Jabiru skin thickness?

[rtfm]

Hi guys,

 

Does anyone know how thick the composite skin of the Jabiru is? For that matter, does anyone know how thick composite skins usually are? I have received advice that a fuselage skin thickness (sandwich) of 3mm is more than enough, and other sources insist on 12mm. Are either of these in the ballpark of what would be considered "normal"?

 

Regards,

 

Duncan

 

 

[damkia]

Hi guys,Does anyone know how thick the composite skin of the Jabiru is? For that matter, does anyone know how thick composite skins usually are? I have received advice that a fuselage skin thickness (sandwich) of 3mm is more than enough, and other sources insist on 12mm. Are either of these in the ballpark of what would be considered "normal"?

Regards,

 

Duncan

I would hazard a guess it would depend on the type of composite structure and what loads it is carrying.

 

I know for a fact that Nomex honeycomb typically makes about 7-8mm thick composite for the skin (Lancair 360), but it was a stressed skin design with minimal longerons and other supporting structures with only three bulkhead structures along the inside (firewall, seat back, and one in the tailcone. These are always post cured/vacuum bagged.

 

Polystyrene foam sandwich is not something I am that familiar with, but I would imagine the thickness would vary depending on the load, and whether it is "cold layup" or post cured (baked)/vacuum bagged for better penetration of the epoxy or vinyl ester adhesive.

 

 

[fly_tornado]

Aren't you building your plane to some sort of engineering formula?

 

 

[Guest Maj Millard]

The layups around a honeycomb sandwhich generally end up around 2-3 ml, but like anything else with aircraft design, and construction, have to be tested and found suitable for the application.

 

The vacuumed bagging of an item especially when combined with auto calving or heat blanket ramp up and ramp down, is to ensure controlled curing and a more precision- moulded part. Generally pre- preg glass (resin pre-inpregnated cloth)is used in these applications anyway in preference to a hand lay up, to ensure the correct resin distribution throughout the cloth. If making many of the same parts, IE: wings or fuselages, consistency is the key, both with weight and required strength.

 

One must ensure when doing a hand lay- up, that the final item doesn't end up 'resin rich' or for that matter 'resin poor'. To 'scrape out' excess resin is to make the final product stronger and lighter.

 

The strength is not in the resin, but the encapsulated cloth or sandwich honeycomb structure . A resin rich layup (often seen in marine boat applications) is just simply carrying too much weight in excess resin, for no additional strength. A resin-poor layup will

 

Lack final structural strength. Remember in all aircraft finished structures, lighter is stronger, and heavier is weaker. It is the weight that ultimately exceeds G limits and breaks an aeroplane........you have to 'engineer' a composite structure just as you would any other structure using other materials, then test it for the qualities you need..........................Maj...

 

 

[rtfm]

Aren't you building your plane to some sort of engineering formula?

Hi,

This is my own design. I have been advised that the "standard" (in the US anyway - eg: Glasair) is 1/2 inch sandwich for the fuse. I went ahead and bought 10mm foam designed specifically for infusion. However, a local builder said that this was way overkill, and that the 3mm foam I bought for the wing skins was more than adequate for the fuse as well.

 

So the question remains - does anyone know how thick the sandwich skin of the Jabiru is?

 

Regards,

 

Duncan

 

 

[Guest Maj Millard]

Duncan, I'm not sure you got the message that I was trying to convey in my post #4. You don't simply build a composite aircraft by just ensuring you have the right thickness of composite structure. The whole thing is engineered from scratch and wall skin thickness is just one factor in the whole picture. Composite structures come in many different forms, especially these days with some of the exotic fabrics and combo weaves available to the builder. It is almost impossible to answer your question accurately , without viewing the whole structure. To simply 'copy' the wall thickness of some other design would be a stab in the dark at best.......................Maj...

 

 

[Guest ozzie]

Knock knock anyone home?

 

How thick is the skin on a Jabiru?

 

 

[turboplanner]

What these people are trying to tell you is correct. A honeycomb structure consists of an inner skin, core, and outer skin, and many different materials can be used for each, and there are a few different manufacturing methods as well.

 

All these methods produce different strengths for impact and bending.

 

The design itself can multiply strength by hundreds of percent if angles or curves are built into the moulds

 

The principle of honeycomb design, simplified, is that two stressed members act in tension, held apart by the core, so the tensile strength of the outer and inner skins determines the strength for a given core strength, and total strength can be increased by separating the two skins further apart.

 

So with the same skin thickness and tensile strength a 12 mm core will produce much greater strength than a 3 mm core.

 

Conversely, if you used a 3 mm core, but the skins, through material strength or skin thickness had much higher tensile strength, the 3 mm core combination could be stronger.

 

That's why you have to engineer the design.

