Prather QM Little Toni build thread - Page 5 - RC Groups
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Apr 02, 2012, 02:34 PM
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Here are my top motor choices at the moment. I want to be able to go at a competitive speed for this airframe back in the day, or maybe a little more within reason. I'm trying to stay near the original .15 weight of 5-6 oz.

Here's a back of the envelope weight comparison. The original engine+tank+misc (motor mount, extractor pipe) probably came in around 11-12 oz (that includes 5-6 for the engine, 4 for the full tank, and let's say 1 each for mount and extractor), whereas the weight of a 2500 mA LiPo pack alone is about 10 oz. But I can delete the old NiCad battery pack the airplane was designed for, which weighed around 4 oz. So the old total of motor + tank + misc + Rx battery would be 15-16 oz, the new equivalent will be 10 + motor, so I figure I'll be OK with a motor in the 5-6 oz range. (The weight of the ESC, just an ounce or so, should be offset by the lighter servos of today: my 9650's have about twice the torque and speed of the old servos, and weight just under 1 oz. each compared to 1.5 oz per servo of old). And I think the plane should still balance properly as long as I keep that big LiPo pack right at the CG, which is the best place for it anyway to minimize polar moment. Of course I'm calculating based on a full tank of fuel... but the batteries do not get lighter as they discharge, unlike fuel!! (Edit: I forgot I can also delete the throttle servo, so my weight savings on servos will be about 3 ounces... which should really start putting me in the ballpark on meeting the old minimum weight).

The motors I am considering at this point in time are polar opposites on the price spectrum:

Turning 3648 1450kV up to 1600W 6.4 oz 7x7 prop:

Neu 1115/2.5D 2100kV up to 1400W 5.8 oz 6.5x6.5 prop:

The Neu setup is as used on the Trick ( It's definitely a hotter setup, really likely more than I need. But a lighter motor -- have my cake and eat it too?

Note to self: Use plenty of carbon on that center section...
Last edited by rjtw; Apr 02, 2012 at 03:31 PM.
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Apr 05, 2012, 12:58 AM
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I picked up the fuselage the other evening and was starting to test fit it to the fuselage when I noticed that the right (removable) cowling is larger than the left (non-removable) cowling. I was stunned... after the many times I had looked at the fiberglass parts over 30 years I had never noticed this before! The removable cowl is, of course, made larger to fit over the engine. It's not just a little different, either -- it sticks out further out from the fuselage and has a much more square cross-section than the left side. And of course it has the built-in cutout for the extractor pipe discussed earlier.

Once I noticed the difference between left and right, it really annoyed me. Why not make it symmetrical now that I won't have an engine sticking out one side? The left cowl has a great aerodynamic shape, all I have to do is copy it to the right side. And as luck would have it, the cowl itself is symmetrical top to botton, as is the surrounding fuselage area (to a good approximation). So if I lifted a mold off the left side and flipped it over to the right side it should fit. I'll know for sure when I pop the new cowl out of the mold I made.
Apr 06, 2012, 02:39 PM
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What do others use for a mold release? Maybe I should have waxed the cowl and the mold first, but using this spray alone the pieces were stuck together and I had to really pry them apart after curing. This was true even getting the mold off the original fuselage, which had a smooth glossy finish to begin with.

But, the finished cowl came out well and after some trimming and a coat of resin it's ready for final trimming and fitting. I want to eventually glue it on and fair it in so that it's indistinguishable from the other side.

I may need to wait to purchase and mount the motor before I take the plunge and attach the new cowling, though. Once I glue on the cowling, the only access to the motor will be through the inside of the fuselage, down that long narrow nose, and via any openings I make behind the spinner (I intend to front mount the motor to keep things simple). Until then, I have the option of cutting out the area in the fuselage itself (under the cowl) originally intended for glow engine access. It would give great access to the motor but I'm concerned about weakening that area, especially with a front mount. I'll obtain the motor and get it mounted before gluing on the cowl.

