I have a question about throttle lag on an MW54 Mk III. I am sure that I did something wrong, but not sure what it is. The engine starts fine and will eventually run up to full RPM, but it takes literally about 13 seconds for it to get to full RPM at the factory default settings. Once there it seems to be putting out full thrust. It is a little faster to ramp up from about 1/3 throttle, but is still taking longer than I know a wren should as I hear they have excellent throttle response. I reduced the "Acceleration Delay" from the default of 25 to 15 and then all the way down to 7 and it seemed to help a little, but it was still taking about 7-8 seconds to get to full RPM.

Any suggestions from the Wren veterans out there?

Thanks for any feedback!

Woody

I would run it by Sara at WREN.

Hi Woody,

Sorry I missed answering your PM - I had a couple of days out of the office last week.

With an acceleration delay of 7 the throttle should be about 3.5 to 4 seconds, so there may be something wrong. Check the running temperature as the engine accelerates - if it's rising towards 800 the ECU will hold the engine back.
Check the position of the engine relative to the tailpipe, it should be about an inch and a quarter away. Which tailpipe are you using and what is the diameter? - it needs to be large enough.

Sara Parish
Wren Turbines

If the temps are high, as Sara suggested to check, be sure you do not have the thermocouple inserted too far.

Check what the max temp is set. I set mine at 852 and this does help with the acceleration. Also as said previously, check the thermocouple is only in about 3mm.

Jason

All of the above is good advice, overtemp during throttle ramp will cause the ECU to 'hold back' the ramp to avoid overtemp.

DO check the tailpipe distance and ensure it is as recommended.

Paul

So is the distance to which the thermocouple is inserted is critical to accurate temps? When I built my MW-54 kit recently, I mounted the thermocouple so that it was inserted about 4mm, but I didn't measure exactly. I'm getting pretty high EGT temps, that seem to be limiting the max rpm to about 156,000 - where the EGT is measuring ~730 degrees. So from the above comment, I take it I should have less of the thermocouple tip inserted? How can I tell whether or not the engine is actually overheating?

Jason

Does that mean by resetting the max temp. to 852 degrees that this temp. could be achieved in an overtemp. start? Is it a good idea to allow the turbine to reach 852 degrees?

John

The 156k won't be limited by the temp. Its more likely to be the PW is near the 1000 mark, see what the PW gets to running flat out.If its above say 875 then I would look at lowering it. You may have a restriction in the fuel lines or the cells may not be man enough if you are using a manual set-up. 730 deg is not mega hot but it could do with being closer 600 deg. Try the thermo-couple at 3mm, it should help.

Jason

Jason

Although none of my engines get anywhere near even 800 deg I have used the 852 setting for years and I do find it better for acceleration. I don't think the 50 odd deg is much to worry about. Infact I am sure the last ECU I had from Wren which was for a 44 was set at 852 anyway. Is that right Sara?

Jason

Yes, Jason, the 44s are set at 850 - I've just checked with Mike Murphy. He tells me that the others are usually 800 but as you say, the temperatures don't actually go anything like that high but it gives more room when accelerating.

Sara Parish
Wren Turbines

Try pulling the thermocouple out a bit and try a run. If it accelerates faster, that may have been it.

Thanks to everyone for all the great advice. I will try them this weekend to see what happens. I did set the distance between the engine and tailpipe at 30mm as recommended by Sara. Not to worry about the delay responding Sara, I figured you all must be on vacation like half the world at this time of year! I set the thermocouple at 3mm when I built the kit, but it may have gotten pushed in during install or since I completed the build. I will check that as well. Thanks again to all for the tips and I will report back on my success.

Woody

success?

Have incorporated all suggested changes but have not had a chance to try it out yet. Will run it by next weekend and let you all know.

Woody

hello pantherman. I have a wren mw54 mk3 and here in aussie becauce of the weather difference we normally have the thermocouple in at about 1.5 mm.. If at 4mm like suggested it would not ramp up quick because the ecu would read it as running to hot

Thanks for the feedback. It is very hot here too and I am certain this is the issue. I have backed it out to about 2mm and will see how it responds. Thanks again for taking the time to post!

Woody

Just wanted to update everyone. I finally got to run the MW54 this weekend at the Greater Southwest in Waco and it ran great. No lag issues whatsoever. I did change the max temp to 850 and moved the thermal probe out to about 2.5mm and it worked fine. Engine ran great all the way through the flight. Thanks to Sara and all for the tips.

Now I just need to buy the upgraded fadec and whatever else is needed to convert it from onboard manual start to full autostart! Too uncoordinated to work that starter button and worry about all the things going on at one time during startup. Almost fried my jet at one point not paying attention. [X(]

Other than my lack of starting skills all worked very well and I am pleased with the turbine which is especially surprising since I built it!

Woody

congratulations!!