What does a wing do that an engine can't?

Question

This isn't a question of how a wing works -- vortex flow, Bernoulli's principle, all of that jazz. Instead, it's a question of why we need a wing at all. A wing produces lift, but why is that necessary?

I got to this by thinking of an airplane at a coarse level. The wing produces lift through some interesting physics, but it needs energy to do this. The engine is what ultimately provides all of this energy (let's assume no headwind, and in "ultimately" I'm not including chemical energy in the fuel, yadda yadda "it all comes from the sun"). That means the engine pushes enough air, and fast enough, to (a) offset gravity and (b) still propel the plane forward. So the question is: why can't we just angle the engine down a bit and get the same effect?

To slightly reword: why do wings help us divert part of an engine's energy downward in a way that's more efficient than just angling the engine?

One answer is that we can do exactly that; I'm guessing it's what helicopters and VTOL airplanes like the Harrier do. But that's less efficient. Why?

One analogy that comes to mind is that of a car moving uphill. The engine doesn't have the strength to do it alone, so we use gears; for every ~2.5 rotations the engine makes, the wheel makes one, stronger rotation. This makes intuitive sense to me: in layman's terms, the gears convert some of the engine's speed-energy into strength-energy.

Is this analogy applicable -- is the wing on a plane like the gearbox in my transmission? And if so, what's the wing doing, more concretely? If a gear converts angular speed to increased force, what X does a wing convert to what Y?

None of the answers I could guess at satisfied my intuition. If the wing converts horizontal speed to vertical speed, tipping the engine downward would seem to have the same effect. If it's changing the volume/speed of the air (more air blown slower, or less air blown faster), it would still have to obey the conservation of energy, meaning that the total amount of kinetic energy of the air is the same -- again suggesting that the engine could just be tipped down.

EDIT

In thinking about this more from the answers provided, I've narrowed down my question. Let's say we want a certain amount of forward force $S$ (to combat friction and maintain speed) and a certain amount of lift $L$ (to combat gravity and maintain altitude). If we tilt our engine, the forces required look like this:

The total amount of force required is $F = \sqrt{S^2 + L^2}$. That seems pretty efficient to me; how can a horizontal engine + wing produce the same $S$ and $L$ with a smaller $F'$?

Answer 1

Let's look at the relationship between momentum and energy. As you know, for a mass $m$ kinetic energy is $\frac12mv^2$ and momentum is $mv$ - in other words energy is $\frac{p^2}{2m}$

Now to counter the force of gravity we need to transfer momentum to the air: $F\Delta t = \Delta(mv)$

The same momentum can be achieved with a large mass, low velocity as with small mass, high velocity. But while the momentum of these two is the same, THE ENERGY IS NOT.

And therein lies the rub. A large wing can "move a lot of air a little bit" - meaning less kinetic energy is imparted to the air. This means it is a more efficient way to stay in the air.

This is also the reason that long thin wings are more efficient: they "lightly touch a lot of air", moving none of it very much.

Trying to replicate this efficiency with an engine is very hard: you need compressors for it to work at all (so you can mix air with fuel and have the thrust come out the back) and this means you will have a small volume of high velocity gas to develop thrust. That means a lot of energy is carried away by the gas. Think about the noise of an engine - that's mostly that high velocity gas. Now think of a glider: why is it so silent? Because a lot of air moves very gently.

I tried to stay away from the math but hope the principle is clear from this.

Answer 2

The key point is that wings allow you to "tilt the engine" much more efficiently than actually tilting it. Tilting an engine converts the power only at 1-to-1 ratio, but wings do it better - a Boeing 747 has a lift/drag ratio of 17 at cruise speed, the wing is generating 17 times more lift than the applied engine power.

Answer 3

It is merely a long comment but hopefully it gets your intuition on the right track. I try to detail the physical part of the reason why having fixed wings is a good thing:

You basically ask, why are airplanes more efficient (hence people still produce them despite their less nice maneuverability)

well you probably noticed helicopter rotors work pretty much as wings too. But for the most efficient flight you want to optimize your wing for some average flight scenarios, say flying straight 1000 km/h. Now, when it comes to optimizing the "good" thing a wing can do is provide lift, and the "bad" thing it does is provide drag. So you want to optimize for highest lift-to-drag-ratio. But your problem is that helicopter wings dont always face the direction of your flight speed. it spends a lot of time going backwards(relative to the aircraft at least) so that cannot receive air flows at the same speed as the rotor blades that go forwards, yet they are at exact same blades with the exact same same angle as the ones going backwards(facing slower air flow)

notes:

1. If you think about it, this applies to any aircraft you could imagine that relies on "rotating wings" for lift.

2. So it basically means that aircraft don't really need static wings for staying in the air, only when going at high speeds.

