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nick14
28th Oct 2008, 11:31
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

Flambards
28th Oct 2008, 11:51
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!)

ACMS
28th Oct 2008, 15:25
better view for the first class pax :)

sorry, couldn't resist.:O

barit1
28th Oct 2008, 16:31
They snaproll faster (Harvard, PT-22, ...) :}

ChristiaanJ
28th Oct 2008, 19:15
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

FE Hoppy
29th Oct 2008, 14:48
google HORTEN for more info on flying wings.

ChristiaanJ
29th Oct 2008, 15:42
Good one, Hoppy.

You can also directly try:
Horten Ho 229 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Horten_Ho_229)

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

CJ

TURIN
29th Oct 2008, 18:19
Have a go at THIS :uhoh: (http://uk.youtube.com/watch?v=8dVQKtgsMng) you'll soon learn the practicle.

ChristiaanJ
29th Oct 2008, 18:42
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

ORAC
29th Oct 2008, 18:55
And the, of course, there are forward swept wings.... (http://www.desktopaero.com/appliedaero/potential3d/fsw.html) :}:}

ChristiaanJ
29th Oct 2008, 19:14
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

TURIN
29th Oct 2008, 19:53
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. ;)

18-Wheeler
30th Oct 2008, 05:07
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 hanger!"

He didn't get the job and I met him flying Twotters in PNG. Great bloke though. :)

Yamagata ken
30th Oct 2008, 06:41
Wikipedia has this to say:

Swept wing - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Swept_wing)

Well out of my area of expertise (geologist), so I'll let others judge the validity.

knox
30th Oct 2008, 06:59
It looks cool :cool:

Knox.

nick14
30th Oct 2008, 12:02
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

ChristiaanJ
30th Oct 2008, 17:44
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.... :ok:

groundfloor
30th Oct 2008, 22:27
Low drag at high speed = sweepback.:p - then go on about less fuel burn and getting the pax in on time.

parabellum
30th Oct 2008, 23:30
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'.

Mach trim
1st Nov 2008, 21:35
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 ?

Brian Abraham
2nd Nov 2008, 03:59
the airflow passes over the wing in a diagonal direction and remains in contact with the wing longer = more lift for same chord
While the effect is to increase the Renyolds Number (and lift) you have at the same time a reduction in camber which has the effect of reducing lift (for a given AoA). The end result is anybodys guess I would say unless referring to a particular case tested in a tunnel.

parabellum
2nd Nov 2008, 11:31
Hi Brian, I take your point, unless the shape of the wing is modified to allow for the loss of camber?
I was just trying to give a simple and backed up, (Davis), reference that might be suitable for an interview.

I went to an airshow at Thorny Island way, way back and when a Swift flew over I asked, "Why the swept back wing?" and the answer from an RAF chap nearby was, "So that it can go faster".

Keith.Williams.
2nd Nov 2008, 13:24
Advantages include.

a. Increased Mcrit (due to reduced effective thickness to chord ratio).
b. Increased Lateral Stability.
c. Increased Directional Stability. But b and c do not necessarily result in
an overall improvement because of the greater tendency to Dutch Roll.
d. Reduced tendency to structural divergence (one way flutter), so the
structure can be lighter.
e. Increased stalling angle (but this is offset by reduced Cl at any given
angle of attack so the downside is an overall increase in Vs.
f. Less profile drag (but at the expense of greater induced drag) due to
reduced aspect ratio)

FlyingLapinou
26th Oct 2011, 21:02
Hi guys and gals,

Way too late to help the OP, but as I'm having to read up on swept wings myself, the following might be useful to someone, somewhen?

One of the multiple choice questions in the aerodynamics section of the exam I'm currently swotting up for reads*:

Modern airliners have high aspect ratio swept wings. This solution:
a) is a good compromise on condition that the sweep does not exceed approx. 30 to 35
b) increases the critical Mach number whilst minimising drag
c) ensures fuel economy at high subsonic speeds
d) all of the above are true, but the structure of a swept wing requires reinforcement compared to a straight wing

The answer is....




....d)

*free translation from the original French :{