[FF-6] Performance and Energy

G-Loading. Load factor, expressed in Gravity (G) units, is the total load imposed on an aircraft, divided by the aircraft weight. Load factor during a turn varies with the angle of bank. Airspeed during a turn does not affect load factor, because for a given bank angle, the rate of turn decreases with increased airspeed and results in no change of centrifugal force. For a 60-degree bank, the load factor for any aircraft is 2 Gs regardless of airspeed. This means that a 20,000-pound H-60 in a 60-degree bank will, in effect, exert 40,000 pounds of force on the aircraft structure. Load factors do not begin to increase significantly until bank angles exceed 30 degrees. Above 30 degrees, the load factor rises at an increasing rate and may produce unacceptable rotor disk loading, depending upon the aircraft gross weight and flight conditions.

Turn Performance and Energy. A level turn is one in which altitude is kept constant, which
could be accomplished with or without a constant airspeed. To cause the helicopter to turn, a turning (centripetal) force must be applied perpendicular to its flight path. This force is created by banking the rotor system in the direction of turn. The horizontal component of the thrust is the turning force and the vertical component is equal and opposite in weight. A steeper bank angle will produce a greater turning force, but will require additional power (thrust) to maintain altitude. The percentage of increase of power required during turning flight, as opposed to straight flight, is directly related to the bank angle. A 15° bank requires only a 3.6 percent increase in power, while a 45° bank will require a power increase of over 41 percent (refer to Table 1). Sufficient power may not be available for a sustained 45° bank angle at very high or low (high drag) airspeeds, and a sustained 60° bank angle may only be maintained at speeds near the maximum rate-of-climb/maximum-endurance (least drag) airspeed. Sustained turning maneuvers that require more power than is available can be completed only at the expense of losing altitude. During lowlevel flight, such an altitude loss could be disastrous. 

Table 1. Effect of Bank Angle on Thrust Required.

BANK ANGLE	INCREASE IN MAIN ROTOR THRUST REQUIRED
0° 15° 30° 45° 60°	0 %  3.6 %  15.4 %  41.4 % 100.0 %

WARNING: At steep bank angles (over 60°) a rate of descent will occur. If the descent is not arrested shortly after its onset, it can increase to 5000 FPM. Recovery from these high rates of descent can only be made by rolling to a near wings level attitude.

 

Indicated versus True Airspeed. For any aircraft, the indicated airspeed (IAS) is influenced by the density of the surrounding air. As the air becomes less dense (higher density altitude), IAS will decrease for a given true airspeed (TAS). For example, at a DA of 12,000 feet, an aircraft traveling at an IAS of 100 knots is actually traveling at a TAS of 120 knots. At high DA, if you continue to fly the same IAS you use at low DA, several problems can result, because you are actually moving faster than you are accustomed. If you don't shorten the time of each traffic pattern leg, your patterns will get larger. Your rate of closure on final will be faster. Due to the increased inertia, you may overshoot the intended landing area. Power changes on final will be greater.