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 Post subject: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Fri Aug 22, 2008 5:21 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
As a pilot, I understand the POH claims that in order to achieve max rated horsepower, the prop must be set at or above 1800RPM. Keeping in mind that obstacles are cleared and a cruise climb is desired, Cessna only provides performance data for 1750RPM at 115KIAS climb. Does anyone have any performance data on torque/prop/fuel flow/ITT temp other than 1750RPM?

The reason I am asking is because the POH only mentions that the pilot may select 1600-1900 RPM as long as we observe engine parameter limitations as per POH, but there is no mention on whats best for economy/performance/engine health.

What is the best climb/descent prop RPM at close to max gross weight from PA of 1000 up to 12000 in order to achieve the highest IAS/Climb/Fuel Economy/Engine health ? Are there any documents which would support any answers?

Thanks,


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Fri Aug 22, 2008 6:57 pm 
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Joined: Fri Oct 29, 2004 3:13 am
Posts: 56
Location: Niagara Falls Area
The min rpm would be for cooling concerns (keeping stable temp) ... where tendencies to approach max-hp inside of a max-weight climb will heat-up more-rapidly with the smallest positive variations in pitch esp at higher IAS. Economy is likely going to remain secondary-concern to the more-extreme cost (or risk to safety) where peak ITT arises from too low of a shaft rpm, the slower engine airflow.

(If you can study the real live 1/4 cut-out section of a PT6 engine at the science centre ... it makes you realize just how skinny/small its interior really is ... for making all that power while cooling adequately.)


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Sat Aug 23, 2008 1:46 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
PDW,

Thanks for your perspective on this one. It makes sense as I did notice in cruise climb that a slightly lower ITT temperature will exist for the same IAS/FTminute the lower the prop RPM is set. Since I couldn't find anything solid in the documentation, I'm still in search of some figures to support the reduction of prop RPM for more than just "passenger comfort" as mentioned in the POH.

Have you ever seen any figures or charts about this?

Thanks again for your input,


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Sat Aug 23, 2008 1:42 pm 
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Joined: Fri Oct 29, 2004 3:13 am
Posts: 56
Location: Niagara Falls Area
The opening statement of this thread.

Quote:
I understand the POH claims that in order to achieve max rated horsepower, the prop must be set at or above 1800RPM.


To get the best horsepower: if the POH is stating 1800RPM as the minimum rpm, then if everything else stays same except going to 1900RPM, the fuelflow should draw a bit higher too ... and give ITT the chance to dwell somewhat lower.(my thinking anyway)

Climb performance (torque) charts were discussed recently on another thread .... after some recent updating/changes have apparently tightened torque-limit downward some more ... which brought up some questions from max-gross operators at certain hotter/higher elevations.


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Mon Aug 25, 2008 1:47 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
Thats along the lines of what I was thinking too. Now if I could only find some published info on this it would be great.

Do you remember the name of the other thread dealing with this?

Thanks,


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Mon Aug 25, 2008 3:00 am 
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Joined: Fri Oct 29, 2004 3:13 am
Posts: 56
Location: Niagara Falls Area
The recommended power settings are in the POH.

In the Caravan there is the short/lighter scenario and the longer/heavier possibility, where the former is for floats, so in winter many of those don't have a pod.... but the extended frames usually do; ... that can make two very different power-need/setting-requirements esp for loading and eventually even bulkier/etc in icing.

Fuel/fuel-economy did not seem to be in operational safety concern (much as rising fuel costs are serious), given that main focus was sound power management routines (torque settings) for safe ITT-trends ... ongoing effort in prevention of heat-deterioration within the turbine-engine usually when forcing the higher hp's.


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Sat Aug 30, 2008 1:50 pm 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
PDW,

I'm with it, but the POH doesn't specify anything for climb other that 1750RPM and 115 KIAS in terms of performance for the 208B (heavier/longer) with/without cargo pod. The prop setting and recommended RPM for 1600-1750-1900 have figures for cruise, but again I'm looking for climb data. I've been hearing different things about keeping the ITT below 700 for cruise, but the POH gives the recommended power settings by torque/prop rpm. Any published stuff on this? Again, I really appreciate your replies and explanations.

Thanks,


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Sat Aug 30, 2008 9:42 pm 
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Joined: Thu Aug 14, 2008 2:49 am
Posts: 1
Location: BHM
The way my company wants us to fly the van is prop always at 1900rpm max ITT in climb 765 max ITT in cruise 700 with of course NG below 101.6% and Torque below redline.


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Sun Aug 31, 2008 11:46 pm 
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Joined: Thu Jan 31, 2008 11:59 pm
Posts: 15
Bassman, shoot me a PM if you want. With some more information from you, I might be able to shed some light on your question. Might is the key word....no warranties!


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Tue Sep 30, 2008 4:18 am 
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Joined: Fri Oct 29, 2004 3:13 am
Posts: 56
Location: Niagara Falls Area
Starchecker wrote:
The way my company wants us to fly the van is prop always at 1900rpm max ITT in climb 765 max ITT in cruise 700 with of course NG below 101.6% and Torque below redline.


1)"1900 rpm max ITT in climb 765 "

2) "max ITT in cruise 700 degrees "

Here's the thing:
The way I understand it is that no two identical PT6 engines, both doing the '1900 rpm up to a max of 765 ITT', will be producing the same horsepower (hp) due to an engine wear-state that takes them both to "765" where one (despite identical) achieves less hp than the other by the time that temperature raises up that far (or else better-said where one reaches maximum sooner than the other while set at the same hp that-then also forces a levered power-cut-back/hp-reduction sooner (for the hotter engine) to stay under that ITT/temp limit.

