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Foxtrot11
Topic Author
Posts: 5
Joined: Wed Jan 24, 2018 7:18 pm

### Typical Epicyclic gear ratios

If we were to design a theoretical power-split device (PSD) for an aircraft, how can we decide on the Ring:Sun ratio? Assuming that the Input Speed is 20000 rpm; Output Speed is 2000 rpm and for power of 1000 HP.

From my literature review, It's pretty clear that there has been no significant work done on PSDs for aircrafts. But, there are planetary reduction gearboxes that are commonly used. Would it be right to approximate the parameters of a PSD from those of a reduction gearbox?

WIederling
Posts: 9291
Joined: Sun Sep 13, 2015 2:15 pm

### Re: Typical Epicyclic gear ratios

So you want to
run the sun gear @ 20krpm
have the planet carrier run @ 2krpm
and use the ring gear as reversed secondary take off ?

then:
there is this planetary gear differential around, symmetric.
A differential is a PSD device.
https://en.wikipedia.org/wiki/Epicyclic ... fferential
Murphy is an optimist

Foxtrot11
Topic Author
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Joined: Wed Jan 24, 2018 7:18 pm

### Re: Typical Epicyclic gear ratios

Apologies. Should have mentioned this earlier. I am trying to design a Hybrid power train with a Gas turbine connected to the Planer Carrier, one electric machine (generator) connected to the Sun gear, the second electric machine (motor) is connected to the output shaft which itself is connected to the Ring gear.

WIederling
Posts: 9291
Joined: Sun Sep 13, 2015 2:15 pm

### Re: Typical Epicyclic gear ratios

Foxtrot11 wrote:
Apologies. Should have mentioned this earlier. I am trying to design a Hybrid power train with a Gas turbine connected to the Planer Carrier, one electric machine (generator) connected to the Sun gear, the second electric machine (motor) is connected to the output shaft which itself is connected to the Ring gear.

What kind of task do you want to accomplish?

IMU the gas turbine would be the highest rpm item, wouldn't it?

Have you looked at the Toyota Prius Powertrain ( that uses two generator/motor e-engines. convoluted but brilliant imho.)
Murphy is an optimist

zeke
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Joined: Thu Dec 14, 2006 1:42 pm

### Re: Typical Epicyclic gear ratios

And what is it going to drive ? A propellor ?

Do you want to have a small gas turbine drive a generator and a propellor through a PSD, and then use the battery turn the generator into a a motor to assist the gas turbine for takeoff etc ?

Still essentially a hybrid car setup
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wingscrubber
Posts: 823
Joined: Fri Sep 07, 2001 1:38 am

### Re: Typical Epicyclic gear ratios

There aren't really any 'typical' ratios - you've got to decide that yourself for your target speeds and select the ratios and number of gear teeth yourself.

The Prius drivetrain simulator here used to work well for explaining three-way gearboxes. Won't currently work for me on my work computer.
https://www.mmatsubara.com/prius/ThsSim ... _i18n.html

I used the three way differential gearbox principle as the basis of my electric motor-generator PTU patent, which I understand was being test flown on the Cessna Longitude, not sure if it went into full production though;
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Foxtrot11
Topic Author
Posts: 5
Joined: Wed Jan 24, 2018 7:18 pm

### Re: Typical Epicyclic gear ratios

WIederling wrote:
Foxtrot11 wrote:
Apologies. Should have mentioned this earlier. I am trying to design a Hybrid power train with a Gas turbine connected to the Planer Carrier, one electric machine (generator) connected to the Sun gear, the second electric machine (motor) is connected to the output shaft which itself is connected to the Ring gear.

What kind of task do you want to accomplish?

IMU the gas turbine would be the highest rpm item, wouldn't it?

Have you looked at the Toyota Prius Powertrain ( that uses two generator/motor e-engines. convoluted but brilliant imho.)

So, finally I would want to check whether using the PSD will be benefical or not, especially when you consider the weight of the additional electric machine and the PSD itself.

And, yeah, the Gas Turbine (Turboprop) would be the highest rpm machine.
My baseline powertrain configuration is infact the one that is used in the Prius, but I am still trying to decide on where to keep the motor: before the final reduction or after.

Foxtrot11
Topic Author
Posts: 5
Joined: Wed Jan 24, 2018 7:18 pm

### Re: Typical Epicyclic gear ratios

zeke wrote:
And what is it going to drive ? A propellor ?

Do you want to have a small gas turbine drive a generator and a propellor through a PSD, and then use the battery turn the generator into a a motor to assist the gas turbine for takeoff etc ?

Still essentially a hybrid car setup

Yes, a propeller. And, yeah essentially, there might be another electric machine instead of having multi-modes like charging and discharging (required if you have a single electric machine) which makes the control strategy a bit more complicated.

WIederling
Posts: 9291
Joined: Sun Sep 13, 2015 2:15 pm

### Re: Typical Epicyclic gear ratios

Foxtrot11 wrote:
And, yeah, the Gas Turbine (Turboprop) would be the highest rpm machine.
My baseline powertrain configuration is infact the one that is used in the Prius, but I am still trying to decide on where to keep the motor: before the final reduction or after.

You could place the motor/generator right on the turbine shaft and go from there with a simple reduction to the propeller.
Murphy is an optimist

Foxtrot11
Topic Author
Posts: 5
Joined: Wed Jan 24, 2018 7:18 pm

### Re: Typical Epicyclic gear ratios

WIederling wrote:
Foxtrot11 wrote:
And, yeah, the Gas Turbine (Turboprop) would be the highest rpm machine.
My baseline powertrain configuration is infact the one that is used in the Prius, but I am still trying to decide on where to keep the motor: before the final reduction or after.

You could place the motor/generator right on the turbine shaft and go from there with a simple reduction to the propeller.

You mean like a Series configuration? If so, my aim is to employ the PSD to an aircraft powertrain and analyse its benefits, if there are any. Placing the motor/generator on the turbine shaft will not require a PSD, if I understood you correctly.

WIederling
Posts: 9291
Joined: Sun Sep 13, 2015 2:15 pm

### Re: Typical Epicyclic gear ratios

Foxtrot11 wrote:
You mean like a Series configuration? If so, my aim is to employ the PSD to an aircraft powertrain and analyse its benefits, if there are any. Placing the motor/generator on the turbine shaft will not require a PSD, if I understood you correctly.

If the generator/motor is a synchron machine ( I assume that ) you can program the VFD that drives to just create additional torque
on the shaft, respective decrease torque for battery regen. That is not really a serial hybrid.
( IC engine -- E-generator - copper - E-motor -- propeller )

What you have is a shared hybrid.
( IC engine --E-generator/motor--propeller , reduction gearing left out )
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