Over the past years whilst building my modified Cozy, I have listened to much discussion about drag minimization and reducing weight but I have heard little about propellers. I started looking for propellers a few years ago to see what was out there. I contacted many of the manufacturers and was dismayed at the lack of understanding that existed. Several of the manufacturers just wanted to know how fast was the aircraft and how much power was there to turn the propeller. I explained that I was using an automotive engine and that the power peaked at a different RPM to that of the Certified Lycoming or Continental. This seemed to make little or no difference and a pitch and diameter was quoted, by the manufacturer, with no reference to the helix, blade planform or any other parameter. On delving deeper, many of the manufacturers were unable to support the figures that they gave and just tried to assure me that they would ensure that I would be satisfied. I rapidly came to the conclusion that a high proportion of the manufacturers were actually fine craftsmen who built propellers and relied on customer feedback to arrive at the optimum solution. With the LyContinentals so well understood, most of the manufactures were probably close enough for most of the slower homebuilts that filled the skies back in the pre 90's era. With the super speedsters of today the situation is becoming different but the designs are still largely anecdotal with a few exceptions. These exceptions are expensive, at least for my wallet, and understandably still seem to align with the proven LyContinentals. With my automotive power plant providing more than just some expensive ballast I needed to dig deeper into the understanding of how a propeller produces thrust and how to optimize this thrust. With the automotive configuration and a reduction drive the rotational speed with which the propeller turns is no longer directly tied to the peak power of the engine. The engine must now be matched to the propeller through a Propeller Speed Reduction Unit (PSRU) with some, yet to be determined, ratio to provide the optimum configuration.
During my search for the optimal propeller design, Nigel Field had done some excellent work in creating a construction technique that brings the manufacture of a glass/wood composite propeller within the grasp of the home builder. In addition to Nigel's good work Don Bates had developed a very useable software program which seemed to understand the issues that I felt lacking when talking with the propeller manufacturers. With Nigel's technology the builder is no longer faced with the horrendous costs associated with the procurement of a propeller, and with Don Bates' program, the builder is now in a position to optimise his propeller to the ship he/she flies. These web pages are written to provide an account of my thought process in this arena and are not necessarily proven nor accurate. Constructive critique is always welcome and may be incorporated within these pages at a later date.
During my literature search for the propeller design methods I found numerous myths. These myths although sounding very solid seemed to fall apart when considered bore thoroughly.
I found continual reference to the three bladed propeller having a better climb performance but worse cruise performance. We've all heard that claim, and under most conditions that is born out in fact except that the whole story is not given. Typically the comparison is made for a metal prop where the manufacturer is forced into using a smaller diameter propeller when transitioning to a three blade design. This is necessary because the blade area increases by 50%, when adding the third blade, thus loading the engine more heavily if the same diameter is utilised.
So what happens when the diameter is reduced? Apart from the efficiency being reduced, the power bandwidth is also reduced because the tip velocity is lower. Lower tip velocities yield lower power bandwidth.
Since the true comparison is when
Smaller propellers are less efficient that larger propellers. Although this is not exactly a myth it behooves the builder to determine the change in efficiency as a function of diameter. The impact of diameter on efficiency, for high speed aircraft is not as significant as the rumors would suggest and in many cases the choice of a smaller propeller allows a greater engine speed which means more power. The rate of growth in power as a function of engine speed is far greater than the loss in efficiency resulting from the diameter change. For this reason the small racing aeroplanes use relatively small diameter props even though you would have expected the designers to strive for maximum efficiency.
The figure to the right indicates the induced velocity through the propeller from which the momentum efficiency may be calculated. The four curves are for a 230 hp engine and four propeller diameters ranging from 40 inches to 70 inches in diameter. The second (lower graph) is momentum efficiency for the same propeller set. Notice that at maximum speed the propeller efficiency is almost independent of diameter, however during the climb the efficiency dose vary significantly but these curves are for a very large range in propeller diameter.
Thursday August 31, 2006