Originally posted by Figher Master FIN
Zap since I'am 15 years old I can only count 1+1.

This brings up an other question, So If I udnerstand you zap you say that wings that are mounted up high make the plane more manouverable,

I don't want to be ignorant to Zaphod, but he's simplified it too much, and its kinda incorrect.

Did you try that link I posted - its got some nice graphics that might help you.

fin, whatever the rest of us have tried to tell you, I say listen to kilcoo. He really knows what we 'think' we know when it comes to aerodynamics

Nice pictures too Fin but you are kind of incorrect when you say there are no fighters with top mounted wings;

F-35
F-22
F-18
F-15
F-14
Tornado
Jaguar
Orao
MiG 23,25 &27
Mitsubishi F.1
Mirage F.1
Harrier

and I reckon loads more than that.

Originally posted by kilcoo316

Originally posted by Figher Master FIN
Zap since I'am 15 years old I can only count 1+1.

This brings up an other question, So If I udnerstand you zap you say that wings that are mounted up high make the plane more manouverable,

I don't want to be ignorant to Zaphod, but he's simplified it too much, and its kinda incorrect.

Did you try that link I posted - its got some nice graphics that might help you.

Yeah, I know I oversimplified it, but I had about 10 minutes before I was leaving for work so it was a little rushed.

This is in my undergrad aero notes, which are in the lab, but off the top of my head:

The complicated explanation [skip to simpler one at bottom if yez want]


The desire for swept wings was led by the possibility of optimising the wing sweep through the flight profile.

There is an equation for aerodynamic efficiency, well, induced drag IIRC, I think the factors in it include the wing volume, flight mach number, aspect ratio and lift coefficient. It reads something like [I'm very rusty here, I think there are other variables]


Cdi = CL*(1+X)


X is just a factor dependant on wing volume, aspect ratio, Mach number etc - using variable geometry wings allows you to keep X at 0, meaning you pay theoretically a 0 penalty for supersonic drag rise.

* I'll check my notes next week and correct this if anyone is that keen.*



Simpler explanation:


The air speed at 90 degrees to the wing leading edge is what creates the lift and drag of the wing, ideally you want to run the aircraft as much in its design zone as possible [for sake of argument lets say thats 150 -> 400 mph at sea-level].

So, if we have a wing swept at 60 degrees, your optimal aircraft speed is actually 300 -> 800 mph.


Obviously 300 mph is too quick for a good take-off speed, but by sweeping the wing forward to 0 degree sweep, aircraft optimal speed is back to 150mph up to 400, which is much better.


As you increase up the speed range, and transonic drag rises start to move the aerofoil/wing out of its best operating zone, you sweep the wings back to lower the "wing speed", taking it back into optimal zone.

[edit on 11-7-2006 by kilcoo316]

I can't top what kilcoo has told you there but in answer to the other part, there is more wing area in the unswept position because as the wing sweeps futher back the inboard trailing edge goes inside the fuselage.

To illustrate what Kilco was saying, there was a B-1B doing a low level training flight with the wings swept all the way back, when they had a hydraulic failure. IIRC a hydraulic pump blew apart or something like that. They tried to swing the wings forward, but they stopped at 55 degrees. When they went below about 220 mph they started to stall, and have flight control problems. They wound up flying out to Edwards AFB and landing on the dry lake bed out there. They touched down at 230 mph, used all the "paved" runway, and went off into the lakebed before they were able to stop.

Essentially fin, by sweeping the wings back, you have less area of the plane to push through the air, and it can go faster. That might be oversimplified again, but it's essentially what's happening.

Looks like I haven't learned enough in flight school. :/

Well, that's why I'm going back, anyways, from my understanding, those are the dihedrals and anhedrals. That's all I know.

Shattered OUT...

Basically, the simplest explanation is that swept wings have low drag (high speed) but also generate less lift. Extended wings have high drag (can not go supersonic) but generate more lift. The reason you need the variable geometry is that you can take off from short runways, go very fast and then have low approach speed and land on the same short runways (or carriers).
Of course nowadays you can achieve that without the variable geometry. Vortex generators (or lately canards) are used to create more lift while still having a wing with a high angle sweep. F-16, F-18, Su-27 & MiG-29 use their knive like leading edge of the wing blending into the fuselage as vortex generators. Newer designs use canards.
You have seen the jets of air going over the inside part of the wing of F-16's when they pull up sharply. That's the vortexes which give it lift and help it maneuvre at high angles of attack.

