Prop RPM at idle

[Guest]

Hello Twotter fans,

I wanted to share the following in case it saves someone else a lot of time.

I was experiencing strange engine behaviour on the ground. The props would cycle up to 100% RPM with power levers at idle. I could feather the props, but as soon as they unfeathered, they'd go back up to 100%. I spent hours (well a long time anyway) troubleshooting FSUIPC, FSX control assignments, etc. to make sure there wasn't something hidden there which was causing this.

Eventually, I pulled back my mixture levers (Saitek quadrant mapped to the mixture axis, normally Ctrl-Shift-F1 to F4) and the props returned to a normal idle range. It became clear to me that I couldn't use the levers on my physical quadrant and would have to click on the condition levers in the cockpit during start and shutdown.

I could deactivate the mixture levers in the turboprop profile altogether, but I would like to use them for the King Air, which has low idle and high idle detents for the condition levers. For now, I am keeping the levers in a dead zone at maximum, hoping that the software doesn't suddenly recognise they're there while I'm on the ground and spin the props up to maximum. If that happens, I can still pull the levers back until things calm down.

In that case, normal engine idle values at ISA would be about 44% NG giving 67% NP?

Thanks, and happy flying.

Chad

[cbd80 258]

Ideally you don't want any Mixture assignment with the Twotter, as the aircraft doesn't use mixture like other props.

If you're using FSUIPC why not create a separate profile for the Twotter?

[spike9 13]

I have a CH control quadrant with two levers assigned to the the mixture control and they give me no problem whatsoever. In the twin otter they seem to work fine as the fuel levers and I enjoy having them, same as on the king air series aircraft just without a low idle detent.

[Guest]

Ideally you don't want any Mixture assignment with the Twotter, as the aircraft doesn't use mixture like other props.

If you're using FSUIPC why not create a separate profile for the Twotter?

Thanks Chris, I'm using the workaround as explained in my post for now, I'd like to spend a little more time flying than I've spent with FSUIPC the last 2 weeks

Of course creating separate profiles for the Twotter, the ATR, the Cheyenne, the Dash 8 and the King Air (which sees the mixture controls as condition levers) is also possible, and I'll get there, in time.

My post also applies to anyone using Ctrl-Shift-F4 to start the engines. Note it sets the condition levers to on normally, but behind the scenes it's also setting high idle which causes this prop runaway.

Chad

[Finn 873]

First and foremost always ensure that all axis' are properly calibrated.

When set to cut off - hardware lever move full back

When turning on - hardware lever move full forward

We use the default FSX turboprop model, though it has been tweaked quite a bit.

The default FSX turboprop is based on the Pratt and Whithney PT6 where the condition lever (FSX Mixture) has 3 detends - Cut off, Low idle, high idle.

Since the Twin Otter PT6 only have Cut off and On, where On equals Low idle, we need to set mixture to 50% whenever it´s sensed to be higher.

If Your mixture axis isn´t calibrated correct or is spiking, then it will constantly send changing values around maybe 98 and 100%.

For the Twin Otter You should either assure that the mixture axis' are propelr calibrated, or even beter simply unassign them from the FSX controller setup menu and then use mouseclick on them instead.

For most useres there really shouldn´t be any problems with hardware axis' assigned.

Finn

[Guest]

First and foremost always ensure that all axis' are properly calibrated.

When set to cut off - hardware lever move full back

When turning on - hardware lever move full forward

We use the default FSX turboprop model, though it has been tweaked quite a bit.

The default FSX turboprop is based on the Pratt and Whithney PT6 where the condition lever (FSX Mixture) has 3 detends - Cut off, Low idle, high idle.

Since the Twin Otter PT6 only have Cut off and On, where On equals Low idle, we need to set mixture to 50% whenever it´s sensed to be higher.

If Your mixture axis isn´t calibrated correct or is spiking, then it will constantly send changing values around maybe 98 and 100%.

For the Twin Otter You should either assure that the mixture axis' are propelr calibrated, or even beter simply unassign them from the FSX controller setup menu and then use mouseclick on them instead.

For most useres there really shouldn´t be any problems with hardware axis' assigned.

Finn

Hi Finn,

Thanks a lot, it looks like it's a spike from my quadrant at some stage then that forces the value above the intended 50%. I've created a dead zone on my levers near the top now, so I can still use the levers at start-up, but it'll then remain at 50% instead of constantly increasing the value.

I'm off to do some testing and tweaking.

Thanks,

Chad

[Guest]

I've done some testing and noticed that a slew (or something relating to it) triggers the 100% mixture setting. In that case I can pull the condition levers on my quadrant back to about 50% to achieve the low idle again. Now, when I return the levers to the max position, the engines remain at low/normal idle.

Chad

[taguilo 12]

What's happening to you is really weird.

