XJet

I've had a lousy week -- a bout of flu, rats infesting the house and all manner of other hassles -- so I should have figured that today was not a good day to go flying.

Right from the start the signs were there.

The normally happy little Norvel was hard to start wouldn't run right and I was down about 2K RPMs.

When I launched, the trim was all wrong and I had to crank in all the downtrim I had and half the right.

Then, in the middle (or what was supposed to be the middle) of an uneventful flight, I was doing a pass at about 20 feet over the nice sealed runway we have when *wham*. Without warning it appears that all the controls went to full deflection and the damned thing snap-rolled right into the tarmac.

The airframe is a total right off but (remarkably) the four HS81 servos seem to have escaped without losing any teeth. The prop-bolt was bent on th eengine and as it was pushed backwards by the impact, the silencer wiped away a goodly part of the small exhaust port stack that is supposed to locate it properly -- but other than that it *appears* undamaged.

All the RC gear seems to work just fine -- which is a *real* worry -- because I was flying with the FMA M5 and the only other flier around at the time was on 40MHz (I was on 72) so it shouldn't have been interference.

I did notice a little condensation in the LCD window of my Hitec Eclipse transmitter though -- although nothing could induce either the transmitter or receiver to glitch again when tested afterwards.

At least if you can find the cause of a crash you feel more confident about flying again -- right now I'm going to retire this receiver and battery pack -- or at least put them in a very slow foamy until I'm happy that they're okay.

Oh well, back to the building board :-)

BuzzBomber

My Feedback: (4)

Bummer. I have had a crash or two where I *know* I didn't do anything to cause a, um, high-G arrival, but nonetheless, it occurred. Very frustrating to to a post-crash analysis when there are no obvious answers. Good luck at the building board.

BMatthews

I'd move the transmitter over to a cheap foamy as well. At least for now.

The other guy was on 40 but it may be that some harmonic or other produced some problem. If you can talk the fellow into some testing with you it may produce some answers. If so at least you'll have a concrete cause.

XJet

I've flown on numerous occasions with the other guy and we've had no problems at all so I don't think that was the problem.

I'll try to get a foamy going this week and give the gear some flight-testing. It may be that it was "just one of those things".

Bipe Flyer

I'm sad to hear about your day.

It sounds to me like your servo tray/rails may have come loose. This would explain the need for all the trim at the beginning of the flight and, assuming the tray moved again during flight, the resulting crash.

The exact same thing happened to me while flying my bipe stick, including the re-trimming and then the almost total deflection of elevator and rudder. Luckily I was high enough and throttled back right away so I was able to nurse it back to the runway unharmed. I pinned the servo rails with toothpicks after that!

XJet

No it wasn't that. I was using servo rails that were glued to the fuselage sides and they were still firmly attached after the crash.

I broke my best prop too!

It's not easy to find small props around here and after the guy at Ultimate********* took my money for some APCs and never delivered I'm back to a tired old black cox 6x4 for my next plane.

Japanman

Bummer.
Any chance there was some condensation in the rx or servos?
water in servos in my experience causes a change in the neutral point, and water in a rec. causes a full lock-lock up.
\but I have only experienced that in boats... is it a wet winter this year?

J.M

XJet

I'm investigating the condensation aspect now. I noticed afterwards that my transmitter (Hitec E7) had some condensation behind the window that covers the large LCD and when I pulled the back off I see that although they were dry, the PCBs have some bloom on them which is indicative of condensation forming at some point.

I'll clean the bloom off the PCBs and leave everything in a low-humidity environment for a couple of days.

I find this a little disturbing actually, since the transmitter has never been out in the rain and I always return it to the polystyrene tray in which it was packed when I've finished flying for the day -- so as to reduce the effect of varying temperatures in my hangar (which is also nice and dry).

