Main and Tail Rotor Thrust Information

[65FlightMech]

Hello everyone. This is my first post here and am excited to be part of this forum. I am a Flight Mechanic for the U.S. Coast Guard stationed in Alaska. It truly is a great oppurtunity to be stationed in the world's last frontier. Anyway,....on to my post...

I am looking for any information available regarding calculating thrust production in Main and Tail rotor systems. I am sure that I am going to be deluged with a bunch of rubber room formulas and nutty theories so I am looking for fairly easy to understand information. I would appreciate any input that anyone has that would send me in the right direction.
I looked at another post from a while back and noted some of the thrust calculators, but it didn't seem to fit my quest.

I am interested in finding out the blade size (length, width and chord), speed and angle of attack to produce a certain amount of thrust. I am also considering what the affects of having a dual concentric rotor (one atop another rotating in opposite directions) How would you calculate that? Obviously there will be turbulance concerns between the two systems, but I just want to calculate that actual thrust that is produced.

Again, I am looking forward to reading your responses!

Thanks,
Stephen

[Chuck Roberg]

Hi Stephen, Sorry I don't have the knowledge to answer your question.

I only want to say I really admire all you people do for us in the Coast Guard. Mention to all the people you work with. You all have my admiration and respect.

[C. Beaty]

Stephen, rotor power is usually separated into two components; (1) induced power, the power expended in accelerating air mass downward to create lift and (2) profile drag power, the power expended in dragging the rotor blade airfoils through the air.

Exact calculations are complex, time consuming and difficult but approximations based on momentum theory for induced power and mean values rotor profile power are quite simple.

The slipstream velocity of a hovering rotor, not influenced by ground effect is:

Square root of: weight/(air density x rotor area)

Air density at sea level is .0023 slugs/cubic foot (W/g)

Say the machine weighs 500 lb and rotor disc area is 300 sq. ft.

Then: (500/(.0023 x 300))^ ½ = 26.9 fps

The power expended is:

Slipstream velocity x weight/550 = 26.9 x 500/550 = 24.5 hp


Rotor tip speed depends upon blade loading and blade lift coefficient. For top speeds in the range of 100 mph, necessary blade mean lift coefficient is about 0.5.

In that case, rotor tip speed is approximately equal to:

Vt = 66 x (blade loading)^½

With rotor disc area equal to 300 sq. ft., rotor radius is 9.77 ft. and if the chord was 7 inches with a 2 blade rotor, blade area would be 11.4 ft.²

Vt = 66 x (500/11.4)^ ½ = 437 fps = 427 rpm

The power required to spin the rotor at 427 rpm is very approximately;

Hpo = 7.172 x 10^-9 x tipspeed^3 x blade area

Then:

Hpo = 7.172 x 10^-9 x 437³ x 11.4 = 6.8 HP

Total rotor power = 24.5 + 6.8 = 31.3 HP

Bear in mind, these calculations are based on very simple assumptions and rotor profile drag HP can vary widely with airfoil quality.

The same calculations will also work for tail rotors.

[Rotor Rooter]

A pair of closely spaced 2-bladed coaxial rotors will be comperable to one 4-bladed single rotor, if the details are identical.

The coaxial's loss due to rotor-rotor interaction will be partially offset by swirl recovery. The tail rotor consumes approximately 10% of the power and the twin-main-rotor configurations can apply this power to lift.

[Ga6riel]

nice job Chuck