Dear Loyal Customers, January, 2011
Business has been extremely slow the past two years and I have been trying to expand my product line during this time. While developing a new aerobatic prop this year I discovered some interesting things about my prop making process.
I originally got in the prop business 20+ years ago to solve the problem of broken Formula One racing propellers and I believe I have accomplished that. In 1994 I began to offer my lightweight and robust construction methods to the aerobatic community who were looking for ways to save wear-and-tear on their expensive engines without compromising performance. I believe I have accomplished that. In 1999 I made it a goal to build safe fixed pitch racing props that are also as identical blade-to-blade as possible. In 1993 I was asked by Bruce Bohannon to make molds from his favorite wooden prop and to make him a carbon fiber duplicate from the molds. He set three world records with the composite prop but he absolutely hated it! He said it had a terrible vibration and was causing parts to fall off of Pushy Galore even though the prop had perfect static balance. We finally agreed that I had just copied what he gave me and he admitted that the wooden prop had been damaged and cosmetically repaired to take a mold off of it. In 1999 Jim Miller asked me to do the same thing. This time, I carefully analyzed the wooden prop he sent and I found glaring differences between the two prop blades. I started with that prop to develop a unique molding process in which I use a one-bladed master to generate a two-bladed, one-piece, fixed-pitch propeller mold. Jim only flew his new prop a couple of times and was happy with it but it was destroyed in his landing accident at Reno that year.
I started to measure every prop I could and found that there are often large and easily measurable differences between blades on props of any given type- wood or metal- but the wooden ones seem to have the biggest differences. I reasoned that these differences are not noticeable in the highly damped wood props but become very noticeable in props made from stiffer aluminum and carbon fiber. I believe this was the underlying problem in the prop I made for Bruce.
If you have ever seen a prop shop re-pitch an aluminum blade you are familiar with the gruesome process. If you haven’t, just imagine a tall, heavy steel bench bolted to the floor (or a 50 ton shop press) and two 6 foot 5 inch tall, 300 pound guys named Bubba wielding 10 foot lever bars with airfoil shaped cut outs in them. They use all their weight and strength to bend the blades to the new pitch at a few stations then spend an hour grinding out all the wooies in the trailing edges and re-polishing the blade. The typical general aviation prop blade has a flat-bottomed airfoil which accommodates a standard prop protractor to measure the pitch but our high performance props have curved bottom airfoils requiring the use of factory templates (yeah, right) or tricky but suspicious work-arounds to allow the use of a standard prop protractor like layers of masking tape on the leading and trailing edges of the prop to space the protractor below the “hump” in the airfoil. This all adds up to less than identical blades and uncertainties in pitch measurement in my experience.
By this time I had a successful product line made from two-bladed masters. When I made the first composite props, I found that the solid composite hub area was subject to warpage during cure. A process was developed by which the prop was mounted on an adjustable fixture on a milling machine. The position of the prop was adjusted such that the tips had the proper tracking and the same angle of attack which could be measured without a traditional prop protractor. Then, the mounting surface of the hub was trued up at this angle and the holes were drilled and counterbored. I made props this way up until 2004. That year, my good customer Scotty Crandlemire returned a 5466 race prop that his prop shop had measured and claimed was way off in pitch from one blade to the other. I was a bit upset at this- imagine the nerve of a customer actually scrutinizing my work! But, when I got the prop I measured it and indeed the blades had embarrassingly different pitches! I ended up laminating some material on the hub and installing it on the adjustable milling fixture. This time I measured 5 stations along each blade and adjusted the prop until the angles on each blade were as close to the same as possible and re-trued the hub. Since then, I have machined all new props using the 5-station method and I immediately scrapped the 5466 molds. The next summer I made a new 5468 prop for one of my original customers, Gary Hubler. He had a 5468 I made in 1999 that he was apparently unhappy with but he had never said anything to me about it. I machined the new prop using the 5-station method and he loved it. I also adjusted my process regarding post cure temperatures.
I have continued to refine the Half-Master™ process with a new 5465 mold in 2001, new 5466 and 5467 molds in 2007 and the new 5468HP molds in 2008 and I am generally pleased with the resulting props which have two blades of nearly identical chord, thickness, and pitch distribution and are also well statically balanced right out of the molds. The average static balance of the 5465 props went from about 9 grams to about 3 grams using the new molds.
During 2010 I worked very closely with one of my aerobatic customers to develop a new high pitch acro prop for 200+ HP monoplanes. He did lot of testing with one of my standard pitch acro props and a standard pitch Sensenich metal prop and a high pitch Sensenich wood prop. Based on these tests we settled on a new prop design with a pitch of 68 inches. This would also be the first of my acro props to use the Half-Master™ process. I used every trick and refinement I had learned and even added a couple of new features to make this the best set of molds I had ever produced. I was very busy all summer making the master and the new molds. I updated my customer by emailing him pictures on a daily basis or at least as each major step was completed. The prototype prop was beautiful and perfectly statically balanced right out of the molds. When I mounted the prop on the milling fixture to true up the hub I decided to compare the actual angles to the desired ones. I was shocked to discover that the prop had only 65 inches of pitch! All that work and expense to develop a high pitch acro prop and all we had was a moderate increase in pitch.
This started me on a frenzy of data analysis. I had been measuring and recording the ANGLES of all props since 2004 but I had never converted them to PITCH or done any other analysis or comparison of the data. Pitch measurement is difficult because one inch of pitch change is only about three tenths of a degree! Combine that with the fact that the high performance props do not have flat-bottomed airfoils and it adds up to uncertainties in pitch calculations. I invented a special pitch measuring tool which attempts to pick up the chord line of the airfoil and, using a high quality digital protractor, I have tried to be at least CONSISTENT in all my measurements. The pics below show my setup. I also use a laser to align the tips in the tracking direction and have started putting permanent, accurate station marks in the molds to be sure measurements are made in the same location from prop to prop and from blade to blade.
So, even taking all this care and using the best methods available the fact remains that these are plastic props which are subject to the forces of resin shrinkage and temperature. The molds are also plastic and subject to the same phenomena. My props have a complex construction, (not homogeneous like metal) a complex shape, and have unpredictable reactions to these forces. I have also replaced the heat producing lamps on my milling machine with with fluorescent ones for illumination.
I have never claimed to build anything but “bullet-proof” props not the “fastest” props and I believe this is still a legitimate and valuable service. The fact that my props have been used on over 20 record setting flights just shows that the aircraft, engine, and pilot can put everything they have into the flight and the prop will take it.
All this has caused me to RENAME some of my molds based on a study of the ACTUAL pitch of the props produced in them (as measured by me using my method). While this is a difficult decision, I think it is best to name the props in a way that reflects their true pitch. There is a cross reference table below for your convenience.
To attempt to fully characterize the issue and gather as much data as possible, I would offer to anyone with one of my props that was built prior to 2004 (i.e., not in the table below) a free measurement of the pitch distribution along with re-labeling if desired. You pay for the shipping to me and I will pay for the return.
While this seems like an unfortunate event I choose to view it as a continued refinement of the process and another step towards the goal of building the highest performance, most accurate fixed pitch racing and aerobatic propellers in the world. I hope you, too, can see it that way and choose to continue on the journey with me.