300px|thumb|Flight envelope diagram.
In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft or spacecraft refers to the capabilities of a design in terms of airspeed and load factor or atmospheric density, often simplified to altitude.
The term is somewhat loosely applied, and can also refer to other measurements such as maneuverability. For example, 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. During vehicle test programs, flight envelope simply means that part of the aircraft or spacecraft's design capabilities that have already been successfully tested, and have therefore moved from theoretical or designed capability into a demonstrated/certified capability.
Flight envelope is one of a number of related terms that are 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 flight. It is closely related to more modern terms known as extra power and a doghouse plot which are different ways of describing the flight envelope of an aircraft. 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
Extra power, or specific excess power,
At higher temperatures, air is less dense and planes must fly faster to generate the same amount of lift. High heat may reduce the amount of cargo a plane can carry, increase the length of runway a plane needs to take off,
and make it more difficult to avoid obstacles such as mountains. In unusual weather conditions this may make it unsafe or uneconomical to fly, occasionally resulting in the cancellation of commercial flights.
Side notes
Although it is easy to compare aircraft on simple numbers such as maximum speed or service ceiling, an examination of the flight envelope will reveal far more information. Generally a design with a larger area under the curve will have better all-around performance. This is because when the plane is not flying at the edges of the envelope, its extra power will be greater, and that means more power for things like climbing or maneuvering. General aviation aircraft have very small flight envelopes, with speeds ranging from perhaps 50 to 200 mph, whereas the extra power available to modern fighter aircraft result in huge flight envelopes with many times the area. As a trade-off however, military aircraft often have a higher stalling speed. As a result of this, the landing speed is also higher.
"Pushing the envelope"
This phrase is used to refer to an aircraft being taken to or beyond its designated altitude and speed limits. By extension, this phrase may be used to mean testing other limits, either within aerospace or in other fields e.g. Plus ultra (motto).
See also
- Coffin corner (aviation)
- Helicopter height–velocity diagram
- Küssner effect
- Maneuvring speed
- Corner case
Notes
ja:飛行性#飛行包絡線