CARF Pitts 12 Build Thread

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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Wed Jul 17, 2019 3:49 pm

I also found time to mold up some carbon holder for the coke bottles I will be using as fuel and smoke tanks. In the end I decided to go with a 1.25 litre fuel tank and a 1 litre smoke tank.

I debated whether a larger 1.5 fuel tank should be fitted, but in the end decided to go with something just a bit smaller (to save weight on fuel being carried), as apparently the Moki engines are pretty frugal on fuel.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Wed Jul 17, 2019 4:01 pm

So, as of yesterday I'd made enough progress with the build that I was able to put the model together and do a test balance. Not yet posted on here are the progress with the wings and stabs. Both are basically done with the servos and linkages fitted.

I had planned to fit some JR servs I have to the ailerons (1 servo on each) and use the new Futaba A700 servos on the tail surfaces. But, after a moment of absent mindedness I fitted the Futaba servos to the wings and by the time I realised, its was kinda too late to take them out (the JR servos are a bit taller and would have put the output arm in the wrong location) :doh:

So, the better part of £800 later I'd got another 4 of the Futaba servos for the tail....

I'll try and remeber to put some images of the winmgs and stabs up here when I am next posting. Also, regarding the stabs, CARF have modified the Pitts 12 design so there are now hatches in the stabs allowing for eay mounting of the servos there. Whereas previous the servos were mounted in the fuselage and drove the elevators using very long 10mm carbon pushrods.

Evidently, with the progress made in servo design its now easily possible to drive one of these surfaces with a single servo, hence the stab mount option that has now been added.

Coming back to the test balance, not unexpectedly, with the 'weighty' Moki 300 up front, the Pitts was very nose heavy. To cut a long story short on achieving a good CofG, the Powerbox is going to be mounted just behind the canopy and the two RX LiPo batteries just in front of the tail former (at the front of the fin). The Pitts has a short tail and with all that engine up front, its needing weight distributed a long way back to compensate.

Anyway, here are a few images of the assembled model, without the flying wires. And, also a list of the remaining jobs I have to do. All being well I should be finished next week and ready to fly at the start of August, once the LMA/CAA over 20kg paperwork has been completed.

B.

Cut the spinner
Secure ignition leads
Make up ignition switch lead
Fit choke servo
Fit ignition switch
Fit Velcro around the smoke tanks
Fit fuel filler
Fit fuel filter
Plumb in fuel tank
Plumb in smoke tank
Fit RX batteries
Fit ignition battery
Mount the Powerbox
Solder EC5 connectors onto the PB leads
Sort out the elevator servo hatches
Order steel rudder servo pushrods
Make up and fit all servo leads
Fit tail wheel wires
Secure all pipes and cables
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Re: CARF Pitts 12 Build Thread

Postby steverobertsuk on Fri Jul 19, 2019 9:13 am

Looking good Stuart- cant wait to see it at the field.

Steve
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 1:53 pm

Thanks, Steve

The first outing with this should be too far off now. The Pitts is basically finished. Its just needs the canopy fitting and is back at M&R now, as Dave kindly said he'd do this for me. At the same time its being inspected for the LMA over 20kg scheme.

I was surprised at just how straightforward the build turned out to be - - it went much faster than I expected.

I spent quite a bit of time reading the (very long) CARF Pitts 12 build thread on FlyingGiants and saw some changes ot the design when mine arrived. This included the addition of servo hatches in the stabs allowing the servos to be easily fitted there. Previously other owners of this model who were using large engines and thus needed the elevator servos in the stabs, we having to make servo cutouts and hatches themselves, so it was nice to find my model with this update and a few others thrown in.

The weight of the Pitts has come out at 23.5kg. This is about 2kg heavier than this model with a lightened DA200, with all of the additional weight just being due to the Moki 300 - Its a big lump of engine!

