How does an aircraft's wing work?

[RogerS]

My theoretical understanding is, apparently, wrong!

What do you think?

 

[mailee]

:?: :?: :?: it would help to know what your theory is first? Air travels faster over the upper surface of the wing creating a lower pressure so lifting the wing, as far as I know :?

 

[Eric The Viking]

It's all down to the bolts inside, which you can't see, that fix the wing securely to the fuselage, thus preventing it falling off.

The other wing is usually exactly the same, only a mirror image of the first one.

Simples really

E.

 

[Tom K]

Yeah just blurt it out Rog did you think air passing under the wing held the plane up?

 

[duncanh]

:?: :?: :?: it would help to know what your theory is first? Air travels faster over the upper surface of the wing creating a lower pressure so lifting the wing, as far as I know :?

Agreed.

My theoretical understanding is, apparently, wrong!
What do you think?

I assume that your understanding was that this difference in air speed was due to: for adjacent air particles at the front of the wing, the one that goes over the top of the wing has to travel faster so that it can meet up again at the back. This causes less air pressure above the wing. This is the explanation is often given but it doesn't make sense as there's no reason for the 2 air particles to need to be together at the back of the wing - they're not connected in any way.

 

[James C]

The particles aren't connected in any form it just an easier way of imagining the actual situation. The plane moves through the air at a constant speed.

 

[Lons]

Thanks for that question just a week before our flights :roll: #-o

Anyway, loads of info on the web - heres an interesting argument on the theories;
http://iopscience.iop.org/0031-9120/38/ ... _6_001.pdf

Bob

 

[RogerS]

Theory 1

Think about a light aircraft like a Piper Cub. Flies at, say, 125 knots. Seats 4. Think about the wing area. Visualise it.

Now think about a Jumbo. Flies at, say, 500 knots. So that's 4x the speed of the Piper. So that's 4 x 4 seats. = 16 seats. But a Jumbo can hold 480 passengers. So thats 30x the wing area needed compared to that of the Piper. Visualise it. Doesn't fit, does it?

So Theory 1 says that it is the belief of the passengers that it can fly. That's what keeps it up in the air.

Theory 2

The commonly held theory. Top of the wing is curved so the air has to go faster thus creating an area of low pressure above the wing which creates lift. If that is the case then why is it that aircraft with wings of a symmetrical cross-sectional area fly? Because they do.

Theory 3

It's all to do with the speed of the aircraft and the angle of the wing from the horizontal. It is the downward thrust of the air flowing beneath the wing that, following Newton's Law 3rd law, there is an equal thrust upwards and that is what keeps the plane in the air.


Personally I believe Theory 1.

 

[Eric The Viking]

Seriously, the "low pressure on top" thing has been discussed as a bit of a canard (ouch!) for a while now.

Aircraft wings haven't had a 'bird's wing' cross section since the early days of flight* and the smart money seems to be on a different mechanism entirely, as most planes are fairly happy flying upside down, even if crew and passengers probably aren't.

I've seen discussion of the air current from forward motion being reflected into downthrust, but that still doesn't explain inverted flight well. It makes most sense that wings work because of a number of different aerodynamic effects, and different ones dominate at different speeds, attitudes and air pressures. That's most certainly true of aircraft capable of speeds approaching and exceeding the sound barrier. Like many things, it looks simple but is in reality pretty complex.

You can't really use sailing boats as a complete analogy either, as an aircraft has no equivalent to a keel or centreboard/plate. In that instance though laminar flow is very important. Boats with foresails (jibs) perform better going towards the wind, i.e. getting 'lift,' than those with a single sail. The jib encourages laminar flow over the leeward side of the leading edge of the mainsail, considered to be more important than the extra 'lift' it creates on its own. It's also true that boats with streamlined masts, where the mast is aerodynamically shaped and faired into the leading edge of the mainsail, perform better to windward than boats with the same sail area but a simple, circular mast.

But I digress (typically!). I too am watching this thread with interest.

E.

*OK the bottom part of the wing was sometimes open in the early days (still is in microlights, many kites and hang gliders), but that wouldn't affect the bird's wing theory, apart from adding turbulence to the discussion. That said, I've got a couple of Flexifoil kites, which are self-inflating bird's wing designs, and the lift compared to traditional kites is phenomenal. In light winds, with the right spars installed, they're easily capable of flying with the lines beyond vertical, and two of them stacked (six-footers) can lift me off my feet.

 

[Eric The Viking]

So Theory 1 says that it is the belief of the passengers that it can fly. That's what keeps it up in the air...
... Personally I believe Theory 1.

It's a good theory. After all, a very similar mechanism is keeping the British economy going (and more so the Euro), even as I type. ;-)

 

[duncanh]

Loads of stuff on the Nasa website including some interesting applets which simulate different wing profiles in air flows, including a rotating cylindrical wing and a ball.

