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Aug 29, 2005, 12:20 AM
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gsmith's Avatar
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Mini SR from SkyKing - Build

A while back I got this prototype kit of the Mini SR offered by Ed at SkyKing and it waited in the on deck circle while a couple of other projects were finished. Now is the time for the Mini SRís place on the bench.

The Mini SR is a Mark Grand design based on one of my favorite ships from my first life as a sloper in the early Ď80ís, the Bob Martin SR-7. It has been scaled down hence the Mini designation. The Mini SR also uses a more modern airfoil in the thinned down RG-14. She should cook!

The kit is complete and has a molded fiberglass fuse, white foam cores, laser cut balsa and plywood, hardwood for leading edges as well as vacuum formed canopy and hardware. My kit is a pre-production version and there may be slight differences when Ed gets the final version shipped but there will not be major differences.

The build on the Mini SR is pretty standard PSS fare. If youíve built a plane similar to this one you may do things differently that I am. If you know a better way to do a step by all means go for it. Then tell us about it so we all know!

I started with the wings.

There are 8 sheets of 1/16th balsa. I glued four sheets together, twice, with CA after checking the edges for straightness. I like the Great Planes Easy Sander and use it often. There are a couple of sizes and I have several of each. Of course you could use any kind of sanding block you are comfortable with.

Then I used the core beds to mark the wing planform on the balsa sheets leaving about ľ inch extra all around. Align the sheets so the grain is parallel to the Trailing Edge.
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Aug 29, 2005, 12:37 AM
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Laminating the Skins

After I had 1/16Ē balsa cut to shape I used 3M 77 (old formula because the new doesnít play nice with white foam) to laminate the skins to the core. I coated both the core and the balsa skin with the 3M 77 got the core beds ready, let the 3M get tacky for about 5 to 7 minutes and then laid the skin down and pressed the core onto the skin. Do the top skin, put on the top core bed and then use some weight the press the whole shebang for a while as the 3M sets.
Aug 29, 2005, 01:05 AM
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Trim the Skins

After the skins are laminated, take the wing out of the cores, trim the excess balsa skin and sand even with the foam core on the Leading Edge, trailing Edge and Tips.

FYI, the grey mat on the worktable is a cool material that is used for commercial kitchens or bar floors. It is great to cut on and is self healing. I got it from a friend who installs it but donít remember what it is called. Itís pretty heavy and stays flat without having to glue it down.
Aug 29, 2005, 01:25 AM
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I'll Have a Different Scheme But This is What it Will Look Like.

Thought a peek at what the finished plane will look like might be in order.
Aug 29, 2005, 02:05 AM
Ninja of the Nasty
KingOfTheHill's Avatar
way cool greg.... ive never done this before and its always kinda looked like a lot of work... but after seeing you do it, it doesnt look like that much...

is this the same for the PLY sheeting? do you guys not wrap around the LE and just glue in a basswood LE that you can get at the LHS?

also, your using the old 3m77 formula.... CAN you use the new stuff if you spray from a distance? or is there something else out there that i can use if i cant find the old 3m77?

Aug 29, 2005, 07:08 AM
Detail Freak
target's Avatar
You can just use EPOXY resin, JOe, and then there is no way to get a twist in the wing. With either 3M77 or Southern's Sorgum, its kinda easy to get a twist; if the core isn't perfectly flat when you put on the second skin, it'll lock in a twist when you put on the second skin...
With the epoxy, the bond isn't instantaneous, so you can get the layers weighted down in the beds before the final bond occurs. Also, if you are off in the aim a bit, and one skin isn't covering an edge completely, you can fix it...
No, the ply never wraps the LE, at least not that I've seen. They take either a hardwood, or balsa LE.
If you mist the new 3M from far away, you can use it on EPS, I've heard...

Sorry for the interruption Greg, looks pretty cool, I was thinking of getting one of these....

Aug 29, 2005, 08:16 AM
Registered User
TFLG's Avatar
No way to get a twist with epoxy??? You've not seen some of the wings I've seen

It's still easy to get bows and twists. You need to make sure you work surface is flat AND make sure the core beds AND the cores are flat and straight. I've had cores where the core beds (the foam block) was not flat. So when weighted down flat the cores would actually bow. This is actually quite common as many of the foam sheets are not perfectly flat to start with. CSD had so much trouble with this that he started cutting the top and bottom skins off the foam sheets prior to cutting the cores out.

Pro Bond PU glue is another great adhesive for skins. Again with this adhesive you need to put the cores into the beds while it sets so you need to be sure you surface is FLAT and the beds are too. I actually prefer the PU glue to epoxy for skining wings.

Aug 29, 2005, 09:27 AM
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More on Sheeting the Wing

Thanks for chiming in guys. It is good to hear from builders who have a ton more experience than I do.

Getting a straight wing is totally key to a good flying plane. Fortunately, these are nice and straight. I think the cores may have been shaved or CNCd flat because they were nice and flat.

Of course there are several ways to sheet a wing. On this plane the cores are white foam and when I use epoxy I vacuum bag the sheeted wings in the core beds. I like how my bagging setup is working for composite or any other wing that uses a pink, blue or Spyder Foam core and didn't want to mess with it. I use about 18 inches Ė 20 inches or what ever I can pull with the pump for those cores. The white foam needs only about 6 or 7 inches of mercury or the foam will crush. Didnít want that! Also, I hadnít used 3M before and I liked how fast it was. Might be slightly lighter too but I donít have a direct comparison. When I use epoxy I usually add a thin layer of glass on the inside, between the core and the skin.

