Pushing the envelope

In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft refers to the capabilities of a design in terms of airspeed and load factor or altitude. The term is somewhat loosely applied, and can also refer to other measurements such as maneuverability. When a plane is pushed, for instance by diving it at high speeds, it is said to be flown "outside the envelope", something considered rather dangerous.

Flight envelope is one of a number of related terms that are all used in a similar fashion. It is perhaps the most common term because it is the oldest, first being used in the early days of test flying. It is closely related to more modern terms known as extra power and a doghouse plot which are different ways of describing a flight envelope. In addition, the term has been widened in scope outside the field of engineering, to refer to the strict limits in which an event will take place or more generally to the predictable behavior of a given phenomenon or situation, and hence, its "flight envelope" .

Extra power, or specific excess power, is a very basic method of determining an aircraft's flight envelope. It is easily calculated, but as a downside does not tell very much about the actual performance of the aircraft at different altitudes.

Choosing any particular set of parameters will generate the needed power for a particular aircraft for those conditions. For instance a Cessna 150 at 2,500 ft (800 m) altitude and 90 mph (140 km/h) speed needs about 60 hp (45 kW) to fly straight and level. The C150 is normally equipped with a 100 hp (75 kW) engine, so in this particular case the plane has 40 hp (30 kW) of extra power. In overall terms this is very little extra power, 60% of the engine's output is already used up just keeping the plane in the air. The leftover 40 hp (30 kW) is all that the aircraft has to maneuver with, meaning it can climb, turn, or speed up only a small amount. To put this in perspective, the C150 could not maintain a 2g (20 m/s²) turn, which would require 120 hp (or somewhat more) under the same conditions.

For the same conditions a fighter aircraft might require considerably more power due to their wings being inefficient at low speeds, for argument's sake it might require 10,000 hp (7.5 MW). However modern jet engines can provide considerable power, the equivalent of 50,000 hp (37 MW) typically. With this amount of extra power the aircraft can achieve very high maximum rate of climb, even climb straight up, make powerful continual maneuvers, or fly at very high speeds.

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Wikipedia