Inertial Particle Separator & TO Distance
Join Date: Apr 2008
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there are many times where we don't get above 92% n1
What is stopping the engine from getting above 92%? I can't think of anything obvious.
Do you still get 2230 ft-lbs torque for takeoff?
Nothing, except overtorquing the engines prior to reaching column switches(92%) for AFX. KA200's with AFX often suffer the same problem. Easy fix FO flies the leg captain holds AFX in test positon bypassing the column switches
[QUOTE=Virtually There;9084270]Law of thermodynamics anyone?
Why do ITT and fuel flow both increase for the same torque when intake flow is restricted? Because you have less mass airflow to maintain torque, add fuel to increase mass flow itt rises due to less cooling air. put EAI "on" your rpm and temp will remain the same, torque will drop. to maintain the same torque you must increase N1
This is because the extra mass airflow entering the compressor is tiring to slow it down. To maintain it set condition of n1 speed an increase of Wf is required. This now will increase itt. Same can be shown when a airconding compressor is turned on. What happens. ?
[QUOTE=Virtually There;9084270]Law of thermodynamics anyone?
Why do ITT and fuel flow both increase for the same torque when intake flow is restricted? Because you have less mass airflow to maintain torque, add fuel to increase mass flow itt rises due to less cooling air. put EAI "on" your rpm and temp will remain the same, torque will drop. to maintain the same torque you must increase N1This is because the extra mass airflow entering the compressor is tiring to slow it down. To maintain it set condition of n1 speed an increase of Wf is required. This now will increase itt. Same can be shown when a airconding compressor is turned on. What happens. ?
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I was talking to our engineer the other week about why the PT6 gives hotter ITT starts as the battery starts to deteriorate. If you don't allow Ng to spin up and stablise before you introduce fuel, or if the battery is low and Ng doesn't spin up as high as it should, both will give hotter starts.
The layman's explanation was the reduced mass airflow between turbines meant less cool air was being introduced, whilst heat had more time to build up between the turbines.
You'll note ITT also rises in relation to torque as you gain altitude or on less dense days. With the bypass engaged, the incoming air mass would also be less dense than with ram-air - which has the same effect.
The difference is, with an intake restriction (bypass engaged), the engine is less efficient and has to work harder - burn more fuel (and create more heat) - to make the same power under the same ambient conditions. That's why ITT and fuel consumption both go up.
At higher altitudes, the engine has less mass airflow (higher ITTs), but is more efficient, so actually burns less fuel.
That's my rather non-scientific understanding of it
The layman's explanation was the reduced mass airflow between turbines meant less cool air was being introduced, whilst heat had more time to build up between the turbines.
You'll note ITT also rises in relation to torque as you gain altitude or on less dense days. With the bypass engaged, the incoming air mass would also be less dense than with ram-air - which has the same effect.
The difference is, with an intake restriction (bypass engaged), the engine is less efficient and has to work harder - burn more fuel (and create more heat) - to make the same power under the same ambient conditions. That's why ITT and fuel consumption both go up.
At higher altitudes, the engine has less mass airflow (higher ITTs), but is more efficient, so actually burns less fuel.
That's my rather non-scientific understanding of it
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In the PT6 the Power lever sets the speeder spring in the FCU. Fuel is added or reduced to hold the N1 at requested RPM.
The question of what the power lever does is a very, very fundamental thing about the PT6 engine and the single fact that promotes the most complete understanding of the operation of the engine.
The power lever does not control fuel flow. It does not control ITT. It does not control torque.
It controls Ng (aka N1) speed, via a speeder spring / flyweight mechanism, known as the "Ng governor".
When the pilot positions the power lever to a particular setting, he is in fact commanding a particular Ng value. The Ng governor will then increase or decrease the fuel flow so as to bring the Ng speed to equal the speed commanded by the power lever position.
With those variations to the fuel flow, of course, you will get variations to the ITT and torque (and prop RPM too, if not yet in the governing range).
I kind of think that the discussion about whether or not the 3% should be added and how you would look like a dufus if you needed the 3% but didn't allow for it is a discussion where people are not seeing the wood for the trees.
There are so many variables that go into operating off limiting strips, runway length and TODR are such a small component.
You could allow the 3% and then lose all of that 3% and much more as you slowly advance the power levers in order to stop the prop governor from hitting the stops.
You could be taking off from a country runway with patches of water on the runway and as you run through the puddles you compromise the acceleration of the machine - you are effectively operating off a contanimated runway.
You could be using standard weights to determine the weight of the aeroplane and be well over the weight you think you are. Not a biggie in a jet but in terms of scale a much bigger issue in an aeroplane that weighs in the 5700 kg mark.
You could have a cargo pod on the aircraft which hasn't been properly accounted for in the performance calculations or operate it into icing conditions with the pod fitted. Many operators in days gone by had cargo pods fitted which if you read the STC said do not operate in icing conditions, which everyone to a man conveniently disregarded.
And none of that even starts to consider the fact that in those types of operations there is generally little to no consideration given to terrain clearance procedures in the event of an engine failure i.e. Pre planned escape procedures nor do the performance figures derived from the book consider obstacles so while the aeroplane might be able to get off the runway if anything goes wrong you are in no mans land.
I appreciate that there is a difference between transport category and part 23 certified aeroplanes, but in my view focusing on whether or not a 3% reduction in TODR is required due to a particle separator is not looking at all the components of the equation.
Having trained a lot of pilots on a number of turboprop and Jet types, I often saw people totally invalidating the performance calculations with line up and power application technique.
The biggest problem is that they very rarely, if ever, saw the aeroplane operate at performance limited weights, and when they did it was with both engines operating so it wasn't a true indication of how the machine would perform in the failure case.
In my experience people are horrified when they sit down and calculate how many track miles it would take to get to a 3 or 4000 ft LSA when the aircraft was performing at its minimum certified performance.
There are so many variables that go into operating off limiting strips, runway length and TODR are such a small component.
You could allow the 3% and then lose all of that 3% and much more as you slowly advance the power levers in order to stop the prop governor from hitting the stops.
You could be taking off from a country runway with patches of water on the runway and as you run through the puddles you compromise the acceleration of the machine - you are effectively operating off a contanimated runway.
You could be using standard weights to determine the weight of the aeroplane and be well over the weight you think you are. Not a biggie in a jet but in terms of scale a much bigger issue in an aeroplane that weighs in the 5700 kg mark.
You could have a cargo pod on the aircraft which hasn't been properly accounted for in the performance calculations or operate it into icing conditions with the pod fitted. Many operators in days gone by had cargo pods fitted which if you read the STC said do not operate in icing conditions, which everyone to a man conveniently disregarded.
And none of that even starts to consider the fact that in those types of operations there is generally little to no consideration given to terrain clearance procedures in the event of an engine failure i.e. Pre planned escape procedures nor do the performance figures derived from the book consider obstacles so while the aeroplane might be able to get off the runway if anything goes wrong you are in no mans land.
I appreciate that there is a difference between transport category and part 23 certified aeroplanes, but in my view focusing on whether or not a 3% reduction in TODR is required due to a particle separator is not looking at all the components of the equation.
Having trained a lot of pilots on a number of turboprop and Jet types, I often saw people totally invalidating the performance calculations with line up and power application technique.
The biggest problem is that they very rarely, if ever, saw the aeroplane operate at performance limited weights, and when they did it was with both engines operating so it wasn't a true indication of how the machine would perform in the failure case.
In my experience people are horrified when they sit down and calculate how many track miles it would take to get to a 3 or 4000 ft LSA when the aircraft was performing at its minimum certified performance.