 

Just cutting a Jab structure might tell you the skin materials, core material and dimensions FOR THAT LOCATION. The beauty of composite design is that we can use different laminate and core thickness right through the product to save weight and cost. You would want to cut either the thinnest or thickest section then try to build a fuselage; it would finish up too light or too heavy.

 

 

[Guest Maj Millard]

Well said Turbo, I think that was what I was trying to say also................Maj...

 

 

[damkia]

Well said Turbo, I think that was what I was trying to say also................Maj...

Ditto...

 

 

[flyvulcan]

Hi Duncan,

 

It looks like you may have to call Jabiru to get the answer to your question.

 

For what it's worth, the Lightning Bug fuselage does not use any sandwich and the skin is (I am estimating) simply 2 layers or possibly 4 (we are about to measure skin thickness to confirm) of uni cloth oriented appropriately. The forward fuselage has a central structural keel to which pretty well everything is attached but that keel stops just aft of the pilots seat. The rear fuselage consists only of the skin, a few bulkheads and what is essentially a longeron on each side where the fuselage top and bottom halves have been joined at their flanges.

 

That said, the original owner of the Lightning Bug kit that I have built my Komet project from added a 6" wide sandwich longeron running from nose to tail along each sidewall which uses 1/4" foam. I expect that this mod would have been with the blessing of the original designer of the Bug as my kits former owner built the prototype of the Bug for the designer.

 

My wing leading edge D-cell, which is the main structural component of the wing uses 1/4" foam in its sandwich structure.

 

When I get home, I shall email you some photos of my fuselage where you can see the centre structural keel and also the foam sandwich longerons that have been added to my kit only. All current flying Bugs do not have that longeron, only the skin.

 

This info is for your interest and consideration as an example of a different method of construction to straight sandwich construction.

 

Cheers,

 

Dave

 

 

[bexrbetter]

I believe they are 3.864mm at 29.7 C.

 

Or you could contact AVOCET who has actually worked with them and built his own composite plane sucessfully.

 

 

[Dafydd Llewellyn]

Hi guys,Does anyone know how thick the composite skin of the Jabiru is? For that matter, does anyone know how thick composite skins usually are? I have received advice that a fuselage skin thickness (sandwich) of 3mm is more than enough, and other sources insist on 12mm. Are either of these in the ballpark of what would be considered "normal"?

Regards,

 

Duncan

Pal, you do NOT build an aeroplane by comparison with other aircraft if you want to stay alive. The Jabiru structure is extremely carefully worked out, and it's exactly as strong and rigid as it needs to be, (taking into account crashworthiness as well as flight and ground loads), there's no margin. Also, composite aircraft are often designed by stiffness, rather than strength.

If you have sufficient understanding of how to calculate the loads on the aircraft structure, you could perhaps make some test specimens using whatever materials you want to use, and test them. To do that you also need to know how to perform meaningful structural tests.

 

If you do not have that level of knowledge, then either stick to a kit that somebody else has tested - or build yourself a structural test specimen of an entire wing, an entire fuselage, etc, and static load test them. You may need to pay an engineer to work out the number of sandbags and where to place them.

 

If you choose to do that, then support whatever it is you're testing with a few hydraulic jacks whilst you're putting the sandbags on; then let the jack down a little bit at a time. That way, when something cracks, you won't destroy the whole test specimen by having it fall on the floor with a ton or so of sandbags on it; so you'll be able to beef it up and try again.

 

Bear in mind that a composite structure normally needs to be able to carry around 225% of its design limit loads, in order to have the required safety factor (150%) at the temperature of a white aeroplane standing in the sun.

 

Also bear in mind that the structure needs to be tested for the worst possible combinations of manoeuvre and gust loads, for all points on the flight envelope. See FAR 23 subpart C (Structures).

 

 

[Old Koreelah]

...a composite structure normally needs to be able to carry around 225% of its design limit loads, in order to have the required safety factor (150%) at the temperature of a white aeroplane standing in the sun...

Good point. Even my light yellow wing is hotter than a white one. Darker colours would be risky. Even if metal wings can cope, internal fittings like fuel lines must get pretty hot.

 

 

[Keith Page]

Hi RTFM

 

Best for you go and see Rudi at Lavender Composites at Acacia Ridge.

 

He will put you on the correct track will cost you but well worth it.

 

Warn you he will blind you with knowledge re. Design, composites, construction, actually the whole plane. You will enjoy him.

 

Regards

 

Keith Page.

 

 

[Ultralights]

when it comes to vacuum bagged parts, 1 ply usually equals 0.010 of an inch. plies required depend on the material, the weave, the resin, cure pressures, and temps. a carbon fibre 3 ply panel cooked to 500deg to cure, under 50 PSI will be far stronger than just 3 plies of room temp cured fibreglass.

