Figure 3-4

HELICOPTER AERODYNAMICS WORKBOOK

CHAPTER 3

Best rate of climb airspeed is formed at the point where the difference is a maximum between

power required and power available. This rate of speed can be estimated from the change in

potential energy. The increase in mass flow from forward flight reduces climb power required as

opposed to vertical flight. Induced power is already low in forward flight, so there is little to be

gained from a significant increase in mass flow. Also, since a climbing condition produces a

significant increase in parasite drag and tail rotor power requirements, excess engine power is

concentrated toward those efforts instead of vertical flight.

At this speed, minimum rate of descent in an autorotation is also found, since the power

required to keep the aircraft airborne is at a minimum. At this speed, the potential energy

corresponding to height above the ground and gross weight can be dissipated at the slowest rate.

Since the goal of achieving maximum loiter time is making the available fuel last as long as

possible, and since fuel flow is proportional to engine power, maximum loiter time should also

be at this point.

Stretching the glide distance in an autorotation is a totally separate situation. Maximum glide

range is found at a point tangent to the power required curve on a line drawn from the origin.

This gives the highest lift-to-drag ratio.

Figure 3-4

Maximum range speed is found on the fuel flow curve (figure 3-4) by drawing a line tangent

to the curve from the origin. This ratio of speed to fuel flow shows the distance one can travel

on a pound of fuel on a no-wind day. If there is a head wind, the line should be originated at the

head wind value, which derives a higher speed and lower range. For a tail wind, the optimum

airspeed decreases, but the range increases significantly.

HELICOPTER POWERED FLIGHT ANALYSIS 3-5