More pictures next time!
Apr 06, 2012, 03:01 PM
This is my Nemesis....
ViperZ's Avatar
Great thread, Great read!
Apr 07, 2012, 02:29 AM
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Wing bolt dowels

I placed the wing bolt dowels. I ended up using a sharpened piece of 1/2" brass tubing to start the two front holes (these need to go through a fairly thick layer of epoxy and fiberglass under the balsa skin, and the tubing really just got these holes only started), then finished up with a Dremel with a 1/8" flat tipped milling bit following the round outline from the tubing cut. Worked great on the trailing edge cut though.

After trimming the dowels, I'll finally be ready to glass the wing center section, then the rest of the wing. Not sure if I'm looking forward to that or not, it's a big step!! I've purchased some 2-ounce cross-weave carbon fiber (basically interwoven 1/2" tow) which I want to use in the center section, and I have a bunch of 3/4 ounce cloth for the rest. I'm still not sure whether to put it both layers on at one go or do the center section separately, let it cure, sand it down, then do the full wing in 3/4 ounce cloth, but I have a theory that if I do both layers all at one go I'll have less of a bump in the middle where the carbon fiber fabric is. I'm even toying with the idea of using the carbon fabric on the entire underside of the wing, but then I might have to deal with a faint checkerboard-type pattern due to the over-and-under weave in half-inch tow (even though the tow only measures two thousands thick!!)

Question for those who have gone the 3/4 ounce cloth route: Do you thin the resin? With alcohol or acetone? I'll be using Z-poxy finishing resin. Its consistency is not bad for most work but for this really fine cloth it might be nice to get it almost as thin as water.
Last edited by rjtw; Apr 07, 2012 at 02:35 AM.
Apr 08, 2012, 02:39 AM
Registered User

Wing hold down

To make the hardwood dowels flush with the surface of the wing, I first cut off the excess using my X-acto saw about 1/64" above the wing sheeting surface. I then used 100 grit sandpaper on a small block, again protecting the wing sheeting with a piece of paper, to slowly bring the dowel close to the surface. The dowel is so much harder than the surrounding balsa that you really can't touch the balsa at all in this step or you'd end up removing mostly balsa and little of the dowl. In fact, they are so hard to sand, that I cheated a little and took out my Dremel with a sanding drum installed to knock off most of the high points first (though I had thoughts of Steve Martin in The Jerk in the back of my head while doing this ) Kids, don't try that one at home. Finally, when I could only just feel the dowel slightly proud of the sheeting, I switched over to my long block with finer paper and slowly, with the entire block on the wing from root to tip but only applying pressure right on top of the dowel, worked it right down to the sheeting.

Terry has a good old fashioned wing mounting system, just exactly the way I did it on my slope gliders back in the 70's -- with nylon bolts and threaded plywood. You first drill out the holes in the wings, then epoxy in some plywood bars across the fuselage just under the wing saddle, drill through the plywood using the wing as a guide, then tap out the plywood. Who needs blind nuts?

To mount the bolt hole locations, I carefully lined up the center seam of the wing with the fiberglass fuselage center (the line where the halves meet) and used blue painter's tape to tape the wing in place. I then measured from each wing tip to the back of the fuselage to make sure the measurements were identical, then drilled right back through the front holes I already had in place into the front part of the wing saddle. The fiberglass in the fuselage is only cut out to within an inch and a half or so of the leading edge, so the front two holes drill into that front part of the saddle. In the rear, there's very little space between trailing edge and the aileron torque rods, so you have to keep the rear bolt really right at the trailing edge. I used the nylon bolt supplied to determine the outside head diameter of the bolt, then marked the hole location so that the head will be 1/16" forward of the trailing edge. Not much to spare! I then drilled through the rear dowel and into the rear edge of the wing saddle.

Finally, I sized the supplied plywood crossbars to fit and epoxing them in centered over the holes I had drilled in the fiberglass. Later, I'll add an additional wood block over each hole to give more meat for the threads, drill out wing and block holes, and tap out the blocks.

I almost forgot, but there's one other point I ran across. I put in much less dihedral than Terry calls for. He instructs us to prop up one wing by 2 3/4 inches. I felt that was a bit excessive and wanted a flatter wing, mostly to ensure as axial a roll as possible, so went with flat on top which equates to just over an inch under one tip. I later discovered that the fuselage's wing saddle is designed for the greater dihedral. Imagine the wing saddles, especially at the front and rear, making a shallow V all the way to the middle. Thus my wing contacted the middle more than the outside. It was OK, though, if I mildly pressed the wing onto the fuselage as this basically deformed the middle of the saddle ever so slightly (probably about 1 mm in the middle). Again, it was only an issue at the front and rear. So, when gluing in the wing mount blocks, I'm using a brace to force the saddle into the shape I want -- hopefully this will allow the wing to fit in effortlessly now.

