The Mark 2 kit from Wren

When I decided to pursue the Wren TurboProp, I chose early to buy the highly prefabricated kit for the gas generator section of the turbine. I had already machined an MW54 from scratch, and decided this time to focus my energy on the prop and gearbox section. This decision will easily save 3 months or more of effort, and the price of the Mark 2 kit (NO machining AT ALL!) is very attractive, especially to those outside Europe. Is VAT as "painful" as it appears to those outside the UK? ;-)

I am not going to devote a lot of space or photos of the kit. Other web sites have already done this, and the kit is so nicely put together that success is almost assured. I will, if necessary, post some of the more interesting aspects of creating the gas gennie section of the turboprop. This page is devoted more or less to impressions, and especially the differences in design, of this turbine over the thrust turbine I machined 18 months ago.

First, let me state unequivocably, the Wren MW54 kit is superb in every way. Instructions are excellent, with dozens of color photos to lead one through the process of assembling the turbine. Initial inspection of the machined parts indicates a high finish and apparently a quality tolerance control. The assembled portions too were superior, with the combustion chamber exhibiting excellent craftsmanship. The parts were packed in a safe manner, and Wren thoughtfully included 2 cap screw drivers of the correct size.

One of the first things I noticed was that the lubrication section and plumbing has been extensively reengineered, for the better I might add. Gone are the rather tricky, angled lube delivery holes. In its place is a wider screened area, and a spot-faced access channel for the lube pipe.

Here is the new filter cover (top) compared with the older filter cover. Note the increased width to accomodate the spot-face channel. Note too under my grimy index finger the small hole for the pipe itself, which originally was angled through a shoulder in the diffuser.

A close view of the lube spot face. You can see how it does in fact cut through the diffuser into the front spacer cavity.
The propane delivery pipe originally was a two-pronged affair which was inserted into two dedicated holes in the front of the CC outer. The new kit has a much simpler (and I suspect, more effective) delivery of the propane to one of the sticks at the rear of the CC.
Wren includes enough 3mm Festo fittings to complete the turbine. Since I have and maintain a huge selection of 4mm Legris fittings, I decided to machine adapters for this turbine so that I may use fittings and tubing common to my thrust MW54.

In this photo, we see a stock 3mm Festo from Wren at the bottom. This is equipped with a female 3mm tapped hole designed to accept the male threaded service adapters, one of which may be seen screwed into my aluminum adapter upper right. To adapt the larger, 10-32 Legris fitting, I machined 4-off aluminum stub fittings, turned 6mm at the bottom, threaded there 3mm, for the service connectors. The 6mm base allows direct use without being forced to spot-face or modify the front cover and inlet cone of the turbine. The wide portion of the adaptor is threaded 10-32 for the Legris fitting, upper left. Since the Legris has a built-in hex channel inside for a hex wrench, my plan is to loctite the legris permanently to the adaptor.

The compressor nut has been profiled for a better fit with the stock Wren bendix assembly.
Finally, the rather complex 2-piece exhaust cone assembly is replaced with a single cone. Aesthetically, I prefer this new one, even if the original is more efficient. This cone will probably go right onto my thrust MW54, since the gas gennie doesn't use an exhaust cone!

Test Run 23 Apr 2002: Mounted to the stand, the cone was fitted to the completed turbine. An EGT probe was also fitted, and the turbine plumbed. My 5 Bears ECU did the honors; this was somewhat of an acid test for the latest firmware revision, in that it was being asked to run a new turbine with unknown characteristics.

I loaded the ECU turbine data set 3 with data appropriate for the Gas Gen. Really the only difference from my thrust MW54 was a suspected lower EGT; hence, I dropped the EGT MIN RUN to 400. At this time, I haven't added the electric starter, so I decided to use an air start, which is always smart for a new turbine anyhow. With the ECU announcing READY, I applied the air. At 3,000 RPM, the gas and glow came on. Ignition was instantaneous, much faster than my thrust MW54. With the air reapplied, the ECU took care of everything else and the turbine was at idle with no fuss about 10 seconds later.

The big question? VIBRATION! There was NONE! I was delighted with the results of my meticulous balancing. I ran the turbine up to 140,000 RPM, checking vibration levels every 10K or so. Absolutely smooth! EGT's were higher than I expected, with levels only slightly lower than my thrust variant. The cone obviously helps boost EGT and will not be used for the turboprop.

In a nutshell, it was a non-event. Instant start, right "out of the box" after maybe 5 hours of construction and 3-5 hours of balancing. The Wren Mark2 comes highly recommended.

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