rcfs

You're absolutely correct!

rcfs

This illustrates what I believe you're describing http://www.rcflightschool.com/SetupPDFs/MAC.pdf

rcfs

Here's some graphics to consolidate what you're saying: http://www.rcflightschool.com/SetupPDFs/CG%203pg.pdf

da Rock

Unfortunately, the issue of pitch stability really isn't that simple. And the wing's center of pressure is much like "the man behind the curtain" in Alice in Wonderland for the average modeler of today. It really isn't true the CG should always be on a wing's maximum thickness.

Finding a starting point of a CG for a model today should actually take into consideration the things that affect pitch stability, like the model's tail, the part of the model that controls pitch. And it's best if figuring that is simple. All that's needed in fact is a yardstick or tape measure. The link below actually is that simple to use. All the modeler needs to know is where the tip, root, span, and sweep are and be able to measure them. By plugging in 5 and then 15 to the green box gives the suggested range for where to locate your start CG.

And it takes into consideration what your model has to control the pitch, the size and location of the horizontal tail.




http://adamone.rchomepage.com/cg_calc.htm

rcfs

As I'm sure you know, the objective of balancing a symmetrical wing airplane near the center of pressure is aimed at achieving more neutral handling rather than stability. Anyhow, I work with many very bright individuals, and yet even my simple illustration to determine the C.P. is beyond the scope of what most of them are able to comprehend. That said, what's most important is that those who abide by the rule of thumb of balancing at the wing's thickest point will be safe and achieve handling very close to optimum, and since flight testing is seldom performed before airplanes are released to the market, they will most likely fly better than with the manufacturer's recommended CG.

da Rock

That's an interesting belief.

Actually, it a pretty amazing belief.

MarkU6299

So if I input all the dimensions into the calculator for my Phoenix Models Sbach 342 .120 size, I get a CG of 6.62" to 5.29" from the leading edge of the wing, yet the manual states the CG to be 4.53" (115mm). Then in some threads related to this plane, some people state that the plane feels tail heavy with the instructions CG of 115mm.

What to do. Follow the instructions or go with the calculator?????

Mark

da Rock

No law says you're required to choose one or the other. Suggested CGs are just that, suggested.

With the info you got from the calculator, you know the Phoenix suggested CG is really, really "stable". (In fact it might be so "stable" the elevator throw is less efficient. When that happens, at slow speeds the elevator throw is inadequate, which some interpret as tail heavy.)

I don't have a Phoenix mfg'd Sbach, but my Hangar 9 Sbach 342 with 62" wingspan has a suggested CG of 4.75-5.25" from the LE at the root. Yours and mine have almost the same wing area. Interestingly, H9 recommends elevator throws about 13degrees Low, and 44degrees High rate. Phoenix recommends 12 low and 15 high. That's not much difference at all and suggests their elevator throw and significantly forward CG are probably hurt more than help.

Phoenix 4.53" is most certainly going to be very safe. However, if the plane was mine, I'd make the elevator high rate about twice the low. That way, if what might be an excessively nose heavy CG won't overload an elevator with too little throw. High/low rate switches give you lot's more range and control but the surface throws need to help not hurt.

What to do? Maybe get an experienced flyer (whom you've seen maiden for someone else) to help, if you're not comfortable with your final choices.

da Rock

BTW, my Sbach hasn't actually flown yet either. However, in spite of the fact it was designed by a known US flyer for Hangar9, and I've measured it and run the numbers through http://adamone.rchomepage.com/cg_calc.htm, it's CG is set somewhat aft within the resulting CG range. Not what H9's designer suggests. this hobby is more art than science The numbers help, but don't guarantee anything.