Monday, July 11, 2005

Masters & Slaves: Three More Things About Propellers

This is probably my final chapter on the propellers. They do have every right to be this complicated as they are one of the most important systems on the airplane. These last three things are autosynchronization, blade latches, and prop ties, which I just thought of so I had to go back and change both this sentence and the title of the post.

The propellers, as previously and repeatedly mentioned, spin around. And they make noise. And noise is actually a wave in the air, not an up and down wave like on the ocean, but a squishy back and forth wave, like a pulse going along a slinky that is stretched out along the floor. If two slinkies interfere you get a tangled mess of coiled metal, and your mom comes and solves the problem with wire cutters, so you end up with more slinkies, only shorter. (I know this from an unsuccessful slinky race with a friend when I was little. The stigma of having a shorter slinky than all the other kids haunts me still.) If different sound waves interfere, you get a pattern of sound called a beat. So if your two propellers are going at exactly the same speed, you hear a lot of normal propeller noise. If they are going at slightly different speeds, the captain hears a beat, and I hear a lot of propeller noise, and the captain complaining. So the correct way to deal with this is to randomly tweak the propeller levers until the captain gets irritated enough that he syncs the propellers himself. But if I'm lucky, there's an autosynchronization package installed and it will do it for me.

In this airplane the autosynchronization package has magnetic speed pickups (like the kind on bicycle speedometers except way more expensive) on each overspeed governor, an actuator on the right engine and a trimmer on the right primary propeller governor. The pilot need only get the propellers within 2.5% of one another, and then the difference between the two speed signals controlls the actuator to trim the right governor to achieve an rpm identical to the left. Thus the left is the master and the right is the slave. If the propellers go significanly out of sync, the system is inhibited, so that an engine failure or overspeed condition on the left engine won't affect the right. In such a case, as well as during take-off and landing and on the ground, the pilot is expected to have the autosynch turned off.

You can understand from the description of the propeller system that when the engine is shut down that the oil pressure will drop and the propeller blades will be pushed into the feathered position. When the airplane is restarted, max rpm will be selected, so the blade angle will reduce, but there will momentarily be a sudden lurch forward, undesireable for some operations. So if you postion the propeller levers opposite the zero thrust markings, corresponding to approximately a one degree blade angle, the latches will engage and hold the blades in that position. After start, if the propeller lever are kept behind the zero thrust line, when the propellers reach about 30% Np the latches disengage.

My friend has a Jeep convertible (are all Jeeps convertible?) and won't park it anywhere without attaching a steering wheel lock. This airplane does not have an antitheft device. Heck it doesn't even have keys. The so-called control "lock" is just a bar that keeps the rudder, ailerons and elevator from flapping about in the wind. It's also important that the propeller not be allowed to spin in the wind, for two reasons. One is that a spinning propeller could whack someone in the head, and that might convert them into a unsatisfied customer, or a non-productive co-worker. The second reason is that there is no oil pressure when the engine is shut down, so a spinning propeller equates to the front half of the engine spinning with no lubrication, shortening its life. So this is where the prop ties come in. There are different variants of this sophisticated system. Some use a loop of rope around a propeller blade, fastened down to something. Some run a bungie cord around a blade and hook it into an opening in the cowling. In nations where slaves are cheaper than rope, you could station a slave on each side of the airplane to hold the propeller still. Whatever works. But then you'll need extra slaves to load baggage. or you could hold the left one yourself, then the left would be the master and the right the slave, just like the autosynch.


aasmodeus said...

I don't understand, I thought _you_ were the captain?

Lord Hutton said...

There's nothing worse than a short slinky;-)
And, hey, I nearly understood this lesson!

Aviatrix said...

Alright, Andy, me too!

Aasmodeus: sometimes yes, sometimes no. Sometimes what I talk about happened yesterday, sometimes it happened years ago.

I'll be the FO on the new job, at first.

Anoynmous said...

"I'll be the FO on the new job, at first."

That sounds less like a prediction and more like a promise. Are congratulations in order?

Aviatrix said...

Ah no, anoynmous, it's somewhere between positive thinking and and lazy sentence structure. If I do get the job, I will be an FO. With the elided clause, the English verb doesn't make that distinction.

The chickens remain uncounted.

Stu said...

Cool. I _am_ reading these really. And to think I just thought propellors went round and round like my lego ones.