Hi all,
Greetings from rainy England. New member, good to be amongst fellow WWI fanatics!. A gripe here, in Halifax yesterday, spent some time in a superb modelshop there called 'Halifax Modellers World', thoroughly recommended. Anyway, i was tempted by a few Eduard models lurking there on the shelves. Opened the boxes, as expected from Eduard superb moulding etc. but - incorrect representation of fabric 'sag' on undersurface of mainplanes!. Eduard seem to be of the opinion that 'sag' is inwards on an undercambered concave surface, this seems to be universal on all their WWI subjects and is of course a physical impossibility!. Roden have got it right though, they correctly portray the fabric as 'sagging' downwards, in effect trying to regain it's 'flatness' between L/E and T/E. Roden also correctly vary 'sag' according to degree of camber, i.e. less on areas of shallower camber, good old Roden!. I was tempted to purchase an Eduard DVII or Albatros DV but was put off by the degree of correction that would be required in this area, how much easier it would be if they got it right in the first place, after all it's a pretty obvious principle!. Does anybody out there have any opinions on this?, any 'quick fixes' etc., cheers for now, Al.

Hi ! I was curious about this, so looked at various WW1 photos, taken from a forward view. ( Albatri, Fokker Tri, Fokker DVII, ) and there was no discernable sag in either direction. I am going to the Shuttleworth Collection at the end of the month, and will keep an eye out.
I seem to remember discussion on this site - why not do a search - this site only ??

If the fabric on a real aircraft was to sag as much as Roden and Eduard protray it, it would tear off the first time you tried to fly it. Head down to your local airport and look at any fabric covered aircraft. Note that the fabric is drum tight between the ribs. If it weren't, the wing countours would distort and the wing would lose lift, just before the fabric tore off in the slipstream. Bue Max kits capture the true shape of fabric covered wings best.

Don't stress about it. When you're building those wonderful Roden and Eduard kits, remember to repeat the Monty Python mantra: "It's only a model."

Thats a riot They ought to sell something like the Staples "that was easy" button to modelers with that python sound bite.

Actually Allen the fabric never sagged downwards either as the dope applied to the fabric pulled everything drum tight.

Hi all and thanks for your comments regarding this. There seems to be a misunderstanding regarding my use of the word 'sag', 'sag' implies slackness, which obviously wasn't the case !!. Regarding Warren's comment on the dope pulling the fabric tight and thus no downwards sag (Warren - did you mean on the lower or upper surface?) i can assure you it did, if it didn't i've been seeing things all these years!. To anyone who's built a flying model with a tissue covered (that's going back a bit!) wooden structured wing this won't need explaining. After tissue covering, water spraying, and a couple of coats of dope all is revealed. 'Sag' is a natural feature of a fabric covered wing structure and is a deviation from the ideal, i.e. the particular aerofoil section used. It is a natural result of fabric shrinkage in the doping process, also to a degree initial tautness of application and direction of application, e.g., spanwise, chordwise or diagonally. Thoretically 'sag' is also affected by air temperature and humidity and therefore could vary minutely from day to day. It varies according to degree of camber and rib spacing (which is why for example fastidious Rudolf Geringer incorporated those lovely 2/3 - approx- chord formers between ribs on the Pfalz DIII wing upper surface, to 'preserve' the design aerofoil section ). Of course, it 'sags ' inwards on a convex surface , but more relevant to our current discussion 'outwards' for want of a better word on a concave surface, e.g. on an undercambered wing. This isn't always easy to see on photos and depends on lighting angle etc., but is there, believe me. The only time fabric doesn't sag is when applied to a flat surface e.g. ailerons, elevators and fuselage sides, for example a Fokker welded tubular structure, which was a 'succession of flats' towards the sernpost, each bay being a flat, thus - no sag. To sum up, representing sag correctly is as important as getting the panel lines in the correct position on a metal skinned A/C. It's part of the external configuration of the aircraft, the shape, the character, nuances etc. Again, look for 'outwards' 'sag' on undersurfaces, it's there, really!, (if it's not, then WNW are wrong - which they ain't!). Think about the mechanics of it all, it's merely a result of the fabric 'trying to regain it's flatness' where unsupported between L/E and T/E, and of course on an undercambered surface it has to be the reverse of what happens on the uppersurface. All the best and cheers for now, Allen.

To sag or not ???

Here is an interesting picture from Vintage Aviator site. seems the fabric has an Unsag in this view.

Is it loose fabric? or aero dynamics?

I tend to believe that sag is in the I of the beholder. Also if you paint it a certain way (highlighting) you can adjust you sag to suit YOUR purposes.

I for one ignor the fact, Sag is sag, and it depends on the weather and humindity and age, and in the picture which way you are going.

Irregardless enjoy yourself
Captn Tommy



***If you can't see these photos go to my photo gallery. CT

I have just returned from the Shuttleworth Collection, where the under-wing fabrics on the Sopwiths had no visible sag whatever !
During the flying display, I happened to stand next to a man who claimed to have training in aeronautical design, and he stated that gravity is very small compared to the effect of drying dope tightening up. Not only does this occur between the ribs, but also between the leading and trailing edges of the wing.
If the lower side of the wing is concave, then the rib tapes will hold the fabric up, but the stretch between front and rear will try to keep the inter-rib material flat, and the combination might resemble sag.
If the lower surface is more "modern", eg convex, the tightening between front and rear will resemble upwards sag, with the ribs holding the fabric down !!

Bang on Clive,it is a case of the fabric trying to regain it's flatness between L/E and T/E after doping. Where not restrained from doing so, I.E. between rib positions,it, for want of a better word 'sags'. As mentioned by myself earlier the degree of 'sag' is amongst other factors, dependent on the degree of undercamber present. By far the easiest way to grasp this is to imagine we've just doped the upper surface of a wing, and in doing so of course, created 'sag'. We then imagine what the inside surface would look like (if we could look inside!). We would obviously see the reverse, 'sag' in the opposite direction. If we could look inside at the other side of the top surface of the wing we'd be looking at in effect - an UNDERCAMBERED surface!. What prompted me to repost here was an erroneous statement by someone reviewing the Eduard 1/48 Fokker Dr.I and stating that the representation by Eduard of the undersurface fabric is correct when compared to Dragon's effort, it is not correct, Dragon have it correctly represented in principle, although grossly overdone. I might add that apart from Eduard's error in the fabric 'sagging' in the wrong direction (which does seem to be generic to their models - Pfalz DIII - AAARRRGH ! ! !) their Fokker Dr.I is absolutely SUPERB, matching the late great Ian Stair's plans in the DF Special almost exactly, a little work of art, but, no top longeron dip to the tailplane shelf, this WAS there, but easily rectified. There are of course numerous other little 'niggles' with the model, most mentioned since it's introduction eg, the more forward position of the top wing 'celluloid' window etc. but again, nothing too serious. One point i will perhaps expand upon in a future post is the correct appearance of the triangular ply fuselage fairing with relation to the fuselage covering eg visible contact points through the fabric with regard to the top and bottom edge of same and areas of 'sag' created by various curvatures in this area, again exaggerated in the Eduard kit but better in the Roden 1/32 offering, although neither have it correct, more later.
Cheers, AL