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        Discussion The Comet Stinson SR-7 - Another Free Flight conversion project - Video Addeded

#1 PaulBrad Aug 21, 2010 08:17 PM

The Comet Stinson SR-7 - Another Free Flight conversion project - Video Addeded
 
10 Attachment(s)
Earlier this year I decided to revisit a model from my youth and built a rubber powered Free Flight Comet 25 span Stinson SR-7. It became such a good flier that I decided an RC version was in order. Between the recently released Pat Tritle Stinson SR-10 kit and the ParkZone Stinson Reliant I guess I am in good company in terms of wanting an RC Stinson. As a result, I thought I would do a build log of the project.

The Comet kit dates way back to the 30s and has held up well over time in terms of the overall balance between scale and flyability. The Comet designers certainly took some liberties relative to scale fidelity, but the model does retain the distinctive lines of the full scale airplane while providing for a very good flying platform.

With the availability of the great gear these days my thought was a fairly simple conversion of the Free Flight design could be accomplished. My Free Flight Stinson weighs just under 40 grams without a rubber motor. Using something like a brick receiver/servo package and one of the popular brushed RTF motor packages, my goal is to build an RC Comet Stinsion that does not exceed 45 grams flying weight.

My equipment choices are the Specktrum AR6400 receiver, the motor/gear drive from the ParkZone Micro P-51, and a 150 mah lipo cell. I plan to only use rudder and elevator flight controls so the AR6400 is over kill, but it may get used in a future project that will need the added channels.

Now on with the project. I use iron-on transfer paper to transfer the parts to sheet balsa. After cutting out the parts I began construction with the fuselage. Both sides get built over the plan. The second side is built on top of the first side to make sure they are exactly the same.

Once each side is built they are joined using the formers in the center of the fuselage section. Once that is complete the sides are joined at the rear and the remaining formers are added.

With the fuselage sides joined the nose section gets built. I developed a simple jig to help that process. The transition from the rectangular fuselage cabin area to the round nose requires something more in my opinion than eyeballing cracking and bending longerons as called out on the original plan. The jig holds the round forward former while nose longerons and stringers are added. Once the nose is completed the jig is removed and the remaining stringers are added to the fuselage. Also added is the cabin roof and the landing gear support plate.

#2 PaulBrad Aug 21, 2010 08:23 PM

4 Attachment(s)
Next up is building the cowl. That process begins by building the sub-structure. The sub-structure is simply two balsa rings and four strips of 1/16x1/8 balsa. Once the sub-structure is complete two layers of 1/32 balsa sheeting are wrapped around the round structure. The nose ring is made from two laminations of 1/8 balsa. The nose ring is shaped using a Dremel tool. After shaping the balsa disk that makes up the back of the nose ring is opened up for the motor. More on that later.

Next on the to do list is setting up the equipment installation in the fuselage.

#3 PiperCub49 Aug 21, 2010 08:43 PM

Looks great Paul. Thanks for bringing your talents over here!

I've gotten started on the fairings that I asked you about on HPA. I'll use pink foam for mine I think. I need to keep them LIGHT. I'm hoping that I can make my final set look half as good as yours. :rolleyes:

http://www.rcgroups.com/forums/showt...260414&page=10

Can't wait to see you build this one!

-PC49

#4 Slider2732 Aug 21, 2010 08:49 PM

Some fine free flight conversions going on recently, classic lines again :)

For those who may not have seen NC-15173, including me until just now, here's a link to a large pic of the original aircraft.
(photo says there is stringent copyright on copying, so fair enough).
http://www.airliners.net/photo/Untit...ext_id=1127927

#5 ckreef Aug 21, 2010 09:41 PM

Very nice can't wait to see a video of the finished plane.

#6 PaulBrad Aug 21, 2010 11:12 PM

PC49 - I have been following your build of the 30" Stinson. It is coming along nicely and should be a fine flying model. You effort and the work done by Glenn Lewis have been inspirations for this project.

