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Tuesday, April 1, 2014

Picture the Power

By Frank Lombardi

Frank Lombardi
The Power Required curve is not typically included in the rotorcraft flight manual, but contains a wealth of information and can help to explain some of the mannerisms of the helicopter. It plots the amount of engine power required to maintain level flight at various airspeeds for a given weight and altitude (see figure). It can be created by recording a series of airspeed and power settings in level flight. Add up the individual power required to overcome profile, induced, and parasite drag (including ancillaries like generators, transmission drag, etc.), and this curve would be the result.

Air friction on the skin of the rotor blades creates profile drag. Profile power is what is needed to keep full RPM at flat pitch on the ground. It remains fairly constant until speeds get very high.

Induced drag is created when the blades produce lift. As air is forced down through the rotor and lift is created, the resultant lift vector is tilted somewhat aft. This aft component acts as drag, requiring additional power. Induced drag is highest in a hover and decreases rapidly as speed increases.

Parasite power is the power required to overcome any other drag not associated with the spinning rotors, i.e., the fuselage, landing gear, etc. It is zero in a hover and increases very rapidly as airspeed increases, at the rate of velocity cubed. A cleaner fuselage design will have less parasite drag.

Summing up the three drag curves into one total power required curve reveals some important points. The lowest point on the curve occurs at the “bucket” speed (point 1), where total power required is a minimum. Flying at that speed will give maximum endurance in level-flight, being at/near minimum fuel flow. As that speed also gives the greatest surplus of available power, it will produce the best rate of climb (VY) when the collective is pulled in. Since the rotor power requirements remain essentially the same when unpowered, it is also the speed that will produce the minimum sink rate, should you find yourself in autorotation.

To maximize range, you need the best combination of maximum speed at minimum power (actually min fuel flow, which is at a slightly higher speed in turbines, to be perfectly correct). This is found by drawing a line from the origin of the graph tangent to the curve (point 2). Since lift-to-drag is a maximum at this point, this airspeed will also be the best range glide speed when the engine quits. Maximum horizontal speed is reached when power required meets power available (point 3).

If power is limited due to an inoperative engine or high-density altitude, then the power available line drops down and two points of intersection dictate your speed range (points 4 and 5). If unable to hover/takeoff vertically, the speed for best angle of climb (VX) happens at a combination of max power margin and minimum speed, found by drawing a line from the beginning of the limited power available, tangent to the power required (point 6).

The shape of the curve reveals some characteristics as well. In the bucket speed range from approximately 75-45 knots, the curve is fairly flat, as the power required to maintain level flight in that range doesn’t change drastically. This is why on an approach it can seem difficult at first to get the aircraft to descend as it slows, but once you get to about 40 knots “the bottom drops out.” At that point you are on the “backside” of the curve, where aft cyclic increases your descent rate. On either side of the bucket speed, the power requirements rise rapidly within a given airspeed range. Because of this, you’ll find that in these speed ranges, you can control your vertical flight path with small cyclic changes much more effectively.

Next time, whether you are trying to fly a precision instrument approach, steep approach, or some advanced autorotational spot landings, picture the power curve. It may come in handy.

 

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