Topic Author
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A/C Engines - Gearbox?

Tue Sep 09, 2003 9:23 pm

Ok, while driving to town today, I suddently got this idea/question stuck in my head:

How does the gearbox on a aircraft engine works? I mean, is it like automatic gear, where the fuel injection doesn't stop while the gear is switched, or?

[Edited 2003-09-09 14:28:48]
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RE: A/C Engines - Gears?

Tue Sep 09, 2003 9:30 pm

most piston singles do not have gear reductions. however, a lot of turbines do. they need it to keep the prop spinning slower than the engine. a prop simply could not spin as fast as the engine. the gears don't switch, they just provide reduction between the prop spinner and the engine itself.
"...cannot the kingdom of salvation take me home."
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RE: A/C Engines - Gearbox?

Tue Sep 09, 2003 11:10 pm

On turbojet/turbofan engines, the gearbox is used to drive the engine/aircraft accessories such as the starter, constant speed drive, generator, hydraulic pump(s), and the hydromechanical engine fuel control.
It is usually mounted on the bottom of the engine and is driven by a shaft that is geared to the engine drive shaft. On single spool engines, it is geared to the only engine shaft; on twin spool engines, it is geared to the N2 shaft; on three spooled engines, it is geared to the N3 shaft.
On P&WA engines this gearbox drive shaft is called the tower shaft, since it is mounted vertically between the engine dive shaft and the gearbox.
On turboprop/turboshaft engines there is a reduction gearbox to reduce the output RPM to propeller or rotor; often this reduction gearbox also has drives to operate the accessories in a similar way the to turbojet/turbofan engine accessory gear box.
On the RB211, the accessory gearbox was originally intended to be mounted on the core engine casing inside the fan bypass area. The gearbox was relocated to outside the fan case and this required an additional angled gearbox on the back of the accessory gear box to connect the accessory gearbox to the tower shaft.
This angled drive gearbox is called the step aside gearbox and had been a problem area on the RB211-22B. I experienced one in-flight shutdown on a L1011 when the step aside gearbax failed resulting in the loss of the connection between the engine and the accessory gearbox.
On bigger reciprocating engines, where the engine RPM is greater than that necessary for the operation of the propeller, there is a reduction gearbox mounted on the nose of the engine to keep propeller RPM's in the correct range. On these engines, that is the only purpose of the gear box as, usually, the accessories are mounted on the back of the engine and are driven by geared drives from the engine.
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RE: A/C Engines - Gearbox?

Wed Sep 10, 2003 12:04 am

Not really much to add to Broke's very excellent reply.

Piston engines used two types of reduction gearing.
All large radial engines (and the Lycoming IGSO/GSO 480/540 series) use planetary reduction gears, whereas the TCM GTSIO520 series use spur gears, for the propeller shafts.

An interesting web site for those interested is...
aaron atp
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Wed Sep 10, 2003 9:30 am

So who wants to tell him how a CSD works?

(anyone except VC-10 of course, because he could probably assemble one while blindfolded and asleep)

As for being on topic, on the TFE731 (most popular bizjet engine ever), the fan is driven by planetary gear reduction.

For pistons, two aircraft come to mind, the Helio-Courier and the Cessna 421 (which had the GTSIO520's 411a mentioned)... I've never flown the former, but the latter didn't appreciate chopping the throttles on short final due to the effect of the gearing. It's not good for the engines, your altitude/airspeed, or the airport fence.

The Helio pilots probably think it's fun to use "reverse," but it is a much different beast than the golden beagle. Actually, IIRC, the early helios used v-belts for reduction. I'm not sure how they fly that thing, other than ssslllloooowww.

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RE: A/C Engines - Gearbox?

Wed Sep 10, 2003 11:30 am

Watched with some amusement in 1968 when a Helio-Courier called LAX tower on the helicopter frequency (127.6 at that time) and requested landing instructions.
He was cleared to land on the helipad opposite satellite 6...when the tower noticed it was NOT a helicopter...called him twice, were ignored, and he landed on the helipad and rolled exactly twenty feet to a stop.

