Tyres and canopy

[David Glen]

Hi, everyone
As a new member I feel something of an imposter, since I build static scale models with all the aerodynamic qualities of a brick. However, I do build big and some of you may have seen my ca. 1/5th scale Spitfire which is on permanent display at the RAF Museum at Hendon.
My current project is a P51-D destined for Washington DC, and it is in the final stages after seven years in the building. However, I have one or two major challenges yet to overcome, which is the purpose of my message:

1. Can anyone advise me with regard to the best technique (materials, etc.) for pattern making and moulding rubber tyres. I think they will have to be solid, since I suspect that my model being largely of metal, is not far off scale weight!!
2. I would also appreciate advice on the best way to tackle the bubble canopy. I have the original drawings, but I know enough to appreciate that a poorly finished pattern means a poorly finished product, no matter how good the vacform process. I also appreciate that if the end result is not close to being optically perfect, it could compromise the entire model. I have a commercial company to do the moulding but thay are leaving the pattern to me.

Anyway, thanks in anticipation. As a lifelong model maker, I don't know why I haven't joined LMA before now. From what I've seen, the technical wisdom on the forum alone is worth the fee.
If anyone is interested in seeing my models, my website is www.spitfireinmyworkshop.net
Cheers for now
David Glen

[Joshua Henderson]

Well them planes are horrible;-)
I'm not jealous honest;-)
If I were doing it then I would draw a 3d model on computor to the exact shape and get the mould cnc cut
Then vaccuum form it, as I believe that is the most accurate way
Where as the wheels I don't have a clue
By the way they are some of the nicest models I have ever seen, you definately very talented
Thanks alot
Josh
Ps keep the pics coming

[Stuart Solomon]

As for the tyres, do what Mick Charles and Dave platt did for scale detail. Make an original from whatever you like, for the lettering use six or seven layers of Letraset and then take a plaster of paris mould from this. When the plaster has dried completely, spray it with a good release agent. Dunlop 'Flexane' would be an ideal material for tyres, this is what Platty has used, depending on the grade it is also used to repair the tyres on fork lift trucks. I think that you get some release agent when you buy the pack of Flexane. Hope this helps.

[David Glen]

Hi, Stuart and Josh
Josh, the cnc approach looks spot on for the sliding hood and for the Hamilton Standard cuffed prop blades! Arthur Bentley might well be able to do the CAD work for me. Thank you.
Stuart, do you mean Devcon Flexane? Plenty about that on the net. Again this looks very promising. Do you know where I can access more detail about the Mick Charles Dave Platt technique? You mention Scale Detail?
Thanks again
David

[Robin Woodhead]

Have a look at these vids on YouTube, they are in German but are about Flexane.
http://www.youtube.com/user/Avteh#p/u/0/n4_zFcFGuWc
Hope these are usefull
Robin

[Stuart Solomon]

Dave, knew it began with a 'D' doh. Dave platt did a series of softback books on how to do scale detail. I'm sure I saw it in one of these, not sure which one. He went through the pattern, moulding and casting process in one article which was extremely informative. If I come across my copy I will certainly let you know. Will have to start looking now Try googling Dave Platt or American Scale sites may be able to give you more details. Just remembered, the book was published by RC Modeller mag in the states. Cheers

[Phil Clark]

Dave

If you don't want to go down the 'machined CNC tool' route for the canopy, have a look here....................

http://www.rcscalebuilder.com/forum/forum_posts.asp?TID=11359&PN=1&TPN=50

Starting on page 50 of the RCScalebuilder thread (with a short break until part way down page 53), I document how I produced tooling for a 1/4 scale Sea fury canopy. Hope it's of use...........

Phil

[David Glen]

Phil
This looks exactly what I'm looking for! I discussed the cnc route with Arthur Bentley (the aviation draughstman) this morning and he is snowed under with work so I'd have to wait a long while for a cad model, but he did send me a full set of sections for the P51-D hood. So at least I now have the drawings I need.
I tried to register with RCScalebuilder as you advised, but must have messed it up, because it won't let me in, or re-register!!
Is there any other way to access your documented material.
Best regards
David

[Phil Clark]

David

I ran a duplicate build thread on the UK based RCMF forum.................some of the detail was lost in a forum crash a few years ago, but all of the details relating to the canopy are still there (no need to register to view this one....sorry, I forgot about that with RCSB )

The canopy tool production starts in post # 216 part way down this page.........

http://www.rcmf.co.uk/4um/index.php/topic,51403.200.html

Cheers

Phil

[David Glen]

Phil, thank you... You're a star!!

I think I can handle this OK, and have taken the notes and pictures to read and ingest. One thing occurs: You say that the mould you made was for production runs. What are the key shortcuts if I just want to make one pattern, or tool? I have no need for more than one canopy for the model with, say, two held as spares. And since the P-51D hood has no undercut (I'm not using the blown version), could I get away with a one-piece rather than a two-piece laid-up mould or would extraction prove a problem?

David

[Phil Clark]

Yes, the tool produced was for production use (I'm the European agent for Jerry Bates plans).......and is still in use today having had I'd estimate 15-20 pulls taken from it over the past 12-18 months.

