# Factors Affecting Performance A helicopter’s performance is dependent upon the power output of the engine and lift production of the rotors. Any factor affecting engine and rotor efficiency affects performance. The three major factors affecting performance are density altitude, weight, and wind. *** ## **Density Altitude** As air density increases, engine power output, rotor efficiency, and aerodynamic lift also increase. Density altitude is the altitude above mean sea level (MSL) at which a given atmospheric density occurs in the standard atmosphere. It can also be interpreted as pressure altitude (PA) corrected for nonstandard temperature differences. PA is displayed as the height above a standard datum plane, which in this case, is a theoretical plane where air pressure is equal to 29.92 inches mercury (Hg). PA is the indicated height value when the altimeter setting is adjusted to 29.92 inches Hg. PA, as opposed to true altitude, is an important value for calculating performance as it more accurately represents the air content at a particular level. The difference between true altitude and PA must be clearly understood. True altitude means the vertical height above MSL and is displayed on the altimeter when the altimeter is correctly adjusted to the local setting. For example, if the local altimeter setting is 30.12 inches Hg and adjusted to this value, it indicates the exact height above sea level. However, this does not reflect conditions found at this height under standard conditions. Since the altimeter setting is more than 29.92 inches Hg, the air in this example has a higher pressure and is more compressed, indicative of air found at a lower altitude. Therefore, the PA is lower than the actual height above MSL. To calculate PA without use of an altimeter, remember pressure decreases approximately 1 inch of mercury for every 1,000-foot increase in altitude. Four factors affecting density altitude most are atmospheric pressure, altitude, temperature, and moisture content of the air. ### **Atmospheric Pressure** Due to changing weather conditions, atmospheric pressure at a given location changes from day to day. If the pressure is lower, the air is less dense. This means a higher density altitude and less helicopter performance. ### **Altitude** As altitude increases, air becomes thinner. This is because the atmospheric pressure acting on a given volume of air is less, allowing air molecules to move further apart. Dense air contains air molecules spaced closely together, while thin air contains air molecules spaced further apart. As altitude increases, density altitude increases. ### **Temperature** As warm air expands, the air molecules move further apart, creating less dense air. Since cool air contracts, air molecules move closer together creating denser air. High temperatures cause even low elevations to have high density altitudes. ### **Moisture (Humidity)** The water content of air also changes air density as water vapor weighs less than dry air. Therefore, as the water content of the air increases, air becomes less dense, increasing density altitude and decreasing performance. Humidity, also called relative humidity, refers to the amount of water vapor contained in the atmosphere and is expressed as a percentage of the maximum amount of water vapor air can hold. This amount varies with temperature; warm air can hold more water vapor, while colder air holds less. Perfectly dry air that contains no water vapor has a relative humidity of 0 percent, while saturated air that cannot hold any more water vapor has a relative humidity of 100 percent. Humidity alone is usually not considered an important factor in calculating density altitude and helicopter performance; however, it does contribute. There are no rules-of-thumb or charts used to compute the effects of humidity on density altitude. Aviators should expect a decrease in hovering and takeoff performance in high humidity conditions. *** ## **High and Low Density Altitude Conditions** A thorough understanding of the terms high density altitude and low density altitude are required. In general, high density altitude refers to thin air, while low density altitude refers to dense air. Those conditions resulting in a high density altitude (thin air) are high elevations, low atmospheric pressure, high temperatures, high humidity, or some combination thereof. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude (dense air). However, high density altitudes may be present at lower elevations on hot days, so it is important to calculate density altitude and determine performance before a flight. One of the ways density altitude can be determined (CPU-26A/P is another) is through the use of charts designed for that purpose. The graph is used to find density altitude either on the ground or aloft. Set altimeter at 29.92 inches to indicate PA. Read outside air temperature (OAT). Enter the graph at that PA and move horizontally to the temperature. Read density altitude from the sloping lines. * **Example 1**: Find density altitude in flight. PA is 9,500 feet and temperature is 18 degrees F. Find 9,500 feet on the left of the graph and move across to 18 degrees * **Example 2:** Find density altitude for takeoff. PA is 4,950 feet and temperature is 97 degrees F. Enter the graph at 4,950 feet and move across to 97 degrees F. Density altitude is 8,200 feet (marked 2 on graph). {% hint style="info" %} In warm air, density altitude is considerably higher than PA {% endhint %} Most performance charts do not require computation of density altitude; instead, the computation is built into the performance chart. All that remains is to enter the correct PA and temperature.
*** ## **Weight** Weight is the force opposing lift. As weight increases, power required to produce lift needed to compensate for the added weight must also increase. Most performance charts include weight as one of the variables. By reducing weight, the helicopter is able to safely takeoff or land at a location otherwise impossible. However, if in doubt, takeoff is delayed until more favorable density altitude conditions exist. If airborne, land at a location that has more favorable conditions or one where a landing can be made that does not require a hover. At higher gross weights, the increased power required to hover produces more torque, which means more antitorque thrust is required. In some helicopters, during high altitude operations, the maximum antitorque produced by the tail rotor during a hover may not be sufficient to overcome torque even if the gross weight is within limits. *** ## **Winds** Wind direction and velocity also affect hovering, takeoff, and climb performance. Translational lift occurs any time there is relative airflow over the rotor disc. This occurs whether the relative airflow is caused by helicopter movement or wind. As wind speed increases, translational lift increases, resulting in less power required to hover. Wind direction is also an important consideration. Headwinds are desirable as they contribute to the most increase in performance. Strong crosswinds and tailwinds may require the use of more tail rotor thrust to maintain directional control. This increased tail rotor thrust absorbs power from the engine, which means less power is available to the main rotor for production of lift. Some helicopters even have a critical wind azimuth or maximum safe relative wind chart. Operating the helicopter beyond these limits could cause loss of tail rotor effectiveness. Takeoff and climb performance are greatly affected by wind. When taking off into a headwind, ETL is achieved earlier, resulting in more lift and a steeper climb angle. When taking off with a tailwind, more distance is required to accelerate through translational lift. *** ## **Performance Charts** In developing performance charts, aircraft manufacturers make certain assumptions about the condition of the helicopter and ability of the pilot. It is assumed 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 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. *** ## **Hovering Performance** Helicopter performance revolves around whether or not hover is possible. More power is required during hover than in any other flight regime. Obstructions aside, if hover can be maintained, takeoff can be made, especially with the additional benefit of translational lift. Charts are provided for in-ground effect (IGE) and out-of-ground effect (OGE) under various conditions of gross weight, altitude, temperature, and power. The IGE hover ceiling is higher than OGE hover ceiling due to the added lift benefit produced by ground effect. 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 existing conditions. Any adjustment to 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. Being able to hover at the takeoff location with a certain 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. You should be able to predict whether hovering power will be available at the destination by knowing the temperature and wind conditions. Using performance charts in the helicopter flight manual, and making certain power checks during hover and in flight prior to commencing the approach and landing. *** ## **Climb Performance** Most factors affecting hover and takeoff performance also affect climb performance. In addition, turbulent air, pilot techniques, and the overall condition of the helicopter can cause climb performance to vary. A helicopter flown at the best rate-of-climb speed 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 best rate-of-climb speed 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 depends upon power available. If there is a surplus of power available, the helicopter can climb vertically; therefore, the best angle-of-climb speed 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 ground speed and ground track. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://229.gitbook.io/229-hub/knowledge/army-aviation-fundamentals/aerodynamics-of-flight/factors-affecting-performance.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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