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Helicopter Lesson - Performance

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Chapter 8. HELICOPTER PERFORMANCE (CONTINUED)

EFFECT OF GROSS WEIGHT ON HELICOPTER PERFORMANCE

We learned earlier that the total weight of a helicopter is the first force that must be overcome before flight is possible. Lift is the force that is needed to overcome or balance this total weight. It is easily seen that the greater the gross weight of the helicopter, the more lift that is required to hover. The amount of lift available is dependent upon the angle of attack at which the rotor blades can operate and still maintain required rotor RPM. The angle of attack at which the blades can operate at required rotor RPM is dependent upon the amount of power available. Therefore, the heavier the gross weight, the greater the power required to hover and for flight in general, and the poorer the performance of the helicopter since less reserve power is available (fig. 62); or, to state it another way, the heavier the gross weight, the lower the hovering ceiling.

A study of the hovering ceiling chart (fig. 56) reveals the following interesting information for one helicopter, and this is fairly typical for all helicopters (with unsupercharged engines). At 60° F. in dry air, the hovering ceilings for gross weights of 1,300, 1,400, 1,500, and 1,600 pounds are 9,400 feet, 7,400 feet, 5,900 feet, and 4,300 feet, respectively. An increase of 300 pounds in gross weight decreases the hovering ceiling by more than half. At gross weights of 1,300, 1,400, 1,500, and 1,600 pounds at a temperature of 100° F. in air with a relative humidity of 80 percent the hovering ceilings are 6,100 feet, 4,400 feet, 2,900 feet, and 1,300 feet, respectively. In the latter case, an increase of 300 pounds in gross weight reduces the hovering ceiling by almost 80 percent. A comparison of the two examples illustrates vividly the reduction in performance brought about by a combination of heavy gross weight and high density altitudes.

Of the three major factors affecting the performance of a helicopter at high elevations (density altitude, wind, and gross weight), the pilot can control only the gross weight. It should be obvious that the gross weight carried on any flight must be considered - not only for takeoff under the existing density altitude, wind conditions, and power available at point of departure, but also under the expected density altitude, wind conditions, and power available at the landing destination. Smaller amounts of fuel may be carried to improve performance or to increase useful load. It must be remembered, however, that this necessitates a sacrifice in range. The importance of loading a helicopter within the approved center-of-gravity limits, and the ill effects on performance if this is not properly accomplished, have been discussed in the preceding chapter.

EFFECT OF WIND ON HELICOPTER PERFORMANCE

We have seen earlier that when the horizontal airspeed of the helicopter reaches approximately 15 miles per hour, an abrupt increase in lift is experienced. This we call effective translational lift. Actually, from the moment the helicopter begins forward flight, translational lift is present, but is not very apparent or effective under about 15 miles per hour.

Translational lift is created by airspeed, not groundspeed. Therefore, translational lift is also present when the helicopter is hovering in a wind. If the wind velocity is 15 miles per hour or more, the helicopter will be experiencing effective translational lift in a hover. Due to this increased lift, less power will be required to hover than would be required for hovering in a no-wind condition (see figure 63 to the right); or, a greater gross weight could be carried when takeoff is to be made in a wind exceeding 15 miles per hour than could be carried if takeoff is to be made in a no-wind condition.

No-wind conditions increase the amount of power necessary to hover, or require that a lighter load be carried. Thus, no-wind conditions reduce helicopter performance. Since wind decreases the power required for hovering, or permits taking off or landing with greater loads, helicopter performance is improved. If the wind exceeds 15 miles per hour, performance is improved considerably; however, wind gusts over 30 to 35 miles per hour may tend to destroy the additional lift obtained between 15 and 30 miles per hour.

PRACTICAL METHODS FOR PREDICTING HELICOPTER PERFORMANCE

Certain practical methods for predicting helicopter performance were developed through engineering and flight tests for a particular model helicopter used by the Army. These practical methods for this particular helicopter are given in this handbook to give the reader a clearer understanding of factors influencing helicopter performance, and sound principles on which to base flight decisions. We wish to emphasize the fact that these rules are for a particular helicopter used by the Army and the actual figures will apply only to this particular helicopter. Even though such practical aids are developed for a helicopter, they should not be used as substitutes for experience and good judgment.

