Constant speed props

 

Constant speed props are often a source of mystery to people who haven't used them before. They look at all sorts of complicated diagrams and try to memorize arbitrary rules.

It's really pretty simple. From a pilot's standpoint you have another knob - generally blue between the throttle and the mixture control.

All the knobs go forward into the dashboard for takeoff. You can think of the prop control as the stick shift of a manual transmission in a car. When it's all the way forward, it's in first (low) gear, for accelerating from a dead stop, and when it's pulled back, it's in fifth (high) gear, for cruising at high speed.

When you want to increase power (e.g. to takeoff, or climb), you go from right to left - mixture forward, then prop forward, then throttle forward.

When you want to decrease power (e.g. to level off) you go from left to right throttle back, then prop back, then mixture back.

The setting of the throttle is shown on the Manifold Pressure gauge, and the prop knob controls the RPM shown on the Tachometer.

There are a million combinations of throttle and prop settings which will make the same power, but as a rule of thumb, the higher the Manifold Pressure and the lower the RPM, the lower your fuel burn will be, and the quieter it will be, too. Lycoming says a maximum spread of 5 between the Manifold Pressure and the RPM, for the little engines.

 

For example, with 2000 RPM (20 "turns" is very salty) you can run max 25 inches of manifold pressure. As always, the aircraft POH/AFM is golden do what it says to do, with that engine and that prop in that airframe. Especially with the four cylinder Lycomings, which do not have crankshaft counterweights, it is not unusual to have yellow arcs on the tachometer which you cannot operate in, to avoid undesirable torsional harmonic resonances. As usual, oscillations are really bad things in aviation.

It's important to realize that although you have a variable-pitch prop - that is, the blades of the prop rotate in the hub in flight - you really have a constant-speed prop, courtesy of a magical lump of metal, bolted onto the engine, called a "governor". The prop control actually moves an arm on the governor, which meters oil pressure to the prop.

You don't actually directly control the prop blade angle. What you do, is request that the governor maintain a particular RPM, and it will do it's best to do so, within the limits of the prop blade movement, as the airspeed of the aircraft increases and decreases, which is actually pretty cool.

If you reduce the power (with the throttle) a whole lot, what you will notice is that the RPM falls. The governor now has the prop blades at their finest pitch, and what you have now is essentially a fixed-pitch (fine pitch climb) prop, which you control with the throttle, just like you would on a 172. You can ignore the Manifold Pressure now - just look at the tachometer for small power adjustments (e.g. +/- 250 RPM), as you are likely to do on base and final of your approach.

And, now that the RPM has fallen, you can safely advance the prop control all the way in, as part of your pre-landing check. It won't make any difference to the engine speed, because the prop is already at it's maximum fine pitch.

One tiny little detail ... I mentioned that the prop is controlled by the governor, by the oil pressure from the engine.

On 99% of piston singles, if you lose oil pressure, the prop is built so that it goes to full fine pitch, as you would have at takeoff or short final.

However, on a piston twin, or an aerobatic airplane, "counterweighted" props are used, and when they lose oil pressure, they go to the opposite stop, at full coarse pitch. You might think that this is unnecessary, especially on a single, but a friend of mine had a bad experience in his RV-8 one day, when in a dive, he unported the oil pickup on his Lycoming angle-valve IO-360 and wound it up to 4,000 RPM when the prop went fine. Prop was trash, of course, and the engine had to be overhauled and inspected, new connecting rod bolts, etc. He went to a counterweighted prop after that.

Hope this helps. As is often the case in aviation, people make this stuff much more complicated than it needs to be.

 

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acboyd@gmail.com May 2013