The Comet Stinson SR-7 - Another Free Flight conversion project - Video Addeded - Page 5 - RC Groups
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Sep 11, 2010, 11:00 PM
Headed to Naval Flight School!
PiperCub49's Avatar
Looks great, Paul.

I notice that you have the rear motor mount peg in (I'm not sure I know what I'm's the piece that the dowel or aluminum tube goes into to hold the rubber). Is there any reason why you didn't replace it with a normal cross member?

First flights should be exciting!

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Sep 11, 2010, 11:06 PM
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glewis's Avatar
I'l bet it flies as good as it looks too!

After watching your build, I dusted mine off and loaded it in the car for tomorrows trip to the field.
Sep 12, 2010, 02:06 PM
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indoor_fr's Avatar
Wonderfull work, congrats!
I really like these balsa planes, but i've had a bad experience with a cessna 172 from Guillows (crash = litter bin), and i'm a bit reluctant now to build an other wood plane...

My best wishes for the first flights!
Sep 12, 2010, 10:35 PM
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PaulBrad's Avatar
PiperCub49 - The piece that hold the motor peg in the rubber powered model also anchors the forward ends of the two strips that support the stab. I considered thinning it down but an early analysis suggested the pushrods would exit at the location of the piece. As a result I decided to leave them alone and just not cut the holes for the motor peg. When the pushrods were actually installed the exits needed to be a bit behind the motor peg pieces. The difference in material/weight would be difficult to measure .

Glenn - I hope you had a nice flying day and that your Comet Stinson rewarded you with some nice flights.

indoor_fr - I am sorry to hear that you had a bad experience with a model built from balsa. Guillow's designs are actually very good candidates for RC conversion. Typically, though, the balsa needs to be replaced with lighter stock. Not always, but many times that is a way to greatly improve flight performance. Lighter models not only fly better but they also survive crashes better (less inertia to break parts).

Nothing new to report today. The winds gods did not cooperate so I was not able to fly the model. Other life activities kept me out of the shop as well. Hopefully I will have something new to report tomorrow.

Paul Bradley
Sep 13, 2010, 06:45 AM
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glewis's Avatar
Dang weather, we were hoping for a flight report.

I did fly mine yesterday. Mispositioned the battery and made a few laps of the field with a seriously nose heavy model. Landed, repositioned the battery and had a nice flight. One of the guys at the field commented how fast it flies.

I know what you mean about other life activities, same thing prevented me from working on the Skyraider. Oh well, grandkids are only little once!

Sep 13, 2010, 09:04 PM
QuAd FaNaTiC
Razors edge 29's Avatar
Very beautiful.

Wow - I wish I could build like that. I am only on my first Guillows build now lol, the Cessna 180.

Learning a lot though!

Originally Posted by PaulBrad
Glenn - For this project I really did have to do only 18 cowl bumps. One factor in making that happen was not worrying about adding a radius to the bottom of each bump. That greatly reduced the chance of screwing them up. I did do that on the Free Flight version of the model and had waste pieces. The radius is flat enough not to worry about (at least for me that is the case).

Tom - I did see your build thread for the Dumas M-1 Mail Transport. That will be a great model when finished. I am certainly tickled that my Comet Stinson project help you decide to start the M-1 project. I look forward to watching your progress in the build thread.

Not a lot of progress today. Basically the wing panels were mounted. The model is now ready to fly but does need a few more details added to be called complete. I am not sure what will occur first, flight or the additional details. It all depends on my local weather. I want to do the first flights in very calm wind conditions and that has not occurred for a while. Needless to say, while waiting out the wind I will work on the remaining details.

Here a several photos showing the model with the wing finally in place.

Paul Bradley
Sep 14, 2010, 12:29 AM
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PaulBrad's Avatar
The Stinson project is now complete. All that is left on the agenda is to fly the model. That will happen when the wind gods in my area decide to let me have a day/evening of calm weather.

Today was spent completing and installing the remaining details. That included making a prop spinner, building the entry steps, making up some exhaust stacks, and fabricating the tail wheel assembly. Nothing to difficult, just fussy stuff. At some point in time I would like to make up a dummy 9 cylinder radial engine. Just to complete the look and feel of the full size Stinson Reliant. I will post a flight report when that event finally happens.

Here are a few photos of the added details.

Paul Bradley
Sep 14, 2010, 01:36 AM
Lawn dart pilot/builder
Prime_8's Avatar
very nice,
love the tail wheel
Sep 14, 2010, 06:38 AM
Or current resident
glewis's Avatar

I made the dummy engine on mine out of bendy straws. An armature was cut out of 3/32nd balsa. The flex part of the straw was collapsed then slit in half lenthwise. The cylinder half was then glued to the armature. Once all 9 cylinders were in place the assembly is trimmed to fit and glued into the cowl. Add some plastic rod pushrods and done.

Sep 14, 2010, 11:54 PM
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PaulBrad's Avatar
Prime_8 - Thanks. I am very happy with the smooth running of the tail wheel.

