P-51 - RC Groups
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Sep 13, 2017, 08:38 PM
Registered User
Build Log

P-51


Hello All,

I decided to do a build log on the P-51 I am trying to put together from foam blocks cut with my 4 axis CNC using Gcode generated by DevFus Foam. I have been putzing around with this for the last month or so and maybe if I do this build log it will make me commit to finish it, maybe not...

Warning! The fuselage is nothing fancy; no exhaust ports, no removable canopy, etc. I am not a skilled practitioner of this art! The end result will not be a jewel like those planes from the many excellent builders in the community. Just a basic fuselage that I hope will serve well for a reasonable slope plane (which I will promptly lose in a tree anyway). Also, this is only the 2nd fuse I've done this way so maybe you will see many issues with the approach, please comment.

Simply put I cut blocks of foam and assemble them into two separate halves of the fuselage. After final shaping I cover the foam with a layer or two of fiberglass cloth. This serves as the mold halves for the fuse layup. The rest of the build is basically like other builds except the wing will have a kevlar LE, not basswood, and be covered in several layers of fiberglass and no TE stock will be used and it won't be thinned (OK, so it's not basically like other builds...). Hey, but it will be an RG-14 (unless it's not).

Regards,

Eusebio
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Sep 13, 2017, 08:38 PM
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Sep 13, 2017, 08:43 PM
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Here's the process so far (this will continue to be a bit wordy but should be mostly pics after a few posts, I hope).

First I found several nice 3 views of the P-51, some with fuselage cross section formers, on the net. I took the drawings and put them into GIMP and scaled everything to a 38 wingspan although I'll likely cut a 48 wing for this fuse. The P-51 seems to have a narrow fuse but is tall enough from the top of the canopy to the bottom of the intake under the wing so I also 'squashed' down the height to about 90% scale but left the width scale.

The parameters for the fuselage are:

Length: 33.5 tip to tail
Width: 2.75
Height: 6.25

Once I generate scaled drawings using GIMP I can import them into DevFus Foam to generate the Gcode and cut the blocks of foam for the fuselage. I like DevFus Foam; using a good 3 view you can build up a fuselage and then generate the Gcode to cut the blocks of foam used to make the fuse in a relatively short time. If I'm not too fussy about the end result I can do this in about 3-4 hours. Also, if you have a drawing with cross section cuts along the fuselage these can be inserted into the 'drawing' and used to help shape the fuse. Both wing airfoil and tail airfoil section cuts can be cut in the foam blocks and the angle of the airfoil cut can be chosen relative to the datum line of the fuselage. A feature I really like is that you can cut the blocks in 'left' and 'right' halves so that once assembled you have two halves for the fuselage. This is what I use for the layup.

Some pics of the 3D model of the fuselage are shown below. You can see from the pics that when I draw in the fuselage boundaries I usually make them larger than the outline on the scaled drawing. The reason is that I try to minimize the number of foam blocks I have to cut (I'm lazy) and this causes obvious problems, especially with contours (look at the canopy section), since the cuts are all just straight lines.

When I'm happy with the basic shape of the fuselage in DevFus Foam I generate the Gcode for left and right halves and then do the airfoil cuts. After the foam blocks are put together I just sand down the fuselage and try to get it to match the 3 view profile by using a template.
Sep 13, 2017, 08:51 PM
Registered User
I already have the fuselage halves for the mold built out so I am going to start the pics from there. Unfortunately I didn't take any pics of the separate foam blocks, maybe next time...

The foam is HL 60. It cuts and sands well enough and is resistant to the usual dings that occur during the build so I have less work to do with filling / sanding.

Once the foam blocks are cut and assembled, using 5 minute epoxy, I usually fill in the joints at the blocks with light weight spackle then do the usual: sand down the surface of the fuse some and then use a template printed from the scaled drawing to try to shape the fuselage halves. Not much sanding is usually required. Once I think the shape looks OK I lay down a layer or two of fiberglass cloth and wet it out. I used 2.74 oz balanced cloth from Thayercraft (it is very tight weave and looks and feels almost like muslin) cut on the bias.

I let this set overnight and then make whatever corrections have to be made to the fuselage. If I did a good job of shaping then usually only the nosecone tip needs some TLC. I then cover the part with a thick layer of water based polyurethane mixed with talcum powder (hate bondo). This fills in the weave and after it dries is easily sanded. The end result is usually a smooth surface that I can wax and put some PVA onto. It will likely take paint or primer but I don't bother since the outside of the mold is the inside of the fuse layup and doesn't have to be so smooth.

I didn't put any fillets or vertical tail onto this fuse although I did do this for my first attempt at a fuse using this technique, a P-40 shown in the pic.


