For PROP_DESIGN support, please utilize the:
I put a lot of time and effort into the aforementioned material, so that you can answer all of your PROP_DESIGN related questions on your own. The material also covers topics that PROP_DESIGN users may find relevant. The Fortran 77 source code files have always been the primary reference documents. The source code is responsible for the actual calculations. All other support information merely helps to understand what the source code does or how to work with it's output.
PROP_DESIGN contains many examples, to help get users started. The included examples relate to the following products:
These products were chosen as PROP_DESIGN examples, because they cover the typical operating range of aircraft propellers. These products were designed, built, and tested prior to the existence of PROP_DESIGN. The PROP_DESIGN example input files are based on information found on the internet. Thus, the PROP_DESIGN example designs may or may not be similar to the actual product designs.
Three additional examples are provided in the reference information folder and described below. The modern turbofan example was done out of curiosity and proved to be very enlightening. I believe all PROP_DESIGN users will find this example to be very interesting. The VTOL and Hovercraft examples are intended to show how PROP_DESIGN integrates into vehicle design projects.
I used PROP_DESIGN to study modern turbofans. This stemmed from the newly created flying wing concept. I wanted to see what sort of ducted fans I could create using PROP_DESIGN. Below are pictures of an optimized ducted fan design that could be used on typical transport aircraft or the flying wing concept I posted recently. It was modeled in Rhino and rendered in Keyshot. It has a propeller efficiency of 91.4% at cruise conditions of Mach .7 32,000 feet. It makes 26,843 lbf of thrust at takeoff. The cruise thrust is 3,514 lbf. Using four engines, the aircraft could have 14,056 lbf of drag at the cruise condition. Power at cruise for all four engines would be 19,298 shp. Power at takeoff for all four engines would be 118,543 shp. Fan efficiency at takeoff is 67.5%. Mass flow rate at takeoff is 2,107 lbm/s for one engine. Mass flow rate at cruise is 204 lbm/s for one engine. There are 22 straight blades with a diameter of 111 inches. As always, NACA 65A009 airfoils are used, due to their excellent transonic performance. To maximize thrust the radius/chord ratio is 2, rather than the typical value of 10. This yields about 5x the thrust while maintaining the same efficiency. The use of a duct adds about 20% thrust at takeoff with a 10% boost in fan efficiency at takeoff. The strut, hub, and duct geometry were shaped to reduce drag as much as possible. There is a lot more information contained in the download, I don't want to duplicate information, and spoil the conclusions.
Below are some animations of propeller wakes. The black and yellow wake shows a counter-rotating configuration. It is meant to show the vibratory nature of the wake interaction and exemplify why counter-rotating propellers will never work as intended. This is discussed in the example documentation. The other wake animation shows how the propeller wake interacts with the stators. It is meant to show that you need a lot of properly designed stator blades, with ducting, to capture the propeller wake and extract the rotational power.
Below is an example of a VTOL aircraft. The ducted fans are shown in light blue. The thrust vectoring nozzles are shown in white. The parts in black are where the cargo and/or passengers would go.
VTOL aircraft will be more efficient than helicopters and quadcopters. They will also be able to travel at much higher speeds. This is due to the air flow into the blades. An axial flow orientation is ideal. However, VTOL aircraft are much less efficient than traditional aircraft. All VTOL have a lift-to-thrust ratio of one. Wings made with NACA 65A009 airfoils have a lift-to-thrust (i.e. L/D) ratio of 27.5. So VTOL aircraft should only be used when absolutely necessary.
Below is an application of a diffuser. Moreover, a diffuser was used to create static pressure for the plenum chamber of a hovercraft. The diffuser is shown in gray. The ducted fans are shown in light blue. The plenum chamber is shown in white. The parts in black are where the cargo and/or passengers would go.
The VTOL and hovercraft examples use the same fans. The hovercraft example uses half the fans of the VTOL example (3 out of 6 fans). Thus, the hovercraft example uses half the power of the VTOL example. The hovercraft example can lift 4.33x the weight of the VTOL example, while only using 25% of the thrust (1 out of 4 fans). This comes with one major limitation, hovercrafts can only float inches above the ground.
The hovercraft example has a lift-to-thrust ratio of 17.3. Traditional aircraft are more efficient than hovercraft. Wings made with NACA 65A009 airfoils have a lift-to-thrust (i.e. L/D) ratio of 27.5. Traditional aircraft are about as efficient as a typical car or truck. Cars and trucks have a lift-to-thrust ratio of 33.3.
I recommend the pusher configuration, placing the propeller behind the fuselage or wing, for traditional aircraft (those that create lift using a wing). In lieu of tiltrotors, helicopters, and/or quadcopters; I recommend thrust vectoring nozzles. Thrust vectoring nozzles create lift by redirecting the flow from a ducted propeller (a.k.a. axial flow fan), they can be fixed or adjustable. To create static pressure, I recommend placing a diffuser behind a ducted propeller. The use of a ducted fan in combination with a diffuser is a replacement for an axial flow compressor. An axial flow compressor utilizes a rotor and stator to increase static pressure (the combination of a rotor and stator is called a stage). Velocity is constant through an axial flow compressor, this requires area to decrease. The diffuser creates static pressure by decreasing velocity and increasing area. The combination of a ducted fan and diffuser has many benefits over an axial flow compressor; higher thrust-to-weight ratio, higher efficiency, less cost to manufacture, cheaper to maintain. However, since axial flow compressors typically utilize multiple stages, the combination of a ducted fan and diffuser will not create as high of a pressure ratio as a typical axial flow compressor. Axial flow compressors require tight tip clearances. Ducted fans do not require tight tip clearances, regardless if they are used by themselves, with diffusers, or with thrust vectoring nozzles. Below are pictures of these configurations:
Sweeping the airfoils is recommended over sweeping the blade. You should only use sweep when the vehicle velocity is greater than Mach .5. You can not tell visually if the airfoils are swept or not. Many propeller manufacturers create the illusion of sweep by manipulating the chord distribution (one such example is scimitar blades). Many propeller manufactures also tout the virtues of sweep for low speed aircraft, don't let them fool you.
Below are pictures showing the same blade with two different sweep methods. They both perform the same aerodynamically. However, the swept blade configuration may cause structural problems. Therefore, using the swept airfoil method is always advised.