Inertial Particle Separator & TO Distance

[FoolCorsePich]

Hi All,

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



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

[FGD135]

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

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".

[OCTA]

FGD = Nail + Head. Thread done!

[kingRB]

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.

[Capt Claret]

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?

[yr right]

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.

[kingRB]

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

[FoolCorsePich]

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]

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]

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]

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]

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]

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]

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]

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]

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]

Originally Posted by Lumps

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]

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 FCU

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.

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]

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]

Originally Posted by yr right

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.