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

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
 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.
 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.
 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 a_{g} where a_{g}(acceleration due to gravity)
= 32.2 ft/s^{2} )
1) 21 seconds.

2) 283 seconds.

3) 114 seconds.

4) 197 seconds.

Answer:
4
V  V_{o} = (F_{ex} 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
 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 =(F_{ex }x 120 sec)/ 24,844
 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
 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 seventytwo!
