The thread about the Wren semi-auto start questions made we wonder about a couple of things. I don't want to hijack that thread, hence this new one...

Question for customers :
What are your reasons for choosing anything other than a full-blown auto-start system ? Is it primarily to save space / weight in the aircraft, or save money, or just wanting to "be manually in control" more (kinda like why I prefer a manual transmission instead of automatic ;-), or something else ?

Question for Sara / anyone else associated with Wren:
For the models in which starting is done with the handheld starter-on-a-stick approach ... does this show any difference in turbine wear and tear etc ? e.g. with the built-in starter motor approach, the starter motor also tends to be used for cooling the turbine down by spinning it over very soon after shut-down. That can be happening automatically while you're still wheeling the aircraft back towards the pit, whereas with the separate starter the cooling will presumably not be applied until the pilot gets the aircraft back to wherever he can apply a blower or other such cooldown device (just as we used to have with the old generation air-start systems). Does that difference in cooldown strategy have any noticeable effect ? Or is the highly controlled cool-down over-hyped and not really an issue for improving the longevity of any of our turbines ? Or have Wren's boffins developed some magic that makes them tolerate different cooldown approaches, etc ?

Note that I have very high regard for the Wren products, so the above is not in any way an attempt to suggest anything is wrong with the NON-fully-automatic systems - I'm simply trying to plug an "ignorance hole" in my understanding of the cooldown cycle and its tue importance.

Thanks,
Gordon

I found the starter wand to be very difficult to use especially when the turbine was installed in a F86 with a full bypass. Space was too tight and the starter wand is alittle clumsy. I choose the Wren 54 as my first turbine primary because of the cost and secondly, I enjoyed the opportunity to put it together. There is also a significant weight advantage not having to carry an onbound starter battery, and associated equipment. One small 4-cell pack for the pump is all that is needed. I resolved the issue by installing a speed 400 starter motor with Wrens small aluminum mount. Everything is still manual start with all equipment outside the airframe. I made a simple battery box which plugs into the starter motor with the FADEC mounted ontop. Simply plug in the ECU and battery lead and spin her up. After landing I can pop the hatch and plug in the starter for the cool down cycle or use a small electric leaf blower. This turbine is very easy to start and performs as advertised.

You can also buy the engine with the onborad starter motor but manual start.

Well it does save money as well as space and weight. And if you're a purist.... [sm=72_72.gif]

I have a semi-autostart. That is a starter motor mounted on my engine but I connect to external gas for starting. I spin the starter and monitor the gas manually and the rest is automatic. It's easy and I really can't see any reason for full autostart.

As for the reliability/cooldown question... I have some 100+ hours on my engine. Replaced the bearings once because they had been damaged in a crash. No issues with overheating.
I haven't done any testing but I bet that we are way too concerned with this issue anyway. I bet that the fuselage will suffer more damage from a hot engine after shutdown than the bearings (mine's exposed ).

I have a semi-auto wren 44 gold. I does save weight, space and money.
When you have to sort out problems with your turbine,
I think it is more convenient to have the full control of the starting sequence.
I might not think that way if the wren engines were not that easy to manually start.
About cool down, well, I shut off the engine after taxi back to the pit.
Then I cool it down. There is no difference to me...

Yann

I can think of 2 reasons to have the starter on the engine. The first? My friend had just started his MW44 when the rubber grommet thing fell out of the wand & started bouncing around in the fuselage getting closer & closer to the inlet [X(] Fortunately it didn't get sucked in.

The second is if you use the wand you cannot have a FOD screen fitted to the front of the engine & the Wren screen is the neatest around.

I don't think the lack of auto cool down is an issue, the same friend has a Lambert Micro Turbine & it has 9 hours of running so far with no problems so far.

Full auto start? I thought given the cost of a turbine what's a few bucks extra? I was ready to buy the full auto start MkIII when the Super Sport was released. The auto start means less things for me to stuff up! I did leave the gas tank out & start from a Coleman. - John.

I have 3 autostart engines, and a Wren 44 Mk 1 manual start with on-board starter motor rather than the wand. I only bought the manual start 44 because at the time I needed a 44, that was all there was. Had the autostart Gold version been available at that time I would have chosen it in preference. Battery weight for autostart is not a problem, my Wren Super Sport came with a 1500mah lipo the size of a matchbox, similar in size and weight to the 4 cell AAA nimh pack that the manual 44 uses just to run the pump. The only real weight saving in manual start is if you use the starter wand rather than an on-board starter, and the effect of 3 or 4 ounces of starter motor on my 11lb Skyhawk is neither here nor there. Give me full on-board autostart every time.
I think the options offered by Wren are for historical reasons. They started as a kit maker rather than ready to run engines, many customers were gas turbine enthusiasts not fliers, when many engines were still air-start, and Wren offered builders choices so the customer could choose the cost, the weight, the complexity or hands-on approach to play with, and so on. The market has changed a lot in these last few years, away from the engine enthusiasts tinkering with new technology, to the fliers who just want a ready to run product. But the engine enthusiasts are still there and since Wren has the product for them they may as well continue to make it available. There are always going to be people with little interest in flying but with a huge interest in building and manually starting and running a fabulous piece of engineering.

