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HomeTurbineAircraftHelicopterTurbine Helicopters › Back to my First Turbine Pod and Boom Hotcat
07-21-2017 02:32 PM  12 months agoPost 1
MattJen

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UK

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This was one of the first Turbine pod and Boom back in early Noughties,
I bought it in as part of a deal..

I sold it when I went over to Wren Scale Longranger and I bought it back a few months ago and it has sat in my shed..

Finally got it up and running, had to replace the fuel bottles as they had cracked around the nipples ( common problem ) I found a source on Ebay that did the exact same size with tube and filter already attached.

I also replaced the Robbe head with its weird 45o offset, and replaced it with a Quick Uk Raptor 90 Metal head and swash plate, I re-setup the 4 servo cppm and it sounds ok with no binding at the limits.

I did have an issue with the turbine hunting and giving me a temp time out error.

After a long discussion with Modtron ( Jetcat Uk ) I popped over and we went into the settings, we discovered the probe was giving spurious temp readings before, during,after starts.. so we replaced that along with a worn starter motor and all seems to be running fine, now it is a case of tracking the blades and getting some hours on it.. it still has only done 19hrs in the 12 yrs I had it.
It is still running on Version 4.9 software and is working fine. So a big thanks to Modtron for his assistance.

Matt

All The Best

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07-21-2017 07:59 PM  12 months agoPost 2
helihenry

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uk

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Yes, he can be quite helpful at times.

Simon

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07-23-2017 05:58 PM  12 months agoPost 3
Machinehead01

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Lower Michigan

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Single stager!
What is your preference 1 or 2 stage? Other then lower fuel consumption are there any real advantages to a 2 stage over a single? Seems to me the single would be more responsive, is it?

Tom

"You judge according to the flesh; I am not judging anyone."

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07-23-2017 06:16 PM  12 months agoPost 4
MattJen

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UK

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two stage is quieter and more efficient more powerful.

All of the thrust of gas generater is used to drive second stage where as on the single stage all the thrust is wasted,also the thrust has to be diverted and counteracted on certain scale models as it pushes heli forwards and you have to counteract that forward movemet, where as on second stage it don't..

The other big (huge in my opinion) benefit is you are not loading down the engine as you would with the direct drive method (single stage). There is no mechanical connection between the engine and the second stage/gear box. The second stage turbine only drives the load in other words and can tolerate a considerable mismatch between the engine and the second stage load.

This makes for a much smoother running model turbine engine, better engine management, better speed control, longer bearing life, reduced stress loads on the turbine.

Of course a two stage turbine is more costly (both to purchase & when servicing) as there are two more expensive ceramic bearings on the second stage half (4 ceramic bearings in total for the entire assembly). That said, the second stage doesn't spin nearly as fast as the engine thereby eliminating large amounts of gear reduction.

Typical speeds would be engine @ 120,000 RPM and second stage around 50,000 RPM), so the second stage bearings last longer.

I will always prefer second stage.. as to responsiveness I found second stage very responsive.

Matt

All The Best

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07-23-2017 09:19 PM  12 months agoPost 5
Machinehead01

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Lower Michigan

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Not how I thought it would be....
So I learned something today.

Tom

"You judge according to the flesh; I am not judging anyone."

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07-24-2017 04:14 PM  11 months agoPost 6
Chris Bergen

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cassopolis, MI USA

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Matt is definitely Spot on with his description. I can't speak to the responsiveness of a single stage vs a 2 stage, but it's always important to keep the turbine spooled up, not drag it down to an RPM that would cause severe turbine lag.

JetCat have the advantage with their built in governor system, but the Wren can certainly be managed with the use of cyclic-throttle mixes to maintain Rotor and Turbine RPM.

Talking about the single stage Turbines, one wonders if a movable exhaust system could be affixed, to increase the forward speed of the heli, see what speed one could get out of it... Hmmm....

Chris D. Bergen

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07-24-2017 06:32 PM  11 months agoPost 7
MattJen

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UK

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one wonders if a movable exhaust system could be affixed,

Interesting, I went to Jetpower in Germany a few years ago and on some of the Fixed wing demo models they were doing the equivalent of a PROP HANG with a jet, the exhaust was totally moveable and was moving to inputs from pilot..

Interesting to watch...

As to Governors, in the truest sense None of the jetcat single stage has a governor, they set it to run at 100% and then via gears and pullies gear it down to a set rpm in my case 1280rpm,you have to send it back for a gear change if you change to say a 1/4 scale model where the head speed is re geared to run at 950rpm on some.

Under load I found that the head speed fluctuated, I had a video of it ranging on the hotcat by 1500rpm.(PHT3)I will admit it was probably down to collective management when I was new to flying heavy models..

You could use exactly the same principle on the wren, as Bellbloke demoed it to me by working out the gearing needed to run at a pre set head-speed, and then set the throttle curve to 100% straight line in an idle up setting, purely experimental.

On my Long-ranger and Predator Wren we used a good throttle/pitch curve, as Chris has done and demoed many times over the years very successfully, I always used to say if you want to see responsiveness with a good throttle/pitch setup look at one of Chris videos in the early days with the intrepid where it is in an idle up on ground (IRCHA I THINK) and he bangs in collective and it shoots up like a rocket with a constant RPM into a aerobatic routine which for a turbine was rare and took guts.

We got it down to where it fluctuated by 150rpm, again we used a video camera strapped to the boom. It was in the days when Bell was running a Arial photography company and he did loads of testing in different configurations, this was before a lot of the electronics came onto the market that we have today and turbines were in their infancy and very new onto the model sector..

That was one of the reason I liked the WrenMw54 in one test I wanted to know how low I could get the head-speed on the pod and boom zealous, we got it down to 750rpm from 1240 by adjusting the fadec, obviously this is not ideal as turbine not in its ideal power band but it was a test to see how good my set of 950 MAH blades were and before the tail lost authority.

But for a good and simple truly governed setup the two stage jetcat SP5 was brilliant, you tapped in the gear ratio into the GSU and presto it run flawlessly, it was a great engine and still is.

All The Best

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