Iron Dog

Just like when applying iron-on covering, the relief cuts make it much easier to overlap layers to better conform to the compound curves of the surface without puckers, wrinkling, or other air gap issues that would fatally weaken the structure due to separation of layers.

Iron Dog

After the .7 and 1.4 oz. layers were completed, I let the mold partially harden overnight.

Iron Dog

I the morning, the epoxy is partially hardened, yet far from fully cured. This means that it can be handled but is only very slightly tacky.

Ridges and fibers that wouldn't lay down totally flat were lightly sanded in preparation for the final layers.

Iron Dog

Next I applied a couple layers of 3 oz. tight-weave cloth:

Iron Dog

This is the plug after two layers of 3 oz. cloth.

Iron Dog

The last step is to build-up layers for strength and thickness in the mold. I am using 6 oz. cloth for this purpose. Tighter-woven cloth would likely be better; but years ago, shortly after I first started working with fiberglass, and long before he passed away, "Electric" Dave (SAM guys will know who I am referring to) gave me a monster-sized roll of loose-weave 6 oz. cloth. It worked well on the original production mold (pictured below), so it should work fine this time, as well.

I cut my thicker fabrics on a 45 degree bias to the weave of the cloth. This helps them better conform to complex curves. (Note that the roll is laid-out 45 degrees to the cutting mat and to the general orientation of the cuts.) Using the Olfa rotary fabric cutter, I cut 30 - 32 pieces of 6 oz. cloth - approximately 3" wide at the base and 2" at the tip. They are angled this way beings the leading edge of the cowl is less in diameter than the trailing edge. I cut them slightly longer than the length of my original cowl mold (pictured below) so that each would overlap at the center. (I forgot to mention/show this earlier, but the lighter-weight cloth layers were cut similarly.) This center is what is oriented in most of my pictures to appear as the "top." When production cowls are produced, this section is the open, front section that will be cut out, allowing the engine drive shaft and cooling air to pass.

But a thickly built-up part here is important so that when high-pressure compressed air is applied, this flat -- and therefore relatively weak -- section does not "buckle" and weaken under the pressure. When removing the mold, I will use a fine-tip pressure nozzle to separate the sides, but the front face tends not to want to separate without something to get air to this opposite end. The original plug has a piece of brass tubing passing through it from the bottom (or rear of the cowl) and then flush with the flat face. This acts as a nipple, to which I attach fuel tubing. This allows air to pass through to this section, finally allowing the mold to "pop" off the plug. I ensure this small hole is temporarily "plugged" by the wax, prior to applying PVA and initial tooling coat.

speedracerntrixie

My Feedback: (29)

Looks good Ollie, you may actually put more effort into your molds then I do. I have never had an issue with PVA and humidity. The only issue is when I got impatient and did not let it completly dry in a corner of two. Using slow hardener is always the way to go IMO. It does tend to yeild a better part but it helps solve another issue. To degas your resin you can warm it up with a heat gun and just watch the air come up to the surface. It will decrease pot life and that's where the slow hardener comes into play. The only thing I would question is the use of micro balloons in your surface coat. You could end up with some porosity in the surface. Don't buy any Cabosil though, I have a 5 gallon bucket of it and will gladly give you some. Keep up the good work bud! Soon I will be just a 1/2 hour away and we can start sharing knowledge, I'm sure there are some tricks I can learn from you too.

Iron Dog

I am VERY much looking forward to that, Shawn.

Thanks for the feedback. I'm so glad you brought up the heat gun technique, because in the past I've tried heating my resin, and it worked great on the air bubbles, but the significant cut in pot life was frustrating to me. But upon further reflection, I remembered that this was frustrating to me when I did it with larger batches . . . such as when mixing enough to do the initial layer for an entire fuse. While it wasn't worth it to me when doing larger parts -- especially beings larger batches already "kick-off" faster than smaller ones, it probably would have been ideal for small parts, such as this cowl mold. It's been so long since I've done this kind of work, I had forgotten some of the minor details of what I learned from my past trials and errors. Thanks for reminding me.

Iron Dog

Here is the first piece of 6 oz. cloth going on. You can clearly see the difference in the tightness of the weave.

Iron Dog

Most of the way around with the first layer. You can see the layers of 3 oz. in the middle, and the 6 oz. to either side.

Iron Dog

30 or so pieces later, I have gone around the surface 3 times with the 6 oz. cloth. Add to that: 2 times with the 3 oz., twice with the 1.4 oz., once, once with the .7 oz, had a chopped fiber and graphite layer, and a tooling layer of epoxy and graphite.

It is a very similar process when doing a larger part, such as a fuse; or, any other more complex part. The only difference is having to make a parting surface so that the part can be made in (usually) two- or sometimes more- pieces, to avoid locking-in - and potentially ruining - your plug due to undercuts.

