From: megazone@coal.WPI.EDU (MegaZone)
Newsgroups: sci.aeronautics
Subject: Re: Want information of Foward swept wing
Date: 28 Jan 1993 06:37:54 GMT
Organization: WPI Discordian Society, Undocumented Cabal
NNTP-Posting-Host: coal.wpi.edu

In article <1993Jan27.225152@IASTATE.EDU> cffitzge@IASTATE.EDU (Charles F Fitzgerald) writes:
>Takuya Kitamura writes:
>>Does anyone know about Foward swept wing? I just know some 
>>experimental airplane exist such as X-29A. What is advantage 
>>and disadvantage? Could you tell me about it?
>The only forward swept aircraft I have heard of is the X-29.
>It was a proof-of-concept aircraft built to prove that a
>forward swept wing could fly (at least that is what I have heard).

Not quite... The X-29 (actually 2 built) is a research aircraft to probe the
high speed and high alpha advantages of the FSW. (forward swept wing) However,
it is not the only FSW aircraft.

In WWII the Germans built a large FSW bomber, however this of course was a 
large and slow aircraft.

There is also a business jet with a FSW, I think teh VFW-614, I have all the
data on it on my DAT, I can untar it if anyone is really interested. It uses
the FSW since this allowed the wing carry-through box to be located behind the
passenger compartment rather than create a hump, ala a Learjet.

>The advantage to forward sweeping is the same as the advantage
>of backward sweeping (but it looks neater :-)), _ie._ the velocity

True, when swept wing research was first carried out it was found that forward
or aft swept wings provided the same speed advantage.

However, there is one major difference. With an aft swept wing the root catches
the airflow first and the flow proceeds at an angle back and outwards along the
wing. At high alpha (alpha basically means angle of attack) the flow becomes
turbulent before reaching the tips. There for the tips stall before the root
does. The problem is that the airelons are located at the tips and this can
cause loss of roll control.

Using forward sweep the tips catch the airfirst and the flow is inwards and
back. In this situation the root stalls first and the tips remain in 'clean'
airflow allowing roll control in what could be considered a 'post stall'
condition. 

You can probably see where a combat aircraft would find this useful. Another
way to acheive this is vectored thrust, as in the F-18 high alpha research
aircraft and the F-22. (I do think the F-22 has asymetrical vectoring...)

>One major disadvantage to forward sweeping is it leads to 
>dynamic instabilities, thus requiring a lot of computer control
>during flight to be implemented.

Not necessarily... The bizjet is stable... It is a factor of the individual
design. But at high speed the problem is divergence.

See, at high alpha the wing tips can flex.. With an aft swept wing they flex
up, but the aerodynamic forces keep them in line. With forward sweep they bend
up, which increases the angle of attack, which increases the force, which 
bends them more, which increaes the angle of attack... This can continue until
the wings rip themselves apart.

This calls for a very strong wing. Using metal the wing is then too heavy and
you loose any advantage to FSW (except moving the wing box like the bizjet)
But modern composites have circumvented this problem. The X-29s wings were
layed out specifically to counter this tendency.

Most modern fighters are unstable. The F-16 needs the fly-by-wire system to
remain controlled. An unstable aircraft changes vector quicker, meaning it is
more agile. Of course this is desirable...

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