Vacforming is an issue as the material used for the tool really needs to be free of any grain. I've tried 'quick' tools in the past from glass skinned balsa and pine and as soon as they get hot, the grain lifts and is transfered to the inside of the clear moulding which as I'm sure you'll know looks terrible. I've tried using multiple layers of a much heavier weight cloth over the timber tools followed by plenty of rubbing down etc....same issue, as soon as the cloth heats up, the weave starts to show through and the tools surface is no longer smooth. The use of a 'resin' tool which has no grain/weave texture & is also thermally stable avoids this issue and gives good results every time....only issue is it's very time consuming.

The only other method I have used (which was good for 2-3 pulls before the tool started to break down) was the use of 'Therolite Turbo' building blocks covered in P-38 body filler. The blocks are airated concrete and very light. Cut with a panel saw & shaped with a surform to approx 1/4" undersize all over. You then 'plaster' them in a thick coat of P-38 and sand to shape. This does work as the P-38 is grain free, but the tool ends up pretty delicate and quite easily chipped. You also have to remember to seal the underside as when forming and 'blowing' the PETG to stretch it before pulling down, any dust on the underside of the tool (in the porous grain of the block) will be blown out all over the inside of the plastic.

I can't see an issue with a 1 piece tool for a non blown P-51 canopy. As long as the plug is well waxed etc....the GRP tool should pop off fine. When pouring the resin, is shinks very slightly when curing so releases itself so once fully cured and cool, it drops straight out of the GRP tool. If you do go down this rote, try not to have much over 1/2 - 3/4" of thickness in the resin as when curing, anything thicker than this will generate A LOT of heat.

1/5 Hellcat & Bearcat tools recently produced using exactly the same method as shown on the Sea Fury

Phil

[Mike Booth]

David, whilst Phil has showed you perfectly the route for producing canopies, the only way you are going to get the clarity you are after, is to blow the PETG in a frame man enough to take pressure.
Not too difficult to make up and will be very similar shape to your aluminium P51 frame.
The only reason I mention this is that even in the best enviroment it is impossible to stop fine particles being trapped between the master tooling and the PETG.
There will always be small imperfections with this process, its just a case of what level of finish you require.

[David Glen]

Thanks, Phil
I can see that it makes sense to go the extra mile. Who knows, some of the guys on this forum might even want a 1/5th scale P-51D hood!!
I recall in my own local model shop recently, someone talking about pulling a rubber balloon over the master before mould making, but I can't imaging that working with a glass laid mould.

Mike, Yes, that's a good point. I can imagine contamination being a problem. And I seem to remember Rob Millinship describing blowing the hood of his circa 1/6 scale DB Spitfire MkII, which I drooled over at the Model Enginner Exhibition some twenty years or so ago. As I recall he got a gold medal for his pains and the hood was pristine. When I built my Spitfire Mk I I cheated blatently and put a flat hood on it, on the grounds that some of the very early machines had flat hoods.... What a cop out!
Hmmm.. this one's a real headach for me. There's so much of the clear stuff on top of a P-51D that even small imperfections could screw up pretty well seven years of work, and I don't want that. There's no denying though, from the photographs, Phil's finished product looks the bees knees !!

David

[Mike Booth]

True David but no picture on a forum will show you the imperfections that vacforming will surely bring.
Its worth considering that you are only building a P51 to that standard once and only need one canopy that is right.

[David Glen]

Mike, can you advise where the frame blowing method is described, so I can check it out? And how would you be sure that the finished article conforms to the true contours of the original in the longitudinal and transverse planes?
David

[Mike Booth]

David here you go, pictures tell a thousand words, the most important point is that the base of your frame conforms to the plan view of the canopy frame.
I am asuming that you are only going to blow the opening portion of the hood, so you would also make the vertical frame section as well as the rear horizontal, that way you will end up with something that will fit the forward screen when it is shut.
I am also sure that a chat with the people at the UK link will bring forth some assistance.
Regs Mike.
Copy and paste link below.

http://autospeed.com/cms/title_Custom-B ... ticle.html ...........for pictures of 'How to'

http://www.ampcomouldings.co.uk/

[Phil Clark]

Imperfections on vacformings are far from imporrible to avoid................

1) Find a GOOD vacform company, ideally one who does a lot of model work as they generally know the clarity we are after, a regular industrial vacform company who makes endless chocolate box inserts are a complete waste of time. If you need pointing in the right direction, let me know.

2) Leaving a clear pull on the tool and having a 2nd pulled over the top can also improove clarity.

Good luck...............

Phil

[David Glen]

Hi, Phil I have a company recommended by Eddie and Judy Stocker at DB Sport and Scale, but I'm more than happy to be quided by your experience.
Mike's point is a good one. Blowing the thing would clearly circumvent any issues of this kind, but I can't see how I could do that in this case: The early P51D hood (not the later bubble version) is flush laterally with the canopy frame - ie in the same plane and with no discernable bulge - a bit like an inverted and splayed U shape. So I think vac forming is my only route here.
David

[Mike Booth]

Pressure and an accurate frame David, will control shape.
They were all blown, Spit's Mustang's Tempest's.

Best of luck lets have a look at the results when you have done it.

Rolly at Sarik vacforming in Bristol does just about everything thing thats ever been vacformed in the model industry.
0117 9605402. They will easily manage a 1/5 mustang .

[David Glen]

Good to hear Sarik, Mike. That's the firm recommended by Eddie Stocker, so now I'm certain I'm in good hands. I gave Sarik a call a month or so ago to discuss this and they were very helpful. I'll certainly keep you informed of progress.
David