Lift vs Thrust Force: How Is More Lift Than Thrust Possible?

[Puglife]

The thrust of an airplane changes the speed of the airplane, or you can look at it as it is increasing the airspeed relative to the airplane. The airspeed across the wings is only as much as the thrust allows it to be.

The airspeed across the wing is what produces thrust, and I realize how it does so, via either pressure difference, camber, angle of attack, or a vortex lift system.

What my question is, how can it be possible that a plane can produce more force in lift, then it has in thrust. I ask because the lift is directly dependent upon the thrust, so where does the extra force come from?

Can someone break it down, via both calculations, an example, and a conceptualization?

Thank You All Very Much!

 

[SteamKing]

The thrust of an airplane changes the speed of the airplane, or you can look at it as it is increasing the airspeed relative to the airplane. The airspeed across the wings is only as much as the thrust allows it to be.

The airspeed across the wing is what produces thrust, and I realize how it does so, via either pressure difference, camber, angle of attack, or a vortex lift system.

It does? Then why do they put those engine things on planes?

What my question is, how can it be possible that a plane can produce more force in lift, then it has in thrust. I ask because the lift is directly dependent upon the thrust, so where does the extra force come from?

Can someone break it down, via both calculations, an example, and a conceptualization?

Thank You All Very Much!

Lift is a force which usually acts perpendicular to thrust.

For an aircraft in level flight cruising at constant velocity, the thrust provided by the engines balances the drag of the aircraft at that cruising speed, while the lift produced by the wings and horizontal tail surfaces equals the weight of the aircraft and its contents.

 

[russ_watters]

What my question is, how can it be possible that a plane can produce more force in lift, then it has in thrust.

I can't imagine why they should be related. Are you aware that gliders have glide ratios of 20 or 30 to 1? Drag is due to friction and a pressure unbalance need not be significantly related to lift.

 

[Puglife]

I realize that they have the high drag to lift ratio, but I am asking how it is physically possible for it to produce more force from lift than thrust.

The lift comes from the thrust of the airplain, or glider, so how is it that the force can come out of no where?

I am just trying to understand, thank you all for your help

 

[Puglife]

It does? Then why do they put those engine things on planes?Lift is a force which usually acts perpendicular to thrust.

For an aircraft in level flight cruising at constant velocity, the thrust provided by the engines balances the drag of the aircraft at that cruising speed, while the lift produced by the wings and horizontal tail surfaces equals the weight of the aircraft and its contents.

I realize that, what I do not understand is how it is possible to have more lift than thrust. Doesn't energy have to be consetved? Where does the extra energy come from?

 

[SteamKing]

I realize that, what I do not understand is how it is possible to have more lift than thrust. Doesn't energy have to be consetved? Where does the extra energy come from?

Lift and thrust are properly forces, not energy. The lift of a wing is determined by among other things, its shape and the speed of the air flowing over it. Thrust is created by some sort of engine, and the power output of this engine or engines is not dependent on the amount of lift being generated by a wing.

If a plane is not moving, or a wind is not blowing, its wings cannot generate lift. However, it can still generate thrust by having its engines operating and the pilot standing on the wheel brakes.

When aircraft are launched from an aircraft carrier, the engines are running at full power before the catapult is engaged to fling the plane forward and off the ship.

 

[A.T.]

I realize that, what I do not understand is how it is possible to have more lift than thrust. Doesn't energy have to be consetved?

Do you understand the relationship between energy and force?

https://en.wikipedia.org/wiki/Work_(physics)#Mathematical_calculation

What is the work done by the lift force if that force is perpendicular to the planes velocity?

 

[Puglife]

Lift and thrust are properly forces, not energy. The lift of a wing is determined by among other things, its shape and the speed of the air flowing over it. Thrust is created by some sort of engine, and the power output of this engine or engines is not dependent on the amount of lift being generated by a wing.

Yes, but the lift generated by a wing is from the airflow, which is produced by the thrust.
So how can their be more force in thrust than the force of the airspeed, or the force of the thrust?

 

[SteamKing]

Yes, but the lift generated by a wing is from the airflow, which is produced by the thrust.
So how can their be more force in thrust than the force of the airspeed, or the force of the thrust?

