Inertial Particle Separator & TO Distance [Archive]

FoolCorsePich

10th Aug 2015, 09:35

Thanks for the responses but perhaps I asked the wrong question. I'll rehash my question. The question was not why does torque reduce with the intake in bypass. I dare to think I'm a smart enough cookie to understand that one.

Two identical aircraft to be piloted identically line up. Both are set at max torque and max rpm for the take off. One is in bypass and the other is not. Both ITT's are within limits though the one in bypass is or will be at a higher ITT.

Brakes are released.

Which aircraft will reach 50 foot first?

FoolCorsePich

10th Aug 2015, 10:10

Thanks Slam, yeah I get it now.

I'm not done yet though. I've got some more verbal diarrhoea that needs dealing with.

Say I'm operating in a cold environment where ITT limits never become a limiting factor. The strips are so short that the 3% required addition for the separator is the difference in being able to take off or not. Seeing as you've mentioned the performance with or without the separator will be identical would I be displaying gross negligence in deciding to omit the 3% from my take off calcs?

I'm just trying to stir some technical discussion that might assist me with my understanding before my endo. Most of my colleagues claim you never get temperature limited unless operating in 35C+ weather. It just perplexed me as to why apply such a blanket requirement with such a small overall effect when it really only might have an effect on the warmest of days.

UnderneathTheRadar

10th Aug 2015, 10:49

Without knowing for sure, I'd suggest they've proved that no matter what the outside conditions, it's never worse than 3% - ergo, blanket 3% added.

Much easier than doing all the testing again.

UTR

Lumps

10th Aug 2015, 10:54

You might then say, "but I can still get takeoff torque with the separator open!". I would then say, "not always".

Then I would say "then why does it have in the excerpt shown by FCP say increase TO distance by 3%?"

It does not mention temperature, either ambient or ITT. If 'not always' then why a blanket 3%

A head scratcher for me too this one. If it was an ITT / OAT combo then it wouldn't always be 3% - TO distance would increase as a variable of the ITT limited power output.

UTD - just read yours. Can't think of any other reason, sounds plausible to me

Captain Nomad

10th Aug 2015, 12:28

Guys, I think you are assuming that in most cases, except for hot temps, you will get max take off torque with the separator in BYPASS. I would think this would be the exception rather than the rule especially when you consider the torque limiter or lack thereof.

As far as I know, unless things have changed, the 750XL does not have a TQ limiter. This means your meticulous engineer would have done ground runs and adjusted engine settings so that you can ram your power lever forward and not exceed the maximum TQ (or at least not by much) with the system in NORMAL. This means, with the disrupted airflow when the separator is in BYPASS (actually 'closing' a door in the 750XL), the TQ WILL drop and be less than what it just was with the system in NORMAL. It may not be a drop by much at SL on a cool day, but the effect would compound with increased altitude and temperature. By the way, speaking from experience, you can get the reduced take off TQ as a result of: separator in BYPASS, increased field elevation, or increased temperature and still not be ITT limited.

Two identical aircraft to be piloted identically line up. Both are set at max torque and max rpm for the take off. One is in bypass and the other is not. Both ITT's are within limits though the one in bypass is or will be at a higher ITT.

The bit you have missed is that, although both are at 'max torque' (power lever fully forward), there is a different number on each TQ display. The aircraft in BYPASS achieving the lower torque. Think of TQ as being like your manifold pressure power measurement on a piston and you can answer this question.

3% on 500 metres is 15 metres so not a whole lot to be bickering about...

would I be displaying gross negligence in deciding to omit the 3% from my take off calcs?

Braver than me...

Lead Balloon

10th Aug 2015, 23:14

would I be displaying gross negligence in deciding to omit the 3% from my take off calcs?Don't know about "gross negligence", but you'll look like a "complete doofus" and receive a "big bill" if you need the extra 3% but don't have it. An unnecessary, self-inflicted hole in the Swiss cheese.

OCTA

10th Aug 2015, 23:26

Captain Nomad,

I hope the engineers are not adjusting the engine not to exceed max Tq. The power lever should be set that max Ng or gas generator speed is reached with the power lever all the way forward. Remember the power lever just controls the amount of fuel going into the engine nothing more.... (Ignoring beta and reverse of course)

FGD135

10th Aug 2015, 23:54

Remember the power lever just controls the amount of fuel going into the engine nothing more.... (Ignoring beta and reverse of course)
Actually, it controls the target Ng speed. The fuel control unit then varies the fuel flow so as to keep the engine "on speed".

