Back to basics...

I have jsut recently discovered how big the whole area of "aircraft projcets" is. When I have read some of your posts I have discovered that I don't know anything about the planes. Or how they work. I have really no idea what "lift" is. Or how an engine of an F/A-18 works... it really sucks that we aren't studying this at school...

How does a plane lift

When a plane flies, it needs and engine (daah and wings. The engine gives the power, and the wings make the plane fly.

Could somebody explain what happends with the air and the wings, so that the plane actaully lifts...??

Adn what is acually lift to-drag-ratio etc etc...

And what are these wingstips good for...??





The engine

I'd like to know how a jet engine actually works. How the thrust is created. And also how thrust is calculated.



What is the tail actually good for...??




What is counted in this picture...??



THIS IS NOT A SCHOOL ASSGNMENT, I'am just interested...

[edit on 30-1-2006 by Figher Master FIN]

Its funny, some of the material you included in your post answer your questions.

As far as how a wing works, it’s really simple. It’s called the “Bernoulli” effect or principle.

If you look at your diagram, the top of a wing is curved, and the bottom is straight. What happens is the air above the wing has to travel further than the air under the wing. And it needs to do it in the same amount of time so that air must travel faster. And the faster moving air above the wing creates less pressure than the air under the wing creating lift.

The faster the wing moves, the greater the lift.

Factor in different wing shapes and varying speeds and you can understand why different types of planes have different looking and shape wings.

Look at an A10 for example. It’s a slow moving plane that needs to carry a lot of weight. Now look at the wings. The wings are long and straight with a very pronounced curve on the top of the wing.


external image

It needs that curve to generate more low pressure above the wing at slower speeds.

You get the idea.

Mod Edit: Image Size – Please Review This Link.


[edit on 30/1/2006 by Mirthful Me]

Uhm, ok, I'll try to keep this equation free and "wordy" - and short


Lift:



Ignore that stuff about distances travelled and speeds over the wing surface. Sorry skippy, but its well, rubbish

This is how subsonic lift is made, I'll not deal with supersonic/hypersonic here to keep it simple.

It is the bound vortex that results in the speed differentials between upper and lower surface, simply put, the vortex is working with the aircraft speed above the wing, and against it below the wing.

Dynamic pressure is negative and dependant on speed squared.

Total pressure is the sum of dynamic and static pressure, static pressure comes from the ambient conditions (altitude, temperature etc). Therefore a "higher" dynamic pressure results in a lower total pressure, and thus a helpful differential between top and bottom - giving lift.

[edit on 30-1-2006 by kilcoo316]

That bottom picture is a bit technical for me but it seems to be showing how the tailpane is used to achieve level forward flight. The cg is show to be well forward of the centre of lift, known as a 'forward cg'. This should induce a nose down attitude, technically known as a 'dive', however the amount of downforce produced by the tailplane (called 'lift' in the picture despite the arrow pointing downwards, duh!) is sufficient to counteract this and push the tail back down equalling a differential of 0 and thus level flight, the aircraft attitude can be altered by increasing and decreasing the amount of downforce produced with tailerons. A simple illustration of this is to balance a model plane on your fingertips behind its cg, naturally it will tip over nose first, but if you hold your fingertips of your other hand over the tailplanes you can prevent this and hold it steady, same principle.

A wing makes an aeroplane fly basically because the air travelling over the curved top surface of the wing has farther to go than the air going under the flat underside, of course the air still has to get around the wing at the same time so the upper surface flow has to be faster , as air travels faster pressure is naturally reduced so the slower moving higher pressure air under the wing lifts the plane up.

Lift-Drag ratio.

Uhm, pretty basic - if the wing generates 100 Newtons of lift, and 10 Newtons of drag, its L/D = 10.

A commerical aircraft will have a L/D of around 10-15.


edit: An aerofoil will always produce more L/D than a wing - due to 3D effects, so if you get better results in something like xfoil, don't be surprised!

[edit on 30-1-2006 by kilcoo316]

The differential in pressure over a moving surface as described by me and skippy also applies to a car in a lake, even if the car is filled with water you cannot open the doors while it is sinking as the pressure only equalises once the car settles on the bottom, A s demonstrated in Top Gear on the BBC, moral = if you drive into a lake DO NOT wait for the car to fill up with water so you can open the doors, it doesn't work. Get straight out.

Winglets.


From my above post, you can see the wingtip vortices (also referred to as trailing or tip vortices). Note the direction of circulation from top to bottom - reducing the lift production of the wing near the wingtip will reduce (but not eliminate) this circulation.

The winglet presents a blockage to the vortex, and thus immediately reduces its effect, even better, the winglet can be angled and curved to produce a thrust from the wingtip vortices. By reducing the wingtip vortex, the total pressure on the upper surface of the wing near the wingtip is kept down, so the lift of the wing increases, as well as a drag reduction - a pretty neat huh?

Turbine engine:

Operates on exactly the same principle as a reciprocating engine (as in, the one in your car).


Suck, squeeze, bang, blow.

For where the thrust comes from, its mainly from the compressor (or fan in a turbofan design), some more thrust comes from the combustion chamber, the rest is drag (including the nozzle [unless its underexpanded - but we'll not go there]- hard to believe).

A turbine sucks its way along, not pushing - a common misconception.

Tailplane & Balance:

Well, your diagrams and Waynos' explanation covers it all pretty much.

Again, to keep it simple, I don't think its necessary to start thinking of reduced static margin aircraft (to keep it simple) - unless you want explanations.

Originally posted by kilcoo316

Ignore that stuff about distances travelled and speeds over the wing surface. Sorry skippy, but its well, rubbish

Its funny, its not wrong by any definition, but in your opinion its rubbish. LOL

Originally posted by skippytjc
Its funny, its not wrong by any definition, but in your opinion its rubbish. LOL

Believe me, it is wrong (I do this for a living BTW).

