Wren 44 + 800T

[lperagallo]

I believe that CB is correct that you need a clutch. The reason is so you have a way to disconnect the head from the engine at idle speed. Don't want a rotor system turning when the turbine is running at idle. What if you can't get the turbine to shut down? I had an issue with vBar settings once and I couldn't get it to shut down. At idle I was able to get to the heli to disconnect the power to the fuel pump. I guess I could have waiting until the fuel ran out but that would have been quite a long time at idle.

Here is the copy of the AMA safety document: http://www.modelaircraft.org/files/0...regulation.pdf

Statement: "For rotary wing aircraft: The rotor head must be disengaged from the power source and remain stationary when the engine is at idle."



Lou

[jockstrap]

The gearbox of the turbine is like a clutch - there is no physical direct connection between the turbine and the gearbox.. The air pressure from the turbine drives the gearbox..

The gearbox will not turn whilst the turbine is at low rpm...

[KevinChiu123]

Originally Posted by jockstrap

The gearbox of the turbine is like a clutch - there is no physical direct connection between the turbine and the gearbox.. The air pressure from the turbine drives the gearbox..

The gearbox will not turn whilst the turbine is at low rpm...

To Lou~ Thank you very much for your sincere advice.

To jockstrap~ "The gearbox will not turn whilst the turbine is at low rpm".~~~ Yes, yes, yes.

[flyingrhino]

Must be something wrong with my turbine because the my gearbox turns at idle.

[desertstalker]

Originally Posted by flyingrhino

Must be something wrong with my turbine because the my gearbox turns at idle.

If there is no load on it, it will. Add some more inertia and there will need to be more thrust from the turbine to spin it up, but is suspect the blades will still turn slowly at idle (there is not THAT much friction in a rotor setup).

[helibeli]

I can't get my rotor to not turn at idle with my Predator turbine either no matter what idle rpm i choose, which means i have to shut down the engine to re-fuel.

[lperagallo]

Originally Posted by helibeli

I can't get my rotor to not turn at idle with my Predator turbine either no matter what idle rpm i choose, which means i have to shut down the engine to re-fuel.

Well that a good thing that you have to shut down to refuel. I'd hate to see someone attempt to refuel a running turbine. With exhaust temps running in excess of 350C I don't want to be around a small fuel spill.

For the record, my rotor assembly stays still in my Bergen with a clutch. It wouldn't without it. Also at 55,000 rpm idle you would be building excessive heat within the turbine without the second stage spinning. Heat is tough on turbine components.

Lou

[KevinChiu123]

Originally Posted by helibeli

I can't get my rotor to not turn at idle with my Predator turbine either no matter what idle rpm i choose, which means i have to shut down the engine to re-fuel.

Does your heli have a clutch? If yes, is it possible that your clutch is too tight?! The margin between clutch and clutch holder/cup is too small?!

[jockstrap]

ok guys - need clarification..

I'm new to turbines, have a new wren 44 but yet to complete build..
So as always i can be mistaken.. However, i'm led to believe that the 2nd stage (gearbox) is not direct coupled and is like " a fan blowing a fan" to turn it..

so under no load, i suspect it will turn, but under any sort of load - like hands on blades / rotor button, the blades will slow down and can be held still..

Am i incorrect ? Happy to be proven wrong, it's just the way i thought the 2nd stage works???

any input appreciated..

found this simple explanation: http://www.rchelicopterfun.com/two-stage-turbines.html

[KevinChiu123]

So as always i can be mistaken.. However, i'm led to believe that the 2nd stage (gearbox) is not direct coupled and is like " a fan blowing a fan" to turn it..
******
Yes.

so under no load, i suspect it will turn, but under any sort of load - like hands on blades / rotor button, the blades will slow down and can be held still..
******
Yes.

Most important, the output of "ratio".
Engine, 33,000 ~ 195,000. Gear box, 4,000~ 15,000. Engine gear vs. main gear~ 1:8.2.

