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Old Dec 27, 2009, 12:39 AM
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Twf wing build calculations

**** UPDATE ****

Post maiden tuning has resulted in a a CG location at 36% MAC (61mm) in order to achieve good dive test, inverted flight test, and axial roll results. A heck of a lot further back than I would have thought, but that's where it flies best. New version R1 updated the camber compensation from 1mm per % camber to 1.5mm per % camber.

************************************************** ************************************************** ******************************

I wanted to document my TWF wing build progress/process, and encourage more RCG and SD builders to go for it, and build a TWF for themselves.

After some thought, I realized that the tried and true build process documented by Sleddriver (Jay Decker) on slopeflyer.com (http://www.slopeflyer.com/soaring/ol....com-site.html) is difficult to improve on, so I won't. Jay's process is well documented, and produces a solid TWF wing.

What I did find, is there is not one single source for calculating the wing tube location for a TWF. The process is straight forward once you know what to calculate.

The wing tube should be located on the aerodynamic center (AC) of the your wing. The AC is the aerodynamic neutral point of the wing. Adam from Wyowindworks illustrates what AC is nicely in these three graphics in the ZMAX build thread:

http://static.rcgroups.com/forums/at...g?d=1245880955

http://static.rcgroups.com/forums/at...g?d=1245880969

http://static.rcgroups.com/forums/at...g?d=1245880969

So, to locate the your TWF wing tube per Dr. DrelaMark Drela you 1) locate the pivot point relative the mean aerodynamic chord (MAC), 2) the pivot point should be located at 25% MAC plus 1.5% more for every degree of airfoil camber. This isn't an exact science, but you want to make your best calculation to minimize the aero load on your servos, and the following get's ya pretty close.

So, I used the ISR CG calculator to determine the AC at the wing root, and then add 1mm per percent camber. The AC at the root is calculated by calculating .25 of the MAC Cord length and adding the sweep distance to the MAC.

NOTE: The AC is not the same as the CG.....

So, you can use the attached to calculate the AC by plugging in your wing info into the attached to calculate your wing tube location.

For maiden CG, I use about 28% or 29% MAC to calculate CG....nice and safe, usually gets you in the ball park.

EXAMPLE TWF WING TUBE CALC:

Root=177.8mm
Tip=127mm
Sweep=22.225mm
Wing 1/2 span=725mm
Wing camber=1.58%

Per calculations, MAC Cord= 153.81, so AC = 25% of MAC or 0.25*153.81=38.4525mm....add sweep to MAC of 10.5mm, AC at Root = 38.4525+10.5= 50.51......add camber compensation of 1.58*1.5, and your wing tube location is 48.95+2.34=51.29mm from LE at root location.

This would give you a AC of 48.95mm and a TWF Wing Tube location of 51.29mm from the LE at the root location. CG of 28.5% MAC would be at 54.33mm from LE at the root location. So in my case, I use a 5/16's (8mm) wing rod, so the maiden CG would be at the aft edge of my wing tube.

Hope this helps.
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Last edited by dc812; May 13, 2012 at 01:19 AM. Reason: title change
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Old Jan 14, 2010, 05:33 AM
Launchpad McQuack
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Great info. Thanks a lot!
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Old Mar 05, 2010, 04:49 AM
Twisted and Confused
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There is a neat little program for calculating multi panel MACs/CGs here at top of page. Simply put 25% in and it will give you the pivot point (add camber compensation manually).

Steve
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Old Mar 06, 2010, 01:51 PM
Launching!
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Chula Vista, CA
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Cool little program Steve

I had to download the .dll file to get it working on my system....but it was available on the same page for download.

I had initially used a calculator that I converted to Excel just cause most people have it and it would allow me to add my own tweaks in a familiar format.

Cheers!

Thanks!

DC
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Old Jun 28, 2010, 10:50 AM
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Thanks for posting that stuff!

WD
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Old Jun 28, 2010, 06:23 PM
Launching!
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Chula Vista, CA
Joined Jul 2006
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RE:Calcs

WD,

Putting that stuff together was an act of desparation on my part.....when I setting up TWF wings, I could not find a single source of the calculation(s) process for locating the wing tube location.......so I compiled the info as I found it, and punched it all into a spreadsheet.

Not very creative on my part, but I hoped to save people some time searching so they could spend more time building

Cheers!

DC
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