I will let my cowl mold cure until next weekend, and will then attempt to successfully remove it from the plug. There is always a little anxiety in the waiting for me.

I ruined the original P-47 plug that I built, when I put recessed panel lines in it. I was never sure if some of the panel lines caused enough of an undercut that the plug locked, or beings it was my first (an resultantly the last) time trying gel coat which "alligatored", that the parting barrier became ineffective and the mold partially bonded to the plug. But because of this experience, and part of my P-39 fuse's fillets getting stuck, I'm never quite 100% certain that I won't be able to remove my plug without ruining it.

Wish me luck.

airraptor

My Feedback: (66)

ollie i will look to you for some help when i go to make some prop molds if I cant get anyone to make them for me. going to try ZZ props next.

Iron Dog

Jeff,

I'd be happy to help any way I can. But this is getting into an unfamiliar area, representing a whole new learning curve for me:

At the RPMs we turn, precision would be absolutely critical. Both in the plug and in balance of the castings.

I assume that glass-filled nylon is made by melting nylon then mixing in glass fibers? What would the mold need to be made from to withstand the heat of the castings?

This just begins the questions we'd need answers to.

Hopefully you can find someone who has a far greater knowledge in this facet of the hobby than I; or you have much that you bring to the process.

airraptor

My Feedback: (66)

well these molds will be for hand laid up carbon roving. there is a special epoxy/resin that is used for this. It would take about a day to lay up one prop but well worth it.

Iron Dog

In my limited experience with carbon fiber, I've found it difficult to thoroughly wet-out all the fibers to completely fill the weave of the cloth and get the fabric or tow to conform precisely to the surface of the mold, strictly as a hand layup. I expect we would need to vacuum bag them. This would also better regulate the resin content, and improve the balance of the blades. I have two vacuum pumps, but have yet to use either of them. Vacuum bagging techniques are the next intended evolution of my skill set.

speedracerntrixie

My Feedback: (29)

I just did a mold at work where I used a special high temp epoxy mixed with aluminum powder. The parts I made from the mold were bagged and cured at 350 degrees. There is some really cool stuff like this out there but is cost prohibitive. This stuff is only sold in 10 gallon kits. I've gone over the CF prop thing in my mind for a while now and always come to the same conclusion. The molds have to be CNC cut from billet aluminum and then nickel plated. This would allow the use of liquid release agents and provide the stability to keep tolerances tight. It's not very difficult just expensive.

speedracerntrixie

My Feedback: (29)

Ollie, we will get you up and running with your bagging equipment. Soon you will be bagging all your wings and tail sections. Soon you will be designing parts that can only be done via bagging.

Iron Dog

Yay! That's pretty exciting to me! I expect these techniques would further improve my productivity. I've already made a foam core cutting station, and it works good, so bagging these assemblies would seem the next logical step.


(The actual automatic cutting bow to this station is in my main shop, not here in my composites room. The cutting bows pictured are hand-held types.)

airraptor

My Feedback: (66)

Ok i found a guy that will make a mold for me for 250 dollars and then each prop is 18 dollars in carbon. save me some time now lol

Iron Dog

Very good, Jeff. Glad it was able to be worked out that way.

(It is rewarding to be able to do a lot of these activities ourselves, but I think you will be happier in the long-run to have that extra time available to invest in other ways.)

Iron Dog

Success!

The plug was successfully removed, and the mold was trimmed back. Afterwards, the exterior was rough-sanded to remove the many loose ends of the 6 oz. cloth that stick up and were poking into my hands like scores of little needles. (The after-effects of the sanding is why the external surface looks white and blotchy in the pictures.) The mold can now be handled comfortably, and will soon be waxed, PVA'd and a layed-up for a new cowl to be created . . . just as detailed at the very beginning of this build thread.

Iron Dog

So, these processes have come full-circle. For those who have asked my process for making a mold, I hope I have answered your questions.

Some have asked how I make a plug from scratch. That process I largely detailed on the "P-39 Anything" thread, with a direct link to page 7 accessible here:

http://www.rcuniverse.com/forum/rc-w...-thread-7.html

and beginning with post # 169.


