In
aerodynamicsAerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
, the
flight envelope or
performance envelope of an
aircraftAn aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil An aircraft is a vehicle which is able to fly by being supported...
refers to the capabilities of a design in terms of airspeed and
load factorLoad factor is the ratio of the lift on an aircraft to the weight of the aircraft. The load factor is expressed in multiples of g where one g represents conditions in straight and level flight. In straight and level flight the lift is equal to the weight so the ratio of lift to weight is one, and...
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.
In
aerodynamicsAerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
, the
flight envelope or
performance envelope of an
aircraftAn aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil An aircraft is a vehicle which is able to fly by being supported...
refers to the capabilities of a design in terms of airspeed and
load factorLoad factor is the ratio of the lift on an aircraft to the weight of the aircraft. The load factor is expressed in multiples of g where one g represents conditions in straight and level flight. In straight and level flight the lift is equal to the weight so the ratio of lift to weight is one, and...
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.
Extra power
Extra 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 150The Cessna 150 is a two-seat tricycle gear general aviation airplane, that was designed for flight training, touring and personal use.The Cessna 150 is the fourth most produced civilian plane ever, with 23,839 aircraft produced...
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 2
gThe g-force experienced by an object is its acceleration relative to free-fall. The term g-force is considered a misnomer, as g-force is not a force but an acceleration....
(20 m/s²) turn, which would require 120 hp (or somewhat more) under the same conditions.
For the same conditions a
fighter aircraftA fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets by dropping bombs. Fighters are small, fast, and maneuverable...
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 climb straight up, make powerful continual maneuvers, or fly at very high speeds.
Doghouse plot
A doghouse plot generally shows the relation between speed at level flight and altitude although other variables are also possible. It takes more effort to make than an extra power calculation, but in turn provides much more information such as ideal flight altitude. The plot typically looks something like an upside-down U and is commonly referred to as a doghouse plot due to its resemblance to a doghouse. The diagram on the right shows a very simplified plot which shall be used to explain the general shape of the plot.
The outer edges of the diagram, the envelope, show the possible conditions that the aircraft can reach in straight and level flight. For instance, this aircraft can fly at altitudes up to about 20,000m, at which point some external influence means it can no longer climb. The aircraft can also fly at up to Mach 1 at sea level, but no faster. This outer surface of the curve represents the
zero-extra-power condition. All of the area under the curve represents conditions that the plane can normally fly at, for instance, this aircraft can fly at Mach 0.5 at 10,000m, and doing so would require something less than full power.
Flying outside the envelope is possible, since it represents the straight-and-level condition only. For instance diving the aircraft allows higher speeds, using gravity as a source of additional power. Likewise higher altitude can be reached by first speeding up and then going ballistic, a maneuver known as a
zoom climbA zoom climb is a climb where the rate of climb is greater than the maximum for a sustained climb, as determined from the thrust of the aircraft's engines. During a zoom climb, the aircraft gains potential energy at the expense of kinetic energy...
.
In the case of high-performance aircraft, including fighters, this "1-g" line showing straight-and-level flight is often overlapped with additional lines showing the maximum performance at various gee loadings. For instance, the
F-16 Fighting FalconThe Lockheed Martin F-16 Fighting Falcon is a multirole jet fighter aircraft originally developed by General Dynamics for the United States Air Force. Designed as a lightweight, daytime Visual Flight Rules fighter, it evolved into a successful multirole aircraft...
has a very small area just below Mach 1 and close to sea level where it can maintain a 9-g turn.
Stalling speed
All
fixed-wing aircraftA fixed-wing aircraft, usually called an airplane, aeroplane or plane, is an aircraft capable of flight using forward motion that causes air to pass over its wings to generate lift. Planes include jet engine and propeller driven vehicles propelled forward by thrust, as well as unpowered aircraft...
have a minimum speed at which they can maintain level flight, the stall speed (left limit line in the diagram). As the aircraft gains altitude the stall speed increases; since the wing is not growing any larger the only way to support the aircraft's weight with less air is to increase speed. While the exact numbers will vary widely from aircraft to aircraft, the nature of this relationship is typically the same; plotted on a graph of speed (x-axis) vs. altitude (y-axis) it forms a diagonal line.
Service ceiling
Inefficiencies in the wings also make this line "tilt over" with increased altitude, until it becomes horizontal and no additional speed will result in increased altitude, this maximum altitude is known as the
service ceilingIn aeronautics, a ceiling is the maximum density altitude an aircraft can reach under a set of conditions.-Service ceiling:The service ceiling attempts to capture the maximum usable altitude of an aircraft...
(top limit line in the diagram), and is often quoted for aircraft performance. The area where the altitude for a given speed can no longer be increased at level flight is known as
zero rate of climb and is caused by the
liftA fluid flowing past the surface of a body exerts a force on it. Lift is defined to be the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is defined to be the component of the fluid-dynamic force parallel to the flow...
of the aircraft getting smaller at higher altitudes, until it no longer exceeds gravity.
Top speed
The right side of the graph represents the maximum speed of the aircraft. This is typically sloped in the same manner as the stall line due to air resistance getting lower at higher altitudes, up to the point where an increase in altitude no longer increases the maximum speed due to lack of oxygen to feed the engines.
The power needed varies almost linearly with altitude, but the nature of
dragIn fluid dynamics, drag refers to forces that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming flow velocity...
means that it varies with the square of speed -- in other words it is typically easier to go higher than faster, up to the altitude where lack of oxygen for the engines starts to play a significant role.
Sidenotes
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 aviationGeneral aviation is one of two categories of civil aviation. It refers to all flights other than military and scheduled airline flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
aircraft have very small flight envelopes, with speeds ranging from perhaps 50 to 200mph, whereas the extra power available to modern fighter aircraft result in huge flight envelopes with many times the area. As a tradeoff 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, and perhaps 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.
Pushing the envelope has also become an
expressionExpression may refer to:* A statement or sentence * Idiom* Facial expression* Artificial discharge of breast milk; see breastfeeding* Expression...
in other fields, referring to the act of introducing new ideas in readily established concepts. An example would be a marketing campaign which advertised its product in a way never done before, or a video game which interacted with the player in a completely original fashion.