Happy New Year to all our Waco and antique aircraft friends!
Let’s open the New Year with some talk about propellers. It’s a subject that’s of interest to all of us since each of our airplanes has one! We all have a rudimentary knowledge of props; low pitch, high pitch, wood, metal, cheap, expensive (although most are in the expensive category!).
Props are a subject that can fill a book, or several books. So let’s begin with just one…the constant speed Hamilton Standard, commonly referred to as the “2B20”. This tells us that the hub is a type 2B that fits an SAE no. 20 shaft (20 spline). This is a good place to start.
Calling the prop a “2B20” only identifies the hub, with no reference to the blades. The blades, in a sense, ARE the prop. That’s what makes the airplane go. So let’s address the blades.
The common blade model number mated with a 2B20 hub is the 6135A. Following this blade model there is always a “dash” number, which causes some confusion.
It would be more correct to call the “dash” a “minus” because it actually tells us the length of the prop below full spec. Full spec of a 6135A prop is 108 inches. Therefore, a -15 (dash 15) is 15 inches less than 108, or 93 inches long. A -9 (dash 9) is 9 inches less than 108, or 99 inches long. A -6 (dash 6) is 6 inches less than 108 or 102 inches long. Pretty basic, huh! Now let’s inject a little history.
The most common WWII aircraft using the 2B20/6135 prop was the venerable Cessna UC-78 Bamboo Bomber. The UC-78 primarily used -15s. There were 5200+ built which means that the production generated about 10,000 props. Some UC-78 wonk will probably e-mail that there were variants with other props, which I acknowledge. The point is, however, that there were a BUNCH of -15s available surplus, and many made their way to our treasured antiques.
Numerous Waco Aircraft Company models left the factory with the 2B20/6135 prop pre-WWII. Several of the cabins specify -6 (102”), others -9 (99”). The exceptions are some Wright powered models that required -13 (95”) because of their need to reach 2400 RPM on takeoff. I’ve examined records of a couple C6s and they both left the factory with -6s (102”). OK, so what’s the deal about length?
Prop length is one of the single most important items relating to aircraft performance…a BFD to quote our VP. We all know that the prop converts power to thrust. That’s pretty basic. So let’s consider some statistics.
A -15 (93”) has a prop disc area of: 47.17 square feet.
A -9 (99”) has a prop disc area of: 53.46 square feet.
A -6 (102”) has a prop disc area of: 56.75 square feet.
So…
The -9 has 6¼ more sq ft of prop pulling you than a -15.
The -6 has almost 10 sq ft more pulling you than the -15.
I’ve been quoted as saying “A dash 15 doesn’t fly a Waco worth a s---.” Maybe now you can see why.
That’s why we equip all our Jacobs powered Wacos with the -9. It makes the UPF-7 (more correctly a ZPF-7) a dramatic performer! No Jake powered open cockpit Waco can touch it!
A fixed pitch prop on a 275 Jake? Doesn’t make any sense to us. Why have the 275 horsepower and not be able to develop it for takeoff & climb? Especially considering the Jake is all set up for a 2B20/6135 and a governor.
For the Cabin guys…A ZQC-6 came to us several years ago with a -15. It had been operating in that configuration for many years. The first thing we did when we got it home was swap the prop for a -9. The performance change was dramatic. Cruise increased over 10 mph. Takeoff performance improvement was striking as well, and we didn’t need a stopwatch or a tape measure. Not surprising when you consider that the airplane was designed to fly with a -6.
Now, I’ll be the first to admit that finding a 102” 2B20/6135 is nigh unto impossible these days. The 99 inchers are out there, though. Yes they are expensive, but what good prop isn’t?
Hope I haven’t worn you out. I don’t have a reputation for brevity.
Cheers,
Roy
