PT6 FCU during start

[m39462]

I've read references that say during startup the fuel metering valve will be at the low flow stop, but I don't see how that can be since Ng will be well underspeed which should drive to max flow. I thought it might be a special case when Px and Py are near ambient, but that doesn't make too much sense because the pressure reference they are working against isn't ambient, it's the low vacuum in the acceleration bellows, and I don't see any indication of springs that might explain a different result at low pressures. What am I missing?

[5LY]

Bleed valves, low air flow?

[rigpiggy]

The fcu is basically off until you get Py air around 50 ish percent. There is a bypass "min flow valve" that once it recieves pressure 100ish psi if i remember. It will give roughly 50% Ng. On the -28's around 90pph, to around 180pph on the -67. From there on the Py acts on the pneumatic bellows"FTG" allowing the valve attached to the governor to control fuel flow.

[m39462]

rigpiggy, I'm aware of the bypass valve in the fuel side that maintains constant fuel pressure differential across the metering valve but has no air connection, and the minimum flow stop on the metering valve itself, but you seem to be talking about a separate Py-operated minimum flow valve. Am I understanding you right? I would expect though, if there were a second minimum flow valve, there would be a second minimum flow adjustment somewhere.

I found a different explanation here. The cross arm that connects the bellows to the metering valve runs inside a torque tube that is connected to the cross arm on one end and to the casing on the other. That torque tube is the (torsional) spring that I couldn't find earlier, and is rigged to bias the metering valve to the low flow stop.


So, whenever Px = Py the bellows does indeed tend to collapse and increase fuel flow, but at low absolute pressures that tendency is overcome by the torque tube contribution. When those pressures get close to the normal operating range though the torque tube effect becomes negligible and the bellows respond to the Px/Py pressure ratio.

[rigpiggy]

Basically the fuel topping governor acts as a stop for p3/py air. When you exceed the 106% it lets out the p3/py. Without the bellows the fcu is choked off. So the min flow is the governing until you start to get pressure, and the secondaries cut in.

Take two paper coffee cups, "fuel cup" cut a square thru both" metering orifice". Turn opposite, that is the primary governor movement. Lift it up, that is the bellows.

Yes is it simplistic, but explain like your dealing with a 6 year old.

[rigpiggy]

I had a great pic from the maintenance training manual that showed the "fuel cup"

[rigpiggy]

http://freebee.ucoz.com/_ld/0/5_PT6A-BendixFCU.pdf

[m39462]

Thanks, I've seen that presentation. It's a great basic introduction, but some of the details it omits in the interest of simplicity I find are important if you want to dig in a level deeper.

[MX Trainer]

Always good to see that someone is inquisitive as to the inner workings of things like fuel control units.

So to clear up some confusion on this post you must understand there are 2 makes of fuel controls used on the PT-6.

The smaller engines use the Bendix fuel control - which has a needle valve in a contoured seat to meter the fuel to the engine. This is the system as explained in the FreeBee publication.

The larger engines use a Woodward fuel control unit that uses a 3D cam and the rotating / lifting (Coffee cup with a square hole reference) fuel metering valve.

The 2 fuel controls operate differently.

So to answer the original question as to how the fuel metering valve is held against the minimum flow stop during engine start the answer is simple. When the valve train is installed the torque tube and metering valve is set to pre-load the valve against the min flow stop.

The information was in the United Turbine document - page 47

reference start
BELLOWS ASSEMBLY
The bellows assembly consists of an
evacuated (acceleration) bellows and a
governor bellows connected by a
common rod. The end of the
acceleration bellows opposite the rod is
attached to the body casting. The
accelerating bellows provides an
absolute pressure reference. The
governor bellows is secured in the
body cavity and its function is similar to
that of a diaphragm. Movement of the
bellows is transmitted to the metering
valve by a cross shaft and associated
levers.
The cross shaft moves within a torque
tube which is attached to the cross shaft
near the bellows lever. The tube is
secured in the body casting at the
opposite end by means of an
adjustment bushing. Therefore, any
rotational movement of the cross shaft
will result in an increase or decrease in
the force of the torque tube. The torque
tube forms the seal between the air and
fuel sections of the control. (The torque
tube is positioned during assembly to
provide a force in a direction tending to
close the metering valve. The bellows
act against this force to open the
metering valve.) Py pressure is applied
to the outside of the governor bellows.
Px pressure is applied to the inside of
the governor bellows and to the outside
of the acceleration bellows.

Reference end

The torque tube will transmit the force from the air side to the fuel side - sealing is done with an o-ring which is a simple effective seal method. The torque tube is not a torsion tube - ie. - it is not a spring and simply serves to transmit the forces.

Having had many of these fuel control units apart I can attest that the valve is pre-loaded exactly as the manual says.

Hope this helps.
author of the FreeBee Bendix FCU document.

[m39462]

MX, thanks for coming to the rescue. I had read p 47 and my earlier post was an apparently faulty attempt to understand what was being described. I hope you'll be willing to shed a little more light on the subject.

Part of the reason I read it the way I did was this bit:

The cross shaft moves within a torque
tube which is attached to the cross shaft
near the bellows lever. The tube is
secured in the body casting at the
opposite end by means of an
adjustment bushing.

That sounds to me like one end of the torque tube is fixed to the cross shaft and the other end is fixed to the case (at least that is what I figured "secured in the body casting" meant). If one end twists with the cross shaft but the other end doesn't move, that sounds a lot like a torsion spring.

You say the torque tube "simply serves to transmit the forces", but transmit them where? To the body casting? The diagram I linked to shows the metering valve being actuated by the far end of the cross shaft, not by anything the torque tube drives, so I'm scratching my head over this too.

The reason I was looking to find a spring somewhere was that without one I couldn't see how the bellows/metering valve would avoid travelling all the way to a stop whenever the Px/Py ratio was off. I think there are two ways to prevent that result in a control system such as this:

1. Rate limit the control (how quickly the metering valve moves) so that the system under control can respond before it reaches the stop. In this application I think that rate would have to be pretty slow because the response times (fuel flow affects N1, then N1 adjusts bleeds that modify Px/Py) are slow, but I didn't see anything that would accomplish this.

2. Add a spring that supplies a proportional counterforce to the movement. Now a small pressure imbalance can cause a small valve motion, and a larger imbalance a larger motion.

The second seemed a more likely candidate, but if the torque tube doesn't do the job is there another answer?