Gee and velcro

[Shaun Walsh]

50 minutes ago, Geoff S said:

It's surprising how high values of G can reach under even apparently innocuous operation.  Some years ago (many, now!) I was heavily involved in the design of a radio telemetry system to measure temperature, stress etc in gas turbines.  Our electronics were housed in an annular containment ring about 400 mm diameter IRC.  It rotated at about 7k rpm and apparently subjected the electronics to 20,000G!   All the circuits were embedded in epoxy and we got no failures due to G other than the wires which led to the strain gauges and thermocouples,which progressively failed during a test.

 

We had a graduate who was intending to do a Phd studying failure due to G of integrated circuits and he had a centrifuge to conduct the tests.  Unfortunately, he was unable to record any failures which made his proposed thesis a bit thin! 

 

The only aircraft I've lost batteries in flight have been, one: a very small Depron biplane with the battery attached by Velcro underneath.  The battery fell out and the model did a perfect landing unaided by anything bar gravity!  And two, my Multiplex Fun Cub which shed its battery part way through a bunt.  It floated gently to earth inverted, again with zero damage.  I've made alternative arrangements for battery security and had no further problems.

 

I really don't think there's any need to measure the G forces on a model.  I use Velcro straps round the battery and Velcro on the battery and platform to stop sliding. It just isn't a problem.

Before retirement, one of the pieces of equipment I was responsible for was an ultracentrifuge, 100,000 rpm in a sealed vacuum chamber producing 800,000xg. The rotors were machined from solid Titanium and had a limited service life after which you used them at your own risk (we decommissioned them!) the rotors cost more than £10k each. The vacuum chamber was half inch thick steel and the whole thing was epoxied to the floor just in case a rotor failed resulting in the total destruction of the machine.

[PatMc]

On 18/09/2021 at 20:46, PatMc said:

Is the G force always measured in one plane (not aeroplane) parallel to the fixed plane of the G meter ?

@Dickw Serious question, is it ? 

[Dickw]

9 hours ago, PatMc said:

@Dickw Serious question, is it ? 

A good question, and I don't know the answer.

Both my home made G sensor, and the Gyro assisted receivers I have, claim to have 3 Axis accelerometers, so probably each axis is measured separately. Interestingly the G meter in my home-made unit reports all three axes separately but the Rx only reports a single value (but then it is not a dedicated G meter).

 

The receiver manual also suggests that although the accelerometer is factory calibrated you might like to recalibrate it at your own location as the Earth's gravitational field can vary slightly. The recalibration process involves placing the receiver on each of its six sides in turn on a level surface. Once installed in the aircraft you have to tell the Rx which way round it is installed.

 

Perhaps I should investigate more, but I don't really use the G force data for anything and only tried it out of curiosity.

 

Dick

[Robin Colbourne]

There used to be a very simple G meter in the form of a hollow transparent cube containing a sponge ball soaked in ink.  If subjected to an impact, the foam would squash, leaving an imprint on the inside of the cube.  By measuring the diameter of the imprint it was possible to determine the maximum G force to which it had been subjected.  As I recall, the application was to go in parcels containing delicate equipment, so the receiver could see if it had exceeded the safe load in transit.
Some smart phones contain accelerometers these days and there are apps available for measuring G force and storing the results in the app's data logger.

[Gary Manuel]

On 18/09/2021 at 20:46, PatMc said:

Is the G force always measured in one plane (not aeroplane) parallel to the fixed plane of the G meter ?

Well the graph that Dickw posted shows 3 data streams for X, Y and Z axes, so it looks like G force is measured in three planes (not triplane).

 

 

Edited September 20, 2021 by Gary Manuel