Almost as interesting as watching DC6's approach, land on (what was then) runway 34, a few years before.

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RE: A/C Engines - Gearbox?

Thu Sep 11, 2003 2:51 am

So who wants to tell him how a CSD works?

I'll take a stab at it.

A CSD or constant speed drive works much like a automatic transmission in your car. A jet engines speed varies from a few thousand rpm's at idle to over 10,000 at takeoff power. Inside a CSD is a variable displacement pump and a hydraulic motor. The CSD senses the engines speed and adjusts the pumps output to the hydraulic motor to maintain a constant speed at the output shaft.
Fwd to MCO and Placard
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RE: A/C Engines - Gearbox?

Sat Sep 13, 2003 3:57 am

Broke mostly said it....

Turbofan engines has a forward and aft gearbox located on the bottom part of the engine. I do recall of them having an inlet gearbox. Basically they drive your acessories, that's why it's called the acessory gearbox and it also drives your low pressure turbine (LPT) and your high pressure turbine. (HPT)

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RE: A/C Engines - Gearbox?

Sat Sep 13, 2003 6:08 am

There is no hydraulic motor in a CSD/CSU/PCU. A hydraulic motor is one which turns due to a stream of fluid causing it to rotate.

The pump inside of a CSU is a gear type pump (variable displacement, it depends on engine speed for it's output).

Here is a brief summary of the operation of a Constant Speed Unit (CSU). Also known as a Constant Speed Drive (CSD) and on turboprop aircraft it's known as a PCU/PSU (Power Control Unit)

A CSU can be broken down into three basic parts:

The governor flyweight assembly;
The oil control valve;
The idling circuit.

All connected on the same engine driven shaft.

The governor flyweight assembly consists of two L-shaped flyweights that sense changes in prop speed (centrifugal force acting on the flyweights as they rotate) and causes the oil control valve to adjust the flow of oil to/from the propellor hub. The flyweights are hinged at the point near where the long arm of the L meets the short arm. The short arm of the flyweights is in contact with the head of the oil control valve.

The oil control valve controls the flow of oil into the propellor hub and back into the sump. On the head of the oil control valve is a conical spring that opposes the force applied by the governor flyweights. The CSU can be rigged by turning an adjuster screw. The screw controls the compression on the spring by way of a rack and pinion assembly.

The idling circuit is provided for when there is a state of equilibrium (the prop is at the correct RPM and the centrifugal force acting on the flyweights is in balance with the conical spring). The pressure buildup in the CSU causes a spring loaded valve to offseat thereby opening an idling circuit within the CSU - the pressurised oil is sent back to the pump inlet.

Imagine you have an aircraft in straight and level flight, in clear air, engine set to the required manifold pressure and prop set to required rpm. The prop is said to be onspeed in this situation.

The aircraft now enters a cloud. The air becomes denser and as such the prop slows down slightly (termed underspeed - indetectable usually to the human ear/eye). The centrifugal force on the governor flyweights is lessened and as such the force exerted by the conical string overcomes the flyweights and pushed the oil flow control down. Oil enters the port into the propellor hub and cause the prop blades to move towards a finer setting (due to centrifugal twisting moments acting on the prop blade). The prop speed increases as a result, causing centrifugal force acting on the governor flyweights to increase, overcoming the force from the conical spring. This means that the flyweights now cause the oil control valve to move upwards, until a point is reached where the centrifugal force and the force exerted by the spring are equal - the oil port to the propellor blade hub is closed, and no oil can flow into or out of the propellor hub.

