Reynolds number correction for propellers + Script for converting APC PER3 files

[Scavanger]

Adds the missing Reynolds numbers correction in FGPropeller. This applies to small UAV/RC propellers with low Reynolds numbers and should result in a 10–15% improvement.

In the propeller XML, two additional tables, CT_RPM_FACTOR and CP_RPM_FACTOR, have been added, these are normalised to 1.0 at the RPM for which the baseline C_THRUST/C_POWER data were measured.

I have also added a script that converts APC PER3 data files into clean propeller XML files and calculates the corrected Reynolds numbers and an example XML for a APC 18x8E Propeller.

Unfortunately, XML files for small propeller XML files generated by Aero-matic are incorrect, as Aero-matic is not designed for such small propellers and calculates a shaft power that is far too high.

#1466

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.09%. Comparing base (3e1a5cf) to head (794aecc).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/models/propulsion/FGPropeller.cpp	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1470      +/-   ##
==========================================
- Coverage   25.10%   25.09%   -0.02%     
==========================================
  Files         171      171              
  Lines       18843    18851       +8     
==========================================
  Hits         4731     4731              
- Misses      14112    14120       +8     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[bcoconni]

Thanks for your contribution.

It looks good to me.

Using RPMs to get the Reynolds number correction makes perfect sense because it is assumed that:

• The air viscosity is constant which I think is a reasonable assumption for UAVs/MAVs which are designed to fly at low altitudes.
• The propeller blade velocity is mainly driven by its circumferential velocity due to the vehicle velocity being negligible in respect.

These 2 assumptions stand because we are dealing with UAV/MAV with small propellers and because the Reynolds number correction only applies to (very) small propellers. So I am wondering if this should be documented ?

In particular, tables CT_RPM_FACTOR and CP_RPM_FACTOR would need to be tuned if one propeller would be used to model Ingenuity flying in the Mars atmosphere and for another UAV on Earth. That is now a possibility following the PR #1462 😉