Question for turbine guys

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Let's say we have four turboprop engines. Three are flat rated to 850HP and one is 867 HP. In no particular order they can maintain that power at ISA plus 37,28,49,24 degrees. Flat rating (a number less than an engine's thermodynamic maximum), as I understand it, is established to increase an engine's longevity while also preserving takeoff and climb horsepower in hot/high takeoff. That horsepower number can then be maintained to higher altitudes without overstressing the engine.

The engines are all from the same Pratt and Whitney - Canada PT6A family as some of you will have guessed. They are:

Dash 140 @ ISA +24C

Dash 66B @ ISA +28C

Dash 52 @ ISA +37C

Dash 66D @ ISA +49C

The 66D is kind of an outlier due to its mission but maybe useful for comparison.

My question is (finally, you are saying), that given the closeness of some of these specs why are there so many different ones doing more or less the same job? More importantly, what would the internal differences be to effect these numbers? I ask this because I can read specs but know nothing of the materials, blades, angles, etc. that make them.
 
As a pilot, the technical discussion of this question isn't something I can tag specifically. But to speculate....

Development; I would think that some of the engine variants were early, some were later and the later ones could operate at full power at higher temperatures without overtemping.

I am sure that there could be differences in fuel delivery, compressor design, angle and composition and number of blades/etc. as well as combustion chamber efficiency and materials.

There could also be differences based on a planned aircraft - one may need bleed air for climate control but another may not, which would affect the performance.

There may also be a torque requirement based on the planned propeller usage, so one engine (for example) could be planned for a larger prop and need higher torque limits.

I know that in the Pilatus PC-12 that I flew for 2 1/2 years, the PT6A-67B was derated from 1600SHP to 1200SHP for durability, and that surely would factor into the equation.

There's a mechanic I know that has made some similar inquiries, and apparently the engine builders keep that sort of technical info behind their closed doors and it rarely gets out.
 
Thanks Kuato, appreciate that. I can see how the various platform requirements drive the choices. It's just the closeness of some of these that makes me wonder how it makes sense to P&WC economically. So it seems likely these are modular such that they can mix and match compressor stages, blade makeup, fuel mapping, etc. Three of those engines I referenced are even the same physical size if I recall correctly.

Another comparative example is the -66B (Avanti) and the -66D (TBM) which are nearly exactly the same output-wise except the Avanti has two of them, backwards. I think you're right about the differences. The customer (aircraft manufacturer) generates their requirements for power, obviously, but also for things like warranty risk, marketing considerations to differentiate the product and so on. I'd place my bet on marketing being the driver right behind power. Still, the nagging question remains, why so many of differing engine thermodynamic ratings flat rated to 850HP. Some unspoken convention out there somewhere? Some connection to insurance ala muscle cars? Just kidding.

The PC-12 is sure up there with my wish-list airplanes. Care to share any insights on flying it? Thanks again for your response.
 
I'd also think that accessories mounted on the engine might make a difference as well. Certainly having the Avanti's rear props makes a difference.

PC-12: Great flying airplane. Roll rate a little slow but that make you look smooth.
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Low stall speed and forgiving envelope. Simple and durable construction. Controls unboosted and cable operated, so they get heavy at high speed but the autopilot is very good. Tight getting in but comfortable once in the seat. Can beat a King Air to 25000' but is slightly slower. Fantastic glide ratio for its speed (2.4nm for every 1000') so if you do lose the engine you have plenty of options.

Burns around 380 pounds per hour and can take off with pretty much whatever you can load inside, with ability to go into and out of shorter fields. In some parts of the world they use it on dirt strips; I never did but understand it's very good there also. Ours had a Garmin 430/530 setup, so navigation was a cinch and we also had WAAS so could use GPS approaches like an ILS to get into many back country airports in bad weather.

If I hit the lotto, I'd consider getting one for my personal airplane.
 
Originally Posted By: DeepFriar
Thanks Kuato, appreciate that. I can see how the various platform requirements drive the choices. It's just the closeness of some of these that makes me wonder how it makes sense to P&WC economically.



