Calculating Rotor Downwash

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How do you calculate the rotor downwash speed during hover, assuming a 7 in. chord rotor, a rotor diameter of 24 feet, all up weight of 650 lbs, rotor rpm of 500, with 0 wind speed ?

Using the formula below, which is found in helicopter texts, I get 17.4 ft/sec.

Apparently the downwash can be estimated without knowing the blade chord or rpm. It is mainly a function of disk loading and air density.

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Al hit the nail on the head.
What are you calculating?

al, your equation is similar to this one
lift = Cz * 1/2 *R * V2 * S
cz depends on airfoil, i guess its the 0.002378
V wind speed
S surface (main disc - central disc)

but this equation (i gave to dean first then, deleted it) is not accurrate as it is used to calculate the lift for WINDMILLS, assuming a wind normal to the disc.

like christian said in other post, another method should be more accurate

Victor, your and Al's formulas are derived from momentum theory; Force = Mass x Acceleration.

The factor; 0.002378 is air density, slugs/ft³ (weight of air, lb. per ft³ divided by acceleration of gravity)

Momentum theory assumes uniform flow through the rotor disc and ignores the influence of ground effect.

Power required is down wash velocity x gross weight divided by 550. The answer will be optimistic by 10% or so and to that must be added rotor blade profile drag power.

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c BEATY, thats right, i think thats a similar calculus.
ok for air density (i m dumb considering a Cx of 0.002 )

why 550 ? can you explain or give a link ?
i have various formula and the only method i consider is : perform them all and design a little above of all.. (hope my english is understandable)
thanks

James Watt discovered that Welsh mine ponies could move coal carts at a rate of 550 ft/lb/sec over the course of a working day. A drawbar pull of 100 lb., moving at the rate of 5.5 fps (3.75 mph) = 1 HP.

Therefore, one Imperial HP = 550ft/lb/sec.

Velocity (fps) x force (lb.) divided by 550 = HP.

*That would be 76 kg/meters/sec but you round off to 75 kg/m/sec.

During the prime of the Citroen 2 CV, I saw horses pulling sugar beet wagons in the south of France that would equal 3 Welsh mine ponies.

The 2 CV, by the way, probably was 15-20 hp. The 2 CV came from the tax collector's formula.

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LOL okay C beaty, i m in Metric system
thanks

Way cool thread, and you are right on, C. Beaty, except that the helicopter is actually far less efficient than the 10% you estimate (the rotor itself loses 20%, the main transmission 2%, the downwash on the fuselage 3 to 5%, the tail rotor about 3 to 5%). For most helicopters, the entire system is only about 65 to 75% efficient, so take the momentum theory horse power value and increase by about 40% to get at the real number.

Nick, I was only discussing that portion of rotor power used in producing a momentum change in the air mass (induced power).

The ~10% figure results from slipstream rotation and non uniform velocity over the rotor disc.

I agree that blade profile power, fuselage negative lift and transmission losses can substantially increase hovering power. Profile power alone can amount to as much as 40% of induced power, depending on blade loading and resultant tip speed.

Welcome aboard, Nick!
Although, I don't visit PPrune as often, I usually seek out your posts there.

For those of you who don't know of him, Nick is the S-92 Program Manager at Sikorsky Aircraft.

With all this loss of power it's obviously that the tail rotor has to go.

For decades, Western aerodynamic texts have mathematically shown that the tail rotor wastes 8 - 10% of the power. Recently, Kamov has stated in an article entitled Aerodynamic Features of Coaxial Configuration Helicopter that the tail rotor wastes 10-12% of total power. In addition, the latest aerodynamic text (year 2000), by Leishman (University of Maryland) supports Kamov's position.

If a craft's gross weight to empty weight is 2:1, then the above might result in a difference in payload of up to 24%.

Dave J.

Welcome Nick, looking forward to your input in the forum.

hello dave, christian, nick i v not be introcuded but pleased any way .. did i miss something ? coax or not coax? hoax ?
cheers

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