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Swept wings advantages

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Swept wings advantages

Old 28th Oct 2008, 10:31
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Swept wings advantages

Hello all,

Just ran a search on the advantages of swept wings and found only 1, the increase in Mmo, higher cruise speeds.

Are there any others as teh Ryanair assesment guys love the topic apparantly.

Many thanks
Nick
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Old 28th Oct 2008, 10:51
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Increased lateral stability (though too much and it ain't an advantage, you'll need to offset too much stability with anhederal)

Have a look at the CL curves and min drag info on one of those aero sites you find all over the web, they may reveal a little more.

Many disadvantages too - may well be part of your answer. Example high sweep equals hi AOA for same lift therefore high pitch attitudes (Concord!)
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Old 28th Oct 2008, 14:25
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better view for the first class pax

sorry, couldn't resist.
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Old 28th Oct 2008, 15:31
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They snaproll faster (Harvard, PT-22, ...)
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Old 28th Oct 2008, 18:15
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nick14,

I can think of one other 'advantage' : you can get rid of the horizontal tail!

A swept-back wing can be longitudinally stable on its own, without needing a horizontal tailplane as a straight wing does.
Think of examples like the Messerschmitt Me-163, or the Northrop flying wings, or the B-2, or the F7U Cutlass They don't rely on artificial stability.
Just try with a few paper aeroplanes, if you don't believe me. A rectangular piece of paper by itself flutters down, because it's unstable. Folded into the right shape, it'll glide.

You don't always want to delete the horizontal tail surface (see present-day jet airliners), most of all because you complicate the control in pitch.
For a start, you'll now have to control the aircraft in pitch with the ailerons, which become elevons, with a far smaller moment arm.
Secondly, IIRC, you have a far smaller CG margin, which is not a good idea on an airliner, unless you have an efficient fuel transfer system, as did Concorde.

But....
If the wing itself is already longitudinally stable it will mean you can to a large extent "offload" the horizontal tail in cruise, and only use it for control and trim.
This is the opposite of a straight wing, where longitudinal stability is obtained using a horizontal tail with a negative incidence relative to the wing, hence a down-force, resulting in increased drag.

CJ
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Old 29th Oct 2008, 13:48
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google HORTEN for more info on flying wings.
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Old 29th Oct 2008, 14:42
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Good one, Hoppy.

You can also directly try:
Horten Ho 229 - Wikipedia, the free encyclopedia

Clicking through from there to the other articles about "flying wings" and "tailless aircraft" will give you more info.

CJ
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Old 29th Oct 2008, 17:19
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Have a go at THIS you'll soon learn the practicle.
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Old 29th Oct 2008, 17:42
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TURIN,
I thought of that too....
Also one of the articles mentions that's one way of getting longitudinal stability:
hang the CG way below the center of lift!

CJ
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Old 29th Oct 2008, 17:55
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And the, of course, there are forward swept wings....
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Old 29th Oct 2008, 18:14
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ORAC,
Nice article! I bookmarked the site, may come in useful again.

It won't help nick14 much, since it quotes the advantages and disadvantages of forward vs aft swept wings, but not those of swept wings as such.

CJ
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Old 29th Oct 2008, 18:53
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Also one of the articles mentions that's one way of getting longitudinal stability:
hang the CG way below the center of lift!
That is quite correct, however the hang-point on a hang-glider is not fixed as such therefore the mass below the wing does not actually act in the same way. Good job really cos if it did my arms would drop off trying to hold it in a turn.
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Old 30th Oct 2008, 04:07
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And as mate of mine said in the technical part of his Qantas interview in the 80's, "so you can fit more in the hangar!"

He didn't get the job and I met him flying Twotters in PNG. Great bloke though.
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Old 30th Oct 2008, 05:41
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Wikipedia has this to say:

Swept wing - Wikipedia, the free encyclopedia

Well out of my area of expertise (geologist), so I'll let others judge the validity.
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Old 30th Oct 2008, 05:59
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It looks cool

Knox.
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Old 30th Oct 2008, 11:02
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Many thanks for all your replies, I do love this forum.

Ok so far we have: High speed
Stability (or disadvantage)
Dispensing with the tail

In terms of modern Pax/cargo jets, If asked, "what are the advantages of wing sweep besides the higher Mcrit?" I would still be stuck for an answer.

I always thought that wing sweep created the Dutch Roll problem (obvious disadvantage) And I cannot see boeing/airbus chopping off the tail any time soon.

Thanks again guys and girls
N
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Old 30th Oct 2008, 16:44
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Originally Posted by nick14
In terms of modern Pax/cargo jets, if asked, "what are the advantages of wing sweep besides the higher Mcrit?" I would still be stuck for an answer.
So would we all, it seems....
What's more, pretty well everything that does not need to get up to Mcrit has a straight wing.

I always thought that wing sweep created the Dutch Roll problem (obvious disadvantage)
Yes and no. Swept wing aircraft are prone to Dutch roll by nature, but straight wing aircraft can also exhibit it.
Check "Dutch roll" in Wikipedia : better write-up than I could do. (BTW, I don't know who does the aeronautical write-ups in Wikipedia, but generally they are quite good.)

Oh, and I see they only quote the "polite" explanation for the origin of the term (i.e., it looks the motion of a Dutch skater). The more likely explanation is that it's like a drunken Dutch sailor making his way from the pub back to his ship....
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Old 30th Oct 2008, 21:27
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Low drag at high speed = sweepback. - then go on about less fuel burn and getting the pax in on time.
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Old 30th Oct 2008, 22:30
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Starting at the very beginning, take a straight wing and fly it through the air, the airflow will pass over and under it at 90 degrees and generates lift for the time that the airflow is in contact with the wing chord. Now sweep that same wing back, (same chord), and the airflow passes over the wing in a diagonal direction and remains in contact with the wing longer = more lift for same chord which can then result in all of the above answers you already have! Draw it out on a piece of paper. That is a very, very simple swept wing 101 and interviewers may expect more!

See Handling The Big jets, third edition, page 90, fig.5.5, also, same book, page 84, 'Sweep'.
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Old 1st Nov 2008, 20:35
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Question Critical mach increased: Dont sweat the small stuff

In my own words, same thing and,......

Also from handling the big jets,

" the velocity vector normal to the leading edge
is made less than the chordwise resultant "

What's important ? In my humble opinion.

" the airpeed can can be increased before the effective chordwise component
becomes sonic.

Thus the CRITICAL MACH NUMBER IS RAISED.

Curious Nick14 why look for other things ?

when the interviewer may be asking for your knowledge on things such as local speed, compressiblity, transonic speeds, wing thickness chord ratio, shock waves, longitiudinal stablity.

I wouldnt go too deep or you could get yourself into a hole. you are not expected to be an expert on aerodynamics

Maybe I am wrong. What the hell do I know.

What about the disadvantages

For me a swept wing is worse with a heavy crosswind. I remember that flying the 727.

Which airliner had the highest sweepback and has now ?

Why is a swept wing thin etc ?
Duch roll ?

Last edited by Mach trim; 1st Nov 2008 at 21:01.
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