Aerodynamics Flashcards
Transverse Flow Effect
- In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than the air passing through the forward portion.
- Less AOA and lift in the aft portion
- More induced flow in aft compared to front area
- Higher AOA and lift in the front portion
- Results in vibrations
- Occurs between 10 and 20 knots
- Due to gyroscopic precession, right rolling motion.
Dissymmetry of lift
- The differential (unequal) lift between advancing and retreating half of the rotordisk caused by the different wind flow velocity across each half.
- Compensated with:
- Blade flapping and cyclic feathering
- Blade flapping (aerodynamic):
- Upward and downward flapping motion changes induced flow. This changes AoA.
- Due to gyroscopic precession, nose up pitch (blowback) unless compensated for by cyclic.
- Cyclic feathering (mechanical):
- Changes the angle of incidence differentionaly around the rotor system
Effective Translational Lift (ETL)
Occures with the helicopter at about 16-24 knots, when the rotor - depending on size, blade area, and RPM of the rotor system - compleately outruns the recirculation of old vortexes and begins to work in relativly undisturbed air.
- Airflow more horizontal, therfore induced flow and induced drag are reduced.
- AoA increase.
- Nose pitch up and rolls right (during transition), unless compensated for with cyclic input (due to combined effect of Diss. of lift and Transv. flow effect).
Airflow during a hover
General
- At a hover, the rotor-tip vortex reduces effectiveness of the outer blade portions.
- During hover, rotor blades move large amounts of air through the rotor system in a downward direction. This movement of air also introduces another element—induced flow
- Ground effect permits relative wind to be more horizontal, induced drag to be reduced and lift vector to be more vertical.
Airflow during a hover
IGE
- Rotor efficiency is increased by ground effect to a height of about one rotor diameter.
- Figure 1-49 shows IGE hover and induced flow reduced. This increase in AOA requires a reduced blade pitch angle (angle of incident). This reduces the power required to hover IGE, due less induced drag (lift vector more vertical).
Airflow during a hover
OGE
- Induced flow velocity is increased causing a decrease in AOA.
- A higher blade pitch angle is required to maintain the same AOA as in IGE hover.
- The increased pitch angle also creates more drag. More power to hover OGE than IGE is required by this increased pitch angle and drag.
Settling with power
Settling with power is a condition of powered flight in which the helicopter settles in its own downwash.
Three conditions needed (1,2,3)
- Vertical or near vertical descent of at least 300´/min
- slow forward airspeed - less than ETL
- 20-100 % available engine power applied.
Recovery
- Initial state, a large application of collective,
may arrest rapid descent. - Cyclic to gain airspeed and/or lowering collective
Conducive (3 app, 2 hover, 1 fun thing)
- Steep approach
- Downwind approach.
- Formation flight approach
- Hovering above the maximum hover ceiling
- Not maintaining constant altitude control during an OGE hover.
- During masking/unmasking.
Retreading Blade Stall
- Definition
- Eventually in forward flight the retreating blade will tend to stall at high speed because of the high AoA needed to compensate for less lift area.
- Conditions (3 highs, 1 low and one T) - “LT. LAG” (Martin)
- High GW blade Loading (GW)
- High DA (Altitude)
- High G-maneuvers
- Low rotor RPM
- Turbulence
- Recovery (4 reductions and 1 increase - “I Reduce 4”)
- Reduce collective
- Reduce airspeed
- Reduce severity of maneuver
- Reduce altitudes (if possible)
- Increase rotor RPM
Dynamic rollover
Human factors
-
Inattention.
- Dynamic rollover is more likely if the aviator at the controls is inattentive to aircraft position and attitude when lifting off or touching down to the ground, effectively losing situational awareness (SA).
-
Inexperience.
- Most dynamic rollover accidents occur while inexperienced aviators are at the controls. The pilot in command (PC) must remain vigilant.
-
Inappropriate control input.
- Applying inappropriate or incorrect control input is the root cause of nearly all dynamic rollovers. If the aviator applies appropriate control input smoothly and carefully, dynamic rollover is avoidable.
-
Failure to take timely corrective action.
- Timely action must be exercised before a roll rate develops.
-
Loss of visual reference.
- Loss of visual reference may allow the aircraft to drift unnoticed by the crew. If the aircraft contacts the ground while drifting sideward, rollover can occur. Therefore, if visual reference is lost while the aircraft nears the ground, the aviator should execute a takeoff or go-around using instrument techniques if necessary.
Dynamic rollover
Physical factors
- M/R Thrust
- Aft C/G/Low fuel
- Surface area/Slopes
- T/R Thrust
- Crosswind component
Martin: (CG Loves CMT)
- C - CG (aft)
- G - Ground Surface-/ Slope area
- L - Low fuel
- C - Crosswind component
- M - Main rotor thrust
- T - Tail rotor thrust
DYNAMIC ROLLOVER
DEFINITION
Defined as the susceptibility of a helicopter to a lateral-rolling tendecy. Three condition must be present:
- Pivot point
- Rolling motion
-
Exceeding the dynamic/critical rollover angle
- if exceeded, recovery is impossible
Certian factors influence dynamic rollover:
- Right skid down
- High Roll rates
- Left pedal input
- Lateral loading
- Crosswind
Translating Tendency
What? CCW rotating single rotor helicopter has a tendency to drift laterally right.
Why? Results from right lateral tail rotor thrust, which compensates for main rotor torque.
Compensation: Tilt rotor disc to the left.
UH-60: MMU - Collective to roll coupling.
Relative wind
Relative wind is airflow relative to an airfoil.
Rotational Relative Wind
The rotation of rotor blades as they turn about the mast produces rotational relative wind
Induced flow
As blade pitch angle is increased, the rotor system induces a downward flow of air through the rotor blades creating a downward component of air that is added to the rotational relative wind.
Total aerodynamical force (TAF)
As air flows around an airfoil, a pressure differential develops between the upper and lower surfaces. The differential, combined with air resistance to passage of the airfoil, creates a force on the airfoil. This is known as TAF
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