914 doesn't go above 4500RPMs! Help needed!

Joaquim

Don't confuse TCU position to throttle position.  Read the heavy manual for the type 914 and it will explain the internal mapping of the throttle based on the 7 sensor channels.  You can also read on you BUDS the airbox temperature, that is the one that will tell the TCU to back off boost if it exceeds 90C.  Head temperature is a function of the coolant and not the incoming air.  

0 throttle position is self evident.  The 115 is a %, not to be used unless you are in WOT for takeoff.  The 100 % is for maximum continuous running.  So 0 is no throttle, 115 is max throttle (set points to be entered when you calibrate the cable to the TCU) 100 % max continuous.  

O = idle, 115 = 5800 RPM and 5500 is 100 for max continuous.  WOT should only be used for takeoff.  This is explained in the manual.

Cheers

Joaquim

This might help, from the operators manual.

Cheers

Hi Joaquim,

The lack of power you experienced could be 1 - due to a fault somewhere in the turbo "system" OR 2 - the "system" operated as designed and the TCU reduced boost due to high airbox temperature, you just need to determine which it is. (It could be a combination of both or something completely different but I'll stay with these two possibilities for now!). 

As Stemme decided a MAP gauge is not necessary, the new instrument you have ordered will help greatly in deciding which of the two options I mentioned above is the culprit.

FAULT IN THE SYSTEM?

Yes, we still don't know the designed waste-gate position for 115% with engine stationary. There have been some issues in the past with servo motors failing both electrically and/or mechanically.

There is a procedure in the MMH 78-20-00 page 13 for inspection of the servo motor including checking the electrical resistance of internal components.

Here is a thread where someone discovered internal damage of the servo motor. 914 Turbo Servo problem

The MML 12-20-00 p65 reference I gave was for inspection, lube and adjustment of the waste-gate cable and lever, nothing to do with the oil system. Perhaps we are looking at different editions of the manual. I use the current online version from flyrotax.com, Edition 3/Rev.0.

Your point:

1. Something wrong with the TCU/sensors - your new instrument should tell you if there is a problem there.

Your plan:

1. Run the engine with BUDS connected makes sense to me or better if you can wait to do it with the new instrument installed giving even more information.

2. I would also carry out the servo electrical check I mentioned above and recheck the cable adjustment before the engine run.

Servo position target vs actual will be interesting to see and could perhaps point to a failure of the servo/cable sub-system if they don't match.

If you find the airbox temperature high then we need to find out why.

HIGH AIRBOX TEMPERATURE?

Your point:

1. Could be a leak.

2. It could be residual temperature (heat soak) from extended ground running.

Your plan:

Check all hoses and leak check, yes but please be careful.

I once made the VERY BIG MISTAKE of running a Stemme S10-VT for a leak check with the cowling removed. It overheated in a VERY short time with the coolant boiling and making a huge mess. If you choose to run it for a leak check, please make sure you have the cowls installed!

Maybe if you can somehow perform a static leak check using low pressure compressed air at the induction inlet would be better. 

That leads me to say they are prone to getting hot under the cowl, even with low ambient temperature and as you mentioned your CHT was high maybe there was some residual heat in the top of the engine from extended ground running.

The intercooler will only be efficient with high external airflows, the inlet doors are small as you know, a long way behind the propeller and somewhat shielded from propeller slipstream at low speed/crosswind on the ground so heat build up under the cowl and in the induction system from relatively high power/slow speed taxi could be an issue for you. Could you reduce the amount of time you run the engine with the cowl flaps not fully open? 

Please continue to keep us informed of your progress.

Cheers,

Des.

I think I found the issue! 😃 Thanks @Des It was your input that sent me in the right direction. It was probably the airbox temperature too high! Today I ran the engine connected to BUDS with only the top cowling and the left one. With the oil temp clearly rising but without having reached the green I tested the run up RPMs. No issue. Then tested full throttle. Airbox temperature was well below 90C and the RPMs went to 5500 as expected! 😃 I used another PC with BUDS and sent the log to my PC. But only tonight I will be able to analyse it. I thought that my PC with TLR was not working. But it reads all the parameters. It just doesn’t read the history. But in BUDS a big part of history is also gone. I hope the weather is flyable on Saturday so that I can test.

I think what happened was a mix of warming up the engine with flaps partially closed and needing too much RPMs for taxing. The CHT close to 110C just before takeoff should have be my clue to not attempt a takeoff. If the CHT was this high, the engine bay temperature was also high and thus it is easy to understand that the airbox temperature was above 90C.

With the turbo monitor this would have been diagnosed in the moment! Anyway I think I need to change my taxi strategy, at least with soft ground (and uphill) when I have to go sometimes to more than 4000RPMs. Maybe start taxing when the oil temperature is in the middle of yellow zone. And not takeoff if airbox temperature is above 80C.

It also seems that my tachometer is underreading by, at least, 100RPMs

Great news Yoaquim, I hope you have a nice flight this weekend. Please report back after you have used the turbo monitor, it sounds very interesting to me.

Cheers,

Des.