Still, the more time I spend with this kit, the more I'm impressed with how well its many details are thought out, and the great attention that has been paid to the smallest details. The wing saddle is a great example -- or the landing gear plate's trapezoidal shape that is designed to give proper clearance, even adjusting for the increasing depth of the airfoil -- or the neat little scribe lines around the motor cutout area and on the removable cowl to show you where to trim. Terry had this down so that you practically couldn't go wrong as long as you follow the directions -- and I'm making extra work for myself when I don't!! The quality of the fiberglass work is top notch as well, perfectly flawless, and quite strong. I can only imagine how much work must have gone into those plugs and molds and getting the production methods perfected!
Apr 13, 2012, 01:40 AM
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Cowl (again)

I decided to re-mold the cowl. The first one wasn't bad, but I wasn't happy with the quality of the fiberglass and I really wanted to nail down the mold-and-release technique. The spray-on release that the guy at TAP Plastics had recommended was not at all up to the job and the parts were more bonded together than not. If I end up doing more molding in the future, this was a pretty good way to do some more experimenting to find out what works. I wanted to see that part practically fall out of the mold with a perfect finish!!

Even before doing the first cowl, I had done some research on the Internet and found that most people used a combination of wax and PVA on the molds. I purchased some special mold wax (it was on the shelf right next to Maguiar's mold wax, probably the same stuff as car wax) and a small bottle of liquid PVA.

You first coat the mold (or plug) with several coats of wax, then spray or brush on a thin coat of PVA. I think what's happening here is that the PVA essentially forms a thin plastic-like skin over the wax. The fiberglass/epoxy bonds with the PVA layer and stays with the fiberglass part you pull off. The wax merely provides a smooth seperation point for the PVA; the PVA prevents the epoxy from touching anything directly. The PVA is water-soluble and you just rinse it off the fiberglass part you've just made when done.

I found that after waxing, brushing on the PVA simply caused it to form droplets. It wouldn't initially form a continuous thin layer. But I found that if I first brushed on PVA everywhere, then used a paper towel to wipe it all out again, just enough was left behind to form a kind of cloudy base. It's probably akin to a first fog spray coat. Then, I could use an only partially wetted brush to brush in a very thin, smooth and continuous layer which would hold. I spread it as thin as I could and still see gloss. The PVA can be spread very thin but will stay liquid and glossy just long enough to flow out the tiny brush marks. By the time it dries, in about 30 minutes or so, it's a completely smooth, transparent, glossy layer.

Hopefully the pictures show the difference -- I'm much happier with how this one came out. Shoulda done it this way in the first place!
Apr 13, 2012, 11:37 AM
Kinetic Sculptor
Originally Posted by rjtw
I found that after waxing, brushing on the PVA simply caused it to form droplets. It wouldn't initially form a continuous thin layer. But I found that if I first brushed on PVA everywhere, then used a paper towel to wipe it all out again, just enough was left behind to form a kind of cloudy base. It's probably akin to a first fog spray coat. Then, I could use an only partially wetted brush to brush in a very thin, smooth and continuous layer which would hold. I spread it as thin as I could and still see gloss. The PVA can be spread very thin but will stay liquid and glossy just long enough to flow out the tiny brush marks. By the time it dries, in about 30 minutes or so, it's a completely smooth, transparent, glossy layer.
I've found that a sponge brush works very well for PVA. Don't try to wet the brush or thin the PVA with water (although it is water-soluble) because it will form droplets. Sometimes there will be little streaks of incomplete coverage as it dries, and as you have already noted, a second light coat will take care of that. But once the mold has been used a few times, there will be enough wax buildup on it that some gaps in the PVA won't matter much. In fact, with some molds, you can use wax only after a while.

This is a fascinating thread! Thanks for posting!


Apr 13, 2012, 03:49 PM
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Wing glass

Thanks for the feedback, Duane! I always love reading build threads and it's fun to be doing one this time. There's always something to learn.

