The Power Struggle

The performance envelope of every helicopter is defined by the relationship between the power required and the power available at some flight condition. Power required can be thought of as being airframe dependent, while power available is engine dependent. In other words, the helicopter does not care what kind of engine powers it, any more than the engine cares what it supplies power to.

It takes the same amount of force to hold a 5,000-pound helicopter one foot off the ground as it does to hold it at any cruising altitude. But to create that lift force, we need to turn the rotor and accelerate a mass of air. In physics, it is said that the rotor does work on the air. It takes power (the rate at which work is done) to overcome the drag of the rotors and create lift. Power is that parameter we can measure in the cockpit, and changing conditions of air density, airspeed and flight condition will all cause a change in power requirement.

Generally speaking, the maximum power available from the engine can be assumed to be constant. Circling back to the idea that power required is airframe dependent, it can be further broken down into the subcategories of profile, induced and parasitic.

Profile power is required to overcome the friction drag on the blades and push the rotor’s shape through the viscous air. It does not change significantly with a change in angle of attack and accounts for 15% to 40% of the main rotor power required in a hover. It stays relatively constant with airspeed until, when at high speeds, compressibility and/or blade stall drive it up.

Induced power is required to overcome the drag developed during the creation of rotor thrust. With an increase in angle of attack, the airflow that moves down through the rotor causes the total reaction lift vector of the blade to tilt rearward, creating induced drag. It takes about 60% to 85% of total main rotor power in a hover to overcome it.

Parasitic power is additional power required to move everything else attached to the rotor through the air — the fuselage and everything attached to it. It rises with the cube of airspeed.

Adding up these components of power — as well as miscellaneous power consumers like the tail rotor, hydraulic pumps, gearbox losses, generators, etc. — results in the familiar total power required curve that defines our flight envelope.

Moving from hover into forward flight brings a rapid decrease in required power, due to a change in inflow angle of oncoming air. This is the induced power decreasing as the rotor becomes more efficient (translational lift).

Total power continues to decrease with increasing airspeed, until you reach the “bucket” speed. This is the point of greatest difference between power required and power available, which can be translated into maximum rate of climb. Beyond this speed, the rotor continues to become more efficient, but wind resistance begins to prevail, and parasite power swaps places with induced power as the main contributor to total rotor power required. Total power then begins to rise until it meets with power available, defining the helicopter’s maximum horizontal speed.

Although the blades and airframe move easier through the thinner air of higher altitudes, more pitch in the blades is required to create thrust, and an overall increase in total power required can be expected with altitude increase.

Having a keen understanding of an aircraft’s power requirements can help perfect technique, maximize performance and minimize maintenance. Knowing that induced power will decrease with greater airflow through the rotor disk in forward flight — and similarly in a max performance vertical takeoff — a pilot can finesse a heavy aircraft into the air by watching the power margin increase on a decreasing torque meter allowing more “pitch pull,” while still staying within limits. Knowing that left pedal or a left cyclic roll will drive power requirements up on rotors rotating clockwise (looking up), a pilot can avoid an overtorque by not operating too close to a limit while maneuvering. Opposite pedal/cyclic will, of course, do the same on opposite spinning rotors.

Multi-engine helicopters bring even more need to understand power, especially in one-engine-inoperative flight, and require their own discussion. The performance envelope of every helicopter is defined by the relationship between the power required and the power available at some flight condition. Power required can be thought of as being airframe dependent, while power available is engine dependent. In other words, the helicopter does not care what kind of engine powers it, any more than the engine cares what it supplies power to.

It takes the same amount of force to hold a 5,000-pound helicopter one foot off the ground as it does to hold it at any cruising altitude. But to create that lift force, we need to turn the rotor and accelerate a mass of air. In physics, it is said that the rotor works on the air. It takes power (the rate at which work is done) to overcome the drag of the rotors and create lift. Power is that parameter we can measure in the cockpit, and changing conditions of air density, airspeed and flight condition will all cause a change in power requirement.

Generally speaking, the maximum power available from the engine can be assumed to be constant. Circling back to the idea that power required is airframe dependent, it can be further broken down into the subcategories of profile, induced and parasitic.

Profile power is required to overcome the friction drag on the blades and push the rotor’s shape through the viscous air. It does not change significantly with a change in angle of attack and accounts for 15% to 40% of the main rotor power required in a hover. It stays relatively constant with airspeed until, when at high speeds, compressibility and/or blade stall drive it up.

Induced power is required to overcome the drag developed during the creation of rotor thrust. With an increase in angle of attack, the airflow that moves down through the rotor causes the total reaction lift vector of the blade to tilt rearward, creating induced drag. It takes about 60% to 85% of total main rotor power in a hover to overcome it.

Parasitic power is additional power required to move everything else attached to the rotor through the air — the fuselage and everything attached to it. It rises with the cube of airspeed.

Adding up these components of power — as well as miscellaneous power consumers like the tail rotor, hydraulic pumps, gearbox losses, generators, etc. — results in the familiar total power required curve that defines our flight envelope.

Moving from hover into forward flight brings a rapid decrease in required power, due to a change in inflow angle of oncoming air. This is the induced power decreasing as the rotor becomes more efficient (translational lift).

Total power continues to decrease with increasing airspeed, until you reach the “bucket” speed. This is the point of greatest difference between power required and power available, which can be translated into maximum rate of climb. Beyond this speed, the rotor continues to become more efficient, but wind resistance begins to prevail, and parasite power swaps places with induced power as the main contributor to total rotor power required. Total power then begins to rise until it meets with power available, defining the helicopter’s maximum horizontal speed.

Although the blades and airframe move easier through the thinner air of higher altitudes, more pitch in the blades is required to create thrust, and an overall increase in total power required can be expected with altitude increase.

Having a keen understanding of an aircraft’s power requirements can help perfect technique, maximize performance and minimize maintenance. Knowing that induced power will decrease with greater airflow through the rotor disk in forward flight — and similarly in a max performance vertical takeoff — a pilot can finesse a heavy aircraft into the air by watching the power margin increase on a decreasing torque meter allowing more “pitch pull,” while still staying within limits. Knowing that left pedal or a left cyclic roll will drive power requirements up on rotors rotating clockwise (looking up), a pilot can avoid an overtorque by not operating too close to a limit while maneuvering. Opposite pedal/cyclic will, of course, do the same on opposite spinning rotors.

Multi-engine helicopters bring even more need to understand power, especially in one-engine-inoperative flight, and require their own discussion.

This column originally appeared on R&WI's website. RWI