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Finding chord line of wing

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Old 01-24-2008, 10:19 AM
  #1
c-jack
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Default Finding chord line of wing

Hi, everyone

I am presently building a Balsa USA Citabria Pro and am stopped by setting the wing incidence. I am not an aeronotical engineer and BALSA USA say to find the chord of the wing to set the incidence of the wing relative to the fuselage. WOW! They don't show this on the plan, but tell you that, to use the trailing edge of the wing for the rear of the chord. Fine, but what about the leading edge? This wing is semi symetrical and I have no clue on how to find the point for the front position of the chord.
Anybody with a degree in airplane engineering out there? I need help!!!!!
I also don't want to email BALSA USA as I did it for some other item on the plan and I got some dumb answer, like how come I didn't know that!!! What a way to treat customers.

Thanks to anyone replying to this! It will be greatly appreciated.

[>:][:-]
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Old 01-24-2008, 11:09 AM
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Mike Connor
 
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Default RE: Finding chord line of wing

I would think it would be the most forward point of the leading edge.
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Old 01-24-2008, 11:13 AM
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Default RE: Finding chord line of wing

This might help. Get a long piece of scrap balsa that is a little bit longer that the airfoil. You'll be making a cheap compass using the balsa and two pins. Push a pin thu the balsa at one end to serve as a starting point. Secure it with a little CA so it wont wiggle. Then push in the other pin near the opposite end close to the distance of the leading edge. Using the plans airfoil as reference, stick the "fixed" pin at the center of the trailing edge. Then set the other pin to where it just reaches a little past the leading edge. Making sweeping movements (and making sure the TE pin stays fixed at one point), you will be slowly pushing or tilting the pin until it just touches the front of the leading edge. Note where it first touches the LE without slicing through the airfoil. That is where the LE intersection point it located. Then you just connect that point with the TE and youre done!

Hope this helps

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Old 01-24-2008, 12:42 PM
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Default RE: Finding chord line of wing

The instructions OR plans SHOULD state how much incidence you need. MOST parasol wing planes use a 2 degree or 3 degree POSITIVE incidence.

I use the Great Planes laser incidence meter.

I block the plane until the HORIZONTAL STAB is at 0 degrees (level), then I put the GP incidence meter on the wing and adjust the incidence from there. Be prepared to use washers for shimming and/or to cut the cabanes as needed to get the proper incidence.

When I did my Stosser, I used ring terminals soldered onto music wire. To set them properly, I used a drop of CA on the terminal/wire junction to "lock" the ring terminal in place so I could remove the wing and mark the location for soldering.

I didn't have the exhaust manifolds on when I took the photo.

Hope this is of some help.

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Old 01-24-2008, 01:01 PM
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Default RE: Finding chord line of wing

Chords...
and incidence..
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Old 03-27-2012, 06:31 PM
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Default RE: Finding chord line of wing

Yours truly had been staring at the instructions (#5 on page 26 of the manual) and I too was struggling with the method described to determine the wing incidence on the Balsa USA Citabria Pro.

I used the method described by Funkworks (post 3 above), to determine the chord line, using the profile for the wing as shown on the fuselage plan.

I then measured (at right angles) from the datum line specified in the instructions, and got the following results:

datum line to forward chord line: 4 29/32"

datum line to rear chord line: 5 3/16 "

Thus the wing as drawn has a NEGATIVE incidence relative to the datum line (trailing edge is higher than leading edge when aircraft is upright); Using trigonometry I come up with an angle of 1.15 degrees. That is reasonably close to the one degree stated in the instructions.

The cited instructions state "(t)he incidence angle in the Citabria Pro is 1 degree positive as shown on the plans (remember the airplane is upside down)." AHAH, MAYBE, JUST MAYBE, he meant positive when viewed UPSIDE down?????

If this remains of interest to anyone, I will scan my sketch of the chord line and post it.

The question remains for me, do I use the method described in the instructions. I have emailed BUSA.
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Old 03-28-2012, 10:05 AM
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Default RE: Finding chord line of wing

Why not use an incidence meter? Am I missing something?
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Old 03-28-2012, 10:25 AM
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Default RE: Finding chord line of wing

The instructions say an incidence meter will not measure the angle accurately because of the shape and leading edge radius of the airfoil.
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Old 03-28-2012, 12:29 PM
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Default RE: Finding chord line of wing

Buy a Robart incidence meter and attach it to your wing which is attached to your fuselage. Level wing/ fuse assembly to '0' and weight the assembly so it won't move. Remove the Robart meter and stick it onto the stab and adjust stab for 1.5 degrees incidence where the leading edge of the stab is higher than the trailing edge. That'll give you a great starting point for flying.
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Old 03-28-2012, 12:50 PM
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Default RE: Finding chord line of wing


