In-Flight CG Calculation

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

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.

JJ's trim handle technique seems the best in-flight indicator, and requires no additional sensors.
What happened to the "That Looks About Right" technique? Isn't that how it all ends up?
The Blanik L13 was great for TLAR. If it went over on the nose when you climbed in the C/G was too far forward, so put the big pilot in the back. The DG1000 is similar, but waits until the wheel brake is applied on landing... Ask the USAF TPS.
From experience, an LS4 with 70 liters of water in the cockpit is beyond the forward C/G limit. Also from experience the Nimbus 3 at 108% and ASW27 at 112% fly nicely.
Wondering what the CG position on XX's test flight of the same N3 with the broken Mercury weight shift system was.
Jim

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

In flight CG measurement is also known as the trim position.

Tom