What’s the deal with that prop?
What’s the deal with that prop?
What´s the deal with that prop?
You may remember when it took me forever to fly home I figured out the propeller was set to a climb setting, not a cruise setting. Most props are fixed pitch or constant speed. Either the blades are fixed and that’s what you get or you can adjust the pitch via a lever in the cockpit; climb setting for takeoff and cruise for, well, cruise.
There’s a third prop out there, not as common, and that’s the ground adjustable propeller. You can adjust it to a climb or cruise setting but only on the ground and once it is set you have to put hands and tools on it to change it again. Very simple really, instead of a lever in the cockpit my propeller control is two big wrenches, a torque wrench, machinist’s square, digital level, and some pliers. Also a rubber mallet and some time, call it an hour.
This particular hunk of steel was crafted by the folks at McCauley for a couple different engines during the war, mostly found on Stearmans. This prop had slipped in its clamps to a climb setting. This meant I could get off the ground super quick but once in the air my speed was very limited. I was about 10-15mph slower than I could be. I had to measure the pitch of the blades and change it to a cruise setting; for the McCauley they go by the pitch at the 42-inch station, that is 42 inches from the center of the hub. Normally done on the bench at the prop shop you can do this on the aircraft. The prop has to start out perfectly horizontal, hence the digital level. You spend more time measuring your angles than you do anything else with this job. Measure, bump the blades toward level, nope, tenth of a degree too far, bump back. Once level take a reading off the one giant nut that holds the prop to the engine. You compare that angle to the angle you measure on the blade at that 42-inch spot and subtract one from the other. Boom, not-quite-instant blade angle measurement.
That’s only one of the blades though. You have to swing the prop around, go through your leveling process again, find that 42-inch station again, and measure the other blade, then do your math, again. They should be the same, or within a tenth of a degree of each other. If not you probably have been complaining of a vibration for a long time because if they aren’t the same it can rattle your fillings loose. If you need to make an adjustment you take your giant wrenches, loosen of the blade clamps, whack the blade with your rubber mallet with enough precision to effect a couple tenths of a degree change, then tighten your clamps, then do everything I said above again…twice. There is nothing quick about this process. Typically you have the prop shop set the pitch for you when it comes out of inspection then never touch it again. Unless, like in my case, it has slipped and you want it right and you’re a little crazy.
McCauley Manual Cover
To me it’s part of learning about this plane. I enjoy messing around with this stuff and since it was set it hasn’t been touched. The plane is a solid 10mph faster and if there was a hit to the takeoff performance I can’t tell. So why mess with this prop that takes hours to adjust and has an inspection due every 100 hours and has a restriction between 1500 and 1650 rpm, lest you set up a harmonic vibration and toss the blades off the plane? Because it’s probably the best prop for the plane. It is finicky to get dialed in correct and running smooth but if you do it is superb. The wood prop runs super smooth, but wood flexes and you lose a lot of climb performance. The Hamilton Standard prop is a constant speed prop but the prop and governor add over 30lbs of weight. Plus I don’t have a prop control in the cockpit so I’d have to add one, no thanks. MT makes a composite prop for the plane but the cost is mind blowing.
I’d like to find a wood prop some day, just to see what it’s like. The mounting nut is the same so it’s a quick swap between the two propellers. I also have a second McCauley prop as a spare, because why have one when you can have two at twice the price? Plus this limits my down time when inspection comes around every 100 hours, I can just swap them out and the running prop gets inspected and becomes the spare. Another learning experience brought to you by Stearman. When I bought the plane I knew nothing about this prop except it was big and metal and made the airplane go. Now I’m an expert. I love this stuff.
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