Rotors Flashcards
Blade sides
Advancing side - fast relative wind, flaps up (lowest pit h angle)
Retreating side - lowest relative wind, flaps down (greatest pitch angle)
Rotor tip velocity/speed
Roughly 400 rpm at roughly 400 knots at the tip
Total Aerodynamic Force (TAF)
Aerodynamic force resulting from lift/drag
Two components of TAF: LIFT, DRAG
(Resultant Force is inter-related)
Drag is parallel and opposite of thrust, or 90* degrees from lift
We want to lift to act straight up on the center of pressure, drag pushed the vector to the rear
“What is the cause of all general aviation stalls? Exceeding the critical angle of attack”
Think of airfoil picture
Drag
Induced: by product of lift, caused by blade tip vortices and induced flow (*the most critical drag in rotor wing flight - higher power = higher drag). Decreases with forward windspeed. At hover, induced flow ic rease, and the angle of the resultant relative wind causes higher drag and pulls the TAF further back/rear.
Parasite: created by non-lifting components (fuselage skin, struts, armaments…); increases or decreases proportionally with airspeed
Profile: caused by frictional resistance of the lift-producing surfaces (rotor blades)passing through air. Is relative,y unchanged in flight
Total drag: the sum of induced, parasite, and profile drag. Computes SOME airspeed limitations
- *induced flow is the main drag at hover, reduces in forward flight drastically
- *parasite drag increases dramatically as forward speed increases - airspeed limiting
- *profile drag is relatively unchanged throughout flight at roughly 25%
Total drag can be used to compute SOME airspeed limitations
Dissymmetry of lift
Differential lift between advancing/retreating sides of the rotor disk caused by different flow velocities
Sustained hover is symmetry of lift. Forward flight causes dissymmetry of lift
Forward flight reduces retreating blade velocity relative to wind - causes a “no lift” area (retreating blade stall) close to the rotor hub on the retreating side (forward speed matches inner rotor speed)
Rotor compensates for dissymmetry of lift by flapping
Pilot compensates for wind “blowback” by forward cyclic feathering
Two modes of flight
Accelerated: one or more forces are greater than their opposites
Unaccelerated: level flight, thrust equal to drag, lift equal to weight
- *(Semi-Rigid (tilt) System: rotor disk tilts with respect to the MAST)
- *(Rigid and Fully articulated System: tilting of the disk is relative to the HUB)
Centrifugal forces
An outward force starting from the center of rotation and extends outward to the rotor tip
Most dominant force is the turning rotor, other for es may modify it
Proportional to rotational velocity
Provides rigidity to the rotor system (rotors droop when not turning, rotors flat in spin)
*centrifugal fir e is GREATEST just outside the rotor tips
Rotor blade coning
The upward flexing of the rotor blades in the normal process
Is a compromise between LIFT and CENTRIFUGAL FORCE
Just a bit of flex, like a diving board, blade tip bends up slightly
Four Causes Excessive Coning
Low RPM - we lose rotational velocity
High gross weight - require more lift than normal
High G maneuvers
Turbulent air
**Adverse effects: loss of disk area (inverted umbrella), loss of total lift, stress on blades (cracks, fractures or rotor separation)
Gyroscopic procession
Manifestation of an applied force 90* degrees after the application in the direction of rotation
Effects of procession are overcome by offsetting the linkage in the cyclic pitch control to create an input 90* prior to the desired action
Blade twist
Why do we twist the blades in the standard American helicopters? it is necessary to DISTRIBUTE the lifting force more evenly along the blade
Higher blade pitch angle at the root of the blade, lower pitch angle at the tip of the blade (most modern helicopters)
(Angle of incidence decreases from root to tip)
Effects of torque
Torque is a force or combination of forces that tend to cause rotation
**CCW rotor causes CW fuselage motion
Amount of yaw is directly proportionate to the engine power being delivered to the rotor
- more power, yaws right
- less power, yaws left
(Action reaction effect)
Controlling torque reaction
Tail rotor powered from. Ain rotor transmission system
Provides RIGHT THRUST (left pedal, left nose) opposite of torque
Amount of thrust controlled by pitch of tail rotor blades
Pilot inputs:
- left pedal increases tail rotor pitch, for more tail thrust
- straight and level flight, pedals counter changes in torque and keep aircraft in trim
ONE true anti-torque pedal (left pedal)
Translating tendency
The tendency if a single engine helicopter to drift laterally to the RIGHT (while at a hover) (tail pushes air left, thrust is to the right)
Corrected by tilting the main rotor system left by:
- *- rigging of the flight control system
- *- slight tilting of the mast
- programmed mechanical inputs/auto flight control and augmentation systems (not for tests)
- *- left cyclic input by pilot
