Ground Effect and Prop RPM

jheidebrecht

Good day everyone,

A couple of questions:

It appears to me that the TBM 850 uses the default MSFS ground effect values as used in the TBM 930. As many people are aware, the default MSFS ground effect is grossly exaggerated. Right now, I have set the lift_coef_ground_effect_mach_table to 0.127:0.5, to roughly approximate what I would imagine the aircraft would behave like. I only have experience in single-engine pistons, but based on my observations, the TBM settles down pretty quickly at flight idle. Has anyone else experimented with this, or have thoughts on the default ground effect in the TBM 850?
Are there plans to fix the constant-speed prop RPM simulation in the TBM 850? I'm not sure if this is a limitation in MSFS, but as of now, a reduction in prop RPM in cruise flight results in a significant decrease in airspeed, which shouldn't be the case according to the manual. A reduction from 2000 RPM to 1900 RPM results in a 16-17 knot loss in TAS.

Regards,
Jonas

Buzz

Same power but less RPM equates to less horsepower, doesn't it? And after reading your question I had a look in the manual and couldn't find any mention to aa power setting using 1900 RPM.

severniae

What you are seeing would be expected if you maintain the same torque setting. In order to maintain TAS then you will need to increase power if reducing RPM to maintain the same...

The prop is also most efficient at 2000RPM, in fact IRL most operators just leave it where it is...

The general rule if you want to reduce is to wait until you have put climbed your torque limit, then reduce RPM to maintain max torque.

Eseem

Now I might be wrong here, but AFAIK aircraft with the PT6 generally are controlled with a power lever and prop lever. The power lever controls power output, while the prop lever does not. In other words, when reducing RPM without touching the power lever, the RPM goes down and the TQ goes up by the same factor.

This was incorrectly modelled in the last version, haven't tested the new version yet, but from the above and from the change log it sounds like it's not fixed.

Discussion between people way beyond my knowledge level: https://www.pprune.org/archive/index.php/t-426635.html

Eseem

Found a video of it, see how the TQ increases as a result of RPM reducing here: https://youtu.be/lJLM56U12x0?t=1006

sg_aviation

I just tested the same in the new version:

Reduce RPM from 2000 to 1900, torque increases by approx. 10%
Reduce torque by approx. 10%
Airspeed remains same.

severniae

Yeah it works about right. I think what people are doing is reducing RPM at cruise and having to reduce power lever to maintain below 120%Trq - this will reduce airspeed..

jheidebrecht

For a given power setting, a reduction in RPM (with a proportional increase in torque) yields a lower TAS in my aircraft. You’re saying that you’re maintaining the same TAS with that change? Not sure what I’m doing differently…

severniae

@jheidebrecht

You have it right. If you leave the power lever alone, then IRL you would likely see a slightly lower airspeed. Really this is down to the efficiency of the prop - it's designed to run at 2000 RPM so anything less and it will provide less power. That said, it shouldn't be a huge reduction...

If you were flying along at 120 torque, and you reduce RPM, you would need to reduce power in order to maintain within the torque limit - in this instance you would see less airspeed, as you have reduced the amount of power overall.

Eseem

@severniae Do you have sources to this claim about efficiency? Either for the TBM specifically or for constant speed props generally? I'd like to understand this more.

To quote Daher from the cruise performance pages in their TBM 850 POH:

Propeller RPM utilization between 1600 and 2000 RPM is possible without changing
performance. Display the TRQ indicated in table with Np = 2000 RPM, then reduce Np
without exceeding 121.4 % TRQ.

In other words, any theoretical reduction in propeller efficiency is not worth considering when reducing RPM, at least not in this aircraft, as long as you're not butting against a limitation like in your example. And above ~FL250 you're no longer able to reach 121.4% at 2000 RPM, so reducing RPM should increase TQ for (within reason) the same performance, not the loss of TAS that we currently see.

What is yet to see is if this is something BS can remedy without going completely outside the sim like Majestic did for the Q400, or if the age old issue with Microsoft sims' poor rendition of turboprop engines will make this a permanent thing (and if FS24 will be any better than it's forebears)