First I'll describe the behaviors and damage, post a few pics and then get to our final (we hope) hypothesis.
In the below blog entry titled "Flat out -- the end" at the 2:30 mark of the accompanying video, the vehicle has begun to exhibit some strange behaviors (unseen in the video). Rick has just reported passing through 43mph and though not reported over the radio, is feeling some rather large, low frequency power surges -- strong enough to rock the entire chassis.
About the same time, out the driver's window of the chase truck, I've noticed that the chain tensioner has begun to swing drastically forward and aft -- not in random bounces, but in large, regular arcs about its lower pivot. This motion has reached a point so dramatic that I am reaching for the radio to tell Rick to abort so we can have a look at the drive train.
Bang!! At 2:39 on the video, the propeller loses it's connection with the drive axle and we can see that the top sprocket has been bent, spitting the chain off it's teeth. The run is over and we pack for home.
As we pack the vehicle away, we notice that the right ratchet hub has some damage -- not enough to keep it from functioning, but damage none-the-less. I carefully check the left hub for damage and find none (later, you'll find out why I wish I hadn't checked the left one).
On the way home, Rick was sort of giving me more detailed narative about what he was feeling and thinking during the run. Playing it through in his mind he couldn't separate the moment when he applied the brakes from the moment the chain came off. I was surprised because I had assumed he had applied the brakes *because* the chain came off. Turns out he started braking because he felt the surging had reached a critical point and wanted to abort the run. As soon as he braked, he felt the sprocket let go.
Let's go back to a few days before when we broke the ratchet hub at the New Jerusalem runway push tests -- the one where we stripped the ratchet while pushing it at a speed we really never expected to reach. As you remember, we decided to simply lock off (bypass) the ratchet on the prop shaft for this trip. It wasn't required for the chain drive (it's designed into the system assuming a spool drive, something we're still aiming for) but locking it out does have one possible downside -- braking can now shock load the drive system to far higher loads than the wind can. Previously, one could grab a handful of brakes and the prop would just freewheel to a stop -- now the chain has to also take the load of the slowing propeller.
After downloading and reviewing the GPS data that had been logged, we began to consider the possibility that the sprocket didn't let go under power, but under braking. Rick and been quite gentle on the brake application, but the GPS data clearly showed a far more rapid drop to zero than the climb to speed.
When we got back to Mountain View, as we unloaded the truck I noticed something very strange -- the right side tire had extreme wear as compared to the left side tire. Now, as previously mentioned, let's remember that we likely ran the vehicle in anger no more than five miles and above windspeed perhaps two of that -- hardly a high mileage vehicle.
Another intersting aspect of the tire wear was that it wasn't consistent all the way around the circumference -- the knobs were in far better shape at say 3 o'clock than at 7 o'clock. It was as if the tire had been out of round and yet it spun true as could be.
To further the mystery, Ken noticed on the video that even at lower speeds, before we passed and went in front of the vehicle, there was significantly more dust coming out from behind the right tire than the left tire.
We considered a number of hypothesis but none fit all the data. We did know that due to the torque reaction of the propeller, the right drive tire would have been loaded vertically less and less as the speeds and prop rpms rise. This would have led to slip on the right side -- except that our ratchet hubs don't allow for any power transfer from side to side like a differential would -- under most circumstances it behaves as a straight through axle. Also, why did the side that was more lightly loaded (and thus unable to tranfer as much power) end up with the damaged hub?
Well, today Rick produced the hypothesis that fits all the data perfectly.
(first, a bit of background on how the ratchet hubs on the drive axle are used: Our vehicle has a very wide stance. The hubs allow for one side of the drive to freewheel and catch up if the rig is turning. We don't want the axle to 'bind up' if we are steering to stay on a DDW course and lose the hard gained momentum. *Also* (and this is key here) we reverse the ratchet hubs after the run to allow us to move the vehicle around and even tow it back upwind, without the prop spinning.)
In short, it appears that before the run, the Car Chief on duty (or not on duty as the case may have been) failed to either reverse the left side ratchet, or at least set it to the wrong direction. This means that we made our final run with a one wheel, right-side-only drive vehicle. This explains the asymmetric dust trail. It also explains the right side ratchet hub damage and pristine left side - the right was double loaded while the left not at all. The surging was the result of the right side getting lighter and lighter (remember the torque reaction of the prop) until the tire would lose traction ... briefly suspend prop thrust ... regain traction ... repeat. These rather violent surges are what tore the knobbies off the right side tire -- but not quite all the way around ... just where the tire first hit before regaining traction.
Now, we'll never know for certain if this hypothesis is correct. Because I played with the left side ratchet before we left Ivanpah (remember I said I regret this now?) we can't check to see what position it was in during the run. But if we go out to our next tests and blow through 45mph without any such surges, we can be pretty sure Rick nailed it.
There is actually quite a bit of relief all around -- it's frustrating to not have all the pieces fit and now we can move forward fixing the problems.
Just so you know, we're not going to reveal who the Car Chief was at the time of this incident -- it we did he might just stop updating the blog. :-)
