Turboprop engine tuning

Roy Holmes

Resource contributor
As I pointed out in the original post, I was working on the C-2/E-2 turboprops which, above about 45% PLA, run at constant prop speed of 1105 RPM. The way that RPM is maintained is similar to that used in the sim and it works as follows. If the Prop RPM is above 1105, then the pitch angle is increased slightly which creates more power required and so the prop slows down to the normal 1105. These airplanes do not vary Prop RPM for noise abatement nor range considerations.
From what you say and I have learned by studying the King Air Flight Manual the Pt-6 behaves differently.
So I assume that the sim does a generic Turboprop just like it does a generic turbojet or turbofan, one model fits all.
My approach in developing engine FDE is to simply create the best performance match between an airplane and its performance manual figures and pay less attention to the niceties. If the speeds are correct and the climb and range data is within 3 to 5% I'm happy.
If the speeds are correct and the climb and range data is within 3 to 5% I'm happy.
That's the problem with many armchair pilots. They dismiss an FDE as unrealistic if the fuel flow is off by 1%, since they don't know (or don't accept) that even on long range heavies like the 767, FF can be easily higher than 5% of the book value.

@jcomm, that's why I 'love' formulas..since IRL they apparently don't always work ;) The quoted values are from a standard PA-31T with PT6A-28 engines.
Another question is what a pilot might consider 'no change'. If we take e.g. Roys 3-5% margin, the PA-31Ts 2% difference might be simply ignored by some pilots.
I see and fully agree with your approach Roy.

My post wasn't actually meant to be "against" it, quite on the contrary, juat to point out some "limitation" on ACES's approach when they modelled the turboprop.

Your view of a more generic approach, suiting both free-running à lá PT-6 and single shaft à lá Garret, types makes a lot of sense.


Resource contributor
If one changes the prop RPM the Power does not chance so the torque must. If the RPM is decreased the Torque will rise. No Fuel flow variations... And it makes sense to reduce RPM when transitioning from climb to cruise, at constant throttle...
This is completely correct.
Fuel Flow on PT6A engines is governed by FCU; changes depend exclusively on Power Lever position AND gas generator RPM status at different flight levels (air density)

With a constant altitude and power lever position, changing prop RPM must not produce any FFlow noticeable change, appart from a momentary variation just after props are repositioned, because of a brief disruption of flight conditions.

PA-31T S.L. ISA 2000 RPM 246ktas 1628lbft = 754lb/hr
PA-31T S.L. ISA 1900 RPM 246ktas 1628lbft = 736lb/hr
That is correct. Lower Prop RPM with same torque means lower Gas Generator RPM (N1), which in turn means lower FFlow.

Actually so far the only desktop sim I've used that properly models this feature of a free-runing ( PT-6 like ) trubine is ELITE IFT and it's B200.
Milviz Turbine Otter models PT6 engine almost exactly like real, including accurate FFlow values and behavior.