Wings can generate lift if there is no thrust being applied to the aircraft. All it takes is air flowing over the wing from the wind to generate lift. This may not produce enough lift to make the plane fly, but it can reduce the takeoff speed or distance required to become airborne. This is why when carriers launch aircraft, the ship is pointed into the wind and is traveling at full speed (usually around 30 knots) before launching aircraft with catapults. The extra wind speed over the deck (30 knots + ambient wind) helps to get the plane airborne after launch.

After a plane is flying at altitude, sometimes its engines lose power and there is no more thrust available. The plane does not immediately drop out of the sky, but it can glide for a certain time, trading altitude for speed to keep air flowing over its wings. When an aircraft is gliding, by definition, there is no thrust being applied, although lift is generated by air flowing over its wings and drag is created from the passage of the plane thru the air.

 

[gianeshwar]

Engine provides the power and we can fix arbitrarily powerful engine which can provide forward thrust and hence accelerate the plane consequently due to Bernoulli effect net upward pressure can be increased.So all depends on Engine.

 

[A.T.]

So how can their be more force in thrust than the force of the airspeed, or the force of the thrust?

Have you heard of levers?

https://en.wikipedia.org/wiki/Lever

You can put in a small force, and get out a greater force. There is no force conservation.

 

[boneh3ad]

There is no "conservation of force" rule in physics. Lift is, in a certain sense, independent of thrust. Lift depends on forward motion of the plane. Thrust must only overcome drag in order to provide that forward motion. It's also perfectly feasible to get forward motion without thrust, in which case you have zero thrust and positive lift.

 

[Puglife]

Wings can generate lift if there is no thrust being applied to the aircraft. All it takes is air flowing over the wing from the wind to generate lift. This may not produce enough lift to make the plane fly, but it can reduce the takeoff speed or distance required to become airborne. This is why when carriers launch aircraft, the ship is pointed into the wind and is traveling at full speed (usually around 30 knots) before launching aircraft with catapults. The extra wind speed over the deck (30 knots + ambient wind) helps to get the plane airborne after launch.

After a plane is flying at altitude, sometimes its engines lose power and there is no more thrust available. The plane does not immediately drop out of the sky, but it can glide for a certain time, trading altitude for speed to keep air flowing over its wings. When an aircraft is gliding, by definition, there is no thrust being applied, although lift is generated by air flowing over its wings and drag is created from the passage of the plane thru the air.

So the extra force comes from the additional wind that is their? How then is it possible for it to take off on a non-windy day?

 

[rcgldr]

I think the intended question here is how can lift / drag ratio be greater than 1.0? Consider how much smaller the profile of a aircraft as seen from the front (drag), is compared to the profile as seen from above (lift). The wings are relatively thin, but long (wing span). Lift is generated by a wing diverting the relative airflow downwards. Wings operating in an efficient mode divert a relatively large amount of air downwards by a relatively small angle.

 

[A.T.]

So the extra force comes from the additional wind that is their?

No. It comes from mechanical advantage:

https://en.wikipedia.org/wiki/Mechanical_advantage

 

[boneh3ad]

So the extra force comes from the additional wind that is their? How then is it possible for it to take off on a non-windy day?

The aircraft carrier sailing into the wind at 30 knots creates at least a 30-knot headwind that is helpful for takeoff. Any additional wind speed on top of that is just bonus and is not necessary for takeoff.

I think the intended question here is how can lift / drag ratio be greater than 1.0?

No, his question is how can lift/thrust be greater than 1 and the issue is that these two quantities are not really related in any sense other than thrust is what generally accelerates a plane up to a speed where lift is significant.

 

[SteamKing]

So the extra force comes from the additional wind that is their? How then is it possible for it to take off on a non-windy day?

I believe naval catapults are designed to launch aircraft with weight restrictions even if the carrier is not sailing into the wind, creating additional wind speed.

Some carriers do not use catapult-assisted launching at all. The forward end of the aircraft deck has a special "ski jump" ramp built on it. The aircraft takes off like it would on land, but when it reaches the "ski jump", it is given a momentary upward boost as it runs up the ramp, which boost is sufficient to keep the aircraft aloft as it begins flying.

Aircraft can take off from land runways without any wind blowing. It just takes an aircraft a longer distance to build up sufficient speed to allow the wings to generate enough lift so that the plane can fly. This is not to say that any aircraft of any weight can take off from a given airfield on a calm day, only that it is possible to do so under certain conditions. Whatever wind is present just makes it easier for takeoff to occur.

