jesse
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Look forward to it.
King Air C-90
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wabower
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Let's say that the maintenance guys left the condition levers in high idle position and the pilot didn't notice prior to engine start and left them there during the start sequence. Would a hot start result from this oversight?
I'm not a turbine expert by any means, I've never torn into them, but to the best of my understanding, the Power lever operates as a Mixture lever and the fuel controller acts as similar to the throttle since the only thing regulating air through the engine is RPM (no throttle plate) so the controller matches the fuel to the air available at a ratio to provide the power level you select with the power lever (why they don't call them Throttles in a turbine). The power lever says what percentage of fuel flow you desire, then the fuel controller uses that to match fuel mixture required to the air delivered for that power setting. If you are asking for the same power at a lower RPM, you have to flow more fuel to make more torque to make up for the reduced 'time' component of Torque * Time=Power.
If I've got this seriously wrong, I'd appreciate an education.
No clue, but I would think not unless of course you weren't making speed when you start to feed fuel, then it wouldn't matter.
Dave Siciliano
Final Approach
I'm looking forward to your answer Wayne. Of course, we practice aborting a start all the time in training and I've never seen one including on the Garrets in the military. Now, I always heard from someone that knew of someone (g). It seems to be a very low probability event for which we constantly train. Not saying we shouldn't know what to do. I've also never had an engine fail; can't tell you how many times I've practiced for that.
I've also been hit by enemy fire; SIMOM doesn't throw that one at me (g).
Best,
Dave
I've also been hit by enemy fire; SIMOM doesn't throw that one at me (g).
Best,
Dave
We all watched a guy melt down a TPE 331 in a Thrush. When we saw the flames on start up the boss sat down on the ramp and did a double face palm.
Everskyward
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In my observation, the best way to avoid a hot start is to have the batteries charged up or to start on external power. That goes for all the turbine engines I have started. Also, they naturally tend to start hotter at high altitudes. So when you take that King Air to Aspen, don't be surprised. 
Dave Siciliano
Final Approach
Wow! You have me going into Aspen already with the high rollers. It's all down hill from there (g). Actually, I prefer Winter Park, but your point is well made. I did some GPU unit starts here when it was really cold. In summer, GPU is great to get the AC going before pacs arrive (g).
Best,
Dave
Best,
Dave
wabower
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The engines will start normally, assuming the components are working properly. The 80 PSI fuel valve won't open until N1 reaches 12%, which is the sustained RPM necessary prior to the pilot advancing the condition levers from cutoff to idle in the normal start sequence. Selecting high or low idle really doesn't matter either, since the engine will spool up normally to whichever setting you selected. Bear in mind that I'm not advocating any of this stuff, but if you're asking if I have been in a King Air when any/all of this stuff has happened, the answer is jas.
If you want some excitement during a start, simply depress the starter button to motor the engine without spark, then introduce fuel when N1 reaches 12%. After 10 seconds or so in this configuration, quickly raise the starter button from the motor position to the start position, thereby triggering the igniters with an engine full of unburned fuel.
Once you understand what will happen, you'll also understand even better why the aborted start checklist calls for motoring the engine with the condition lever in idle cutoff in order to clear all of the unburned fuel.
PS: I've seen/heard that too.
If you want some excitement during a start, simply depress the starter button to motor the engine without spark, then introduce fuel when N1 reaches 12%. After 10 seconds or so in this configuration, quickly raise the starter button from the motor position to the start position, thereby triggering the igniters with an engine full of unburned fuel.
Once you understand what will happen, you'll also understand even better why the aborted start checklist calls for motoring the engine with the condition lever in idle cutoff in order to clear all of the unburned fuel.
PS: I've seen/heard that too.
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GPUs are excellent things to have around turbines. Not like you can prop them up, and there's no slow cranking hoping it's gonna start.
wabower
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It depends. A simpler explanation is to use the SHP calculation to determine the impact of N2 (prop RPM) changes. Using the power settings that Dave mentioned earlier, his 800# torque X 2,000 N2 X .00199 (constant) equals 318 SHP or 57% power based on his 550 SHP PT6-A-21 engines. If he pulled the props back to 1,900 and torque increased to 840# as a result, the SHP would remain at 318 using the same formula and fuel flow would remain constant. If torque increased to more than 840#, SHP would increase and fuel flow would increase as well, and vice versa.
As I figured, and with that increase in torque will come an increase in temp since the airflow is reduced and fuel/air ratio increased.
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wabower
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If so, the difference would be so insignificant that it wouldn't show on the analog gage in the C-90.
Interesting.
wabower
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Does this mean that you think 318 SHP would burn more/less fuel depending on power control configuration?
gismo
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I think you're forgetting that the free turbine speed isn't tied to prop RPM.
What Wayne describes in post # 169 is referred to as a CEM. (career ending manuver)
Henning, if you decrease N2 without touching the power levers, torque will go up but temps will stay the same. As N2 decreases, torque has to go up to maintain the same HP. HP is a function of N2 times torque.
Once you are temp limited you will not adjust power levers again during the climb but, will simply see a decrease in torque. As you are climbing in a temp limited condition the temps will rise slightly due to the compressor being unable to supply constant air supply so you may have to tweek the power levers down slightly. In a cruise configuration N2 speed will not affect temp as long as power levers are unchanged. This is the reason that temps aloft affect turbines so much. Last November going to Cody WY. I had outside temperature of -40 at FL260. I had a TAS of 296 knots. I took a picture of the ADC just to be able to show off. A summer time speed is more in the 286 knot range. The simple reason is I can burn more fuel at my max ITT with cold air.
Also as I believe Wayne said the engines do not start any richer in high idle than low idle. The fuel control units prevent the high pressure nozzles from activating until enough N1 is achieved to need them.
Henning, what may be confusing you on the temps is that lowering N2 will not change N1. When you reduce the prop speed (N2) the compressor speed (N1) does not change. The Pratts are a free turbine unlike the Garretts.
It is not a rich/lean thing that changes ITT. It is the amount of cooling air available from the compressor section. I don't know if it is true but, one instructor at Flight Safety said approx 60% of the air flow at take off is for cooling. The hundreds of little holes in the burner can "shape" the flame and keep the burner cans cool. It is magic to me.
This is more than you want to know but, this is the purpose of flat rating. I will use the example of the plane I fly. The Cheyenne III has -42 engines where as the IIIA has -61 engines. Both are limited to 720 HP each, max. They will make a lot more power than that. I think the thermodynamic rating of the -61 is close to 900 HP. However, the -61 has bigger compressors and will maintain the 720 HP about 5000 feet higher than the -42. That also makes the IIIA about 20 knots faster and will climb quite a bit quicker to FL280 than the straight III. Turbines are naturally aspirated engines, less air, less power. I don't know if all of this made things more or less clear
Henning, if you decrease N2 without touching the power levers, torque will go up but temps will stay the same. As N2 decreases, torque has to go up to maintain the same HP. HP is a function of N2 times torque.
Once you are temp limited you will not adjust power levers again during the climb but, will simply see a decrease in torque. As you are climbing in a temp limited condition the temps will rise slightly due to the compressor being unable to supply constant air supply so you may have to tweek the power levers down slightly. In a cruise configuration N2 speed will not affect temp as long as power levers are unchanged. This is the reason that temps aloft affect turbines so much. Last November going to Cody WY. I had outside temperature of -40 at FL260. I had a TAS of 296 knots. I took a picture of the ADC just to be able to show off. A summer time speed is more in the 286 knot range. The simple reason is I can burn more fuel at my max ITT with cold air.
Also as I believe Wayne said the engines do not start any richer in high idle than low idle. The fuel control units prevent the high pressure nozzles from activating until enough N1 is achieved to need them.
Henning, what may be confusing you on the temps is that lowering N2 will not change N1. When you reduce the prop speed (N2) the compressor speed (N1) does not change. The Pratts are a free turbine unlike the Garretts.
It is not a rich/lean thing that changes ITT. It is the amount of cooling air available from the compressor section. I don't know if it is true but, one instructor at Flight Safety said approx 60% of the air flow at take off is for cooling. The hundreds of little holes in the burner can "shape" the flame and keep the burner cans cool. It is magic to me.