One of the best questions you can ask a newcomer to turboprops is:

"What does the power lever do?"

Sounds simple, but when you do, you tend to get a range of answers back - none correct.

It does not control fuel flow. It does not control power. It does not control torque.

I once heard a pilot describe the power levers as "the torque levers"(!). I was stunned. Seems he had noticed that, when you moved them, the torque changed.

Well, so had the ITT and Ng!

The levers that would have most claim to a description like "torque levers" would in fact be the prop RPM controls.

yr right

11th Aug 2015, 08:51

As the power lever is increased this moves the metering computing section in the FCU, this now allows extra fuel (Wf} which in turns increases N1 and as such ITT. This increase follows till the computing section reaches the position set by the power lever. Tq there fore follows to increase. However Tq can be adjusted via the Prop Gov via the prop lever.
The prop is set via the Flight angle stting on twins and via the prop shop on most singles, this setting WILL NOT STOP an over Torque.
The Tq limiter only drops Px air px which inturns resets the FCU computer and takes fuel away from the Fcu and there fore a decrease in Wf and slows the engine down.
The engines performance really all comes down to the condition of the air gap at the compressor turbine. This is what you damage when you start when your voltage is below what it should be and you hot start.
And remember in a PT6 the engine is in two parts the compressor section and the power section and its air coupled between the two sections,
this is just a basic run down of the PT6

GTang

11th Aug 2015, 09:46

Then I would say "then why does it have in the excerpt shown by FCP say increase TO distance by 3%?"

It does not mention temperature, either ambient or ITT. If 'not always' then why a blanket 3%

A head scratcher for me too this one. If it was an ITT / OAT combo then it wouldn't always be 3% - TO distance would increase as a variable of the ITT limited power output.

UTD - just read yours. Can't think of any other reason, sounds plausible to me

Assuming not being temp limited, to achieve the same torque with IPS on you will have to push the power lever further. One effect I can think of is increased spool up time, although might be a very small effect.

With regards to power lever controlling target Ng, how come when I push it forward more it stays and never goes above 96% on a certain PT6 I've flown?

SPUDO

11th Aug 2015, 10:30

how come when I push it forward more it stays and never goes above 96% on a certain PT6 I've flown?
Torque limiter in action. It acts by limiting the fuel flow.

As the power lever is increased this moves the metering computing section in the FCUNot in the PT6 - unless your definition of "metering computing section" is very broad. The cable from the power lever goes to the speeder spring tension. Nowhere else.

Remember the power lever just controls the amount of fuel going into the engine nothing more.... I believe this is true for the Garrett/Honeywell TPE engines (in the flight range). Have never flown one but spent a few hours searching online for this information today. Perhaps a Conquest or Metro pilot could enlighten us.

If true, it would give rise to the need for these pilots to be especially watchful of the ITT during the climb.

Captain Nomad

11th Aug 2015, 13:16

I hope the engineers are not adjusting the engine not to exceed max Tq.

Fair comment. My comment probably better suited to describing the process of setting the TQ limiter for an aircraft which actually has a TQ limiter.

Having said that, without a maintenance manual to reference, I assume there would still be a 'target TQ' and max power would be assessed against a chart when conducting engine performance runs. It would be expected to conform within a certain range.

From the 750XL manual:

The power lever is used to control the compressor speed and to control the propeller
pitch in reverse. The power lever is connected to a cam-cluster located on the
accessory gearbox via push/pull cables. The cam transmits power lever movement to
the fuel control unit which modifies the fuel flow to the engine and Ng speed. In the
forward operation mode, the power lever controls Ng speed only and has no effect on
the beta valve.

NG GOVERNOR. The Ng governor controls Ng as a function of the
power lever angle. Driven by the accessory gearbox via the fuel pump,
the Ng governor flyweights rotate at a speed proportional to compressor
speed. The cockpit power lever is linked to the speed-scheduling cam
located inside the governor section of the fuel control unit. Movement of
the power lever from idle to a higher position increases the tension on the
spring and closes the governor bleed to prevent Py from venting to
atmosphere. The centrifugal governor flyweights apply a force in the
opposite direction to reopen the Py bleed.

