Aeromatic Prop – and a point

Builders,

Well known, 800 hour Corvair/Pietenpol pilot Bob Lester and his girlfriend Dusty flew across the top of Florida on Saturday, from their grass airstrip to ours. The took Bob’s classic Stinson 108, which he has owned about 15 years. The Piet was not used for the trip because he brought over its leaky 40 year old fiberglass wing tank from it in the Stinson, so vern and I can use it as a template to make the Piet an aluminum one.

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Bob recently spent $6,000 to buy the prop which is pictured below. If you don’t know what an Aeromatic Prop is, google the wikipedia page and read up. I know a lot about props, and these are arguably the most elegant and beautifully simple automatic props off all time. Open the throttle wide, and it will go to the full redline rpm of the engine, pull it back for cruise and it automatically drops rpm and increases the pitch for efficient cruise. The great part of all this: it accomplishes all this with no controls, no governor, no oil, no electricity. It works very elegantly, simply on the balance of forces.

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Above, the business end of the 108; the engine is a 165HP Franklin, which is the closest certified aircraft engine to a Corvair that you will ever see. The slot on the spinner is for the counterweight on the prop blade to swing when the blade changes pitch. On the ground, you can walk up to this prop and change the pitch with your fingertips. The hub is alloy, but the blades are wood. They are a 1940’s design. They work very well on a certain speed range of planes. They were largely superseded by constant speeds, because they can cover a wider speed range and can feather, but these features of constant speeds came this greatly increased cost, weight, pilot workload, maintenance and complexity.

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Above, the sticker on the prop. If you look at the logo it shows an exaggerated feature of the aromatic design: the blades do not line up with each other in the hub, nor is their axis of pitch rotation perpendicular to the crank.  Without these geometric features, the prop could not be made to work automatically. Understanding how this geometry could be harnessed in proportion to the pitch centrifugal and aerodynamic forces is the brilliant part of this very unique design. Notice you can see my reflection in the prop finish. This was not an overhauled prop, it was new.

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The point:  Bob bout the prop, which cost about 25% of the plane, because it provides a critical safety margin over the fixed pitch prop when operating out of short fields on hot Florida days. $6,000 is a lot, but it isn’t much to make a dramatic performance improvement, and thus a safety improvement in the plane. It could now lose a cylinder on climb out and not have a forced landing. to better understand the concept, look at this story: Critical Understanding #5, Knowing “+ROC/5” Rate of Climb on Five cylinders.

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Likewise the gas tank we are making for Bob’s Pietenpol isn’t going to be ‘cheap’ either, unless you factor in what happens to the passenger if the dated, softened polyester fiberglass tank pours fuel on them. Bob has been around planes for a number of decades, his dad was a B-25 pilot in WWII, he has a family history of knowing that gambling and aviation don’t go together when there are improvements available which keep all of your operations within a reasonable aviators assessment of acceptable risk.

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