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What a fabric covered Wellington wing actually looks like!


Dave Fleming

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This pic from the Key Publishing forum of the RAFM Wellington shows what a fabric covered geodetic wing actually looks like - not the weird, waffle like fabric-under-the-ribs surface that Trumpeter and Special Hobby gave us. Matchbox wasn't too far off!

 

Any manufacturers planning a Wimpey should take note

 

http://forum.keypublishing.com/attachment.php?attachmentid=250008&stc=1&d=1480628715

Edited by Dave Fleming
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Although that shot certainly shows the geodetic pattern well, rather than the fabric being depressed between the ribs, that looks to me as though it is being lifted above the plane of the ribs, ie, bulging outwards against its fastening, due I'm guessing to the lower pressure over the top off the wing, especially aft of the centre of lift at about 1/3 chord, exactly the opposite as depicted on geodetic wings by most model makers. Less is certainly more in this rerspect, especially when building a model at rest, the shot off Key Publishing website is likely to be more representative, ie like the Matchbox Wellington & Wellesley

Steve.

Edited by stevehnz
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You're correct, Steve ... but I wonder if that effect is caused by the airflow over the wings?  The link posted by Dave shows a wing just sitting there ... and there is some evidence of the geodesic frame, but not much.  I'd be interested in reading about the explaination between the two photos.  Can anyone with more knowledge of the effects of airflow vs fabric covered surfaces, address this?

 

Scott

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Hi

   Random thought, if the airflow pulls out the fabric in flight, might it stretch, then when the aircraft was parked the fabric would sag ? 

   cheers

      jerry 

Edited by brewerjerry
typo
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3 hours ago, top turret toddler said:

Please scroll down to the very bottom and make up your own mind. Looks like waffles and a bit of peace & love to me. Cheers !

Look at the lighting. The light is at a VERY low angle to the wing surface. This will accentuate even the smallest variation in texture with highlight and shadow. Like walking along at sunrise or sundown and casting an extremely long shadow.

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Hello

Fabric on the wing's upper surface lifts due to an airflow, which is also very evident on other photos of fabric covered aircraft in flight. The photo on the first link shows, at least in my opinion, the wing which had been recently covered with a new set of fabric and has never been subjected to aerodynamic forces in flight since. Cheers

Jure

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Interesting, never looked that closely and just accepted what was made in plastic, although never looked that close at one either, can someone post pictures of the different kit parts? If the Matchbox is good, how is the rest of the kit? Or could you use the Matchbox airframe and kit bash it with better parts?

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Nice find Finn, that photo also shows the bulging aft of the spar line. looking at the angle & direction of the sun, it shows a small shadow on the lower right side of the geodetic diamonds as we view it, the opposite to what it would show if the fabric sagged between the ribs.

Steve.

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Do remember that an operational bomber parked out in the open in all weathers will look rather different to a newly resurfaced  wing inside a museum restoration hangar..

 

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Often forgotten is the fact that a fabric covering once doped and painted will become taut and as rigid as thin aluminium sheet , this means that any "sag" will be fixed and will probably not exaggerate with age.

 

Andrew

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Sorry Andrew, but they do indeed sag with age.  The in-flight photo showing the bulging due to the lower pressure on the wing uppersurface proves that there was indeed some stretch in the fabric, or this could not happen, and thus the alternative state of "at rest" would therefore see some sagging.  To further confirm this is the regular need to recover fabric covered wings after a time in service.  That certain models exaggerate this effect or just get it wrong completely is unfortunate but a model with a completely smooth surface would also be unrepresentative.  It would however display the desired design outline (ie as the project designers and aerodynamicists intended, before structures and production weighed in with their contributions), be more attractive, and not represent an aircraft that would be grounded as unserviceable were it real.

 

It's a difficult task for the model designers to be faced with.