Stikybeke

11th Aug 2015, 23:53

Internal separator opens the by pass and as such you loose ram air affect. If you install raisebect ram air recovery it doesn't matter then.

I'll be blowed. That's exactly how it was explained to me way back👍 I was also told to always add the distance just in case something got sucked in because with it in the bypass position the air doesn't go into the engine but rather back out one of the cowls.

yr right

12th Aug 2015, 02:40

[QUOTE=SPUDO;9078533]Torque limiter in action. It acts by limiting the fuel flow.

Not in the PT6 - unless your definition of "metering computing section" is very broad. The cable from the power lever goes to the speeder spring tension. Nowhere else.

Oh what about the speed scheduling cam. Or dose that not matter. Which in turns sets the fcu fuel schedule for the computing section of which it is a part off. Prehaps you could enlighten us of what p3, px , py air dose and how it's use in the computing section and how it's changes effect the wf or is that too detailed. What I said what broadly speaking. If you like a full detail I'll let you know.

FGD135

12th Aug 2015, 03:53

Oh what about the speed scheduling cam. Or dose that not matter. Which in turns sets the fcu fuel schedule for the computing section of which it is a part off.You're talking about beta/reverse. SPUDO was probably referring to the flight range. Yes, there is mechanical linkage from the power lever to the fuel control unit, but it only applies during beta/reverse. Captain Nomad gave us this information a few posts ago.

A question for you, yr_right: What is the purpose of that mechanical linkage (which only applies during beta/reverse)? I know the answer, but am giving you the opportunity to put it.

yr right

12th Aug 2015, 07:25

Pre haps you should tell me what the reverse lever pickup dose on the topping gov. Ive only held a engineering lic on the whole PT6 series for a bit over 20 odd years work on them in extremely remote areas of the world and Australia. here is your chance to shine.

yr right

12th Aug 2015, 07:40

how come when I push it forward more it stays and never goes above 96% on a certain PT6 I've flown?

Torque limiter in action. It acts by limiting the fuel flow

Whilst the Tq limiter removes fuel via air py bleed air line the fact of 96% is not a case of tq limiting. This is a fact of either engine condition or environment conditions. And with out have the details I would think more engine condition. Generally Tq limiter is set around 104% and above, it differs for each aircraft model. For example a tbm which has a derated engine will never over tq the engine but can over Tq the airframe.

So saying the tq limiter is in action is completely incorrect.

GTang

12th Aug 2015, 09:27

FGD135

12th Aug 2015, 13:50

Generally Tq limiter is set around 104% and above, it differs for each aircraft model.
yr right, Torque limit is not, and cannot, be based on engine speed. There is no direct relationship between engine speed and torque. On a cold day, for example, an engine in good condition could achieve max torque at a speed much lower than some other engine, in lesser condition, on a hot day. There is no direct relationship.

The same engine, even, under two different sets of ambient conditions could reach the torque limit at different speeds. For example, a takeoff at low elevation in cold temperatures and a takeoff at higher elevation in hot conditions on the same day. The Ng is guaranteed to be different between these two scenarios. How can a torque limiter possibly be based on engine speed, given such typical circumstances?

So saying the tq limiter is in action is completely incorrect.The poster gave almost no supporting information, so SPUDO was quite justified in giving the *most likely* reason as torque limiter, given that this is the only time, on a properly operating PT6 engine, that power lever advance should have no effect.

The next most likely reasons are mis-rigged power lever, crook Ng governor or instrument error.

GTang, some more information please. What aircraft type? Does it achieve rated takeoff torque? Have you flown other aircraft of the same type? If so, how do they compare?

yr right

12th Aug 2015, 20:28

yr right, Torque limit is not, and cannot, be based on engine speed. There is no direct relationship between engine speed and torque. On a cold day, for example, an engine in good condition could achieve max torque at a speed much lower than some other engine, in lesser condition, on a hot day. There is no direct relationship.