This is more than you want to know but, this is the purpose of flat rating. I will use the example of the plane I fly. The Cheyenne III has -42 engines where as the IIIA has -61 engines. Both are limited to 720 HP each, max. They will make a lot more power than that. I think the thermodynamic rating of the -61 is close to 900 HP. However, the -61 has bigger compressors and will maintain the 720 HP about 5000 feet higher than the -42. That also makes the IIIA about 20 knots faster and will climb quite a bit quicker to FL280 than the straight III. Turbines are naturally aspirated engines, less air, less power. I don't know if all of this made things more or less clear
No, I expect the fuel for the power to be the same, but I would suspect the temperature to rise measurably with the increase in torque and pressure.
There will be an increase in pressure regardless the mechanism of transfer.
There was a 10% reduction in time for work however you want to divide it up.
Henning, let me try again. An increase in torque caused by a decrease in N2 does not affect the power the turbine is producing. N2 decreased? Then torque will go up and horsepower remains constant. Fuel flow nor temps will change.
Another way to say it: If you change the speed of the prop you only affect the speed of the gearbox, and the PT wheels. The gearbox / PT wheels are not mechanically connected to the CT wheel nor the compressor wheels. You said you expect a temp rise with increase in torque and pressure. What pressure? Only a change in fuel flow (power levers) will change N1 and cause a change in temps.
You seem to think decreasing the speed of the prop will somehow "load up" the gas generating section or reduce the airflow through the gas generating section. You can start a PT6 and run at normal idle with the prop stationary. Your idle speeds, idle fuel flow and temps will be the same with or without prop rotation. Edit: The torque will be higher than if the blades are moving. PT6's are normally started with the props in feather. It may take several seconds after normal idle is established before the prop speed gets up to normal idle speed. As the prop speed is increasing torward normal idle speed you will see torque start dropping. Idle fuel flows remain unchanged during this process.
The 10% comment I did not understand either. If you mean the N2 speed slows 10%? Again if the prop is turning slower then torque must increase to maintain constant horsepower (temperature). Prop speed does not affect the amount of horsepower the turbine section is producing. Only a change in power levers or a change in pressure altitude will do that. This is about the best I can do.
Another way to say it: If you change the speed of the prop you only affect the speed of the gearbox, and the PT wheels. The gearbox / PT wheels are not mechanically connected to the CT wheel nor the compressor wheels. You said you expect a temp rise with increase in torque and pressure. What pressure? Only a change in fuel flow (power levers) will change N1 and cause a change in temps.
You seem to think decreasing the speed of the prop will somehow "load up" the gas generating section or reduce the airflow through the gas generating section. You can start a PT6 and run at normal idle with the prop stationary. Your idle speeds, idle fuel flow and temps will be the same with or without prop rotation. Edit: The torque will be higher than if the blades are moving. PT6's are normally started with the props in feather. It may take several seconds after normal idle is established before the prop speed gets up to normal idle speed. As the prop speed is increasing torward normal idle speed you will see torque start dropping. Idle fuel flows remain unchanged during this process.
The 10% comment I did not understand either. If you mean the N2 speed slows 10%? Again if the prop is turning slower then torque must increase to maintain constant horsepower (temperature). Prop speed does not affect the amount of horsepower the turbine section is producing. Only a change in power levers or a change in pressure altitude will do that. This is about the best I can do.
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That's all well and understood, but to increase torque would require an increase in pressure which causes an increase in temperature. If the same work (horsepower (work)= torque (pressure) * time (rpm). If the same fuel flow produces the same TIT at 10% lower time (2000 revolutions of the prop instead of 1800 to transfer the same work) the prop would be about 3% more efficient at that change in RPM for the TIT to not change.... Maybe, but that's a lot.
Henning, what pressure? If you slow the prop, the torque must increase to keep horsepower constant and vice versa. The gas generator does not care what the prop is doing. The prop speed has NOTHING to do with the power output of the gas generator. You are thinking that when you increase torque you put more load on the turbine. Not so. The gearbox sees more torque over a slower speed thus actual work done remains the same. Your equation is correct. Lets say torque is 2 and RPM is 3 so horsepower is 6. If we decrease RPM to 1.5 then torque must increase to 4 to maintain the same horsepower of six. Torque and RPM are inversely propotional to maintain conatant horsepower. You are not increasing torque due to more power from the gas turbine but, rather due to the slowing of the prop the torque measurement goes up per the equation to maintain constant horsepower.
