Aerodynamic Effects on Propeller Efficiency in DCS Code

[Kortana]

I am curious if things like compressibility, dynamic pressure, slip ratios, and AoA are modeled for propellers in DCS, and to what extent if they are. Testing in the sim seems to suggest that this might be the case, as slightly higher speeds are achievable with slightly lower than max RPM (tested in the P-51D-25-NA at sea level using WEP at 3000RPM and 2700RPM), indicating that a slightly coarser pitch is resulting in improved efficiency. 

 

Edited July 7, 2021 by Kortana
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[M1Combat]

Yeah us racers have been trying all kinds of things with the TF-51 for a while...  I have a complete theory as to why we would get more speed at 2700...  but most people just say "Nah the .gov says 3K is fastest so it IS..."  

 

So yeah...  

 

I personally think it could be right...  but I'm not certain.

 

Keep in mind though...  the lower you go below 3K the longer you're exerting combustion pressure on the con-rods and the less long your engine will last.  I always ran 2830 for endurance races, 2750 for sprint races.  I would reduce to 2700 slowly around the last 1/2 of the last lap if it was going to be close.

 

Also you're a little too close to the ground for low alt max speed.

[Kortana]

Yeah, I initially started playing with this in the TF-51 specifically for an air race and came up with very similar numbers to what you posted. In terms of the altitude, yeah I was just being lazy and wanted to make sure I had a consistent altitude for the test since I was looking for a comparison between the two RPM settings rather than a true determination of max speed.

[M1Combat]

Understood :)...  Just trying to put everything out there.

 

I've also found that closing the coolant rad, then opening for a proper 4 second hold to be good...  but that's on a dive into a racetrack at top speed.  You may need a 5-6 second hold to keep it cool enough at ~330.  This will also make the aero more consistent for your test if you weren't already doing it.  It looks to me like maybe you were just closing it??  based on your coolant temps being pretty high.

 

Oil rad doesn't affect enough to matter...  Kinda...  I always started closing that one in tight races too.  Just in case it did something...  but never really found anything in testing it.

 

Anyhow...  I think the auto prop pitch will increase the prop pitch to keep resistance on the manifold pressure at reduced RPM...  so it ends up finding more thrust at lower RPM because there's still enough HP to max the MP at higher blade pitch.  Basically...  If I'm remembering my thoughts correctly.

I wrote a post about it a long time ago...  2015 maybe??

[Kortana]

Cool, that more or less lines up with my thoughts on what is going on. I did notice that I had more or less identical manifold pressures for both tests. Feel free to drop a link to your original post if you have one handy

 

I am still curious to know exactly how ED has this coded. Propeller aerodynamics can quickly get too complicated to simulate in a timely manner as advance speed and RPM increase. So either this effect is an artifact of the coding or they are applying some form of heuristic to allow for the sim to account for this without sacrificing too much performance. 

[M1Combat]

Nah for sure I'd LOVE to know all the details as well...  For sure.

 

I do know that this specific issue has been brought to the attention of the person responsible...  And they didn't say "We'll look into it to see if it's correct"...  and that particular person would have if they thought it wasn't correct.

 

As far as how the performance arrives where it does...  whether physics based or canned...  I'm not sure...  but I would like to be sure

 

[Kortana]

Well, if you happen to hear any specifics from those responsible, feel free to drop them here. I would like to be sure as well