The same engine, even, under two different sets of ambient conditions could reach the torque limit at different speeds. For example, a takeoff at low elevation in cold temperatures and a takeoff at higher elevation in hot conditions on the same day. The Ng is guaranteed to be different between these two scenarios. How can a torque limiter possibly be based on engine speed, given such typical circumstances?

The poster gave almost no supporting information, so SPUDO was quite justified in giving the *most likely* reason as torque limiter, given that this is the only time, on a properly operating PT6 engine, that power lever advance should have no effect.

The next most likely reasons are mis-rigged power lever, crook Ng governor or instrument error.

GTang, some more information please. What aircraft type? Does it achieve rated takeoff torque? Have you flown other aircraft of the same type? If so, how do they compare?

Clearly you don't understand how and why the Tq limiting works and how it senses it. I would suggest that you find out how it works before you make any more comments. This is because you are incorrect with your statements.

yr right

12th Aug 2015, 21:12

You obviously have never done a preformance run for maintenance either. The difference between two engines with different Tq is more than likely the condition of the engine. It's a lot easier to see a engine loosening power on a twin as you can see the them together as you run them. This is where trend monitoring comes into play.
When you fly what do you set too.
You don't set to N1. However this is the first thing as an engineer we look for , this tells us what the condition of the engine is. For a set Tq ,np , wf we should have a N1 in a region if it's out we ethier have a sad or good engine.
Tq is in the power section in the prop reduction gearbox. Helical gears that allow the disc to move. The more Tq the more it moves. It will not vary the Np when it's in the governed range. Hence you set Np then add power Tq will rise Np will say at the set range. N1 will rise itt wf will rise. Np will stay the same.
If the engine is running to 96% and is itt limited it's a sign of a poor gap seal at the compressor turbine. Not Tq limited

Greedy

12th Aug 2015, 22:11

Getting back to the original question. Could it be that the 3% difference separator on/off is due to account for the change in mass flow through the engine? With the separator on even though the same torque and rpm may be set for take off the mass flow through the engine could be less. Is residual thrust provided by the gas generator exhaust ? ( I'm not familiar with the PT 6 or 750TX installation)

GTang

12th Aug 2015, 23:37

It's an tired Pt6-34 on a PAC XL750. I've flown it in conditions on takeoff from max torque 58psi to much less and Ng will not go above 96%. Also done runs with the engineer. This is the only XL I've flown but the other pilot has over 4000 hours in XLs and the newer engines perform better. Yr right is right on this point.

yr right

13th Aug 2015, 00:15

The 3% loss is what I first said. It's the loss of ram air affect. This means that now you air bypassing air and the engine is basically sucking air in. With the bypass in the closed position the air is being rammed into the inlet. This is where the gain is.

I'll make this clearer. Tq limiter is set via oil px from the Tq ring in side the prg. The Tq oil px equals at over Tq at around the 104%ish mark. Tq limiter wf loss before this is a problem with the limiter and not the engine or and indication problem. You will never over Tq at 96%.

FGD135

13th Aug 2015, 00:16

Getting back to the original question. Could it be that the 3% difference separator on/off is due to account for the change in mass flow through the engine?That reduced mass flow can be compensated for by just pushing the power lever slightly further forward - provided the engine is not already at one of its limits (torque, ITT or Ng). Most of the time, it will be possible to make such compensation.

Does an open separator create slightly more airframe drag? This could be the reason for the 3% factor.

GTang, thanks for that. What happens when you continue to advance the power lever beyond 96%? Does ITT, fuel flow and torque continue to rise, or do they also cease increasing?

If the answer to this is YES, then clearly you have an Ng instrument error.

If the answer is NO, and the torque is less than the limit (if the aircraft has a torque limiter fitted), then clearly the Ng governor has a fault.

yr right

13th Aug 2015, 02:53

That reduced mass flow can be compensated for by just pushing the power lever slightly further forward - provided the engine is not already at one of its limits (torque, ITT or Ng). Most of the time, it will be possible to make such compensation.

Does an open separator create slightly more airframe drag? This could be the reason for the 3% factor.

GTang, thanks for that. What happens when you continue to advance the power lever beyond 96%? Does ITT, fuel flow and torque continue to rise, or do they also cease increasing?

If the answer to this is YES, then clearly you have an Ng instrument error.

If the answer is NO, and the torque is less than the limit (if the aircraft has a torque limiter fitted), then clearly the Ng governor has a fault.

