Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius — and a lot of courage — to move in the opposite direction. – Albert Einstein
I went back to the schoolhouse yesterday. I’m always excited to learn new things and was looking forward to getting some time with my CFI again. Since I’ve only had my license since August/2009 I haven’t had a need to fly with him in seven months. This week I had a couple excuses.
My club has a requirement that all members go on a check ride with an instructor on a biannual basis and get a sign-off in order to use club planes. Since we’re in the spring check ride period I had to get this sorted out by the end of April. The check ride isn’t associated with our FAA supervisors, so it’s a pretty low stress event. Just something that has to get done.
The other, more fun part, was that I started the training I need in order to fly our Piper Arrow IV, a complex airplane. The supervisors say that a complex airplane is one that has all the following:
- A retractable landing gear (land aircraft only; a seaplane is not required to have this)
- A controllable pitch propeller (which includes constant speed propellers)
- Movable or adjustable flaps.
The planes I trained in have adjustable flaps. So, no problem there. The complexity is in the variable pitch propeller and the gear.
In the Cessna 172s I usually fly, we do a GMC on approach. Gas (select both tanks), Mix (full rich) and Carb (turn on carb heat, if the plane has it. One of the ones I fly does, the other doesn’t). In the Arrow we do a GUMPs. Gas (select the fullest tank), Undercarriage (put down the landing gear), Mix (full rich) and Prop (on short final push it forward in case of a go-around).
The other change is in engine/propeller management. In the 172 we have one knob for the throttle and that’s it. Push it to go forward, pull it to stop. The propeller goes in circles, but the pitch always remains the same. This is a bit inefficient. A propeller is an airfoil and in fixed pitch systems, a compromise has been struck. You might not be as efficient in cruise, climb or take-off, but you don’t have to manage the pitch.
With variable pitch, you can gain better efficiency by adjusting the propeller so that it is attacking the apparent wind at the right angle. This allows you to get maximum efficiency at take-off when you want to get off the ground and in cruise when you want to save fuel. You measure your pitch in RPMs. Yes, that isn’t intuitive. This kind of system it also called a constant speed propeller because you set for an RPM and the airplane adjusts pitch to maintain that RPM as power and apparent wind change.
There is a nice little subtlety in that. If you have trained in a fixed pitch plane, you are used to getting some audible feedback as the plane climbs or descends. With variable pitch, because the RPMs stay the same, the engine noise also stays the same. So, as you should have been doing anyway, you really have to stay more on top of your scans and your sight lines to make sure the plane is doing what you want.
Independent of the pitch, you adjust the engine power. This is measured by the unfortunately named, manifold pressure. It’s unfortunate, because what’s being measured is vacuum. It’s a measure of how hard the engine is sucking at the atmosphere. If you are at a low power setting, it’s sucking very hard and not getting much. If you are at a high power setting, it’s getting as much air as it wants.
The two settings combine to push air backward and make the plane go forward. You can throw air backward hard enough by either choosing a high RPM and a lower manifold pressure or by choosing a lower RPM and a higher manifold pressure. For example, you can get to a 65% power setting by setting 2400 RPMs and 20.2 inches of mercury manifold pressure *or* by setting 2100 RPMs and 22.7″.
The advantage of lower RPMs is fuel efficiency and, if you are billed on tach time, 14% lower bill. The advantage of higher RPMs is that you will be warmer in the winter because you will be throwing more CO2 into the air.
Ok, that’s a rather modest benefit…