FYI, Ed at SkyKing has used Southern Sorgum quite a bit and he probably has tips for using that. I believe you brush or roll it on. Getting a nice even coat would be key. So far I still have a few cans of old formula 3M 77. Haven't tried to renew the supply lately but about 6 months ago I could still finds the old stuff.

BTW, I am doing a Rodent kind of along side this plane and will be bagging those wings with epoxy, glass and 1/32 ply. Pretty similar idea. Sheet the wing, sand the sheets flush with the core and add the leading edge. The Rodent is sheeted all the way to the trailing edge and then you cut the ailerons free and face them. A bit different but should make a good wing as well. Just need to make sure the whole Bed, core, bed assembly is flat and true.
Last edited by gsmith; Aug 29, 2005 at 10:31 AM.
Aug 29, 2005, 09:44 AM

MiniSR information

Greg, glad to see you got started on the MiniSR. I thought I'd post some specific information for anyone that might be interested in this kit.

I expect to have about 25 kits ready to ship early in October. The cost will be $149 plus $12.00 shipping.

Balsa wing skins are included with this full kit but should you desire poplar skins I can provide them at an additional cost of $20. I would not include balsa and poplar. The balsa would be substituted with poplar.

I do not suggest poplar skins if you do not have bagging equipment. Poplar will produce a beautiful and very tough wing skin and it is all one piece so it doesn't require gluing individual pieces to make up a wing skin.

I do not have this kit set up on my shopping cart at this time but I will by October 1st.

The wing now sports a 42" span thinned RG14 instead of the older and much thicker section that was used on the larger SR7 kit that was produced several years ago.

Aug 29, 2005, 09:49 AM
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Wing Facing

The Mini SR uses a sub trailing edge and a sub leading edge as well as tip facing. There is 3/8 x 1/8 or so spruce in the kit for this. I used 30-minute epoxy here. Pretty simple. Make sure the pieces are straight, glue Ďem and tape Ďem down while they set. I used 30-minute epoxy and micro balloons here.

Hereís a little tip. I like the blue 3M painterís tape and have it on a nice dispenser. I fold over one edge so I can get it off easily when it is time to remove it.

I weighted the now faced wings back in their cores while the epoxy dries.

Another tip. I like the Great Planes Easy Sander aluminum sanding bars because they are great for sanding but they also work well as weights because you can slip a 5/8 or so steel rod in the hollow of the handle. Nice!
Aug 29, 2005, 10:29 AM
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Main Leading Edge

Trim the sub trailing edge, sub leading edge and tip facing. I use a Zona razor saw.

Attach the main hardwood leading edge with epoxy. The leading edge stock is 3/8 x 3/8 basswood included in the kit.

After the epoxy dried I roughed out the shape of the leading edge with a Master Airscrew Razor Plane. I also really like the David Razor Plane that Ed at SkyKing sells but mine is MIA after the recent shop move. Canít wait to unpack the box it is in. Wherever that is!

I also used the Great Planes Easy Sander here. The 12 incher with 60 or 80 grit is good for getting close to the final shape. The long version with 150 is nice for smoothing it all out. I do a final swipe with 220 by hand.
Aug 29, 2005, 08:45 PM
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The Trailing Edge

There is 1-1/2 inch trailing edge stock in the kit for the ailerons.

There is a right side up for the ailerons. The aileron on the left is correct the one on the right is upside down.

I extended the lines where the aileron will be cut on the plans and then cut the tip pieces, aileron and root pieces.

After I cut out the pieces, I glued the little tip portion of the Trailing Edge on.

I then traced the upper airfoil onto the tip pieces that will give the Mini SR on of the distinctive visual bits from the original.
Aug 29, 2005, 08:51 PM
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Aileron Work

I taped the aileron on the bottom and sanded the aileron to match the airfoil shape on top.
Aug 30, 2005, 11:18 AM
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Aileron Control Rods

For the aileron control rod I used a Du-Bro #556 3/32Ē strip aileron control rod. You need to cut the plastic sleeve to length. Slide it on and bend the control rod so that you can insert it into the aileron. I made the bend slightly rearward so that there is a small amount of positive differential in the aileron throw (more up than down). We will only be using one servo in the wing so electronic mixing is not an option. I made an educated guess at how much to use. Not much since the span is only 42 inches.

Next, I took the root Sub trailing Edge and routed out the front of it to allow the control rod assembly to fit in. Half the control rod sleeve gets recessed in the front of the Trailing Edge stock and the other half gets recessed in the wing Sub Trailing Edge.

Bevel the root end of each piece so they butt together when you install them on the wing.

Also note the clearance for the control rod to move fore and aft.

After these are fit properly glue those thing on. I used Epoxy. You could use thick CA as well. Just make sure not to get any on the moving parts
Aug 30, 2005, 12:56 PM
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Making the Wing Tips

Here I glued on the wing blocks and shaped them to the distinctive shape of the original. The tip turns under in a Horner shape.

I roughed it out with the Dremel and a large drum sander.

For the final shape a ĺ inch piece of PVC pipe with sand paper 3M 77ed to it was the perfect size.

Note that I taped the inboard balsa panel with a piece of masking tape so I did not gouge it while sanding.

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