qprop.c

🌐 Run in your browser   •   ⇩ Download   •   ✅ Validation cases

qprop.c is a simple, lightweight library for analyzing the aerodynamic performance of propellers. It uses the same mathematical formulation as Mark Drela's QPROP, which makes it well-suited for slender rotors that operate at low Reynolds numbers and do not have strong tip vortices.

QPROP belongs to the family of Blade Element Methods (BEM). Unlike traditional BEMT methods, which incorporate actuator disk theory, QPROP utilizes vortex theory instead. This approach offers several numerical advantages, such as avoiding the infamous singularity at $\phi=0$, and provides more accurate results.

💡 Motivation

This project was created with the goal of building a minimal tool to quickly estimate thrust and torque of drone propellers and that could be easily integrated in conceptual design workflows and optimization algorithms.

The original QPROP is a great tool, but it is not designed to be used in this way. It is a standalone program that requires some setup and manual effort to use. In contrast, qprop.c is a plain C implementation in one single file with no dependencies, making it easy to include in other projects.

✨ Key Features

• Lightweight and portable: One file in pure C, no dependencies, no installations
• Easy to use: simply copy the library into your own project directory
• No file I/O required: perform analyses and retrieve results without writing input files or reading output files
• Accurate airfoil polars: unlike the original QPROP, which requires users to tune oversimplified analytic models, qprop.c gets the aerodynamic coefficients of the airfoils by interpolating XFOIL polars
• Python and Julia support: Bindings are included for both languages, maintaining native C performance

🚀 Getting Started

To begin using qprop.c, you have two options depending on your needs:

🌐 Web Interface (Quick Analysis)

For a fast, no-setup analysis, use the interactive web GUI:
Try it in your browser

This is ideal for quick evaluations without installing any software.

💾 Local Integration (For Development & Workflows)

To integrate qprop.c into your projects, download the latest release from the GitHub Releases page. The package is completely self-contained and requires no external dependencies.

After unpacking, you will find a set of example scripts to help you get started:

📚 qprop-portable-latest.zip/
├── 📂 qprop-portable/
│   ├── 📄 LICENSE
│   ├── 📄 qprop.h
│   ├── 📄 qprop.jl
│   ├── 📄 qprop.py
│   ├── ⚙️ qprop-lib-linux-x64.so
│   ├── ⚙️ qprop-lib-macos-arm64.dylib
│   └── ⚙️ qprop-lib-windows-x64.dll
├── 📄 example_julia.jl
├── 📄 example_matlab.m
├── 📄 example_python.py
├── 📄 LICENSE
└── 📄 README.txt

For a quick start, consider the following Python snippet showing how to read airfoil data, load a propeller model and compute thrust and torque:

import math
import os
import sys
sys.path.insert("path/to/qprop-portable/")
import qprop

#read airfoil polars from a folder named "airfoil_polar_naca4412_Ncrit=6"
polar_filenames = sorted([
    os.path.join("path", "to", "airfoil_polar_naca4412_Ncrit=6", f) \
    for f in os.listdir("airfoil_polar_naca4412_Ncrit=6") \
    if f.endswith(".txt")
])
naca4412 = qprop.import_xfoil_polars(polar_filenames)

#import propeller geometry from file
apc10x7sf = qprop.import_rotor_geometry_apc(
    os.path.join("path", "to", "10x7SF-PERF.PE0"),
    naca4412
)

#calculate propeller performance using qprop.c
Uinf = 0.00                 #freestream velocity (m/s)
Ω = 6014*math.pi/30         #rotor speed (rad/s)
results = qprop.qprop(apc42x4, Uinf, Ω)
print("Thrust: ", round(results.T, 5), " N")
print("Torque: ", round(results.Q, 5), " N-m")

You can also explore the validation folder in this repo for further examples.

🛠️ Build from Source

qprop.c is a self-contained, dependency-free C implementation in a single file. Unlike many projects, it does not require a complex build pipeline. To build qprop.c, simply compile the source file using your favourite C compiler. Here's an example using GCC on Linux:

gcc qprop.c -o qprop-lib-linux-x64.so -shared -lm -fPIC -O2 -Wall -Wextra

The Zig compiler (zig cc) is recommended when cross-compilation is needed (see build/build.sh).

📄 License

While the mathematical formulation of qprop.c is the same as the original QPROP, the implementation is written from scratch without looking at the original code.

This allows qprop.c to be released under the MIT License, allowing its use virtually without restrictions.

See the LICENSE file for details.

🤔 Issues

If you encounter any issues while using qprop.c, please open an issue ticket and provide detailed information about the problem you are experiencing.

Any question, feedback or request you may have are also welcome. Contact info: here.

📑 References

[1] M. Drela, "QPROP theory document", MIT Aero & Astro, 2006, Online

[2] APC Propellers, "Propeller Geometry Data", Online

[3] J.B. Brandt et al., "UIUC Propeller Database", Volumes 1-4, University of Illinois at Urbana-Champaign, Department of Aerospace Engineering, Online

[4] S.A. Ning, "A simple solution method for the blade element momentum equations with guaranteed convergence", Wind Energy, Volume 17, Issue 9, Pages 1327-1345, Wiley, 2014, DOI: 10.1002/we.1636

[5] N. Moëllo and J. Liscouët, "Multi-Fidelity Approach for Aerodynamic Optimization of Propeller Blades in VTOL UAVs", More Electric Aircraft Conference, 2024, Toulouse, France