How to install hardpoints to honeycomb core panels?

[LSfan70s]

I already search the forum but did not find something that may help me with this request. If you know about a thread with this info please post it.

I want to know what is the best way to install hard points to aluminum core CF panels.

Like this:

http://dpcars.net/dp1v8/dp1044.jpg

Should I just bond them to the panel, or also use a mechanical holder?

My goal is to get the best in tension, compression and shear.

Thanks in advance for any input.

 

[RPstress]

There are a couple of common methods used in aerospace. The usual choice is between a 'pan-down' or an insert.

For a pan-down the core is removed locally and the skins ramped down and reinforced (including the ramp-downs) for the attachment, usually a fitting.

For an insert, 'microballoon blocks' can be used. These are usually room temperature-set epoxy with about 10 or 15% of phenolic or glass microballoons. This mixture is cast to a suitable shape to fit into the panel full depth and big enough to distribute the force applied such that the stresses will not fail the core at the block's periphery. Sometimes higher density core is used local to the block to keep the block size and weight down (extra money of course). Foaming adhesive is used at the block's periphery to make the bond to the core. Sometimes a machined Tufnol-type block can be used instead of making a custom block.

If the load is very low you could just attach to the skin with no reinforcement. A light fitting could be glued on or through-bolts used, perhaps with distance tubes. I've seen little clips bonded on for non-structural attachments such as light electrical wiring. As a rather (very?) 'low-rent' solution blind rivets just to the one skin could be used, but that's a bit shonky. I once saw that used for clips for a nacelle fire-wire, where a bond could not be used, since a fire might destroy it. Fluid ingress is a problem for this sort of thing.

I have seen some very fancy inserts used which were not full depth. This was to absolutely minimise block weight. I didn't think that the very small weight save was worth the complication, but the designers (Dornier) did.

Another alternative I've seen used for moderate loading is 'cotton reels,' basically a small cylindrical block for each through-bolt. Locating these in the panel with sufficient accuracy is usually a problem.

NB: larger blocks can be a problem because of disbonding from the skins. It's partly for this reason that pan-downs are common.

 

[LSfan70s]

Awsome! Thanks so much for the great information RPstress. Do you know what is a good book or a link abook where I can find more info on this matter?

 

[rerig]

The Hexcel web site has a wealth of info.

 

[SandEngXp]

Make you panels with medium density foam inserts and edge fill. This is lighter and requires less labor but requires some planning.

Chris

 

[KirbyWan]

Another method which works if you start with precured panels before cutting shapes out of them is to drill a hole and insert an allen key in a drill bit to spoil the core and then fill with potting compound like RPstress's epoxy compound with microballons. after it cures in place you can drill it out and bold support plates on.

And don't forget to electrically isolate the aluminum from the carbon fiber or you will get galvanic corrosion. This is true for both the core inbetween the CF and any attachments and fasteners.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.