3. Airplanes are arguably more resilient in general then helicopters so they cost less in insurance and maintenance, which makes them overall less costly besides being better optimizable(which I attempted to detail in the main text).

If you have questions fell free to comment. I hope I helped.

Answer 4

I've wondered about this a bit before. I think it's good to hugely simplify things to think about it. Incidentally, I'm absolutely not a source of authority here. I'm just thinking it through from what seems apparent to me.

A falling plate

You've said specifically that you don't want to think about how wings work, so let's not. Forget about the wing. Forget about the airplane. Forget about the thrust, rocket or otherwise too.

Think about a plate of flat, rigid, fairly light material. It "magically" doesn't tip: it remains parallel to the ground. I can only slide up and down.

I'm pretty sure you'll agree that this will "fall" a lot slower than a ball of the same weight? And yet, it is a completely passive device – it does not require energy or expel thrust.

It falls more slowly because, to fall, it must move air molecules out of the way, or squashed them up under it. Both of these require force to be applied, which it must supply, resulting in an equal and opposite force providing it a gradual descent.

It's easy to see that making the plate larger (and keeping the same weight) will increase the staying-in-the-air quality. Changing its shape in other ways will have other effects that are beyond me to speculate on, because I'm not a real physicist or aeronautical engineer.

In air, a falling thing has a terminal velocity. The terminal velocity of our plate will be small. Much smaller than a solid ball the same weight. If it's terminal velocity if v_fall, and it is "launched" from a height h, it will take at least t=h/v_fall seconds to fall to the ground (I am neglecting any time required to accelerate to its terminal velocity).

Adding thrust

Let's attach something that provides thrust. Our plate will slide along, and again, magically stay parallel to the ground. Provided there is no significant friction created by sliding the plate through the air, and provided that the thrust source isn't heavy, we'll have got t completely free seconds of "flight" (or, if you prefer, falling with style) from the plate. That sounds pretty good! It the trust pushes us at v_thrust, we'll travel t.v_thrust before we start sliding on the ground.

Flying

But what if we start on the ground, and we want to travel for longer than the t seconds?

If we are on the ground, we could angle our plate just a touch so that as it is pushed along, the fall (from the plate's terminal velocity in air) just matches the amount that the plate climbs (due to it being pointed up a bit rather than parallel to the ground) as it is pushed along. If we did that, we'd lose some of our forward speed. We'd end up with:

v_forward^2 = v_thrust^2 - v_fall^2

(Pythagorus, right?)

If we can decrease v_fall, perhaps by making the plate bigger, then we can make make v_forward (the useful bit) bigger.

Just an engine

Without the plate at all, v_fall would be large – whatever the thrust source's terminal velocity is, in fact. So v_forward (the useful bit) would be much smaller.

Real Wings

Real wings, living in the real world, don't have access to magic to maintain their necessary pitch. They have to be built from available materials instead of ideal ones, and they need to contend with slipping through a fluid that adds drag. For these reasons and probably many more, they have a clever shape.

Obviously, everything here is grossly simplified.

I'm not claiming the tiny amount of maths here is useful for modelling reality and would be extremely surprised if it is! Nor am I claiming the lift producing mechanism of a real wing works in this way (presumably, it works better).

However, I am claiming that by working with an extremely simplified model, you can see that an airplane's wings achieve something that thrust alone does not. They provide an extremely "cheap" source of lift that would otherwise require much more of the thrust to be diverted downwards.

You could probably summarise their function as being a parachute that is rigid enough to push through the air :-) Further support that cheap lift is available comes from engineless gliders, sycamore leaves, and dandelion seeds.

Answer 5

The important quantity in determining the effectiveness of a wing is its lift to drag ratio. It turns out that the key contributer to a large lift to drag ratio is a large wing span ($b$ in the below equation). As such the large wings on the aircraft can be far more efficient at generating maximum lift with minimal drag that the smaller "wings" of the engine.

