Formulation:

PROP_DESIGN does not rely upon momentum theory, Theodorsen's theory of propellers, or the Betz condition. PROP_DESIGN has no light loading limitations either. Therefore, PROP_DESIGN can easily calculate the static condition. PROP_DESIGN utilizes a novel implementation of lifting-line theory (a.k.a. vortex theory). The formulation is purely analytical with a few exceptions. Empirical data was used to create the airfoil, atmospheric, and stall models. The empirical data, utilized by PROP_DESIGN, is freely available online. Reputable sources were referenced:

  • Airfoil Model; NACA (Predecessor of NASA)
  • Atmospheric Model; United States Committee on Extension to the Standard Atmosphere (COESA)
  • Stall Model; Sandia National Laboratories

You can use your own airfoil and stall models, if desired. You will have to edit and re-compile the Fortran 77 source code, in order to do so, however. Editing the airfoil and stall models is not too difficult, as the source code is very clean. The portions which define the airfoil and stall models only consist of a few lines of code.

Using your own airfoil and stall models will require wind tunnel testing and/or 2D CFD. You need to know Cl, Cd, and Cm (i.e. lift, drag, and pitching moment coefficients) for angles of attack of 0 to 90 degrees. These coefficients must be calculated for Mach numbers of .1 to 1.3 and a wide range of Reynolds numbers. Traditional airfoil analysis programs are not capable of accurately determining this information.

PROP_DESIGN executes quickly, and requires little computational resources, because it does not attempt to solve the Euler, Navier-Stokes, or Boltzmann equations. Rather, the Biot-Savart law and the Kutta-Joukowski theorem are utilized to iterate upon induced angle of attack and circulation. As of May 21, 2013, PROP_DESIGN includes the affect of wake contraction. The affect is very small but included for completeness.

PROP_DESIGN is based on a code called PROPSY, described in the following publications:

  • 'The numerical determination of circulation for a swept propeller', 1996, by Markus Tremmel
  • 'Numerical Determination of Circulation for a Swept Propeller', 2001, by M. Tremmel, D. B. Taulbee, J. R. Sonnenmeier

I have made many corrections and improvements to PROPSY. In fact, at this point, PROP_DESIGN is almost an entirely different program. Therefore, one should not expect the results of the two codes to be the same. Rather than publish these changes via a traditional outlet, I decided to publish them online. This was done to make it easy, and free, for people to obtain them.

Limitations:

PROP_DESIGN does not calculate aircraft propeller noise levels. This is due to the fact that an efficient aircraft propeller is also a quiet aircraft propeller. Since you normally strive for an efficient design, there is no additional effort required to achieve a quiet design. To calculate aircraft propeller noise levels, I recommend the procedure described below (the document is based on empirical data collected by Hamilton Standard and also contains information showing what aircraft propeller characteristics increase aircraft propeller noise levels):

  • 'Prediction Procedure for Near-Field and Far-Field Propeller Noise', 1977, SAE AIR1407 1977-05-01

The paper shows that, if you have an efficient design, there is only one variable left that drives noise. This variable is shaft horsepower. Noise is proportional to shaft horsepower. So the higher the shaft horsepower is the more noise the aircraft propeller will make. Again, from an aircraft propeller design point of view, this means that all you need to do is create an efficient design. Shaft horsepower is dictated by the size of the aircraft. The size of the aircraft is dictated by the max. payload. As an example, if you want to fly four people around, that is going to be much quieter than flying four hundred people around. There is nothing you can do about that. This is why it is a waste of time to worry about noise levels. It is more useful to minimize noise by creating an efficient design. PROP_DESIGN_OPT will automatically find the optimum geometry for any operating condition that a solution exists. So PROP_DESIGN makes it very easy to maximize fuel efficiency and minimize noise.

 

Below are the other limitations of PROP_DESIGN:

  • Does not have a GUI
  • Does not export complete CAD software files
  • Computes using only one CPU core
  • Does not include nozzles or diffusers
  • Does not include the affect of back pressure, pitch, or yaw
  • Does not apply to tiltrotors, helicopters, or quadcopters
  • Does not apply to marine propellers or wind turbines

Creating a GUI, exporting complete CAD software files, and computing in parallel are outside my programming abilities. PROP_DESIGN already runs very fast using only one CPU core. So there is little reason to spend the time to re-write or modify the code to run in parallel. It is easy to compute the performance of nozzles and diffusers by hand, so there was no reason to include them in PROP_DESIGN. The reference information shows how to perform these calculations. It is best to avoid situations with back pressure, pitch, or yaw. Therefore, I did not specifically address them. I was only interested in creating the best performing propellers. Part of that involves avoiding poor operating conditions as much as possible. This is why I do not believe in tiltrotors, helicopters, and quadcopters. They operate in some of the worst operating conditions, most of the time. Using thrust vectoring nozzles is a much better approach. Thrust vectoring nozzles will increase efficiency, thrust to weight ratio, and safety. They will also decrease cost. Although PROP_DESIGN could be modified to handle marine propellers and wind turbines, I have no interest in these devices. There are many other codes dedicated to marine propellers and wind turbines, so there is little value in expanding the focus of PROP_DESIGN.

