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Old Jan 07, 2013, 07:12 AM
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Symmetrical wing conversion Citabria 1/4 scale HELP !

I need the engineers' help..

I'm going to build a 1/4 scale Nosen Citabria,as a Decathlon.

The Decathlon has symmetrical wings for better aerobatic flight.

To make my life easier,I'm going to draw a centerline on the kit ribs, and add partial ribs to the bottom of them, to fill it out to fully symmetrical..

How much positive incidence should I install ? The lifting,flat bottom wing is set at +2 degrees...
I'll start with a theroretical statement. High wing, requiring down thrust,stab at 0,-Wing still at +2 ? Consider there'll be a LOT of inverted flying, and tell me where to start. Thanky.
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Old Jan 07, 2013, 08:13 AM
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I have no training in the field, but I would go with no engine offset and 0 incidence, both on the wing and stab.
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Old Jan 07, 2013, 10:08 AM
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upside down

Okay-here's a thought though...Since the wing is high above the thrust line,I thought some 'down' thrust (3 deg),would help drag the wing along,without the wing trying to lift the nose.
When inverted,the 'down' now becomes 'up' thrust and it shouldn't take so much elevator to hold the airplane in a level attitude.
Present all answers considering full throttle...I'm gonna fly the bejesus out of it !
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Old Jan 07, 2013, 10:53 AM
Scale Builder
United States, AZ, Litchfield Park
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Hi Earl,

Just my .02 cents but I would probably go with 0-0 on the flying surfaces and probably 2-3 degrees of down thrust. This has worked very well for me on low and mid wing types but I think it would be just fine on the Citabria. Just FYI, like Tom, I have no formal training but rather lots of "on the job" training!
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Old Jan 07, 2013, 11:17 AM
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Not a huge deal breaker but according to my haphazard research (yahoo) the Decathelon has the NACA 1412 ( @ 12% thickness/chord) over the standard Citabria's 4412, which is undercambered and also 12%thick.

Also, according to this site:http://airfoiltools.com/airfoil/deta...il=naca1412-il

The Decathelon's 1412 section is not quite a full symetrical section.

Also the Citabria has -3.5deg stab incidence while wing has 1 deg. positive. I would bet the the Decathelon's setting are the same.

With scale settings and airfoil I suspect the model Decathelon will be as fully aerobatic (contraction Citabria spelled backwards) as the real aircraft. Are you going over to the dark side??!!
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Old Jan 07, 2013, 12:41 PM
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I have to assume some right thrust is already built in. Some where between 2 & 3 degrees will be needed. As far as down thrust, I'm uncertain. IMO the airplane has no idea if it is right side up or down side up. Down thrust will in effect add some positive incidence. I would be inclined to go +1 on the wing and 1 down on the engine and set the stab to zero. It might be wise to make an adjustable motor mount so the thrust angle could be adjusted.

Ken
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Old Jan 07, 2013, 01:17 PM
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Earl, I'd strongly advise you to simply make new ribs. By adding bottoms to the present ribs your wing is going to come out VERY thick. Far better to make up some proper NACA 0012 or whatever you come up with ribs to avoid this.

If you want to try it with the CG well back to where the model is right on neutrally stable then you can reduce the decalage between the wing and tail to 0-0. If you don't want to get that crazy then stick with the +2 in the wing. You can "force" the model to less than the 2-0 setup in any case by simply using some downtrim in the elevator in response to moving the CG back.

The downthrust you use in a low pitch stability setup is then going to be mostly used to compensate for the high center of drag found on a high wing design. The good news is that when inverted the downthrust will then be upthrust which compensate for the LOW center of drag when inverted. So don't try to overthink it.

In the end the trim setup and final thrust angle is all going to be somewhat flexible. By altering the CG to a more rear position and re-trimming the elevator trim point and fine tuning the downthrust angle you should be able to obtain a pretty close to neutral handling. It won't be up for competing in a world class precision aero competition but it'll get the serious fun flying job done nicely.
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Old Jan 07, 2013, 02:30 PM
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Epoxyearl,

For us modelers there is no limitation when modifying one model to another so why not one little step more?