Propeller RPM vars and Mixture vars are not directly related. There is no way that setting Mixture to 100 % will set PROP RPM to 100 % when Power levers are at idle. Even removing the 50 % limitation, RPM go no further than 70 %. Also removing vibration and peak correction vars makes no significant difference.

I tested Slew mode as well and nothing.

The problem must be elsewhere, maybe related to Power/Condition levers situation. It would be great that you make exhaustive tests and report the results.

Tom

[Guest]

It is really weird yes, that's why I started the post, in case anyone else was experiencing it. The feedback from Finn has made things clear to me.

You're right, normally there is no way to achieve 100% NP at idle. As an experiment, please start the Twin Otter normally, then press Ctrl-Shift-F4. What are your NG and NP values?

Thanks.

Chad

[taguilo 12]

Finn's feedback advises the reason why not to force a 100 % rich mixture, but it is not directly related to your problem.

If you have a payware version of FSUIPC, you can use the Log to watch for the events being called.

Just check Buttons, Events and Axis control boxes, together with Send to console window box.

So, with the console open, move you mixture lever and confirm that only AXIS_MIXTURE_SET is being fired, and after that MIXTURE1_SET and MIXTURE2_SET are fired three times -some of them are Finn's corrections that pull the 100 % mixture to 50 % in an autoresponse by code.

FYI, my normal values for IDLE power are PROP RPM 58 % and ENG Ng 45 %

Tom

[Guest]

I've checked the FSUIPC console and only MIXTURE1_SET and MIXTURE2_SET values are present when I move the levers.

I can see that the value is forced to 8192 (50%), until I reach about 9800 on the way down, at which point the 8192 no longer appears.

Chad

[Finn 873]

In FSX a setting of 100% mixture will force Prop RPM tp 100%.

I´m not sure if high idle in the real PT6 engine, that have both low and high idle, behaves that way, but it could be an FSX turboprop issue, though when axis' has been calibreated correctly it shouldn´t affect the Twin Otter, since it only uses the low idle position.

But to emphasize:

With correctly setup controllers, the PT6 engines of the Twin Otter extended will work correctly.

Finn

[taguilo 12]

In FSX a setting of 100% mixture will force Prop RPM tp 100%.

No.

I repeat: Mixture Lever position is not directly related to PROP RPM (whatever unit). Mixture position affects ENG N2 in jets and ENG N1 in turboprops. In turboprops, max PROP RPM is about 70 % with condition levers at 100 %, mixture at 100 % and idle power.

I've checked the FSUIPC console and only MIXTURE1_SET and MIXTURE2_SET values are present when I move the levers.

I can see that the value is forced to 8192 (50%), until I reach about 9800 on the way down, at which point the 8192 no longer appears.

Chad

If you don't see AXIS_MIXTUREn_SET when you move your controller, something is wrong or is different that standards in your configuration,

I can't help you more here because I don't have a Saitek unit.

Anyway, if you can solve the problem just by not operating the mixture levers, so be it

Tom

[Guest]

I meant AXIS_MIXTUREn_SET in my post, so there's nothing wrong with the controller. As mentioned in an earlier post, I am now using the mixture levers quite happily without problems.

What remains a mystery is why when I exit from slew mode, the FSX mixture value shoots up to 100%. In the default King Air, this would be no problem. In the Twin Otter, this extra Ng is enough to run the prop up to 100% (due to necessary tweaks according to Finn).

To conclude, when this happens now, all I do is reduce the 'mixture' until the values are back to normal and the Twin Otter code limiting the output to 50% takes hold again. I have no idea what causes the override in the first place, according to FSUIPC it is not my controller.

Chad

[taguilo 12]

I meant AXIS_MIXTUREn_SET in my post, so there's nothing wrong with the controller. As mentioned in an earlier post, I am now using the mixture levers quite happily without problems.

What remains a mystery is why when I exit from slew mode, the FSX mixture value shoots up to 100%. In the default King Air, this would be no problem. In the Twin Otter, this extra Ng is enough to run the prop up to 100% (due to necessary tweaks according to Finn).

To conclude, when this happens now, all I do is reduce the 'mixture' until the values are back to normal and the Twin Otter code limiting the output to 50% takes hold again. I have no idea what causes the override in the first place, according to FSUIPC it is not my controller.

Chad

Now I detected the reason of your problem.

In aircraft,cfg, there is a line like this:

minimum_on_ground_beta = 0.0

Actually, beta range starts at around 14 % and goes from this value down to -5 %, when it turns into Reverse Mode.

Initially I have corrected the beta range by modifiying that line with:

minimum_on_ground_beta = 12.0.

to obtain a more realistic beta range, even with a better sound from the propellers. Also I will never have full rpm with any mixture set with this value.

A 0.0 range is very close to Reverse range, so commanding a full mixture will make propellers come to full, certainly.

I apologize for not remembering this issue.

Tom