Supporting Software:

PROP_DESIGN is used in combination with several other software programs:

  • *.bat, *.DAT, *.plt, *.txt, *.TXT, and *.XYZ files can be viewed with Microsoft Notepad or other text editors
  • *.f files where created using Force and can be viewed and edited with Force or other Fortran editors. You can even view the files with Microsoft Notepad, although it is much harder to read
  • *.ods and .odt files were created with LibreOffice and can be viewed and edited with LibreOffice or other programs that support these file types
  • *.pdf files can be viewed with Google Chrome or other programs that support this file type
  •  *.xps files can be viewed with Microsoft XPS Viewer or other programs that support this file type
  • *.plt files are for use with gnuplot. When opened in gnuplot, plots are automatically created
  • *.XYZ files can be imported into many CAD programs. They are in units of millimeters and designed to start the geometry creation process. User knowledge and work is needed to generate the blade geometry. The *.XYZ files simply provide a starting point

Aerodynamic Performance:

PROP_DESIGN and gnuplot can be used to create plots which aid the understanding of aircraft propeller performance. Some of the available plot types are shown below. The performance maps were made with PROP_DESIGN_MAPS_CS. This is the version of PROP_DESIGN that lets you evaluate constant speed propellers. It is available in the PROP_DESIGN_UNDOCUMENTED folder. All of the screenshots below are for the included General Atomics Predator B example.

Aerodynamic Loads:

PROP_DESIGN outputs steady state aerodynamic forces and moments along the span (a.k.a. the quarter chord line) of the blade. These are for use with FEA software. All loads act at the airfoil's quarter chord point. As of 11/29/18, PROP_DESIGN now outputs aerodynamic force and moment in Cartesian coordinates. This makes it a lot easier to use them in FEA software. PROP_DESIGN also outputs lift, drag, and pitching moment coefficients along the span of the blade. These coefficients are non-dimensional. They are output for completeness, more than anything.

Hot Shape CAD Software Models:

Below are renderings of hot shape aircraft propeller and hot shape fan CAD software models. The renderings are of four of the included examples; Airbus A400M, Delta Computer Case Fan, General Atomics Predator B, and the Piaggio Avanti II. These renderings should give you an idea of all the different shapes PROP_DESIGN can create.

Four different chord distribution options are available; constant, elliptical aligned along chord / 4, elliptical aligned along chord / 2, and scimitar (elliptical aligned along the trailing edge).

Design Process:

The geometry PROP_DESIGN outputs is termed the hot shape. The hot shape is the geometry required to provide the desired aerodynamic performance. FEA is needed to find the corresponding cold shape. Centrifugal force and aerodynamic loads will deform the cold shape into the hot shape, at a user specified shaft angular velocity. The cold shape is the geometry that should be manufactured.

PROP_DESIGN is unproven. It is up to the user to verify his or her design. To do this, I recommend comparing the results you obtain with PROP_DESIGN against test data that you collect. The typical aircraft propeller design process is as follows:

  • Determine the required hot shape, using PROP_DESIGN (or equivalent)
  • Create a hot shape CAD (computer aided design) software model, using output from PROP_DESIGN (or equivalent)
  • Perform additional aerodynamic analysis, on the hot shape CAD software model, using CFD (computational fluid dynamics) software
  • Perform noise analysis, on the hot shape CAD software model, using CAA (computational aeroacoustics) software
  • Use FEA (finite element analysis) software to create a cold shape CAD software model that yields the desired hot shape under steady state loads
  • Ensure that the cold shape CAD software model can withstand the steady and vibrational loads acting upon it, using FEA software
  • Ensure that the cold shape CAD software model is not excited by flutter, using FSI (fluid structural interaction) software
  • Use the cold shape CAD software model to manufacture prototype(s)
  • Test prototype(s) to make sure they perform as desired
  • Ensure that the final design meets all applicable regulations (ASTM, EASA, FAA, etc...)

This is an iterative procedure that is usually conducted by a number of qualified engineers. Many of these steps are performed in parallel, in order to save time and money. Many people manufacture the hot shape, this may cause you to miss your performance target. As you can imagine, this process is very complex, time consuming, and expensive. Many people have been killed due to mistakes in this process.