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Aircraft Performance

Performance generally refers to the motion of the airplane along its flight path, fore and aft, up or down, right or left. The term "Performance" also refers to how fast, how slow, how high and how far. It may also refer, in general sense, to the ability of an airplane to successfully accomplish the different aspects of its mission. Included are such items as minimum and maximum speed, maximum altitude, maximum rate of climb, maximum range and speed for maximum range, rate of fuel consumption, takeoff and landing distance, weight of potential payload, etc. There are specific maneuvers which are used to measure and quantify these characteristics for each airplane. In many cases, flight testing takes place in a competitive environment to select the best airplane for accomplishing a particular mission. Since all of these performance measurements are strongly affected by differences in the weather conditions (that is, temperature, pressure, humidity, winds), there are some very specific and complex mathematical processes which are used to "standardize" these values.

Aircraft Performance

One of the most important considerations in flight is the balance of forces maintained between thrust, drag, lift, and weight.


Balance of Forces

An aircraft in flight retains energy in two forms; kinetic energy and potential energy. Kinetic energy is related to the speed of the airplane, while potential energy is related to the altitude above the ground. The two types of energy can be exchanged with one another. For example when a ball is thrown vertically into the air, it exchanges the kinetic energy (velocity imparted by the thrower), for potential energy as the ball reaches zero speed at peak altitude.

When an airplane is in stabilized, level flight at a constant speed, the power has been adjusted by the pilot so that the thrust is exactly equal to the drag. If the pilot advances the throttle to obtain full power from the engine, the thrust will exceed the drag and the airplane will begin to accelerate. The difference in thrust between the thrust required for level flight and the maximum available from the engine is referred to as "excess thrust". When the airplane finally reaches a speed where the maximum thrust from the engine just balances the drag, the "excess thrust" will be zero, and the airplane will stabilize at its maximum speed.

Notice that this "excess thrust" can be used either to accelerate the airplane to a higher speed (increase the kinetic energy) or to enter a climb at a constant speed (increase the potential energy), or some combination of the two.

Excess Thrust EnergyExchange

Excess Thrust Energy Exchange

There are energy exchange equations which can be used to relate the rate of change of speed (or acceleration) to the rate of change of altitude (or rate of climb). (These equations are introduced later.) In this way, level flight accelerations (accels.) at maximum power can be used to measure the "excess thrust" over the entire speed range of the airplane at one altitude. This "excess thrust" can then be used to calculate the maximum rate of climb capability for an aircraft.

Takeoff

The takeoff is a critical maneuver in any airplane. The airplane will usually be carrying a payload (passengers, cargo, weapons) and often a full load of fuel. The resulting heavy weight means that a high speed must be reached before the wings can generate sufficient lift, thus a long distance must be travelled on the runway before lift-off. After lift-off, the heavy weight will result in a relatively slow acceleration to the speed for best angle of climb.

After lining the aircraft up on the runway, the pilot applies the brakes (accomplished by applying pressure to the top of the rudder pedals - each pedal controls its respective wheel). The throttles are then advanced to military power (100% RPM). As the engines wind up, the engines and instruments are given a "last minute" check. (Pilots do a lot of "checks" to ensure that everything is going OK. After all, if something were to happen, you can't just pull off to the side of the road!) When everything is ready, the brakes are released and the airplane accelerates down the runway. At a pre-determined speed, the pilot pulls back on the stick to pitch the airplane upward about five degrees. Although the nose wheel is off the ground, the main gear remains on the runway because there is not yet enough airflow over the wings to create sufficient lift to raise the aircraft. After a little while, the airplane reaches the speed (90 knots) at which its wings produce lift slightly greater than its weight and it takes off.

While the airplane climbs away from the runway the pilot must raise the landing gear (this decreases the drag) and the flaps, then let it accelerate to the desired climb speed. Once this speed is reached, it is maintained by raising the nose slightly and "trimming" off all control stick pressures.

Maximum Performance Takeoff illustration

Straight and Level Flight

If an airplane maintains a given altitude, airspeed, and heading, it is said to be in "straight and level flight." This condition is achieved and maintained by equalizing all opposing forces. Lift must equal weight so the airplane does not climb or descend. Thrust must equal drag so the airplane does not speed up or slow down. The wings are kept level so the airplane does not turn. Any imbalance will result in a change in altitude or airspeed. It is the pilot's responsibility to prevent or correct for such an imbalance.