Omg

By facts given it is tried. Sounds more like A HSI is required. I would think that your trends should be looked at. That will tell you your proplem straight up.
If you know your troubleshooting and how to find what the problems with indicated instruments it's not hard to figure out.

yr right

13th Aug 2015, 02:56

Please explain the location of the NG gov.

GTang

13th Aug 2015, 09:31

That reduced mass flow can be compensated for by just pushing the power lever slightly further forward - provided the engine is not already at one of its limits (torque, ITT or Ng). Most of the time, it will be possible to make such compensation.

Does an open separator create slightly more airframe drag? This could be the reason for the 3% factor.

GTang, thanks for that. What happens when you continue to advance the power lever beyond 96%? Does ITT, fuel flow and torque continue to rise, or do they also cease increasing?

If the answer to this is YES, then clearly you have an Ng instrument error.

If the answer is NO, and the torque is less than the limit (if the aircraft has a torque limiter fitted), then clearly the Ng governor has a fault.

Yes, itt, fuel flow and tq increases. When they changed the vane class, Ng and tq improved slightly.

FGD135

13th Aug 2015, 13:44

Yes, itt, fuel flow and tq increases.
GTang,
If this is what you see when doing a static engine run, then it can only be an instrumentation fault. It is physically impossible to get a further rise in those parameters without the Ng also further increasing (in the case of a static engine run). You may be seeing the indicated Ng cease rising, but the actual Ng must still be rising.

On a takeoff roll however, it is perfectly normal to get a rise in those parameters whilst Ng stays constant - this is the ram effect.

Please explain the location of the NG gov.It may not be called that, but I am referring to the mechanism with the speeder spring and flyweights that signals the FCU to increase/decrease fuel flow.

The 3% loss is what I first said. It's the loss of ram air affect.No. I think you will find that the performance tables for this aircraft make the assumption that rated torque is achieved by the engine for every takeoff, including those with the separator open (this can be confirmed by a 750 pilot - he just needs to confirm that the AFM gives one torque value for all takeoffs).

In practice, this is achieved, for a separator-open takeoff, by just slightly increasing the Ng (thus compensating for the reduced air flow). The pilot does what he always does for takeoff - advances the power lever in order to set the target torque.

With the separator open, of course, he may notice a slightly higher ITT. If he is really sharp-eyed, he may notice the Ng is also slightly higher.

That 3% factor on the TO distance, queried by FoolCorsePich, can only be due to the aerodynamic drag of the open separator.

yr right

13th Aug 2015, 22:24

[QUOTE=FGD135;9081079]GTang,
If this is what you see when doing a static engine run, then it can only be an instrumentation fault. It is physically impossible to get a further rise in those parameters without the Ng also further increasing (in the case of a static engine run). You may be seeing the indicated Ng cease rising, but the actual Ng must still be rising

Completely incorrect. If the wrong class ngv is installed you can have itt and Tq within parameters and a low n1. And if they changed the ngv recently this may be the problem. Once again look at your trends. It will tell you the problem

FoolCorsePich

14th Aug 2015, 03:46

That 3% factor on the TO distance, queried by FoolCorsePich, can only be due to the aerodynamic drag of the open separator.

That's an interesting thought. Thank you.

yr right

14th Aug 2015, 06:10

That 3% factor on the TO distance, queried by FoolCorsePich, can only be due to the aerodynamic drag of the open separator.

How do you figure that. If anything the drag will be less not more.
This extra required length in takeoff is consistant of every aircraft I know off fitted with a separator.
The reason for this is simple. Please read.
It's the loss of ram air effect. Nothing more nothing less. And like I said. In a b200 if you fit a raisebeck ram
Air recovery you don't loose any engine preformance with the inertia separator in the open position. If there was a case of drag then this would not make any difference.
This the separator open you loose toque. By memory in a b200 it's around 40lbs. Hence the lose of preformance.

FoolCorsePich

14th Aug 2015, 06:59

Yr right, so the reduction in thrust as a result of losing the ram air effect cannot be recovered in any way?

yr right

14th Aug 2015, 08:39

If you have a beech you can stc the ram air recovery kit. For any thing else not that I know off. It depends where you are if you in hot weather prop not. But they need to look at your trends 1st. That's a must. If the engine tried your behind it straight up. -34 is a small engine and will suffer more than a -42. It's the same with a king air with -41 as to -42. Same power @850hp but -42 higher itt.

