A bitch of a pitch

It was the best Ercoupe takeoff since the JATO tests of 1941. That’s when the National Academy of Sciences strapped rocket pods under the wings of an Ercoupe and lit the fuses in a series of successful tests that led to the military use of rockets to help heavy planes get off of short runways—and to the founding of the Jet Propulsion Laboratory.

First_JATO_assisted_Flight_-_GPN-2000-001538

But there were no rocket pods on Tessie’s sturdy metal wings, and we were rising off the ground at nearly 1,000 feet per minute. Granted, in other airplanes that’s nothing to write home about. But in my beloved, but heavy and underpowered ‘Coupe, the only time I’d seen a number like that was when a thunderstorm tried to suck me up into its jaws when I foolishly tried to slip under it.

So I was ecstatic about the climb rate. The new stroker engine was really showing its stuff, lifting Tess off the runway like never before.

But, as always with airplanes, there was a problem. And the problem was my propeller. Oh gosh, where to start… Where to start? OK, the angle at which a propeller cuts the air is called its “pitch.” A flatter pitch bites the air in a way that creates better climb, but at the cost of speed. A steeper pitch gives more speed, but less lifting ability. Pitch also has a complicated ménage à trois relationship with torque and rpm that I don’t even pretend to understand, but the upshot of all of this was that while we had JATO-like takeoffs, we were hitting our engine’s redline at about half power. My mechanic advised me that I needed to re-pitch the prop.

What’s involved in that?

Well, fancy airplanes have variable pitch props that let the pilot change the pitch of the propellers in flight using a lever in the cockpit so that they can have both strong climb on takeoff and fast cruise in flight. Less fancy modern planes have what are called ground-adjustable propellers. The pitch can be easily and quickly changed on the ground to best fit the mission at hand.

But I have neither.

I have a metal prop whose pitch can only be changed by having an expert literally bend the metal blades to change the angle, thus “re-pitching” it. Luckily for me there’s just such an expert an hour and a half’s flight away and there’s no limit on how many times my particular model of prop can be re-pitched, other than the limits imposed by my bank account balance. Unluckily for me, this is not an exact science. It’s more of an art. Adding to the complexity of the situation, propeller performance is affected by weight, temperature, altitude, the whims of the Gods of Aviation, and who knows what else.

Of course, in my innocence at the beginning of this particular Plane Tale, I knew none of this. I trustingly flew to the prop shop and talked to the Master Metal Bender, giving him what data we had. Tessie’s prop was measured. She was wearing a 46-pitch prop. Yeah, the numbers meant nothing to me either, don’t worry about it. All you need to know is that would be considered an “extreme” climb prop for an Ercoupe, which is what she needed at our altitude with a largely worn out engine. Given our data, the prop was re-pitched to 51, which is completely at the other end of the spectrum for ‘Coupes. I now had a fast cruise prop.

And boy, was Tess ever fast. Wearing her new pitch, she cut through the air a full 10 miles per hour faster than ever! It was amazing. Race trophies danced in my eyes.

But, as always with airplanes, there was a problem. And the problem this time was the runway. Tessie didn’t want to leave it. We used up thousands of feet of concrete, and then she could barely lift into the air. I had cartoon visions of Tessie furiously flapping her metal wings to get airborne.

This just wouldn’t do.

So back to the prop shop I went. The Master Metal Bender took Tess’s propeller off again and re-re-pitched. Logically, it seemed we needed to be halfway between where we’d been and where we went (although these things aren’t necessarily linear). As half way would be 48.5, and things don’t work that way, I had to choose between 48 and 49. I went with 49, on the fast side of middle of the road. OK, forget what I said a few minutes ago. We really do have to all talk more about these pitch numbers to drive the story forward. Here’s your background…

Historically the Ercoupe wisdom was that:

48 was a climb prop.

50 was a normal prop, and…

52 was a cruise prop.

But ‘Coupes have gotten fat. New electronics and gadgets have made them heaver over the decades, and that affects prop performance. While there’s no official data, for modern weights, many Coupe folks now consider that:

46 is a climb prop.

48 is a normal, prop, and…

50 is a cruise prop.

Adding to the confusion is that no one seems to know what prop best suits the stroker in an Ercoupe. Given the fact that this whole prop thing is more of an art than a science in the first place, I’m sure you can see where this is going.

So how’d the re-re-pitch go? Rio said it best when he told his grandmother that it was, “Less miserable.”

The new pitch, as expected, reduced the speed and increased the climb. But it was a marginal change at best. So we have to re-re-re-pitch. What a bitch.

So picture me standing in the Aviation Maintenance Casino. I’m standing at the propeller roulette wheel, and there are only two numbers left to bet on: 47 and 48. I know 46 is too flat. I know that 49 and 51 are too steep, and that even though we skipped 50, the change between 49 and 51 wasn’t much. This suggests that going from 49 to 48 wouldn’t net much of a change either. Of course, by the same logic, a 47 shouldn’t be much different from a 46, which was where all of our troubles started in the first place.

The Croupier calls out, “Ladies and gentlemen, place your bets…”