Rigid, hard coating?

[BillClark]

Any ideas for a very rigid, hard coating that can be applied by conventional methods? the coating would also need to be fairly high in viscosity to avoid saturating the core material. The application is a propeller blade 7" long by 1" wide and .040" thick. To put the loads in perspective each blade produces about 25g of thrust at 1000 rpm. The current design uses 1 ply of spread tow unidirectional carbon fiber/ epoxy (.002" cured thickness) on either side with a balsa core in the center. this nets a blade that weighs 1.0g and is more rigid, stronger and durable than it needs to be. A coating(.002-.004" thick) with similar tension, compression and modulus of the laminate capable of the loads mentioned above would be ideal.I would assume if such a coating existed it would be very brittle and thats fine. The hopes are the end result would be a slight reduction in weight and a less intensive process for making them.

 

[Andrew1974]

I would use an Epoxy Mastic Coating. CM-15 Epoxy Mastic from Gulf Coast Paint is a good product we've used in our facility. Self priming, great for immersion.

 

[dik]

The epoxy might be the item. You do not necessarily want a hard rigid coating. I suspect most of your loading would be from cavitation and not pressure.

Dik

 

[Compositepro]

Nothing can compare to the stiffness to weight ratio of carbon fiber. But if, as you say, that is overkill for your application there may be alternatives. Saying epoxy does not really say much. Epoxies can be formulated to be rubbers or hard resins. Epoxy designed to be used as an adhesive or sealant will be very flexible and not suitable for your application. Laminating resins are much stiffer but still fall in a wide range.
Unidirectional fibers in a propeller is not nearly as effective as bidirectional. You need torsional stiffness as well as axial stiffness. I suggest you coat your core with a laminating resin and then lay on some 104 style glass fabric, which is only 0.0013" thick.

 

[MikeHalloran]

I'm thinking that epoxy mastic, sprayed over a styrofoam core, with no further reinforcing, might meet your specifications.

First, you might try just burning the balsa out of the cured composite shell.


Mike Halloran
Pembroke Pines, FL, USA

 

[BillClark]

Gentlemen, thank you very much for your feedback. It is greatly appreciated.
Andrew, these are air propellers. I am assuming from yours and Mike response Mastic is a rigid coating. I will give the CM-15 a try.
Dik, Compositepro,
I am using a Clear coat/laminating epoxy

https://www.systemthree.com/collect...cts/clear-coat-low-viscosity-laminating-epoxy

I selected this because I have experience with it and its low viscosity would make it easier to remove excess resin although I havent verified the low viscosity is a requirment. I have not tried any other "laminate specific" epoxy though. Perhaps I should try a different one. Any suggestions? I did try vinyl ester as its supposed to be stiffer but the solvents in it dissolved the binder in the carbon making a big mess...and it stinks. The resin content now is 30% and thats all thats left after I compress the top and bottom laminates in a press (individually) to 300psi then the core is sandwiched between the 2 halves in a matched mold at 100psi (compression molded). I have substituted the carbon uni for carbon veil and the carbon is definitely stronger. Torsional stiffness is good even with just the uni carbon. If I add any bias plies it becomes too heavy.I will get some of the 104 glass too and try that. It would be lighter at .5oz compare to the carbon which is .88oz. I am also looking into differnt core materials, possibly urethane foam (expanding type) as the compression strength of the core plays a big part. All of this being very weight critical. here is what the end result will be. the entire plane weighs 40 grams

https://www.youtube.com/watch?v=ygSjDsHkWYc&t=90s

 

[MikeHalloran]

That's a nice airplane, and a very skilled pilot.

The prop is obviously producing a little over 40 g of thrust in hover, and a fair fraction of that in level flight, given the AOA.

A former employer used a two-part Rustoleum industrial epoxy 'mastic' that may be rigid enough for your purposes. I was never able to play with a chip of it, because it wouldn't let go of a clean surface for anything, but it sure was tough to get out of a sprayer that was abandoned in mid-job over a weekend.



Mike Halloran
Pembroke Pines, FL, USA

 

[dik]

Sorry... I was thinking boat... not drone.

Dik

 

[MikeHalloran]

Could we please see some photos of the propeller at rest?


Mike Halloran
Pembroke Pines, FL, USA

 

[BillClark]

Mike, if you look closely you'll notice there are 2 propellers on the plane (contra rotating) to cancel torque and aero imbalance. Here is a link to my thread on rc groups showing some recent blades

https://www.rcgroups.com/forums/showthread.php?2784297-Bill-Clark-s-F3P-build/page2

mine have an adjustable pitch hub so I am making individual blades