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Flight Performance - Level 3

Airplane Performance-Section 3

CRUISING SPEEDS

Between an airplane's top speed and its stalling speed, there is a wide range of speeds that have often been referred to as the working speeds. Each is the answer to a particular problem of practical flying.

Best Gliding Speed or Normal Glide. The speed at which an airplane with power off will glide the farthest distance. This is the speed at which the lift/drag ratio is best and therefore might be called the "speed of least drag".

Speed of Slowest Descent. The speed at which the airplane loses altitude at the least rate. In a nose high attitude, near the stalling speed, the airplane sinks most slowly. This speed is made constant use of by glider pilots.

Maximum Range Speed. The speed at which the airplane travels the most miles per gallon of fuel. This speed is about 5 to 10% faster than the best gliding speed.

Maximum Endurance Speed. The speed at which the fuel consumption is the least per hour. To achieve this condition of flight, the airplane is flown quite slowly, with minimum power, very nose-high, with the wings at a high angle of attack. The airplane is not covering much distance for it is too slow but its endurance is substantially extended.

Maneuvering Speed. The maximum speed at which the airplane should be flown in rough air. It is the speed least likely to permit structural damage to the airplane in rough air or during aerobatic maneuvers. Maneuvering speed for most airplanes is set at about twice the flaps-up, power-off stall speed.

Optimum Cruise Speed. The speed at which the best balance between use of fuel and maximum range is achieved.

Best Rate of Climb Speed. The speed at which the airplane will gain the most altitude in the least time. This speed is rather fast: for most airplanes it is approximately the same speed as that of best glide.

Best Angle of Climb Speed. The speed at which the airplane climbs most steeply, gaining the most altitude for distance covered over the ground. In routine flying, it is seldom necessary to use best angle of climb speed but it is of importance when it is necessary to climb out over a high obstacle at the end of a runway. It is, however, not a climbing procedure that should be used except in emergency. Firstly, it is a speed that is not much above the stall and the pilot must be very vigilant that he does not permit the airplane to stall. Secondly, at such a slow speed, with full power, insufficient air is passing over the engine to cool it properly. The engine is therefore likely to overheat and the resulting engine wear is undesirable.

Air Endurance. The maximum time an airplane can continue to fly under given conditions with a given quantity of fuel.

Range. The maximum distance an airplane can fly under given conditions with a given quantity of fuel.

Given Conditions. Both airspeed and altitude affect the range and endurance. The best airspeed for maximum endurance is generally less than that for maximum range. Generally speaking, a propeller driven airplane may be flown at a relatively low altitude for maximum endurance. For maximum range, a propeller driven airplane should be flown at that height at which, at full throttle, the indicated airspeed is about 5 to 10% better than the best glide speed. This is realized at a relatively high altitude. A jet, on the other hand, would fly at high altitude for both range and endurance.

Range and endurance, when stated in the airplane specifications, are usually based on still air, standard atmosphere conditions. However, they may be computed for any assumed wind direction and wind speed.

Airplane performance is always a compromise between such variable factors as range vs. payload, endurance vs. airspeed, etc..


USE OF PERFORMANCE CHARTS

The performance information that is available to you, as a pilot, in the form of the tables and graphs that have been illustrated in the preceding sections is very valuable. You should, therefore, thoroughly familiarize yourself with the tables and graphs published in the Airplane Flight Manual for your airplane and should practice using them at every opportunity in order to gain confidence in their use and to determine for yourself that your calculated or anticipated performance equals the actual or realized performance on any particular flight.

The charts are particularly useful in planning a cross country flight and in solving the navigational problems of range, fuel consumption, etc. that are part of proper preflight planning.

WAKE TURBULENCE

The theory of wing tip vortices was introduced in the discussion of induced drag. As the lift producing airfoil passes through the air, the air rolls up and back about each wing tip producing two distinct counter rotating vortices, one trailing each wing tip. The intensity of the turbulence within these vortices is directly proportional to the weight and inversely proportional to the wing span and the speed of the airplane. The heavier and slower the airplane, therefore, the greater is the intensity of the air circulation in the vortex cores. The most violent vortices would be generated by airplanes during take-off and landing and at near gross weights when the aircraft are flying at high angles of attack.

Air density influences vortex strength. In cold air, therefore, the vortices can be expected to be more severe. Vortex strength is also affected by the configuration of the airplane producing the vortices. The position of flaps and undercarriage, as well as the location of the engines and the tail configuration, modify vortex pattern and persistence. For example, the high tail of the DC9 increases the persistence of the vortices.

The greatest vortex strength occurs under conditions of clean configuration, high weight and slow speed.

Similar vortices are generated by rotary wing aircraft. These vortices have the same internal air circulation as those generated by fixed wing airplanes, but are potentially more dangerous because of the helicopter's lower operating speeds.

The vertical gusts encountered when crossing laterally through the vortex can impose structural loads as high as 10 g on a small airplane flying at a high angle of attack. The combined effect of an upgust immediately followed by a downgust has been estimated as high as 80-feet-per-second. Most small planes are designed to withstand vertical gusts of 30-feet-per-second at their normal operating speeds.

There is a distinct possibility of structural failure when an airplane crosses a pair of vortices at a large angle (about 90 ). The severe up-down-down-up forces and the pilot's attempt to counteract them could result in airframe design limits being exceeded. Loss of control is another result of encountering these unseen monsters. Vortex cores can produce a roll rate of 80 degrees per second, a situation with which the small light airplane is structurally unable to cope.

Vortex generation starts with rotation (the raising of the nose from the runway) and reaches its peak intensity at lift off when the full weight of the airplane is sustained by the wings and when the airspeed is low. It ends when the airplane touches down. The vortices settle below and behind the airplane but tend to level off about 1000 feet below the aircraft's flight path. They may trail the generating aircraft by 10 to 16 miles depending on its speed and, in still air, decay slowly. Wake turbulence usually dissipates after about 2 minutes although it has been known to last as long as 5 minutes after passage of the generating aircraft. Atmospheric turbulence does, however, help to break up the vortices more quickly.

The vortex flow field covers an area about 2 wing spans in width and one wing span in depth. The rotating vortex may, therefore, have a diameter as great as 200 feet behind a very large airplane.

When the vortices sink to the ground, they tend to move laterally outward over the ground at a speed of about 5 knots. This characteristic means that the laterally moving vortices may position themselves over a parallel runway and constitute a hazard to airplanes operating there. In a crosswind condition, the lateral movement of the upwind vortex will be decreased while the movement of the downwind vortex will be increased. As a result, the upwind vortex may remain stationary in the touchdown zone. A tailwind condition can move the vortices of the preceding airplane forward into the touchdown zone of the airplane following even though the latter takes the precaution of landing beyond the touchdown point of that preceding airplane.

The phenomenon of wake turbulence is of immense importance to a pilot. In fact, it can be a matter of life and death. It is of prime importance, therefore, that you, for your own safety, learn to envision the location of the vortex wake generated by large aircraft so that you can avoid encountering the very serious hazards that are associated with this situation.

 

WAKE TURBULENCE AVOIDANCE

Remember that the vortices, in still air, have a downward and outward movement but that the ambient wind will alter their normal pattern of movement. Pilots should be particularly alert to wind conditions and try to picture mentally how those wind conditions will affect the vortices. Will they remain in the touchdown zone, or drift onto a nearby runway, or sink into the landing path from a crossing runway, etc.?

During flight. Although wake turbulence is most likely to be encountered during the arrival and departure procedures, it can be a hazard during cruising flight as well. Avoid crossing behind and less than 1000 feet below the f light path of a large, heavy airplane or of a helicopter, especially at low altitude when even a momentary wake turbulence encounter could be hazardous. Alter course or climb to be above the expected vortices.

During taxi. Stay well behind large airplanes that are taxiing or maneuvering on the ground. Do not cross behind a large airplane that is doing an engine run-up. Avoid taxiing below a hovering helicopter. The downwash from its rotors is significant and hazardous.

During take-off. On the same or a parallel runway. Start the take-off roll at the beginning of the runway and plan to be airborne before the rotation point of the previous airplane. Stay upwind of the large airplane. This action and a normal climb should keep you above the descending vortices of the preceding airplane.

Do not make intersection take-offs when large aircraft are using the same runway.

On an intersecting runway. Plan to be airborne before you cross the intersection, remembering to keep your flight path well above that of the aircraft that departed on the other runway

When following an aircraft that has just landed, plan to become airborne beyond the point of touchdown of the preceding aircraft.

During landing. When following a heavy aircraft that has just taken off, plan to touch down before the rotation point of the preceding airplane.

When following an aircraft that has just landed, plan to, touch down beyond the point where the preceding aircraft touched down, remembering to keep your approach path above that of that aircraft.

When landing on an intersecting runway behind a large airplane that has taken off, note the airplane's rotation point and, if it was past the intersection, continue your approach and land prior to the intersection. If the rotation point of the large airplane was prior to the intersection, it might be best to abandon the landing unless you can assure a landing well before the intersection.

Departing or landing after a large airplane that has executed a missed approach or a touch-and-go landing. The vortex hazard may exist along the entire runway. Ensure than an interval of at least 2 minutes has elapsed before making your take-off or landing.

Above all, avoid a long dragged-in approach. The largest number of dangerous encounters with wake turbulence has been in the last half mile of the final approach.

However, ATC cannot guarantee that wake turbulence will not be encountered. When the tower controller advises, "Caution Wake Turbulence", he is warning you of the possible existence of this phenomenon.

Even when the controller advises of the possibility of wake turbulence, it is solely the pilot's responsibility to avoid encountering it. Don't hesitate to ask for further information if you believe it will assist you in analyzing the situation and deciding on a course of action. Even though you have received a clearance to land or take off, if you believe it safer to wait, or to use a different runway, or in some other way to alter your operation, ask the controller for a revised clearance. The controller is interested in the prevention of accidents too and will assist you in any way he can while accomplishing his job of expediting traffic.

JET BLAST HAZARD

The area immediately behind jet airplanes can be particularly dangerous to small airplanes maneuvering on the ground or about to take off or land. Pilots should exercise caution when operating near active runways or taxiways. With the increasing use of intersecting runways, there is always the possibility of jet blast or propeller wash affecting other aircraft.

Light weight airplanes with high wings and narrow track undercarriages are particularly susceptible to jet blast accidents. Stay at least 600 feet away from a jumbo jet when its engines are idling and 1600 feet away from it when it is at full throttle for take-off.


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Updated: March 12, 2004