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.[1][2]
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,[3] 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-foot (760m) altitude and 90-mile-per-hour (140km/h) speed needs about 60 horsepower (45kW) to fly straight and level. The C150 is normally equipped with a 100-horsepower (75kW) engine, so in this particular case the plane has 40 horsepower (30kW) 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 40hp 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 (20m/s²) turn, which would require a minimum of 120 horsepower (89kW) under the same conditions.
For the same conditions a fighter aircraft might require considerably more power due to their wings being designed for high speed, high agility, or both. It could require 10,000 horsepower (7.5MW) to achieve similar performance. However modern jet engines can provide considerable power with the equivalent of 50,000 horsepower (37MW) not being atypical. 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.
Doghouse plot
Altitude envelope (H-M diagram). Contour is load factor.Turn rate envelope, described in an E-M diagram (doghouse plot). Contour is specific excess power.
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 kennel (sometimes known as a 'doghouse' in American English). 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, the aircraft described by the black altitude envelope on the right can fly at altitudes up to about 52,000 feet (16,000m), at which point the thinner air means it can no longer climb. The aircraft can also fly at up to Mach 1.1 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 fly at with power to spare, for instance, this aircraft can fly at Mach 0.5 at 30,000 feet (9,100m) while using less than full power.
In the case of high-performance aircraft, including fighters, this "1-g" line showing straight-and-level flight is augmented with additional lines showing the maximum performance at various g loadings. In the diagram at right, the green line represents, 2-g, the blue line 3-g, and so on. The F-16 Fighting Falcon has a very small area just below Mach 1 and close to sea level where it can maintain a 9-g turn.
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 climb.
Stalling speed
All fixed-wing 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 additional speed will not result in increased altitude. This maximum altitude is known as the service ceiling (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 lift 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 drag 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.
Velocity vs. load factor chart
A V-n diagram showing VS (stall speed at 1G), VC (corner/maneuver speed) and VD (dive speed)
A chart of velocity versus load factor (or V-n diagram) is another way of showing limits of aircraft performance. It shows how much load factor can be safely achieved at different airspeeds.[3]
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.[4][5]
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 200mph, 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.
This phrase is used to refer to an aircraft being taken to or beyond its designated altitude and speed limits.[6] By extension, this phrase may be used to mean testing other limits, either within aerospace or in other fields e.g. Plus ultra (motto).[7]
Aerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in conventional passenger-carrying flights. The term is a portmanteau of "aeroplane" and "acrobatics". Aerobatics are performed in aeroplanes and gliders for training, recreation, entertainment, and sport. Additionally, some helicopters, such as the MBB Bo 105, are capable of limited aerobatic manoeuvres. An example of a fully aerobatic helicopter, capable of performing loops and rolls, is the Westland Lynx.
In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.
In aerodynamics, wing loading is the total mass of an aircraft or flying animal divided by the area of its wing. The stalling speed, takeoff speed and landing speed of an aircraft are partly determined by its wing loading.
Cruise is the phase of aircraft flight that starts when the aircraft levels off after a climb, until it begins to descend for landing. Cruising usually comprises the majority of a flight, and may include small changes in heading, airspeed, and altitude.
With respect to aircraft performance, a ceiling is the maximum density altitude an aircraft can reach under a set of conditions, as determined by its flight envelope.
Aerobatic maneuvers are flight paths putting aircraft in unusual attitudes, in air shows, dogfights or competition aerobatics. Aerobatics can be performed by a single aircraft or in formation with several others. Nearly all aircraft are capable of performing aerobatics maneuvers of some kind, although it may not be legal or safe to do so in certain aircraft.
Air France Flight 296Q was a chartered flight of a new Airbus A320-111 operated by Air Charter International for Air France. On 26 June 1988, the plane crashed while making a low pass over Mulhouse–Habsheim Airfield as part of the Habsheim Air Show. Most of the crash sequence, which occurred in front of several thousand spectators, was caught on video.
Basic fighter maneuvers (BFM) are tactical movements performed by fighter aircraft during air combat maneuvering, to gain a positional advantage over the opponent. BFM combines the fundamentals of aerodynamic flight and the geometry of pursuit, with the physics of managing the aircraft's energy-to-mass ratio, called its specific energy.
