In-Flight CG Calculation

Just curious if current technology could easily/cheaply support this calculation ... using a couple of accelerometers ... kk

Getting tired of leveling, measuring, weighing and calculating, KK?

John, Q3

On Monday, May 29, 2017 at 9:28:25 AM UTC-4, John Carlyle wrote:
Getting tired of leveling, measuring, weighing and calculating, KK?

John, Q3

We don't do it all that often - so not really ... but it is a little bit of a process when you do have to ... so just wondering.

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

are you concerned with dumping water and changing your CG?

On Monday, May 29, 2017 at 11:28:43 AM UTC-4, Matt Herron Jr. 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

are you concerned with dumping water and changing your CG?

Hi Matt ... not really, the factory numbers and methods we all use are sound and time tested ... and I just used these methods to calculate CGs for an empty ship - and with pilot and chute - just to know 'exactly' where the CG is located. I was just curious if CG could be calculated in flight ... using hardware/electronics that are available today. Just a question on a rainy afternoon in Delaware. kk

There is a simple in flight test used with RC gliders.

Some of which are getting to be quite big these days and a subsection of
which fly faster than 500 mph (up hill, into wind, that was measured at)
So, probably, has some application in our sport.

Fly at whatever speed you are normally trimmed at and put the glider
into a dive (not particularly steep, say 30 deg nose down) and let the
glider do what it wants from there. (staying below Va or Vne depending
on conditions)

If the glider stays at that angle an RC glider has it's CofG perfect (for
us
that means our CofG is a bit too far back as we need more stability in
pitch than the RC gliders)

If the glider pitches down and increases the dive, your CofG is too far
back (flying surfaces are keeping the nose down and their effect
becomes more pronounced as speed builds)

If the glider recovers back to flat n level then the CofG on ours is about

right, on an RC glider it's a bit too far forward.

The other check to make before you do this: note where your trimmer is
in normal flight, after landing put the trimmer back there and look at the

elevator. Ideally, when your CofG is right, your elevator should be in line

with the horizontal stab at "Normal" trim (it probably isn't as your CofG
will be a bit forward to give the glider longitudinal stability)

So, the RC glider test will show where your inflight CofG is (note, your
center of pressure will move relative to your CofG depending on speed)
Modern gliders are designed to be stable.
Older gliders too (but usually not so well)

A slightly forward CofG gives you stability
A very forward CofG gives you lots of drag.
A rearward CofG gives your wife hope for the next husband to be better .

Ehhhh...that can work, but.........doesn't help to determine CG envelope when your butt is in the glider. Static on the ground (done properly) tells you where you are before getting in.

-Centered CG is nice.
-Forward CG can be good for training/low timers. If you don't pay attention to pitch and relax on the stick the glider will speed up, staying away from stall.
-For XC, sit on the ground, mark the stick where the elevator is neutral to the stab, fly, see where the stick is flying a constant speed used for XC, change weight (CG) until the glider flies hands off at the required speed (stick relative to mark made before on neutral controls). This is usually an aft CG.

Yes, I have accidentally flown a CG out the backside before....once (ASW-20). Over the top spin entries when doing zoomies into thermals told me something was not right. The glider owner (flying tow at that time and saw a couple spins) asked over the radio, "spin much Chuck?!?!".

On Monday, May 29, 2017 at 8:47:42 AM UTC-7, wrote:
On Monday, May 29, 2017 at 11:28:43 AM UTC-4, Matt Herron Jr. 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

are you concerned with dumping water and changing your CG?

Hi Matt ... not really, the factory numbers and methods we all use are sound and time tested ... and I just used these methods to calculate CGs for an empty ship - and with pilot and chute - just to know 'exactly' where the CG is located. I was just curious if CG could be calculated in flight ... using hardware/electronics that are available today. Just a question on a rainy afternoon in Delaware. kk

May I humbly recommend a C of G program I wrote specifically for gliders? It's called SeeG and is available he http://www.glideplan.com/styled-2/page12/index.html

Lots of discovery tools for adding moments, playing with water, charting effects of changing CG, etc.

Matt

On Monday, May 29, 2017 at 2:55:51 PM UTC-7, Matt Herron Jr. wrote:
On Monday, May 29, 2017 at 8:47:42 AM UTC-7, wrote:
On Monday, May 29, 2017 at 11:28:43 AM UTC-4, Matt Herron Jr. 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

are you concerned with dumping water and changing your CG?

Hi Matt ... not really, the factory numbers and methods we all use are sound and time tested ... and I just used these methods to calculate CGs for an empty ship - and with pilot and chute - just to know 'exactly' where the CG is located. I was just curious if CG could be calculated in flight .... using hardware/electronics that are available today. Just a question on a rainy afternoon in Delaware. kk

May I humbly recommend a C of G program I wrote specifically for gliders? It's called SeeG and is available he http://www.glideplan.com/styled-2/page12/index.html

Lots of discovery tools for adding moments, playing with water, charting effects of changing CG, etc.

Matt

Matt, you beat me to it.
Was going to ask Kilo Kilo the millennial question:
"Don't they make an app for that?"
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

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