In other words,
If you set up your engine at 50,000 ~ 180,000,
Your gear box's rpm is 3,846 ~ 13,846. (Practical min. rpm range > 4,000.). So as such, <4,000 is not sufficient to turn your main blades.
Your main gear's max rpm is 1,688.

Or, we may say~ the blowing power of engine rpm <50,000 does not offer any sufficient energy to gear ->main gear ->blades yet.

I am not an experienced turbine pilot yet..... Hope to receive some more comments from real experts. :o)

On the other hand, "I'm also new to turbines, have my 2nd. new wren 44 but yet to complete build" (cited from jockstrap).

For my 1st turbine, Jetcopter (Jetcat PHT-3)~ it weighs >9kgs, it used to swing 810mm main blades. We made some modifications then successfully installed Align 700 rotor head system. I fly 850mm main blades now.

For my 2nd one, from Align 700 (Wren 44 that has the same hp/output as that of PHT-3) became 800~ its weight is 6.5kgs. It swings 810mm.

For my 3rd one, I decided to buy Wren 44 again due to less engine weight, the same main blades for the same thrust. MD-8 is even lighter than Align 700.

[cbergen]

All are correct, it IS possible to hold the second stage still while the turbine is running, HOWEVER....

It is NOT recommended that you hold the second stage still, blowing the hot exhaust at that non-spinning wheel and possibly creating back pressure into the turbine itself.

For this reason, AMA does not accept a "2 stage" turbine as having satisfied the requirement of separating the power from the rotor system.

[sunnyville]

Very interesting!

[KevinChiu123]

Dear cbergen: Greatly appreciate your precious information.

[aviatorflyer]

How does the 700 version handle compared to the 800? I have a 700 Align wren 44 & it boggs down when you flip it or do any hard cyclic maneuver . I've turned the pitch down to 9 degrees & turned the max rpm up on the engine.

[KevinChiu123]

Originally Posted by aviatorflyer

How does the 700 version handle compared to the 800? I have a 700 Align wren 44 & it boggs down when you flip it or do any hard cyclic maneuver . I've turned the pitch down to 9 degrees & turned the max temp up on the engine.

What is the size of your blades?! 710 or 720mm? If so, your high pitch should be 9.5~10 degrees. Engine rpm~ >185,000 should be good enough.

What is your total weight? 5.8, 6 or 6.2 kg?

For approximately the same weight, of course, 800 size offers much better "thrust" for flying.

[jockstrap]

wow - don't tell me that.. i'm collecting my pieces to build my wren 744.. at 6.8hp I thought bogging would be the last issue i'd have to worry about

[KevinChiu123]

Originally Posted by jockstrap

wow - don't tell me that.. i'm collecting my pieces to build my wren 744.. at 6.8hp I thought bogging would be the last issue i'd have to worry about

Since you are building your Wren 744, I strongly recommend you modify your bird into Wren 844. It is easy. Not too much for you to spend more.....

[aviatorflyer]

I' m using 690 blades , rpm is 195,000 , never weighed it . It's a Heli Chris conversion . If the 800 has more thrust for flying I would consider converting it.

[KevinChiu123]

Originally Posted by aviatorflyer

I' m using 690 blades , rpm is 195,000 , never weighed it . It's a Heli Chris conversion . If the 800 has more thrust for flying I would consider converting it.

Wow!!! Your setting had to be wrong...... Hope you don't mind my saying this statement.

In the past, mine had 710mm, rpm was only 175,000. 6.3kg. Flew smoothly.
You can do the math~ "the outer diameter runs faster" (means more thrust) than the inner part of blades.
I believe that your 690 is too small & engine rpm is too high.

Now, my 800 has 810mm. rpm is only 180,000. 6.5kg. The thrust is much much more powerful and flies much much more stable. The engine still has 15,000 rpm to go.......

On the other hand, your heli must be lighter than mine.

[aviatorflyer]

I turned up the turbine RPM per other advise & turned down my throttle settings in the transmitter to get the the desired head speed. At max throttle up my head speed is 2100 rpm ,next one down is 1950 RPM.