Vacuum pump & Alternator Q

[PGT]

What's the difference between a dry vs. wet?

I'm studying for my multi comm and the BE76 is a dry pressure type, what does the pressure mean?

I know dry fails more quickly, a wet pump fails slowly but that's about it.

Also, I was asked what's important about the self exciting feature of the alternators? Other than it provides power at low rpm, am I missing something?

 

[USMCmech]

What's the difference between a dry vs. wet?

A wet vacuum pump uses oil to make a good seal between the vanes of the pump. It works better and lasts longer, but it makes a mess.

I'm studying for my multi comm and the BE76 is a dry pressure type, what does the pressure mean?

Beechcraft doesn't use vacuum, instead they use pressure to power their instruments. Everything works the same way, except the lines are hooked up different.

Also, I was asked what's important about the self exciting feature of the alternators? Other than it provides power at low rpm, am I missing something?

Modern brushless alternators do not require a charged battery to work. The old style would not charge a dead battery.

 

[N18040]

What's the difference between a dry vs. wet?

I'm studying for my multi comm and the BE76 is a dry pressure type, what does the pressure mean?

I know dry fails more quickly, a wet pump fails slowly but that's about it.

Also, I was asked what's important about the self exciting feature of the alternators? Other than it provides power at low rpm, am I missing something?

Supposedly the pressure systems are suppose to work better at high altitude because it can produce a larger pressure difference between it and ambient, than the suction side can.

As far as the alternator goes, the field is an electromagnet. The self-exciting type has some residual magnetism in the field so that it does not require any current to be produced from the battery for it to start produce power.

 

[Fr8dog44]

Fact about dry pumps; The vanes are on an angle. It is theorized that rotating a prop backwards could break the vanes. There is controversy on this! But that's what an A&P told me once!

Pressure could be pnuematic. Navajo's, have a pressure pump. This is so the pressure generated can be used not only to drive gyro's but also provide pressure for de-icing boots. BE Turbines (PT-6) derive their pressure from bleed air (P-2 or sometimes P-2.5).

Remember, alternators cannot generate power like generators. The outter windings must first be energized with a minimum of 4 volts before they will provide power. Non of the planes I flew had an exciter. I would imagine an exciter would allow an alternator to be able to provide power without that initial external current being provided.

I hope this helps!

 

[Roger, Roger]

Fact about dry pumps; The vanes are on an angle. It is theorized that rotating a prop backwards could break the vanes. There is controversy on this! But that's what an A&P told me once!

SOME have the angled vanes. Not all. In fact most do not.

 

[patthepilot]

I've heard that too about rotating a prop backwards. Good because the mags won't fire, but bad becuase ______ and my mind draws a blank.