It could be the guidelines might not be as accurately defineable as we'd need them to be, mostly since the 'times' left to overhaul are very different from one engine to another, since the precise variation in overhaul state is unique to each individual turbine.

It's OK to remain with time tested guidelines to maintain the engine-life however there are a lot of simpler considerations that are involved in reaching best cost efficiency ... ways to try and curb the big 'input', the daily fuelling. We certainly can all relate to searching out better methods in reducing fuelflow in any legitimate way that is less-expensive/safer.


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Wed Oct 01, 2008 3:13 am 
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Joined: Wed Nov 24, 2004 2:25 am
Posts: 18
Location: Central Texas, U.S.A.
My goal is to make our engine outlive me. With our plane, when taking off a runway with 2500' or more, 7000lbs+- gross weight, I set the ITT at 700o and let her go, climb to altitude usually 10,000-15,000' also at 700o. The first few thousand feet of climb is 120 knots reducing to about 100 knots until reaching our cruising altitude. Only on heavier loads and shorter runways do I find it necessary to raise the ITT as much as 740o. We also cruise at 680o. This is a little slower (168-170 True Airspeed), but pushing the throttle will only produce 5-10 knots. I prefer to keep our engine cool.

Our engine has approximately 1000 hours and we fly the short model with no pod. At this time, when we set the engine to the highest allowed numbers for cruise at a given altitude and outside temp, our ITT is at/below 700o. Our engine is in great shape and it's a fantastic bird!

You guys flying the longer Caravan with the pod probably do not have this luxury.

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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Wed Oct 01, 2008 7:06 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
We are flying the B model with the pod with limited take off weight at 8000 lbs at 30c to clear obstacles on a 2400ft runway which requires us to set the torque to the limit at prop 1900. We never exceed the ITT of 740 even in these conditions and in climb we don't push the ITT higher either. For cruise we set the ITT at or below 700 at elevations of 8500-12500 feet and our true airspeed is in the low 160's. If we push we gain 5-10 knots but the fuel consumption doesn't really make sense for the speed gained.

The thing I'm wondering about though is the prop setting for the cruise climb. What factor do you set your power by on cruise climb assuming you aren't redlining on any parameter?

I know that per climb speed and feet per minute when I reduce the prop below 1900, the torque rises a bit as does the fuel flow, but the ITT seems to drop. We climb at 1750-1900rpm at 110-120KIAS not exceeding 740 ITT and then we have lots of room below redline figures on Torque and Ng. Fuel flow is secondary and doesn't increase that much.

So, the question comes up again, where can we find some written info on the best way to care for the engine in a cruise climb? Is it always a matter of keeping the ITT lowest possible for climb as well as cruise? I wonder why the POH doesn't publish cruise values in terms of ITT (based on a healthy engine)...

Out of curiosity, on batter starts with quick turn around what it your start peak ITT on a 30c day at pressure altitude of 1000 ft?

Thanks for the info and would appreciate any reference to documentation on these matters,


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Wed Oct 01, 2008 2:27 pm 
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Joined: Wed Nov 24, 2004 2:25 am
Posts: 18
Location: Central Texas, U.S.A.
...For our 1500' strip and a hot day (100o F OAT and 1300 MSL), I shoot for the ITT to be no more than 740o during takeoff, and immediately bring it down to 700o during climb out. Torque and Ng are well within limits. This cannot be achieved if we are heavy on our short runway, as Torque and ITT would be max out!
...Long runways and medium loads, I set the ITT at 700o and let her climb on her own.
...1900 RPM for all takeoffs and landings, reducing to 1800 during the climb and cruise for noise. Below 1800 produces more vibration than I prefer.

...As for battery starts for quick turn around on a hot day such as 100o F, ITT will be 800o-820o. If it is 200o or above before starting, I motor it (fuel pump on) for 15 seconds, wait 30 seconds, and then start with the Ng at least 16% and the ITT at or below 100o before adding fuel. This usually takes another 10-15 seconds on the starter. Gotta have a good battery for all this to work! For a turn around start, mine will be showing 26 volts before all this begins and 20 volts during the actual cranking before fuel is added. And it makes a big, big, difference if we can be facing the wind, as the temp will 800o or below during the start.

How and what do others do?

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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Mon Oct 06, 2008 3:33 pm 
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Joined: Fri Oct 29, 2004 3:13 am
Posts: 56
Location: Niagara Falls Area
For that, also need to identify heavy or light, pod/no pod (long van) or whether with floats, pod or plain (short van).

[Lazy Eagle flies plain and light ... fastest and cheapest (fuelwise)]


Bassman: (podded and heaviest)
When going "5-10kts" faster than "160" the above configurations will all affect engine differently .... and would make such request (thread topic) likely impossible ... ie "documentation" of precise performance beyond those already listed as we find in the POH and revisions.

(Might need to take in the turbine training course or somekind of 'refresher' to get the most accurrate/recent details on this.)


 
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 Post subject: Re: Prop setting in a cruise climb/descent - Benefits vs Cost
PostPosted: Tue Oct 07, 2008 4:00 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
I spoke in person with a PT engineer/maintenance specialist which is responsible for hot section inspections in our area of operation. Performance wise, we refer to the POH. In terms of raw engine care regardless of the installation aircraft, lower ITT during all phases of flight is the most important factor in extending engine life. Like many of you and PDW explained before: ITT temps as low as practical will help the turbine life, while the performance for maximum continous ITT temps are safe and calculated for operation they do wear the engine quicker.

All in all, I still don't have any documentation from PT or Cessna to support this, it seems to be the basic principle of turbine health depends primarily on ITT temps being as low as possible.

The rep did mention a book by PT regarding power management for pilots, which he is looking for and will share with us soon. I will be happy doing followup posts when I have more info. Thank you,


 
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