 

[Tom K]

The truth is all aircraft have to have their wings left slightly loose. Combined with a slightly bumpy runway the oscillation induced allows your plane to mimic the humble sparrow thus taking us airborne. All other theories are hokum. (homer)

 

[nev]

I'll go with E's Pratchetesque theory, and extend the belief factor to multiplied by a % proportionately to the fewer people on board, or in the case of an unmanned craft, the person who's footing the bill

 

[Jonzjob]

All you have to do is to join the Royal Air Force and they will teach you the complete theory of flight like wot they did me 8)

If you think of a glider, that's the easiest one cause it didn't got an engine unless it's a powered glider then it has, then it's never going up as far as the air around it is concerned, it's always going down, and the only time it will go up, when not being towed or winched, is when the surrounding air is going up faster than it is going down.

With me so far?

If you have a single engined aircraft,say like the Piper Cub Roger mentioned, the only reason it's got a fan on the front is to keep the driver cool. This is easy to see because if it stops when it's flying you can see the driver start to sweat profusely until he can start it again, then he cools down again.

If you are going to go flying with someone you don't know, like a big airline, then the best seat you can choose is the one directly above the 'black box', because it's designed to withstand all sorts of rough handling, vertical landings, etc., and if that's the only thing they want to retrieve then it's probably a good idea to be strapped to it :mrgreen: By the way, it's BRIGHT orange.. I used to have to service those in their early daze.

The theory that the air going over the top of the wing compaired tends to go out of the window when you havea symmetrical wing section come ça, third one down

Then, unless you point the front up the air has the same distance to go on either side. They are used on aircraft that fly upside down just as well as upside up and don't normally carry travelling passengers. They normally rely on brute force to get them into the air and can be quite fast.

The 747 and A380 don't have symmectrical section wings because if they did fly upside down 2 things would happen. 1. the drivers coffee would leak out of its cup and 2. all of the passengers would complain bitterly when they hit the roof, literally!

While we are talking flying. Does anyone know why the toiley windows are opaque? Who the hell's going to look in at 35,000 feet up? And just like trains, you aren't supposed to use them when you are on the ground..

 

[Steve Maskery]

John
But does you symmetrical wind fly when it is level like that? Surely it only flies when it is tilted up at the front? Or at least, tilted down at the back (ailerons, are they called?). In which case the pressure differential still holds.
S

 

[RogerS]

John
But does you symmetrical wind fly when it is level like that? Surely it only flies when it is tilted up at the front? Or at least, tilted down at the back (ailerons, are they called?). In which case the pressure differential still holds.
S

They require an angle of attack. It still isn't anything to do with pressure differential between top and bottom though.

The droopy things you are referring to are called 'flaps'. Flaps go down together (usually). When they are extended they let the aircraft slow down but still have enough lift to stay up in the air. That way the aircraft can land at a much slower speed.

When they aren't able to extend the flaps (as sometimes happens) then you land really really fast. And then they keep you waiting at the end of the runway with lots of red lorries with flashing lights to see if your wheels are going to catch fire because the brakes got hot as a result of trying to brake a several hundred ton monster from hurtling off the end of the runway. And then when they finally let you taxi to the airport terminal, you find waiting for you another shiny red lorry with flashing lights....just in case the firemen in the other red lorries were asleep and failed to see the flames. DAMHIKT.

The waggly bits at the extreme end of the wings are the ailerons and when one goes up..the other goes down..

 

[Jonzjob]

It relies on a slight angle of attack and, normally, high speed. Ailerons are the movable controls at the ends of the wings normally and one moves up whilst the other moves down and they are used to start a turn. There are variations to that though. The ones on the trailing edge that normally only go down are flaps and they are used to produce more lift at slow speeds like take off or when fully lowered both lift and drag to help the aircraft in landing. In some cases both ailerons can be moved up and the flaps lowered, called 'crow breaking', and it's a very efficient way of slowing down without loosing control and stalling.

Then you can have things like flaperons, flap/aileron mix, elevons, elevator/aileron mix used on flying wings and ruddervators, rudder/elevator mix used on 'V' tails. Good 'ere 'init :mrgreen:

The theory of flight is a fascinating subject and I really enjoyed it all those 52 years ago!! :shock: It still gets used because of my interest of RC gliders.

Edit : - Didn't see your reply Roger.. You're learning though ain't yer :mrgreen:

 

[RogerS]

......
Edit : - Didn't see your reply Roger.. You're learning though ain't yer :mrgreen:

Cheeky bugger! I used to fly gliders.

 

[Jonzjob]

That's wot I said :mrgreen: Yer lernin in't yer.... =D> =D>

Since when did glider guiders ever know about the theory :twisted: :twisted:

This is one of mine

On this, the ailerons are so programmed so that on landing both of them can be moved upwards so as to spoil the lift. They still work as ailerons and if you want/need to turn left for instance the Starboard aileron goes down and the port wing drops because the Starboard wing now has a bit more lift than the Port jobbie.. There aren't any flaps or spoilers on it and before I programmed it to do this it was a very fast landing at times!! :shock: Wingspan 2.2 meters. Weight, just over 2 lbs..

 

[Dibs-h]

Wow - missed this! LOL! Reminds me of the past, when someone I know, asked the same question ages ago. In the end almost told him to fcuk off as he was doing my head in. Not that this thread is going that way. LOL!

I always understood it to be - as a few have pointed out - due to the differential pressure between the top surface and the bottom surface generates the lift (pressure being force over area) and lift has to equal or exceed the gravitational effect.

Angle of attack, wing profiles - man that takes me back to my R\C days!

Dibs