Helicopter Power Required
An introduction to helicopter power required
There are a number of ways power is used in a helicopter, a brief summary of these follows.
- Rotor Profile Power
- Induced Power
- Parasite Power
- Ancillary Power
With the exception of ancillary power, these items are speed dependant varying as airspeed is increased.
Power In Detail
Rotor Profile Power is the power required just to turn the rotors. It is the power required to overcome the rotor aerodynamic drag force.
Power has dimensions of Nms
-1, and as we know power is Rate of Rotation * Torque.
The drag force we measure in Newtons, and if we apply element theory and integrate the force along the blade, We can calculate the blade and thus rotor torque.
The Torque is measured in Nm.
If we convert RPM into radian s
-1 we can now multiply by this and have a figure for Rotor Profile Power.
Induced Power is that portion of the power required to produce lift. It is the power required to overcome the portion of rotor drag which is caused by the induced flow tilting the total reaction rearwards.
Induced power is the
force required to move a mass of air through the disk at the induced velocity.
If T is the rotor thrust (in a hover equal to weight (Mass * G)), which is a force, and this force moves the air at a velocity V
i , P
i = TV
i .
This example is off the cuff for the H300, with some remembered figures.
Mass of A/c = 980kg
Weight = 980 * 9.8 = 9604N
Vi = 8.42 m/s
P
i = 9604 * 8.6
P
i = 82941 w
P
i = 111 Horsepower to hover !
Parasite Power is that portion of the power required to move the rest of the airframe through the air. It is the power required to overcome parasite drag.
If the Fuselage Drag Force= C
D½ρv
2S measured in Newtons, we know that
power is force * displacement and we know that velocity is displacement in a period of time so we can calculate the power required to move the fuselage through the air by multiplying drag force by its velocity (v) this will give us a value in Watts [Nms
-1] .
From this we conclude that Parasite Power is in fact C
D½ρv
3S.
Parasite drag = C
D½ρv
2S
If we assume that the frontal area of a helicopter is its width * height then we have S.
If we assume C
D is that for a square flat plate. (1.18 varies dependant upon aspect ratio)
We can now calculate the force required to move that plate throught the air at a given speed (V).
This is the power required to drive any ancillary items such as generators, alternators, air conditioning etc.
The power required to drive an alternator (assuming no losses) would be Voltage * Current, once again in Watts.
Taking a 28V alternator, with a maximum current of 70 amps (H300 again).
P = IV
P = 28*70
P = 1960W
P = 2.62 Horsepower
What this means
The total power required is the sum of all the items mentioned above. All of these items are speed dependant with the exception of ancillary power which is controlled either by the designer or the pilot. A graph showing a typical power required curve is shown below. 