Virtually There

14th Aug 2015, 11:07

In the Caravan there is a yawing moment to the right when you engage the inertial separator that needs to be offset with a bit of left rudder trim. This does seem to create a bit of extra drag, as the ASI tends to read lower for the same torque settings.

Conversely, on take-off, you need less right rudder, possibly reducing drag. (But that's not as easy to quantify.)

If you set the engine at take-off power, the PT6 will build an extra 50ftlb or more of torque by the end of its roll-out at Vr due to ram-air effect. You won't see this extra power with the inertial separately engaged, so if you don't touch the power lever, then an aircraft in bypass will need more runway.

The POH says nothing about pushing the power lever forward after setting take-off power with the inertial separator engaged, so this is likely one of the reasons it says to use an extra 3% of runway. That may be due to the fact that an engine in bypass will never make as much power as one without, and will reach its ITT limits faster if you try (by pushing the power lever forward during take-off and accidentally over-cooking it). The PT6 is governed by max torque (which can be exceeded for up to 20 seconds during take-off), max ITT (805C with a five-minute limit over 765C), and max Ng (101.6%) - whichever limit is reached first.

Now, before I posted the above ^ this morning, I thought I'd gather a bit of empirical evidence to confirm all this . . . so I put the Caravan in a low cruise setting (1300ft/lb @ 1750rpm - inertial separators are a bugger to engage/disengage at higher power settings!) wrote down some numbers, and then engaged the bypass. The aircraft immediately yawed to the right, as expected, though this was attributed to the initial loss of power (about 135ft/lb) changing propeller pitch. The aircraft continued to yaw slightly right after increasing power, indicating the likelihood of a front-end drag bias (more drag on the right/oil cooler side, less on the intake/bypass side).

Here are the numbers:

Alt: 9500'
Torque: 1300ft/lb
RPM: 1750

No bypass -
ASI: 128kts
ITT: 642C
Fuel flow: 291lb/hr

Bypass -
ASI: 125kts
ITT: 676C
Fuel flow: 300lb/hr

So there is a small drag penalty (possibly due to rudder trim), but most importantly, a large increase in ITT - almost 35C.

So my guess is, if you set take-off power with the bypass open, you will lose ram-air effect - and power - and risk overcooking the engine if you try to compensate by pushing the power lever forward during roll-out. Thus, you will need 3% more runway with the reduced power without risking overheating the engine with the bypass on.

Lumps

16th Aug 2015, 12:12

propeller RPM x torque x k = shaft hp

Torque is measured in PSI in these kiwi kites. For the PT6A-34AG in this formula k=0.00581

yr right, there does not seem to be a variable in this formula to account for bypass being open or not, which leads to the conclusion that if you can still get the same torque bypass open or closed, the engine is producing the same power and performance will be... the same.

Aerodynamic drag? Maybe. Still going underneath the radar for me

FoolCorsePich

16th Aug 2015, 12:48

What Lumps posted makes so much more sense to me.

If torque is torque and you're under max ITT then have you not just recovered the lost power due to the lack ram effect?

Anyhow, just how much ram effect is there anyway when you're starting to fly passing through 50kts.

Captain Nomad

16th Aug 2015, 14:50

if you can still get the same torque bypass open or closed

...is once again an assumption containing a variable...

Those proposing aerodynamic drag have obviously never looked at the lower cowling of a 750. Nothing extends into the airflow to create drag when the separator is in bypass.

Virtually There

16th Aug 2015, 16:29

Torque is measured in PSI in these kiwi kites. For the PT6A-34AG in this formula k=0.00581

yr right, there does not seem to be a variable in this formula to account for bypass being open or not . . .
Law of thermodynamics anyone? :O Why do ITT and fuel flow both increase for the same torque when intake flow is restricted?

I'm not an engineer, but ram air isn't really an "effect" - it's an intrinsic part of the design of a turbojet that relies on mass airflow for compression and cooling. An inertial separator is a compromise measure that protects said engine whilst hurting efficiency (and any power reserves you might need in the event of an emergency during take off).

