Need ideas for a aircraft design project.

[wewe2011]

Hello,
I'm currenly working on a small project, i'm trying to figure out a mounting design for the tanks (pressure vessel) in an aircraft wings (to the ribs or skin), so that the pressure tanks could take some of the aerodynamic loads. The problem for me is the requirement, that the tanks could be easily disassembled and expansion of the tanks need to be accounted.

The image shows, the current structures that i'm working with, problem with the mounting at the root is already solved, but since the tanks is "hanging" inside the wings, a mounting structure that accounts for expansion and ease of dissasembly is needed, so that the tanks does not come to contact with the skin in flight. My ideas was just to place a rubber material under the bottom skin, and just let the tank come into contact with the skin. But i don't know how realistic this is. Any insights would be helpful

Thank You

 

[GregLocock]

" the pressure tanks could take some of the aerodynamic loads". I think the fundamental idea is <insert past tense of procreational verb here>. If the tank is usefully supporting load when pressurised, then what happens when the tank is empty?

So assuming you pressurise the tank all the time, so you have an inflatable structure, comme ca,

Yes I think some sort of foam would work

But, as the wing loads and unloads, for instance when landing, the foam will scrape across the tank. That doesn't sound unsolvable, but it does need consideration.

Cheers

Greg Locock


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[wewe2011]

The idea is that the tanks act so like a spar, even when empty. If the foam scrapes the tanks, but doesn't damage or cause structural issues, then it should be fine. What kind of foam are you reffering to?
Thank you though for the insight, i'll take it into consideration.

 

[rb1957]

what type of fuel are you thinking of ? If liquid, why the cylinder ?? Nearly all wings already do this (carry fuel and react airloads).

If gas, then "surely" not H2; what, LNG ??

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[wewe2011]

Its pressurised gas (H2) @ 350 bar.

 

[zeusfaber]

What sort of flight envelope are you designing to (especially in terms of Normal Acceleration limits)? What's the chances (my guess would be high) that this mounting arrangement has to cope with some degree of negative g?

 

[rb1957]

Oh, ok. is the H2 liquid at that state ? if so, how will you insulate the cylinder ? (presumably it is very cold too ?) And you want this tank to be " easily disassembled" ? Sure, let's have lofty goals ... H2-proof disassembly joints are Very difficult to achieve.

I would suggest (ok, my first suggestion is pick another project !) an elliptical tank, being more aerodynamic. Possibly Ti, maybe steel; and I'd have some hardpoints mounted on the tank, to mate with a track on the wing structure (for thermal expansion). If you are very clever (too clever ?) you could morph the ellipse into a sufficiently aerodynamic shape that all you have to add is a trailing edge ... we're talking a low speed plane, right ? (this is when you add "no, hypersonic").

350 bar is 5200 psi (ish), say a 10in radius cylinder, stress = 5200*10*FS/thk ...

so Ti (160ksi), FS = 2 (minimum), thk = 0.65in ... weight per foot = pi*10*2*0.65*12*0.16 = 78 lbs (wow). And this without any aero loads. (or insulation) ...

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[wewe2011]

So, the tanks are made with composites, and the H2 are non cryogenic, so insulation and stuff are not required. I‘m expecting maximum wing loading around 6750 N/m^2 including safety factor. Honestly, the tanks are someone else problem (it's already been determined), all i need to do is to think about the mounting in the wings, so that the tanks could take the aerodynamic loads, and allow the tanks to expand, with ease of disassembly in mind.

 

[GregLocock]

As you use the fuel up the spar will depressurise won't it?

Cheers

Greg Locock


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[IRstuff]

Honestly, the tanks are someone else problem, all i need to do is to think about the mounting in the wings, so that the tanks could take the aerodynamic loads,

THIS is how BAD implementations arise; you need to design both as a unified system. Otherwise, the tank guy might simply give you a steel tank that weighs more than the entire plane, and where would you be?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[wewe2011]

THIS is how BAD implementations arise; you need to design both as a unified system. Otherwise, the tank guy might simply give you a steel tank that weighs more than the entire plane, and where would you be?

The tanks are are made with composites, ca. 10mm thick.

 

[rb1957]

welcome to airplane design ... it is all about trade-offs. ok, someone else is designing the tank, and making decisions that optimise their project (like a cylindrical tank). In this case I think an elliptical tank will help you out (at having something that'll accommodate an airfoil).

