Need help with Aerodynamics 101: finding the CG

[dj_d]

I'm starting to dabble in scratchbuilding designs. What's the best way to determine the desired CG of a plane? I think I sort of get it for a standard design, but for something shaped more like an IFO, for example, I'm clueless.

If someone's written up a good "Aerodynamics for Modelers" type intro thing, I'd love a link to it, or just a good set of steps for figuring it out.

Thanks!

[slipstick]

Sorry, old-fashioned approach here. No computers involved except the crude one in your head.

Make up a small glider from foam or wood the same shape (usually 6-10" span will do). Chuck it around, adjusting CG, until it glides right. CG will want to be in the same place on the larger model ;).

Steve

[ciurpita]

here's a link to a description of the neutral-point calculation from martin simon's book. the CG is typically 5% of the mean wing chord forward of the neutral point.

http://ciurpita.tripod.com/rc/notes/neutralPt.html

[Julez]

http://home.arcor.de/d_meissner/schwerp.htm

[CloudyIFR]

http://h1.ripway.com/cloudyifr/files.htm

Download the article/tutorial then the MS Excel file "Sailplane Calc".

All the best.
Curtis

[dj_d]

Do these techniques apply equally well to something shaped unusually, like an IFO?

http://www.wildrc.com/

[slipstick]

The calculators don't usually have any way to even input such shapes. That's why I suggested using the old-fashioned way.....which works for any shape.

Steve

[JetPlaneFlyer]

One factor that is overlooked in most/all modelling neutral point/CG calculators is the effect of the fuselage. Scale models with lots of fuselage planform area ahead of the CG (like many of the jets I like to build) usually need CG to be well forward of the position calculated on wing and tail area alone.

I simple chuck glider is potentially better in this respect but you would need to build the glider with some fuselage volume rather than a standard profile type.

If the model you have in mind quite closely resembles some other flight tested model then it's often possible to estimate where the CG should be based on the actual position measured from the other model. For scale models a rough estimate of the CG position can be arrived at from the location of the main landing gear. CG will be just aft of the main gear on a tail dragger and just forward on a tricycle.