Thrust vs. weigth

[QuEsT147]

Hi guys

Maybe a silly question, but could you please explain to me the difference between thrust and weigth of the airplane?

For example a B737 has engines with 20k of thrust each which is about 2000 kgs of thrust in kilograms. So we have 4000 kgs of total thrust available. But the T/O weigth of the airplane may be more than 60 000 kgs. How is it possible that 4k kgs thrust engines can move 60k kgs plane and even hold it in the air? As I have said, it is a silly question but Iwould like to understand it.

Thanks for help
QuEsT147

[JohnFTEng]

This really is Aero101! There are four basic forces acting on your a/c - thrust and drag in fore/aft plane, and, lift and weight in vertical plane. The lift opposes the weight - so your 60 tonnes is opposed by the lift of the wing so the a/c goes up. You don't the lift free - there is an associated drag force that is opposed by the thrust - your engine thrust of 2 tonnes. As the L/D is about 10 - 20 you can see why the thrust is only a portion of the weight (lift). Of course you need quite a large thrust to accelrate your 60 tonnes up to flight speed so its a bit larger than the cruuise thrust needed.
This is really day 1/page 1 stuff and its whole lot more complicated when you get down to the details. I only you're not a pilot or we're all doomed

[Dont Hang Up]

Thrust only needs to equal weight if you wish to achieve vertical take-off. For a fixed wing aircraft, thrust only needs to overcome drag (air resistance) when travelling at flying speed. The lift (which of course must still equal the weight) is generated by the wings as they move through the air.

[QuEsT147]

Thanks for answer... You are right, I am not a pilot, just interested in planes and all around that. I even understand the four forces that you have mentioned but I simply did not realize the relationship between them. There are things which I understand and some that I do not, that is why I am asking the pros Tank again for your answer.

Any other comments?

[Lemurian]

For example a B737 has engines with 20k of thrust each which is about 2000 kgs of thrust in kilograms.

This is what happens when people have no tech background.
20 k of what ? I presume lbs, right ?
So that's 20,000 lbs of thrust, equivalent to some 9,000 kg, but more correctly, as thrust is a force, 88,200 N or 88.2 kN.
Then, total installed thrust, MSL / Standard conditions : 176.4 kN.
So, quite a lot more than you thought.

[QuEsT147]

Sorry, I see, my mistake, I meant kN, but wrote a value in lbs

[Dont Hang Up]

Anyone who has been slightly patronising in their answer to QuEsT should be made to write a short essay on why the coefficient of lift is not exactly equal to 1 for any wing design. In aerodynamics nothing is as obvious as it seems. Someone who claims to fully understand it is probably fooling themself.

[barit1]

Exactly. As my dad said (he was a flight instructor 1942-45) "You think you know it all until you start to teach someone else!"

[QuEsT147]

Thank guys for your replies.

I do not think any of these answers are patronising although sometimes it may seem so. For me, information given there are more important, that is why I am asking here.

Once again, thanks for answers.

[enicalyth]

..er you lost me there

G' Day Don't hang up

Do you include flaps for example in your lift generating components? If so you will find that CL can be made to exceed 1 by quite a large margin? So it must pass through one when increasing and one again when decreasing.

I'm all ears

[Dont Hang Up]

Okay, so in my rush to be flippant I got my terminology wrong. I shall write a thousand times "I shall not try to be clever on Prune again."

The actual point I was trying to make was actually to do with induced drag. It is not as immediately obvious as one might think as to why the drag induced by the wing as it generates lift is only a fraction of the lift obtained. At first glance it appears one is getting something for nothing. And hence QuEsT's question is definitely very pertinent.

Yes I do understand it does not really work out that way, but, as I said, nothing is as obvious as it seems in aerodynamics.

[Brian Abraham]

Maybe a silly question, but could you please explain

QuEsT147, no such thing as a silly question, only silly answers. NASA have a very good educational web site that may be right up your alley. You should be able to spend hours exploring and learning the wonders of aviation. Good luck and don't be afraid to come back with any questions you might have. They are a good bunch of experienced people here.
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