The key equation for the theoretical maximum lift to drag is:

$(L/D)_{max}=0.5 \sqrt{\frac{\pi\epsilon}{C_{fe}}\frac{b^2}{s_{wet}}}$

where $(L/D)_{max}$ is theoretical maximum lift to drag ratio, $C_{fe}$ is the equivalent skin friction coefficient, $b$ is the wing span $S_{wet}$ is the wetted area and $\epsilon$ is the span efficiency factor (a number near to 1 in the optimal case.

Refs:

• Loftin, LK, Jr. "Quest for performance: The evolution of modern aircraft. NASA SP-468".`
• Raymer, Daniel (2012). Aircraft Design: A Conceptual Approach (5th ed.). New York: AIAA.`

Answer 6

Why is an airplane better than a rocket? Because the plane grips the medium.

For a rocket to remain at a fixed altitude, it must continually thrust upwards in order to counteract the fall caused by gravity. It does this by pushing down a large amount of air and fuel each second.

An airplane can create this same upward force to counteract gravity, but in a more efficient way, by pushing against the local medium of air. Forget for a moment about the shape of the wing providing lift, just think about how the flat planar shape cutting horizontally through the air keeps the altitude from dropping.

The wing pushes down on the air below it, and the air briefly resists, pushing back upwards against the wing. A motionless plane would quickly begin to fall because the air below it would soon stop resisting, and would begin to travel downwards with it or move out of the way. But when an airplane is moving, it is getting a continual fresh supply of static medium. And all this fresh air resists the attempt of the wing to fall downwards.

A similar effect is used by the keel or centerboard below a sailing boat when it is tacking perpendicular to the wind. The planar board has the effect of "gripping" the water as it cuts through it, preventing the boat from being pushed downwind (leeward) in the axis perpendicular to the motion.

This effect is present even when the boat or airplane is not moving, but is far more pronounced the faster the plane is cutting through the medium. So the faster you go, the less horizontal area your wing needs. (Consider the relative area of the wings on a biplane versus those on a modern jumbo jet.)

You can also consider how a glider can achieve much greater distances than a rocket, whilst expending no energy at all! Or how a flat sheet of paper falls more slowly than a rolled (or crumpled) sheet of paper.

Answer 7

I think You have Your intuitions a bit scrambled.

Engine and wings do not have the same function.

If You need to make a parallel then wings have the same function of wheels on a car: they provide an efficent way to remain above ground by pushing down on some medium.

Wheels push on ground and have no problem keeping your car above it even when the car is still.

Wings, on the other hand, manage to produce an high pressure on their underside and a low pressure on the overside, this difference of pressure converts in a net force keeping the plane from falling. Unfortunately to maintain this difference of pressure the wing has to move in the air (somewhat less solid than ground) at least with a certain speed ("stall" speed); below that airflow is not laminar anymore and the plane becomes a paperweight. Energy has to be provided to maintain the speed against friction; that's the purpose of the engine.

Wings are very efficient in their job: an albatros can fly for thousand miles without moving their wings (with no "engine") just using the tiny difference in windspeed before and after oceanic waves.

Using brute force to keep airborne, as hummingbirds do, needs MUCH more energy even if you use wings. It's the same difference between a suitcase with or without weels.

Answer 8

Wings have a much larger surface area than engines, so they are better at preventing a plane's gravitational potential energy from being converted into downward kinetic energy. A plane with gliders instead of wings and engines pointed downward at an angle should be just as energy efficient as a regular plane. The glider might be less practical because the engines would need to be more powerful in order to produce enough thrust to lift the glider by themselves.

Answer 9

Consider helicopters, which are simply aircraft whose wings go in a circle.

Then consider those flying platforms consisting of a fan pointed down.

The only real difference is whether the wings are big and slow versus small and fast.

Lift consists of the momentum (per second) of air directed downward. Momentum is $mv$. That air has kinetic energy proportional to $mv^2$. You can get the same lift for less energy by directing more air mass $m$ with less $v$.

If you want to see an aircraft intermediate between a fixed-wing and a helicopter, whose engines tilt, look at the Osprey.

Answer 10

How about this explanation (not that detailed, but simple):

The reason is that, if force of lift and force of gravity cancels each other out exactly, then staying airborne requires zero energy. Basically, energy and force are different things. A helium balloon can stay in the air indefinitely, without needing energy to do so, and without the air supplying any energy. The air pressure just supplies force, which is enough.