 

[Puglife]

The aircraft carrier sailing into the wind at 30 knots creates at least a 30-knot headwind that is helpful for takeoff. Any additional wind speed on top of that is just bonus and is not necessary for takeoff.
No, his question is how can lift/thrust be greater than 1 and the issue is that these two quantities are not really related in any sense other than thrust is what generally accelerates a plane up to a speed where lift is significant.

They are completely related, because the headwinds, or airspeed is directly determined by the thrust. So how can it have a higher ratio than 1?

 

[Puglife]

Have you heard of levers?

https://en.wikipedia.org/wiki/Lever

You can put in a small force, and get out a greater force. There is no force conservation.

Ohh, but levers sacrifice speed for force, what do planes sacrifice?

 

[SteamKing]

They are completely related, because the headwinds, or airspeed is directly determined by the thrust. So how can it have a higher ratio than 1?

You put a bigger engine on the plane, or even a rocket like the X-15.

https://en.wikipedia.org/wiki/North_American_X-15

The X-15 had a max. loaded weight of 34,000 pounds, which is the amount of lift its wings had to generate to keep the craft flying. On the other hand, the rocket engine which powered the X-15 could develop max. thrust of over 70,000 pounds.

 

[SteamKing]

Ohh, but levers sacrifice speed for force, what do planes sacrifice?

Not always.

 

[A.T.]

Ohh, but levers sacrifice speed for force, what do planes sacrifice?

Same thing:
thrust < lift
speed parallel to thrust > speed parallel to lift

 

[boneh3ad]

They are completely related, because the headwinds, or airspeed is directly determined by the thrust. So how can it have a higher ratio than 1?

Why did you ask the question if you are just going to disbelieve the answer?

They aren't related. At least, they aren't in the way you are treating them. Now that I think about it, @rcgldr was onto something talking about the lift to drag ratio. Thrust only has to overcome the drag on the plane. A typical wing produces much more lift than it does drag, as indicated by the lift-to-drag ratio. So thrust only has to overcome the drag, which is typically several times smaller than lift.

 

[DarioC]

I have a classic aircraft and fly it regularly. It weighs 1200 lbs loaded. We were talking on our forum and came to the conclusion that the propeller for this class of aircraft would develop (perhaps) 350 lbs of thrust. If the plane only weighed 300 lbs it could go straight up.

Even if Puglife makes his argument poorly, he does have a good point. How does my airplane produce 1200 lbs (and more) of lift by using 350 lbs of thrust. What is the conversion factor and how does it work?

Oh also, there is at least one error repeated here; a glider does produce thrust. It produces a vector along the length of the aircraft from the downward pull of gravity.

DC

 

[jbriggs444]

Rhetorical question: How can a sailboat tack upwind?

 

[russ_watters]

The lever analogy is good, but it is still an analogy. Lift is not as directly tied to thrust as a lever.

For example, if you pitch a glider horizontal it will no longer receive thrust from gravity and will begin to slow down. But it will still generate lift.

Conversely, you can lower a plane's landing gear to increase drag and simultaneously throttle up to maintain speed, but that will not affect lift. The thrust is not redirected as lift.

It is best to try to decouple these issues: the engine exists to counter drag, whereas the wing creates lift while also creating drag. It is best to set the engine aside and concentrate on what actually creates the lift.

 

[boneh3ad]

Think of it this way: an airfoil, at the very basic level, essentially works by moving air around it in order to create forces. It pushes and drags so air forward along with it, which is essentially drag. It also directing air downward, and it is the upward reaction force to this downward change in air momentum that is lift. A well designed airfoil will direct far more air downward (lift) than it takes along with it (drag).

These forces come from the movement of the air (or the wing through the air), so thrust is needed to generate that movement. Since thrust only has to overcome drag in order to create movement, it generally doesn't have to come anywhere near matching the amount of lift that is generated by the airfoil. It just has to overcome the drag and the airfoil does the rest.

 

[David Lewis]

Puglife wrote: "The thrust of an airplane changes the speed of an airplane..."

David Lewis wrote: Moving the elevator changes the speed of an airplane. Trimming in up elevator causes it to slow down*, and down elevator trim makes it speed up.

In most situations increasing thrust causes the airplane to climb. Reducing thrust causes it to descend.

