'Stressed Skin' – What Does It Mean?

Thank you to all your replies to my question – I am now suitably illuminated. It’s very handy, to say the least, when those with working knowledge are on hand to assist.

I think it’s easy for the non-engineers amoung us to regularly read engineering terms (monocoque, stressed skin etc) without knowing what they actually mean. The example of a Spitfire collapsing when skins are removed without a jig in place is an excellent illustration.

I am now emboldened to ask one or two more numpty questions when they occur.

Cheers,

This is one of the reasons I like this forum so much. If you have a question just ask, there is a good chance someone will have the answer or tell you where the answer is likely to be found but they never make you feel stupid for asking or making a remark which is not quite correct. They just politely point out where you are wrong with no recriminations.

I had wondered that also.

I understand it is because historically Supermarine made ‘flying boats’ and it is the way with boat building.

Can anyone confirm?

Mark

That sounds like it might be correct, seems to fit doesn’t it?

I’ll second that!
When Airbidane explained why the Hurri was more prone to ground looping than the Spit I was happy as a pig in sh!t 🙂
I have heaps of tech questions…for example, why do the fuselage skins on a Spit overlap the “wrong” way?—
Allan
(Picture credit to Garry R Brown)

I had wondered that also.

I understand it is because historically Supermarine made ‘flying boats’ and it is the way with boat building.

Can anyone confirm?

Mark

Hamtech -A perfect monocoque structure occurs naturally. An egg!

As already mentioned by Chiron in post 6.

Hamtech. Nice gift. For the Australian Imperial or UK metric version. 🙂

Hi all

“This type incidentally was called ‘composite’ in it’s – very confusing.” should read”This type incidentally was called ‘composite’ in it’s day – very confusing.” Hopefully that makes more sense. I’ve seen the term used in several 20’s and 30’s engineering texts.

As for asking questions about engineering, I get the impression from the surprising number of replies to this thread, that there is probably a lot of interest in the technical issues – so ask away.

Chiron

Hamtech -A perfect monocoque structure occurs naturally. An egg!

Thanks. BTW Mark, is it pronounced ‘Longer-on’ or ‘Lon-jer-on’? I’ve heard both. (Please resist from quips terminating the thread 😉 pretty please? 😀 I’ve done it now)

Pronounced ‘Lon-jer-on’ according to The Concise Oxford.

Mar

To ‘Longeron’ or to ‘Lonjeron’

Thanks. BTW Mark, is it pronounced ‘Longer-on’ or ‘Lon-jer-on’? I’ve heard both. (Please resist from quips terminating the thread 😉 pretty please? 😀 I’ve done it now)

Confused – you will be.

Thank you to all your replies to my question – I am now suitably illuminated. It’s very handy, to say the least, when those with working knowledge are on hand to assist.

I think it’s easy for the non-engineers amoung us to regularly read engineering terms (monocoque, stressed skin etc) without knowing what they actually mean. The example of a Spitfire collapsing when skins are removed without a jig in place is an excellent illustration.

I am now emboldened to ask one or two more numpty questions when they occur.

Cheers,

Seafuryfan,

If you are talking Spitfire structures, that’s ‘frames’ where most will say formers, ‘intercostals’ where most will say short stringers between frames and ‘longerons’ where some might say longitudinals.

Mark

Thank you to all your replies to my question – I am now suitably illuminated. It’s very handy, to say the least, when those with working knowledge are on hand to assist.

I think it’s easy for the non-engineers amoung us to regularly read engineering terms (monocoque, stressed skin etc) without knowing what they actually mean. The example of a Spitfire collapsing when skins are removed without a jig in place is an excellent illustration.

I am now emboldened to ask one or two more numpty questions when they occur.

Cheers,

Kevlar = Aramid FYI.

Thanks – I wasn’t sure of that, but too lazy to check 🙂

JC

“Composite” (meaning an amalgum of Glass Fibre, Carbon Fibre, Kevlar and Aramid

Kevlar = Aramid FYI.

Another reason for fabric covering the control surfaces would have been serviceability I guess, although weight would have been the main consideration.

JC

Actually, speaking of control surfaces, a guy in Florida built himself a 60% scale Spitfire MkIX replica, completely out of composites (although I think the spars were wooden). He took it up for the first flight, but crashed and was killed when it appeared the aircraft couldn’t gain much alititude.

It was found later that the elevators were quite heavy (something like 25 pounds in weight) and this may have been a contributing factor in the crash as the aircraft was quite tail-heavy.

A sad end to what could have been a huge success – he’d already built a MkV replica, and was considering kitting the MkIX.

:rolleyes:

Because they’d be lighter and would require less stick force from the pilot to move them.

Although, having said that, we all know that the Spit later had metal skins fitted to the ailerons to stop them ballooning.

On a similar issue, can anyone explain why, long after stress skin structure was usual for the rest of the aircraft, control surfaces were still fabric covered?

Paul

Well yes actually! 😀

JC

Modern light aircraft designs made from the modern usage of the word “Composite” (meaning an amalgum of Glass Fibre, Carbon Fibre, Kevlar and Aramid weaves all bound together by resin) are more true to the term monocoque, as they can be thought of as a shell.

JC

Bit like a Mosquito then. 🙂

Mark

What they all said – Simply put, the skin forms a major structural part of the assembly, whereas in a “composite” (that’s a good term for it Chiron!) structure the tubular chassis (for want of a better word) would be self supporting. In other words, don’t strip the skin off a Spitfire without it being properly supported in jigs.

Modern light aircraft designs made from the modern usage of the word “Composite” (meaning an amalgum of Glass Fibre, Carbon Fibre, Kevlar and Aramid weaves all bound together by resin) are more true to the term monocoque, as they can be thought of as a shell.

JC

Seafuryfan,

Aircraft structures come in several different types.

The oldest is the type seen in the Hurricane etc. where the aerodynamic loads are transmitted up through a structural hierarchy. The lifting force on the wing on the fabric skin, for example, is transmitted through the rib stitching to the rib, and then to the spars and finally to the fuselage.
This type incidentally was called ‘composite’ in it’s – very confusing.

The next and still the most common is the ‘semi-monocoque’ or ‘stressed-skin’ type. If you look in the back of a Harvard fuselage you can see the same longerons, formers and stringers that an earlier aircraft would have had, except that they are all made of metal, and they share the load with the metal skin.The advancement from there has largely been a matter of reversing the ratios in which skin shares the loads with those components, i.e. the individual longerons have all but disappeared in modern aircraft, formers (which give the circular shape to the fuselage) and stringers (which run fore and aft to give rigidity) have all become proportionally smaller and structurally less significant. In the rear fuselage of a Cherokee for example, the stringers are formed by bending the edges of the skin sheets inside the fuselage, very clever and very simple. The skin is then used to take up the remainder of the loads.

The ultimate structure is full-monocoque where the skin takes all the loads.
The classic example of the perfect monocoque structure by the way is an ordinary old egg shell.

Hopefully this answers more questions than it raises.

Chiron.