Anyway, I'll now spend a bit of time getting caught up with posting some more of the build images here, which I continued to take along the way.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 1:58 pm

Here are a few images of the Powerbox install and the final location of the receiver batteries. With all that engine weight up front the batteries had to go in just head of the fin former in the tail. I used a pair of Secraft battery beds to mount the packs , cutting out a pair of ply formers to screw them to, which were then glued to the fuselage sides with Hysol. With some surgery the ply formers cam out pretty light at 6 grams each.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 2:09 pm

Here are a few images of the engine mounting, throttle servo, tanks and smoke pump.

Starting with the engine I'm very pleased with how the mount came out. I'm sure I could have made something lighter, but with all that weight and power I just decided I'm better off with a nice solid arrangement. I have used titanium bolts, nuts and washers to mount the engine, which saved me about 100 grams over the steel equivalents.

The tanks are Coke bottles with Secraft cap fittings. The carbon holders I molded up are glued (with Hysol) straight to the tank tray that comes fitted to the Pitts. The tray is made of a carbon-form-carbon sheet - very light and easy to work on. Just was well as I had to Dremel a couple of bits of it away up against the engine firewall, to allow the washers and bolts to be added to the engine mounting bolts.

The throttle servo is mounted to the underside of the motordome, again using a Secraft product. The motor dome on this model is designed from the get go with an opening for headers and pipes, which for anyone using a two/quad two stroke will make the exhaust system install much easier than normal (no surgery to the motor dome required to make header openings).

If you take a look at the image of the firewall, you can see I cut a scoop into the bottom/right hand sise of the circular opening. It was to allow me to run a pushrod backwards out of the carb to a servo, for operation of the choke. However, later on I realised that I could drop the servo and just use a carbon pushrod for a manual choke. In this case running the pushrod sideway through an opening in the mount and a hole in the side of the fuselage. I'll post images of that later. As a benefit I was able to save the weight of the servo etc. up front, on a model already nose heavy.

Again, with an eye on nose weight, I decided to mount the smoke pump the UC mount/former within the fuselage. Usually I always mount smoke pumps outside of the motor dome, so if there is a leak it won't ingress within the fuselage. However, in this case I decided I'd rather get the weight further backwards, hence picking a point that is very close to the CofG.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 2:19 pm

I opted to mount the Powerbox switch and ignition switch (also a Powerbox item) at the rear of the canopy opening. Ordinarily I would fit the ignition switch in the nose, ahead of the wings. But, standing at the front of the assembled model I was found there wasn't much distance between the leading edge of the top wing and the back of the prop. I was thus concerned that if I need to get into that space with the engine running, to access the switch, I'd be in a tighter space than I would like, with that prop whirling away (not something I want to get 'nipped by...!). So, the decision was made to position this behind the wings as explained here and show in the images.

When making up the ignition lead, I used along length of heavy duty Powerbox wire, which was then soldered to an MPX connector for connection to the Moki ignition unit. I'm always wary of soldered wires fracturing from vibration, so I ended up potting the wire & solder connection with Hysol. A pretty easy job to do. I just put a double wrap of masking tape around the connector and then injected Hysol into it using a syringe. I forgot to take a photo of the finished item, but I can explain that the wire and solder join is fully encapsulated at the point it joins the MPX connector. I don't ever see the solder joint failing.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 2:23 pm

I just found an image of the manual choke. Pokes out of the right hand side of the fuselage, just above the bottom half of the nose. Very simple and protected from vibration with a rubber grommet added into the fuselage side.

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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 2:26 pm

The Moki exhaust required that the exits be extended to reach outside of the cowl/fuselage bottom. I initially looked at using lengths of flexible extension pipe, but couldn't get a descent bend to locate the exits where I wanted them. After a bit of research it turned out that Zimmerman make bespoke extensions and so on similar to in the image below was organised.

B.
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Re: CARF Pitts 12 Build Thread

Postby Billy Bunter on Sat Jul 27, 2019 2:29 pm

Here are a few more images of the assembled model. I do plan on putting up some other interesting images, but need to get the model back here to take them. So, more to follow next week.

B.
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