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Old Mar 10, 2012, 12:30 AM
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Prather QM Little Toni build thread

I purchased this Prather Little Toni QM kit brand new sometime in the late '70s and thought it would be great to take it out of the attic and put it together using many of the traditional building and finishing techniques of the day -- but as an electric conversion. It'll probably end up being a hangar queen, but I want to build it the way I always pictured it. I'm especially interested to see how the glass-over-balsa finish will work out with electric power.

Now, on to some pictures...
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Old Mar 10, 2012, 12:34 AM
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Originally Posted by rjtw View Post
I purchased this Prather Little Toni QM kit brand new sometime in the late '70s and thought it would be great to take it out of the attic and put it together using many of the traditional building and finishing techniques of the day -- but as an electric conversion. It'll probably end up being a hangar queen, but I want to build it the way I always pictured it. I'm especially interested to see how the glass-over-balsa finish will work out with electric power.

Now, on to some pictures...
Awesome! There is a nitro version of that plane hanging in my local Hobby World. It's one awesome fast looking plane, especially with those super narrow wheel pants and skinny aluminum wheels.
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Old Mar 10, 2012, 12:36 AM
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Kit and parts.
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Old Mar 10, 2012, 12:45 AM
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Wing skins

The kit instructions call for putting together three parallel 3x22 inch 1/16th balsa sheets for each skin. However, I must have cannibalized some of those sheets way back when, and have only five instead of twelve -- though I also have some other very good 3x48 balsa sheets from the same period I'd like to use. Also, one of the five sheets weighs about 25g, while the other four only weigh about 17g. After a little creative thinking, I decided to go with putting one sheet parallel to the leading edge (using up the four matched original sheets), another sheet parallel to the trailing edge (from my other old balsa supply) and using the leftover super-stiff but heavy sheet for the little triangular section left in the middle. I put the sheets together using slow set CA glue directly on their edges and holding them together flat over wax paper for assembly.

I forgot to take pictures of the total weights, but each pair of skins ended up within a gram of the other this way.
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Old Mar 10, 2012, 01:05 AM
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Skinning the wings

I really debated on whether to go the 3M Spray 77 or the epoxy route. Both are techniques I used back in the day, although I used Spray 77 far more of the time than epoxy because I made a lot more gliders than power ships and always felt 77 ended up lighter. However, I was also concerned about the strength of the wings. Sure, these things were designed to go screaming around at 150 mph or so for the QM versions and even faster for the .40 size, but these original wings do not even have spars. And the wing is so thin at the tip (only 3/8" sheeted) that it really gave me pause for thought. Especially when it's so easy to find old stories on the Web about how this plane or that had its wing fold.

After considering various ideas, I finally decided to go with something not even available back in the day -- carbon fiber -- after spotting plain/unimpregnated fiber at the local hobby shop. This also dictated going with epoxy.

I don't have pictures, but I first coated a thin, even coat of 45-minute epoxy on each wing skin. I was concerned about weight, too, and first tried a fairly hard push while scraping the epoxy (I used a 4 inch or so section of 1/16th balsa stick as a scraper). This actually removed so much glue that it left the surface with no gloss and a fairly dry appearance. So I went back and used just enough pressure on the scraper to leave a gloss finish... any more pressure and I would lose the gloss... so I think this is about the minimum required for an excellent bond.

I then laid in the carbon fiber tape (only .004" think and about a half inch wide -- no problem in potentially creating any bump in the upper surface) down where I had previously marked as the eventual high point of the airfoil at root and tip, then pressed hard with the scraper right on the carbon strip to get epoxy to seep up from underneath. I drizzled on a bit extra and worked it in throughly on each strip, again ending up with just enough to leave a gloss. Finally, I weighed each wing skin to make sure I was putting the same amount of epoxy on each and again made them the same to within 1g.

Here are the results.
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Old Mar 10, 2012, 01:25 AM
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Le, te

After final trim, on go the leading and trailing edges!

Trailing edge is 3/16 by 3/8 tall balsa, followed by 1 inch aileron stock. Leading edge is 3/8 square balsa. Both are epoxied on.

Terry calls for two interesting points here. First, the aileron will eventually get cut out of the middle section (leaving a fairly large tip area without the aileron, presumably for better stability and less possibility of flutter) so Terry has you fully glue the tip section but spot-glue everywhere else ("every two inches" with epoxy). I'm not sure I'm quite ready to commit to permanently attaching even the tip at this point, so I spot glue with slow CA every two inches over the entire trailing edge.

Second, Terry briefly mentions that you should line up the 3/16 TE stock vertically (and trim/sand the core as necessary) so that the aileron stock is square. Boy, does he mean it, this is an extremely important step... you'll end up shaping the entire aileron and trailing edge based on this single step and you better get the TE of the aileron stock lined up exactly down the middle of the airfoil.

The instructions also call for gluing on the tip blocks at this point, but I'm going to wait until after I completely shape the leading and trailing edges as I feel I can more accurately match the halves this way by being able to see and measure the thicknesses of the skins and curvature of the LE and TE.
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Old Mar 10, 2012, 03:45 AM
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Give Kevin Matney at Matney Models a call. He has already successfully converted some of his QM15 kits to electric. Progress looks good. That Prather Little Toni is a sweet flying racer.
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Old Mar 10, 2012, 02:01 PM
This is my Nemesis....
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Looking good! Your attention to detail is amazing!