With regard to the PT6,

Don't forget the prop RPM requirement differs markedly between airframes. And heli versions have an entirely different gearbox.

Some of the older engines have simply been replaced by newer and more efficient ones. Same HP, better efficiency.

We operate 2 identical PC-12's, one with the -67B (older) and one with the -67P (newer) versions of the PT6. The newer engine has the same 1600 HP rating, derated to 1200HP.

1) For the -67B max continuous ITT is 760*C but a practical limit is 720*C. (older pilatus) flat rating 1000HP continuous
2) For the -67P Max continuous ITT is 820*C but a practical limit is 780*C. (newer pilatus) flat rating 1200HP continuous

The newer pilatus is faster, by about 10Kts and climbs a bit better, all while consuming fuel at a slightly higher rate. However, it does not consume as much more fuel per mission as the flow rate might indicate, due to the better climb/cruise and shorter flight.

Interestingly, if we match ITT between the aircraft at say, 720 degrees, the older pilatus is considerably faster. I don't recall if the older pilatus is burning more fuel at this point, but I think it might be (just can't remember) . In any case, the older engine is the more efficient choice for overall operations. I don't believe the newer engine is more efficient, only more powerful at higher temperatures and higher altitudes.

Conclusion: Same ratings, vastly different max cont ITT. Some difference in real world performance at altitude by burning more fuel.


Here is a brochure on the engine upgrade: http://www.finnoff-products.com/files/products/Engine-Brochure.pdf
 
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Thanks Cujet. Yes, the gearbox differences of each is something I tend to forget. These we're talking about are -6A so not for helis but important. I've read that some of the speed improvement in thr TBM900/930 was due to matching the prop to the airframe, both along the empennage and the power pulses into the intake.

About the older/newer engines, although the ones mentioned are all still in production and being delivered it is apparent from your examples that they differ in efficiency/ power which continues to make one wonder about the black art of selection.

As to the -67 variants you mention and their operating temperatures I found a few refs to some DC-3 turbine converters shying away from them and toward the -65AR for instance specifically to increase reliability via lower ITT's. Oh well....

I noticed that Finnoff pioneered the 5 blade prop before it became factory standard. They must be some aggressive folks. The PC-12 is a great canvas to work on. As we hurtle toward the stupid absolute reliance on GPS (with only some VORTACS/DME's remaining) it would interest me to know, due to my background, if an independent inertial could be hooked to the APEX cockpit to increase coasting time accuracy after signal loss/interruption. I asked that very question to both Duncan and Pilatus USA last week and the answer from both was "well, we have dual GPS units". Good Lord.
 
Originally Posted By: DeepFriar
As we hurtle toward the stupid absolute reliance on GPS (with only some VORTACS/DME's remaining) it would interest me to know, due to my background, if an independent inertial could be hooked to the APEX cockpit to increase coasting time accuracy after signal loss/interruption. I asked that very question to both Duncan and Pilatus USA last week and the answer from both was "well, we have dual GPS units". Good Lord.



DeepFriar, we also operate a G650ER and G550. Both aircraft have the Honeywell Planeview, primus epic flight deck. The Pilatus APEX uses a nearly identical set of components. Just one MAU (modular avionics unit) instead of three. And no inertial reference systems, just AHRS. I can't imagine it would be difficult to replace (for example) one AHRS with a IRS, as these components are all from the same family.

You are correct, when the GPS system gets hacked or goes down for military reasons, GPS dependent aircraft may be at a disadvantage. That's why civilized countries plan to decommission only half of the VOR's. Most of which are in areas of significant overlap.

It's said that the remaining VOR network will be sufficient for accurate navigation everywhere in the US, at 5000 feet AGL or above.
 
Might better go look again. I believe both those aircraft have triple redundant Honeywell laseref ring laser gyro IRU's. Full inertial grade. And have had since forever. The full-up nav unit is probably close to $1M by itself. It provides both blended nav, free nav if necessary (about 1NM/Hr or better IIRC) and inertial outputs to the needing systems like the FMS/Autopilot/vision systems, etc.

