The flight envelope of an airplane is one of the many characteristics an airplane has in flight. It refers to the set of parameters of a plane in flights such as maximum speed, maximum altitude, etc.
There are two general ways of calculating a plane's flight envelope: the extra power method and the doghouse plot method. Here, we will be focusing on the doghouse method as it is more accurate. A simplified explanation will be presented here.
A doghouse plot is a plot which generally is used to show the relation between level flight speed and altitude, although other variable are also shown with it. It is known as a doghouse plot because the plot generally takes the shape of a doghouse, an inverted U.
The image to the right shows a simplified doghouse plot. The various lines on the edge of the plot indicate the flight envelope of the aircraft for which it was calculated. The y-axis here shows the altitude (in thousands of feet) while the x-axis shows the speed of the aircraft (in Machs). The plane I am referring to is shown with the thin black line.
The upper edge of this plot shows the service ceiling of this aircraft, or the maximum altitude it can attain. This plan can theoretically reach at 60,000 feet at which point it will stall. The max altitude it can maintain while flying straight is around 52,000 feet.
The right edge of the plot shows the top speed of this aircraft. This one can reach up to Mach 1.6 at altitudes in between approximately 19,000 and 60,000 feet.
The left side of the plot shows the stalling speed curve. If a plane is flying at a certain altitude and speed such that that point falls on that curve, it will not be able to maintain steady flight and will stall i.e. not generate enough lift and start falling down.
For high-performance aircraft, additional lines are included under the envelope lines which indicate performance at various G-forces. For example, the green line here indicates this aircraft's maximum altitude and speed under a force of 1G, while the blue line indicates it for 2G and so on.
This plot shown here only shows the flight envelope for fixed, stable straight-line flight. It can be broken through other methods, such as going faster than Mach 1.6 (for this plane) by diving with the throttle at full, using gravity to provide extra speed, or by going above the maximum altitude using a technique known as zoom climbing.
(Left): A plane undergoing a zoom climb.