Puglife wrote: "...lift is directly dependent on thrust."

David Lewis wrote: In level flight, lift is solely dependent on weight.

* Depending on how quickly up elevator is fed in, there may be a brief, transient climb before the airplane levels out at a slower speed.

 

[A.T.]

The lever analogy is good, but it is still an analogy. Lift is not as directly tied to thrust as a lever.

For example, if you pitch a glider horizontal it will no longer receive thrust from gravity and will begin to slow down. But it will still generate lift.

Yes, a simple lever doesn't store or dissipate energy, while a plane has KE and PE. But the energy conservation (which seems to be the OPs concern) can still be explained using the work theorem (just like with lever).

 

[rcgldr]

Some carriers do not use catapult-assisted launching at all. The forward end of the aircraft deck has a special "ski jump" ramp built on it. The aircraft takes off like it would on land, but when it reaches the "ski jump", it is given a momentary upward boost as it runs up the ramp, which boost is sufficient to keep the aircraft aloft as it begins flying.

That upwards boost doesn't help if the aircraft doesn't have sufficient speed or sufficient thrust at the launch angle to keep it airborne and accelerating. I think the ramps are used for Harrier jets, and the ski jump orients the Harriers for climbing at slower than normal flight speeds without having to orient the jet engines downwards (so they don't heat up the deck), relying on jet thrust to keep them airborne and accelerating until they reach normal flight speed.

 

[russ_watters]

While originally designed for Harriers, they can be/are used for other aircraft as well:
https://en.wikipedia.org/wiki/Flight_deck#Ski-jump_ramp

 

[boneh3ad]

That upwards boost doesn't help if the aircraft doesn't have sufficient speed. I think the ramps are used for Harrier jets, and the ski jump orients the Harriers for climbing at slower than normal flight speeds without having to orient the jet engines downwards (so they don't heat up the deck), relying on jet thrust to keep them airborne and accelerating until they reach normal flight speed.

There are also other countries with the ski-jump carriers that use planes on them that are not VTOL capable. The ramp really just gives it a larger positive angle of attack at launch so that it can generate more lift at its low speed when it leaves the carrier.

 

[rcgldr]

That upwards boost doesn't help if the aircraft doesn't have sufficient speed or sufficient thrust at the launch angle to keep it airborne and accelerating.

While originally designed for Harriers, they can be/are used for other aircraft as well:

There are also other countries with the ski-jump carriers that use planes on them that are not VTOL capable. The ramp really just gives it a larger positive angle of attack at launch so that it can generate more lift at its low speed when it leaves the carrier.

The key issue here is that for a given launch angle θ, the thrust has to be greater than m g sin(θ) + aerodynamic drag. The ramp is more efficient at putting the jet into a climb orientation than it would on it's own. Getting back to the Harrier jets, although they could use downwards vectored thrust, this would heat up the flight deck.

 

[boneh3ad]

The key issue here is that for a given launch angle θ, the thrust has to be greater than m g sin(θ) + aerodynamic drag. The ramp is more efficient at putting the jet into a climb orientation than it would on it's own. Getting back to the Harrier jets, although they could use downwards vectored thrust, this would heat up the flight deck.

I mean, sure it would heat up the flight deck but they do it anyway. Imagine what it will be like when the F-35 has to land on a carrier deck. In that plane the LiftFan doesn't blow any hot air, but at the rear they vector the main engine exhaust straight down during vertical landing.

 

[rcgldr]

heat up the flight deck

The issue is a hot flight deck could affect the tires of the next jet that is taking off, creating a potential delay between launches. Similarly, the blast deflector on a catapult launch is angled a bit backwards to divert the hot thrust away from the deck (note the image in the wiki article):

http://en.wikipedia.org/wiki/Jet_blast_deflector

Maybe the newer carriers for VTOL type jets have solved this problem.

 

[rcgldr]

Getting back to the original question:

how can it be possible that a plane can produce more force in lift, then it has in thrust?

Compare the situation to that of a frictionless wedge, the force that the wedge pushes upwards (and/or downwards), can be a multiple of the horizontal force applied to the wedge. Similarly, the wings on a aircraft only divert the air downwards by a small angle. This somewhat correlates to the wedge situation, a much greater downwards force (versus the horizontal thrust) is applied, but at a much lower speed.