Speaking of something to learn, I'm starting to come up with a new plan from an idea Izzy mentioned -- namely vacuum bagging the wing and going full carbon. I picked up a foot of 2 1/4 oz. carbon weave already (1/2" by 1/2" woven tow) which I intended to use only to reinforce the center section as a thinner/stronger alternative to the original 4-6 oz. glass. (The entire wing is then supposed to be glassed with 3/4 ounce glass). But as it turns out, the carbon weave came off a roll which is just wide enough to cover the entire span... so if I picked up another foot of this stuff... I could do the entire wing top and bottom, bag it, and ideally end up with a smooth glossy finish nearly ready for prime & paint.

I found these links pretty informative:
(I already have an automotive vacuum pump, maybe I can use that!!)

Edit: Some more

Question for those who have done this: As the wing is joined and the shucks only usable on half at a time, can I get by without shucks? And what's the best way to divide this up... all in one shot, or maybe the entire top then the entire bottom? Or only one surface or wing at a time?

I'm also thinking about how to handle the leading edge. I'd like to wrap the fabric over the leading edge from the top, extending partway (1/2" to as much as 2 inches) on the bottom and have the bottom layer come right up to the leading edge, but I haven't seen any such examples on the web. Not sure if the Mylars are flexible enough to wrap around that leading edge. I'm also working on how, alternatively, I would get the top fabric to wrap smoothly over in place if the top and bottom Mylars only extend to the leading edge but not wrap around.

Finally, what's the best way to handle the aileron? I'm considering whether I can just bag with the ailerons in place, as that would give me a nice uniform trailing edge, and I could clean up any flash in the bevel. Just not sure if the gap itself causes any issues with trapped air etc.
Last edited by rjtw; Apr 13, 2012 at 08:45 PM.
Apr 13, 2012, 07:12 PM
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Starting to do research into best bang for the buck on epoxies.

Z-Poxy Finishing -- this is what I've been using but a bit on the thick side for what I feel I need to do an entire wing panel, also fairly short pot life (20-30 mins) which is too short for me to wet out & bag for sure.

MGS -- good things on the net said about it but I've only seen it available by the gallon at CST, I will never go through a gallon! Also the slow-cure requires hazmat shipping.

West Systems -- conflicting reports on the net, some say it's way too soft for composite-only structures, think I'll steer away from this one.

EZLam -- still need to do more research -- available from ACP Much cheaper than West, MGS and ProSet.

ProSet -- I'm leaning towards this right now. 125 resin with 229 hardener will give a working time of up to 160 minutes, and has a low viscosity (only 450 cps, versus over 700 for West, for example). Said to cure very hard and good reports on the net.

Some of these epoxies (perhaps all?) give better physical characteristics under heated cure or post-cure heating. It's surprising to me at how big some of the property differences can be... for instance the tensile strength of the ProSet epoxy above is 7550 psi under room temp cure, but 10247 psi with a 15 hour room temp cure followed by an 8 hour 140F cure -- a 35% increase in strength!! I think I'm going to draw the line on putting together a curing oven though

PS Izzy -- I'm starting to like your live hinge idea more and more... but it's too late for me...
Last edited by rjtw; Apr 13, 2012 at 08:02 PM.
Apr 13, 2012, 08:56 PM
Registered User

live hinge

Rick hi
firstly let me say again , your post has me watching my iphone for updates on this build at least every half a day , hoping to see a new post from you

regarding live hing e, its not too late , as you can check this post, since you ave not glassed your wing yet

link skin hinge

if only i was in usa , shipping stuff is closer , ad then i would also buy a vacuum pump ..

let me know how this works for you

by the way i am using what they call 81 epoxy , and left the airconditioner at 25 degrees celcius on in the building room while the wings were curing , and it came out super strong
i dont think a carbon wing is necassary ,fiberglass is more than enough
and will also be easier to sand and healthier

hwever i would try and maybe strengthen the motor area with carbon especially me and my rossi engine

looking forward to more....
Apr 18, 2012, 02:29 AM
Registered User

Glass choice

Hi Izzy,
Sorry I don't have anything to post more often Seems like progress is pretty slow sometimes. Can't believe it's been over a month now and I don't even have the wings done!! On the other hand, in a couple of coats of glass they will be ready for prime and paint, so maybe not so far. I have barely begun to think about paint designs though...