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The two forces are not comparable though. An airworthy fabric surface reflects the fact that on the ground the only force creating sag is the weight of the fabric itself, and this is minimal. If it is in good order and with good original workmanship he fabric contour does not sag but it does follow the path of least resistance between hard structural components and if they are curved like ribs  then this highlights the rib contours slightly. But that's not a form of sag, for the dictionary definition of the word.

 

In flight, the wings are supporting the entire loaded weight of the aeroplane. The force pulling the fabric up is therefore vastly greater per square inch of fabric than the force creating any static sag on the ground, so even a very fresh and well maintained fabric wing will show some fabric rise on the upper surfaces, but won't look baggy or loose on the ground. 

 

You do see some poor fabric work, though, in photographs of ww1 types in particular, some of which I would not consider fit for flight in a modern context. Especially on fuselages.

Edited by Work In Progress
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Agreed that the in-flight load greatly exceeds the static case, but how elastic is the fabric?  As long as their elastic limit is not exceeded metals will bend, and then go back to their original shape, but fabric is less efficient at this.  Once the material has stretched to cope with the in-flight loads, it will not revert to its ideal original tautness, and will therefore sag under gravity.  The effect of one flight is small, but this is incremental.   Once it gets to looking "baggy or loose" then it is overdue for a recovering.

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16 hours ago, top turret toddler said:

Please scroll down to the very bottom and make up your own mind. Looks like waffles and a bit of peace & love to me. Cheers !

 

http://sas.raf38group.org/forum/viewtopic.php?t=1295

 

 

 

The effect on the 2nd last photo is caused by under pressure above the wing and this results in bulging of the fabric sections.

 

It's enhanced in that pic due to the flat angle the sun light comes in ...

 

The Matchbox kit is still the best in that respect if one wants to build an AC standing on ground.

 

Edit:

Oops hadn't read most of the replies above, sorry for the repeat of what others have stated too

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6 hours ago, Finn said:

Here is one that shows from the top:

 

http://www.iwm.org.uk/collections/item/object/205260183

 

you can click on the pic and zoom in.

 

Jari

 

Yep, aerodynamic pressure is pushing the babric up, giving that quilted effect as the fabric is above the level if the frame it is attached to. Quite the opposite of what is depicted by Trumpeter and SH.

 

At rest, the fabric will be taught or slightly sag, giving an impression of the ribs. What it won't have is the fabric below the level  of the surrounding metal skin or ribs.

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Hallo!

 

NACA Technical Note 428 "Charactersistics of an airfoil as affected by fabric sag"

http://naca.central.cranfield.ac.uk/reports/1932/naca-tn-428.pdf

Figure 2 is one of the best if not the best numerical description of the "sagging" phenomenon.

 

Taut fabric covering between curved ris/frames will always have visible "sag". The maximum "sag" coincides with the largest frame/rib curvature point (i.e. the smallest radius point). This is simple result of the Newtons III law.

 

Take example of fabric covering on upper side of a wing rib span. At the center, between ribs, leading and trailing edges of fabric do have downward line force compontent. Fabric covering creates the opposite and equal magnitude reaction force by "sagging". Then the left and right side edges do have upwards component  (in line forces toward ribs) making the sum of forces zero. The fabric covering centerline is inside (below in this case) the rib curvature even in perfectly taut surface!

 

If the curvature is zero - or in other words it is flat surface - there is no fabric "sagging" whatsoever on the covering. Majority of control surfaces fall in this category. In engineering sense, that is.

 

 

In-flight photos of fabric covered airplanes are suitable references only for in-flight models what comes to the "sagging" or (rather bulging or ballooning).

 

I am of the opinion that fabric covering shall have only elastic stretch when ballooning in-flight. If there is plastic deformation i.e. the fabric really is sagging, it is no more airworthy.

 

Cheers,

Kari

Edited by Kari Lumppio
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Agreed> Actual sag, in the sense of the fabric having lost its tension, rather than simply taking up its natural taut geometic form between structural members at rest, is a no-go item on pre-flight inspection. Apart from anything else it will no longer be contributing to rigidity, and will increase the risk of flutter.

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