We know lift can be generated either using wings or using engines. Generating a constant upwards force using engines requires a constant expenditure of energy. But if you use wings, you can generate a strong upward force for a fraction of that cost, provided that you don't try to convert this upward force into energy (i.e. you can't climb for free with wings, you need engines (or an updraft), but you can maintain a constant altitude without engines, disregarding drag).

Answer 11

You can also consider how a glider can achieve much greater distances than a rocket, whilst expending no energy at all!

Ah, no. One expends energy in the launch phase - an aerotow or winch launch being typical - and gains energy in slope, wave, dynamic or thermal lift and then expends it in overcoming drag. Energy management is one of the most important factors in high performance gliding and if you're not good at it you won't do well in competition!

Simply 'not having an engine' does not equal 'expending no energy'. Not by a long way.

And I think the Apollo crews might argue that they covered rather greater distances than any glider flight is likely to manage.

Answer 12

There are planes that tilt the engine down to take off those are called VTOL (vertical Take Off and Landing). These are usually military fighter craft.

The reason why they aren't used for general aviation is multifold:

First every regulation aircraft must be able to touch down safely after losing an engine, if the engine is what provides the lift then the craft turns into a falling brick (a helicopter uses the rotational energy stored in the rotor to touch down safely).

Second you need a large thrust to weight ratio (>1 to even lift off) creating an engine that powerful isn't cheap especially when you want to transport cargo.

Answer 13

The lift of a wing is proportional to the square of the air velocity passing over it. If you have a wing it is thus very easy to get a lot of lift just by increasing your speed. In this way, rather than using engine force to lift the aircraft directly, you use the engine force to push you in the direction you want to go as fast as possible. This means you get there faster, because more of the engine's force is used to push you forward rather than push you up (which the wings are doing for you as a by product of moving forward rapidly).

Answer 14

The accepted answer explains how wings can help reduce the required energy for flying by pushing a lot of air slightly, in place of forcing a little air strongly.
The idea that the required thrust can even be made less than the weight of the body is intriguing, and this is not possible without wings. The following discussion attempts to explain it.

The formula $F_{drag} \propto v^2$ holds the answer to the question what wings can do but engines can't.

It tells us that in air, velocity of a body can be used to generate a force that, in turn, can be used to fight the weight of the body.

And since velocity, a component of momentum, is conserved, it should be theoretically possible to have a flying body with no external power needed!

So, we need a mechanism to generate resistance, and redirect it (or at least some of it) opposite to gravity.
Both of these things are done by wings. In very simplistic terms (not considering Bernoulli's principle and friction etc.), the picture below roughly shows what wings do.

So, I would rather ask the question - what does an engine do that wings can't!

Both of these cannot be done in any significant way by the engine - generating resistance and redirecting it. Yes, engine can generate thrust, and a lot of it, so much so that we won't even need air resistance to generate lift, and that's what helicopters do. But, if we take the help of air resistance, we may not need thrust at all, or at least we'll save a lot on thrust.

The wings do generate resistance, but unfortunately they can't redirect all of it vertically - some it is left out to act horizontally - as drag - opposite to velocity. This is where engines are needed. They just counter the drag that is less than the weight of the aircraft.

So, it's wings that do the flying given an initial velocity, and engines just help wings maintain that initial velocity. The wings are always falling in a horizontal direction onto the air in front of them, and due to the special angle of attack, in spite of the horizontal fall, some of the force of resistance is redirected vertically upward, and that does the trick.

Answer 15

While @Floris has answered it in almost totality, I wanted to add one more significant aspect of the wings.

On earth, when we try to generate movement in a body (or even to keep the body stable), in general, there are two forces we need to tackle with (ignoring friction etc.) - the inertia of the body and the weight of the body.
It is possible to find ways to outsource fighting-the-gravity part (i.e. balancing weight fully or partially) to something, so that we can direct our forces in overcoming the inertia as much as possible. Wings represent such an arrangement.

If we bring an aircraft to zero-speed mid-air and stop the engines, it will not have the same downward acceleration as it would have had if it didn't have wings.

So, wings fight gravity on their own (even if they don't get any help from the engine), by using their shape and angle of attack against some properties of the surrounding fluid (air). This is something that engine can't do.

So, a good part of the work done by engine can now be spared from fighting gravity. This is how a thrust-to-weight ratio of less than $1$ can lift the aircraft.

