Double Trouble Rebuild

Before the crash.

I crashed my Double Trouble, my favorite glow-powered airplane, in August 2020.  I crashed it out of stupidity and laziness.

I had been flying this plane for 8+ years, and I was using the same battery pack I had flown with the previous weekend on another plane.  That battery pack was on regular charge after the previous weekend and then on trickle charge during the week to keep it fresh.  The charge indicator showed it was fine, and surface checks before the flight were fine, so I didn’t bother to do a full battery status check with my expanded scale voltmeter (ESV).

I took it out on 16 August 2020 to fly in between rain showers.  Everything worked fine on the ground and for the first 45 seconds of flight, but as soon as I put a heavy load on the servos, the voltage dropped too low and the receiver browned out.  The airplane was snap rolling up and to the right at full power when it stopped responding to commands.  It continued in the snap roll, quickly inverting and going straight at the ground at full power.  It ended up just off the east side of the hill at the RCMB flying field, in many pieces.  The ground was soft from the rain, and the engine buried itself well into the mud.  I disassembled the engine and did a lot of cleaning, but it continues to run well on the plane after the rebuild.  It is even using the same glow-plug, but the spinner did break in the crash.

This is the first plane I had totally smashed in many years, and it was all my fault for not doing a full battery check before flight.  After picking up the pieces and doing that full battery check, like I should have done before the flight, I could see that one of the cells had gone bad sometime in the previous week.  The pack had just enough output to seem fine on the ground through surface checks, but not enough to keep the receiver and servos alive in a high electrical load condition.

After getting home, I did more digging into the electronics and found that the culprit was both the battery and the switch harness.  The battery was low but not so low that I would have thought it completely bad.  The switch harness had a voltage drop across it too.  I was checking the battery condition through the switch harness at the field when I diagnosed the battery as the sole problem.  That switch harness killed the current when used with a questionable battery.  With the battery by itself plugged into the ESV, it was technically good, but on the edge.   Based on this finding, I am going to run a separate line off the receiver buss from now on and check the battery in parallel with the receiver and servos all turned on.  I also bought new NiMH AA batteries and built new receiver packs.

So, moral of the story is, DON’T GET COMPLACENT IN YOUR PREFLIGHT CHECKS!  If you have an ESV, make that battery check before each flight.  Do everything you should do to be safe, before putting that 5+ pound object in the sky.  It could hurt someone, or at the very least, ruin a perfectly good airplane.

When I picked the wreckage up, I thought “That’s not worth rebuilding”, but I changed my mind after more inspection.  It was close to a write-off but was not so far gone as I feared.  I decided I could rebuild it with a little effort.  First I built a new fuselage from the cockpit forward, embedding some carbon fiber runners along its length. Then I cut out the broken pieces of the wing, rejoined the wing halves, and added carbon fiber doublers on the main spar.  I resurfaced the whole wing D-tube structure with 1/16" balsa, added filler, and did lots of sanding to get it all smooth again.  After some time with my Monokote iron and new covering, it is almost as good as new.  It flew again for the first time after the rebuild on 4 September 2020.

As a footnote, I had to buy some black Monokote to recover the plane.  I bought a roll via Amazon, but it came directly from Horizon Hobbies. It was not included in free shipping under our Amazon-Prime account, so I had to pay $2.95 for shipping.  This is the giant box that Horizon used to ship a single roll of Monokote.  It would have held 30+ rolls easily.  It did arrive within 5 days though, so that was pretty quick non-Prime shipping.