Turboprop - Can't calculate Prop RPM :-(

[rickalty]

Hi Pete / everyone, I'm obviously doing something wrong here, and would appreciate some help.

I'm writing a turboprop Eicas panel, and have everything working except the Prop RPM gauges :-(

I see from the SDK that I should be able to get this for Engine 1 by reading 0898 - Engine1 N1, and multiplying that by 08C8 - Engine1 prop scaler.

Well, I can't seem to make that work.

As it sits right now, FSInterrogate tells me that N1 at 0898 is 10189, which factors to 62.2%, and the scaler at 08C8 is -11392, which would be 54144 as a positive number. Well, the propRPM gauge on the actual MSFS cockpit shows 1058 RPM, and I just can't seem to come up with any way to take those two values - 10189 or 62.2% and -11392 or 54144 - and get 1058 :-(

Someone please tell me what I'm missing?

Thanks

Richard

[Pete Dowson]

I see from the SDK that I should be able to get this for Engine 1 by reading 0898 - Engine1 N1, and multiplying that by 08C8 - Engine1 prop scaler.

Eryou missed out "then dividing by 65536". Makes quite a bit of difference!

As it sits right now, FSInterrogate tells me that N1 at 0898 is 10189, which factors to 62.2%, and the scaler at 08C8 is -11392, which would be 54144 as a positive number.

FSInterrogate only interprets the N1%, it doesn't take the scaler into account. This is okay for Jets, where the % is all that is needed. 10189 is 62.2% of 16384, the max value for that offset. You can see the calculation FSInterrogate uses and work that out for yourself.

Well, the propRPM gauge on the actual MSFS cockpit shows 1058 RPM, and I just can't seem to come up with any way to take those two values - 10189 or 62.2% and -11392 or 54144 - and get 1058 :-(

The scaler is a positive number, always. It varies from aircraft to aircraft. For example, for the default FS2004 Cessna 172, and the Baron, it is 10800. For the Caravan it is 928. These give correct RPM values.

However, I just checked the default King Air and it is giving 54144, which is interesting! However, I still get the correct results -- N1 gives 1471 when the RPM on the FS panel shows just over 1200:

1471 x 54144 / 65536 = 1215.

I don't know what aircraft you are trying this with, but it sounds as if it may be a jet model pretending to be a turbo and not doing a good job of it. Is it one from FS2000 or FS98 perhaps? I don't think the turbo model goes back a long way in FS history so folks use to fiddle things.

Check it out with the default FS aircraft. Then check the model you are using. Okay?

Regards,

Pete

[rickalty]

Eryou missed out "then dividing by 65536". Makes quite a bit of difference!

I'd actually remembered to do that, just forgot to mention it in my post.

Well, the propRPM gauge on the actual MSFS cockpit shows 1058 RPM, and I just can't seem to come up with any way to take those two values - 10189 or 62.2% and -11392 or 54144 - and get 1058 :-(

The scaler is a positive number, always. It varies from aircraft to aircraft. For example, for the default FS2004 Cessna 172, and the Baron, it is 10800. For the Caravan it is 928. These give correct RPM values.

However, I just checked the default King Air and it is giving 54144, which is interesting!

Yes, that's exactly the same value as I'm getting for the Dash-8

However, I still get the correct results -- N1 gives 1471 when the RPM on the FS panel shows just over 1200:

1471 x 54144 / 65536 = 1215.

I don't.... current reading is N1 10763, so 10763 * 54144 / 65536 = 8892, but the cockpit gauge reads 1117 :(

I don't know what aircraft you are trying this with, but it sounds as if it may be a jet model pretending to be a turbo and not doing a good job of it. Is it one from FS2000 or FS98 perhaps? I don't think the turbo model goes back a long way in FS history so folks use to fiddle things.

Check it out with the default FS aircraft. Then check the model you are using. Okay?

Maybe thats the problem - I'm using FS2002, which doesn't include a default Turboprop, so I'm using a downloaded Dash-8. I'll try a few more and see what I come up with.

Thanks,

Richard

[Pete Dowson]

Maybe thats the problem - I'm using FS2002, which doesn't include a default Turboprop

What about the King Air? That was in FS2002 as well. Hang on, let me find an FS2002 and check that ...

[later]

Hmmm -- it is different. The N1/N2 values for the King Air in FS2002 do seem to be similar to the jet N1/N2 values. They've changed something in the turbo stuff between FS2002 and FS2004.

On the King Air panel there are two RPM values for each engine -- a prop RPM and what I presume is a turbine RPM. However, even applying the scaler to the N1 values doesn't produce either. I assume you have to know the max RPM rating for the turbine and calculate using the N1% for a turbine RPM gauge. I don't know where the prop RPM comes from in FS2002.

In FS2004 the same two gauges read similarly, but the N1 value gives the correct Prop RPM when scaled. I can't see what gives the turbine RPM.

I'm afraid I'm a bit lost here. We need an aircraft/panel designer who understands these things to chip in and see if it can be clarified. Maybe the turbo gauges do their own calculations using these input figures, and knowing the specific aircraft.

Regards,

Pete

[rickalty]

In FS2004 the same two gauges read similarly, but the N1 value gives the correct Prop RPM when scaled. I can't see what gives the turbine RPM.

I'm afraid I'm a bit lost here. We need an aircraft/panel designer who understands these things to chip in and see if it can be clarified. Maybe the turbo gauges do their own calculations using these input figures, and knowing the specific aircraft.

The KingAir must have been in a different release of FS2002.... I don't have one.

However, I tried out half a dozen diffeent turboprops and found that dividing the prop RPM from scaling N1 by 7.96 gave the right Prop RPM in each case. Why 7.96 I don't know, but it worked, and the app is now on the Avsim Library.

Richard

[Pete Dowson]

The KingAir must have been in a different release of FS2002.... I don't have one.

Ah, yesthe Professional Edition had extra aircraft.

However, I tried out half a dozen diffeent turboprops and found that dividing the prop RPM from scaling N1 by 7.96 gave the right Prop RPM in each case. Why 7.96 I don't know, but it worked, and the app is now on the Avsim Library.

7.96? Doesn't seem to mean anything, does it? :) Still, if it is consistent that's fine. Well done.

Regards,

Pete