If air was liquid, absolutely no work would be needed by the engine for fighting gravity, and all its thrust could be used for forward movement.
On the other hand, if air had no fluid properties (like pressure etc.), all the fight against gravity would be required to come from engine (thrust $\ge$ weight). An aircraft in air is somewhere in between.

This is somewhat like inclined plane where the plane takes some fighting with gravity (i.e. balances part of the weight), and hence, additionally, less force than the weight of the body is needed for supporting the weight of the body.

Answer 16

This is a very interesting question, and I do not think aerodynamic lift is something for nothing. See my explanation below: -

1. Air consists of gas molecules moving at random speeds between 0 and roughly twice the speed of sound (0 - 2,500 km/H) in random directions.

2. Collisions and the rebounding of these molecules with everything around them is what causes pressure. But there are so many molecules impacting, it just seems like a steady force.

3. Any surface at molecular level is rough, so the rebound angle of air molecules is also pretty random.

4. Air pressure at sea level (due to these collisions) = 10,330 Kg/m2.

5. Wing area of Boeing 737 = 125m2.

6. Force due to air pressure on top of B737 wing = 125 x 10,330 Kg = 1,291,250 Kg or 1291 Tonnes.

7. Pressure force on the underside of the wing is the same, so they cancel out.

8. The weight of a B737 is around 55 Tonnes.

9. So, the trick with aerodynamic lift is to reduce the air molecular impacts on the top surface and/or increase the air molecular impacts on the bottom surface.

10. Imagine a wing angled upwards at 15 degrees and moving left to right through the air which has no wind (i.e. the average speed of all air molecules is zero). Because the bottom surface is travelling faster left to right than the average air speed and moving towards the air, it collects more collisions. The top surface is moving away from the average speed of the air and collects less collisions. (Think about a car driving in the rain. The front windshield gets very wet because it hits more raindrops, and the back windows gets much less wet.)

11. You don't need an airfoil shape to get this effect, but airfoils obviously optimise the effect.

12. Because the underside of the wing picks up more air molecule collisions than the top, it experiences a higher pressure - but only because it is picking up more collisions. This is not vectored engine thrust.

13. Since the pressure on the top and bottom of the B737 wing is around 1291 tonnes on each surface, it only requires a slight imbalance in the pressure to lift the aircraft i.e. 1318.5 underneath and 1263.5 on top.

14. So where does the lift energy come from? The engines move the aircraft along and allow the wing to create the pressure imbalance, but the force lifting the aircraft is from the impacts with the air molecules hitting the wing at random angles and at speeds between 0 and 2,500 kmH (+- the aircraft speed of course).

15. And we know the lift energy did not come from the engines because the B737 engines do not generate enough energy to directly lift the aircraft (i.e. if pointed straight down).

16. So, it looks like aircraft (and birds) actually extract energy from the random kinetic energy in the air molecules and turn it into lift. And they do this merely by moving the right shape of wing through the air, effectly putting the random molecule-impacts off-balance.

17. Since, energy cannot be created from nothing, the air should be slightly cooled behind the aircraft (very difficult to measure though) to account for the energy transferred to lift. My simple computer model suggests that this is actually what happens.

18. Of course some people will argue that this must be wrong because it breaks the 2nd law of thermodynamics - but I think aerodynamic lift probably does break that law!

Anyway, thats my theory!

So, the first paragraph of the question is basically asking, "Where does the extra lift energy come from?". My explanation is that it comes from the conversion of random kinetic energy in air molecules into a vectored thrust (lift) by the action of the wing moving through the air at an angle.

The wing is not 'more efficient' than the engine. It is a separate energy conversion machine being powered by the engine.

This is a controversial idea because the 2nd law of thermodynamics suggests that we cannot use the random kenetic energy in air except by using processes that transfer the energy from hot to cold bodies.

In the case of the B737 it gets 17 units of lift energy for every 1 unit of engine energy at cruising height (i.e. its lift to drag ratio is 17:1). Where else does this energy come from?

Notes: Speed of air molecules averages out at around the speed of sound which is 1250 km/h. There is no top speed for an air molecule but not many will exceed 2,500 if the average is to hold (https://en.wikipedia.org/wiki/Speed_of_sound).

Air pressure (https://en.wikipedia.org/wiki/Atmospheric_pressure) 14.696 psi = 10332.299613018 kgf/m2.