Effect of Helicopter Weight and Flight. Altitude on Performance

With increase of the helicopter weight there is an increase of the power

* ^ ^

required for horizontal flight, since TV/ — —. Figure 63b shows curves of the power required for the Mi-1 helicopter for flight weights of 2200 kgf and 2300 kgf. In comparing these curves we can say that with increase of the flight weight:

the maximal horizontal flight speed decreases;

the minimal speed when using rated power increases;

the economical and optimal speeds increase, although only slightly;

the horizontal flight speed range decreases;

the excess power decreases;

hovering of the helicopter outside the air cushion influence zone is impossible even when using takeoff power.

 

These variations of the helicopter flight characteristics should always be taken into account, particularly in those cases when a large fuel supply is carried. If the flight performance is based on takeoff weight, the values obtained will be too low. Therefore, if a large fuel supply is carried, the flight performance is based on the average flight weight with consideration for the fuel consumption

 

Gfuel

2

 

G = G av to

 

/95

 

where

 

G is the average flight weight;

cLV

G is the takeoff flight weight;

(Jfuel is the fuel weight (tanks completely full).

 

Effect of flight altitude. The helicopter flight characteristics depend on the flight altitude and also on the air temperature and humidity. The air density decreases with increase of the altitude; therefore, the parasite drag decreases, as does the power required for motion

 

where

 

Since the power required for motion has a large value at speeds above the economical speed, a change of flight altitude will have an effect on this speed

In studying the hovering regime, it was established that the thrust developed by the main rotor depends on the flight altitude, i. e., this thrust decreases with increase of the altitude, and this means that the lift force will decrease. But since the horizontal flight conditions specify that Y = G, it is necessary to increase the induced velocity V_^. Consequently, the induced power N. = GV. will increase in proportion to 1/Д, i. e., yV. = //, . The

і і *il *0 La

profile power changes very little with increase of the altitude.

Thus, with increase of the altitude the power required for motion decreases, while that required for creating the lift force increases. These conclusions are illustrated by the plot of power required for different altitudes (Figure 63c). This figure shows also how the power available varies with altitude.

For the supercharged engine the effective power increases up to the critical altitude and then decreases. As a result of this variation of the power available and the variation discussed above of the motion power and the induced power, we can say that with increase of the altitude up to the critical altitude:

1. For speeds lower than optimal, the power required for horizontal flight increases owing to the increase of the induced component of this power.

2. For speeds above optimal, the power required for horizontal flight decreases as a result of decrease of the motion power.

3. The magnitude of the optimal speed changes very little with change of the flight altitude.

г

4. The maximal and minimal horizontal flight speeds increase.

5. The excess power increases up to the engine critical altitude and then decreases.

Consequently, if flight must be accomplished at high speed, this should /96 be done at high altitude.

Increase of the air temperature is equivalent to increase of the altitude, since the air density decreases as its temperature increases. Increase of the air humidity leads to reduction of engine power and of the maximal horizontal flight speed.

All these conclusions are valid if we ignore the factors which restrict the maximal horizontal flight speed.

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