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Beginner's Guide to Propulsion
Thrust to Weight Ratio and Excess Thrust

1.   As we are waiting to take off, what is the F/W (Thrust to Weight Ratio)?
 1) 0 2) .5 3) 1 4) Cannot be determined.
Answer: 1 since the thrust is zero
2. After our seat belts are fastened, the plane begins going down the runway and starts to accelerate. The drag is 1,000 pounds. What must be true about the thrust?
 1) Thrust < 10,000 pounds. 2) Thrust > 10,000 pounds. 3) Thrust = 10,000 pounds. 4) 1,2, or 3 - depends on F/W.
Answer: 2, if aircraft is accelerating the thrust must be greater than the drag.
3. As we sit back, relax, and dream about our vacation, the 747 climbs to it's desired cruising altitude. What happens to the thrust?
 1) It decreases. 2) It increases. 3) It remains constant. 4) It depends on the lift.
Answer: 1, the thrust of an engine decreases with altitude.
4. When the jet reaches it's cruising altitude, it's velocity is 540 mph. If the excess thrust to reach this altitude averaged 100,000 pounds, approximately how long did it take to reach this altitude? (Hint: Weight = m x ag where ag(acceleration due to gravity) = 32.2 ft/s2 )
 1) 21 seconds. 2) 283 seconds. 3) 114 seconds. 4) 197 seconds.
V - Vo = (Fex t)/m and 540 mph = 792 feet per second
m = 800,000/32.2 = 24,844 slugs so 792 - 0 = 100,000 t / 81,632.6
5. It takes the pilot about two minutes to accelerate to 580 mph. What amount of thrust did he have to use?
 1) 12,145 pounds. 2) 58,440 pounds. 3) 84,520 pounds. 4) 125,830 pounds.
Answer: 1, use same equation as part D
580 - 540 =(Fex x 120 sec)/ 24,844
6.  For the remainder of the flight as you are enjoying your food and drinks, the pilot continues cruising at 580 mph. During this time,
 1) The thrust must be greater than the drag. 2) The drag must be greater than the thrust. 3) The thrust must be equal to the drag. 4) The relationship between the thrust and the drag depends on the cruising speed.
Answer: 3, since plane is cruising
7. As the aircraft is landing, the pilot wants to decrease the speed by cutting back the throttle. What has to happen?
 1) The F/W Ratio must increase. 2) The excess thrust must increase. 3) The drag must be greater than the thrust. 4) The thrust must be greater than the drag.
Answer: 3, since speed is decreasing

Now that you have enjoyed your flight and safely landed, check to make sure you answered the questions correctly. If all answers are correct:

1. The sum of your numerical answers will be fifteen, AND

2. the product of your numerical answers will be seventy-two!

Related Pages:
Standards
Activity
Worksheet
Propulsion Activity Index
Propulsion Index

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