? MU-2 Electrical System

[PA31Ho]

I noticed that there is a switch for "Parallel" and "Series" for the battery. I tried to get somebody to explain why it's there and I really couldn't get a great answer. The only thing that was said was that you use "Series" for starting when the temperature outside is cold. (like 5C below). Can anybody give me an explanation on why this would be?

Thanks!
-Nick

 

[Lead Sled]

Simple - more amps for starting. The Garrett is a fixed shaft engine and it takes a lot of ommph to get everything turning (including the propellor) on start up.

Lead Sled

 

[PA31Ho]

So why would a series circuit provide more amps?

 

[EagleRJ]

Not more amps- more volts.

Many planes have a way to put the batteries in series to double the voltage going to the starter. This results in a shorter start, lower peak temperatures, and less chance of a hung start. For many free-turbine engines, it's used for starting a heat-soaked engine or starting with a tailwind, but a direct drive engine may use it for cold weather to make sure it accelerates enough for a stable start.

The batteries are already in parallel for normal starts, and Ni-Cad batteries can put out a huge amount of amperage for start. Some planes can even start both engines at the same time!

 

[Jedi_Cheese]

A series circuit draws power from both, thus you get the full power from all the batteries. A parallel will draw from both batteries equally.

http://ourworld.compuserve.com/homepages/g_knott/elect27.htm

 

[skyking1976]

The downside of a series start is that you have to wait longer for the batteries to recharge before starting the other engine. Kind of annoying, but if you need to get it started without a GPU and weak batteries that's the best way to do it.

SK

 

[Lead Sled]

Oops, my bad. I've got a lot of time in MU2s but that was like 20 years ago. I was running from memory and I could have sworn that it doubled the amps. (I really don't remember that we hit the starter with 48 volts on low temperature starts, but like I said it was a long time ago.) Oh well. I haven't had to worry about that switch for a long time.

Lead Sled

 

[RJones]

POM
Section 3
Systems Description

"An external power source for starting is recommended in cold weather. When a battery start is to be made, it is recommended to place the Battery Select switch to SERIES when outside air temperature is less than 10c (50f)."

It is my recommendation to try your best to find a GPU to start. Series really zaps the battery and if you have NiCad's you may get a runaway after starting.

RJ

 

[PA31Ho]

Thanks a lot fellas! I understand the reasoning now.

-Nick

 

[KigAir]

The downside of a series start is that you have to wait longer for the batteries to recharge before starting the other engine. Kind of annoying, but if you need to get it started without a GPU and weak batteries that's the best way to do it.

SK

Ah, this explains why I see MU-2's on the ramp with one engine running. I've wondered why it takes soo long to start the other engine.

 

[RJones]

Start one engine and wait, get the batteries charged-up to some degree and start the other engine. Unlike the King Air, the Mu-2 can't (uh ) do a "cross generator" start...the electrical system can't handle the high initial amperage load that's created when starting the second engine.

RJ

 

[Vector4fun]

Ah, this explains why I see MU-2's on the ramp with one engine running. I've wondered why it takes soo long to start the other engine.

As I recall from the MU-2J, the battery temps get quite hot, and it sometimes took 4-5 minutes to cool enuf to start the other engine. Always used a gpu whenever possible.