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Author Topic: scratch build balance  (Read 1612 times)

Offline Phil Krankowski

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scratch build balance
« on: October 07, 2012, 08:41:17 PM »
I am build a pair of Zinger I
http://www.aeromaniacs.com/Zinger.htm
but made a scale error while printing, and didn't catch it till late.  My wing measures 6 1/4 LE to TE instead of the 5 1/2 inch that appears indicated on the website.  I am planning on using a Cox .049.  I have two, one with the long 8cc tank, and the other with the short 5cc tank. 

The plans seem to indicate that there should be 1 inch in front of the LE, the wing cord of 5 1/2 inches, then the wing chord again, 5 1/2 inches, to the back tip of the fuselage.  I just finished the second set of wing, vertical stabilizer, horizontal stabilizer, and elevator.

I plan to use balsa wood blocks to beef up for a plywood firewall, and apply a 1/16 plywood doubler to at least the thickest part of the wing, since these are going to be bashers, and hopefully stunt capable.  I plan to cover the wings in a plastic shrink film, I think I bought "Monokote" but might have a different product.  I plan to use polycrylic, then rustoleoum on the exposed wood, and I am aware of the 2 week cure time, and that snow might fly before these planes are cured off.

Is there an "easy" way to aid balancing, or pre-balance the airframe BEFORE cutting the balsa wood fuselage? 

Cutting an inch long in the nose, then trimming IS an option.  Adding some weight after finishing is OK, but I obviously don't want to add too much.  I don't expect to get a flight test in before winter with the long cure on the paint. 

Thank you

Phil
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Offline Larry Renger

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Re: scratch build balance
« Reply #1 on: October 08, 2012, 08:29:04 AM »
The fuselage contributes little to the balance of an airplane, so you could just rubberband all the other components to a stick of roughly the correct length and set up your cg that way.  Yardsticks are either very cheap, or even free.  Possibly paint stir sticks might be long enough and usually are free.
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Offline Phil Krankowski

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Re: scratch build balance
« Reply #2 on: October 08, 2012, 09:49:21 AM »
That crossed my mind too.  I like the idea.  I have a 1/2 inch stick of balsa that I have no clue why I bought...seems perfect for this!

Phil
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Offline Phil Krankowski

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Re: scratch build balance
« Reply #3 on: October 08, 2012, 10:24:52 PM »
I have the Cox engine with the regular run tank on it, the engine with the extended run tank is on a bench board currently.  I am sure this makes some difference.

I cut the balsa stick to the intended length: wing cord + wing cord + 1 inch, so 6 1/4 + 6 1/4 + 1 = 13 1/2 inches

The directions say balance 15% from the leading edge, so .94 inches...I measured 1 inch and marked the wing.  I know that a 1/2A can balance on the leading edge and fly quite nice.

I positioned everything as intended and the stick was tail heavy.  I moved the wing back 1 inch, and the tail forward 1 1/2 inches so the wing is 2 inches from the motor, and the tail is 1 1/2 inches closer to the wing.   

Does this seem reasonable?  The fuselage ends up only 12 inches long.  I even cut blocks for the engine mount and pinned them, the engine seems way in the front using 2 inches of projection. 

Phil
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Offline Larry Renger

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Re: scratch build balance
« Reply #4 on: October 09, 2012, 08:25:19 AM »
I would only move the wing back, maximizing the wing/tail spacing.  In fact I would have kept the wing/tail as it was and just lengthened the nose.
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Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

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 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Phil Krankowski

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Re: scratch build balance
« Reply #5 on: October 09, 2012, 10:01:52 AM »
I haven't cut the fuselage out yet, so I'll give that a try. 

 :!
I have enough of my stick left that I can fasten the wing and tail on, then position the motor last and see if that looks any better. 

Phil
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Offline Phil Krankowski

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Re: scratch build balance
« Reply #6 on: October 09, 2012, 09:45:52 PM »
I redrew my fuselage at 15 inches long with 2 1/2 inches of nose.  The nose is not much longer than my lil wizard.  Hopefully this will turn a lot better than the lil wizard.

My sig skyray has 2 inches of nose, and a much much smaller elevator.  the lil wizard is in the garage so I can't compare.

If it looks good tomorrow, and a better idea isn't shown to me, I'll cut out a fuselage!

Phil

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Offline Phil Krankowski

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Re: scratch build balance
« Reply #7 on: October 10, 2012, 03:30:45 PM »
That was easy...sorta.
Phil
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Offline Larry Renger

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Re: scratch build balance
« Reply #8 on: October 10, 2012, 07:38:17 PM »
Looks great! The airplane too.  ;D
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Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

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 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Phil Krankowski

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Re: scratch build balance
« Reply #9 on: October 10, 2012, 10:04:20 PM »
That is my daughter, 4 and a bit.  She flew a Platter Extreme we made from 4mm fluted plastic with me a few days ago.  Interesting getting that to launch with a stooge.  She pulled a fast one on me and

I changed the tail, and shortened the space between the wings by an inch and cut the second fuselage out.  I ran out of 1/16 plywood for doublers...can't get that thin in town.

I am looking for the rubber band on motor mount that came with the original Lil Wizard.  I would like to use those because I am winding up short an engine or two...5 engines, 7 planes.

I have a scan of the part dimensioned...but not on this computer.

Phil

sorry, huge pictures.
Phil
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Offline ray copeland

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Re: scratch build balance
« Reply #10 on: October 11, 2012, 06:59:25 AM »
Phil, you can just make those rubber band mounts out of 1/8 inch 5 ply, works just fine, may have to file the screw ends down on the side the engine
mounts to. 
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Offline Larry Renger

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Re: scratch build balance
« Reply #11 on: October 11, 2012, 08:11:32 AM »
#2 Self tap screws should work just fine, and then drill out pockets in the rear firewall to clear the ends.
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Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Phil Krankowski

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Re: scratch build balance
« Reply #12 on: October 11, 2012, 09:33:48 AM »
I've made them out of 2 pieces of 1/8 inch plywood.  I have a hard time with plywood and precision.

use sandpaper and take my time...yes.

Dimensioned pictures as promised, this is a scan from my 30yo Lil Wizard mount when I was cleaning the engine.

Phil

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