In developing performance charts, aircraft manufacturers make certain assumptions about the condition of the helicopter and the ability of the pilot. It is assumed that the helicopter is in good operating condition and the engine is developing its rated power. The pilot is assumed to be following normal operating procedures and to have average flying abilities. Average means a pilot capable of doing each of the required tasks correctly and at the appropriate times.
Using these assumptions, the manufacturer develops performance data for the helicopter based on actual flight tests. However, they do not test the helicopter under each and every condition shown on a performance chart. Instead, they evaluate specific data and mathematically derive the remaining data.
Most autorotational performance charts state that autorotational descent performance is a function of airspeed and is essentially unaffected by density altitude and gross weight. Keep in mind that, at some point, the potential energy expended during the autorotation is converted into kinetic energy for the flare and touchdown phase of the maneuver. It is at that point that increased density altitudes and heavier gross weights have a great impact on the successful completion of the autorotation. The rotor disk must be able to overcome the downward momentum of the helicopter and provide enough lift to cushion the landing. With increased density altitudes and gross weights, the lift potential is reduced and a higher collective pitch angle (angle of incidence) is required. The aerodynamics of autorotation has been presented in detail in Aerodynamics of Flight, Autorotation.
Helicopter performance revolves around whether or not the helicopter can be hovered. More power is required during the hover than in any other flight regime. Obstructions aside, if a hover can be maintained, a takeoff can be made, especially with the additional benefit of translational lift. Hover charts are provided for in ground effect (IGE) hover and out of ground effect (OGE) hover under various conditions of gross weight, altitude, temperature, and power. The IGE hover ceiling is usually higher than the OGE hover ceiling because of the added lift benefit produced by ground effect. See Aerodynamics of Flight, Hovering Flight, for more details on IGE and OGE hover. A pilot should always plan an OGE hover when landing in an area that is uncertain or unverified.
As density altitude increases, more power is required to hover. At some point, the power required is equal to the power available. This establishes the hovering ceiling under the existing conditions. Any adjustment to the gross weight by varying fuel, payload, or both, affects the hovering ceiling. The heavier the gross weight, the lower the hovering ceiling. As gross weight is decreased, the hover ceiling increases.
Sample Hover Problem 1
You are to fly a photographer to a remote location to take pictures of the local wildlife. Using Figure 1, can you safely hover in ground effect at your departure point with the following conditions?
A. Pressure Altitude...........................8,000 feet
B. Temperature..................................+15 °C
C. Takeoff Gross Weight....................1,250 lb
|Figure 1. In ground effect hovering ceiling versus gross weight chart|
First enter the chart at 8,000 feet pressure altitude (point A), then move right until reaching a point midway between the +10 °C and +20 °C lines (point B). From that point, proceed down to find the maximum gross weight where a 2 foot hover can be achieved. In this case, it is approximately 1,280 pounds (point C).
Since the gross weight of your helicopter is less than this, you can safely hover with these conditions.
Sample Hover Problem 2
Once you reach the remote location in the previous problem, you will need to hover OGE for some of the pictures. The pressure altitude at the remote site is 9,000 feet, and you will use 50 pounds of fuel getting there. (The new gross weight is now 1,200 pounds.) The temperature will remain at +15 °C. Using Figure 2, can you accomplish the mission?
|Figure 2. Out of ground effect hover ceiling versus gross weight chart|
Enter the chart at 9,000 feet (point A) and proceed to point B (+15 °C). From there, determine that the maximum gross weight to hover OGE is approximately 1,130 pounds (point C). Since your gross weight is higher than this value, you will not be able to hover in these conditions. To accomplish the mission, you will need to remove approximately 70 pounds before you begin the flight.
These two sample problems emphasize the importance of determining the gross weight and hover ceiling throughout the entire flight operation. Being able to hover at the takeoff location with a specific gross weight does not ensure the same performance at the landing point. If the destination point is at a higher density altitude because of higher elevation, temperature, and/or relative humidity, more power is required to hover there. You should be able to predict whether hovering power will be available at the destination by knowing the temperature and wind conditions, using the performance charts in the helicopter flight manual, and making certain power checks during hover and in flight prior to commencing the approach and landing.
For helicopters with dual engines, performance charts provide torque amounts for both engines.
Sample Hover Problem 3
Using Figure 3, determine what torque is required to hover.
Use the following conditions:
A. Pressure Altitude . . . . . . . . . . .. . . ..9,500 feet
B. Outside Air Temperature. . . . . . . . . . 0 °C
C. Gross Weight. . . . . . . . . . . . .. . . . . . 4,250 lb
D. Desired Skid Height . . . . . . . . . . . . . 5 feet
|Figure 3. Torque required for cruise or level flight|
First, enter the chart at 9,500 feet pressure altitude, then move right to outside air temperature, 0 °C. From that point, move down to 4,250 pounds gross weight and then move left to 5 foot skid height. Drop down to read 66 percent torque required to hover.
Most of the factors affecting hover and takeoff performance also affect climb performance. In addition, turbulent air, pilot techniques, and overall condition of the helicopter can cause climb performance to vary.
A helicopter flown at the best rate-of-climb speed (VY) obtains the greatest gain in altitude over a given period of time. This speed is normally used during the climb after all obstacles have been cleared and is usually maintained until reaching cruise altitude. Rate of climb must not be confused with angle of climb. Angle of climb is a function of altitude gained over a given distance. The VY results in the highest climb rate, but not the steepest climb angle, and may not be sufficient to clear obstructions. The best angle of climb speed (VX) depends upon the power available. If there is a surplus of power available, the helicopter can climb vertically, so VX is zero. Wind direction and speed have an effect on climb performance, but it is often misunderstood. Airspeed is the speed at which the helicopter is moving through the atmosphere and is unaffected by wind. Atmospheric wind affects only the groundspeed, or speed at which the helicopter is moving over the Earth’s surface. Thus, the only climb performance affected by atmospheric wind is the angle of climb and not the rate of climb.
When planning for climb performance, it is first important to plan for torque settings at level flight. Climb performance charts show the change in torque, above or below torque, required for level flight under the same gross weight and atmospheric conditions to obtain a given rate of climb or descent.
Sample Cruise or Level Flight Problem
Determine torque setting for cruise or level flight using Figure 4. Use the following conditions:
Pressure Altitude........................................ 8,000 feet
Outside Air Temperature............................ +15 °C
A. Indicated Airspeed..................................80 knots
B. Maximum Gross Weight.........................5,000 lb
|Figure 4. Maximum rate-of-climb chart|
With this chart, first confirm that it is for a pressure altitude of 8,000 feet with an OAT of 15°. Begin on the left side at 80 knots indicated airspeed (point A) and move right to maximum gross weight of 5,000 lb (point B). From that point, proceed down to the torque reading for level flight, which is 74 percent torque (point C). This torque setting is used in the next problem to add or subtract cruise/descent torque percentage from cruise flight.
Sample Climb Problem
Determine climb/descent torque percentage using Figure 5. Use the following conditions:
A. Rate of Climb or Descent. . . . . . . . . . . 500 fpm
B. Maximum Gross Weight . . . . . . . . . . . 5,000 lb
|Figure 5. Climb/descent torque percentage chart|
With this chart, first locate a 500 fpm rate of climb or descent (point A), and then move to the right to a maximum gross weight of 5,000 lb (point B). From that point, proceed down to the torque percentage, which is 15 percent torque (point C). For climb or descent, 15 percent torque should be added/ subtracted from the 74 percent torque needed for level flight. For example, if the numbers were to be used for a climb torque, the pilot would adjust torque settings to 89 percent for optimal climb performance.
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