Pre-Finish CG check...anybody have a "rule of thumb"?

[Steve Helmick]

If you check the CG before the model is painted, but has all the hardware installed, the CG has to be ahead of the target location. Has anybody got a way to figure how much noseheavy it needs to be?  I'm thinking it could be related to square inches of wing area, like .2" per hundred, for example.  That would be 1.4" for a 700 sq. inch plane.  Steve

[Mark Scarborough]

Steve,
According to my discussions from Pat and Randy, if you design it right it wont matter,, 
However, aside from that, I recall talking to them and agreeing that if it balances about one inch ahead of the projected CG you will be pretty close after finishing, ( well unless you add that extra shiney coat of clear to the tail)

[James C. Johnson]

The center of pressure (Aerodynamic Center) doesn't change, it is fixed in relation to the wing, change the wing (more or less sweep) and the CP will move. ... I have a formula for figuring AC on any airplane some where.. On most planes the Center of Gravity is just forward of the Center of Pressure.

Here is a link that you wrap your brain around.. it has the formula..
http://adamone.rchomepage.com/index5.htm

To calculate MAC of a tapered wing, the following simplified equation may be used:
MAC = root chord * 2/3 * ((1+T+T2)/(1+T))







This works pretty well on the drawing board.. I used to use it to find CG for some of my small RC designs when using large fuel tanks to fuel small engines.. Did you know an OS .10 FSR will run for 23 minutes on a 4 oz tank !!

Jim

[Randy Powell]

Steve,

Mark is sort of right (as he so often is). I generally figure that if the CG is about an inch ahead of my final CG prior to finish (all assembled with engine, gear and various stuff), then you'll be in the ballpark when it's done

[dirty dan]

Steve,

I am pretty sure you have been given CG-shift figures for a painted finish. You won't get as much shift if using a film-based finish.

While I don't have my notes at hand, it is relatively easy to go to the Hobbico site and get specs on the weight of MonoKote per square foot. Even if not using this particular brand of film the weights will be close for other films. From there it is a pretty simple matter to calculate how much weight will be going where and I think you will find the results to pretty accurate.

As an example, the Impact was designed with a painted finish assumed, plus this design tends to come out slightly nose-heavy anyway--presumably because it takes less weight to balance if weight is added to the tail instead of the nose. My 2009 Impact is completely covered in film and took something like 6 or 7 sparkies in the tail to get it balanced properly. More when I am using an APC prop.

Dan

[Steve Helmick]

Thanks, guys. The plot is to remove a Saito .80 (!) and extend the nose (somehow!), to install a Stalker .76RE. The idea is to NOT have to put 10 oz. of nose weight in there with the surprisingly light Stalker.  When I get the Saito out and need ideas of how to graft more wood onto the nose, I'll post more pictures. This is the ex-Gerald Schamp beastie I call "Titan". It's about 750 sq. inches. Not sure I'm going to like it, but I won't know until I try it.
  Steve

[Derek Moran]

Steve,
Perhaps a radial mount will simplify the new engine installation.
Derek

[Mark Scarborough]

A really HEAVY radial mount Steve