A few Caravan / PT6 questions...

[lilflyboy262]

Not too sure if this is meant to be in the maintenance section or not (Mods feel free to move it).... And excuse the English and lack of proper terms but its been a long day in class.

But here goes!

Ok the first.

On the top of the panel, on the pilots side, there is a placard that shows 4 RPM and Torque limitations.
Now I know that (Torque x RPM) / 5252 = SHP
The engine is rated to 675 SHP.

The first total is at a reduced torque, but max RPM, and it = 674 SHP
The second total is at max torque, but a reduced RPM, and it = 675 SHP

The engine can run all day at a reduced torque, and max RPM or vice versa, so that has me thinking, since max RPM and max torque would give around 705 SHP, is the 675 SHP a engine limitation, or an airframe limitation?

Question number two.
Once the fuel goes through the flow divider. It passes around the engine to the 10 primary nozzles and the 4 secondary nozzles.
There is no pump or anything like that, the fuel just passes through the pipes to the nozzles.
How does it find its way equally to all of the nozzles....? Like how does the first nozzles not end up with more pressure, while the top ones end up with next to none?

I just figured since fuel is a liquid and its not compressible, once it reaches a certain pressure in the nozzle as it tries to force its way out of the nozzle hole, it takes the path of least resistance and moves on to the next one, and so on and so on.

But that seems way too basic, but looking at the engine, its just a ring around the outside and into the nozzles... can't see anything special.


Question number 3.

What indications would a blocked fuel nozzle give you?
I realize that this is highly unlikely as the fuel passes through a filter, a 75 micron mesh, and a 10 micron mesh.... BUT... If it happened... What would I likely see?
I'm guessing a reduced torque, higher ITT's than normal for the torque setting, and inability to produce full power.

Would this damage the burner can by having an area that is not burning?

[lilflyboy262]

Anyone?....
Usually, you guys are great at this kind of stuff!

[ahramin]

Well since no one else has, I'll tackle a few.

2. Each nozzle has a very small, very precise hole. That hole not only controls the pattern of the fuel sprayed but also the amount. Each nozzle can only use a fraction of the fuel supplied by the high pressure pump. There is plenty of pressure for all the nozzles.

Nozzles must periodically be checked to make sure they are clean and spraying the correct pattern. I have a picture here somewhere of the homemade rig one of the pilot/engineers made for nozzle testing. Basically you take the nozzles one at a time and plug them into a fuel supply and sit in a cloud of jet fuel looking at the spray pattern coming out to make sure that it is circular and even. If you are interested go to your local diesel engine service shop and ask them to see how they check fuel injection nozzles.

3. If the nozzle is blocked or - far more common - not producing an even spray pattern, the max output for the engine will be reduced, and the ITT for a given torque will be higher. Worse case scenario you are going to damage something, but I've never heard of this happening. Maintenance intervals are designed to catch these sorts of things long before they turn into a problem.

[DHQ]

My information is mostly from hangar talk and not based on any reference, but here are some guesses/opinions.

1. I've heard the HP limitation is based on the reduction gearbox.

2. I think you have your own answer there: "I just figured since fuel is a liquid and its not compressible, once it reaches a certain pressure in the nozzle as it tries to force its way out of the nozzle hole, it takes the path of least resistance and moves on to the next one, and so on and so on"

3. I was just discussing fuel nozzles recently with an AME. I'm not sure about a completely blocked one, but we were talking about carbon build-up. Apparently an engine with dirty nozzles will have a much weaker/slower start-up than normal.

[Canoehead]

The derated power may also be due to a limitation of the engine driven fuel pump. I seem to recall this was the case with the -135 engines on the Cheyenne IIXL. But that was a while ago- I may be off base.

[letsfly]

Lets see if this makes sense.
1) HP is based on gear reduction and the fuel to air ratio, subject to the condition of the fuel pump. Weakened it will loose power.
Airframe not limited as we are now seeing supervans at 900HP.
2) Fuel nozzles when new or clean have equal amount of flow. Should one become plugged or restricted the amount of the others does not change. Example> Take a garden hose full flow and the drop of the end will be 3-5 inches dependant on the pressure. Take you finger and try to plug it and the spray will be much further, yet the pump output has not changed at all. PSI output unchaged but FPS has changed.
3) First indication of plugged nozzles could be a longer start up and certainly a potentially hotter startup as well. When all is clean, time the startup and the temp and there after use those numbers as references when you feel a change occuring.