
Settling with power def Conditions Maneuvers Prevention Recovery
def: helicopter settles in its own downwash, aka vortex ring state Conditions: - 300 fpm or greater rate of descent - low airspeeds (near 0) - power applied (20% to 100%) Maneuvers (DSOMFH): - downwind approaches - steep approaches - formation flight/approaches/take offs - NOE flight - Masking/unmasking ops - Hover OGE Prevention: descend on flight paths shallower than 30* Recovery: with power - collective increase. without power- collective reduction & cyclic in any direction to gain A/S
Dynamic Rollover def
Human factors
Physical factors
Recovery
def: occurrence of a rolling motion when there is a pivot point, and the aircraft exceeds the critical angle Human factors (IFLII): - inattention - inexperience - inappropriate control inputs - loss of visual reference - failure to take timely action Physical factors: - slopes - CG/loading - crosswind - M/R thrust and design - T/R thrust Recovery: smooth moderate collective reduction
Translating Tendency def
Compensation
def: during hovering flight, single rotor helicopters tend to drift laterally to the right (direction of T/R thrust)
Compensation:
- mechanical mixing unit: coll to roll
- pilot: tild cyclic/M/R disk to the left
Retreating Blade Stall def Conditions Indications No lift areas (from inside out) Prevention Recovery
def: retreating blade operates at a slower rel. wind velocity than advancing blade. Effect of stall is displaced aft bc of gyroscopic procession Conditions (LTLAG): - Loads (high GW) - Turbulence - Low rotor RPM - Altitude (high DA) - High G (steep turns) Indications: - severe vibrations - left roll of fuselage - nose pitches up No lift areas: reverse flow, negative stall, negative lift Prevention: avoid abrupt maneuvers, fly slower when conditions are present Recovery: - reduce power, A/S, severity of maneuver - increase RPM toward upper limit - ensure aircraft is in trim
Autorotation def TAF is tilted: forward or aft? 3 regions (inside out)
def: using the flow of air past the rotors, not the engine, to turn the rotor system TAF: tilted forward 3 regions: - Stall (inner 25%) - Driving (middle 45%) - Driven (outer 30%)
Effective Translational Lift def
Factors
Outcome
def: at about 16-24 knots the rotor completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air Factors: - lift much more effective - drag is reduced - airflow more horizontal Outcome: - nose pitches up - right roll
Transverse Flow Effect def
Corrective action
Cause
def: transverse flow is the differential of lift and induced flow that exists bw the FWD 1/2 and the REAR 1/2 of the rotor disk bw 17-20 knots. Gyroscopic procession causes right roll
Corrective action: left cyclic
Causes: bc of coning and fwd tilt, the air has to flow for a longer distance to the rear portion of the disc, there is greater deflection of the air. Air hits the front portion more horizontally and has less induced flow while it flows through the aft portion more vertically and has greater induced flow. Reducing the induced flow over the nose causes an increase in AoA
Airflow during a hover facts
2 factors
IGE
OGE
facts: lift produced at a hvr must equal the total weight, AoA is the same IGE & OGE
2 factors:
- induced flow: vertical flow of air through rotor syst
- rotor tip vortices: airswirls from high to low pressure, reduces the lift efficacy of the rotor tips and creates turbulent air
IGE:
- induced flow vel is less bc ground deflects downwash out, and restrics vortices creation
- main tip vortices are less, blades operate in less turb air, larger lifting surface, more efficient-requires less pwr
- AoI required is smaller
OGE:
-induced flow vel is greater bc there is no ground effect to slow and deflect downwash out, larger main tip vortices
- smaller lifting surface, less efficient-requires more pwr
- AoI required is larger
Compressability def Explanation Conditions Indications Recovery
def: phenomenon resulting from the advancing blade approaching Mach 1 bc of excessive forward speed Explanation: when airflow strike sthe leading edge, it is split into 2 streams (above and under the blade). At low speeds, this splitting happens easily and requires little energy. At high speeds, the air is compressed before splitting into 2 streams, and requires more rotor thrust. Conditions: - high A/S - high RPM - high DA - high GW - low temp - turbulent air Indications: - severe vibrations - roll of fuselage Recovery: - Reduce blade pitch (lower coll if possible) - Reduce A/S - Reduce RPM - Reduce severity of maneuver
Transient Torque def How the helicopter experiences it Causes of increase in torque 4 factors of transient torque Recovery
def: temporary change in torque when lateral cyclic is applied
How the helicopter experiences it:
- LEFT cyclic: increase in pitch, increase in induced drag on adv. blade, RPM slows, sensed by ECU, sends info to HMU to increase FF and NG, momentary increase in TQ
- RIGHT cyclic: decrease in pitch, decrease in induced drag, RPM increases, sensed by ECU, sends info to HMU to decrease FF and NG, momentary decrease in TQ
Causes of increase in torque:
- increase in AoI and induced drag
- RPM tends to increase as G forces are applied
4 factors of transient torque
- amount of power applied
- rate of movement of the cyclic
- magnitude of cyclic displacement
- drag is increased of decreased by the factor of velocity^2
Recovery: Collective change
- reduction when L cyclic
- increase when R cyclic
Dissymetry of lift def
2 factors that cause dissymetry of lift
Dissymetry of lift is corrected through
No lift areas (inside out)
def: diff lift that exists bw the adv and retreating halves of the rotor disk in directional flight due to different air flow velocities.
2 factors that cause dissymetry of lift:
- velocity
- no lift areas
Dissymetry of lift is corrected through:
- blade flapping (change in induced flow vel)
- cyclic feathering (change in AoI)
No lift areas (inside out): reverse flow, negative stall, negative lift
