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Probably a question for gurus here:
I've heard a rumour that when taking off with a 914 with the throttle at 115 %, I have to get back under 100% not earlier than after 30 or even 60 seconds. I was trying to search here for this but didn't find anything.

Is there at least a bit of truth in that?
And if yes, could anyone explain the reason?

Many thanks in advance

  • Re: R914 - Length of Take-Off run @ 115%

    by » 5 weeks ago


    All this information can be found in the Rotax 914 Operator’s, Installation, and Heavy Maintenance Manual, available here https://www.rotax-owner.com/en/support-topmenu/engine-manuals#current-engines

    The position of the throttle valve is divided into a linear progression from 0 to 115%.

    The TCU registers the duration of full engine operation with boost pressure. Full-throttle operation for longer than 5 minutes will cause the red warning lamp to flash. Engine speed over 5500 RPM is restricted to 5 minutes. Exceeding the “take-off” time limits must be recorded by the pilot in the logbook, stating the exact time and duration of the limit exceedance.

    • Take-off performance is 84.5 kW (115 hp) at 5800 RPM (39 in.HG) (115% throttle position) (max 5 minutes).
    • Max continuous performance is 73.5 kW (100 hp) at 5500 RPM (35 in.HG) (100% throttle position).
    • 75% power setting is 55.1 kW (74 hp) at 5000 RPM (31 in.HG) (approx. 67% throttle position).
    • 65% power setting is 47.8 kW (64 hp) at 4800 RPM (29 in.HG) (approx. 64% throttle position).

     

    The Rotax 914 engine limits operation above 5000 RPM to 5 minutes primarily for reasons related to engine longevity and reliability. Here are some of the main reasons:

    1. Heat Management: Operating at high RPM generates more heat. Sustained high temperatures can lead to thermal stress on engine components, potentially causing overheating and reducing the lifespan of parts such as pistons, cylinders, and valves.

    2. Wear and Tear: Higher RPM results in increased mechanical stress and faster wear on the engine’s moving parts. Limiting the duration of high RPM operation helps to reduce wear and tear, ensuring the engine remains reliable over its service life.

    3. Preventive Maintenance: The 5-minute limit acts as a precautionary measure to ensure that pilots do not inadvertently cause long-term damage to the engine. It encourages more conservative operation and helps to maintain the engine within its optimal performance parameters.

    Overall, these limits are set by Rotax to balance performance with durability, ensuring the engine operates safely and efficiently within its design specifications.

     

     


  • Re: R914 - Length of Take-Off run @ 115%

    by » 5 weeks ago


    Thanks!

    But I'm afraid I wasn't clear enough. I know about the 5-minute limit and read through the available documentation.

    I was asking for the information/rumour I've heard that you should not get back under 100% earlier than 30 seconds. This does not make much sense to me, therefore I am asking here.

    >>The Rotax 914 engine limits operation above 5000 RPM to 5 minutes primarily for reasons related to engine longevity and reliability. Here are some of the main reasons:

    I believe the limit is 5500 rpm, not 5000 rpm. Correct me if I'm wrong.


  • Re: R914 - Length of Take-Off run @ 115%

    by » 5 weeks ago


    Hi Premek

    As covered by the manual you can run 5 full minuets at 5800 RPM and WOT.  This is predicated on you having an inflight adjustable or constant speed prop.  There is no Rotax requirement to limit this to only seconds.  

    The engine is very happy to run at 75% power, 5000 RPM, and you will save a lot of fuel.  While you can run at max continuous, 5500, the gain in speed and performance for cruise is a matter of aircraft type and configuration.  

    Cheers


  • Re: R914 - Length of Take-Off run @ 115%

    by » 5 weeks ago


    Thanks!

    Okay, it seems obvious I didn't properly explain what I'm asking for. So, let me try to reword what I'm trying to say:

    I'm not talking about max. time the TCU can run.

    What I'm saying is that I was told that when I push the throttle to 115% at take-off, I should not pull the throttle back to 100% earlier than after 30 seconds of full power. It sounds odd to me, as I do not see any technical reason why it should be that way.


  • Re: R914 - Length of Take-Off run @ 115%

    by » 5 weeks ago


    Got it, no worries!

    Throttling back before the 30-second mark doesn't have any impact, and there's no supporting documentation for this notion. It appears to be nothing more than an unsubstantiated rumor.

    However, rapid throttle movement and unnecessary cycling of the propeller governor puts a relatively high load on the engine. You want to have nice, smooth, and constant throttle movements.

    As per the 914 Operators manual: "To avoid unstable boost, the throttle should be moved smoothly through this area either to full power (115 %) or, on a power reduction, to max. continuous power (100 %). In this range (108 to 110 % throttle position) small changes in throttle position have a big effect on engine performance and speed, but are virtually not apparent for the pilot from the throttle lever position"


    Thank you said by: Premek

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