It's use has limits, which are reflected in the POH.

I'd also argue that when any bypass is engaged on any aircraft, it must have an effect on aerodynamics (beneficial or otherwise), because it introduces airflow passage to a part of the aircraft where there was none prior.

That effect can be measured by a change in airspeed. Or not.

FGD135

17th Aug 2015, 01:02

Virtually There,

I reckon you get the prize for deducing the correct answer. Congratulations! This was the winning statement:

You won't see this extra power with the inertial separator engaged, so if you don't touch the power lever, then an aircraft in bypass will need more runway.

And, if you are following the 750 AFM (the book) precisely, you won't be touching the power lever. That 3% factor, called for by the book, assumes you are complying with the other things directed by the book for takeoff. Those are:

1. That the takeoff power (64.5 psi Tq, 2006 Np) is set prior to brakes release. (Note that you will almost always be able to achieve this power setting. If the separator is open, that will merely mean a slightly higher ITT and Ng).

2. That the power lever is not touched after brakes release. This means that, during the roll, the ram effect will cause the torque to increase beyond 64.5 psi. This does not mean the engine is being over-torqued.

With the separator open, however, the ram effect is not as pronounced. The result is that at rotation time, the power is slightly less than it would have been if the separator was closed.

The power at brakes release is exactly the same in the two cases, due there being no ram effect. It is the change to the power throughout the takeoff roll that gives rise to the different distances.

yr right

17th Aug 2015, 03:01

Have you actually looked at an intake. If there was no benefit for ram air why have it to start with. When you select an open by pass you are directing the airflow so it dose not enter the engine. In some cases it flow by and in others it also has a cane that comes down.
Preformance loose with out ANY forward movement on a b200 is around 40 Ibs. Close the vane it increase 40 lbs.
but clearly pilots no more than the people that make and fix em.
And there fore it's all magic. Tq can be measured in psi , lbs or even %. But it is comes from the same spot on the prg.
But the fact is ope the by pass and you will loose engine Tq. And if you are itt limited that's your job lot. You can't increase power.

Mach E Avelli

17th Aug 2015, 03:44

Yr right, I have not read anything above that suggests pilots here are claiming to 'no' more than the people who make and fix 'em, though I would venture that some pilots know much more about takeoff performance than some engineers. As one would expect.
In posts 11, 43 and 48 I have read clear, rational replies that tell me all a pilot needs to know to understand the effects of the PT-6 particle separator system on takeoff performance.
In fact those three replies are so good, I will unashamedly use their explanations if I am ever asked the question or need to teach performance.

FoolCorsePich

17th Aug 2015, 04:22

This means that, during the roll, the ram effect will cause the torque to increase beyond 64.5 psi. This does not mean the engine is being over-torqued.

So what you're saying is you'll happily operate the engine beyond its take off torque limitation?

yr right

17th Aug 2015, 05:20

[QUOTE=Virtually There;9084270]Law of thermodynamics anyone? :O Why do ITT and fuel flow both increase for the same torque when intake flow is restricted?

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. ?

FGD135

17th Aug 2015, 05:54

So what you're saying is you'll happily operate the engine beyond its take off torque limitation?By setting the torque to 64.5 prior to brakes release, then allowing the torque to increase with ram effect, you're not exceeding any engine limitation provided:

1. The torque does not exceed 68.4 psi;
2. The time spent above 64.5 does not exceed 20 seconds.

Refer AFM page 2-6. Note that this is the ONLY way to be strictly in compliance with the AFM.

Some pilots believe the published takeoff figure must not be exceeded at any time during the takeoff. These pilots will set a torque figure slightly less than the published, in the knowledge that it will creep up during takeoff. Their aim is to set an initial torque value that, after ram increase, becomes equal to the published limit. This takeoff technique is not in compliance with the AFM. If you want to achieve book performance figures, you have to operate as per the book.

Virtually There

17th Aug 2015, 06:48

Take-off torque is transient in that, once set, you will first see an increase due to ram-air, and then torque will start to decrease as you gain altitude (for the same power lever setting). Normal take-off procedures take this into account.