Over the years that this thread has been going on, some other questions that I have been asked, regarding the building methods I've incorporated in this thread, were mainly from those who have yet to use a wing jig. A couple have asked how I design and scratch-build my wings, such as those used on both the P-39 and P-51; and a found the prospect interesting and specifically asked if I "would please explain how to build wings using a wing jig." Addressing this seems timely in that "Bulldog" just picked up a 2nd prototype P-51 "Speed Wing" laser-cut short-kit last weekend, has never used this method, and wants to start it soon; and, two others still have my 1st-prototype wing kits won as prizes at my VALLEY OF SPEED warbird race. From the feedback I have received, so far, none of those receiving these kits have jig-built a wing before. This may be a significant factor as to why they have delayed building it. As I will be giving three more kits away as prizes to contestants at this year's VALLEY OF SPEED II, on August 2nd at SAMs, and want everyone to have a rewarding experience with them, it would likely be helpful to explain this process sooner, rather than later. I can certainly do that, but think that it would be better to do so on a separate thread that these builders can very easily access and refer to as an on-line construction manual, rather than have it buried in this rather large thread; while hopefully simultaneously answering the questions for any others that want to know more about this construction method.

"007" was the first to finish one of the original four prototypes, and has been flying it for several months now. He stated that it felt "Immediately comfortable." Mine is built, but after mounting it to my "Galloping Ghost", I decided not to risk the damaged and repaired fuse, and have decided to use it as a new plug, instead. So I will take another minor detour from this thread in order to build the 2nd prototype wing and detail the process, then finish up my P-47s. Later this summer, I will make a new Galloping Ghost P-51 mold, and lay-up two new fuses. With luck, I will be flying these on two brand new "Ghosts" at my VALLEY OF SPEED II race in August.

The YoungGun

Ollie , Jim told me it was important to have washout on your wings. I don't really understand much about this, but he said it helps with landing. I thought this is the bernoulli's principle but i could be wrong. Another concern i have is also how much washout you have to have? He showed me on a p47 wing ,this has to do with the wing jig i'm guessing? Eric.

Iron Dog

Yes Eric. Building in washout helps to keep the wing from stalling. Every airfoil has has its own critical angle of attack (AoA). At this positive angle (which varies somewhat due to the type and relative thickness of each individual airfoil), proper airflow over the airfoil is disrupted, resulting in a stall. The reason washout is so beneficial towards improving low speed maneuvers/landing is that as a pilot flairs it is easy to exceed this AoA. If one of the tips stall first, that wing drops, and the plane could enter a spin; or when landing, result in a cartwheel, rather than just coming down harder on the mains. Washout serves to angle the airfoil at each tip of the wing downward (at a negative angle relative to the root), effectively reducing the AoA at the tips. This helps the tips maintain proper airflow to keep "flying." The wing stalls first at the root, and controls become "mushy" giving the pilot a warning . . . and hopefully at least a chance . . . to take action, before all control is lost, to improve the situation and hopefully avoid serious damage.

The wing that you and Jim built has my original, thicker airfoil on it. It is in the same "family" as the one on my newer laser-cut wing kits. But, because that design used a thicker airfoil, I did not feel the need to put quite as much washout in the tips. Because everything on this version was hand-cut, when I built these wings (such as on my original "Galloping Ghost" wing), I used a scrap piece of 1/8" plywood as a spacer under the rear jig rod at the tip section of the wing jig. This is a quick and easy way to add a very gradual twist to the wing ribs from root to tip, and build in washout into any wing. It has been awhile, but as I recall, this introduced somewhere between 1 3/4 - 2 degrees of washout. You'd have to talk to Jim to ask him if he used the same thickness of ply, or not. If so, yours has washout within this range. It is possible that he used a bigger spacer, to increase the washout.

My CAD-designed, laser-cut, wing short-kits come with a matched pair of laser-cut secondary jigs (I refer to them as "cradles") that hold both rods at the correct washout angle so when built as suggested on a wing jig, there is no variance from one wing panel to another. (My goal has been to learn from every mistake I have made over the last 20 years of building, and incorporate these lessons into improving my designs towards approaching making everything relatively "idiot-proof.") The cradle at the root is level; but the one at the tip is angled, holding the rods at 3 degrees washout. Because this new wing design uses a thinner, faster airfoil, I increased the amount of washout accordingly.

Does this help to answer your questions?

Ollie

Iron Dog

Perhaps this will help you better visualize what I am trying to explain:

This is a close-up the of the wing assembly that is clamped to the jig, showing how the cradle holds the rear rod near the tip only (on the left side of the picture) angled three degrees higher than the front.

When building the older design (such as on your wing, Eric, if you didn't see it while Jim still had it on the jig), imagine the cradles under the 1/4" rods are not there. On both ends, the rods lay flat on the aluminum surface -- except the rear rod only at the tip end of the jig would have a 1/8" piece of ply under it to introduce washout at the tip -- and the (very nearly) 1/4" thick ply plates at the top would be inverted so that the smaller plate would be on the bottom, allowing room to accommodate the rods, while still holding them snugly against the aluminum, without crushing them.


The top piece is the laser cut-sheet containing the two different cradles. One at zero degrees, the other cut to 3 degrees.