Moving the CG ok?

[HIGHFLYER99]

I wanted to run this important quest by the experts. We have a plane kinda heavy and do not want to add more weight as might never leave the ground. I am told the balance point is supposed to be one third back from the leading edge of the wing. But holding the wing at this place has the tail pointing severly down. I was wondering if we just move the wing backwards a bit on the body, till the whole plane balances at the one third point would be ok?

[tim hooper]

Herbie,

That sounds good for a basic model, and I've done it myself!

FWIW many of the old freeflight model were trimmed in this manner to avoid adding lead.

tim

[HIGHFLYER99]

ok, we'll try it, this thing is at 9 pounds already with geared Mega 22 series, home built with industrial foam. It just might look funny with the wing being close to the tail.

[tim hooper]

Herbie,

I'll move this to Modelling Science where the experts hang out!

tim hooper

[die fliedermaus]

A time honored technique. When you shorten the distance from the wing to the tail feathers, you will be making the plane a little less well behaved though.

[HIGHFLYER99]

I knew there'd be a side effect. Is the CG universally 33% back from the leading edge? Any other ideas other then shaving off tail weight?

[Bill Glover]

Originally posted by Herbie
Is the CG universally 33% back from the leading edge?

No, that's an approximation which works most of the time (actually, I think 25% is safer .. but you probably don't want to hear that!).

The CG position is affected by many things including the size and moment arm (distance from the wing) of the tailplane, position of the tailplane relative to the wake from the wing, and the wing and tail sections.

But there's a simplified formula which works OK for most models:


CG position from wing LE =

(wing chord / 7)

plus

(tail area x tail moment x 3) / (wing area x 8)


Use the same units for everything, i.e. inches and square inches, or cm and square cm. The tail moment is the distance between the 25% chord of the wing and the 25% chord of the tailplane. NB the formula gives the CG position as a measurement, not a percentage.

[chemsurfer]

There are a number of calculators on the web to help you find where to place your CG. Many of them can only handle a single tapered swept panel.

There is one freeware program that can handle 5 tapered and swept panels. Get it here:http://perso.wanadoo.fr/scherrer/matthieu/english/mce.html

You enter in the panel dimensions, and choose your desired percentage of the mean aerodynamic chord that you want to locate your cg at. It then draws a planform of your wing and gives you the distance from the root LE.

If your planform looks screwy, it's because you need to enter the total sweep at each panel, not just for that panel: The root panel is easy, the next panel, add that panels sweep to the root panel, next panel add it's sweep to all the previous panels. etc...


Hope this helps, it helped me when I found it.

Brad

[HIGHFLYER99]

thanks, I may give it a try, seems complex formula to arrive at a CG, but then again, a nice flying plane is important.

[Bill Glover]

In the real world somewhere between 25%-33% will work fine, if in doubt you can make a simple scale glider (balsa stick fus, sheet surfaces ... I normally use white EPS ceiling tiles for this) and remove noseweight until it becomes unstable. Often quicker & simpler (and more fun ;) ) than calculating!

BTW that's the simple formula! The real one is rather more complex and actually calculates the Neutral Point. That's the balance point at which the plane would be neutrally stable, i.e. it wouldn't pull out of a dive on its own. The CG is then derived from that, based on how much stability you want.

[die fliedermaus]

Bill G,

In your calculation, is tail area stabilizer area only or should I include rudder area?

[Bill Glover]

Assuming it's a conventional layout, just the tailplane.