G231PUH and 700mm blades, what's the proper gear ratio?

[ImRich]

I'm begining to wonder if I have the proper gear ratio for my G231PUH on my Xcell Gas w/690/700 mm blades (short boom at this time).

My throttle curve is very flat and at about 20% until after the hover point at which point it takes off in very short order to 100%.

I woke up the other night thinking about this and wonder if I should be using a higher gear ratio to get my engine RPMs up at the hover point (which is about 5/8 stick position for me).

What is the 'proper' gear ratio for a G231PUH on the Xcell?

Or why else would I have such a 'hockey stick' throttle curve?

I'm just striving for perfection!

[bigrcr]

Actually the 6.43:1 ratio that comes stock will work well with the 700's and the G-231 from about 11,500 - 12,700 motor RPM. You flying style will mandate how you set your RPM for this ratio. For 3-D, you will want a high motor RPM, don't worry, the 231 can do it and take it!
The throttle linearity will have a lot to do with your throttle curves as well as you needle settings. Incorrect low needle setting can cause the curve outputs to be very low through mid stick. I have seen throttle outputs in normal mode, nearly in the upper teens and twenties until 3/4 stick, this MAY be an indication of a improper low needle setting. The low needle should be at or very near 1 3/8 open. Set your throttle ATV's so that you have 0-100% output for travel and the carb barrel matches, this means your servo horn and throttle arm will match throws from full cut to full throttle. Make sure that you are properly set on the low needle and are not setting it for a good idle. The low needle needs to be set for a smooth, non-vibrating hover with good transition to full throttle. The idle RPM needs to be then set by the sub trim/trim tabs and this should be a low output, making your throttle hold low as well. If not, a governor will not work properly either.

Operating the motor at low RPM will also make for a low and flat curve and really cannot be helped much if that is the case. At 12,500 RPM, I have a true v-curve in idle up, with the mid stick output in the 40%'s and each end at 100%. In normal, the curve will be much flatter and cannot be helped much due to the throttle barrel opening being as it is on a gas carb. Most of the throttle response happens with very little butterfly movement, and past 3/4 opening there is little gain in output with the last 1/4 throttle opening. Some pilots such as Malorie have gone in and done some mixing/expo work to make the throttle more linear and this should work fine for normal/hover mode. To me this is only much advantage at lower RPM and in normal mode (not in idle-up).

Let me know if any of this helps, or contact me if you would like some help!

[ImRich]

Hi John,

From what you are saying I have the setup correct. I guess I thought that with the low flat throttle curve in normal that I was running the engine too slow. I am running the 6.43:1 ratio, and have been since 1996 when my gasser was new.

Someone at Century mentioned that the 231 wants to be run faster than the 230 to get up on the torque curve and I guess this got me thinking.

I've been tuning my low speed needle to adjust for minimum tail 'wag' during a hover and not to adjust my idle. My throttle mechanics are as you mentioned.

I tried the same thing that Malorie described a while ago and it didn't help much IMHO.

Thanks for your help.

[bigrcr]

The 6.43:1 gear ratio works VERY well in my machine, but my head speed runs at 1875 to 2000 RPM (actually quite fun :D ). The Fury gas will have a 7.15:1 ratio as standard. The G-231 loves to turn up the RPM and really comes on strongest at 12,250-13,250 RPM for the best 3-D performance. There will be other optional ratios available also that will be plug and play. You will be able to play wherever you wish!

Later,

[fitenfyr]

John,
So if I am understanding you right this would be an "acceptable" throttle curve for normal mode on a G26 Predator?
0 20 30 40 70% on a 5 point curve Normal mode.

I have a buddy that is using this as a Camera ship and at those settings we are turning around 1650 or so swinging 700's in a hover and forward flight with no camera gear onboard.
I was thinking the curve was really goofed up, but it sounds like it is correct?

Now with a 3D curve you would run something like this?
80 60 40 60 80%
And shoot for an 1800-2000 headspeed?

Trying to get the learning out of the way so I can get right to flying when that "GasX" comes around. :D

[bigrcr]

Yes, That sounds like an OK curve in normal, except for the 70% on high throttle end. Use 100%, I know that there is little throttle change in RPM from around this point and WOT, but there is some and there are other changes going on with the additional butterfly opening.
The 3-D curve would approximate what you have there also at a good 3-D RPM, but this needs to be established for exact figures,....Your results may vary!..............

Hope this helps!

[fitenfyr]

Thanks John!
I think the top end of his curve is still at 90% so I will move it up next time I see him.

[HeliDriver]

I'm running 6.42 in my Predator SE 231 with 720/115 blades. I'm not expecting a super agile 3D machine with this setup. It is a fast cruiser and really, really stable. Full power vertical climbouts honest to God are very close to in performance to my old Fury Extreme/91 swinging 710s. Loops as big as the sky under full power are a thing of beauty.

The 720s are outstanding for autorotations too, the gasser doesn't feel heavy at all with that large of a disk and it floats down with plenty left at the bottom.

I'm sure the hot 3D setup is a taller ratio with 700 blades. I guess the trick is to keep that RPM up there on the pipe so to speak.

This is my first gasser and won't be my last!

[bigrcr]

I guess the trick is to keep that RPM up there on the pipe so to speak.



That is exactly correct. The Spectra-G has a 7.15 gearing to accomplish just that!

Later,

[ImRich]

John,

thanks for the notes. I have three head speeds which I run:

1550, 1650 and 1800.

The 1550 is my 'normal' curve and 1650 and 1800 are mostly V curves for 3D fun. I started to wonder about my throttle curve as now I'm spending more time to setup the gasser as my camera ship. I don't really need the V curves for that! :D

It sure is a fun ship with the white min-air 20g paddles and the 700 blades. I suppose I'll stop worrying about the 18% throttle at hover at the 1550 head speed.

[bigrcr]

Kool...You don't need a v-curve so that you can do 3-D for your application. Play with your idle up curves to just keep your selected rotor RPM constant so that your camera ship stays smooth. You can also play with some cyclic/throttle mixing to keep the motor RPM subtlety in check and the bird smooth and nimble all of the time. Your head damping will also help with being smooth at a lower rotor RPM. My machine will become very wobbly now at less than 1600 head speed because of the tight damping in the head, so keep this in mind as well.

A friend I was helping a few months ago was starting out in camera work and did all of his flying in normal for camera work. He did not want to set up "v-curves" since he was not flying aerobatics. The machine was becoming a small handful as the rotor RPM changed while lifting the load of the camera and equipment. I had him set up a mild curve and use idle-up to smooth out the machine, now the machine flies great and he's much more happy with the results.

If you choose to do it, you can play with gear ratios to keep the motor on the step and still keep the head speed down, although the machine is much more responsive with the higher rotor speed. The new Spectra-G will be perfect for this as it will easily accept ratio changes without filing and slotting.