CG & the Dive Test
Diagram by Mark Drela, via Gert Nieuwoudt    m
commentary by John Lightfoot    m
It is said that "Everyone knows the Dive Test", but there are those who don't trust it or don't understand it, and there are many who can't or don't use it.  All the same, it is a very useful indication of whether your model is set up correctly.  With intelligent use it can help you get both your CG position correct and the "decalage" or "longitudinal dihedral" right.

Those two unusual-looking expressions both mean "the angle at which wing and stab relate to each other.  The most common setup is to set the wing at between 2 and 3 degrees to the fuselage and the stab at 0 degrees, although it seems that modern designs have their wing set as low as 1 degree.

First, the model must be trimmed so it cruises nicely straight & level, across wind . . . hands off.  Then push it into a gentle dive about 30 degrees nose down and let go . . .

Track A, usually followed by a stall, means the CG is too far forward nose-heavy.
Track B would be shown only by a complete fluke or a perfectly-trimmed aerobatic slope soarer.
Track C would need to be caught and corrected very quickly indeed.  It means the CG is too far back the model is nose-light.
Track D would be the ideal the model raises its nose slowly to above horizontal and, as speed bleeds off, returns to level flight.

The immediate reaction of most people is, "Why does the nose coming up mean it's too heavy?  That's back to front!"

The model was originally trimmed to cruise level.  At higher speed the elevator has more authority and, if it was originally a bit "up" to hold a heavy nose, it will lift it more at the higher speed, and of course, vice versa.

- - - o o O o o - - -