The gears are not under much load when running I agree. However when the impulse coupling is in action the acceleration loads must be considered and this is considered the cause of failure. The Stemme is unusual in that you have to run the engine on the starter till the blades are deployed then go for the ignition so they go though points faster than we do for that reason and it is also harder on the gears than our aircraft are. They also suffer impulse coupling (ware) failures more than we do due to the more time running on them. Cheers. Nige.
From: Rob Thompson Sent: Monday, October 05, 2015 8:47 AM To: [email protected] Subject: RE: [DOG mailing list] Mag Failure I think I might direct a 25mm air hose through the baffle straight onto the magneto. We have our motor very well sealed with silicon and running cool so I think we can spare that amount of air. Each Rotax ignition generates 2 sparks, once when the flywheel magnet hits the coil, second when the trailing edge of the magnet passes the coil. The sparks are reverse polarity. At startup with weak sparks, a zener diode will block the normal slightly advanced spark. This is a very unreliable way of doing it and doesn't alway work that well. Kick backs are the worst of the problems when the Zener doesn't block the advanced spark properly. Starting on one only ignition circuit will help the problem but I have a soft start circuit diagram that your Foxbat friends might be interested in. Rob Thompson 0429 493828 Please note that my new email address is [email protected] -------------------------------------------------------------------------------- From: [email protected] To: [email protected] Date: Mon, 5 Oct 2015 10:24:31 +1300 Subject: RE: [DOG mailing list] Mag Failure A couple of points of interest, 1) I have been doing some delivery flying and test flying of a few Foxbat microlites which have the 100hp Rotax 912 engine. They have electronic ignition. On start up, the engine runs like a hairy dog for a few seconds then smooth … as the electronic pick ups are changed automatically from the TDC start ones to the normal advance ones … maybe a similar idea could be designed for a limbach pickup(s) 2) We have also had a mag failure on a Grob 109 … quite a few years ago though. Could it be that the 109 mag cooling is simply not up to scratch ?? Wolf Hoffman seems to have cunningly used the cooling airflow thru oil cooler to be directed over the mag of the Dimona. While warm air of course, it still seems to offer significant cooling Ian Williams ZK-GCB From: [email protected] [mailto:[email protected]] On Behalf Of Rob Thompson Sent: Saturday, 3 October 2015 11:08 a.m. To: [email protected] Subject: RE: [DOG mailing list] Mag Failure Those gear teeth don't drive much of a load except if the rotor was to jam for some reason? Not easy to tell from the photo but the rotor gear teeth look OK. Maybe if heat is a factor the stripped gear may use plastic that softens at a lower temperature. Did this happen at start up? If so the gear teeth undergo some serious acceleration as the impulse coupling kicks. What about at shutdown? If the engine kicks back the gear teeth also reverse direction suddenly. Some serious analysis needed...I guess all will be revealed in time! Rob Rob Thompson 0429 493828 Please note that my new email address is [email protected] -------------------------------------------------------------------------------- From: [email protected] Date: Sat, 3 Oct 2015 10:33:42 +1300 To: [email protected] Subject: Re: [DOG mailing list] Mag Failure Could well be temperature related Sent from my iPhone On 3/10/2015, at 08:46, Ian Mc Phee <[email protected]> wrote: My question is why does it happen? Is it based on hours or cycles of starts or getting too hot or something else. I can remember once at overhaul he replaced the cog gear as it looked a little damaged Ian m On 3 Oct 2015 12:15 am, "Nigel Baker" <[email protected]> wrote: Attached image of what failed in Rob’s Maggy. The drive gear stripped/lost a tooth and took the distributor gear with it. Cheers. Nige. --------------------------------------------------------------------- You are subscribed to the Dimona Owners Group mailing list. To unsubscribe, send email to: [email protected]