TF
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Old Jan 07, 2013, 03:06 PM
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Memorial Day

Packard Pursuit- I have no intention of jumping in the pool-just my toes in....

The Decathlon is being built in memory of a Lawyer friend who lost his life in his.
This year,we're planning a Memorial Day Celebration for the many friends who've gone on, for various reasons...

I intend to fly it like a wild man, for one flight, then go back to my 30 mph Cub for the rest of my life.

For one brief moment,I'll throw caution to the wind .
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Old Jan 07, 2013, 03:13 PM
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Quote:
Originally Posted by Taurus Flyer View Post
Epoxyearl,

For us modelers there is no limitation when modifying one model to another so why not one little step more?

TF
TF.I'm in the middle of a Mid Wing Conversion,currently...Piper family...
Click on "forums" scroll 'way down to "Balsa Builders", then "Mid-Wing Special"

That is a 1/3 scale Piper Super Cub,converted to a Shoulder wing...scaled from the single full scale one.
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Old Jan 08, 2013, 05:08 PM
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Ribs are showing

I grabbed the kit from the FEDEX driver,said "thank you" and rushed into the shop...I cut all the tape ,opened the box and pulled out a pair of ribs.
Within minutes,I had a new compromise symmetrical rib pattern.I moved the forward spars back 1/2 " and marked holes for a wing tube.I marked the ailerons 1/2" more chord,and moved the rear spars forward a like amount.

With no concern for scale fidelity, I'll build a full "d" tube leading edge,with shear webbing at the rear of the front and rear spars,and an aileron spar at the opening

I'll eventually build the stock wing for some reason,although I'm not sure when..
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Old Jan 08, 2013, 08:39 PM
The Prez....... again
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Quote:
Originally Posted by epoxyearl View Post
I grabbed the kit from the FEDEX driver,said "thank you" and rushed into the shop...I cut all the tape ,opened the box and pulled out a pair of ribs.
Within minutes,I had a new compromise symmetrical rib pattern.I moved the forward spars back 1/2 " and marked holes for a wing tube.I marked the ailerons 1/2" more chord,and moved the rear spars forward a like amount.

With no concern for scale fidelity, I'll build a full "d" tube leading edge,with shear webbing at the rear of the front and rear spars,and an aileron spar at the opening

I'll eventually build the stock wing for some reason,although I'm not sure when..

I think I need you to come live at my house for a couple of months. I just might get some building done. Or not.... You would be flying around the shop at 500MPH building 10 airplanes at once. It would take you about a week to finish my Zlin at the pace you build. A guy could get hurt just watching you go

Ken
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Old Jan 08, 2013, 10:00 PM
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You may as well turn this into a build thread. SO BRING ON THE PICTURES once you've got something vaguely airplane shaped to show....
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Old Jan 09, 2013, 04:06 AM
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Quote:
Originally Posted by BMatthews View Post
You may as well turn this into a build thread. SO BRING ON THE PICTURES once you've got something vaguely airplane shaped to show....
I tried to post these yesterday,but the computer was bein' a snot.

From the input I got from you guys,this is the TLAR profile that resulted.I moved the high point back,went fully symmetrical,and a wing tube.
The entire wing can rotate on the tube,as I 'dial-in' the correct incidence.
Starting with +3 degrees, I'll shim as required until rolling inverted needs as little opposite trim as possible.
I still think 3 degrees down thrust is a fine place to start,because of the high wing placement.
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Old Jan 09, 2013, 05:34 AM
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The 'character' of the Decathlon is augmented by the short landing gear.When I saw the kit for sale,the first thought was to make the gear a little longer,to satisfy MY eyes.
Well,- American Champion,the current owners of the design, agreed.
They hung 210 horsepower on the standard airframe,and named it "Xtreme".
Deferring to the larger prop's demands,they lengthened the gear legs.woo-hoo!
And-the scale prop would be 19",-right in my ball park.
I need a new building board.
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