Propulsion SESA2023

31 flashcards · Physics · by Zack Phipps

1. Why cant an engine be 100% propulsively efficient?

   This would mean thrust is equal to practically zero, and thrust is needed to overcome drag.

2. What is momentum flux?

   Transport of momentum that acts in a direction perpendicular to flow.

3. How is thrust generated?

   Thrust is generated whenever the momentum flux leaving an engine is larger than the momentum flux entering the engine

4. How can the thermal efficiency be increased? and what is it limited by?

   increasing the pressure ratio increasing the temperature ratio It is limited by the material used in the turbine blades being able to handle to temperature and pressure , this can be handled using cooling methods

5. What is the equation for thermal efficiency?

   Nth = 1/2 ((Vj2 - V2) / (f * LCV))

6. Total thrust for a rocket is F = m*Vj + Aj ( Pj- PA)

   This means that thrust is dependant on atmospheric pressure and therefore altitude m is mass flow rate

7. What is propulsive Efficiency?

   A measure of how well an engine can convert the kinetic energy into displacement work

8. What is Thermal Efficiency?

   How well an engine can convert heat into kinetic energy

9. What is thrust equation using the fuel air ratio?

   F = ma [(1 + f ) Vj - V ]+ Aj ( Pj- PA )

10. For a rocket, what is the momentum flux entering the control volume?

    Zero, a rocket has no inlet

11. Propulsive efficiency is defined as:

    The ratio of aircraft power to the rate of kinetic energy of the working fluid produced by the engine

12. Aircraft power is defined as:

    Waircraft = F x V = V ( ma [(1 + f ) Vj - V ]+ Aj ( Pj- PA ) )

13. Wjet is defined as:

    Wjet = 1/2( (ma +mf) Vj2 - maV2)

14. Using the fuel to air ratio what can Wjet be defined as?

    Wjet = ma/2 ( (1 + f ) Vj2 - V2)

15. When the exhaust is fully expanded and f<<1 what can the overall efficiency be simplified to?

    No = (Vj - V)V / f * LCV

16. Overall efficiency can be defined using mass flow rate of fuel:

    No = F x V / mf LCV

17. what is the overall efficiency in terms of thermal and propulsive efficiency? No =Np x Nth

    By separating the two it allows easier optimisation of each component to increase overall efficiency

18. How do you increase propulsive efficiency without reducing thrust

    Increasing the mass flow rate of air allows the thrust to remain constant whilst decreasing the jet velocity. This is why engines are getting bigger.

19. The simplified equation of propulsive efficiency is Np = 2 /( (Vj / V) +1)

    This is a simplified version assuming that: f <<1 Pj = Pa

20. What is the simplified version of the thrust equation?

    F = ma ( Vj - V)

21. What happens to the thrust when propulsive efficiency?

    Thrust decreases as propulsive efficiency increases this is because Vj = V for 100% efficiency and for this thrust would equal zero

22. The momentum conservartion equation is: Mx,out -Mx,in = Σ F

    MX,out is the momentum flux in the x direction leaving the control volume, Mx,in is the momentum flux in the x direction entering the control volume, Fx is the sum of all forces in the x direction

23. What is the momentum flux entering the control volume? [words] Mx,in = mass flow rate of air x velocity

24. What is the momentum flux leaving the control volume? [words] Mx,out = mass flow rate of air and fuel x jet velocity

25. Total thrust equation for a jet engine: F = (ma +m f ) Vj - maV + Aj ( Pj- PA )

26. How can the lift to drag ratio be replaced in the Breguet range equation?

    CL/CD

27. What is Breguet range equation?

    s = -L/D ( V / TSFC) (1/g0) ln(W2/W1)

28. Using TSFC how can the overall efficiency be written?

    No =V / (TSFC *LCV)

29. What is thrust specific fuel consumption?

    TSFC = Mass flow rate of fuel / Thrust

30. In steady flight at constant velocity, what is the relations between: Weight, Lift, Drag, and Thrust

    W = L F =D W = D (L/D) W = F (L/D)

31. What is the fuel to air ratio?

    mass flow rate of fuel / mass flow rate of air