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Beginner's Guide to Propulsion
Maximum Flight Time

1. The Maximum Flight Time for an aircraft depends on the specific fuel consumption, net thrust, and the mass of the fuel load. If all other factors for the following aircrafts remain the same, which of the 2 aircraft has the longer maximum flight time? Place an "X" on the correct line.

A.
 AIRCRAFT A - The throttle setting is 58% _____X_____ AIRCRAFT B - The throttle setting is 75% ___________

B.
 AIRCRAFT A - The engine is producing 1500 pounds of thrust _____X______ AIRCRAFT B - The engine is producing 2000 pounds of thrust ________

C.
 AIRCRAFT A - has a turbojet engine with an afterburner ________ AIRCRAFT B - has a plain turbojet engine _____X_____

D.
 AIRCRAFT A - has a turboprop engine ___X_______ AIRCRAFT B - has a rocket engine ___________

E.
 AIRCRAFT A - has a L/D (lift to drag ratio) of .5 __________ AIRCRAFT B - has a L/D (lift to drag ratio) of .8 _____X_____

F.
 AIRCRAFT A - has a fuel mass flow rate of 2500 lbm/hr_____X_____ AIRCRAFT B - has a fuel mass flow rate of 3500 lbm/hr__________

2.  For a given mission, the required flight time could replace the maximum flight time in the required equation. This idea may be needed to solve the following:

A. For mission I, we need 4500 pounds of thrust for 3 hours. If the Thrust Specific Fuel Consumption is .8, how much fuel is needed for this mission?
M= t x TSFC x F
M = 3 x .8 x 4500 = 10,800 lb mass of fuel

B. For the second mission, the engines are being fed from a fuel tank that delivers 3200 pounds mass per hour. If the aircraft is producing 2500 pounds of thrust, what is the TSFC?

TSFC = mf /F
TSFC = 3200/2500 =1.28

C. If the aircraft can carry 20,000 pounds mass of fuel plus the amount needed for takeoff and landing, what is the limit for the time the mission can take?

tmax = M / (TSFC x F)
tmax = 20,000/(1.28 x 2500) or 20,000/3200 = 6.25 hours

D. The aircraft plans to maintain a cruise speed of 120 mph. What is the aircraft's maximum range?

R = V x tmax
R = 120 x 6.25 = 750 miles

E. The aircraft was forced to land unexpectedly. At that time, there were only 15,000 pounds mass of fuel in excess of that needed for takeoff and landing. All other conditions remain the same. If the final destination of the mission is 630 miles away, what cruise speed should be maintained?

Step 1: tmax = 15,000/3200 = 4.6875 hrs,

Step 2: R = V x tmax

630 miles = V x 4.6875 hours

V = 135 mph

Related Pages:
Standards
Activity
Worksheet
Propulsion Activity Index
Propulsion Index

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