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Old Jan 01, 2014, 06:31 AM
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Norfolk, England
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Build Log
Scale Build-off 3 - 45" Polikarpov PO-2

This will be a joint effort with me doing the drawing and Pat Lynch doing all the hard work of getting the model built.
I've been threatening to draw up one of these for years and this comp, together with some prodding from Pat, has provided the impetus to finally make a start.

Pete
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Old Jan 01, 2014, 08:26 AM
North East England
Joined Feb 2004
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Just looked the aircraft up to see exactly what it was like - l thought it would be a monoplane (only kind of Polikarpov I know) but what a nice bipe it is. A quick Google image search shows every colour scheme from all-white to various camo's so plenty to pick from.

Will you be drawing it with both wheels and ski details? The versions with skis are particularly attractive and would work well at 45" size.

Look forward to the design and build.

Steve
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Old Jan 01, 2014, 09:04 AM
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Although my design techniques are nothing special, I don't do 3D or technical stuff like that, I'll attempt to go through it more or less as it happens. No, don't panic, it won't be a line by line description of drawing a plan.
The idea with this one is to try to get away from the need for laser cut parts. In other words, while the parts will ultimately become available, I want to try to keep the construction simple enough that they aren't essential for a successful build. They'll make life a lot easier, but won't be an absolute requirement.
Over the last few models I feel I have come to rely on the accuracy of CAD and the availability of laser cut parts in my designs. This, to some extent, has lead to quite intricate builds with precisely fitting parts. On this model I want to go back more towards my designing roots. Those early models were all designed to make cutting parts by hand relatively painless and I'd like to make this model the same - but still retain scale outlines, rib count, etc. What it won't contain, if I can avoid it, is lots of intricate ply formers and parts that need to be precisely cut if the thing is to go together at all.

So far all I've done is enlarge the 3 view to model size, trace off the basic fuselage details and start tidying that and filling in construction details. I'm aiming for a relatively lightweight model (around 28 ounces) that will be powered by a 450 size outrunner and have 4 function control. Control linkages will (hopefully) follow scale practice and all be external.

Pete
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Old Jan 01, 2014, 09:32 AM
AKA Jon Rider
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This will be fun to watch! -
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Old Jan 01, 2014, 01:07 PM
71% of the world is runway . .
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Nederland, ZH, Oostvoorne
Joined Jun 2009
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Good luck to you both !

Cheers,

Bart
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Old Jan 01, 2014, 02:25 PM
a.k.a Maltone
Pat Lynch's Avatar
Australia, NSW, Goulburn
Joined Jan 2005
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Quote:
Originally Posted by Shnokey View Post
This will be fun to watch! -
Hopefully it will also be fun to build

The PO-2 is typically Russian of the period - fairly straightforward, no frills and seems to have been a fairly tough airplane. I liked it from the start with its 5 cyl radial all exposed up front, external linkages to the tail, general no-nonsense construction and a wealth of available data. As Steve stated - there is an almost unlimited range of schemes, although I'll probably opt for a common camo version. My input to the project so far has been to request the type, supply some 3-views and discuss scale, motor, battery size and hatch placement. From here on it is mostly up to Pete to wave his wand over the 3V and produce something buildable, and more importantly, flyable Just how much detail will be included is unknown at this point - as usual it will depend on how the design pans out.

I'm pottering around on existing projects to keep the bench clear(ish) for when the s..t hits the fan (or PO-2 prop). I hope, in addition to Pete giving ideas about how the design involves, we can see some interaction between designer and builder as the task progresses.....should be fun

Pat
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Old Jan 01, 2014, 08:29 PM
a.k.a Maltone
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Australia, NSW, Goulburn
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A Picture..

For those who don't know what a PO-2 looks like - this is typical.

The aircraft was designed in the late 20's and continued in service beyond WW2 - estimates of the number built vary between 30-40,000! - amazing for an aircraft not so well known!

Pat
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Old Jan 02, 2014, 03:02 AM
This is a fine fiddly business
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I love the PO-2. What a beauty. Pete and Pat, this will be really fun to follow.
Robert
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Old Jan 02, 2014, 08:40 AM
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Norfolk, England
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Well, although still very basic, and with things I want to change, this is where it stands at the moment.
After discussing pack size with Pat I've moved it back to above the u/c and between the rudder servos. With a 3" wide fuselage and micro servos hard up against the sides that should leave enough room to get the pack in and out through a hatch between the u/c legs.

