Composite Spray Booms for Helicopter 3

[DickRocket]

I am currently designing 15 ft. carbon spray booms to be used in the crop dusting industry. I have constructed a fiberglass mandrel which I will put 4 layers on carbon sleeve over and bag with nylon bungs every 10 inches (these will be tapped later for each spray nozzle). Here are a few questions:

1) what specific resin would be best for this object that is subject to both the hot summer sun and the possibility of freezing while being used in the fall?

2) The booms are a tapered airfoil shape (2.2" at the root down to 1" diameter at the tip). Is this shape a good case for resin infusion? it is somewhat difficult to lay up each sleeve and slide that far wet. Would I be giving up any strength going to an infusion resin?

3) After cure, I remove the mandrel and attach the root (most inboard) part to a 8 inch stainless steel insert that matches the shape of the boom and is slightly smaller so it will slide inside. Any suggestions on a good adhesive? Hysol? Also, do I need a layer of glass to prevent galvanic corrosion where the two meet or does the adhesive provide enough of a barrier?

Any input would be greatly appreciated and thank you in advance to the many knowledgeable guys on this forum.

 

[MikeHalloran]

My first thought is that the nylon bungs present an opportunity, to use them as infusion ports and overflow ports, if you insist on infusion.

My second thought is that the symmetry, shape, width, and length of the part scream filament winding.



Mike Halloran
Pembroke Pines, FL, USA

 

[RPstress]

Some fairly general comments that may be helpful:

Infusion resins tend to be untoughened and usually have higher strengths and glass transition temperatures than, say, prepregs. However, their lower toughness means that they do have a tendency to crack on cool-down from cure, especially with thicker sections. They also have much lower strength after an impact.

I'm afraid that I lack the experience to recommend a specific resin, especially if you're not going to go for an epoxy, but the ones usually used in my industry (RTM6, Cycom 890) are quite tolerant of high and low temperatures (they seem quite happy from -100°C to +150°C). Their high cure temperatures (180°C) gives the high temperature tolerance. Lower temperature cure epoxies such as those from Gurit (part of whom used to be SP Systems), often cured at about 60 to 90°C for things like wind turbine blades, are much less tolerant of high temperature, being limited to about 80°C.

Hysol 9396 two part paste adhesive is generally well regarded, though it has lower shear strength than a film adhesive.

If your stainless steel is good (austenitic such as 304 or A-286) it should be unfazed by the presence of carbon fibre, though not quite as unfazed as titanium. If a 400-series stainless or a precipitation hardening stainless such as 17-4PH is used, it should probably be protected with primer or at least sealant. Generally a layer of glass is only needed when attaching a carbon laminate to aluminium alloy or non-stainless steel or similar.

 

[zgzomb]

Do you have a curing oven and a good vacuum source available? Cytec's PRISM EP2400 is a newer low-cost one-component worth considering, call Cytec or check

http://www.cem-prism.com/.

Be very careful if you go with a wet hand layup with any resin as it will likely result in the worst quality parts especially with carbon (assume a very high scrap rate).

The use of a fabric (PW or a 5/8 harness satin) will let you drape and shape way better than a sock, you just wrap it with the necessary ply drops and overlaps built in based on your layup patterns.

If you are designing a sharp trailing edge, you will want to extend a portion for trimming and edge coating.

Also, a long,thin, tapered beam airfoil shape will usually involve ply drops of some fashion from the root to the tip. Why would you want to carry root bending stiffness out to the tip?

And remember that bolted and bonded is always best.

Good Luck, post a picture if you get a chance.

 

[berkshire]

Dick,
Rather than using a braided ply, have you considered a unidirectional pre preg cloth laid up diagonally one left ,one right, for torsional stiffness.
From your other post you now have an autoclave and should have no trouble curing a pre-preg.
As someone else mentioned, you can get a smooth external finish with nylon heat shrink tape wrapped around your part after you have wrapped it on your mandril.
If you go that way you could dispense with vacuum bagging.
You will however have to do some post finishing to get a totally smooth part.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[berkshire]

Dick,
Thinking about what you are trying to accomplish,
You may be better off with a female mold and an inflatable center.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[DickRocket]

I am inserting nylon bungs and want to try having them sandwiched between layers and not having to do any post cure machining or adhering.
I tried the shrink tubing but got bridging of the shrink tubing and really poor resin results around each bung. (also, since my part is tapered, the shrink tubing is reaching its shrink limit on the small end)
If I were to go the female mold route, i don't see how i could have the bungs work. they need to be in specific spots and I don't know how to keep them in place inside a mold and bladder system.
Am I wrong for being so set on everything curing together? my understanding is that this will achieve the most resilient part.

Also, i have been told twice now that i should consider using fabric instead of sleeve. I'm including a drawing of how the bungs are interlaminary and don't see how i could do that as easily with a fabric.

always open to learning
thanks, and here are some pics

http://i.imgur.com/ZQJc2.jpg

http://i.imgur.com/J10AF.jpg

http://i.imgur.com/vcNn0.jpg

 

[berkshire]

Dick,
If you use a female mold with an expanding bag and pressure molding, you could insert the bungs on tooling screws from outside of the mold. Since there would be no vacuum any resin leak would be to the outside of the mold, lay down your outer layers, screw on your bungs and insert additional plys of fabric to anchor them in place.You would drape your top layers out side the mold until your bag was in place then fold your other layers over the bag, close the mold then inflate.
I am presuming that these bungs are where you insert your spray nozzles,
and that these bungs have a tapered shape, so as to not present an abrupt change in direction of the carbon plies, also that they would be threaded for the spray nozzles, you would use the same screws to fixture.
I realize these are an awful lot of assumptions to make and I have been an ass before. I also recognise that you are making an extremely small part at the tip for this method.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[berkshire]

I should have read your previous post. Yes lay them in like your second diagram, chamfer the edge of the plug to lessen the angle or use a resin paste to avoid that direction change on the edge of the plug. That area is going to encourage bubbles in the layup.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor