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Posted: 4/2/2006 7:54:52 PM EDT
[Last Edit: 4/2/2006 7:55:41 PM EDT by captainpooby]
Why as the airplane climbs and the air gets thinner do the pilots open the throttle more? I'm talking PT6 here.

I know there is a magical fuel box that meters fuel based on a number of parameters including barometric pressure but in a piston engine as altitude increases you would want less fuel to run at optimum mixture.

In a DHC-6 the torques pressure drops from 45 as we climb to altitude and the pilot pushes the throttle forward to increase torque. Percent prop RPM, gas gennie RPM and temp remain constant pretty much.

Does the throttle just increase airflow and not fuel? In a piston engine aircraft the pilot leans the mix as you get higher. Explain this to me.
Link Posted: 4/2/2006 7:58:58 PM EDT
The inverse lapse rate (gets colder as you climb) causes density altitude to increase, allowing more fuel based on turbine inlet temperatures. The compressor is pulling in cooler air which although less dense based on pressure, is adequately compensated by the lower temperature.

Odd since recip pilots sense lower partial pressure of O2 and have to lean but the compressor erases this loss.
Link Posted: 4/2/2006 8:09:07 PM EDT
As you mention there is a magic box that measures lots of things. But pay no attention to the man behind the curtain. No really the fuel control adjusts the fuel flow for a given throttle setting. It does this, in the case of an electronic fuel control, by getting signals from the EEC as to how much fuel should be delivered to the fuel nozzles based on inlet pressure, inlet temperature, burner pressure etc... On a hydro-mechanical system the same thing happens just in a different way. Sorry if that is not the answer you are looking for but you would be very bored if I spent the time it would take to fully explain it all.
Link Posted: 4/2/2006 8:16:54 PM EDT
Since you're talking about a turbo prop, you need to remember density, density, density! True the higher a turbine engine goes, the more efficent it is, yet due to lower density, it needs to make more power to equal the same thrust. Also for more efficent cruising, the pilots will reduce the pitch of the throttles, to take less of a bite out of the air, again for efficency, yet they need to increase the throttle to get even more of a bite.


Granted I haven't messed with a turbo prop in years so I might be a bit off.
Link Posted: 4/2/2006 9:28:58 PM EDT
Man even I am confused now. As a jet engine or any engine for that matter gets higher the air gets thinner and the engine has to adjust the fuel flow to maintain the proper mixture of fuel and air. That is where the efficiency comes in, it takes less fuel to operate because there is less air to burn. At some point there is so little air to burn that the engine will flame out. Ideally engines love very cold very wet air both are dense and both allow for ideal compression.

As far as props a lot of turbo props have constant speed variable pitch props that respond to engine speed. i.e. the prop tries to maintain a constant speed. So when the throttle is advanced the engine receives more fuel and tries to increase in speed. The prop being attached to the engine senses this increase and changes/increases pitch to reduce the operating speed back to the normal range. It does the opposite when the throttles are reduced as long as the prop is not going faster than the engine i.e. negative torque. The biggest limiting factors to any propeller driven aircraft are speed and altitude.
Link Posted: 4/2/2006 10:02:11 PM EDT
As an A&P I have no answer. All I do is replace parts and deffer discrepancies.
Link Posted: 4/2/2006 10:03:37 PM EDT
Link Posted: 4/2/2006 10:22:34 PM EDT
[Last Edit: 4/2/2006 10:37:45 PM EDT by BillofRights]
I'm not sure what you mean by "open the throttles more" Turboprops are limited by Torque and Temp.

When you are low, you are (usually) more Torque limited, as you climb, you become more Temp limited. Torque limiting is the answer you seek. You don't wan't to generate so much power that you damage the gearbox.

The fuel governer automatically reduces fuel as you climb. Take a look at the fuel flows, they will decrease.
The mixture and the prop pitch is automatic. The pilots are not increasing torque, they are telling the fuel governor to try to maintain torque up to it's derated max ITT (temp), by adding more fuel.

The throttles control Fuel. The air is unrestricted just like any other jet engine.

eta- It really is PFM.
Link Posted: 4/2/2006 11:19:14 PM EDT

Originally Posted By ARChoo:
As an A&P I have no answer. All I do is replace parts and deffer discrepancies.



Till the last day of the MEL and then get an extension.
Link Posted: 4/2/2006 11:22:42 PM EDT
I guess between us all we have managed to really confuse the issue. If you did not know anything about engines you would be lost in this thread. And we did not even get into syncrophsers or pulse generators or temperature datum controls. I have to laugh when someone talks about how complicated a Rocket is. Jet engines are very complicated yet simple machines that take a while to truly understand.
Link Posted: 4/3/2006 12:52:41 AM EDT

Originally Posted By HercMech:
I guess between us all we have managed to really confuse the issue. If you did not know anything about engines you would be lost in this thread. And we did not even get into syncrophsers or pulse generators or temperature datum controls. I have to laugh when someone talks about how complicated a Rocket is. Jet engines are very complicated yet simple machines that take a while to truly understand.



No shit, ask 10 different A&P's and you'll get 10 different answers. Really I have not touched a turbo prop since A&P school. I went from turbofan engines to radial engines and back to turbofan engines, so as you can see my turboprop experience is pretty much nil.
Link Posted: 4/3/2006 12:53:17 AM EDT

Originally Posted By Surf:

Originally Posted By ARChoo:
As an A&P I have no answer. All I do is replace parts and deffer discrepancies.



Till the last day of the MEL and then get an extension.




DONE THAT!
Link Posted: 4/3/2006 1:25:17 AM EDT

Originally Posted By HercMech:
I guess between us all we have managed to really confuse the issue. If you did not know anything about engines you would be lost in this thread. And we did not even get into syncrophsers or pulse generators or temperature datum controls. I have to laugh when someone talks about how complicated a Rocket is. Jet engines are very complicated yet simple machines that take a while to truly understand.



Not all turboprops are as "Rube Goldberg" as the T-56. That has got to the be worst engine design I have ever laid eyes on. The TD amps are always going out, the throttle linkage is a huge clusterfruck, the valve body looks like a machine shop exploded inside it...I am going to work on those pigs again soon, glad I'm not an engine guy though. I keep busy enough on them just fixing fuel quantity and changing tach generators, fuel flow transmitters, and torque indicators.
Link Posted: 4/3/2006 6:39:25 AM EDT

Originally Posted By Chairborne:

Originally Posted By HercMech:
I guess between us all we have managed to really confuse the issue. If you did not know anything about engines you would be lost in this thread. And we did not even get into syncrophsers or pulse generators or temperature datum controls. I have to laugh when someone talks about how complicated a Rocket is. Jet engines are very complicated yet simple machines that take a while to truly understand.



Not all turboprops are as "Rube Goldberg" as the T-56. That has got to the be worst engine design I have ever laid eyes on. The TD amps are always going out, the throttle linkage is a huge clusterfruck, the valve body looks like a machine shop exploded inside it...I am going to work on those pigs again soon, glad I'm not an engine guy though. I keep busy enough on them just fixing fuel quantity and changing tach generators, fuel flow transmitters, and torque indicators.




Yeah pretty soon you will have to give up the Gucci airplane and go back to working for a living. And the good ole T56 may be complicated but it does the job. Better suited for the mission than that new fangled contraption that is on the C-130J. LOL
Link Posted: 4/3/2006 7:00:27 AM EDT
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown
Link Posted: 4/3/2006 7:18:12 AM EDT

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown





All the P-3 mechs tell the Herc mechs they are the ones upside down! Don't get the engineers started either.. It might come to blows
Link Posted: 4/3/2006 7:38:15 AM EDT

Originally Posted By Valkyrie:

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown





All the P-3 mechs tell the Herc mechs they are the ones upside down! Don't get the engineers started either.. It might come to blows




Bah! Get a real engine. Pratt and Whitney JT8D-15, or Rolls Royce RB.211-22B!
Link Posted: 4/3/2006 7:43:48 AM EDT

Originally Posted By DeltaAir423:

Originally Posted By Valkyrie:

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown





All the P-3 mechs tell the Herc mechs they are the ones upside down! Don't get the engineers started either.. It might come to blows




Bah! Get a real engine. Pratt and Whitney JT8D-15, or Rolls Royce RB.211-22B!

Never thought I'd see the day when there are no DC-9s,727s and L1011s,by the dozens parked at the gates of airports. Sucks to get old!
Link Posted: 4/3/2006 7:45:39 AM EDT

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown

Remember Zantop?
Link Posted: 4/3/2006 7:58:10 AM EDT
[Last Edit: 4/3/2006 8:06:00 AM EDT by ipschoser1]
The PT6 thrust indication is torque. It has a single spool compressor in it's gas generator. As the air thins, more speed is required to maintain optimal torque (more gas to the power section turbines). At some point, ITT will be the limiting factor as more fuel is added. Actually, cruise power is set at 775C, a reduction from takeoff/climbs 785C so, normally, power lever settings are reduced at altitude. (numbers are for PT6A-60A)

On split spool turbofan engines, the N1 (Fan/LP compressor) automatically speeds up to compensate for reduced ambient air pressure at altitude as it's independant from the N2 spool. Less dense air=higher N1. Typically, jets require a reduction from climb power in cruise due to airframe airspeed limits.

BTW, FWIW (not much), I've been to factory PT6 maintenance school. That was a while back though...



Link Posted: 4/3/2006 8:29:04 AM EDT
The good ol' T-56 is the first turboprop to ever go into mass production in this country. Bound to be a little more complicated than stuff thats come along later.
Changing a starter (Air Research brand) on a rickety folding ladder on a dirt runway in Honduras is not one of the Ol' Crew Chiefs fonder memories!
Link Posted: 4/3/2006 8:50:18 AM EDT

Originally Posted By Valkyrie:

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown





All the P-3 mechs tell the Herc mechs they are the ones upside down! Don't get the engineers started either.. It might come to blows



I already started...
Link Posted: 4/3/2006 8:51:54 AM EDT

Originally Posted By MattyMattel:

Originally Posted By ZitiForBreakfast:
The PT6 runs at 100% RPM....Speed, and other limitations ...is based on maintaining that 100%. Need to go faster? Push the throttle ups, pitch changes, taking a bigger bite of air thus slowing the RPM down, more fuel is added to speed her back up...Many turbo prop's are ran like this...

The T-56 is one of the best motors ever made. Not only for the Herc's, but also for oil rigs, pumping stations, etc...

Some dumb reason the guys on P3's mount theirs upsidedown

Remember Zantop?



Zantop is still alive and ( I dont know if well) flying out of YIP. When I operated for Kalitta and KHI we would share parking space together.
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