The aircraft now flies out of the cloud, the air density drops - there is less air for the prop to have to turn through. The prop starts to overspeed (not to be confused with the very dangerous engine overspeed or runaway prop situations - this is just a term used to describe the prop going faster than the setting selected by the pilot). Centrifugal force on the flyweights increases causing them to move outwards. This moves the oil control valve upwards as the force exerted by the flyweights is greater than that of the spring. The oil in the prop hub drains into the engine oil sump. This causes the prop blades to move to a coarser pitch (due to a combination of aerodynamic twisting moments and a spring [or compressed gas] in the prop hub). The coarse pitch means that the prop blade slows down as it has more work to do to maintain the selected rpm. This reduces the amount of centrifugal force acting on the governor flyweights, the conical spring then pushes the oil control back down until the oil control valve closes, sealing off the flow of any more oil to the oil sump.

This activity within the CSU is a constant cycle as the air density is never constant as the aircraft flies through the sky. I just mentioned the clouds as it would be easier to visualise the changes in air density.

What I described above is the action of a single acting CSU on a single acting propellor (oil controls the pitch in one direction, a spring or compressed gas controls the pitch in the reverse direction in conjunction with blade CTMs or ATMs depending on the construction of the blade). There are double acting propellors and double acting CSUs. With double acting CSUs oil from the CSU controls both fine and coarse pitch settings. Single acting CSUs can be used on double acting propellor blades but double acting CSUs can only be used on double acting propellor blades.

[Edited 2003-09-13 00:03:14]
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RE: A/C Engines - Gearbox?

Sat Sep 13, 2003 6:24 am

It's not good for the engines, your altitude/airspeed, or the airport fence.

that's a good one!!  Laugh out loud
"...cannot the kingdom of salvation take me home."
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RE: A/C Engines - Gearbox?

Tue Sep 16, 2003 12:59 am

Well, the CSDs I’m familiar with do have hydraulic motors (some IDGs do it all electronically but I won’t even take a stab at describing what kind of black box magic they use). CSUs/PCUs are something completely different and don’t. Excellent description of the latter though.

Except for one minor nitpick: Clouds typically aren’t denser than the surrounding air. If they were, they’d sink but they don’t. In fact, air inside clouds tend to be rising... at least until they become CBs, and by then their density probably isn’t the property you’re most concerned about.  Big grin But as you said yourself, it is really secondary and does not invalidate your description of the inner workings of a PCU.

With the clouds out of the equation, increased airspeed and/or increased shaft power are the two main reasons for increasing the blade pitch to keep the prop RPM constant.

I’d like to add that propeller control oil pressure can work towards either fine or coarse pitch in a single-acting prop. In multi-engine installations, you want the prop to revert to coarse/feathered if propeller control oil is lost. You have one more engine to fly on, and if the engine dies it won’t get stuck in fine pitch with the associated windmilling and drag. You’ll want to prevent this from happening on shutdown though, as starting a recip engine (or a turboprop engine without a free turbine) with a feathered prop puts too much strain on the engine.

In single engine installations, you want the prop to revert to fine pitch if oil pressure is lost. Your maximum airspeed will drop, but you will be able to continue flying and you will have full power available at low speeds, should you need it for say a go-around.

You can have centrifugal weights on the prop assy, pulling the blades to a predetermined pitch should oil pressure be lost.

Finally, a few questions which might or might not be stupid. How would a single acting CSU be used on a double-acting prop? What would drive the prop back? Would you apply a constant reduced oil pressure to the other side? And if you are prepared to go through that hassle, what would prevent you from using a double-acting CSU on a single acting prop, leaving the “spring side” oil pressure free to drain to the sump (preferably through a restrictor).

I’m fairly sure that the gas coming out of the combustion chamber is what drives the turbines - which in turn transfer power the gear boxes. Interesting typo.  Smile The exception would be that in some engines, power to spin up the engine during engine start might come through a gear box. E g, you can use a generator or hydraulic pump as an electrical or hydraulical engine to get the engine to turn.

I thought I was doing good trying to avoid those airport hotels... and look at me now.

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