But this would never achieve any Mach above 1 for the flow, because as soon as the flow tops Mach 1, it would be decelerated by the convergent part of the nozzle once again. Now, one can argue that if a supersonic flow enters a converging nozzle, it would accelerate so subsonic speed (at about Mach 1 ) after which the convergent nozzle would accelerate the flow. This is required only at the compressor, where flow velocity should be slower, but after that to get more thrust, the flow needs to be accelerated. Please feel free to post questions if I skipped something that should be addressed.įor a supersonic plane, a convergent nozzle with decelerate the flow. So I won't go out and say there aren't any converging nozzle supersonic nozzle jets, but at least now you know why they would be the exception and not the rule :) Having said all that, aerospace designers are very clever and injecting fuel (afterburners) can cause non-isoentropic flow if designed for it. If you see a converging nozzle on a supersonic-capable fighter look at it carefully to see if it is articulated for changing shape in flight - which is pretty darn common. Note that a modern jet engine is fairly complicated in air flow and the "throat" may be deep inside the engine and not look like a rocket engine throat at all.įor these reasons we would expect to see converging final nozzles on subsonic aircraft like airliners and some fighter jets, while supersonic jets usually have diverging nozzles. This sonic point is then put right at the "throat" where converging becomes diverging and the diverging part turns heat and pressure into more speed up to fairly high values of supersonic. The ideal situation for SUPERSONIC flow is converging-diverging (see the de Laval again) because, for isentropic flow, the converging portion builds energy up to the point of the sonic speed. (There are actually about a dozen subcases here that I'm sure others will yell at me on, so consider that P and T decrease to be an inexcusable simplification on my part to avoid another page of explanation.) So for a slower exhaust final speed, a converging nozzle makes perfect sense. In a SUBSONIC flow the air remains compressible and a converging subsonic nozzle increases velocity while pressure and temperature decrease. Let's keep in mind that what we want (ignoring for the moment optimization of air volume to feed combustion) is HIGH SPEED in the OUTGOING JET EXHAUST. If you want to understand nozzle shapes a fun introduction is to read about the shape that inspired modern rocket (and some jet) nozzles, the de Laval nozzle. Pilot here, as well as an engineer who made it most of the way to an Airframe and Powerplant technician license at one point in life and worked a lot on nozzles including at the jet Propulsion Laboratory (very fun). Examples are the Northrop F-5E or the Panavia Tornado. In most cases it would be more efficient to convert the pressure energy of the exhaust gasses into more thrust by accelerating them to supersonic speed.Ĭonvergent nozzles only make sense if the aircraft is designed only for short, limited supersonic dashes, but will spend almost all flying time at subsonic speeds. A convergent nozzle will be shorter and lighter, but means throwing away a good deal of useable energy with the hot exhaust gasses. Only a divergent flow path will then accelerate the flow further to supersonic speed. A narrowing of the flow path will accelerate subsonic flow, but only to the speed of sound. ![]() With an adaptable convergent nozzle the exhaust gasses can be accelerated up to their speed of sound, but not more. The simple pitot intake of the F-16 is just good enough for Mach 1.6 (above that its efficiency becomes outright awful), but still its F-110 engine has a con-di nozzle. ![]() ![]() Both increase efficiency, dramatically so at higher Mach numbers. If the design is meant to fly supersonically, which implicates a lot of design adaptions, it makes sense to go for higher supersonic speed however, this requires both an adjustable intake and an adjustable convergent-divergent nozzle. Once the aircraft crosses the sound barrier, drag increases only slowly since the drag coefficient actually drops. Low supersonic flight speeds are entirely possible with a convergent nozzle. Since thrust is mainly determined by the difference in entry and exit speeds of the air flowing through an engine, a higher speed than flight speed is required for positive thrust. For example, at 700☌ the speed of sound in air is 625 m/s. A convergent nozzle will not allow supersonic exit speeds of the combustion gasses, but due to their high temperature their speed of sound is considerably higher that that of the surrounding air.
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