I have noticed that lower tier jets like the G280 and Cessna 750 series (10+, Sovereign) which used RLG's or would have used RLG IRU's in the past are starting to get by with a fairly low spec Litef fiber optic nortfinder/GPS blended nav to save big bucks by leaning heavily on GPS. It's coasting accuracy is not really in the inertial class and is only a better sort of AHRS. As you point out the GPS can be unavailable for a variety of reasons including a simple 5 watt jammer. As the RNP requirements become the norm with GBAS we will have one more component that can fail as well - the ground station at or near the airport. Low risk? I suppose so but it's still there. As we tighten the traffic lanes and the approaches we are opening ourselves up for some spectacular failures here and there.
 
One more thought. There used to be some nav systems that could do DME/DME navigation by getting the distance and bearing from multiple vortacs and derive a position that way. Maybe we should bring that back. VOR's alone are not going to rell you where you are.
 
My apologies Cujet, I think I misread your very well considered answer regarding the nav structure of the 650/550 versus the APEX system in the PC12. I thought you were saying all of them relied on AHRS vice IRU's. My bad.

I agree with you that it should be easy to add an inertial input to the APEX system because the EPIC platform behind Planeview has those inertial components. It was exactly that thought that led me to ask the question to Duncan, Pilatus and even Viking.

My interest was a client that will be operating a photogrammetry mission in very high latitudes and using a Twin Otter DHC6-400 as the carrying platform. As you probably know that is the only other user of the full-up APEX (that I'm aware of). Knowing its connection to the EPIC had made me wonder if we could use a single source of inertial for both the payload and the aircraft. Their responses to my questions indicated the system as they know it is good enough for them and they haven't even thought beyond that. I have an INS/GPS in mind to use but we'll have to wait and see. My annoyance was caused by asking smart people what they would do if the GPS signal was lost and they tell me that, oh well, no problem, we have two GPS systems! Grrrr
 
Originally Posted By: DeepFriar
My annoyance was caused by asking smart people what they would do if the GPS signal was lost and they tell me that, oh well, no problem, we have two GPS systems! Grrrr


Haha, it's difficult to communicate accurately with non technical people.

On another note, the GPS system has had intentionally introduced inaccuracies here in FL. 3 times (albeit years ago) I've experienced a significant intentional GPS inaccuracy. So it's not just hacking that can be a problem. It's also when the system is being "tested".
 
Originally Posted By: Cujet
Originally Posted By: DeepFriar
My annoyance was caused by asking smart people what they would do if the GPS signal was lost and they tell me that, oh well, no problem, we have two GPS systems! Grrrr


Haha, it's difficult to communicate accurately with non technical people.

On another note, the GPS system has had intentionally introduced inaccuracies here in FL. 3 times (albeit years ago) I've experienced a significant intentional GPS inaccuracy. So it's not just hacking that can be a problem. It's also when the system is being "tested".



Yes, you see it in the NOTAMS from time to time. Most recently I saw "testing" in the Nevada/California area. Wouldn't surprise me if that was for electronic warfare testing given the location. And not to forget, we're getting this great accuracy right now because DOD is allowing the system to deliver best performance. Any number of "national emergencies" would put an end to that. The whole future airspace plan worries me a bit. DARPA is working hard on trying to develop alternative inertial nav solutions and there is funding available from them for investigation.
 
I think the ground based navs should be kept for just that reason - GPS degradation in crisis...
 
Originally Posted By: Kuato
I think the ground based navs should be kept for just that reason - GPS degradation in crisis...


I think that's the plan.


Of course, all I do anymore is fly up and down the East coast, which is rather easy.... N177CJ, how are you navigating??? Visual, down the coastline...
 
The current plan is to do away with as many VOR's as possible. Keep as many Vortacs as needed to preserve DME as a fallback. Then gapfill with a few VOR's as necessary. The FAA envisions GBAS being the final answer with ground based augmentation stations at or near airports. So you'll get ILS performance on every runway without the FAA having to go around constantly recalibrating the ILS infrastructure. What would you care to bet that all this was originally sold as a cost saving initiative? Your tax dollars at work.
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And all predicated on the unwavering availability of GPS.
 
