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I have a 912iS with 3 bladed sensenich fixed prop. I'm going to do a full throttle static run-up the next time I fly to see what's the max RPM I'm getting. My POH doesn't tell me so I figured I'd ask here:

1. Does anyone know what RPM I should be seeing at full throttle on the ground after 10 seconds?

2. What max RPM should I see in the climb right after takeoff? (My POH provides a value here and says 5,100)

3. What max RPM I should see at full throttle in level flight? (I realize this is dependent on altitude. For reference, I was turning 4,880 RPM at 100% throttle at 14,500' pressure altitude / 17,274' density altitude at 11C OAT.)

Thanks in advance!

  • Re: What max RPMs should I be seeing?

    by » 8 months ago


    Hi anonymous...

    The HP of the engine depends on you turning it to the recommended RPM.  If you are low on RPM you have less power, simple enough.

    Look in your Rotax Operator's Manual and they give you the RPM needed.  Part of this is the  type of aircraft and what type of flying you do.  We don't know that so I assume your OEM has calculated these values already.  Since you are dealing with a none turbo engine and your numbers suggest you are flying well over the normal pressure altitude this was designed for there will be some big variables. 

    First, 5100  sounds low as you must achieve at least 5200 on normal climb.     The prop will unload as you roll down the runway and RPM will increase.  On a normal climb you need to get over peak torque, 5200 and a bit higher is fine.  

    Second, do not be confused by the Rotax number of 5800 as max, that is achieved if you have a constant speed or inflight adjustable prop.  With your fixed pitch you should see about 5500 at WOT in normal altitude density.  This is generally considered to be about 7500 feet as I recall.  The loading to 5500 allows for some increase in RPM if you should drop the nose pitch for some reason.  That max 5800 RPM is for 5 mins only.  Remember that the prop is your governor to control the load on the engine and not overspeed it 

    Just my opinion but you are overloaded on your prop pitch from the sounds of it.

    Cheers

    In general terms the prop unloading from less dense air will be about the same as the power loss of your non-turbo engine power loss.  In most cases the RPM will remain the same from sea level to normal flying altitudes.  The notable change will be your rate of climb will drop off proportionately to the loss of power.  (about 1% per 1000 ft of climb) 

    In your number 3 note  this seems way out of balance.  It appears you are flying in very strange conditions.  If you pitch your fixed pitch to fly in these conditions, unload the prop pitch to get 5500, you will be overspeeding your engine at the higher density on take-off.  

     


  • Re: What max RPMs should I be seeing?

    by » 8 months ago


    Rotax Wizard wrote:

    Hi anonymous...

    The HP of the engine depends on you turning it to the recommended RPM.  If you are low on RPM you have less power, simple enough.

    Look in your Rotax Operator's Manual and they give you the RPM needed.  Part of this is the  type of aircraft and what type of flying you do.  We don't know that so I assume your OEM has calculated these values already.  Since you are dealing with a none turbo engine and your numbers suggest you are flying well over the normal pressure altitude this was designed for there will be some big variables. 

    First, 5100  sounds low as you must achieve at least 5200 on normal climb.     The prop will unload as you roll down the runway and RPM will increase.  On a normal climb you need to get over peak torque, 5200 and a bit higher is fine.  

    Second, do not be confused by the Rotax number of 5800 as max, that is achieved if you have a constant speed or inflight adjustable prop.  With your fixed pitch you should see about 5500 at WOT in normal altitude density.  This is generally considered to be about 7500 feet as I recall.  The loading to 5500 allows for some increase in RPM if you should drop the nose pitch for some reason.  That max 5800 RPM is for 5 mins only.  Remember that the prop is your governor to control the load on the engine and not overspeed it 

    Just my opinion but you are overloaded on your prop pitch from the sounds of it.

    Cheers

    In general terms the prop unloading from less dense air will be about the same as the power loss of your non-turbo engine power loss.  In most cases the RPM will remain the same from sea level to normal flying altitudes.  The notable change will be your rate of climb will drop off proportionately to the loss of power.  (about 1% per 1000 ft of climb) 

    In your number 3 note  this seems way out of balance.  It appears you are flying in very strange conditions.  If you pitch your fixed pitch to fly in these conditions, unload the prop pitch to get 5500, you will be overspeeding your engine at the higher density on take-off.  

    Thank you, Rotax Wizard. I appreciate the insightful reply. I come back to you with much data as I just conducted an extensive test flight:

    I am seeing 4,790rpm at max throttle on the ground (did a 10 second full throttle static ground test).

    I am seeing 4,670rpm during takeoff roll. That remains 4,670rpm in the departure climb and I noted still 4,670rpm climbing through 9,000'. For some reason, climbing through 12,000' that rpm increased by 50 to 4,720.

    At minimum idle after engine warm, I'm getting 1,480rpm. I'm considering having this increased by 200rpm so I don't drop like a rock so much when I pull power to idle over the threshold.

    You mentioned I should see about 5500 at WOT around 7500'. I'm getting 5520 at WOT at 8000' so that seems spot on. At 12,000' my RPM is 5,420 at WOT. At 6000' and below I get about 5600 at WOT.

    Based on your saying that 5100 in climb seems low, then I must be very low if I'm only getting 4,700 in climb. What needs to be done to fix this? Repitch the prop I assume, but any guidance for which way and by how much? 


  • Re: What max RPMs should I be seeing?

    by » 8 months ago


    Rotax Wizard, I just created this new thread with more information: https://www.rotax-owner.com/en/general-tech-discussion/9940-think-i-need-my-prop-repitched-any-suggestions

     


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