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Thread: 912 Gearbox Wear at Idle

  1. #1
    Senior Member
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    Default 912 Gearbox Wear at Idle

    Have read multiple places that the 912 gearbox needs to be run at an RPM above idle when landing and during taxi to alleviate wear on the gearbox. I gotta admit that fact alone boggles my tiny brain as an engineer. Why wouldn't Rotax fix that so that their engine could be operated 'normally' like most other aircraft engines. Wonder if the 915 has the same issue since there is more torque and HP. I have also read that Edge Performance has done some work/modifications on the gearbox with regard to their 912sti version. Anyone know if their engines can be run at idle on approach and taxi?

    Am starting to wrestle with what engine I am going to use and this is one of the questions I have.

    Thanks for any input.

    Gary
    Gary (Geek) Phenning
    Leavenworth (Not the Prison), WA
    Kitfox STi in Building Stage

  2. #2
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    Default Re: 912 Gearbox Wear at Idle

    I would swear there was just a thread on this but I can't find it now.

    When I flew at Stick & Rudder last month they did insist that minimum idle was to be 2000 rpm. In practice it really only required attention on the ground. On approach it wouldn't drop below 2000rpm even with the throttle closed, but once you landed you needed to add power to get your 2000rpm. Yes, counter intuitive and doesn't help the ground roll when coming to a stop but it doesn't create much thrust.

    I didn't let it rent much space in my head since I'm not going with a 912, but I could adapt to the procedure if needed.
    Kitfox 5 (under construction)
    Commercial SE/ME, CFII

  3. #3
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    Default Re: 912 Gearbox Wear at Idle

    Quote Originally Posted by alexM View Post
    I would swear there was just a thread on this but I can't find it now.
    Yup you're right - there was a thread that talked about this. I also saw some obscure reference (but dang if I can remember where or find it) that Rotax had done some upgrade/improvement to the gearbox back around 2019 or so. Guess what I am really asking is if that last statement is correct, did it address the gear wear at low RPM and the follow on is if the Edge Performance gearbox improvement addressed it.

    Appreciate your input based on actual flying experience. Right now I am still in the early stages of building and the only flying experience I have on this is sitting in the shop with a beer can and thinking about it.

    G
    Gary (Geek) Phenning
    Leavenworth (Not the Prison), WA
    Kitfox STi in Building Stage

  4. #4
    Senior Member efwd's Avatar
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    Default Re: 912 Gearbox Wear at Idle

    Interesting, S&R had me do 1800rpm. Then last year someone said they were wanting 1900rpm. 2000rpm now? That is a very long way from the idle setting of 1400rpm. Is this a case of engineers not getting it correct or are we just sheep?
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  5. #5
    Senior Member Dave S's Avatar
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    Default Re: 912 Gearbox Wear at Idle

    Considering the history of the 912, the earlier ones ( a really, really long time ago like pre ULS time) did not have a slipper clutch and were specified to be operated with a minimum idle speed (meaning with the carb idle adjustment at the stops) of 1800. With the 80 hp lower compression UL versions this worked out pretty well. When Rotax upped the game to the ULS, the earlier ULS engines did not have a slipper clutch either which made for some clunky shutdowns and kickbacks due to the higher compression with the 1800 idle. If a person operated either of the engine versions which did not have the slipper clutch below 1800, the gears would clatter in a genuinely attention gathering fashion - which meant gear wear and tear. Introduction of the slipper clutch within the gearbox (then introduced to both the UL and ULS at the time) allowed a reduction in idle speed to 1400 without the terrible gear clatter as the small amount of back and forth slippage allowed by the slipper clutch minimized the gear clatter. The reason for the clatter is the crankshaft RPM, at a low RPM, is not uniform throughout throughout the engine's rotation; and, with a geared engine, the prop wants to stay at a uniform RPM resulting in front and back gear contact. For those not familiar with the gearbox, don't confuse the slipper clutch with the dog clutch which is designed to protect the engine somewhat in a prop strike - it is two separate devices.

    Since the introduction of the slipper clutch, (which we all check for correct breakout torque at annual) Rotax has introduced a number of different engines in its lineup - so things could be different. Then consider all the different props we use in combination with those engines.

    A couple additional things to keep in mind:

    1) The RPM callouts are not entirely black and white. The 1400-1800 RPM range on many of the engines is a caution range (yellow) which means that the engine can be operated there for a short time and is not necessarily harmful, and will obviously be crossed on startup and shutdown or maybe on rollout. Below 1400 (red) - just don't operate there. Over 1800 = Green. There are good reasons to have the idle set so the engine can be pulled back to 1400 on the ground: a) it makes for a much smoother shutdown from 1400 than from 1800; b) permits a person to use some engine braking on final approach and rollout. As has been mentioned - an 1800 idle on the ground will be considerable higher with the throttle closed in the air - being able to set the idle to the minimum of 1800 in the air will result in a steeper approach and shorter rollout when needed and the idle will be less than 1800 - after slowing down on the ground so a person should feed in enough throttle while rolling out to keep the idle where it should be.

    2) Training environment and operator established SOPs. I don't know that much about the engines that S & R is operating; however, it is entirely possible that they have determined some modification of the idle speed are beneficial in that environment.

    In the end - it is best to read the book and stick with the instructions for the specific engine - there are differences among the engines Rotax makes. What may seem to be conflicting instructions is more likely due to changes in the engine with different models over time; and, certainly may be affected by whatever prop a person chooses to use. In experimental aviation, it is nearly impossible for an engineer to consider all the possible things a person may do to or with an engine/prop combo.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  6. #6

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    Default Re: 912 Gearbox Wear at Idle

    Having about 2000 hours behind geared Continentals GTSIO-520 and Lycoming GO-480s. Driving the engine with the prop has always been a no-no. All I know in both those type engines, if you got too low on the M.P. The engine sounds like it’s gonna fall off. You will only do that once. I think it has do do with the torque multiplication, and the direction of the forces. But when the Continentals got spun up and pulling hard the bellowing was like a wounded water Buffalo. I could look through the cowling cooling slots and see the turbocharger glowing orange. That plane was the only plane that I was truly afraid of. I would get my freight loaded at 0500 and wallow over to OAK 29 for the departure. Push the power up to 50” MAP, 3400rpms waits for the egt and temps to stabilize, release the brakes, and accelerate at geologic rates, all the time knowing the if an engine failed I was going to hit the water, with half a ton of random freight shortly there after, hitting me in the back of the head. Not survivable. Don’t miss that one bit.

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