Constant Speed Drive Units

Torfinn · February 3, 2006

There's been quite a bit of discussion of Constant Speed Drive Units and helicopters at our Flight School recently. In the syllabus of theoretical knowledge for CPL(H) and ATPL(H), the CSDU is included. However, one of our instructors claims that CSDU's don't exist for helicopters, as alternators are always connected to the Main Gear Box, and as such need no speed control device to produce constant frequency AC electricity, as rotor RPM is (relatively) constant.

I'm unable to find anything conclusive on this, either in my books or on the Internet. So, I'm hoping that someone here might know...

Edited February 3, 2006 by Torfinn

nbit · February 3, 2006

Hmmm. I don't know the answer but... Interesting... I've never seen a CSDU on an aircraft. Not saying they don't exist. Any aircraft I've flown that needed power on an AC bus always had an inverter to convert DC to the required AC power from generator or battery power. A CSDU seems like undesirable extra equipment, as it would be mechanical in nature, with moving parts, etc...

Torfinn · February 3, 2006

I believe they're used mostly on bigger aircraft. Example: http://www.b737.org.uk/generators.htm

I've also heard of frequency wild AC electricity being changed into DC by a rectifier, and then turned back into nice and stable 400 Hz AC by an inverter. The experience of the instructor I was talking about was from the AS332 Super Puma, where the alternators are connected directly to the Main Gear Box, thus eliminating the need for a CSDU or similar device(s).

No matter how hard I try, however, I'm unable to find any evidence for or against the presence of a CSDU on rotorcraft of any kind...

Edited February 3, 2006 by Torfinn

StuandJoy · February 3, 2006

I dont see a reason to have one either. Most helis stay at a constant speed whether it be engine or rotor. CSDs are normally only required for varying RPM engines. We have them on the C5s but I have never heard of one on a rotor.

delorean · February 4, 2006

If we learned anything about them in A&P school, it must have been very brief because I don't remember anything about them.

Also, it's mostly generators versus alternators.......and AC is used in big aircraft in order to save on weight and space. I don't have a chart in front of me, but it would take a serious chunk of wire to run 28V from tail-mounted engines to a nose wheel landing light on a 717, 727, etc. So, just like the power companies with power lines, they boost the voltage (pressure) to prevent voltage drops in smaller lines.

delorean · February 4, 2006

BTW, here's a interesting fact:

Do you know the (or one of the) main reason why Europe uses 220V versus 110V?

It's because of the limited supply of copper. Two small wires carrying 110V in parallel (to form a 220V line) use less copper than a single 110V copper wire with the same voltage drop.

I'm sure there's more reasons, but that was one of the big restrictions that led to the standardization.

Torfinn · February 4, 2006

BTW, here's a interesting fact:

Do you know the (or one of the) main reason why Europe uses 220V versus 110V?

It's because of the limited supply of copper. Two small wires carrying 110V in parallel (to form a 220V line) use less copper than a single 110V copper wire with the same voltage drop.

I'm sure there's more reasons, but that was one of the big restrictions that led to the standardization.

No, I didn't know that. I've been wondering about that sometimes...

brushfire21 · February 6, 2006

-Just to throw something out there for you guys for some needless knowledge on electrical theory.....

-A few things to primer you guys/gals or help you remember some things from H.S. electronics class:

120vac @ 15 or 20amps = Normal US Household current to wall plugs (60hz)

Watts = (Voltage)x(Amperage)

746 Watts/.746kW = 1 US Electrical Motor Horsepower (Theretical)

#14 Gauge Stranded Copper Wire Amp Capacity = 15amps (National Elec. Code Recomendation)

120vac x 15 amps = 1800 watts (2.4hp)

220vac x 15 amps = 3300 watts (4.4hp)

So as you can see, by doubling the voltage, you double the wattage available that can feed a load keeping the wire size the same. Using stranded wire is more efficient than solid copper (something about electrons running around the outside of the conductor or what not), and is more durable becuase it can flex and not break. In addition, using higher voltage, allows motor loads to start easier becuase of the higher voltage potential available (within reason).

Now that I have bored all of you with some useless information, I am sure someone will point out a flaw or two with my calcs, but we all can learn something from each other. G'Day!

Gomer Pylot · March 25, 2006

Turbine aircraft use generators, which make DC current, and also double as the starters. Any needed AC voltage is provided by inverters, which get input from the DC generator or battery. DC generators don't care what the RPM is, as long as it's sufficient to make the DC current. The inverter just takes the DC, so there is no need for anything else. As long as the engine is running, there is DC and AC voltage, and if the engine fails the battery will supply voltage long enough. CSDUs are superfluous.