Follow-Up to First Run

After the successful but a bit eventful first run of the Wren TurboProp, I do what I often do with newly-completed projects... I made a mental list of likes and dislikes with my particular setup, and I went about improving the turbine. The first thing to do was tear the turboprop down (not all the way, just the turbine from the plumbing and stand) and begin!

Step one was to correct the power-turbine tunnel fuel/oil feed, which was excessive. This I did statically by tapping the fuel manifold output ports, and with an elaborate closed-loop fuel system in place, c/w pressure guage and restrictor valve, I simulated running the turboprop. With the inline valve partially closed, I first generated a typical idle pump setting to produce perhaps 5 PSI in the system. I then set up another valve to control the lube feed. This new valve replaces the banjo fitting which connects the fuel manifold to the lube input port. Using clear tubing, I set up a decent drip, drip, flow, perhaps 2 per second. I then ran up the pump to create a 15 PSI manifold and verified that the lube flow was not excessive. With this set, the lube valve was locked in place.

The next step was to correct the pressure switch which was sticking on. I spent a few hours mulling it over, then decided to be lazy and wire the lube pump into the main switch. To do this, I replaced the SPST toggle with a DPDT rocker switch. While I was at it, I moved the switch inboard a bit as it was previously placed in the path of the left exhaust stack, a bit uncomfortable to kill in an emergency. My thought process was this: the ECU must be ON for the turbine to operate, so you may as well turn on the oil pump with the main switch as well. The drawback to this is that progrmming the ECU will also turn on the oil pump.

The propane manifold was modified to remove the tap which fed the pressure switch, as this was no longer needed. Without the tap, the gas solenoid valve was simply hanging in space with no support, so a bracket was made to hold this firmly.

The fuel pump I was using was way too much for this engine. I had installed a pump which came with the Artes FADEC, and I was throttling from idle to 100,000 RPM in only 3 or 4 8-bit units! This results in a coarse throttle response. I replaced the old pump with a new, smaller pump.

Today, after these mods, I had an absolutely flawless run of 14 minutes. Very easy start, no problems at all! I spent the majority of the run gently throttling between 60K and 110K rmp, which produced an idle prop speed of 2,800, and a top-end (at 110K) of 7,600. This is a Graupner 3-bladed 15X8 prop, and is probably a bit small, as extrapolating to 160,000K would produce destructive prop RPM's. EGT was very low, between 480 and 530 C the whole way.

As I said before, with the MW54, thank you to all the gang at Wren, especially Mike Murphy, for this fine design!


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