In the case of high end gliders, lift to drag ratios can be 60 to 1. Wiki article about one of these type of gliders (one option is a motorized glider):

http://en.wikipedia.org/wiki/Schempp-Hirth_Nimbus-4

If lightly loaded, a Nimbus 4DM would only need about 4 to 5 kw of power (applying about 3 kw of power to the air after losses) to cruise in level flight around 110 kph.

 

[Puglife]

Yes, a simple lever doesn't store or dissipate energy, while a plane has KE and PE. But the energy conservation (which seems to be the OPs concern) can still be explained using the work theorem (just like with lever).

First, I would like to apologize for not responding sooner, it has been a really rough week for me.

Now, as for the topic at hand, levers, work off the principal of conservation of energy. They conserve their rotational kinetic energy, as well as torque. The point is, is that the two related forces, do not come from no where, they are related, and are different only because they must conserve energy.

I do get this now, but their has been many conflicting view points, and would like one last concrete answers.

Thank you for all of your help everyone, it is super appreciated.

 

[Puglife]

Why did you ask the question if you are just going to disbelieve the answer?

They aren't related. At least, they aren't in the way you are treating them. Now that I think about it, @rcgldr was onto something talking about the lift to drag ratio. Thrust only has to overcome the drag on the plane. A typical wing produces much more lift than it does drag, as indicated by the lift-to-drag ratio. So thrust only has to overcome the drag, which is typically several times smaller than lift.

It isn't that I refuse to believe the answer, it is a combination between the fact that I am getting multiple conflicting answers, and the fact that some of you do not fully finish your thoughts, and instead comment on irrelevant information, that does not pertain to the topic at hand what some of you are saying, and asking you to rephrase.

 

[russ_watters]

It isn't that I refuse to believe the answer, it is a combination between the fact that I am getting multiple conflicting answers, and the fact that some of you do not fully finish your thoughts, and instead comment on irrelevant information, that does not pertain to the topic at hand what some of you are saying, and asking you to rephrase.

Yes, people often go on tangents responding to bits of points, that aren't complete thoughts. It can be confusing. However, based on your last two posts, I'm not sure where we're at right now. I'll sum-up some of the key thoughts, but we'll need you to tell us what is confusing you:

1. A lever provides a nice analogy, where energy is conserved and one force is translated into a vastly larger force in a different direction. But:
2. A lever is a fixed ratio whereas lift and thrust are not related at all. There is no fixed ratio or even a necessity that there be any thrust for lift to be produced. There is some relationship between lift and drag, but even that isn't straightforward/fixed.
3. So it is best to set aside this idea completely and focus on what actually does produce lift rather than continue trying to understand lift based on thrust.

 

[A.T.]

... they must conserve energy...

And so does the airfoil, because the work done by lift is smaller than the work done by thrust, even if lift is greater than thrust. The difference in work is dissipated as heat.

 

[A.T.]

1. A lever provides a nice analogy, where energy is conserved and one force is translated into a vastly larger force in a different direction.

The key here is not "in a different direction" but "over a different distance".

 

[A.T.]

I am getting multiple conflicting answers

If you see a conflict with conservation of energy, then try show that mathematically. That will give you the best understanding.

 

[rcgldr]

If there's only enough thrust to compensate for drag (ignoring drag related to the prop wash), then the magnitude of thrust and drag are the same, in which case the lift to thrust ratio is the same as the lift to drag ratio (ignoring the issue of the direction of thrust versus drag). I think a wedge provides a better analogy. The angle of attack for a wing in normal flight is relatively small, so the speed of the downwards diverted air is much less than the horizontal speed of the air (or the speed of the wing if using the air as a frame of reference. At the same time the downwards force (lift) is much greater than the forwards force (drag) exerted onto the air.

Energy conservation gets tricky. From the plane's frame of reference, the air is slowed down due to the combined effects of drag and lift. From the air's frame of reference, the air is sped up from zero velocity to some non zero, mostly downwards (lift) velocity. It's easier to see this in the case of a glider at a constant speed and a constant rate of descent. From the ground or air (assuming zero wind) frame of reference, the decrease in gravitational potential energy corresponds to the increase in energy of the air, mostly mechanical energy, and some thermal energy. From the gliders frame of reference, I'm not sure how to take gravitational potential energy into account, and the air is slowed down (energy decreased) with respect to the glider.