As you can see, I like to get the basic wing rib drawn fairly early on and I needed the partial top view to sort out how the rudder cables/horn work out. Yes, the rudder horn is huge, but pretty close to scale.
I have shown laminated 1/4 balsa forward sides because I wanted to get a positive mounting for the motor mount, at a pre determined position. I couldn't think of a simpler way to do it that wouldn't involve tabbed and slotted formers - something I'm trying to avoid on this model.
The more laser cut ply parts there are, the more costly any possible kit would be. Also, that sort of precise cutting makes life difficult for anyone not using laser cut parts. Most of us can manage to cut and glue a rectangle of ply and then do some shaping with a razor plane.
The mount plate will just be a circle of ply and also serve to locate the dummy crankcase. That can either be vac-formed or made using glassed foam. Fits the 450 rather well I think - without any need to alter the scale size.

Pat, if you have any comments or suggestions, feel free to post away. I can't promise I'll take any notice, but you never know.

Pete
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Old Jan 02, 2014, 08:57 AM
AKA Jon Rider
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Pete, Could you have the lower cowl block be removable and put the battery in front of the firewall between the two motor plywood mount pieces? That would open up the space for the servos and you would not have to battle the landing gear every time you replace the battery. With this big old tail I would expect you may need that weight as far forward as possible -
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Old Jan 02, 2014, 09:18 AM
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That was my initial thought mate, but the pack Pat wants to use is unlikely to fit. It is pretty restrictive of pack size if it goes there. Pat should be used to fumbling around between u/c legs on my designs.
The main problem with using the nose blocks, however, is that the cylinder/exhaust details get in the way.

Pete
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Old Jan 02, 2014, 10:09 AM
‹bung macht den Meister..
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James
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Old Jan 02, 2014, 02:56 PM
a.k.a Maltone
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Australia, NSW, Goulburn
Joined Jan 2005
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Pete, as I see it (I have poor eyesight) you've given options for the battery position. It may still be possible to mount the battery forward of the firewall - I might even research a smaller pack. Most of your designs have some flexibility in this area - esp when no slot-and-tab parts are involved. A hatch on the top or bottom is still a possibility depending how the balance and final physical form plays out.
When the pack size is finally decided, a small hole just big enough for the battery might work on the underside - we'll see.

The drawing was printed full-size and I looked at some 1.6 A 3S packs - 3.5 X 1.25 X 1 - they may just fit in front of the firewall - maybe even miss those lower exhausts....since this will all be hand-cut, anything is possible.

...and Pete - despite what you've heard, I am NOT used to fumbling around between legs

The crankcase might be best vac-formed then I can make several copies if I break any cylinders off after a nose-over!

I'll print out the development so far to give me an idea of scale. It looks nice and straightforward so far

Pat
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Old Jan 02, 2014, 03:28 PM
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It looks to have an air cleaner/exhaust or solid cowl piece at the 6:00 position on the cowl, may be a good "strong point" IF you do nose over - no plastic cylinders to crunch. Still think the battery in front of the firewall is a good idea - That's a HUGE tail - no comment on the "Legs" issue -
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Old Jan 02, 2014, 06:55 PM
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Those stub style exhausts would make it easier to use the front block (upper or lower) for battery access. However, having just spent a hour playing with possibilities, the only way to get that battery forward of the ply former (without the nose blocks needing to be very thin) is to mount it flat. Unfortunately, the only way to do that is to mount it to a ply plate and that would require tonge and slot fixings. I've no doubt Pat could cut them easily enough, but not everyone likes cutting 1/8 ply - especially if a modelling knife is the only tool you have. Personally, with the motor roughly 6" in front of the balance point, and with the battery 3" in front I doubt balance will be that big an issue. It isn't that short nosed really, 7.5" in front of balance and 24" behind . Apart from the tail surfaces (mostly 1/8 balsa) there isn't a great deal of structure in the tail.

Pete
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