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What Is Thrust?  
User currently offlineJMJ From , joined Dec 1969, posts, RR:
Posted (13 years 5 months 1 week 4 hours ago) and read 1066 times:

I have a question on thrust for anyone who would like to answer it. I know what thrust is in the sense that it's the power that the engines exert on the plane, how hard they push it so to speak. My question is when they say an engine has say, 20000 lbs of thrust what does that mean? How is it measured? Does that mean that the engine with 20000 lbs of thrust applies the same amount of energy to the plane as gravity would to 20000lbs? What i mean by that is if I tied a strong rope to the front of a plane, tied the other end to a block of cement weighing 20000lbs and pushed the block over a cliff (hey, I'm stronger than I look) would the force then pulling the plane towards the cliff be approx. that of a 20000lbs thrust jet engine? Thanks for your help,

JMJ

5 replies: All unread, jump to last
 
User currently offlineJt8djet From United States of America, joined Jul 2000, 214 posts, RR: 0
Reply 1, posted (13 years 5 months 1 week 3 hours ago) and read 1028 times:

JMJ,

Let me introduce you to Aaron.

http://www.airliners.net/discussions/tech_ops/read.main/2945/

Enjoy!!


User currently offlineJMJ From , joined Dec 1969, posts, RR:
Reply 2, posted (13 years 5 months 1 week 3 hours ago) and read 1010 times:

Thanks Jt8.  Smile Very interesting, though I didn't understand everything. Maybe I should have phrased my question to ask why is the thrust measured in wieght (Lbs, Kgs, etc.). So then is it safe to say that having a 90000lb thrust engine going full blast pointing down on my foot (try to imagine it  Smile ) would create the same pressure against my foot as having a 90000lb block of cement on it ( eeeeyouch!! ) ? Incidentally I figured that you would also have to add the weight of the engine itself to calculate it's total pressure against my foot, if there's any science majors out their about to correct my test results, hehe. I'm going to need a new pair of shoes after THIS experiment.

JMJ


User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 3, posted (13 years 5 months 6 days 17 hours ago) and read 987 times:

>>>Maybe I should have phrased my question to ask why is the thrust measured in wieght (Lbs, Kgs, etc.).

Thrust is a force measured in pounds or newtons, not kilograms -which is a unit of mass.

We have all seen the profile view of a horse pulling a bucket of water up a well. The imaginary horse can pull 33000 pounds up one foot, 33 pounds 1000 feet, or any combination thereof that results in 33000 ft/lbs of work in one minute. That is one horsepower.

When that horse is standing there in static equilibrium with the bucket, it is exerting thrust -it doesn't matter if it stands there for one second, one minute or five days. If the bucket and water weighs 33 pounds, the horse is exerting 33 pounds of thrust. Of course we aren't taking system losses into account, but the theory is simple.

If the horse is pulling on a rope attached to an immovable wall with a scale in the middle, the amount of thrust exerted by the horse could be read directly from the scale.

Thrust of a jet engine can be measured directly using strain gauges.


aaron



User currently offlineJMJ From , joined Dec 1969, posts, RR:
Reply 4, posted (13 years 5 months 6 days 14 hours ago) and read 970 times:

Thanks Aaron. Your help is appreciated.

JMJ


User currently offlineF-WWAI From Andorra, joined Dec 1999, 131 posts, RR: 0
Reply 5, posted (13 years 5 months 3 days 23 hours ago) and read 948 times:

some simple dimensional physics:

thrust is a FORCE expressed in a modern world in NEWTON
(remember: Newton tought us that Force is Mass times Acceleration)

thus by definition: 1 N = (1 Kg* 1 meter)/ 1 square second

thrust of the engine is: airflow * exhaust speed
thus we find kilogram/second * meter/second = (kg m)sec2

clear enough?
 Smile/happy/getting dizzy FW


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