B267

OTHER: Aerodynamics - General

Overview:

General information on aerodynamics that may not relate directly to the specific projects at this web site.

Topics (item):

Circulation Control: Definition

Web Sites:

http://www.aviationtoday.com/reports/rotorwing/previous/0600/06aero.htm

http://www.ae.gatech.edu/research/artlab/artl/dslot.html

http://www.defensedaily.com/reports/rotorwing/previous/0600/06aero.htm

Flap Back (Blow Back, Back Flap) Longitudinal Flapping:

The act of the rotor disc flapping away from the relative wind. It is caused by a change in relative airflow between the advancing and retreating blades. It will occur whenever there is a change in airspeed. During forward flight the advancing blade will have a greater relative airspeed than the retreating blade, it will therefore generate more lift (for any given angle of attack an increase in airspeed will produce an increase in lift) and begin to rise i.e. it flaps up - the highest point being in the direction of travel of the aircraft. Likewise the retreating blade due to its reduction in airspeed will generate less lift and begin to fall i.e. it flaps down - its lowest point being 180º from the highest point.

The UniCopter, with its rigid rotors, does not [may not?] have flap back and its ability to assist with static stability. However, should not the coning angle provide this or will this coning angle cause the rotor to want to roll to the advancing side? ~ perhaps see Robinson's Wee Wa. Look into the possibility of a larger than normal precone.

Lateral Flapping:

"In addition to the natural tendency for the disk to tilt back with a change in forward flight speed, the disk also has a tendency to tilt laterally to the right. This effect arises because of the blade flapping displacement (coning). For the coned rotor, the blade angle of attack is decreased when the blade is at ψ = 0º [rear] and increased when ψ = 180º. Again, another source of periodic force is produced, but now this is phased 90º out of phase compared to the effect discussed previously [longitudinal flapping]. Because of the 90º force/displacement lag of the blade, this results in a lateral tilt of the rotor disk to the right when view from the behind.

Note that in the hypothetical case with no coning, the blade sees the same increase in angle of attack at ψ = 0º and 180º and there will be no lateral tilt."

~ Leishman. See [Source ~ PHA p.140]

Noise: Should maybe move to Vibration

General:

Normal speech is about 70dB (decibels) and physical pain starts at about 140 dB.

The dB scale is logarithmic.

1. Appreciable compression at the advancing tip.

Rotor Induced:

The elimination of the tail rotor on intermeshing helicopters reduces the craft's noise level.

Tip speeds under 500 ft/sec are quiet. The Hughes "Quiet One was down to 430 fps.

Engine Induced:

Enclosing the exhaust inside the tail boom might reduce the noise, particularly if the outlet to free air was spread over a larger area then just the end of a pipe.

Outside Information:

Revolutionary Concepts for Helicopter Noise Reduction - S.I.L.E.N.T . Program

5 states of Flow through the Rotor:

Zero Thrust

Vertical Ascent

Hover

Turbulent Windmill Brake State

Windmill Brake State

Helicopter Aerodynamics - Airflow through a Rotor Disk ~ You Tube

Last Revised: March 7, 2013