aerodynamics pt 1

Along with the types of drag, here's some notes I've saved from reviewing aerodynamics. It's not complete. There will be more later. Mind you I've already learned about this stuff in the past here and there, but lets just say now I'm really cracking down on the material and additionally, refreshing my memory further. Yay. I hope its simple enough to understand.

AERODYNAMICS

R22 gross weight is 1370 lbs, has a governor, symmetrical semi rigid rotor system, asymmetrical tail rotor and airfoils.

Properties:

3^rd law of motion: for every action there is an equal and opposite reaction

Bernoulli’s principal: generation of lift resultant of wind moving faster over the curve of an airfoil (low pressure created).

Solidity ratio: ratio of total blade area vs. total disc area—this measures potential for thrust.

Coriolis Effect: the rotation of an object moving clockwise will make it drift to the right while in motion.

Gyroscopic Precession: when a force is applied to a spinning gyro, the max reaction occurs about 90 degrees to the direction of rotation (R22-counter clockwise).

4 forces of flight:

Lift: lifting force

Lift=area X Wind X (velocity)^2 X CoL(AoA) X 1/2

Weight: opposing force of lift

Thrust: speed/acceleration/power

T/R: pedals required to counteract torque made by rotors

In hovering, thrust also acts as lift until you push the cyclic forward.

Drag: opposing force of thrust

Three types of drag:

Parasite: drag resulted from surfaces NOT involved with creating lift (airframe, skids)

Profile:

- form: drag created from skin/sfc roughness of airfoils

-skin friction: drag created from sfc’s on airframe-sfc’s not involved with creating lift.

Induced: (means to produce, lead/move by influence, assert or establish)

-is an additional, aft result of relative wind flowing over airfoil.

-Drag is always perpendicular to lift, and

-in forward flight, all forces are perpendicular to reference plane

AERODYNAMIC FLIGHT BEHAVIOR

1.) R22 rotors rotate counter clockwise. Drifting tendency is to the right,, so use left pedal and a little left cyclic to counter act, and, these three other ways. the most important thing is just to remember how the mast is built and what control units you use to counter act the drifting.

2.) Translating Tendency--This is when the helicopter will tend to drift in the same direction as anti torque rotor thrust—in R22’s case it is to the left.

3.) Gyroscopic Precession

4.) The initial aerodynamic conditions apply too.

Ways to help control translating tendency: rig rotor mast, Flt control rigging, mix in hover

1.) Rig rotor mast--the rotor mast is built to have opposite tail thrust, and angled THREE DEGREES to the left. The left skid will tend to pick up after the right skid.

2.) Flight Control Rigging--Opposite t/r thrust when CYCLIC is centered.

3.) A mix of both can be made possible if in hover.

Center of Mass on each rotor will move closer to or away from rotor hub, but will never go past it; this is affected by the Coriolis Effect.

Flaps up: CM moves closer to hub

Flaps down: CM moves away from hub.

1.) The R22’s rotors are symmetrical. T/R blades, etc, are asymmetrical to generate more thrust.

2.) Asymmetrical airfoils’ Center of Mass change quicker than symmetrical airfoils.

Centrifugal force: the apparent force of an object moving along a circular path exerts on body constraining the object will move INwardly and outwardly (simultaneously) from the center of rotation. This apparent force and dampers keep the rotor system from coning too much.

Centripetal Force: the horizontal component of lift. In a turn, centrifugal force is apparent in the direction away from the path of the turn, while centripetal force pulls in towards the vertical component of lift, stabilizing the turn.

*Centrifugal force is always present in flight, while centripetal force may only be present during turns.

*When moving into forward flight, push nose down (apply forward cyclic), use pedals and collective if needed.

IN GROUND EFFECT

When hovering, stay in ground effect, lift vector is better for both the main rotor system and tail rotor.

*When the helicopter shifts in Ground Effect (after about half the diameter of the rotors above the ground), drag will increase, more thrust will be needed. Adjust unless moving into forward flight.

The pitch angle is different from the angle of attack: AoA involves angle of chord line to relative wind. Pitch angle is the angle between the reference plane and chord line.

Out of ground effect

Vector for tail rotor becomes more efficient

More power is needed.

Translational Lift

a. t/r affected as it moves out of ground effect and accelerates.

b. Thrust shifts forward

c. ETL

d. Loses a little alt. as thrust is applied

e. Condition: rises up and may roll to right if not caught.

f. Correct by: lower collective, right pedal may be needed to.

5 Induced flow: max displacement of rotors is 90 degrees of rotation

a. Rotor blades generate rotational relative wind

b. Flow parallel and opposite rotor leading edge.

c. Air projected downward as lift is produced and rotors spin.

d. Large masses of air—resultant relative wind, generate extra lift.

6 Transverse Flow Effect

a. In forward flight induced flow drops to near zero and angle of attack increases in the aft disc area, because of torque and acceleration, it will roll to the right.

b. Happens around 20 knots

c. Cyclic correction needs to be made.

7 Dissymmetry of Lift: (an existing but not major behavior)

a. Advanced blade has more relative wind (and lift) than retreating.

b. Due to flapping and retreating the lift generated equalizes as the rotors spin.

c. For and aft positions of reference plane are balanced.

8 Retreating blade stall:

a. Reduced lift from flapping and effect from Gyroscopic Precession behavior in retreating blade area risks retreating blade stall, may pitch up and roll to left.

b. Also avoid Vne speed.