Proper trim is essential for maintaining this balance. If the pilot, by being "out of trim," is forced to maintain a given amount of stick pressure, the arm holding the stick will eventually tire. But in the short term the pilot must very precisely hold that pressure -- any change will result in a change in attitude. If the airplane is properly trimmed, the correct stick position is held automatically, and no pressure need be exerted.

Obviously, an airplane cannot remain indefinitely in this ideal condition. Due to mission, airspace, and fuel requirements, the pilot must change the airspeed, altitude, and heading from time to time.

Speed

Speeding up and slowing down is not simply a matter of changing the throttle setting (changing the force produced by the engines). Airspeed can also be changed by changing the drag. Many aircraft are equipped with a "speedbrake" for this purpose -- a large metal plate that can be extended out into the windstream, increasing parasite drag and slowing the airplane.

As an airplane speeds up or slows down, the amount of air passing over the wing follows suit. For instance, to maintain a constant altitude as the airspeed is decreasing, the pilot must compensate for this decreased airflow by changing the AOA (pulling back on the stick) to equalize the amount of lift to the weight of the airplane. All this works nicely until stall speed is reached, when an increase in AOA is met with a decrease in lift, and the airplane, its weight not completely countered by lift, begins to dramatically lose altitude. Conversely, an increase in airspeed must be met with a decrease in the AOA (moving the stick forward) to maintain a constant altitude. As airspeed increases or decreases, trim must be changed as well.

Mach number is the most influential parameter in the determination of range for most jet-powered aircraft. The most efficient cruise conditions occur at a high altitude and at a speed which is just below the start of the transonic drag rise. The drag (and thus the thrust required to maintain constant Mach number) will change as the weight of the airplane changes. The angle of attack (and thus the drag) of an airplane will become slightly lower as fuel is used since the airplane is becoming lighter and less lift is required to hold it up.

Climb

Climbs and descents are accomplished by using power setting respectively higher or lower than that required for level flight. When an airplane is in level flight, just reducing the power begins descent. Instead of pulling back on the stick to maintain altitude as the airspeed slows, the pilot keeps the stick neutral or pushes it forward slightly to establish a descent. Gravity will provide the force lost by the reduction in power. Likewise, increased power results in a climb.

Airspeed can be controlled in a climb or descent without changing the throttle setting. By pulling back on the stick and increasing the climb rate or by decreasing the descent rate, the airspeed can be decreased. Likewise, lowering the nose by pushing forward on the stick will effectively increase the airspeed. In most climbs and descents, this is the way airspeed is maintained. A constant throttle setting is used and the pilot changes pitch in small increments to control airspeed.

If the pilot were to fly a climb such that the airplane was at the best-climb speed as it passed through each altitude, it would be achieving the best possible rate of climb for the entire climb. This is known as the "best-climb schedule" and is identified by the dotted line.

Best-Climb Schedule

Flying the best-climb schedule will allow the airplane to reach any desired altitude in the minimum amount of time. This is a very important parameter for an interceptor attempting to engage an incoming enemy aircraft. For an aircraft that is equipped with an afterburner, two best climb schedules are determined; one for a Maximum Power climb (afterburner operating) and one for a Military Power climb (engine at maximum RPM but afterburner not operating). The Max Power climb will result in the shortest time but will use a lot of fuel and thus will be more useful if the enemy aircraft is quite close. The Mil. Power climb will take longer but will allow the interceptor to cruise some distance away from home base to make the intercept.

For cargo or passenger aircraft the power setting for best climb is usually the maximum continuous power allowed for the engines. By flying the best-climb schedule the airplane will reach it's cruise altitude in the most efficient manner, that is, with the largest quantity of fuel remaining for cruise.

Range

One of the most critical characteristics of an airplane is its range capability, that is, the distance that it can fly before running out of fuel. Range is also one of the most difficult features to predict before flight since it is affected by many aspects of the airplane/engine combination. Some of the things that influence range are very subtle, such as poor seals on cooling doors or small pockets of disturbed air around the engine inlets.

