bugleboy

My Feedback: (8)

Okay, I need some help.

I am building a Balsa USA Citabria Pro and just finished the metal struts or wing supports, whatever you want to call them. However, the plans show what appears to be a NEGATIVE incidence and the instructions say the wing should have a POSITIVE. I constructed the "struts" per the instructions and it appears to have a NEGATIVE incidence.

Incidence of the Citabria Pro has been discussed in another thread, but there did not seem to be any concensus.

I am open to suggestions/help! How do you all measure incidence without a incidence meter? Maybe that is my problem. [sm=confused.gif]

Thanks,
Jason

BigBird1

My Feedback: (4)

I've got one with a Saito 120 on it..I did all the measuring and everything and still missed the incidence on the wing...had to add down elevator to get it to fly level...kept adding shims to the back of the wing till all of the down elevator was out of it...it's a sweet flying plane..you'll love it..

BB1

bugleboy

My Feedback: (8)

I'll be using a Super Tigre 2000.

So if you had to add shims to the back, you were decreasing the incidence, correct? I built and measured according to the instructions and the plans, but to get the positive incidence referred to in the instructions, it looks like I will have to shim the front of the wing. Weird?!?

Do you know what incidence you ended up with? Was the end result a negative or positive incidence?

Thanks,
Jason

Ed_Moorman

My Feedback: (1)

It has been typical of many Balsa USA kits, and my buddy and I have owned several, that they have the stab at 0 and a flat bottom wing with the bottom at 0. Due to the rounded leading edge, this puts the wing at plus 1 1/2 to plus 3 degrees, making the plane climb under full power. Ours was powered by a Magnum 1.08 and climbed like mad until we reduced the wing incidence. We have build their Swizzle Stick, 3 Eindeckers, 2 small, 1 large, the Citabria, MO-1 and I forget what else. They all flew the same.

By the way, positive incidence is leading edge up, wing or stab.

Get a small level. Lay it on the stab and prop the tail of your plane up until the bubble indicates level. Now measure from the center of the leading edge to the table. Then do measure the TE. If these aren't the same, shim up the TE.

As has been mentioned, you can fly and begin adding shims under the TE until the elevator trim for full power flight is a level elevator.

Another thing you can do during the building process is to add to the bottom of the ribs, making a symmetrical, or nearly symmetrical airfoil.

bugleboy

My Feedback: (8)

I may be showing my ignorance, but I want to get it right. Finding the center of the LE on a sym. airfoil is easy, but it is not so easy on semi or flat bottom airfoils. Here is how I would imagine it would be done:

-Find the fattest part of the airfoil/rib
-Measure the thickness
-Make a mark half way in between
-Draw a line from the center of the TE through the point just marked and continue the line through the end of the LE
-Where the line hits the LE would be the center of the LE.

Correct? See the attached. Forgive the paint shop picture and quality. []

Jason

DaveB

Close, but IMO not quite correct. I believe the line should be drawn from the center of the trailing edge to the forward most part of the leading edge. If you study the leading edge on the "Pro", it is not symmetrical, rather it is curved to match the air foil which has, if I remember correctly, a flat bottom. Therefore the line would be somewhat lower than center.

I built the "Pro" and suffered the same anguish you are experiencing. I finally reached the conclusion that with the stab at zero (0) degrees incidence, the wing leading edge at the most forward spot should be positive. Having said that, I never got around to finishing the covering and flying. Maybe this coming spring. Please keep the post going.

Good luck.

DaveB

bugleboy

My Feedback: (8)

Most forward point? Okay that should be easy enough.

Keep the thread going? I do have some pictures of the build so far. All of these and more can be found on my temp. web host page for the Citabria: http://comp.uark.edu/~jasonc/citabria.php

Jason

bugleboy

My Feedback: (8)

Need to finish yours Dave!

bugleboy

My Feedback: (8)

Many new updated construction pictures:

http://comp.uark.edu/~jasonc/citabria.php

Jason

Shogun

My Feedback: (2)

If your building this kit be aweare that the instructions are correct and playing around with the incidence is not an issue if you build the kit to the plans and per the instructions.

The real problem with this design is in the engine thrust angle when built to plans. Mine has 2.5 degrees of UP thrust and no right thrust in it. After the initial take off the plane wanted to climb and was very difficult to fly. Post flight checks with a Robart incidence meter revealed the thrust issue and the necessity to correct it.

My C-Pro has an OS 1.60 on it and setting the up/down thrust to zero and right thrust to +2 turned a bad flyer into a real sweet plane.

bugleboy

My Feedback: (8)

Thanks for the insight, Showgun!

Jason

bugleboy

My Feedback: (8)

Update:

GoNavy

My Feedback: (15)

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 in thread here: http://www.rcuniverse.com/forum/m_69...tm.htm#6961521), 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.

bugleboy

My Feedback: (8)

Thanks for the help, but this thread is 7 years old.