Calculating G Forces from IGC file?

Poul - Wow. That was intense. I agree that this isn't just an
exercise in the vertical component.
John - I think you are pointing me in the right direction.

I came up with this math for just the vertical component. Does this
math seem correct?

===================

RateofClimb (vertical speed) m/s = (BRecord_Altitude_N meters -
BRecord_Altitude_N+1 meters) / BRecord_interval*

G force = (RateofClimb_N m/sec - RateofClimb_N+1 m/sec) /
(BRecord_interval*) / (9.80665 m/sec2)

===================
* Typical IGC log file interval has B record entries every 4 seconds.
Yours may be different. And in the C302 if you hit the event button,
the interval always goes to 1 second.

At first glance the second division by the BRecord_Interval seems
wrong, but the BRecord_Altitude's are spaced by the BRecord_interval
AND the RateofClimb's are also spaced by the BRecord_interval.

I looked at a flight in which I remembered the vario going to +17
knots (whoop!). I calculated the G force at 8.4. Seems about right.
This gives a four second averaging so no telling what instantaneous
rates are. So for those flying a ridge where they are getting bounced
up/down repeatedly, this 4 second calculation wouldn't say much about
what you are experiencing. Makes me want to change my log rate to 1
second! Or buy a G meter!

If my math is correct (big IF) then I will publish my calculations so
anyone can plug in their B records and calculate/graph the results.

- John

FYI - When you hit the event button on the Cambridge 302 via remote
switch required (and the Cambridge 302A via the front panel PEV
switch?) it will record 15 fixes at 1 second intervals.

This is useful when you are only going to touch a turnpoint cylinder
and don't want to wait around for X seconds (your standard recording
interval) to make sure you get a valid fix. I have heard of people
that missed a turnpoint due to this.

Do other flight recorders also have this feature?

- John

Buy the G-meter to get the actual data because the logic you have used is
not accurate.

First, you need a real time data recorder for the method you have chosen to
sample the event. Within 4 seconds you could load and unload G without the
event being recorded on the logger. Even with 1 second you would not
capture all of the data accuately.

Furthermore, I doubt the positional data being recorded is accurate enough
for what you are doing. There is a tolerance on the GPS signal. It is my
understanding that height position is a lot worse than Lat/Long.

In addition, I don't beleive your formula is taking into account the
deceleration caused by the G loading. High G's decelerate a glider. Looked
like your formula was straight line.

Finally, I doubt 8.4 G's is correct because this is a lot. A recreational
pilot would black out well before you reach this amount. Also, only the
high end aerobatic gliders could with stand over 8 Gs. A typical xcountry
glider is not rated for 8 G's.

As you said, buy a G meter.

Andrew


At 15:42 13 March 2013, JohnDeRosa wrote:
Poul - Wow. That was intense. I agree that this isn't just an
exercise in the vertical component.
John - I think you are pointing me in the right direction.

I came up with this math for just the vertical component. Does this
math seem correct?

===================

RateofClimb (vertical speed) m/s = (BRecord_Altitude_N meters -
BRecord_Altitude_N+1 meters) / BRecord_interval*

G force = (RateofClimb_N m/sec - RateofClimb_N+1 m/sec) /
(BRecord_interval*) / (9.80665 m/sec2)

===================
* Typical IGC log file interval has B record entries every 4 seconds.
Yours may be different. And in the C302 if you hit the event button,
the interval always goes to 1 second.

At first glance the second division by the BRecord_Interval seems
wrong, but the BRecord_Altitude's are spaced by the BRecord_interval
AND the RateofClimb's are also spaced by the BRecord_interval.

I looked at a flight in which I remembered the vario going to +17
knots (whoop!). I calculated the G force at 8.4. Seems about right.
This gives a four second averaging so no telling what instantaneous
rates are. So for those flying a ridge where they are getting bounced
up/down repeatedly, this 4 second calculation wouldn't say much about
what you are experiencing. Makes me want to change my log rate to 1
second! Or buy a G meter!

If my math is correct (big IF) then I will publish my calculations so
anyone can plug in their B records and calculate/graph the results.

- John

8 Gs seems like a lot. And position error will indeed maginfy second derivatives. I'd first fit a smoothed flight path through 10 points or so, then take second derivatives of the smoothed function. Just run a regression with a quadratic function for the x y and z coordinates separately +/- say 5 points, and use the coefficient on the quadratic term to get local acceleration.
John Cochrane

On Thursday, March 14, 2013 3:47:30 PM UTC-7, wrote:
8 Gs seems like a lot. And position error will indeed maginfy second derivatives. I'd first fit a smoothed flight path through 10 points or so, then take second derivatives of the smoothed function. Just run a regression with a quadratic function for the x y and z coordinates separately +/- say 5 points, and use the coefficient on the quadratic term to get local acceleration.

John Cochrane

I think 4 second intervals is pretty tough for trying to generate a view of acceleration in the vertical plane. You need three points to fit a quadratic which is what you need to measure acceleration. 3x4 =12 seconds. Take a look at this to see what happens when you pull just 3 Gs for 12 seconds.

http://www.youtube.com/watch?v=9aDJLDQ-5QU

I think you need a higher sample rate to come up with anything for normal maneuvering. Certainly measuring a circle will give you horizontal centripetal acceleration, but circling gives you a fair number of data points without getting inverted.

9B

I have a little toy app on my iPhone called "Roller Coaster Physics" that is essentially a g-meter recorder that was free if I recall right.

I don't know how accurate the meters in an iPhone are, but you could try recording some maneuvers and using it as a sanity check for your calcs. I think I downloaded it for use in the plane and pretty much forgot about it for 3 years until I saw this thread.

Morgan

On Thursday, March 14, 2013 7:08:35 PM UTC-7, wrote:
On Thursday, March 14, 2013 3:47:30 PM UTC-7, wrote:

8 Gs seems like a lot. And position error will indeed maginfy second derivatives. I'd first fit a smoothed flight path through 10 points or so, then take second derivatives of the smoothed function. Just run a regression with a quadratic function for the x y and z coordinates separately +/- say 5 points, and use the coefficient on the quadratic term to get local acceleration.



John Cochrane



I think 4 second intervals is pretty tough for trying to generate a view of acceleration in the vertical plane. You need three points to fit a quadratic which is what you need to measure acceleration. 3x4 =12 seconds. Take a look at this to see what happens when you pull just 3 Gs for 12 seconds..



http://www.youtube.com/watch?v=9aDJLDQ-5QU



I think you need a higher sample rate to come up with anything for normal maneuvering. Certainly measuring a circle will give you horizontal centripetal acceleration, but circling gives you a fair number of data points without getting inverted.



9B