Harry

Gordon,

I saw your question early this morning but couldn’t answer until the end of the day, as I wanted to spend some time and write a good answer and I don’t have much spare time in the working day at Wren. However, it’s now almost 5pm UK time, parcels have been packed and posted, phone calls and emails have been dealt with, and I have settled down at the computer to look at this thread.

I see that some of our customers have already given some excellent information and have covered some of the points I was going to make. As Harry has said, our use of the manual start ECU is a historical thing: when we first started selling our engine kits they were half the price of any other turbine and the simple ECU helped keep the cost down. Also, it was much lighter than the Autostart and with the 11-12 pound Mk1/2 Wrens it helped keep down the weight in the plane and gave better thrust-to-weight ratios. In those days, batteries were much heavier and keeping the starter battery off-board made sense with a small plane – and you could then use a cheap 6-cell buggy pack instead of something more expensive.

The other advantage of manual start is that you are in charge of the start, and some people like that. You are right, it’s kinda like why you prefer a manual transmission instead of automatic – you get to know the engine better and listen to it more carefully. We found the manual start easier at first on the small engines – the Autostart units had been designed for larger engines and we often got a better start by regulating the gas manually. We’ve now overcome that problem by fitting a manual valve to the gas solenoid on all Wren Autostart engines, so that you can make fine adjustments.

Manual start doesn’t have to use a starter wand. The vast majority of our engines have the starter motor onboard, usually with the integral FOD screen. With a manual ECU and onboard start, the starter motor is operated by a switch in the lead to the motor in exactly the same way as the switch on a wand is used. The starter wand isn't very popular because it's not always easy to get it into position and you can't use a FOD screen, the onboard motor is much better.

So – onto the question that was directed at me, rather than the customers: does the lack of automatic cooldown make any difference to wear and tear on the engine?
No, we don’t think so. Most fliers are like Yann, they taxi the plane back to the pits, switch off and then plug in the starter lead and do a cooldown anyway. But if the engine doesn’t always get a cooldown, it doesn’t seem to make a lot of difference. One of the Wren team once said “If I bring the plane back to my feet, it gets a cooldown. If I land at the other end of the runway - well, it’s cooled down by the time I’ve walked to it!�
Maybe if an engine never got a cooldown there would be a difference over a period of time, but we can't check it very easily - it would be virtually impossible to find two engines of the same age, one of which hasn’t had cooldowns, one which has, so that we could compare them. OK, people might suggest we could try the experiment here at Wren but it would take a lot of time and effort for little return - we need to spend our time and energy building and selling engines.

In conclusion, it does seem that occasionally missing the cooldown for some reason is unlikely to damage the engine.

Sara Parish
Wren Turbines

Thanks very much for the detailed answer Sara - much appreciated.

Gordon

I began doing 'semi-auto' starts with the FOD screen mounted manual starter motor on my MW44. Since it was so easy and FUN, and a lighter installation (no heavy on-board starter motor/ECU Nicad), I converted my older RAM500 to use the Eco FADEC without solonoids, saving complexity and weight in the aircraft. Recently found a used kit built manual start MW54 III which allows for occasionally swapping components between planes, using either starter battery for any plane, or borrowing someone's NiStarter when I forget my 2 V battery. I do not carry propane on board and have not switched to Lipo cells for the weight savings because I don't want the added potential fire risks.

Good to know the occasional failure to do proper cooldowns does not appear to affect turbine lifetime. I'll continue to use my leaf blower through the inlet or the compressor after shutdown mostly to keep the heat down in the aircraft.

When I was flying my original MKII kit WRen MW54, I had my starter battery, propane and glow ignitor off board. After I landed, I would just taxi back, and let the engine idle while I plugged in the dispay and the start motor battery (has some deans connectors built into the fuse) and then kill the turbine, and watch the ecu while I would cycle the starter motor on and off (much like an auto unit) untill the temp read 100 C. It only took maybe 10 or 15 seconds.

When I used to land my Super Reaper on a short field, I would kill the engine on my approach and in those cases by the time the plane was on the ground it was already cool.

I figure as long as there is air passing through the turbine (whether forced or just by gliding in dead stick) its cooling the same.

AJC

PS to answer the OP - I chose the semi auto start because at the time I purchased my engine, it didnt come fully auto start. I later upgarded to an auto start ecu but never bought the solenoids and propane tank. Plus I had my starts down to a science and very quick and easy. But, I just ordered a new MKIII engine last week and went for the full auto package. Im kind of excited to see how it compares.

Out of interest: the full size practice is to idle the engine for a given time (around 3 minutes - usually long enough to taxi onto stand), and then shut it down. There is no cooling cycling as in our engines, and the jet is often tail into wind - sometimes into a very strong tailwind. They sem to survive this treatment OK.