Structural Considerations

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HeliDev

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Jul 7, 2003
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108
Hey Guys, Im doing some cad work no on my fuse for my helo and came up with a couple of questions reguarding the use of composites.
The way I see it I can do the fuse in one of three ways.
1) Monocoque
2) Chassis and panels
3) Combination of both

If I go for the monocoque constructon, what sort of crash resitiance can I expect? I know this will come down to the actual design, depemnding on load paths ect..... but how does a well designed monocoque structure compare with a metal plane? Also what about repaiars? As I understand it the strenght comes from the integrety of the structure, and as such how easy is it to make a good repair. Finally maintenace, how easy is it to monitor the airframe, generally helos have a much shorter airframe life than fixed wing, mainly due to the high vibrations of the big fan wizzing overhead.

The Chassis and panels is the one Im leaning toward right now mainly because I think that crash engineering would be easier. For example, the Apachie has two alumiium rails that run under the pilot and gunners seat, so that on heavy landing, once the gear collapses, they act like skis. It would also make the addition of a rollover bar easier, as well as mounting of engine gearbox, ect.... I also think it would make inspection of the airframe eaiser, and has the benefit if being able to have panels set for testing/repair, should the need arise.

Combining both I could see some benefits from each, although Im not really sure which parts would benefit from each.

Im sorry for the broadness of the question, and I realise theres more than one way to skin a cat, but any thoughts in this area would be greatly appreciated.
Im not really interested in doing a metal plane, I dont relly have much experience working with metal, as opposed to working with composites, plus Im pretty sure the composite frame will be lighter.
 

orion

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Your question is relatively broad since any structural configuration can be designed for a particular service. Monocoque construction is very good and can be designed to be very robust and crash-worthy. This is especially true for the case of composites, where you can take advantage of the materials' energy absorbtion capabilities.

The latter characteristics are also usefull in damping out the typical vibratory issues that you often see with helicopter applications.

It is the metal monocoque arrangement that actually has some issues of structural integrity in case of crash. You can look at a metal reinforced monocoque structure as being similar to a pop-can. I'm sure most of us have done the trick of being able to stand on an empty pop can without crushing it. But when you barely touch the side, the pop can will rapidly collapse upon itself.

Looking over many accidients of GA aircraft, the one thing that stands out to me is that many of the crashes seem to result in fatalities when the structural integrity of the "can" around the occupants fails. At that point there is little or no protection left and the occupants are either killed by penetrating obstacles or by the collapsing structure itself.

On the other hand, when you look at the accident reports for airplanes whose structure consists of sandwich based composites, you often find that the structure is capable of absorbing a tremendous amount of energy, without causing overall failure or any other significant structural collapse.

However the latter is also true for a steel truss type structure covered in a non-structural or semi-structural shell. After all, the crash survivability record for Alaskan bush planes, most of which are the truss type structures, is pretty good.

Basically, the choice in my mind boils down to the structural arrangement that you are comfortable building - the rest is an issue of design. Personally, I would probably recommend that you proceed with the sandwich composite since that structural configuration also has the superior characteristics when designing for the vibratory environment the helicopter will impose upon it.
 

wsimpso1

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For what it is worth, I think the same way as Orion. What do you feel like building your ship in? That is the material to design and build in.

Personally, I both communicate well in composites (what I envision is what I tend to get) and like that the structures are very energy absorbant while maintaining large scale integrity. Plywood airframes behave similarly. Sheet metal structures don't usually do so well due to local buckling, but steel tube truss fuselages have excellent records. One solution is to do a steel tube fuselage around the cabin and powerplant, then shell the cabin in composites or sheet metal, carrying that into the tail boom. This is done in all kinds of ships, NA AT-6, Mooney, Glastar, etc.

Enjoy.
 

HeliDev

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Joined
Jul 7, 2003
Messages
108
Thanx for the replies guys.
Gives me pleanty of food for thought. Im leaning toward the composite monocoque with some sort of roll cage for the occupants.
For helos dynamic rollover is a serious consideration, (in my opinion), and many helos crush like eggs if the go on their side, I also imagine it may help a little if the blade tries to join the occupants in the cab.
 
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