The prop, gearbox and power turbine are not connected to the gas generator. The gas generator does not even care if any of that is moving. The gas comes out of the burner cans through the compressor turbine then through the power turbine wheels and out the exhaust. The same amount of gas is coming from the gas generator regardless of what the PT wheels are doing.
Airflow through the gas generator is not related to nor affected by the speed of the power turbine wheels. In fact you could take those wheels out and let all the exhaust escape through the exhaust pipes and we would call that a turbojet.
But all of this is academic. Anybody that has operated PT6's knows that changing N2 has no affect on the ITT. It is just a fact. Please feel free to believe what you wish.
Edit: Let me add that the speed of the plane is unaffected by the prop RPM. If you are cruising at 2000RPM (N2) and you pull the props back to 1700 your speed will not change because no more power is being produced. (Blades MIGHT be a little more efficient but, not enough to measure by IAS). However you will have a quieter cabin.
In fact the Cheyenne has a delta torque red line. The torque redline is based on N2. At 2000 it is 1950 pounds but at 1700 it goes up to approx 2300 pounds and is marked on the torque indicators. Piper just does not allow more than 720 HP. You can use any N2 between 1700 and 2000. The 2000 is recommended for TO because the blades are more efficient at slow speeds. Once I accelerate to about 180 knots for climb I pull back to 1800 for noise. That has no affect on performance not ITT. Then in cruise back to 1700 for noise. I have climbed at 1700 and have not been able to discern any difference in climb performance.
The prop, gearbox and power turbine are not connected to the gas generator. The gas generator does not even care if any of that is moving. The gas comes out of the burner cans through the compressor turbine then through the power turbine wheels and out the exhaust. The same amount of gas is coming from the gas generator regardless of what the PT wheels are doing.
Airflow through the gas generator is not related to nor affected by the speed of the power turbine wheels. In fact you could take those wheels out and let all the exhaust escape through the exhaust pipes and we would call that a turbojet.
But all of this is academic. Anybody that has operated PT6's knows that changing N2 has no affect on the ITT. It is just a fact. Please feel free to believe what you wish.
Edit: Let me add that the speed of the plane is unaffected by the prop RPM. If you are cruising at 2000RPM (N2) and you pull the props back to 1700 your speed will not change because no more power is being produced. (Blades MIGHT be a little more efficient but, not enough to measure by IAS). However you will have a quieter cabin.
In fact the Cheyenne has a delta torque red line. The torque redline is based on N2. At 2000 it is 1950 pounds but at 1700 it goes up to approx 2300 pounds and is marked on the torque indicators. Piper just does not allow more than 720 HP. You can use any N2 between 1700 and 2000. The 2000 is recommended for TO because the blades are more efficient at slow speeds. Once I accelerate to about 180 knots for climb I pull back to 1800 for noise. That has no affect on performance not ITT. Then in cruise back to 1700 for noise. I have climbed at 1700 and have not been able to discern any difference in climb performance.
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Torque IS pressure, the pressure is what drives the free turbine disk, torque is the measure of the reactive force to the prop's resistance. The increase in pressure is a reaction to the increase in resistance represented as torque. If you increase resistance on one end of the shaft by trying to do the same work in less time, there has to be an increase in the pressure on a fixed pitch free turbine power take off disc. There is no free lunch in the transference of energy. The only way I can figure it can not increase temp is through an equal increase in efficiency, and the only place that can come from is reduction in prop tip drag. If that's the case, where is the point of diminishing returns? That will be your most efficient cruise RPM for a given fuel flow/temp limited.
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wabower
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But heat is measured at the ITT point between the CT and PT wheels, and the only control that impacts temp (assuming constant PA and temp)is the power lever asking the NG section to build a hotter fire in the burner can. Changing N2 with resulting changes to the gearbox speeds doesn't ask the NG section to build a hotter fire. Everything that changed was contained within the gearbox (N2 decreases, torque increases) but nothing changed the number of logs on the fire.
