Inertial Particle Separator & TO Distance
 

Hi All,

I'm about to do a turbine endorsement and part ( amongst others :} ) of an XL manual caught my attention.

http://i.imgur.com/IfBHfcq.jpg

Why would having the intake in bypass increase take off distance when the only variables in the PT6 SHP formula are torque and rpm?

Discuss

And what happens to the torque when the separator is open? There is your answer.


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

FGD = Nail + Head. Thread done!

What happens when it's 35+ degrees OAT and you are temp limited on take off power?

Separator open = less torque in the whole equation.

Why would one have the inertial separators open at 35 C?

It's been a long time since operating an aircraft with them. Where they recommended for FOD mitigation on gravel?

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.

Claret yes, operating on unsealed strips, so a company requirement to have them open when doing so.

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?

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.

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

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

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.

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...

Braver than me...

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.

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)

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.

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

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?

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.


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.

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:

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.

[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.

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.

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.

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.

Spot on yr right

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.

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

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)

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.

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%.

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.

Please explain the location of the NG gov.

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

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.

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.

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.

[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

That's an interesting thought. Thank you.

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.

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