LBS of thrust always confuses me, I haven't really got it down. Okay so lets take the 737-800 for example, each engine generates 26K of thrust or 26,000LBS. Multiply that by 2 because there is two engines so we are looking at 52,000LBS of thrust on a 737-800. I don't really understand how this is measured but it sounds like for 1 pound of thrust can continuously move 1 pound of weight. However, a 737-800 is usually 140,000 pounds or so on takeoff. I guess it doesn't make since to me that 52,000 pounds of thrust can push a 140,000 pound airplane into the sky. Can someone enlighten me on this. I am however very familiar with horsepower but I know there is no way to really do the conversion. Would be cool if someone could give an estimate though.

What that means is that a single engine can lift vertically 26.000lbs of weight. If you think about it, it does make sense. You don't need to push with a force of a ton to move a car. In the same way, the engines don't need to push with 140.000lbs of force to move the airplane. All the lifting is done by the wings.

Hm, well I think it's a little confusing as well. Thrust however is a force, so it can technically not be in pounds (I think). If you want to "convert" that value of 26K to N, do something like 26K lbs / (2.205 lbs/kg) * 9.81 m/s², that is about 115.6 kN, which seems to be correct. Plane weighs say 80K kg. F=ma, solve for a so that will be your take off acceleration. Of course that's not right because it doesn't account for any of the side effects like drag and friction, so in reality it will be less than that. Plus you never need any force even close to what the airplane weighs in order to get the plane up in the air. The plane has a weight of ~700KN, but it's the wings that do the lifting part, not exactly the engines. If you want to do a vertical climb, you need more thrust than weight. What you need to climb normally though is just some more thrust than what you need to overcome the aerodynamic drag of level flight (=> excess thrust). That part of the thrust will "push" the plane upwards, sort of, and it's only the sine of the climb angle so not very much. That by the way is a common and major misconception, that one would think lift is greater during climb. In fact it's less than in level flight, because it's the engines that do the lifting upwards. Wow, anyone may beat me if this doesn't make any sense. Luckily I'm no physicist.

What that means is that a single engine can lift vertically 26.000lbs of weight. If you think about it, it does make sense. You don't need to push with a force of a ton to move a car. In the same way, the engines don't need to push with 140.000lbs of force to move the airplane. All the lifting is done by the wings.

Exactly. Weight acts vertically down, towards the center of Earth. Thrust is working horizontally, mostly. Now rockets, on the other hand...

That by the way is a common and major misconception, that one would think lift is greater during climb. In fact it's less than in level flight, because it's the engines that do the lifting upwards. Wow, anyone may beat me if this doesn't make any sense. Luckily I'm no physicist.

That really depends. Your statement isn't always true.

That really depends. Your statement isn't always true.

Well, that doesn't help much TBH. Please elaborate. EDIT Ah well ok, well in a glider, obviously you can just pull up and still climb (without any thrust, obviously). Is that what you mean? But you can't achieve a steady climb that way, that's just transforming energies there. And thermals etc. obviously don't count here either.

Edited September 5, 201114 yr by badderjet

Well, that doesn't help much TBH. Please elaborate. EDIT Ah well ok, well in a glider, obviously you can just pull up and still climb (without any thrust, obviously). Is that what you mean? But you can't achieve a steady climb that way, that's just transforming energies there. And thermals etc. obviously don't count here either.

No, in the glider you would be trading kinetic energy (velocity) for potential energy (altitude). We're getting into a whole vector of forces discussion that will take away from the OP, but I'll bite. In certain instances, lift vector can be more than weight, and thrust more or equal to drag. That is to say level on all axis horizontally, but at an airspeed that produces enough lift to overcome weight. I'm simplifying here.

That by the way is a common and major misconception, that one would think lift is greater during climb. In fact it's less than in level flight, because it's the engines that do the lifting upwards. Wow, anyone may beat me if this doesn't make any sense. Luckily I'm no physicist.

Well, lift has to be greater, or else you wouldn't... you know.... lift up. But because lift increases, something must change to keep the balance. In this case, it's momentum, which gets translated into speed. And as speed decreases, you need more thrust to keep it up. Or something like that... I'm no physicist either.

Well, lift has to be greater, or else you wouldn't... you know.... lift up. But because lift increases, something must change to keep the balance. In this case, it's momentum, which gets translated into speed. And as speed decreases, you need more thrust to keep it up. Or something like that... I'm no physicist either.

That's not true, either. Thrust can have a vector that resist weight. Weight always points towards Earth. ie Lift, thrust, and drag have changing vectors. Weight does not. Here's how I remember. LIFT acts perpendicular to the wings CHORD line.

In steady state flight, lift is always smaller in climb than in straigtht and level flight.Turbine engine output is measured in force units, e.g. Lb, Kg. Prop and rotor powered- in shaft Horsepower (brake horsepower for reciprocating engine). The latter engines produce power which is used to turn the prop/rotor, which in turn provides thrust. For explanation of the forces acting on aircraft in flight and on powerplants, a short read of an FAA manual for student pilots goes a long way.

Anyways... we are really starting to split the hair here. I guess anyone gets the gist of it by now. For more clarification and some wise words from actual physicist, the Internet is you friend.

Anyways... we are really starting to split the hair here. I guess anyone gets the gist of it by now. For more clarification and some wise words from actual physicist, the Internet is you friend.

Yeah. Why have a discussion and learn something, right?

That's not true, either. Thrust can have a vector that resist weight. Weight always points towards Earth. ie Lift, thrust, and drag have changing vectors. Weight does not. Here's how I remember. LIFT acts perpendicular to the wings CHORD line.

True. Weight always points down no matter what, and just because as you say lift acts perpendicular to chord line during climb (but the total vector pointing UPWARDS is still the same as the weight), hence not opposing weight anymore direclty, the upward force gets split between lift and thrust. Lift get's smaller by cos(climb angle), and thrust providing the climbing force is the excess thrust. Whatever, we don't need discuss this here if you guys don't want to, but Zach I would appreciate if you would enlighten me with one of the cases where it's not true what I said above. If you don't want to make it public PM me. Just if you like, I'd be interested.

Got it, Thanks guys!

PM sent, Etienne. Some folks appreciate these little things! Glad you found it in our back and forth, Aaron. Sorry about that.

LBS of thrust always confuses me, I haven't really got it down. Okay so lets take the 737-800 for example, each engine generates 26K of thrust or 26,000LBS. Multiply that by 2 because there is two engines so we are looking at 52,000LBS of thrust on a 737-800. I don't really understand how this is measured but it sounds like for 1 pound of thrust can continuously move 1 pound of weight. However, a 737-800 is usually 140,000 pounds or so on takeoff. I guess it doesn't make since to me that 52,000 pounds of thrust can push a 140,000 pound airplane into the sky. Can someone enlighten me on this. I am however very familiar with horsepower but I know there is no way to really do the conversion. Would be cool if someone could give an estimate though.

The technical definition is better explained using the SI units, so 52,000lbs thrust = 23,600 Kg (Newtons) of thrust, this means that if the engines were attached to a plane that weighed 23,600 Kg, those engines at max power would give an acceleration of 1ms^-2 [if no external forces act upon the system], however, for simplicities sake, lets just say the 737 weighs 47,200 kg, that means that the engines at max rated thrust would give the plane an acceleration of 0.5ms^-2. The we get a bit more technical, engines only give off their max rated thrust in ISA conditions, ie. Sea level, 15C, 1013.25 hpa ect... Then the bits about external factors, that would be wheel drag, an uphill slope, ect. That help?