Cry for technical help re foward swept wings

would slimline tip tanks (similar in shape to those seen on early F-5's but composite for lightness of structure) be any use on this design or would the wing loading be excessive?

Just trying to think of a way to get you back your lost gallons without a redesign.

Interesting thoughts mate. The Hansa jet had FSWs with tip tanks but then that wasn’t agile. The wing loading is greater nearer the root on FSW but all the same stresses may be a real issue –and transonic drag too. The current design has no wing hardpoints at all –I was thinking to keep the wing simple (well, if a FSW can ever be simple…)
Another option would be shoulder tanks not unlike the conformal tanks of the F-16 but integral (lighter).
Or the fuselage could be made longer…. A bit of a cop out that. At any rate the 10,000lb dry weight seems optimistic.
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(sorry if the tip tanks are bigger than you asked for –I asked for tip tanks, that’s what I got)

You guys have been really helpful, you should join the project at www.armbell.com...
Feel free to come up with your own ideas.

Yes, I did think that when I looked, but it does get the point across that conformal tanks would be better, thats what I'd choose.

I wouldn't choose conformal tanks myself - I'd just enlarge the fuselage - it will generate more internal volume for the same external shape.

No point compromising a design at this stage!!

I was thinking that a moulded composite conformal tank would be lighter overall than increasing the size (and weight) of the aircrafts structure which would result in every centre fuselage frame being bigger and heavier or added fuselage frames and skin if going for the longer option. My ainitial thought on tip tanks was to avoid making the airframe any bigger as this always adds a disproportionate amount of weight.

My assumptions could be wrong though, I wouldn't know any different.

I suppose it may add some weight, probably depending on specific load path changes and such like. But it surely would allow for a greater fuel volume. I don't know to be honest, I've no experience in conformal fuel tank design at all.

Guys, a further refinement to the shoulder tanks idea.
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To save weight they could be fixed (not removable). The advantage they have over simply englarging the fuselage is that their structure need not include 'primary load bearing frames' (my term) structures wheras enlarging the fuselage increases the size of these 'primary load bearing frames' -which is compariatively heavy. The structure of these tanks could be almost identical to drop tanks. It's all guesswork but tanks of that proportion could make the 10,000 lb possible -if not exceeded. the increase in dry weight would be - maybe 500 lb (?).

What about this idea, extending the shoulder tanks forward to become a LERX structure. I think it ‘looks’ right. Does it make sense?
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(the exact longitudinal positioning of the wings can be made later to consider COG etc)

If you use a conformal tank as a LERX the front section of it will have to be pretty strong.

Probably better to have the LERX as internal structure at least (use internals for fuel of course), but it is a load bearing structure.

Good point. We’ve taken that on board –the forward section of the shoulder tanks is structural (includes the whole LERX section) whilst the rest (slightly darker in illustration) is non-structural.

This is the last of the D-0013 illustrations for a while –we need to consider alternative configurations too although I must confess to now being convinced on the FSW idea.
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Have you guys got any suggestions on alternative configurations –the only requirements are S-STOL, Jaguar-like performance and overall dimensions not much different to super-Etendard etc.

PS. Waynos, like the color scheme?

Ha, the classic old Raspberry Ripple! or very nearly anyway.

As for alternative configs, have you considered (try to picture this) if your FSW was shortened in span, maybe 40%, but it then mated at the tip with a matching aft swept wing coming from the shoulders ((LERX position). Alernately you might try an aft swept 'upper' wing meeting an unswept lower portion at the tips. There are many variations on this theme you could investigate and the benefits would be a more compact aircraft with improved strenght for a lighter structure.

Dornier investigated such a layout for their TKF 90 submission and I've always been attracted to the idea. The forward swept portion of the wing would have to be rooted low down on the fuselage, X-29 style, for this layout to be effective. I think if the wings are too close together the aerodynamic benefits are lost and excess drag is induced.


edit, I've butchered your graphic to show what I mean;


Don't you think that looks like it will be agile, light, strong.................and cool


[edit on 6-1-2006 by waynos]

[edit on 6-1-2006 by waynos]

Well, what kind of top speeds are you trying to hit? Are you subsonic/transonic or supersonic?

It may be worth considering swing wings, it helps keep the wing operating better over a wider range of airspeeds. But it is complex -> heavier than a fixed RSW. Dunno if its heavier than a FSW though...

Ah yes, the Dornier Rautenflugel (what’s that word mean in German? Someone once told me it was named after a square harpsichord), a nice radical design. Its lower wings weren’t forward swept though and it had a conventional tail:
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But I have seen the coupled forward-rear swept compound wings on a recent US UAV. Can’t find which one though. The old Outrigger UAV had coupled wings although not much sweep:
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Interesting company also working on similar multi-wing designs:
www.phoenixnavigation.com...

Anyway, yes your idea can certainly be worked up as an alternative for the S-STOL UCAV. I would never have thought of that sort of configuration. Here is a starting point picture:
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The tip-tanks are just random thoughts. There are so many variables to discuss. First off, is the coupled wing inherently good for STOL in terms of low speed control –and what about supersonic flight?

It is essentially a biplane configuration so I’d guess the lift would be very good –anyone know anything technical on this increasingly popular wing type?

