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Author Topic: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk  (Read 1731 times)
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kittyfritters
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« Reply #25 on: August 08, 2018, 10:22:40 PM »

The last one I built was about 50 years ago and I used the angle fixture, no measurements. As I recall at the time with my limited knowledge all I wanted was the tips to the bottom of the canopy (real sound aerodynamic reasoning  Roll Eyes ) and balanced side to side. I do recall the Chipmunk flew better than most of my other models at the time.

Having the wing tips at the level of the bottom of the cockpit canopy is a reliable rule of thumb for a small, low wing model.  The other rule of thumb is 1 inch of dihedral for every 18 inches of wing span.  Lateral balance is something that it seems that most modelers have forgotten in the RC age,  For rubber powered models, do it.

With the low wing, 900 series models it is a good idea not to cover the bottom of the fuselage until the wings are on.  This give you the opportunity  to add a balsa sheet "carry through"  to tie the ends of the spars together.  This makes the wing attachment much stronger.  I have a 900 series Typhoon built this way in 2004 that is still flyable.  In the photo the wrinkles in the wing are painted on.  (Subject of another post.)  The tissue is actually smooth.

Hope this helps!

KF
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Re: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk
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Konrad
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« Reply #26 on: August 08, 2018, 11:03:17 PM »

Lateral mass balancing is important. But what I was trying to get at was that the dihedral be balanced, vertical tail centered between the wings.

All the best,
Konrad
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Dan Snow
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« Reply #27 on: August 09, 2018, 11:19:49 AM »

I believe I have discovered the cause of the dihedral issue on my plane. As drawn on the plans the dihedral block for the root rib measures 3 degrees.  As die cut, the piece measures 6 degrees!!! I'll give it +/- a degree but I measured each root rib in a couple of places and the reading was pretty consistent. So that explains that issue.

I have cut out another pair of root ribs andwill make a simple jig to sand them to 3 degrees then attach them over the existing rib. I put the ribs before sanding on my scale, (it measures in 1/10 grams) and it didn't register.
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DavidJP
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« Reply #28 on: August 09, 2018, 11:24:06 AM »

Well that is good to hear - not your "building" - and the additional rib will make it stronger all round. I like scales that don't register  Cheesy
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Snaky Stringer
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« Reply #29 on: August 09, 2018, 12:13:02 PM »

If your  scales don't register  maybe you need one of those you make yourself from some balsa and some guitar string. I imagine one of those can measure hundredths of a gramme. I have never actually made one but often fiddled with bits of wire to test the idea. Any day now, I expect. Cheesy
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Dan Snow
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« Reply #30 on: August 09, 2018, 09:10:27 PM »

Started covering the wings and tail feathers using a glue stick, and I am amazed at how easy it makes this task. I seem to vaguely remember not being particularly fond of this task  way back in the dim reaches of the past. I'm interested to see how it holds up.

After reading many posts I see there are almost as many methods for shrinking said covering. Water, 70% alcohol, 90% alcohol, and I think I read one that used nitrate dope. Or was that for sealing?

What's the consensus here?
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DavidJP
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« Reply #31 on: August 10, 2018, 08:09:36 AM »

Yes the glue stick is quite a welcome introduction.  As to shrinking if I don’t apply the tissue “wet” at the outset then a “misting” with one of those small spray we can get in the drug stores.  I have never used dope for shrinking as it is a bit robust I find and can screw up a delicate structure so I use it after water shrinking as a “sealant” and diluted with thinners at least 50%.  Dope as in cellulose that is as I have not had good results with the expensive water base dope.  Andany being of advanced years am hooked on the smell of dope and diesel etc.

But you do seem to be very much on the right track from your own researches etc. so it all sounds good for you.  I hope so.
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« Reply #32 on: August 10, 2018, 09:18:19 AM »

Quote
What's the consensus here?

I don't think there is one Smiley For small outdoor models such as you Chipmunk, I have covered with Glue Stick and then water shrunk much as David mentions but not bothered to seal with dope(nitrate). The tissue will sag in humid conditions but is otherwise ok.

If you are going to add decals, etc or paint then it would be best to seal the tissue with thin dope as David mentions.

John
« Last Edit: August 10, 2018, 09:35:55 AM by OZPAF » Logged
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« Reply #33 on: August 10, 2018, 10:00:44 AM »

I haven't decided on a finishing technique yet. Once I have the whole thing covered and shrunk, I'll decide.

Right now it weighs 25 grams with everything but the fuselage covered. Is that a reasonable weight at this stage or did I make it too heavy?
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kittyfritters
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« Reply #34 on: August 10, 2018, 01:13:35 PM »

When I cover dry I shrink with rubbing alcohol and pin the wings and tail surfaces down while shrinking.  I use tabs, made of tab cards oddly enough, to hold the edges of the parts.  You can also shim the trailing edges of the wing while shrinking the tissue to get wash out.  After the tissue is sealed (I use Krylon #1305.) the washout will stay.  (See photos below.)  The washout in the wingtip photo looks excessive but it is only 1/16 inch.

KF
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Re: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk
Re: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk
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« Reply #35 on: August 10, 2018, 01:24:23 PM »

By my standards that seem quite light!  Be warned though those last little bits like ballast in the nose, the motor and prop. etc. all seem to pile on the the grammes! But even so I would have thought you are on the right side of the line.

Will you be posting a picture or two?

John, I find doping a necessity as left simply water shrunk my effort looks decidedly sad so add a little silk to the sows ear.  But also in truth doping is such a habit I have to say it has never occurred to me to dispense with it.  I must try and think more laterally.  I am therefore obliged!

