Flight Envelope - Doghouse Plot

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 climb.

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 g loadings. For instance, 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.