Originally posted by Zaphod58
Essentially fin, by sweeping the wings back, you have less area of the plane to push through the air, and it can go faster. That might be oversimplified again, but it's essentially what's happening.

Well that was what I was wondering about, what you are sahing is totally logical, but how come the plane (wings) have less are when they are swept back?

Maybe a part of the wings is forced to go in the plane and that way the area becomes smaller?

In the case of swept wings and reduced drag it's all in the geometry and you just hold the aerodynamics constant. You're "tricking" the system with geometry.

Let's take a straight wing with NACA airfoil XXXX...chord length Y and hold those two factors constant for either a straight wing or a swept wing.

The parasite drag that a wing generates is based on three things:

1. Cd of the airfoil cross-section,
2. Chord length of the wing (area),
3. Windspeed SQUARED

So, since we're dealing with the same NACA airfoil, our Cd is constant for either the straight wing or the swept-wing. We have two ways now to decrease drag:

1. Shorten chord length
2. Decrease airspeed

But if we, of course, have limits on what we can do with the chord length since, when all is said and done, we need to still have a wing of sufficient lifting area and we've got to hold our root chord and aerodynamic center to some point to ensure some measure of stability. Not only that, but the drag is dependent on the airspeed squared so we've got more bang for the buck in the airspeed than in the area.

Let's decrease the "apparent" airspeed the wing encounters. How do we do that?

we sweep the wing to an angle to the wind to where only a portion of the forward airspeed results in windspeed perpendicular to the airfoil. (We create a vector). The tangential part of the winspeed (the part that flows along the leading edge of the wing) produces no parasite drag across the airfoil...only the part of the wind vector that flows perpendicular to the leading edge (i.e. travels down our NACA airfoil) creates the parasite drag.

So, we can travel at the same airspeed and generate less parasite drag by causing only a portion of the airspeed to travel down our airfoil. And this is all geometry. You can pretty much figure the perpendicular component on your own just with sin and cos of the sweep angle.

But this also decreases lift (because it too is dependent on the same variables as the parasite drag)...but there's one thing that affects lift that doesn't affect parasite drag...

AOA

so if we increase the angle of attack on a swept wing, we can regain our lift while still reducing our parasite drag.

The above applies for subsonic...when you go supersonic there's a bit more to discuss.

[edit on 7-13-2006 by Valhall]

Originally posted by Figher Master FIN

Maybe a part of the wings is forced to go in the plane and that way the area becomes smaller?

Didn't I already say this?

But that is because less area is needed at highter speeds because, basically, more lift is generated per sq ft of wingh area the faster you go.

In terms of reduced resistance if you picture the frontal view of a VG aircraft, with the wings unswept it might be 50ft across but fully swept it is only 30ft, reduced span and higher sweep equals less resistance and higher speeds.

Originally posted by waynos

Didn't I already say this?

But that is because less area is needed at highter speeds because, basically, more lift is generated per sq ft of wingh area the faster you go.

In terms of reduced resistance if you picture the frontal view of a VG aircraft, with the wings unswept it might be 50ft across but fully swept it is only 30ft, reduced span and higher sweep equals less resistance and higher speeds.

Yes you did. For me it sounded so strange that so little could make a so big difference. If you know what I mean. (because I don't believe that very huge part of the wings is hidden under the fuselage only a small part).

Hmm, valhall, I have to get back to you on this matter after I've read thorugh the text a couple of hundreds of times.

It doesn't have to be a "huge" part of the wings. ANY part of the wings pulled into the fuselage will cause a reduction in wing area.

Like zap said, it doesn't have to be a huge amount. Here is a diagram where I have highlighted the fully swept wing on the left hand side. I have copied this outline onto the unswept wing on the right hand side and to show how much of the wing is inside the fuselage when fully swept, this is coloured in green. As you can see it is a sizeable proportion of the total lifting area.

You are totally right Waynos.

I didn't think that the area would reduce so much with just sweeping them.

And Waynos what plane is that, I believe that it is an American jet, because I do remember building a model of it when I was younger, and I haven't been able to find the drawings so I have no idea what plane it is.


EDIT:

I appriciate your drwaings valhall, and promise you I will use them some day when I'am more educated and know more about math etc. But fore the time beeing I'am to stupid. And I prefer an easier explanation, due keep up answearing the harder way because it might be useful in the future when I'am more interested in how it really works.

[edit on 13-7-2006 by Figher Master FIN]