Either way, have several fittings along the span, integral with the tank, or clamped onto it, that provide bosses which fit into tracks that run spanwise supporting both L/E and T/e structures. These will need to be quite sizeable structures as I don't think your tank is cantilevered onto the fuselage. The point seems to me that the tank takes some local aero loads and passes them to some structure (ahead and aft of the tank) that carries these loads into the fuselage.

Is there a reason you chose not to put the tank inside the fuselage ? "Yes, it was a team decision."

One problem I see is that wing deflections will force the tank to bend. Maybe segment the tank into smaller tanks distributed spanwise ?

As fuel is used, yes pressure in the tank will decrease, but not I think to a vacuum.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[MintJulep]

Engineers need to work with quantifiable requirements.

"...could take some of the aerodynamic loads." --> How much load? What directions?

"...mounting structure that accounts for expansion ..." --> Axial, radial, both? How much in each direction?

"...ease of disassembly is needed..." --> How much ease? How many people? How much time is allowed? What tools and equipment are allowed? How much space is available?

Once you have quantified information to work with, you can start evaluating design concepts, like " My ideas was just to place a rubber material under the bottom skin, and just let the tank come into contact with the skin."

So, Will a rubber sheet transfer the loads in the directions needed? Will rubber sheet accommodate the expansion with an acceptable force vs. displacement characteristic? Will a rubber sheet be easy to disassemble (and reassemble)?

As IRStuff has already pointed out, you and the tank designer should have discussed these points (and many others) long ago. "That's someone else's problem" is a crap way to run a project, but a sure way to have a project fail.

 

[rb1957]

if the gas is not cryogenic ... why the concern of thermal expansion ?

Is the tank graphite, and the wing structure Al ? Then there is a thermal question, but answerable in other ways than a "rubber sheet".

What temperature range are you looking at ? What thermal deflection ??

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

I think the growth is from the 350Bar pressurization swing. Getting magnitude for that expansion would be a big help, but I don't see a way to build a wing with that load path removed from it.

 

[wewe2011]

"...could take some of the aerodynamic loads." --> How much load? What directions?

Some if not all, additional structures can be added i.e. spars. But, main load that concerns is the bending moment.

"...mounting structure that accounts for expansion ..." --> Axial, radial, both? How much in each direction?

Mainly radial direction, due to the pressure swing.

"...ease of disassembly is needed..." --> How much ease? How many people? How much time is allowed? What tools and equipment are allowed? How much space is available?

What i meant by ease of disassembly is such that the tanks can be removed from the wings from the joints/connections point of view, so joints/connections like adhesives and rivets are out.

if the gas is not cryogenic ... why the concern of thermal expansion ?

Is the tank graphite, and the wing structure Al ? Then there is a thermal question, but answerable in other ways than a "rubber sheet".

What temperature range are you looking at ? What thermal deflection ??

Gasses is non crypgenic, but compressed at 350bar, due to this pressure, the tanks will expands.
Tanks is made CFRP, structures Al. But can be changed.
Tank gasses is at room temperature.

Expansion of the tanks is around 5 mm.

Thank you all for the insights and feedback, ya‘ll made me think like a better engineer.

 

[rb1957]

ok, I guess the "thermal" expansion was only in my head. Ok, 5mm expansion over what length ? what strain ? ans "not much", so why such a concern ? your wing structure should be able to accept this enforced displacement. If you have cable operated control surfaces then you'll probably have to get a little detailed over this. A far bigger concern IMHO is the bending deflections of the wing, and how these will load up the tank. A way to resolve this is to have several smaller tanks distributed spanwise, then the tanks can linearise the locally curved wing.

Wanting the tank to be removable means you're hobbling this loadpath.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[wewe2011]

5mm radialy, axial expansion can be ignored. Yes the tanks will need to be able to bend with the wing.

 

[rb1957]

"radial" expansion ?? why is this a concern ?? I assumed, from your original picture, that you were adding (structural) fairings to the tank; that you wanted the tank to take some of the airloads. Radial expansion may be a concern if you're going to encase the tank with wing skins, but then the tank isn't carrying any airloads, directly (sure, it may/will have loads driven into it by the deflection of the wing, but that is, in my mind, different.

5mm radial displacement, under pressure ? what radius is this tank ? I had guessed something like 10in (250mm); 5mm radial displacement is a lot ??

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.