Originally posted by kilcoo316


A turbine sucks its way along, not pushing - a common misconception.

Thats not really true though is it, otherwise it would make no difference where the nozzle was as long as the intake was unimpeded. In the early jets it was a primary requirement to ensure that thrust wasn't lost in a long jetpipe, hence the short wing mounted nacelles of the Me 262 and Meteor and the short cutaway fuselages of the Vampire and J-29 and the top mounted podded engine of the He 162, all required to maximise the thrust of early engines. If an engine only sucked its way along this would never have been a problem.

The thrust of a jet engine comes from the continuous directed explosive force of compressed air mixed with fuel being ignited and thrown out of the back. That isn't a misconception. The force of the onrush of air at high speeds is enough to destroy a front fan, that is why intakes are fitted with baffles to slow the air down before it hits the engine, this fact alone means the notion of the plane sucking its way along cannot be right.

Originally posted by kilcoo316

Originally posted by skippytjc
Its funny, its not wrong by any definition, but in your opinion its rubbish. LOL

Believe me, it is wrong (I do this for a living BTW).

Hey, im proud too that you can finally use the word "newtons" in a practical way since your learned it in school, but for all intents and purposes my description of lift is just fine.

Originally posted by waynos
Thats not really true though is it, otherwise it would make no difference where the nozzle was as long as the intake was unimpeded. In the early jets it was a primary requirement to ensure that thrust wasn't lost in a long jetpipe, hence the short wing mounted nacelles of the Me 262 and Meteor and the short cutaway fuselages of the Vampire and J-29 and the top mounted podded engine of the He 162, all required to maximise the thrust of early engines. If an engine only sucked its way along this would never have been a problem.

The thrust of a jet engine comes from the continuous directed explosive force of compressed air mixed with fuel being ignited and thrown out of the back. That isn't a misconception. The force of the onrush of air at high speeds is enough to destroy a front fan, that is why intakes are fitted with baffles to slow the air down before it hits the engine, this fact alone means the notion of the plane sucking its way along cannot be right.

I'm right now looking at a breakdown of thrust for a turbine engine from the book The Jet Engine by Rolls-Royce. I'm kinda wrong in what I said earlier.

Thrust figures are:

Compressor: 19,0479lb
Diffuser: 2,186 lb
Combustion chamber: 34,182 lb
Exhaust/Jet pipe: 2,419 lb

Drag figures are:

Turbine: 41, 091 lb
Propelling nozzle 2,419 lb



So the main thrust is from the combustion, and the nozzle, tailpipe has a net drag. The component making the difference between a net thrust or net drag is the compressor - where my earlier confusion arose from.

Originally posted by skippytjc
Hey, im proud too that you can finally use the word "newtons" in a practical way since your learned it in school, but for all intents and purposes my description of lift is just fine.

I don't want to start an argument over it as I was taught the same error at A-level.


But consider, if the distance method is correct, why are all aerofoils not designed with a straight lower surface - surely that would induce the maximum velocity differential. Most aerofoils are curved on both upper and lower surfaces like this:

Look, my description is just fine.


Do you disagree that it’s the Bernoulli principle that creates the lift?

Do you disagree that the shape of the wing affects the lift?

Do you disagree that speed is a factor?

After all, that’s my description of lift and you called that rubbish.

Please.

The only difference between your explanation of how a wing works and mine is that you reference the turbulence involved.

But the bottom line is that it’s the faster moving air above the wing that creates lower pressures that creates the lift. And you simply cannot refute that.

If you are so confident that my explanation is rubbish, then you need to prove it as rubbish. A simple “that’s rubbish, I do this for a living” isn’t good enough. You opened this can of worms, now prove your point.

If you look at your diagram, the top of a wing is curved, and the bottom is straight. What happens is the air above the wing has to travel further than the air under the wing. And it needs to do it in the same amount of time so that air must travel faster.

Is wrong wrong wrong, simple as.


I'm not going to start regurgatating my undergrad notes with pages of equations to prove this. Go find an aerodynamics text book and see if they talk about travelling the two distances (upper and lower surfaces) in the same time.


Oh, and re-read my post on it - I kept it nice and simple, no mention of Bernoulli (uhm, which incidentally, is usually considered invalid (meaning inaccurate) above around M=0.3 due to compressibility effects), no mention of camberlines, nor equations relating to velocity. Also, see what I said was wrong and what I didn't say was wrong.


edit: Oh, and I had already my post on the aerodynamics thing written before seeing your post, I edited the rubbish part slightly as in its previous form could have been seen as having an arrogant dig at you - I obviously didnt do it well enough.

[edit on 30-1-2006 by kilcoo316]

Thanks... I really appriciate it... but about that engine... Didn't really get it...

I've studied aeronautical engineering for a while; kilcoo316 is right. The Bernoulli effect does not create the lift.

It's the bound vertex that creates the low pressure. When an aircraft stalls, that vortex seperates from the wing, and the aircraft falls down like a brick. Obviously the shape of the wing affects the lift, I don't think kilcoo316 disagreed on that.

As kilcoo said, it's best you find an aerodynamics book and look it up, instead of fighting over it in an arguement.

[edit on 1-30-2006 by Zion Mainframe]

Originally posted by Figher Master FIN
Thanks...

I really appriciate it... but about that engine... Didn't really get it...

Why don't you find it out yourself:

www.grc.nasa.gov...
(You can download a small java applet and play a bit with the shape of the engine to see what effect it has on the thrust.)

How a gas turbine engine works is explaned on this page: www.grc.nasa.gov...