I have Aviation question
if the supersonic air has to be converted(slowed down) into subsonic before it can be fed to the engines from air intakes then does it not cause a lot of drag? what process ensures that the entry and exit of to and from the air intakes is smooth and turbulence free?
My guess is that this (super to subsonic conversion) process is resulting in a lot of fuel and energy being wasted that doesn’t directly contribute in keeping the jet afloat. explan
Dear, Good Question, But really tough to answer considering the audience.
The Supersonic air flowing into an engine is slowed down, but in effect it is also being compressed, and as a result, it is also getting HOT. Thats the whole point of burning fuel in the engine, heat the air so the it gets compressed due to rise of pressure, enabling it to pick up speed when exhausted.
This is how an (non afterburning) engine works.
1. Compress the inflowing air, using compressor stages. This increases Pressure and well as Temperature. The energy supplied to compressor is added to energy content of airflow, which is a product of Temperature, Pressure and Speed of air-mass.
2. Inject Fuel and Burn, Heating the air-mass, thus increasing its Temperature & Pressure tremendously, increasing the energy content of air-mass.
3. Run a small turbine stage from the high-temp-high-pressure air-mass, which in used provide power for the compressor stage. This reduces the overall energy content of the air-mass. But, an important thing to note is that under theoratical idealistic conditions energy added in compression is exactly the same amount as energy extracted in turbine, cancelling each other out.
4. Exhaust nozzle that allows controlled exit of airflow, designed in a way to maximize the thrust generated by the expanding high-temp-high-pressure airflow into high-speed airflow. A significant part of pressure and heat is also carried by this exhaust which is lost-energy but cannot be helped (because of 2nd Law of Thermodynamics), amounting to almost 40~50% of total energy provided by fuel. This is wasted energy. Rest is all converted to thrust.
Now for a supersonic engine a pre-requisite stage is engineered into the engine.
0. Bring the Supersonic airflow to subsonic airflow, to allow for safe and steady compressor ingestion and operation. This slowed down airflow is already precompressed and preheated as a result of inlet design geometry. This heat+pressure is not lost but adds to the total energy carried out by the airflow, but indeed limits the overall max fuel consumption. Remember the higher the max fuel consumption, the higher the energy supplied, and thus higher the max thrust of the engine.
Now the only problem with this is that when subsonic, and engine intake air is cold, lets assume 25C. But during supersonic flight, the intake air is preheated and precompressed by the inlet, lets assume 200C. Now this is bad only beacause it tends to overheat the airflow to a point where turbine components start to show fatigue and failure. Do keep in mind that Turbine-Stage is the Hottest area of any engine, and all safe engine operation depends on keeping the turbine inlet temperature into safe operating limits, which tend to overshoot when operating at supersonic speeds due to heat added by intake geometry.
I assure you that this is a very dumbed down version and the actual thermodynamic calculations are far too complex, even under assumptions of ideal condition. Removing ideal-gas conditions makes those equations nightmares, and warrents use of mainframes and supercomputers to compute data.
Regards,
Sapper