Slider2732- Thanks for the link to the photo for the full size NC-15173. I had located that photo before completing my Free Flight model and used it to develop the markings. It is interesting that the original Comet plan shows the license number to be NC-15137. I looked that number up on the aircraft registration list and found that number was actually used on a Monocoupe 90A. As I looked through the list I noticed that NC-15173 was issued to a Stinson SR-7. Further searching turned up the photo you referenced. I wonder if the Comet designer inadvertently reversed the last two digits of the license number when he drew the plan?

ckreef - I will try to get video of the model when finished and flying. If not before I should be able to do that at the up coming KIEF event scheduled for mid October 2010. At least I sure do hope it is finished and flying by then.

Paul Bradley

#7 Slider2732 Aug 21, 2010 11:59 PM

Interesting number swap. I can imagine he was rushed for time, remembered from memory and left us all with a trivia note for post flying beer talk at the pub :)

#8 reylf_gnijieB Aug 22, 2010 01:04 AM

I like your jig for the nose former. One more trick to add to my bag. Reading other's build threads helps me become a better modeler.

I've always liked Comet models better than Guillows. I have many plans on file, and will get to some of them some day. Too many planes to build!

#9 glewis Aug 22, 2010 12:16 PM

Cool! Another Comet converion. :D Count me in!

The equipment you propose should make for a very light model that could even be flown indoors. Mine is way too heavy and fast or that!

I built mine in 2003 when we were still using brushed motors and round cells. Still flies good using the twin IPS system and lighter lipos. Almost took mine to the field today but took the Cessna 337 instead.


The nose assembly fixture is brilliant! That is the toughest part to build on this model. Well, carving all those little cowl blisters isn't easy either. Getting them all the same size/shape is a challenge.

Glenn

BTW, After the sucess of the Guillows T-28, I'm now building the Skyraider from the same 900 series. Started a thread but there dosn't seem to be much interest?

#10 reylf_gnijieB Aug 22, 2010 12:38 PM

Quote:

Originally Posted by glewis (Post 15855096)
Well, carving all those little cowl blisters isn't easy either. Getting them all the same size/shape is a challenge.

The way to do the cowl blisters is:

1) Grab a stick of balsa that is as thick as the width, and as wide as the length of one blister.

2) Carve/sand it to an airfoil shape that matches the top profile of the blisters.

3) Sand the end to the blister shape.

4) Cut off blister.

5) Repeat.

#11 glewis Aug 22, 2010 01:43 PM

Sounds easy enough, doesn't it? ;)

Glenn

#12 PaulBrad Aug 22, 2010 01:51 PM

Quote:

Originally Posted by reylf_gnijieB (Post 15855277)
The way to do the cowl blisters is:

1) Grab a stick of balsa that is as thick as the width, and as wide as the length of one blister.

2) Carve/sand it to an airfoil shape that matches the top profile of the blisters.

3) Sand the end to the blister shape.

4) Cut off blister.

5) Repeat.

That is my method as well. It is a bit tedious but does work well.

Glenn - I have been following your Skyraider thread. It should be a nice flier like the T-28. Speaking of the T-28, Jay Smith sent me the photos you sent to him. I plan to include them in my December Model Aviation Column. It sure is nice having the model thread here for reference.

Paul Bradley

#13 glewis Aug 22, 2010 03:36 PM

Quote:

Originally Posted by PaulBrad (Post 15855846)
That is my method as well. It is a bit tedious but does work well.

Glenn - I have been following your Skyraider thread. It should be a nice flier like the T-28. Speaking of the T-28, Jay Smith sent me the photos you sent to him. I plan to include them in my December Model Aviation Column. It sure is nice having the model thread here for reference.

Paul Bradley

Cool! Thanks Paul!
I was going to take some in flight video of the T-28 this morning but the weather didn't cooperate.

A BIT tedious? LOL! Now that's an understatement! :D
I used the same method but still must have made 30 blisters out of balsa and a couple for my fingers too before I got enough that matched exactly.

Glenn

#14 PiperCub49 Aug 22, 2010 04:04 PM

It may just be easier to set up a whole vac-forming system. Then it would only take a couple of minutes to make a whole bunch of blisters. That would take all the fun out of it though! ;)

-PC49

#15 glewis Aug 22, 2010 04:17 PM

That's actually even more work. Make one master male blister. Make a female mold from that. Cast many male male plugs from the female mold. Arrange on the thermoforming rig and pull one sheet of plastic with many blisters on it.
They will all be exactly the same though!
Glenn


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