 

almost all structural composites on Boeing aircraft are cured at a minimum of 200 deg. engine cowlings, both internal and external, are upwards of 500 deg cures. you would be surprised at just how thin some panels are in structural areas.

 

also not uncommon to see fibreglass plies over an aluminium honeycomb structure. Never bond carbon to aluminium, they dont like each other very much, if you do, the contact ply on the metal must be fibreglass or kevlar. again, steel fasteners only with carbon fibre.

 

 

[damkia]

... Never bond carbon to aluminium, they dont like each other very much, if you do, the contact ply on the metal must be fibreglass or kevlar. again, steel fasteners only with carbon fibre.

If my basic chemistry is still OK....

 

Aluminium and carbon = electrolysis (a battery of sorts, differing electronegativities)

 

Steel and carbon is OK as normal steel contains an amount of carbon anyway (far less electrolysis, Fe + C + Ni + Mg + others)

 

 

[Dafydd Llewellyn]

If my basic chemistry is still OK....Aluminium and carbon = electrolysis (a battery of sorts, differing electronegativities)

 

Steel and carbon is OK as normal steel contains an amount of carbon anyway (far less electrolysis, Fe + C + Ni + Mg + others)

Yes, carbon fibre is bad news in contact with aluminium (so is lead pencil).

The point about all this is that, in building a composite structure, you are creating the material; so the choice of face layup, core thickness & material, resin, layup and cure process etc will all affect the end result. That is the reason why composite aircraft structures MUST be justified by test, and the test specimens MUST be built exactly the same way as the actual aircraft structures. There are enormous differences in detail design from one aircraft manufacturer to another, and also enormous differences in layup technique, etc, so it is not a practical possibility to reverse-engineer a structure, because you will never know all the factors involved. When you add fatigue-resistance to the equation, it becomes very complex indeed. It's a minefield for the amateur.

 

 

[aj_richo]

(so is lead pencil)... er graphite?

 

 

[facthunter]

Good question. or is it a statement? Graphite of course, which is an allotrope of carbon No go on exhaust pipes as it will infuse into the steel, ( even stainless) when it is hot, and crack ( So I've been told)... Nev

 

 

[Dafydd Llewellyn]

(so is lead pencil)... er graphite?

Yes. Never use one on aircraft-grade aluminium alloy.

 

 

[rtfm]

Pal, you do NOT build an aeroplane by comparison with other aircraft if you want to stay alive.

Pal, I asked a simple question. Do you know the answer or not? I didn't expect (or deserve) a talking to about doing my homework. Sheesh.

Duncan

 

 

[rtfm]

Hi,

 

Thanks for all your helpful insights and answers. I will be using 10mm 3-D Core as the meat in my CF sandwich. My test pieces are extremely rigid, and surprisingly light. The entire fuse (inclusing vertical tail) will weigh in at just on 21kg, and since the fuse is roughly circular, and only 3.9m long (12.8ft), it is going to be extremely rigid. There will only be two bulkheads. Well, three if you include the structural control panel. (Firewall, control panel, seat back). No longerons.

 

Tricycle undercarriage, with the main gear attached to the inverted gull wing. The spar= single piece, although I am considering folding the wing at the elbow just outboard of the landing gear (like the Corsair).

 

Engine = 900cc Generac (Valley Engineering Big Twin. 116lbs wet (ie running), including starter and redrive. 50hp.

 

The wing skins will be made from 3mm 3-D Core foam and CF. Again, extremely light and rigid. It was when I showed my 3mm test piece to another local composite designer/builder, that he thought it would be more than sufficient for the fuse skin also. Hence the reason for my somewhat idle question. Just wondering, I guess.

 

At the moment, I am almost finished the plug. A final dusting of black paint this afternoon, before (hopefully) the final rub-down tomorrow, and onto increasingly fine sandpaper till I have a mirror-finish. Within a week I hope to start making my molds.

 

Regards,

 

Duncan

 

 

[Dafydd Llewellyn]

Pal, I asked a simple question. Do you know the answer or not? I didn't expect (or deserve) a talking to about doing my homework. Sheesh.Duncan

Which model Jabiru? Where on the airframe? Actually, your question is meaningless; some Jabiru wings are completely foam-filled, and have a single-thickness skin. Others are not foam-filled, and have sandwich skins, the make up of which varies according to the local loads. Some of their skin is glass over 3mm coremat - but they adapt according to the need as shown in structural testing.

 

 

[rtfm]

Hi Daffyd,

 

Hi. Hey look... I'm not trying to be awkward... Just asking an inncocent question. You have answered it quite comprehensively above, so thanks.

 

Cheers,

 

Duncan