Speed power illustration

Turns

The aerodynamics of a turn widely misunderstood, since many people think that the airplane is "steered" by the stick or the rudder pedals (probably the result of thinking of the airplane as a sort of "flying car.") A turn is actually the result of a change in the direction of the lift vector produced by the wings.

A pilot turns an airplane by using the ailerons and coordinated rudder to roll to a desired bank angle. As soon as there is bank, the force produced by the wings (lift) is no longer straight up, opposing the weight. It is now "tilted" from vertical so that part of it is pulling the airplane in the direction of the bank. It is this part of the lift vector that causes the turn. Once the pilot has established the desired bank angle, the rudder and the aileron are neutralized so that the bank remains constant.

When part of the lift vector is used for turning the airplane, there is less lift in the vertical opposing weight. If the pilot were to establish a bank angle without increasing the total amount of lift being produced, the lift opposing the weight would decrease, and the resulting imbalance would cause in a descent. The pilot compensates by pulling back on the stick (increasing the AOA and therefore lift). By increasing the total lift, the lift opposing the weight can balance out the weight and control level flight. This increase in total lift also increases lift in the turn direction and results in a faster turn.


Turn Lift Requirements

As the bank angle increases, the amount of pull required to maintain level flight increases rapidly. It is not possible to maintain level flight beyond a given bank angle because the wings cannot produce enough lift. An attempt to fly beyond this point will result in either a stall or a descent.

Physiologically speaking, the most important part of a turn is the necessity to pull "Gs". As the back pressure is increased to maintain level flight, the increased force is felt as an increase in "G" level. In a 30 degree bank, 1.2 G is required to maintain level flight. The G level increases rapidly with an increase in bank; at 60 degrees, it goes to 2.0 G, and it takes 9.0 G to fly a level 84 degree bank turn. As long as there is enough airspeed, the G level can be increased in any bank angle by pulling back on the stick.

Finishing the turn, a simple matter of leveling the wings by using the ailerons and coordinated rudder, takes time; the airplane continues turning until the wings are level, so the roll-out must be started a little prior to reaching the desired heading. Back-stick pressure must also be released as bank decreases or the aircraft will climb.

Maneuverability

Airplanes are not limited to being a relatively fast means of getting somewhere. Long ago thrill-seeking pilots discovered that aircraft have the potential for providing loads of fun while getting nowhere fast. Aerobatics are an essential skill for fighter pilots; and the training that it gives to pilots in position orientation and judgment is considered so vital that a great deal of time is spent teaching these maneuvers. Maneuverability is defined as the ability to change the speed and flight direction of an airplane. A highly maneuverable airplane, such as a fighter, has a capability to accelerate or slow down very quickly, and also to turn sharply. Quick turns with short turn radii place high loads on the wings as well as the pilot. These loads are referred to as "g forces" and the ability to "pull g's" is considered one measure of maneuverability. One g is the force acting on the airplane in level flight imposed by the gravitational pull of the earth. Five g in a maneuver exerts 5 times the gravitational force of the earth.

Maneuverability
Maneuverability

Aileron Roll The aileron roll is simply a 360 degree roll accomplished by putting in and maintaining coordinated aileron pressure. The maneuver is started slightly nose high because, as the airplane rolls, its lift vector is no longer countering its weight, so the nose of the airplane drops significantly during the maneuver. Back stick pressure is maintained throughout so that even when upside down, positive seat pressure (about 1 G) will be felt. As the airplane approaches wings-level at the end of the maneuver, aileron pressure is removed and the roll stops.


Aileron Roll


Loop

Loop A loop is simply a 360 degree change in pitch. Because the airplane will climb several thousand feet during the maneuver, it is started at a relatively high airspeed and power setting (if these are too low, the airspeed will decay excessively in the climb and the maneuver will have to be discontinued.) The pilot, once satisfied with the airspeed and throttle setting, will pull back on the stick until about three Gs are felt. The nose of the airplane will go up and a steadily increasing climb will be established. As the maneuver continues, positive G is maintained by continuing to pull. The airplane continues to increase its pitch until it has pitched through a full circle. When the world is right-side-up again, the pilot releases the back stick pressure and returns the aircraft to level flight.



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Page last modified: 07-07-2011 02:33:18 ZULU