Above is our research paper on advanced propeller designs. It is free to download and share. Below is a short description of the events leading to us taking this on as a full-fledged project.
For several years, Paul and I have discussed how to increase the useful speeds of general aviation aircraft without increasing the cost of flying. At first we focused on the usual suspects: interference drag at the wing root, gear fairings, cowling streamlining and of course, auto engine conversions.
All of these modifications only netted small increases in speed and would have to be customized for each specific airframe. As for converting existing aircraft to an automotive engine would require significant amounts of fabrication, not to mention testing of the engine to meet FAA standards and having to design a gearbox that would reduce the RPM to something that a propeller could utilize. These changes put it well outside of the "affordable for the average pilot/operator" range we had set as an idea criteria.
One day while reading random TCDS files (yes, I do that for fun), I happened upon the data for the GE90 turbofan engine. This massive turbofan engine produces nearly 100,000lbs of thrust, close to 90% of which is generated from the fan. However, at takeoff thrust, that fan rotates at only 2465 RPM. This is a rotational speed similar to that of a Cessna Skyhawk or Piper Warrior's propeller, with the obvious difference that this engine can also accelerate an aircraft to over 80% the speed of sound. Discounting inlet effects and the contribution of the turbine section, there had to be a way to utilize the same principles in light planes.
What started as a side project began to consume more time as hundreds of pages of research from NACA wind tunnel tests, UDF/propfan test flight results and transonic airfoil characteristics were pored over. Eventually, X-Plane was used to test basic theories, optimal blade numbers and thrust to horsepower ratios. What was discovered is that propellers for small planes are built with the wrong intent in mind. They were built to focus on horsepower absorbed rather than thrust created. There were too few blades, the airfoil sections were not optimized for high speed and the sweep was not aggressive enough to delay transonic drag rise at the tips. In fact, we found that the top speed of the aircraft was directly tied to the drag divergence Mach number of the tip airfoil. This is why adding horsepower does not result in a proportional increase in speed.
Our research has shown a possible path to increased speed, lower fuel consumption, better rate of climb and quieter operations. Aside from the obvious aerodynamic benefits, engines will run cooler, last longer between overhauls and emit fewer pollutants into the atmosphere. As of now, we are predicting at least a 20% increase in useful cruise speed, a similar reduction in fuel consumption and a 30% increase in rate of climb. These figures vary based on altitude, aircraft type and atmospheric conditions.
Research will continue and our plan is to have high resolution CFD analysis performed on the basic design and possibly wind tunnel test a 1/4 scale prototype by the end of 2017.