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Experts,

A few days ago I tried to do a takeoff (2 attempts) and noticed very low power (slow acceleration), so I aborted those attempts.  I've attached two data screen captures, one showing the 2 attempts and the other a good flight.

I notice that both the MAP (which I actually think is airbox pressure in my installation) and the fuel pressure (I'm not sure I'm measuring differential pressure as is recommended) drop sharply when pushing to 115%.  In the good flight, those both rise with the RPM.

I also noticed that in the bad sample, the pressures both drop in the middle of the runup, which is also clearly an anomaly.

I'll also note that the "good" flight was after the "bad" flight attempt... The problem seemed to go away on my third attempt (maybe I shouldn't have done the flight, but it was fine). I just don't want this to come back or happen mid-flight...

Thoughts?

Thanks!,

Craig

10162_1_Good.png (You do not have access to download this file.)
10162_1_Bad.png (You do not have access to download this file.)
  • Re: 914 Low Power on Takeoff Attempt

    by » 7 months ago


    I’m no expert but seems fuel pressure is dropping off 

    do you cycle the main and aux pump during runup to see if one drops ?


  • Re: 914 Low Power on Takeoff Attempt

    by » 7 months ago


    Yes, I test both pumps individually during the runup.

    I notice that both fuel pressure and MAP (Airbox really) drop unexpectedly in the middle of the runup and when pushing the throttle forward for the takeoff.

    I think the airbox pressure drives the fuel pressure, so I think that's the root cause (but, I'm not certain).

    Craig


  • Re: 914 Low Power on Takeoff Attempt

    by » 7 months ago


    Hi Craig,

    Short of hearing from a real expert, I offer this insight.  

    Your fuel pressure sender looks to be connected after the regulator, which is correct. However, it appears to be referencing ambient pressure rather than airbox pressure. This makes the information less useful, or at least more complex to analyze. What is important is that the fuel pressure to the carburetors is maintained at approximately 3.6 PSI (7.3 hG) above airbox pressure. If your fuel pressure sender was a differential type and referencing airbox pressure (and installed after the regulator) than you would expect to see fuel pressure remaining steady at about 3.6 PSI. You might consider making this change.  

    That said, your fuel pressure seems to be tracking manifold pressure correctly. There are a couple points where the fuel pressure seems to be the same as manifold pressure, but that is just an effect of the fuel pressure units being PSI and and the manifold pressure being hG, and how that scales on the graph.  When you do the math at that point on the graph you do have sufficient fuel pressure to overcome airbox pressure. 

    You have not said the engine was running rough, so I assume that what you are describing is the engine running smoothly (normal) but not creating full power.  If this is the case I would guess fuel flow is OK, and you should look at the turbo wastegate and related controls.  You can see that at several points in the graph you accelerate to 5000 RPM and the manifold pressure falls off like it would on a non-turbo engine.  Maybe the wastegate is getting stuck open, or maybe a problem with the turbo servo.  


  • Re: 914 Low Power on Takeoff Attempt

    by » 7 months ago


    The engine was definitely running smoothly, just sapped of power.

    I do need to fix the fuel pressure measurement. I have no idea why the installer did not follow the Rotax guidance on it.

    I'll look into the turbo path. Thanks.


  • Re: 914 Low Power on Takeoff Attempt

    by » 7 months ago


    There is an enrichment solenoid on the airbox. Might want to check that.

     


    Thank you said by: Craig E Maiman

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