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Inertial Particle Separator & TO Distance

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Inertial Particle Separator & TO Distance

Old 14th Aug 2015, 09:39
  #41 (permalink)  
 
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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.
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Old 14th Aug 2015, 12:07
  #42 (permalink)  
 
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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.
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Old 16th Aug 2015, 13:12
  #43 (permalink)  
 
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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

Last edited by Lumps; 16th Aug 2015 at 13:13. Reason: Spellin
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Old 16th Aug 2015, 13:48
  #44 (permalink)  
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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.
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Old 16th Aug 2015, 15:50
  #45 (permalink)  
 
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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.
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Old 16th Aug 2015, 17:29
  #46 (permalink)  
 
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Originally Posted by Lumps
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? 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.

Last edited by Virtually There; 16th Aug 2015 at 19:37.
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Old 17th Aug 2015, 02:02
  #47 (permalink)  
 
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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.
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Old 17th Aug 2015, 04:01
  #48 (permalink)  
 
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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.
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Old 17th Aug 2015, 04:44
  #49 (permalink)  
 
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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.
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Old 17th Aug 2015, 05:22
  #50 (permalink)  
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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?
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Old 17th Aug 2015, 06:20
  #51 (permalink)  
 
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[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?

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. ?
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Old 17th Aug 2015, 06:54
  #52 (permalink)  
 
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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.
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Old 17th Aug 2015, 07:48
  #53 (permalink)  
 
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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.
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Old 17th Aug 2015, 08:21
  #54 (permalink)  
 
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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
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Old 18th Aug 2015, 22:56
  #55 (permalink)  
 
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Well, I'm illuminated. Guess it helps reading the AFM

795 horsepower for 20 seconds! did not know that
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Old 18th Aug 2015, 23:53
  #56 (permalink)  
 
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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
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Old 19th Aug 2015, 01:53
  #57 (permalink)  
 
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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 NG
There 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.
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Old 19th Aug 2015, 04:01
  #58 (permalink)  
 
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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 shite load about pt6.
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Old 19th Aug 2015, 05:13
  #59 (permalink)  
 
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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.
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Old 19th Aug 2015, 08:02
  #60 (permalink)  
 
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Originally Posted by rigpiggy View Post
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
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