On Tuesday, May 30, 2017 at 1:33:12 AM UTC-7, wrote:
On Monday, May 29, 2017 at 6:20:59 AM UTC-7, wrote:
Just curious if current technology could easily/cheaply support this calculation ... using a couple of accelerometers ... kk

Changing the CG will have an effect on the longitudinal stability of the glider. In theory if you could measure the frequency of the short-period oscillation you could estimate the location of the CG. I believe this is because changing the CG changes the longitudinal radius of gyration of the glider - more aft CG - larger radius of gyration - longer period oscillation (I think). You can look at some equations for longitudinal modes he

http://www.iitk.ac.in/aero/fltlab/dynamics.html

I'd say it's not a practical thing to hope for. The short-period mode is typically on the order of a second or so and I can't imagine being able to conduct an experiment in-flight that would give a very precise indication....unless of course your CG is far enough aft to be statically unstable, in which case your butt will be all the accelerometer you need.

Now, if you could instrument all the fuselage-to-tail and fuselage-to-wing attach points with strain gauges you might be on to something...

Andy Blackburn
9B

Interesting theory, Andy. My guess is that you get a longer oscillation with the CG more forward because as the glider slows up from the last oscillation, and as the effect of the elevator pulling down goes away, the forward CG pulls the glider into a steeper nose down correction. It then takes longer to bleed off the extra speed as it corrects again. Move it even further forward and you get stalls between the oscillations.