The FSW lower wing is my own spin on the idea I too thought of incorporating tip tanks, though these would have to be stress bearing as one of the chief benefits of the layout is that the wings themselves can be less rigid and thus lighter. Another choice could be some sort of winglet. I also think its worth investigating three alternative anhedral/dihedral combinations; lower wing flat+upperwing anhedral, lower wing dihedral + upper wing flat and equal anhedral/dihedral with the tips meeting on the centreline (my preferred initial option without any actual data to back them up). These will show different stability/agility benefits. I think the shorter overall span would be very beneficial to supersonic drag too, as well as the joined tips reducing the tendency for the wing to try and twist itself off, a problem with pure FSW designs, as the bending moment of the FSW wing is cancelled out by the opposite forces on the upper portion (in theory).

[edit on 6-1-2006 by waynos]

Kilcoo,
Supersonic is cool but not vital, especially with heavy weapons. The basic class of aircraft it is designed to supercede is the Skyhawk/Jaguar/AMX/Harrier. It must be able to operate from small and medium carriers (Spain, Italy, Brazil, India, Thailand) with a secondary land-based market.
S-STOL is the key feature.

Waynos,
Below are a couple of outline drawings with varying features.

The first has the same width body as the previous FSW design (D-0013) but correspondingly narrower wingspan. It retains the R-R AE3007 lift-jet. This gives a very slab-side form. The Canards are interesting –would that make it much more agile and easier o control at high angles of attack (slow speed landing?)
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The second outline retains the 10m wingspan of D-0013 which allows for a much wider fuselage allowing a lifting-body element. The outline also has a lift-fan instead of the jet allowing a much lower fuselage profile.
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These are just thoughts –we can pick and mix features and ideas to see what we can come up with.

I think you're going to have to refine this one more time.

Okay, here's what you need to use to lock down the maximum volume you have to hold for 10,000 lbs of fuel. Use Chevron's data on this page:

chevron.com...

note that the density can range from 775-840 kg/m^3. So use the low end, that would be 6.47 lb/gal. This would give you a volume of 5851 litres.


[edit on 1-7-2006 by Valhall]

The wings look a little too slender to my eyes, but the quick image I posted has them too broad, I envisage a plan view similar to the one I posted but with the wings more tapered towards the tips, as below.

Of the two you have posted I think the lift jet version is the more practical/most easily accomplished for the reason you spelled out earlier in the thread. I also believe that the canard is unnecessary on this layout as the aft swept wing portion serves the same function, possibly with a small curved Lerx (like the AV-8B, slightly bigger)

This is the wing (slightly altered from above) that I think best combines agility strength and low drag, though your wing would be superior in drag it looks more of a cruising wing than a turning wing, what do you think, am I talking rubbish?

Waynos, I’ll take your suggestions on board and see if I can get another illustration better fitting your descriptions.

A few questions:
Is EJ200 with TVC your preferred engine?
Tip tanks (I can commission illustrations both with and without)?
The big R-R AE3007 lift-jet I’ve been using or a smaller less powerful one? (Don’t worry, I’ll keep it British)

Also, re the wingtip overlap. Would it not be best to minimize this?

In the meantime here’s the artist’s own thoughts –has a touch of Gripen about it:
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I have no preference on either engine, British or otherwise. I think the merits of any available engine should be considered against what it is being asked to do. I can't think of any immediate objections to either engine you mentioned though, but if a better one is found it should be used.

I think the connection at the tips should be extremely rigid and right across the entire tip chord, as opposed to the trailing edge of one being connected to the leading edge of the other, which would place too much stress in the centre of the joint. However I agree that the wings should not overlap completely. There is no reason for the tips to meet directly and a small pylon like strcture (imagine an inverted outer pylon from a Hawk) could effectively connect them and maintain aerodynamic separation.

Here's a quick sketch in paint to try and illustrate it. Actually it looks quite close to what your own artist suggested. Alternativley a tip tank could perform the same function except we must bear in mind that the greater the lateral separation of the wings is the greater will be the stress on the tanks central structure.



[edit on 7-1-2006 by waynos]

Waynos,

Excellent sketch mate, you should consider getting stuck into A.TT (have I hinted too much already? Lol)

This is the MS Paint graphic I got when I put your previous description to my artist:
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I must admit I can’t guess what would happen aerodynamically at the wingtips. As you suggest, it would be handy if a harpoint could be incorporated into the wingtip connector for tip-tanks or even ALARM/Brimstone. The interface between the two wings seems to be the complication to all these coupled-wing designs.

Given the TVC and no pilot (fewer g-limits) it could be very maneuverable (weapons tolerances allowing) but I can’t imagine the upper wing have anything like the control efficiency of an all-moving canard –although it could have slats and flaps for take-off/landing.

Provided the lower wing retains the benefits of FSW in terms of low stalling speeds, then this looks like it would be better for lift than the pure FSW design.

What do they call these wings? –Compound wings? Coupled wings?

This is an early configuration drawing but already it is looking like a serious contender to the FSW design –hard to say which is better.

The aft extensions with control surfaces (elevators????) that you drew should definitely become a feature.

I think that we are using a bigger lift jet than we need –both designs (D-0013 and this one, D-0014) could benefit equally from a smaller lift jet in the 3-4000 lbst range.

Where this design is suffering is internal fuel. With the shoulder tanks the FSW design easily achieved the 10,000 lb spec regardless of the fuel density you chose. This one is, I guess, currently in the 8,000lb range (given fuel in the upper wing).

We could christen this diamond wing-plan “Lucy” (Beatles fan, not druggy)


-back to basics, here’s another similar and increasingly discussed wing plan:
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As you can see it is generally associated with airliners. My gut feeling is that this doesn’t suit a strike aircraft but obviously I’m no expert.

And here’s a concept drawing for a diamond-wing stealth UAV (USAF):
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I think that’s a bit far out for us.