CG Myths and "Rules of Thumb"

Throughout this series of articles, we’ve developed an “engineering approach” to predicting an acceptable CG range for a model aircraft.  We’ve shown how to find the “neutral point” (NP) using either mathematical rules or an online calculation spreadsheet.  With that and a “fudge factor” called “static margin”, we can take a pretty precise stab at where our CG should be placed before first flight.

Now let’s compare this to some of the myths and “rules of thumb” often heard at club flying fields (and online discussions), and see how they stack up:

 

“Balance at 25% of the wing chord”

False. 

Remember that 25% is the location of the wing’s aerodynamic center – not the airplane’s.  Unless the horizontal tail is tiny (or you have a flying wing), the NP will always be well behind this point.  A CG this far forward is just asking for heavy elevator forces and a tendency to nose over on the ground.

 

“Balance about a third of the way back”

Maybe – but be careful.

I remember this from a library book about building Free Flight planes back in the 1970’s.  It actually works fairly well for models that look like Piper Cubs and Cessna 150s.  For the mathematically inclined, look at the diagram from the second lesson:

This shows that for reasonable tail volumes (0.4 to 0.6) and low to medium tail efficiency the NP should be somewhere between 45% and 70% MAC.  Throw a 10% -25% static margin on that and you have a CG somewhere around 33%.  That’s on the conservative side so a kid using this rule would be extremely unlikely to end up with an unstable model.

But be careful!  A big tail with a long moment arm will pull the NP significantly further back (many competition Free Flight models had to be balanced behind the wing trailing edge!)  Watch out for wing sweep and taper as well – remember that the wing MAC is not always the root chord!

 

“Balance on the spar”

Maybe, but not always.

This is kind of related to the last one.  For maximum bending strength, you want the main spars as far apart as possible; so they are often placed at the point of maximum thickness.  And it just so happens that many of the airfoils commonly used for model airplanes have their max thickness “about a third of the way back”…

But who says there is only one main spar?  Model designers do crazy things sometimes, and some wings have no spars at all!  Spar placement doesn’t depend on CG, so why use it as a yardstick?

 

“Bipes are balanced with reference to the top wing only”

False.

Remember that the aerodynamic center (AC) of a biplane’s wing system will be somewhere between the ACs of the individual wings.  Most bipes have “positive stagger” (bottom wing behind top wing) so neglecting the bottom wing would give you a safe CG.  However, it’s not necessarily a good CG and is likely to be excessively far forward.

 

 

“Bipes should balance halfway between the cabane struts”

Sometimes this works, but don’t depend on it.

Again, this has to do with usual spar placement, because the cabane struts usually tie into the spars.  Halfway between the cabanes is often somewhere around 40% - 50% of the root chord.  Allowing for a “reasonable” amount of stagger and a “typical” tail volume, that’s often a conservative starting point for a biplane CG.

But be careful!  Many bipes have swept wings (or a swept top wing), and sweep pulls the MAC back relative to the root chord.  Bipes with large tails and long moment arms will also need a more aft CG.  Look how far aft the NP is on the Bücker Jungmann.

 “Balance your model so it flies hands off with neutral elevator deflection”

False!

This results from some misunderstandings about the nature of “stability” and “trim”.  Many modelers want to minimize drag due to a deflected elevator, so they ballast the plane until this condition is met.  I could write a whole ‘nuther article about why this is wrong, but they are putting the cart before the horse.

Your CG should be set for the level of stability and elevator “feel” that you find comfortable.  If you find that requires some elevator deflection, the best thing you can do is adjust the stabilizer incidence angle to make the deflection go away.  If the stab is securely glued in place, you may be able to accomplish the same thing by shimming the wing (in the opposite direction to the stab).

Or simply live with it.  Many full-scale airplanes have adjustable stabilizers (or elevator trim tabs) so the trim deflection is constantly changing with airspeed (and CG location).  There is no one “correct” elevator deflection, and unless you’re flying a competition sailplane, you’ll never notice the trivial difference in drag.

 

 

“Always go by the plans or instructions”

False.

Plans give you the designer’s idea of correct CG location.  While this is always worth serious consideration, many model designers are not engineers.  Nor are they test pilots, and there’s no guarantee someone went back and revised the plans after the prototype was flown – if there was ever a prototype!

Your best guess for an initial CG placement probably comes from other modelers who have flown that specific plane and who have a similar flying style to  yours.  If you’re building an unusual model from plans, you’re on your own!  CG calculators are based on well proven science – so why guess?

 

Next time, we’ll look at some different ways of finding where the CG of your model is actually located.