I am repairing an 82" span Laser 200 model that I was given but do not have any information regarding CG position or control throws. Any help would be appreciated. It was suggested to me by an experienced pattern flyer that he used a CG between 35% & 38% MAC. This seems extremely aft to me and I would have thought it would have made for a very twitchy model. Any comments?

Tony

Whenever you're in the situation you're in, it's quite easy to measure your airplane with a yardstick and plug the measurements into an online CG application. It takes 10-15 minutes and gives you a 100% safe, reliable CG.

http://www.geistware.com/rcmodeling/cg_super_calc.htm




Searched online and......... According to the manual for the Seagull Laser 200 (which is the only Laser 200 that is found with a Yahoo search)

That may or may not be the model you're working on so............................

Find the spar.<div>
</div><div>Balance it at the leading edge of said spar.</div><div>
</div><div>Go fly.</div><div>
</div><div>What you'll get, for any traditionally configured monoplane ever made, is an airplane that will fly safely, and land safely.  You may, of course, prefer a different CG (probably more aft) location as you move forward, but a "LE of the main spar' will get the airplane airborne without any scary stuff, and let you work from there.  </div><div>
</div><div>FWIW, Bipes also have a "safe starting point".</div><div>
</div><div>Find where the cabane struts attach to the top wing. </div><div>
</div><div>Select the point in the middle, between the leading and trailign attach points.  </div><div>
</div><div>Balance the plane there, go fly.</div><div>
</div>

topwinguk,

Your model has a wingspan that differs from the more common Seagull and Lanier ARF’s.

I have compared yours and the 68.75” wingspan model made by Seagull, finding that yours is 19% bigger (Yours is 26% scale of original airplane).
Hence, if you don’t have a better reference, the Seagull specs could be used for a first approach, in the following way:

The CG of your model would be located at 1.79” (1.5” x 1.1927) aft the LE at the wing tip.
The throws of your model would be as follow (Seagull throws x 1.1927):
Aileron: 7/32” (Low) and 7/16” (High)
Elevator: 7/16” (Low) and 1-1/32” (High)
Rudder: 7/8” (Low) and 1-1/2” (High)

I have attached a schematic of a graphical calculation based on a drawing of a full-scale kit.
I have found that the ΒΌ chord for every section of the wing remains on the same line, which can be measured as 4.55” (4-9/16”) from the leading edge at the wing root.
That location of ΒΌ chord of the wing is a safe and conservative first location of the CG, which should be fine adjusted according to what the first test flights indicate.

The graphical location is very close to the location calculated above.

Please understand that these numbers have been determined on airplanes that may have slight differences in shape with yours; hence, take them with a grain of salt.
For a more accurate first location of the CG, I recommend you to use the following on-line calculator:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Other interesting links:

http://www.horizonhobby.com/Products...ProdID=SEA2100

http://www.seagullmodels.com/Admin/c...amImage/28.pdf

http://www.nasm.si.edu/collections/a...d=A19990171000

http://en.wikipedia.org/wiki/Leo_Loudenslager

http://findarticles.com/p/articles/m.../ai_n13641172/

Best luck with your project!

.
Many foam wings have no spar.
Using measured references to chords and semi-spans is more realistic.

Fair enough. Of course, a great many of those that don't have spars have wing tubes and/or joiners....which are spars.

However, certainly I've come across foam core wings that have no spars, no wing tubes, and no joiners...so...on that occasion, you have a good point.

Still...a rule of thumb that works reliably for a large majority of wings/airplanes seemed a helpful thing to pass along.


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