One thing that hasn't been addressed yet is that extended operation at higher ITTs - even within take-off and climb parameters - will reduce the serviceable life of a PT6. This is one reason why some operators prefer to set take-off torque slightly below maximum (assuming enough runway) with slightly reduced climb-out performance.

yr right

17th Aug 2015, 07:21

Itt is the most critical part of the PT6. The Tip clearance at the CT wheel is were the damage will be done. As your clearance is increased your power will decrease causing more fuel for a given Tq. This is what the original problem in this thread is about. If you over temp you can get blade creep then this will pick up on the ct segments and then your clearance will get larger and larger as more metal is transposed from one side to the other, and a over $1000 per blade and 72 of them im cause your employer will love you heaps

Lumps

18th Aug 2015, 21:56

Well, I'm illuminated. Guess it helps reading the AFM

795 horsepower for 20 seconds! did not know that

yr right

18th Aug 2015, 22:53

Yes and that is why the Tq limiter is set and comes in at around 104% ish NG. The over speed dose not allow you to exceed itt though. The pt6 m/m has a graph for over temp and Tq. Depending how long and temp. Then maintenance required if these are exceeded. And if you have a engine that has a trend monitor then it will show up

FGD135

19th Aug 2015, 00:53

yr right,
You claim to be an engineer with expertise on PT6 engines. You may want to brush up on a few things.

Yes and that is why the Tq limiter is set and comes in at around 104% ish NGThere is no torque limiter on the PAC 750 engine. You probably haven't worked on this aircraft, so I will forgive you that one. But, if there was, it would be based on a torque value, not an Ng speed value.

Being based on a torque value, rather than an Ng speed means that differing ambient conditions would see the torque limiter activate at different engine speeds. On a cold day at low elevation, for example, it could activate at 100.0%. On a hot day at high elevation, it may not activate until 102%.

These are theoretical numbers, not based on any particular engine. I relate them to convey the general relationship between torque and Ng.

The over speed dose not allow you to exceed itt though.There is nothing on any of the PT6 variants that prevents you from exceeding ITT. The primary and secondary prop governors, plus the "fuel topping" governor, are all about preventing overspeed of the power turbine.

yr right

19th Aug 2015, 03:01

Are you really sure you know about a pt6.

Preformance charts state that you should have max Tq prop rpm wf NG itt within a a limit. As a rule max Tq is achieved at between 96 to 100% NG. This is altitude temp compensated and if you on a b200 a correction factor is also applied.
Whilst the Tq is a oil px reading as I've said. Max Tq oil px is around the 104% mark period. And saying that you can't exceed ITT during over Tq that is the pilot to control. Meaning if you go over max Tq you still are temp limited which means if you get to max itt that dose not mean you can go any further. As yes I no a ****e load about pt6.

rigpiggy

19th Aug 2015, 04:13

yr right

19th Aug 2015, 07:02

yr right wrt your previous remarks the the torque drops due to reduced mass air flow to maintain torque you replace air mass with fuel mass, this increases itt's. flying in the north there are many times where we don't get above 92% n1, as such there is a passage in the KA manual that has MX reset the AFX PL column switches to allow it to work.

You flying -41 or -42. You increasing mass airflow by making the compressor work harder. Yes I've done a lot of work north.
By increasing more wf you increase itt and thus increase NG. And if your useing 3 blade props it's hard to reach max Tq before being temp limited even more so if you have -41.

You loste on your switches though. Amusing your talking about a b200

FGD135

19th Aug 2015, 10:42

there are many times where we don't get above 92% n1

rigpiggy,

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?

rigpiggy

19th Aug 2015, 21:25

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? :O 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. ?

rigpiggy

19th Aug 2015, 21:26

BTW when I say north I mean arctic/subarctic not broome/darwin

Virtually There

20th Aug 2015, 02:24

Air is obviously denser in colder climes. I suspect that's why Ng also rises with altitude. Less dense air, easier for the turbines to spin.

Virtually There

20th Aug 2015, 02:59

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 :}

FPDO

24th Aug 2015, 23:35

So what does the Power Lever do?

rigpiggy

2nd Aug 2016, 22:42

Thread Resurrection, no answer given.

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.

FGD135

9th Aug 2016, 05:54

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.Correct, rigpiggy, and congratulations, you are one of a very limited number of pilots that know this.

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).

Snakecharma

9th Aug 2016, 22:27

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.