TBM 850 Power Management Observations
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So, firstly I want to say that I'm quite impressed with this - its a beautiful model, and the avionics & systems really work well. I love the engine computer implementation!
A couple of things, that I suspect were design decisions not to make it 'too hard' for the average simmer (as cooking a PT6-66D can be done quite easily if not handled with care, it's quite a different beast from the piston engines most are used to..)
So - my feedback in no particular order:
1. NG is rising by itself, this is not correct... So with the Pt6, throttle position is linked to the gas generator (NG), in this particular engine flight idle would be around (IIRC, I don't have my manuals to hand) 68-72%NG. Full forward on the power lever would equal 104.1%NG. The throttle sets the desired NG and then the turbine governor will hold that NG (through the fuel control unit) such that a constant NG is maintained. This is the reason that in a turbine engine the Torque will decrease as you climb with a static throttle setting. The decrease is actually quite rapid during a quick climb and must be managed to maintain the desired torque.
So, can we have an 'advanced' implementation with correct throttle behaviour?
2. It seems there is some 'black magic' controlling full throttle power at cruise...
So I was trying a bunch of cruise settings and one thing I noticed is that when I reach my cruise level, even with the throttle firewalled, the Torque will only reach the 'Max Cruise' torque level (as defined in the TBM PoH). In real life there is always a little wiggle room, and in fact you most certainly CAN firewall the throttle past the safe level and shoot past the ITT limit, this is a reason to be careful about your throttle setting..
Can this 'auto limit' be removed?
3. Prop RPM bleeding off too quickly
So this was made a lot better in the latest update, but it's still not quite right. In this model at 120KIAS configured for landing, if I bring the power back to around 20% TRQ, the prop starts to come off the governor and lose RPM - this wouldn't start to happen until generally under 100 KTS, I'll have to check some notes to be sure, but it's certainly far too soon, which also likely explains why the aircraft is still floating quite a bit too much in the flare.
Is it possible to correct this?
Thanks for reading, my comments are intended as constructive criticism, and I hope that my comments are taken in the positive sprit they are given :)
Best wishes,
James -
So, I've done some more looking at what is happening and my initial assessment is wrong - the torque doesn't seem to be being externally limited, but rather just reaching it's total limit, but I think a few things are slightly misunderstood...
So, the first problem is that full throttle is not equalling 104.1& NG. Instead it is topping out at around 103% - on a PT6A-66D, firewalled throttle will always be 104.1% NG.
Second, ITT rise with altitude is also misunderstood. So I just tried a cruise at FL310. I firewalled the throttle to see what would happen - in a real PT6 you would expect to reach MAX ITT at this flight level, way before you torque out. In this TBM, with throttle at full, I'm seeing an ITT of 810 and about 104% torque (which roughly matches what the book says I should have for MAX Cruise power).
In a real PT6, the maximum attainable torque at this FL would indeed be around 104% Torque, but the reason you hit the limit would be due to the ITT limit, not a limitation in the engine's ability to produce power. On low ISA days, you could go further, maybe even to the maximum NG, but this is on those incredibly rare sub -20 ISA days..
So, I think without going into too much technical detail (though if you want to, please do reach out, I have lots of documentation on PT6 operation and a little real world experience) I think there is no artificial 'torque limit' going on, it just seems to be the simulated maximum torque, but the ITT value is running around 20-30 degrees too low, and the maximum attainable power from the engine is about 2-3% too low also (if it were changed such that max throttle = 104.1% NG, then I think this would solve the issue as long as the ITT values were tweaked such that you could match book values for cruise speeds)
Hope this has been helpful!
James -
@severniae Hi James,
Thank your for your feedback. I always appreciate such thorough and careful examinations of my aircraft, as they inevitably lead to more complete simulations of almost every system. I have made the corrections you suggested for the gas generator RPM control. You are absolutely correct that there is no funny business going on with torque at altitude, but I have also made the changes you suggest for full throttle position relative to the limiting ITT in those scenarios. With your extra piece of information, I went back to my reference videos and estimated the actual throttle lever position, rather that assuming it was firewalled above ~FL260. I have also noticed the propeller RPM falling off the governor too early in the approach and lading phase, but I haven't tried tackling this issue yet, as it's something I've noticed with many MSFS aircraft. I'm sure you understand that sometimes you can tweak performance endlessly, but something will still lie outside of the expected parameters, since the simulation is just an approximation, after all. That being said, I will do my best!
Thanks for the feedback again, and I will look forward to hearing what you think after the next update.
Thank you!
Nick C. -
Awesome news! Thank you for taking the suggestions - I am more than willing to test and feed back, and if you would like to collaborate further do get in touch, I have quite the passion for PT6 simulation (I've done several mods myself over on X-Plane aircraft to simulate correct turbine behaviours, so have a little experience, though not MSFS specific.)
Many thanks and looking forward to the next update! :)
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@severniae Hi James. Thank you again for your commentary. I was actually going to take you up on your offer to provide more insight over email, but you were my first post to the JF forums directly as a developer, so I don't know the best way to get in touch directly, haha. I will ask what the best recommendations are from the JF team and get back to you. Thank you!
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Haha, yes I just tried to PM you and discovered such option does not exist!
If you use Avsim, I use the same handle over there "severniae" and feel free to PM me and we can share contact details. I actually had some ideas following your previous posting ref throttle positions vs ITT etc...
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@severniae If you click the user name of the person you want to PM it takes you to their profile, on there you should see a chat button on the right-hand side under the cover image - that's the equivalent of PMs on the forum software we use.