Quote:
ORIGINAL: ARUP

Buy a Robart incidence meter and attach it to your wing which is attached to your fuselage. Level wing/ fuse assembly to '0' and weight the assembly so it won't move. Remove the Robart meter and stick it onto the stab and adjust stab for 1.5 degrees incidence where the leading edge of the stab is higher than the trailing edge. That'll give you a great starting point for flying.
Do you not have this backward? In all of my planes, you want the wing to have more angle of attack than the elevator/stab.
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Old 03-28-2012, 02:42 PM
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Default RE: Finding chord line of wing

wing incidence,tailplane incidence.down-thrust,side-thrust, undercarriage location. toe-in, camber, centre of gravity are all taken into consideration by a model builder. All of these are" relative " to fuselage "datum line",plan view centreline,maybe wing chord line,but more importantly the wing foil section "datum line" [check the chord-line shown on the Seelig airfoil section in a previous posting] I am not familiar with that section,so i cant say that the chord-line and datum line are the same. My point is that more information should be supplied by kit manufacturers.and plan draughtsman. Paper plans are notorious for warping and shrinking over time, so may be incorrect regarding accuracy. I am sure all of us have seen spar lengths differ on left wing and right dimensions.
let us not allow this discussion to develop into the " perennial discussion " like the "downwind turn"saga did in past years



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Old 03-29-2012, 08:01 AM
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Default RE: Finding chord line of wing

Just got the response from Balsa USA as follows:

"These days it would be a lot easier to use an “incidence meter” then trying to work off the mean cord line.
I recommend setting it up using an incidence meter so the wing is at 1/2 and no more then 1 degree positive with the tail at 0.0 degrees. You’ll also need a degree or two of down thrust in the engine.
Hope this helps you out!"

(No irony there!)

The question posted by c-jack was how does one locate the chord line at the leading edge. Mike Connor and Funkworks have offered answers to that question.
Wikipedia, quoting a 1975 work (http://en.wikipedia.org/wiki/Chord_%28aircraft%29) says:

"In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil. The chord length is the distance between the trailing edge and the point on the leading edge where the chord intersects the leading edge."

Humm...how does one find the "center of curvature" of the leading edge, especially one like the BUSA Citabria Pro with it's complex shape?



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Old 03-29-2012, 08:45 AM
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Default RE: Finding chord line of wing

Hi Rodney- this works for me. Maybe BUSA has a concern that some folks won't make the LE of wings correctly or symmetrical with respect to each panel. I scratchbuilt a 1/4 RRG Prufling sailplane (105" span) and used this tried and true method for setup during construction. It was maidened at the JR Aerotow last year in front of God and everybody! The only trim changes needed were a reduction in nose weight and an aileron trim change. The elevator was spot on target. I've used this method on all of my builds, some of which can be seen in my Gallery.

Hi c-jack, hope you get it lined out. That's a neat airplane!
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Old 03-29-2012, 08:53 AM
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Default RE: Finding chord line of wing


Quote:
ORIGINAL: Rodney


Quote:
ORIGINAL: ARUP

Buy a Robart incidence meter and attach it to your wing which is attached to your fuselage. Level wing/ fuse assembly to '0' and weight the assembly so it won't move. Remove the Robart meter and stick it onto the stab and adjust stab for 1.5 degrees incidence where the leading edge of the stab is higher than the trailing edge. That'll give you a great starting point for flying.
Do you not have this backward? In all of my planes, you want the wing to have more angle of attack than the elevator/stab.
Hi Rodney... you are of course very right in this instance... ARUP got his thinking crossed up. But... I'll remind you of your Phaeton 90 and the top wing given some negative. But... I'm just pickin atcha to illustrate that there are often if not always exceptions to the general view.
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Old 03-29-2012, 08:57 AM
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Default RE: Finding chord line of wing

I do think the plan notation to be careful in the use of an incidence meter on some airfoils is appropriate.
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Old 03-29-2012, 09:43 AM
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Default RE: Finding chord line of wing


Quote:
ORIGINAL: AA5BY

I do think the plan notation to be careful in the use of an incidence meter on some airfoils is appropriate.
I agree, and am working on the assumption that the designer or instruction writer meant what he or she said so emphatically, that an incidence meter should not be used. The kit sat on my shelf for many years, and I suspect was the work of the late Ron Bush, founder of Balsa USA and who is now deceased. I met Ron about 15 years ago and he gave me a tour of his facilities. We talked a lot, and I got the distinct impression he was very knowledgeable, as well as a man who meant what he said and said what he meant.

Since I don't know how to find the spot on the leading edge that he referred to, I chose a different method to align the wing so it in the position shown on the plans. I realize that this could be wrong if the plans are incorrect, either because of drafting error or possible distortion (as noted by Wilde, above). Subject to such possible errors, the wing will be positioned per the plans, whatever angle of incidence that is.
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Old 03-29-2012, 10:06 AM
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Default RE: Finding chord line of wing

Try this: lay the wing on a flat surface. Best to use two rulers. Place one ruler in front of the leading edge, measure where it touches the leading edge. Raise the trailing edge up that height of the table. Remeasure the LE again and make any adjustments to the TE. Do this until both rulers measure the same height. Mark the LE at that point.
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Old 03-29-2012, 10:26 AM
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Default RE: Finding chord line of wing


Quote:
ORIGINAL: smithcreek

Try this: lay the wing on a flat surface. Best to use two rulers. Place one ruler in front of the leading edge, measure where it touches the leading edge. Raise the trailing edge up that height of the table. Remeasure the LE again and make any adjustments to the TE. Do this until both rulers measure the same height. Mark the LE at that point.
Okay...what have we located, the forward chord line?
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