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

[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.