That's what I have so far...The rest may be slow going!
Last edited by ek123; Sep 13, 2017 at 11:39 PM. Reason: Added caption to P-40 pic.
Sep 13, 2017, 09:34 PM
Faster is Better
Slopemaster's Avatar
Eusebio,

Looking good dude!

Can't wait to see it fly.



Ken
Sep 13, 2017, 11:04 PM
And Repairs
TyFlies's Avatar
Thanks for taking the time to do the build log and post pics. Good luck with the project!
Sep 14, 2017, 01:17 AM
Tail dragger
grrttmyers's Avatar
Ya I'd say it's looking pretty good dude
Sep 14, 2017, 02:26 AM
It's not going to build itself
TRISME's Avatar
Heck ya, Go Eusebio Go!
Sep 14, 2017, 09:04 AM
Dunetop Flyer
jcarstan's Avatar
Nice to have a "different" type of build thread here. This CAD and CNC stuff is interesting.
Jan
Sep 14, 2017, 03:34 PM
Aerotow and sloping Holland
MaartenX's Avatar
Good looking plug you have there Eusebio. Have fun with the build and keep making pictures
Sep 14, 2017, 03:42 PM
Registered User
ian murdock's Avatar
Looking good!
Sep 14, 2017, 06:13 PM
Faster is Better
Slopemaster's Avatar
I wish I had the drive and motivation to take on projects like this, but I guess I'm lazy, I just build kits.
Sep 14, 2017, 11:39 PM
Registered User
First of all, thanks for all the positive comments and the encouragement. It is greatly appreciated.

I did some more work on the fuselage layup today. It seems to me one of the (many?) drawbacks of using a male mold is that you have to use single pieces of cloth that cover the complete mold unless you don't mind bumps in the layup. I don't think this happens when you use a female mold as long as the first piece of cloth you lay down is a single piece.

The layup I did today is single piece of 6.3 oz and single piece of 2.74 oz cloth both cut on the bias. I will let this set overnight then tomorrow I will pop off the layup (hopefully!) and then complete the layup with additional cloth pieces from my remnant box. A bit of a pain but I want to see how it turns out.

The pics are not terribly instructive, just the usual. I use US Composites medium epoxy after seeing wyowindworks very nice study on various epoxies:

https://www.rcgroups.com/forums/show...S-vs-RR-vs-MGS

One note, I wet out the layup with the mold laying on a flat surface and there is usually plenty of cloth overhang (you can see this in the first pic). I found that once wet out the surface tension pulls the cloth away from the mold a little bit where the mold meets the flat surface. If left untouched this would result in rounded edges so I trim the cloth and then put the mold on a support and then I smooth down the cloth so I get a good edge.
Last edited by ek123; Sep 14, 2017 at 11:44 PM.
Sep 15, 2017, 10:18 PM
Registered User
The layup came off without much of an effort but it is very, very flexible as expected. After I popped off the layup I put it back on the mold and sanded the excess cloth that hangs over the edge of the mold. I think I should sand first then pop off the layup... I added some more 6 oz cloth and also some 3 oz cloth from the nose to just behind the canopy. I washed the fuse layup in hot water to get any PVA off and then roughed it up with sandpaper. I rewaxed the mold and then wet out the cloth on the mold and then debulked in vacuum ( I think I forgot to debulk one half). I then pushed the fuse onto the wet out cloth and mold and stuck it in a vacuum bag. Wait for tomorrow to see what it looks like.

Note to self: Who cares about using remnant cloth pieces! Not worth the extra work. Don't do this again.
Last edited by ek123; Sep 15, 2017 at 10:25 PM. Reason: Added info
Sep 16, 2017, 05:29 PM
Registered User
Pulled the layup off the mold pieces. The parts are very stiff, the edges are straight and square except in a few places but there are no really big problems. Generally, the two halves butt together very nicely. I think the fuse will be easy enough to seam together.

The weight of the two halves is 6.9 oz. I'm not sure if that is a reasonable weight or not for a plane this size (feels stiff though!).

The fuse halves don't have that 'Oh, so nice' look that you get with a well made female mold (at least from the build-log pics I've seen). Partially that's because the polycrylic/talc coating comes off the mold, especially in the high spots and sticks to the inner surface. (I don't spend a lot of time sanding the coating down to a really smooth finish. In fact, you can usually see the brush strokes even after I sand). Also, on one side I didn't debulk the layup and it leaked through pinholes in the original fiberglass fuse layup I put over the additional cloth and bagged. I have some sanding to do.

Next up: Reinforce some areas with splooge, cut out the wing saddle and seam up the two halves. I Also have to work on the intake assembly since I haven't done anything on it yet.


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