Last Minute Perf Questions......

[nix-nac]

Hello Everybody...This is my First Post on Here So please go easy on me..I would like it please if someone could confirm the following answers to the following questions before next Monday??? Here goes..


Q1) Flying at a constant mach no.what is true of Thrust and fuel flow??

A) Decrease in proportion to ambient Pressure with constant temp
B)Increase in proportion to ambient pressure with constant temp

Q2) When is the greatest IAS achieved in a piston A/C?

A) At the lowest altitude
B) At the optimum cruise alt
C) At the service ceililing
D) At the Absolute ceiling

Q3) For an aircraft flying at the LRC speed what happens to specific range and fuel to time ratio


thank you all and eagerly awaiting your responses.....

[Angels Zero]

I'm pretty sure the answer to your first question is that the fuel flow and thrust will decrease as the aircraft becomes lighter the less is required to maintain equilibrium at a constant temp.

As for the other two questions...?:confused:

[buttline]

Haven't seen these specific questions before (not an instructor but done all ATPLs). If they are feedback, they may not be accurate questions so beware but anyway, here's my stab...

Q1) Flying at a constant mach no.what is true of Thrust and fuel flow??

A) Decrease in proportion to ambient Pressure with constant temp
B)Increase in proportion to ambient pressure with constant temp

Flying at constant mach (mach 1 is a function of temperature ONLY). With higher pressure for a given mach number, this means the air will be relatively denser and therefore offer greater thrust and fuel flow so I would go with B. On the other hand I'm arguing with myself that the air being denser means that the engine doesn't need to spin so fast to generate the same thrust to travel at the constant mach no.



Q2) When is the greatest IAS achieved in a piston A/C?

A) At the lowest altitude
B) At the optimum cruise alt
C) At the service ceililing
D) At the Absolute ceiling

Really not too sure about this one. Question doesn't seem right. We generally fly at a constant IAS (CAS) in piston aircraft but the TAS will increase with altitude due to the reduction in density.
The highest IAS is VNE for a piston.


Q3) For an aircraft flying at the LRC speed what happens to specific range and fuel to time ratio.

LRC is a speed that is faster than the best range speed but only uses a relatively small amount of extra fuel. (i.e. get there significantly quicker without too big a fuel penalty). If that allows the aircraft to fly more hours a day (higher utilisation) it may actually be a cost saving. Specific range (if I remember right) is tied to specific fuel consumption which is KG of fuel burn per KG of thrust per unit time. SFC will increase at LRC when compared to best range speed. I'm not sure about the fuel time ratio (don't remember it) but it sounds like the same thing??

I only answered because you posted it as a last minute need - hope my thinking is on the right lines but you'd be better checking with a Performance ground instructor such as Alex at BGS or Peter at GTS Bournemouth.

Good luck.

[dorosenco]

Here below my VERY humble opinion (I only got 85% in Perf) ...

Q1)
Thrust = Mass x Acceleration = Volume x Density x (Vout-Vin)

As you can see the thrust is proportional to the density of the air for the same exhaust pipe air speed minus compressor intake.
The Density is also proportinal to the Pressure of the air and inversely proportional to the temperature.

Conclusion : Answer B

Q2)
The highest IAS can be reached at maximum output power from your engine. At high altitudes the output power of a piston engine reduces due the low air density.

Conclusion : Answer A

Q3)
I don't see the anwers proposed but I would say that the specific range increases. I don't remember the Fuel/Time ration but I guess that it is something like pounds/hour. Maximum endurance would minimize Fuel/Time but Maximum Range would have a higher Fuel/Time than Maximum Endurance.

Conclusion : Both increase