Helicopter Aerodynamics Flashcards
Lift Equation & Four Forces
The Lift Equation is used to calculate lift based on the variables of velocity, air density, wing area, and a lift coefficient (that factors in body shape and inclination, air viscosity and compressibility, and is determined experimentally).
L = Cl x (pv^2)/2 x A
The Four Forces are Lift vs Weight and Thrust vs Drag.
Gyroscopic Precession
The principle that a change in input manifests 90° later.
Translating Tendency
The tendency for a hovering helicopter to drift to the side do to the pushing effect of the tail rotor. It can be mitigated by design (such as an off-center mast or cyclic) but in the case of the R22 some left cyclic input may be necessary.
Coning
The upwards angling of the blades due to lift. Facilitated by coning hinges.
Flapping
The up and down movements of blades due to dissymetry of lift, facilitated by coning and (in the R22) teetering hinges. By design this allows the advancing blade, which generates more lift due to higher airspeed, to flap upwards thus reducing angle of attack and decreasing lift. At the same time the retreating blade, which generates less lift due to lower airspeed, flaps downwards thus increasing angle of attack which generates more lift. This allows the entire rotor disk to generate equivalent lift.
Translational and Effective Translational Lift
A rotor disk in undisturbed air will generate more lift. Therefore as soon as a helicopter moves forward it will become more efficient. At an airspeed of 16-24 kias, the entire rotor disk will be in undisturbed air and become much more efficient. This is Effective Translational Lift.
Transverse Flow Effect
This occurs at approximately 20 kias when the front half of the rotor disk is generating more lift due to being in undisturbed air while the rear half is in turbulent air. That lift is manifested approximately 90° later due to phase lag and in the case of a CCW rotor system can result in a right roll and vibrations.
Dissymetry of Lift & Retreating Blade Stall
Advancing blades have a higher relative airspeed than retreating blades and therefore there is dissymetry between the right and left half of the rotor system. This is mitigated through flapping of the blades. However, with enough forward airspeed this can result in retreating blade stall which can result in pitching up and slight left roll of the helicopter. Fortunately the pitching up will generally reduce airspeed and self-correct, but the proper recovery is to simply slow down.
Dynamic & Static Rollover
Rollovers happen when there is a pivot point (usually a stuck skid) and a rolling moment (movement or applied power) and occur once the helicopter passes the critical angle (5° to 8°). Once past the critical angle they are not recoverable. The corrective action is to reduce collective (thus reducing the rolling moment). Static rollovers occur regardless of power when the critical angle is exceeded - typically on slopes - and are not recoverable.
Vortex Ring State & Settling with Power
VRS is an aerodynamic state resulting in rapid descents as much as 6000 feet per minute. It requires slow airspeed (below ETL), high rate of descent (300 feet per minute or more), and power applied. The helicopter becomes trapped in its own recirculating air.
Indications are severe vibrations, loss of flight control, and rapid descents.
The recovery is to gently reduce power (down collective) and apply gentle forward cyclic to move the helicopter into undisturbed air. Once there is forward airspeed (the trim strings “come alive”) one can immediately begin to climb by pulling collective.
Settling with Power is uncontrolled descent due to inertia. It can be avoided through planning and maintaining power reserves and airspeed.
Autorotation
The transition from powered flight (which uses the engine to drive air downwards through the rotor system) to gliding flight which allows air to move upwards through the rotor system.
The rotor disk can be divided into three regions: the driving region, the driven region, and the stall region. Pitch changes through collective input adjust the size of these regions and balance lift with rotor speed (RPM’s).