Some of the pilots in the sport of gliding take part in gliding competitions. These are usually racing competitions, but there are also aerobatic contests and on-line league tables.
A barrel roll is an aerial maneuver in which an airplane makes a complete rotation on both its longitudinal and lateral axes, causing it to follow a helical path, approximately maintaining its original direction. It is sometimes described as a "combination of a loop and a roll". The g-force is kept positive on the object throughout the maneuver, commonly between 2 and 3g, and no less than 0.5g. The barrel roll is commonly confused with an aileron roll.
A radio-controlled glider is a type of radio-controlled aircraft that normally does not have any form of propulsion. They are able to sustain continuous flight by exploiting the lift produced by slopes and thermals, controlled remotely from the ground with a transmitter. They can be constructed from a variety of materials, including wood, plastic, polymer foams, and composites, and can vary in wing loading from very light to relatively heavy, depending on their intended use.
Hot air ballooning is the recreational and competitive adventure sport of flying hot air balloons. Attractive aspects of ballooning include the exceptional quiet, the lack of a feeling of movement, and the bird's-eye view. Since the balloon moves with the direction of the winds, the passengers feel absolutely no wind, except for brief periods during the flight when the balloon climbs or descends into air currents of different direction or speed. Hot air ballooning has been recognized by Fédération Aéronautique Internationale (FAI) as the safest air sport in aviation, and fatalities in hot air balloon accidents are rare, according to statistics from the National Transportation Safety Board (NTSB).
The scissors is an aerial dogfighting maneuver commonly used by military fighter pilots. It is primarily a defensive maneuver, used by an aircraft that is under attack. It consists of a series of short turns towards the attacking aircraft, slowing with each turn, in the hopes of forcing the attacker to overshoot. Performed properly, it can cause the attacking aircraft to move far enough in front to allow the defender to turn the tables and attack.
Coffin corner is the region of flight where a fast but subsonic fixed-wing aircraft's stall speed is near the critical Mach number, at a given gross weight and G-force loading. In this region of flight, it is very difficult to keep an airplane in stable flight. Because the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Because the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch heavily nose-down, and lose altitude.
Flight envelope protection is a human machine interface extension of an aircraft's control system that prevents the pilot of an aircraft from making control commands that would force the aircraft to exceed its structural and aerodynamic operating limits. It is used in some form in all modern commercial fly-by-wire aircraft. The professed advantage of flight envelope protection systems is that they restrict a pilot's excessive control inputs, whether in surprise reaction to emergencies or otherwise, from translating into excessive flight control surface movements. Notionally, this allows pilots to react quickly to an emergency while blunting the effect of an excessive control input resulting from "startle," by electronically limiting excessive control surface movements that could over-stress the airframe and endanger the safety of the aircraft.
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.
A glider or sailplane is a type of glider aircraft used in the leisure activity and sport of gliding. This unpowered aircraft can use naturally occurring currents of rising air in the atmosphere to gain altitude. Sailplanes are aerodynamically streamlined and so can fly a significant distance forward for a small decrease in altitude.
3D Aerobatics or 3D flying is a form of flying using flying aircraft to perform specific aerial maneuvers. They are usually performed when the aircraft had been intentionally placed in a stalled position for purposes of entertainment or display. They are also often referred to as post-stall maneuvers, as they occur after aerodynamic stall has occurred and standard control surface deflections, as used in flight, are not effective.
The drag curve or drag polar is the relationship between the drag on an aircraft and other variables, such as lift, the coefficient of lift, angle-of-attack or speed. It may be described by an equation or displayed as a graph. Drag may be expressed as actual drag or the coefficient of drag.
Dirgantara Air Service Flight 3130 (DIR3130/AW3130) was a scheduled domestic passenger flight operated by Dirgantara Air Service from Datah Dawai Airport, Malinau Regency, East Kalimantan to its provincial capital's airport, Samarinda Temindung Airport, Samarinda, East Kalimantan. On 18 November 2000, the aircraft conducting the flight, a Britten Norman Islander BN-2 sheared tree tops and crashed onto the forest near the airport shortly after takeoff. Search and rescue team immediately found the wreckage of Flight 3130 and the survivors. No one was killed in the crash, but all 18 people on board were injured in the crash; 11 of them were seriously hurt.
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