[Canberrauav] CMAC Helicopter Testing and Airframe Issue.

Greg Oakes captain351 at gmail.com
Mon Feb 15 11:09:47 AEDT 2016


Folks,

On the weekend at CMAC we had successful test flying of the Trex 700 fitted
with the combination of pxihawk and using an external 3-axis gyro
stabilization system.  The idea behind this setup is to provide an
independent stabilization system for emergency bail out, and to counter
some of the additional vibration effects with larger capacity petrol
helicopters.  With the current setup all control channels are directly
passed through to the external 3 axis MEMS Gyro from the pixhawk SBUS out
in Acro mode. With this separation we can apply more efficient vibration
isolation of the Pixhawk which would otherwise compromise the function of
the Gyro's which need tighter coupling to the airframe.

Whilst we have more testing to do, we flew the Trex 700 in both Stabilise
and ALT hold with no issue other than some minor tuning needed to correct
the demanded versus achieved pitch, roll and yaw values and a little Z-axis
oscillation in ALT hold altitude change.

On a different aspect we have encountered an issue with the current
Trex700N engine mounting coming loose due to excessive wear on the actual
carbon fiber frame where the 3mm countersunk bolts, which precisely align
the engine mount bearers, have worked themselves into the carbon fabric
matrix.  Whilst the helicopter is old and done many hours I am wondering if
anyone has seen or encountered evidence the additional heat from a petrol
engine, could break down the carbon fiber faster.  The trex 700N was
originally designed to take a Nitro glow engine which runs cooler.  It is
either mechanical vibration over time under the bolt stress has broken down
the carbon fiber sealed surface on same stress but addition heat contact.
Looking for opinion advice on the likely suspect here?

The issue we have now is trying to stop the problem spreading further and
also repairing if possible the top surface of the carbon fiber around the
engine mount holes.  What I have done so far is to place a larger diameter
and thicker aluminum washer to distribute the load and try an dissipate the
heat transfer from the motor to the alloy motor mounts.  Also because the
frame engine mount holes are larger dia than the mount bolts, I needed to
insert a spacer to maintain the correct engine alignment.  Using a slightly
larger bolt is problematic due to how close the engine mount holes are to
the edge of the CF frames.

If you have any details of heat effects on breaking down carbon fiber
bonding this would be appreciated.  Also are there any epoxies / glues
which you recommend , that have high heat application?(wont soften at the
temperatures transferred from the petrol engine crankcase.  I need to bond
the aluminum spacers to seal the CF surface.  Fortunately during earlier OS
GT15Hz we measured the temperature at 130-140 C around the cylinder hear
and base of the spark plug.  The max temp we could go to before exceeding
the manufacturers engine spec is 160-170C. So epoxy would need to survive
at the fore mentioned temperatures.

Eventually we will rebuild the heli and look at other materials and newer
composites.  But for now need to extend the operational hours (within
safety) if possible.

Greg
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