Airstream Deviation Due to Inflow

A jet or rocket stream issuing from a nozzle acts like a hydrodynamic sink on the surrounding free-stream cold air flow. H. B. Squire and J. Trouncer (1944) produced beautiful isocline maps of the free-stream deviation angles around a jet (Figure 4.6). The sense of the deviation angles is for the surrounding free-stream flow to feed into the jet. Squire and Trouncer’s calculated deviation angles are parameterized in terms of the ratio of jet to free-stream velocities. The larger the velocity ratio, the larger is the deviation angle.

If airspeed is reduced from a trim value at a fixed throttle setting, the ratio of jet to free-stream velocity increases. This increases the free-stream deviation angles into the jet at any given location. In the common case in which the jet passes under the horizontal tail, this increases the effective downwash angle as the speed is reduced. This in turn provides a nose-up pitching moment at speeds below trim, a destabilizing effect. Forward neutral point shifts of as much as 10 percent of the wing mean chord are found for airplanes whose jet exhausts are forward of the horizontal tail. Conversely, only minor stability effects are measured for jet exhausts behind the horizontal tail.

Squire and Trouncer’s calculated stream deviation angles into a jet are for a jet stream at the same temperature as a free stream. A correction is needed to apply their data to the heated jets that come from actual jet or rocket engines. The equivalent cold jet velocity ratio is related to the actual jet velocity ratio by a function involving the ratio of the jet temperature to free-stream temperature.