Originally Posted By: Cujet
...all I do anymore is fly up and down the East coast, which is rather easy.... N177CJ, how are you navigating??? Visual, down the coastline...



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Originally Posted By: Kuato
Originally Posted By: Cujet
...all I do anymore is fly up and down the East coast, which is rather easy.... N177CJ, how are you navigating??? Visual, down the coastline...



lol.gif



At the beginning of the final, longest, and most remote leg of my dual cross country we did a quick run-up and I radioed in my flight plan. At that point my instructor reached over and shut off the radio, gave the directional gyro a spin, put his hat over the compass, and said, "lets go home".

Ed
 
Originally Posted By: edhackett
Originally Posted By: Kuato
Originally Posted By: Cujet
...all I do anymore is fly up and down the East coast, which is rather easy.... N177CJ, how are you navigating??? Visual, down the coastline...



lol.gif



At the beginning of the final, longest, and most remote leg of my dual cross country we did a quick run-up and I radioed in my flight plan. At that point my instructor reached over and shut off the radio, gave the directional gyro a spin, put his hat over the compass, and said, "lets go home".

Ed


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Good test....did you make it?
 
To try and provide a final information dump in this wandering thread (my fault - apologies to mods) this is what was learned on "attaching" an accessory IRU to an already existing integrated flight deck like the PC-12's APEX system to increase highy accurate coasting times in the event of loss of integrity of GPS signal. The bottom line is, as many of you thought, that it is possible. However there are problems - one regulatory and one technical.

Regarding the first, as you know, nav systems are qualified to a standard. It's the reason we have TSO's for many systems and subsystems. For that to be changed either the aircraft manufacturer or an STC developer has to requalify a system with all the pain and expense associated with that. It seems also that the further we proceed into PBN the more critical this becomes and reliance on the aircraft manufacturers increases as well. This will make it more difficult for any second or lower tier suppliers (say, us, with our fictional add-on IRU/INS) to innovate into the space but that's a different story.

The second is technical. Availability of GPS allows us to "clamp" our nav errors to the positional accuracy of the GPS. But it also does more. A variety of data from the GPS including the postion updates and velocity information from the satellites, mixed with the ground based corrective signals and various air data inputs allows us to have a (very good) blended nav solution. Those inputs are mixed together in something called a Kalman filter (an algorithm) to yield position and other information when applied to a navigation database usually loaded monthly into the FMS. The Kalman also continues to calibrate the inertial outputs as well. This is all great stuff and it's the reason manufacturers can use relatively low quality inertial or ADAHRS boxes. But loss of signal or signal integrity means the party is over.

Having said all that, imagine we show up with our super zoomy, much more accurate IRU. It has to talk to the FMS, it needs the right pins, it needs a defined location in the coordinate frame of the aircraft, not to mention the Earth. All of that timing and interface is a bear. Some of the answer can be an interface box sometimes called a "personality module" that contains those data that would allow a generic, so to speak, IRU to tie into different aircraft, different nav systems, etc. My opinion is that those personality modules are a sham meant to differentiate the platforms. Why a sham? Because all that data could be housed in the IRU memory and either the pin code or the FMS could "tell" the nav box what platform it's in and what the parameters are.

Long story, longer.... It's eminently possible to insert an IRU into a weak kneed AHRS/GPS system to vastly increase the coasting time available for accurate RNP/PBN level enroute and approach requirements. But it takes time and money and expertise. And, from a marketing standpoint, it is the difference between a $100K system and one costing three to ten times more (literally). So the incentive is for the airframer and his nav supplier to cost cut their way to profits, which is OK, not saying it isn't, and guard against any upstart comming out of left field with not only a cheaper answer (and there are already many) but one, at the same time, more accurate and more resistant to GNSS failure (not very many at all).

So, back to my current case, I will spec a high data rate IRU, hopefully with a fully integrated GPS, to place directly on the sensor palate and leave my wistful aircraft navigation dreams for another day.
 
I better get my pallets straight (see palate above) before I get splinters in my mouth.
 
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