View Full Version : In-Flight CG Calculation
Just curious if current technology could easily/cheaply support this calculation ... using a couple of accelerometers ... kk
John Carlyle
May 29th 17, 02:28 PM
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.
Matt Herron Jr.
May 29th 17, 04:28 PM
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
Roy Garden
May 29th 17, 08:55 PM
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 .
Charlie M. (UH & 002 owner/pilot)
May 29th 17, 09:48 PM
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?!?!".
Matt Herron Jr.
May 29th 17, 10:55 PM
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 here: 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 here: 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 here:
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 1:04:35 AM UTC-4, JS wrote:
> 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 here: 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
Hey ... I'm not one of those millennials ... I'm a grumpy senior citizen .... :-)
I have found that when the CG is right (best climb), I don't need to trim aft when entering a thermal. If you have a tail tank, keep adding a small amount of water on each flight until you don't have to trim aft anymore, then put that amount of lead in the tail. This is mainly true for flapped ships, but I have found the Genesus-2 no longer needs aft trim when the CG reaches about 90% of allowable range and that's where she climbs best. The Nimbus-3 likes to be near 100%, as does the ASW-20, but the LS-6 likes to be about 85% of allowable. When I flew the 6 with CG near 100%, I found I was trimming forward when entering a thermal, indicating my CG was too far aft.
So, if you buy into this, your trim knob becomes your in flight CG indicator!
JJ
Matt Herron Jr.
May 30th 17, 03:36 PM
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 here:
>
> 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
JJ: can you explain what you wrote about the trim? Clearly the purpose of trim is to be able to adjust your hand-off speed. Thus if your trim is set for cruise and you stop to thermal you'd need to move the trim somewhat aft? If your CG is so far aft that no change in trim is needed, then the trim control becomes useless and you have no hands-off pitch stability. Moreover, in my experience in several glider types with the CG close to the official rear limit (I am lightweight and thus have always flown that way) the trim still needs to be moved.
In flapped ships, the speed is controlled by moving the flaps......down and the ship slows down.......up and she speeds up, this happens when the CG is aft at 85 to 100% of the allowable range. Will Schueman wrote about this in his modications to the H-301 Libelle. I first noticed it in my Nimbus-3, on the first flight after adding tail weight to put her at 100% of allowable CG range, I realized after an hour and a half, that I hadn't touched the trim! I have observed the same thing in non-flapped ships, I can slow down my Genesis-2 to thermal speed without the need to apply aft trim (CG @90%)........still needs forward trim to go faster than 65 knots, but she will slow to 45 with a little back pressure and thermal with no back trim required (trim neutral).
Does that make sense?
JJ
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