Given that *all* the servos seem to have gone to maximum deflection to cause the crash, and that I was using an FMA-M5 which should have locked out any interference that might have produced such a huge glitch, I'm starting to think that the bloom on the transmitter PCB may have been the cause of the problem. Certainly some leakage across the PCB would also account for the change in trim settings and perhaps the poorly performing Norvel was actually caused by the throttle servo not going to its full-throttle position -- but you can't tell with the little wire mesh on the carb.

ajcoholic

My Feedback: (10)

Hey bud,
What prop did you break? I can send you a few and maybe a spare ccase for the Norvel. Anyone know if the old and new (pre and post revlite) crankcases are the same??

I am building up my packages sent to you untill I have enough for one of your CD's he he...

Email me directly, I may miss this post...

Andrew

[email protected]

DICKEYBIRD

Sounds like a battery or switch/connection problem. Buzzy little engines can make switches go "numb." Check your battery thoroughly, then it's wiring and all connections between the battery and rcvr. Good luck!

XJet

Yeah, I'll ditch the battery and switch harness anyway (we're only talking $12 or so) but the receiver seems to have come through with flying colors. One of the advantages of a very small, lightweight receiver is that you can fit lots of padding around it and it doesn't have much inertia in a crash anyway.

To behonest, I'm surprised that the engine crankcase didn't break -- it came in pretty close to vertical onto a sealed runway at about 45mph. There were a lot of little bits of fuselage and wing all over the place though so I guess much of the energy was absorbed by the shattering airframe.

I'll strip down the HS81 servos because after all I've read about them being a little frail, I can't believe that they didn't bust a few teeth. The aileron servo even had one half of the servo arm broken right off it but the others were still in position and all pushrods were still firmly attached to the servo and the control surfaces.

Does anyone want pictures? :-)

combatpigg

My Feedback: (3)

Mass produced battery packs have the cell to cell jumpers that are spot welded, these are prone to failure, and can make intermittent contact. Take a good close look here. The on off switches are [ mostly] double row contact but are likely cause #2. The PCB contact pins that receive the servo plugs have had the solder joints fail for me. Cause #4 is a faulty antenna wire where there is a break inside the insulation. My most common cause is battery failure due to too much fast field charging, people who keep carefull track of their batteries [ not me] can spot trouble before it happens.

XJet

I've already whipped the shrink-wrap off the battery pack and the wiring is fine -- but I find that it had some castor oil on it (bloody leaky Sulivan tanks). It still had a solid 4.9V under load though.

Bipe Flyer

Is there the possibility of any metal to metal contact. I once had a bolt from the landing gear wear through the foam and heat shrink around the battery pack and when it came into contact with the metal casing of one of the cells all of the controls went nuts.

XJet

No, the battery was in the front compartment between the firewall and the first former (under the tank) and the undercarriage was between the first former and the second former.

It's starting to look a lot like some moisture in the transmitter may have sent it into a very brief apoplectic fit.

I'm going to wash down the transmiter PCBs with some CO contact cleaner (doesn't affect components or the PCB laquer) and then soak-test everything for a day or so.

combatpigg

My Feedback: (3)

It is possible to have some kind of slowly developing airframe failure, like a control surface hinge job letting loose, weak rubberbands, even a servo geartrain re-zeroing itself, but I like your idea that wetness could have caused it.

XJet

There was no sign of any catastrophic airframe failure (at least before the crash) and although I'd been throwing it around quite a bit during the flight, the snap roll into the runway occured when there was the least stress on the airframe -- in straight and level flight (it had been flying straight and level for several seconds).

All the control surfaces were still firmly attached to the elevator, rudder and wing and, as I mentioned earlier, all the linkages were still in place from the servos to the control horns.

The only airframe failure I could think of that would cause a snap-roll into the ground would be if the wing itself broke in mid flight but even after the crash the mainspars are intact but the leading edge of each tip is crushed back to the spar-line (although the LE itself didn't break -- it was spruce). This damage occured because one wing tip struck first which pinged the rubber bands and caused the other end of the wing to swing around and hit the ground.

The rest of the damage is simple compression-induced fracturing (well shattering actually) of the fuselage sides and top/bottom sheeting from the firewall back to the second former (rear wing-dowel area).

One good thing about crashes is that you can learn where the stress points are and (hopefully) make things stronger but not heavier next time -- although I don't think any airworthy craft can be built to withstand a vertical descent onto tarmac :-)