Swing wing question

[Shaggy Sheep Driver]

Following on from the 'variable wing area' thread can anyone answer this: on swing wing aeroplanes (such as Tornado) how is the rearward shift of cente of lift handled as the wings swing backwards?

SSD

[Iron City]

Don't know how others do it but on the F-14 when the wings swing back the glove vanes extend and I believe keep the center of lift somewhat more forward than it would otherwise be. If needed I believe my F-14 NATOPS is in the storage locker and I'll look it up. Believe there is some autostab/autotrim that comes into play too.

[Tim Inder]

There would be a corresponding rearwards shift in the c/g which would account at least in part for the c/p movement - with the F-14 the wings are a much smaller percentage of the overall weight compared with, say a Tornado. (they tend to fall over if the wings are swept back on the ground I hear :\ )

[emitex]

they tend to fall over if the wings are swept back on the ground I hear

Especially the 'short' version when fitted with 'big' tanks!

[Iron City]

Wish I had big tanks

[Synthetic]

Er yes emitex you might be right :O :* :O :* and for a very substantial donation to TVOC, I might reveal names :D


Anyway to get back to the original question, I accept that the wings sweep back the c of g goes with them, but so surely does the mean aerodynamic chord. Maybe if some Boffin has polished up his slide rule and got the sums, pivot points etc right, then one effect serves to minimise or even cancel out the other.

[Shaggy Sheep Driver]

I'd have thought the CG movement would be insignificant comapred to the CP movement. Only a small bit of the aeroplane's weight moves back on swing the wings back, but all of the CP moves back.

Unless the CP only moves back a very small amount, while the bit of the weight that moves does so to a large amount, I can't see how one can cancel out the other.

They would also have to cancel each other out at all intermediate positions, as well.

SSD

[normally left blank]

Iron city.

I read somewhere, that the glove vanes on the "newer" F-14's are wired shut. And that the vanes were originally only used in the transonic? regime.

Best regards

[Mad (Flt) Scientist]

Yes, the glove vanes were inop'ed on later F-14s

All of the lift does not move when the wings sweep. Especially on a military aircraft, a considerable amount of lift may be being generated by the fuselage. Also, the inboard wing generates more lift than the outboard, so the shift in wing lift is not as dramatic as might appear. As the sweep increases the lifting ability of the wing decreases too (which is why you need variable sweep in the first place) so the moment change may be quite small.

Then, of course, one will ensure that the aircraft has adequate trim to cope with any remaining moments. As the tail is likely being sized by either a low speed trim or high speed manoeuvre requirement anyway, this probably wont be a problem.

It's not really much different in concept to other config changes, such as flaps. They make a HUGE change to the lift and trim requirements - you design them to minimise the effect if you can, and design e.g. the tail to cope with anything that's left.

And nary a slide rule in sight. :)

[hairyclameater]

Glove vanes were wired shut on the OLDER 'cats - newer F14s didnt even have them fitted at all- great weight saving feature and obviously not needed !! Seems that the wing glove pylons acted as more than suitable aerodynamic aids in all flight regimes.

[normally left blank]

Digging a little: "Vickers-Armstrong perfected the basic geometry with the VG wings pivoted well out from the fuselage on a fixed "glove" (triangular inner wing) in 1958. Handed it to NASA."(Bill Gunston: MiG-23/-27 Flogger, 1986).

F-111: "-- changes in trim caused by sweeping the wings were automatically compensated for by a "parallel trim" system. There was no sensation of wing movement --" World Air Power Journal Vol 14.

It seems "MiG" got the wing position right with the Flogger. (The Ye-231 prototype had them further aft).

The F-14 has been tested with assymetric wingsweep! But back in the early fifties Grumman build one of the worst aircraft ever, the XF10 Jaguar, with VG-wings joined at the centerline. Somewhere I've got an article and a test pilots report on it. The XF10 is - fittingly! - at the bottom of Grumman's history photopage. :D

Best regards