Manifold pressure and payload

Tests on this particular helicopter showed that 1 inch of manifold pressure was equivalent to 6 horsepower (HP), and that 1 HP would lift 13.5 pounds of weight while hovering. When combined, these two facts give rise to this practical rule:

Rule No. 1 - One inch of manifold pressure will lift 80 pounds of payload.

With this knowledge, the pilot can obtain an estimate of the additional weight that can safely be carried to hover and then to enter flight. This rule may be applied before landing at destination in this manner:

Momentarily, apply full throttle at 100 feet, or less, above the ground and determine the maximum manifold pressure that can be obtained. This will be approximately equal to the maximum manifold pressure available for takeoff.
While hovering, check manifold pressure required for the hover.
Find the difference between maximum available manifold pressure and manifold pressure required to hover.
The difference in manifold pressure changed into its equivalent in weight (1 inch of manifold pressure is equivalent to 80 pounds) gives the approximate additional payload which can be carried to lift to a hover for safe takeoff.

Temperature, winds, altitude, and gross weight are included in the above practical method for this particular helicopter, and need not be considered separately.

Manifold pressure and hovering ceiling

By using available manifold pressure to determine hovering ceiling, a pilot can predict whether or not hovering flight is possible at the destination.

Rule No. 2 - If wind velocity at point of intended landing is approximately the same as at point of takeoff, and the flight is made within the same air mass (no radical temperature change), for each inch of manifold pressure in excess of that required to hover, add 1,000 feet to the point-of-takeoff altitude. This computed altitude will represent the approximate hovering ceiling.

This practical rule may be applied as follows:

Check manifold pressure at a normal hover prior to departure.
While hovering, momentarily apply full throttle and note the maximum manifold pressure available.
The difference in these two manifold pressure readings is equivalent to 1,000 feet altitude per inch of excess manifold pressure. This additional altitude added to the point-of-takeoff altitude will give the maximum altitude (above sea level) at which the helicopter may be hovered (in ground effect).

Payload and wind

In winds from 0 to 15 miles per hour, the hovering ceiling of the helicopter will increase about 100 feet for each mile per hour of wind. In winds from about 15 MPH to 26 MPH, the hovering ceiling will increase about 350 feet for each mile per hour of wind.

Rule No. 3 - The payload may be increased 8 pounds for each mile per hour of wind from 0 to 15 miles per hour, or may be increased 28 pounds for each mile per hour of wind from 15 MPH to 26 MPH.

Hovering and skid height

The hovering altitude over level terrain for this particular helicopter is ideal with a skid clearance of approximately 4 feet (height of skid above the ground). Variable hovering altitudes, due to obstacles or rough terrain, have a decided effect on helicopter performance in determining hovering ceiling and payload. These effects are best estimated as follows:

Rule No. 4 - (1) To hover under 4 feet, add 300 feet to the hovering ceiling or 24 pounds to the payload for each 6 inches of decrease in skid height from the 4-foot hover. (2) To hover between 4 feet and 10 feet, subtract 300 feet from the hovering ceiling or 24 pounds from the payload for each foot of increase in skid height.

Hovering ceiling and gross weight

The hovering ceiling will vary in proportion to the gross weight of the helicopter. To determine hovering ceiling for a known gross weight, apply the following rule:

Rule No. 5 - (1) A 100-pound REDUCTION in gross weight increases hovering ceiling in or out of ground effect about 1,300 feet. (2) A 100-pound INCREASE in gross weight decreases hovering ceiling about 1,300 feet.

Service ceiling and gross weight

The service ceiling of the helicopter varies with gross weight. (For all practical purposes, service ceiling is the maximum obtainable altitude.) To determine the effects of gross weight on service ceiling, apply the following rule:

Rule No. 6 - A 100-pound DECREASE In gross weight adds 800 feet to the service ceiling, and, conversely, a 100-pound INCREASE in gross weight reduces the service ceiling 800 feet.

We wish to reemphasize that these rules are for one particular helicopter used by the Army and the actual figures will apply only to this helicopter.

BRIEF SUMMARY

A thumbnail summary of this chapter might be as follows:

The most favorable conditions for helicopter performance are the combination of a low-density altitude, light gross weight, and moderate to strong wind (fig. 64).
The most adverse conditions for helicopter performance are the combination of a high-density altitude, heavy gross weight, and calm or no wind (fig. 64).
Any other combination of density altitude, gross weight, and wind conditions falls somewhere between the most adverse conditions and the most favorable conditions.

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