Glenn - That is a neat approach to making dummy radial engine cylinders. I will do somehting for sure.

I was finally able to get in a flight with the Stinson this evening. Wind was not quite as calm as I had wanted but I felt the model could handle the modest breeze. According to the wind was supposed be moving at a speed of 4 mph. That seemed about right when I arrived at my flying field.

The model has plenty of power with the ParkZone Micro P-51 power plant. The field is all grass and I chose an area with some taller grass. You know, the old instructions always said perform first flights over tall grass. The model was hand launched and all went well. I had to throttle way back to keep it from climbing like a rocket. Cruise was barley half throttle in the wind. Without wind it should fly at even a lower power setting.

I had been concerned that the model would be nose heavy. I guess the addition of the tail wheel helped move the CG back just a bit and it actually flew like it was a tad tail heavy. Easy to fix with the slide out equipment tray. I landed after a few circuits and moved the battery forward in the battery holder. It felt better with the battery more forward. I think the holder needs to move just a bit forward and everything will be spot on. There is plenty of room on the equipment tray to make that adjustment.

Control response is quite good. It will turn very tightly but is not overly sensitive. I was not able to get a full evaluation due to a motor failure about 4 minutes into the flight. This was a brand new motor so the failure was a major surprise. I am using the P-51 prop and a much lower power setting than when flying my ParkZone P-51. The P-51 as a lot of air time and the motor is going strong. I really wanted to get more air time but the motor failure had me grounded. When I order a replacement motor I will be sure to also order a spare.

Overall I am very happy with the modelís performance. With a few more tweaks it will be a very happy flier. I should be able to get some video at the next flying session when the motor is replaced.

Paul Bradley
Sep 15, 2010, 04:01 AM
Lawn dart pilot/builder
Prime_8's Avatar
i have heard ump51 motors are sor tof random as far as quality .
i figure mine is good as my stang can rocket vertical almost for and pure vert for a good bit, but i find my p-51 only needs ~1/4 T ona clam day to fly well to my liking for fun & scale.
are you gonna open the can up and see what died on the motor ? coil vs brushes ?
i'm just the curio sort who would check . LOL nuts i know.
Sep 15, 2010, 06:44 AM
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glewis's Avatar
Congrats Paul! A model that looks that good just HAS to fly well!

Cruise at half throttle is about what I figured, what is your AUW?

Wondering if yours flys similar to mine.
When I turn, the model yaws and there is a delay until the roll starts, almost like it's resisting the turn.
Once the roll starts it turns fine. Sometimes I have to add some opposite rudder to control the bank and keep the inside wing up. It will snap into a spin if I'm not careful.
How fast is it?

About the CG, why not try making that dummy engine before moving the battery? might be just enough nose weight.

To bad your motor croaked, reminds me of the old IPS motors. Some would last a long time and others were duds right out of the bag.

Anyway, another Bradley masterpiece, thanks for sharing.

Sep 15, 2010, 10:51 PM
Registered User
PaulBrad's Avatar
Prime_8 - Like your P-51, mine also has plenty of power and is showing no signs of degradation. I have not yet opened the failed Stinson motor. I suspect the brushes are the problem.

Glenn - The all up weight for the Stinson is 53 grams. I am using one of the 160 mah lipo cells and they are just a bit heavier than the 130 mah cells. Flying speed seems very scale like. Not real fast but enough speed to handle the modest wind that was blowing.

When turning the model I did not experience the condition you describe as occurring with your Comet Stinson. Application of rudder in either direction results in immediate turn with a bit of roll coupling. Release the rudder input and she immediately returns to level flight heading in the new direction. Many of these smaller models have a very bad habit of dropping the nose when turning right due to prop precession issues. The Stinson did not display that characteristic. Apparently the prop diameter relative to the wing span is small enough not to encounter the precession issue.

I have not yet moved the battery holder for the very reason you mentioned. A dummy engine would add a small amount of weight forward of the CG. I was looking the model over relative to a dummy engine and have not yet decided on an approach. The problem, for me at least, is anything I mount to the forward plate will be to far forward to be in the scale position. When you consider the location of the cowl bumps and remember the valve rocker arms were under those bumps you get an idea of where the cylinders should be. I canít mount dummy cylinders that far back due to the forward plate that restrains the equipment tray. Hence my dilemma relative to doing a dummy engine. I may forego the cylinders and just place a dummy crankcase around the prop shaft opening. That would be fairly close to the scale location. I have plenty of time while I wait for the replacement motor to arrive.

Paul Bradley
Sep 16, 2010, 12:46 AM
Lawn dart pilot/builder
Prime_8's Avatar
paul , you could print out a decal of the cylenders for now.
just to fill the black void like spot .
Sep 16, 2010, 06:31 AM
Or current resident
glewis's Avatar
Looking more closely at the pic of your cowl, I see what you mean, not much room. You could cut the straw into 4 - 90 deg pieces and attach the cyilnder 'slice' onto your front plate.
Or go the easier route and print a decal as Prime 8 suggested.

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