I went to the hobby store today to get some materials,and took some photos of the Toni with my phone ( I think it's the same model). I love the pipe in the cowl cheek. I would like this plane, but it's not really for sale, and I would like an electric.


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Old Mar 10, 2012, 02:49 PM
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Thanks for those pictures, Viper! That's a really nicely finished airplane and it does look great. That's the big brother .40 size version of this airplane (you can tell because it has the clear canopy, where the QM size is all fiberglass).

I always loved the little extractor pipe too coming out of that cowl... it just screams speed. The cowl in my kit has a preformed cutout area for the pipe and I may as well use it to house a fake extractor pipe. I was rummaging around in my basement for my old Cox .15 -- which I originally purchased to power the Toni way back when, but it ended up going into other projects -- and was hoping to use its original extractor pipe, but I couldn't find any trace of engine or pipe in my old boxes. Still, a nice pipe there would look nice, and I'm still thinking about whether I want that to act as functional cooling. Alternately for strength I might leave the entire fuselage area (under the cowl) uncut and rely on cutouts only behind the spinner for cooling.
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Old Mar 10, 2012, 03:13 PM
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Wing shaping

Now for what I consider to be one of the most critical aspects of building any model... final shaping of the wing. I really spend a lot of time making sure the entire airfoils match as closely as possible between left and right so that the model will end up flying straight and true.

But before I start, here are the most important tools I have always used: my building board and a long T-handle aluminum sanding block. The board is absolutely flat and smooth, and has 90 degree edges. I often put the LE or TE right against the edge of the board so it fully supports the wing while I'm sanding, but allows the block to go out over the edge. And I use the 90 degree edge angle as a way to sand in perfect 90 degree vertical (and/or horizontal) angles by using the edge of the board as a guide to the sanding block.

As for the block, a nice long block is necessary to ensure a straight or flat edge -- perfect for LE and TE work. You just can't get a perfectly flat contour with a smaller block. I have 240 grit on the big block now for final shaping work.

For rough shaping I've got a smaller block with 180 grit that my daughter made it school. It would be nice to have another one of the big blocks with rough grit as well but this works out fine for rough shaping.
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Old Mar 10, 2012, 03:41 PM
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More shaping

This wing turned out to be a bit challenging to work with. First, although it's almost impossible to see in the pictures from above, there is a compound curve in the leading edge area of the top of the wing due to the radically different root and tip airfoils. Unlike most wings where you can set your sanding block at a fixed angle from root to tip, the top leading edge area has a markedly different contour at root versus at the tip (root = 10% semisymmetrical, tip is about 7.5% fully symmetrical). The root feautures a fairly steep angle at the leading edge of the foam core, whereas the tip is virtually flat. That means I can't use my long sanding block straight across from root to tip on the top of the wing.

Instead, what I did is start with the small block. On the leading edge, I'm always afraid of diffing into the wing sheeting, but at the same time I want to follow its angle at the front of the cores exactly. I always start by taking that entry angle, the angle at the very front of the foam cores, and sanding to extend that angle all the way forward but without initially adding any additional curvature. To do that, I laid some scrap paper on the top surface of the wing and used that to lightly rest my small block as a guide, placing most pressure on the LE material. I stopped when I got right down to the epoxy glue (holding the LE onto the sheeted cores) but making sure not to go into the skins at all.

I then switched over to the big block with the finer grit.. I held it at a slight angle to the leading edge -- just enough so I could easily contour a section of the wing, but also using long smooth sweeping strokes as I got closer and closer to the finish. I'd be able to cover most of the wing from root to tip in a single stroke this way. My goal was to never concentrate pressure in any one area, but to sand as large an area as the curvature of the wing would allow.

I did something similar on the bottom, but its single curvature allowed me to use the large block held parallel to the LE seam from root to tip.

As I got very close, I then held up each side vertically to the light, so that any imperfections would case long shadows and easily show up. The epoxy holding on the LE is a bit harder than the balsa, so I had to be careful to not apply excess pressure when sanding over just the balsa but apply a little more pressure or extra strokes around the seam.

Here's the result on the leading edges. Next step is to do the other wing, match them to make sure the thickness of the untouched part of the LE is the same (this guarantees that my LE profiles/cambers are as close as possible), then do final contouring of the LE.
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Old Mar 10, 2012, 04:31 PM
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This is an incredible build log rjtw! I'm learning just by watching and reading

Thanks for the info on the size versions. That pipe is so awesome and I think you should make a cosmetic one!
It's really making me want to go make them an offer for that plane, and I'm not even set up for fuel
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Old Mar 10, 2012, 07:14 PM
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Thanks Viper! I always thought these semi-scale racers were really sexy -- right up there with, and perhaps even more so than pattern airplanes of the time like the Curare or UFO. Nothing like a blast from the past!
Rick
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Old Mar 10, 2012, 07:21 PM
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Wings getting close

Shaped the leading and trailing edges, installed and shaped the tips.
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Old Mar 11, 2012, 10:34 AM
This is my Nemesis....
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Originally Posted by rjtw View Post
Thanks Viper! I always thought these semi-scale racers were really sexy -- right up there with, and perhaps even more so than pattern airplanes of the time like the Curare or UFO. Nothing like a blast from the past!
Rick
Oh man... Back in the late 70's I had a Curare with a Webra Speed 60 rear Exhaust with a tuned pipe that exited the canopy. I bought ROM Air retracts for it and gave all the parts to a fellow club member to construct... He left town during construction with no where abouts. It was a total loss

One of those life lessons... It would have been slightly more gratifying to fly it into the ground
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