Eng 1 Flashcards
What is total pressure made up of?
Static pressure (potential energy) and dynamic pressure (kinetic energy or velocity).
What does Bernoulli’s theorem state?
It states that an incompressible fluid that passes through a convergent opening increases in speed and decreases in pressure.
Since supersonic fluid is compressible, the opposite happens.
What differenciates a nozzle from a diffuser?
The nozzle increases velocity and diminishes pressure. The diffuser does the opposite.
What are the minimal components of every gas generator?
Compressor, combustion chamber, turbine.
Describe the Brayton cycle.
It consists of four simoltaneous tranformations: intake, compression, combustion and exaust.
How is gross thrust measured?
It is done with a static enging on a test stand. The thrust obtained is the resuld of the exaust gases only. Air velocity at the inlet is ignored.
It is measured at STD conditions.
What’s the difference between gross and net thrust?
Net thrust is gross thrust corrected for the inlet airflow velocity.
What’s the net thrust formula?
Net Thrust = mass * (Vf - Vi)/t
What are the factors affecting the thrust of a turbine gas engine?
Air density, airspeed/ram effect and engine rpm.
How does air density (and consequently temperature and pressure) affect thrust?
A higher density yields more thrust. Since with higher temperatures air density is lower, higher temps lead to less thrust. A similar reasoning can be done with pressure.
How does altitude affect thrust?
While climbing, both temperature and pressure drop. Since the pressure drop is the greater one, thrust decreases. The effect is moderated by the temperature drup up to the isothermal layer (36.000 ft - the optimum cruising altitude).
How does airspeed affect thrust (theoretically)?
Theoretically, as airspeed increases the margin between intake and exaust speed decreases, thus decreasing net thrust.
Describe ram effect.
It occurs at speeds towards transonic or higher. I consists in an air mass increase at the engine inlet, thus increasing engine performance (it basically cancels or even reverses the theoretical airspeed effect on thrust)
How does RPM reflect on a throttle variation?
At low RPMs there is a little increase in thrust with an increase in throttle. The opposite happens at high RPMs. Also, at low RPMs, TSFC is very high.
List the thrust measuring instruments.
Pressure indication gauge (for jets, indicates EPR=Pin/Pout), torquemeter (for prop or rotor, indicates Shaft Horsepower), tachometer (for both, measures engine speed).
EPR = Engine Pressure Ratio
