>Some people can set their engines
>"by ear". Your ear
>is a tach, too, you
>know. (The faster the
>engine runs, the higher the
>pitch). Most people's sense
>of pitch is not that
>accurate, plus it is nice
>to know exactly where to
>launch a particular plane with
>a particular prop. Just
>earlier this week we were
>out flying and using the
>13 inch three blade that
>worked so well last year.
> We then switched to
>a 14 inch three blade
>that Matthew had reworked and
>the launch rpm had to
>drop 1000 rpm in order
>to maintain the same lap
>speeds. That meant the
>pitch was probably slightly high,
>so Matthew dropped the pitch
>slightly on each blade.
That *might* be true. It's much more likely that the extra load on the prop on the ground resulted in more unload, meaning it has to be set slower on the ground to get the right in-flight revs. Of course, reducing the pitch reduces the unloading too.
>The problem is, the Prather
>Pitch gauge doesn't measure past
>about 11.5 inches, so after
>that you are kind of
>on your own.
Really? Mine goes out to about 13", if you hang the slider off the edge.
Still,
>having the tach to know
>what rpm is needed to
>run a certain prop combination
>certainly helps. Even with
>the 35, there was a
>major difference between Zingers and
>By&Os and Rev-Ups and Bollys
>and...
It's even more complicated than brand-to-brand variation of the measured pitch. The airfoil shape has a huge effect on both the ability to measure, and the line of zero lift. Most Bollys, for example, have a semi-symmetrical airfoil that tends to end up being measured at less than it really is just by virtue of the shape causing you to measure lower than the chord line. On others, like the Rev-Up and Brian Eather props, you are more or less measuring the chord line, but even then, the fundamental property you need to make analytical sense out of the effects is the line of zero lift, and that is drastically different between the Brian Eather, Rev-up, and Bolly, even if you measure the chord line perfectly. By drastically different, to get a true pitch of 3.5" at the tip of a 12" Brian Eather u/c prop, the chord line has to be at a negative angle! Now we all know from experience that actually using this sort of angle will result in terrible performance (like you deployed a parachute in every corner), which merely means that we have determined through trial and error that the best pitch distribution is strongly progressive.
I've been doing a lot of research in this area. If anyone has good examples of different competitive props (Bolly, Brian Eather, APC, Bill Lee, or old-style Y&O, BY&O, and Brodak BY&0) that are broken but still have one intact blade, please contact me and I'll see of it would fill in any gaps in the record. I cut the prop into sections, then scan it in using a scanner, then blow it up in order to determine the camber, high point, and thus zero-lift-line position. This was suggested by the illustrious Joe Supercool, on his web site, and it's a real eye-opener, and makes a lot of sense out of some otherwise very inconsistent results.