I've been sidetracked by the choices on how to glass the wings... carbon or fiberglass? What kind of material to use on the wing center joint reinforcement to keep the edge of that reinforcement invisible? Vacuum bag or not? Live hinges?

I picked up some EZ-Lam epoxy and some 6.5 oz carbon cloth from ACP, and already had some 2.25 oz spread tow fabric from CST.

For comparison, tonight I made a test piece by adding a simple, single layer of glass on some 1/16th balsa using the scrape and tissue paper method. I did the two carbon weaves above plus a test of good old 6 oz fiberglass cloth. Each test patch of cloth was about two to three inches square. This is the first time I've done anything with carbon fabrics.

Izzy, I think I am going to agree with you, plain old glass looks like it's going to be the way to go for this particular project.

The spread tow was very thin, only about .002", but was the hardest to saturate with epoxy due to its very closely spaced fine fibers. It was also very easy to get the fibers loose and worked out of place. Still, I was able to squeegee out almost all the resin in the end and keep the fiber in decent shape. While it looks sexy, it seems as if it will have little workability around curves -- it's almost paperlike. At the end of the day, while it wins on thinness, I'm not convinced that it's going to provide the needed strength in a single layer.

The 6.5 ounce carbon cloth is a fraction of the price of the tow (only about 30 bucks a yard) but it's also the thickest of the three, at a whopping .010". It was easy to wet out and looks like it would handle curves very well, but it's going to be way too thick for that center joint and would not flatten or compress at all with scraping. There's no way I can get that feathered in as well as I would like. This stuff is undoubtedly the strongest, probably by far, but just way too thick for this application. And it also unravelled very easily at the edges -- the bundled fibers at the edge of the weave just fall out. For comparison, by the way, ACP also sells a 3.5 ounce weave that might be a better fit but it's three or four times more expensive. I'll save this for a future project where I'll just lay carbon fiber directly on the foam cores and vacuum bag -- it'll be perfect!!

Which leaves good old glass. The 6 ounce cloth was the easiest to wet out, the easiest to see where wet or dry sections are, and overall the easiest to keep in one piece without too many fibers falling out or getting misplaced while wetting and scraping. It is about .005" dry, but after scraping and paper toweling it appeared to lie down extremely flat. I'm looking forward to measuring actual thickness on the test piece compared to the spread tow weave tomorrow. And the glass is going to be more workable and more easily sandable.

So in the end, I think I'll stick with the original methods for finishing the wing and save all my carbon goodies for a carbon wing slope project I'm dreaming up!!
Apr 18, 2012, 04:47 AM
Registered User
six oz glass is awful heavy for a finish coat.. I would look for some 0.6 to 0.75 oz glass. No need for all that heavy glass on the whole wing.

For the center section we used to use two or three layers of 2 oz glass, first a wider layer, maybe 5" wide at the back edge of the wing and 8-10" wide at the front, then another layer maybe 4" wide at the back, 6" at the front, then a final layer narrower than that to top it off (3" or so). Put down all three layers, work the epoxy down through them, don't need to do them a layer at a time.

I would strongly recommend NOT doing 6 oz glass for the whole wing.. too heavy!
Apr 18, 2012, 01:18 PM
Registered User

glassing wing

Rick hi\i would tend to agree with fizzwater2
in all the pylon applications ive seen , they all had 1 thing in common
lightweight glass for the finished coat (0.5oz)
also using 2 oz cloth for the middle section spread out in 2 or 3 layers

the only thing i am waiting to see / learn from you is if you are now going the skin hinge route , ( as i said before , wish i was living in US ) so that i could pick up some live hinge material easily

looking forward to progress
Apr 18, 2012, 01:18 PM
Registered User
Hi Fizzwater,
My post wasn't very clear -- all these tests are just for the center section. I have some nice .75 ounce cloth for the rest of the wing. I was thinking about possibly using the light carbon tow weave on the rest of the wing, but think there will be too many issues with trying to make that work. But thanks for the tip on multiple layers of lighter cloth, and on those dimensions, I'll go that way!!

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