And I have never tried alcohol - for shrinking that is.  What is the benefit please. I would think rubbing alcohol is pretty weak?     
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« Reply #36 on: August 10, 2018, 03:11:32 PM »

Be careful using alcohol as a shrinking medium.  Alcohol is also a solvent for glue stick.  I agree with the krylon as opposed to dope as it is very low shrink.
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Dan Snow
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« Reply #37 on: August 10, 2018, 04:28:37 PM »

Trying to post photos here is beyond me. I got it to work once and after that I can't figure out how to get the file small enough. I recently got a new phone and I guess it makes much larger files than my old one. I'll try but not holding my breath.

DavidJP the 25 grams includes the prop, wheels, canopy and motor.
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Re: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk
Re: Beginners attempt at Scale Rubber Power, Guillows 17" Chipmunk
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DavidJP
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« Reply #38 on: August 10, 2018, 05:48:16 PM »

Well done - you do seem to be well over the right side of the line.  I believe (in my defence) that some people give the weight sans motor for example and any ballast but I am not sure of point in that.  It is the flying weight that is the issue at the end of the day! So that is pretty good on your part.
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« Reply #39 on: August 10, 2018, 05:55:48 PM »

For resizing pictures, see: http://www.hippocketaeronautics.com/hpa_forum/index.php?board=85.0
ian
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« Reply #40 on: August 10, 2018, 06:20:37 PM »

Well DUH!!!! Shocked  After many many tries I read the structions once a gain and a light bulb went of on the resizer link you posted PB_guy. It is the actual resizer, not instructions for the resizer!!  Damn, it works!! 
Once I figured out that browse and choose file were the same thing, now I know.   Many thanks.
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« Reply #41 on: August 10, 2018, 10:07:18 PM »

It was with a bit of trepidation that I grasped my spray bottle and cautiously pulled the trigger, blasting a small but mighty cloud of fine water mist at the tissue covered structure of wings and empennage for my Chipmunk.  As quickly as I dared lest I rip said tissue, or in my haste crushed said structure I placed it on my building board.

Quickly supports of 1/16" in height were placed upon the building board and the structures restrained with slim needles of fine steel. An extra support of 1/16" height was affixed under the trailing edges of the wing tips.

Then but a mere hour or two later all said structures were dry, the covering tight, and behold, still attached!!  "Success" I cried, "not since I was a wee sproutling some 50 or so years ago had I attempted such a task, but again I can savor the joy of the completion of a dreaded ordeal!!
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« Reply #42 on: August 10, 2018, 10:08:42 PM »


...And I have never tried alcohol - for shrinking that is.  What is the benefit please. I would think rubbing alcohol is pretty weak?     

That is the benefit of rubbing alcohol.  Rubbing alcohol is only 30% water.  (Don't use 91% alcohol, that's just for sterilization and electronic cleaning.)  A misting of rubbing alcohol shrinks well but does not take all the "shrink" out of the tissue.  If there are wrinkles, for example in a corner of the wing, you can simply apply some "full strength water" to the wrinkled area.  In really desperate circumstances, water and a judicious application of a hair dryer will get those pesky corner wrinkles out.  For myself, if I have to reach for a hair drier I have to make the decision to do that or rip it off and recover.

The photo shows a covering job done when I was having trouble with numb fingers in my left hand and I did have to reach for the hair dryer.  Yes, there is a patch.  I managed to poke a hole in it with an Exacto knife behind the wing opening after I got it covered...hangar rash and it wasn't even finished. Roll Eyes
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kittyfritters
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« Reply #43 on: August 10, 2018, 10:10:52 PM »


...Then but a mere hour or two later all said structures were dry, the covering tight, and behold, still attached!!  "Success" I cried, "not since I was a wee sproutling some 50 or so years ago haad I attempted such a task, but again I can savor the joy of the completion of a dreaded task!!

Congratulations!  Nicely done!

KF
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Konrad
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« Reply #44 on: August 10, 2018, 10:19:06 PM »


Then but a mere hour or two later all said structures were dry, the covering tight, and behold, still attached!!  "Success" I cried, "not since I was a wee sproutling some 50 or so years ago had I attempted such a task, but again I can savor the joy of the completion of a dreaded ordeal!!
I'm so happy to read this. Actually I find that success today is actually nicer than is was in my youth. As I now have the critical eye to ascertain what quality is.
Again welcome to your first steps back into this rewarding hobby!
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« Reply #45 on: August 10, 2018, 11:28:23 PM »

Success!! - looks perfect  Shocked  Dan Snow - good work!  Cheesy Smiley

LASTWOODSMAN
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« Reply #46 on: August 11, 2018, 01:19:05 AM »

Very neat and tight covering Dan. Don't lose your little atomiser Smiley

John
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« Reply #47 on: August 11, 2018, 04:44:05 AM »

There you go Dan - piece of cake!  But yes isn't it really satisfying when it all works.  I sense you will be hooked somewhat - get a flight to two in and no turning back!!  Again - well done.
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« Reply #48 on: August 11, 2018, 08:14:03 AM »

Thanks gang, all kind words are gratefully received!

Now I know the Chipmunk is a fixed gear trainer, but I am seriously considering leaving the main gear off. The kits recommendation for installing the mains, quite frankly, gives me the heebie-jeebies!! I mean really? Cover wing, shrink covering, cut a slit in the covering, then snake the wire into the wing, OVER the main spar, then glue in place? Visions of an ugly patch on the upper wing surface when I poke the dang wire through it!!

Since this is not a competition model I am seriously considering dumping the gear. If I was to ever make another, (unlikely unless they convert to laser cut) I would install the gear before covering. One teeny hole in the tissue and it's done!!
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« Reply #49 on: August 11, 2018, 09:45:03 AM »

Leave the gear off, and call it a Super Chipmunk. This is a real full size ship.
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