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HPNPilot1200
En-Route
If you don't increase the power levers then your temp should remain constant. You're not burning more fuel so it won't get hotter.
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Henning, there is more pressure (torque) on the PT discs but not on the CT discs. You are measuring torque on the gearbox not the gas generator. You can keep decreasing N2 untill gearbox failure and the gas turbine is happy until the metal parts from the gearbox gets in the oil system. It is a blowtorch with the flame blowing through turbine blades. The blowtorch is a constant regardless of the blade speed. The blowtorch has a flame that is dependent on fuel and air. Slowing or increasing the speed of the PT wheels do not change the speed of the CT wheel and compressor blades which is mechanically conected. I give up. Good luck Wayne.
Ok, look, I think I been saying this wrong. I'm not denying that it may be true that the engine isn't asking for a hotter fire in the reduction of RPM, I'm just saying that's interesting as it is free energy via increased efficiency from the prop tip speeds, about a 3% gain if what you say is true, it makes me wonder what more can be gained in prop efficiency.
Say I do an experiment to try finding most efficient cruise power setting and speed for the temperature (hi performance cruise) or fuel flow (econo cruise). I pick a minimum cruise speed I'll put up with and set power for that speed, now reduce RPM until speed drops off or temp increases. This will be the most efficient RPM for your econo cruise. Now increase the power ask to max temp. That will be your max performance cruise.
Do you think my presumptions to be incorrect?
kyleb
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Drake the Outlaw
Our Chief Pilot cooked one of the engines in my employer's G-IV and tried to obfuscate the facts. That resulted in a pretty fast shut-down of our entire flight department (Gulfstream + Citation + King Air) and put several people out of work.
wabower
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I can save you some time. Beech already did everything you need to know. Simply look at the max range and max endurance charts in last few pages of the cruise performance charts in the POH. They're presented in landscape format rather than portrait format used for other cruise settings.
I'd love to, can you send me a set? The set of manuals you have for the 200 would be lovely.
I would appreciate an Gulfstream, Falcon, Citation....
Is the speed?
wabower
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Dave Siciliano
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I have the C90 charts scanned in. Unfortunately, the power/cruise charts are pretty limited and only show 1900 rpm for two engine operation. There is a chart for long range cruise, so, that only shows one prop setting also.
BTW, this is a GREAT discussion of how these systems operate. I'm leaning a lot.
Best,
Dave
BTW, this is a GREAT discussion of how these systems operate. I'm leaning a lot.
Best,
Dave
wabower
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In which case you can relax. No leaning is required in your new airplane.:wink2:
So what you are telling me is that the correlation between prop speed and power required is linear?
Everskyward
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This was probably the main problem. I know someone who cooked a jet engine but kept his job because he was upfront about what he had done.
wabower
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Nope, I'm telling you what the book says about performance, and what everyone else who actually flies King Airs instead of hypothesizing about them has been telling you since this thread started.
Changing prop RPM by 100 RPM will cause a slight increase in torque but doesn't cause any noticeable increase in temps. If the temp changes as a result, the change is so slight that the pilot won't know or care. , Those familiar with all of the settings, adjustments and thumb-work that control the information shown on the gages know that it's just a best-guess anyway, so getting wrapped around the axle about the theories is a big WOT.
Knowing where the spare currently limiters are located and which lights and pumps are on the hot bus is far more important to a King Air pilot than this BS.
Changing prop RPM by 100 RPM will cause a slight increase in torque but doesn't cause any noticeable increase in temps. If the temp changes as a result, the change is so slight that the pilot won't know or care. , Those familiar with all of the settings, adjustments and thumb-work that control the information shown on the gages know that it's just a best-guess anyway, so getting wrapped around the axle about the theories is a big WOT.
Knowing where the spare currently limiters are located and which lights and pumps are on the hot bus is far more important to a King Air pilot than this BS.
Yep, typically the engine is insured, it's the dishonesty that kills you. Anyone can make a mistake, have the character to own up..
Everskyward
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Actually I don't think it was but trying to hide stuff will get you every time if it comes to light.