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Thread: GA Turboprop

  1. #51
    steveinindy's Avatar
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    Quote Originally Posted by kjensen View Post
    What is the cost of that engine? I assumed that it will be lower cost than what's out there. If that is not the case, then just disregard the question.
    We don't know they cost yet since they haven't started selling them yet and are most likely several years from doing so, if they get that far. However, we're not going to see a turbine that can compete cost wise with a reciprocating engine in that particular class because of the nature of turbine manufacturing and the costs of maintenance.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  2. #52

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    Quote Originally Posted by steveinindy View Post
    We don't know they cost yet since they haven't started selling them yet and are most likely several years from doing so, if they get that far. However, we're not going to see a turbine that can compete cost wise with a reciprocating engine in that particular class because of the nature of turbine manufacturing and the costs of maintenance.
    Oh ok, I see. Thanks.

  3. #53

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    No body has pointed out the really fundamental difference between turbine and piston applications - and that is turbines don't scale down in size very easily and do not have anywhere near the operational flexibility and fuel specific consumption of piston engines. Our aircraft max out in the range of 300 hp and turbines have to operate at very high rpm and fuel consumptions to get to this small corner of their operating area.

    On the other hand, turbines scale up very easily. Compare the complexity of say a turbine powered 747 vs the same thing powered with multiple piston engines. It would obviously be hopeless to do this and piston engines topped out with the complexity of the P-W 4360 28 cyl engine. Spark ignition piston engines have to limit the combustion chamber size to control detonation and that makes the maximum bore about 6 inches. Only diesels can extend the bore way beyond that which they do by controlling the combustion burn rate with the entrance of fuel.

    Turbines do have the distinct advantage of high altitude ops at nearly max power where the drag of a large aircraft can be considerably reduced.

    If the turbine engine had been invented first, the piston engine would have been considered a remarkable advance considering turbine material and flexibility problems.

    And rotary engines suffer from efficiency problems too - even their seal problems could be licked. The combustion chamber dimensions have to be carefuoly controlled and will always cause a much greater heat loss when compared to the small combusion chamber of a round piston.

  4. #54

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    Advantages: Light wt, simplicity, dependability. ( I once flew 45 minutes with 2 NVA bullets in the compressor section. No sweat.)

    Disadvantages: $$$, high fuel flow and no forgiveness for unfiltered fuel, defered maintenance and clumsy oafs do burn them up on starting.

    The common engines run north of $million. GE, Turbomecca and some PT-6s. The Allison/RR can sometime be found for $100K with dubious pedigree.

    Fuel flow on PT-6 runs about 350lbs/hr. Allison/RR 250-C20 (Bell 206) burn 28 gal/hr at 500 ft and 25 gal/hr at 7000.

    The best deal for OBAM is the T-62. Used as an APU in large helicopters and M-1 tanks. Seems like they were not designed to be O/H. They were and are sold as surplus. Simple and easy for a good machinist to replace seals and bearings and you get 100 to 150 hp that you can tote under your arm. Plan on 15 gal/hr. Check out the clipped wing Luscombe Speedbird:http://ronkilber.tripod.com/luscombe/luscombe.htm

    There is a guy that posted YouTube vids of his Turbine Zenith 701 here in this forum.

    A friend of mine had a shop and O/H ed about 18 or 20 of these. He sold one to another pal who was planning to put it in his BD-5. The machinist friend put one in a KR-2. I checked it out in his shop. Excellent work and it followed the turboprop convention for engine control. See the Speedbird link for details.

    I moved the throttle control and the input was made to the cutest little fuel control that I've ever seen. When I moved the prop control, a Teleflex cable made a direct mechanical pitch change on the prop. It performed well on ground tests.

    Sad to say, Joe, a low time PPL ignored advice to not fly it and resulted in a fatal accident. The power/wt ratio was a factor IMHO. One that I talked to described him going vertical off the runway. Search NTSB for KR-2, May 2005, Galliano, LA.

    These turbines were going out the door O/H for less than a VW.

    Bob

  5. #55

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    Quote Originally Posted by kjensen View Post
    What is the cost of that engine? I assumed that it will be lower cost than what's out there.
    Well, if you define lower cost in small percentages, it may fit the bill. An engine that cost 10% less than it's $300k competitor doesn't seem to register in my checkbook as costing less cause it's still unaffordable.

    The problem is current technology is not going to allow a "low cost" gas turbine engine. On the positive, every dozen yrs or so there is renewed interest with ceramics in gas turbine engine development. So there are possibilities. It's just a matter of time.

  6. #56
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    Spark ignition piston engines have to limit the combustion chamber size to control detonation and that makes the maximum bore about 6 inches.
    Really? I learned something new. I always wondered how big you could make the cylinder.

    On the positive, every dozen yrs or so there is renewed interest with ceramics in gas turbine engine development. So there are possibilities. It's just a matter of time.
    There was a really interesting presentation I attended a couple of years back that featured a couple of engineers from one of the research labs over in France that were discussing a new ceramic/metallic hybrid (the description of the material was high-end engineering talk that I didn't fully grasp) that apparently has a "vastly reduced" (their words) rate of manufacturing defects as compared with previous versions and a potentially reduced cost. If it pans out, it might be the thing that finally helps reduce the costs of turbines.

    If you're interested Marty, I'll dig through my file cabinets and see if I still have the handouts from that conference.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  7. #57

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    Steve, You may have been told about the Turbomecca Arriel 2S2. It has a "single crystal" turbine wheel and the 30 second temp limit is over 1,000 C. I don't know much about metalurgy. The instructors at Flight Safety couldn't explain it either.

    Bob

  8. #58
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    That might have been it. It had some almost obscenely high tolerance limits. I'll dig around and see if I have the handouts.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  9. #59

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    Quote Originally Posted by steveinindy View Post
    Here's the kicker. It seems that very few pilots get themselves into serious trouble because of those additional levers. Saying that those additional levers are a problem is pretty sketchy. I'd argue that the fuel tank selector knob causes far more problems.


    The issue I was getting at with a turbine is the lack of "instant" response like you get with a recip. You have to wait a couple to a few seconds to "spool" it up (at least in the larger engines from what I have been told; I'm not a turbine pilot yet myself so I am just repeating what I was told) before you get the full response. Given how many of us manage to get ourselves into the side of the power curve where we really shouldn't be, it would seem that adding something into the mix that makes recovering from that even less likely (because of the slight delay) isn't necessarily a good thing. However, I think it's a minimal risk and inattentive pilots will be inattentive regardless of what they are flying.
    Yeah, fuel tank selection is a good killer....the other levers you mention with turboprops are the speed lever and the condition lever (fuel cutoff/feather). Those two handles are only messed with on the ground, or if you have to kill an engine. So I would say for the large majority of the time with turboprops, in flight, it's a single lever deal.

    The turbine lag you talk about is true, especially with turbines and PT-6 style (the technology would be free power turbine) does have lag. The Honeywell TPE style is that of a direct drive essentially does not. It seems like the majority of these smaller turbines or anything that we are talking about in terms of size would be of the direct drive style, thus eliminating those issues.

    And yeah, there is potential to blow up turbines. Hot or hang starting...but hey, it's your money and if you want to blow up your engine, go ahead

    fact is there are PLENTY of ways to die in an airplane.....i don't think the masses flying turboprops would make it any worse.

  10. #60

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    Electronic engine controls and single power control lever are real technologies, which go a long way toward idiot proofing engine operation. It's just a matter of pilot/operator culture catching up with technology. Have to put the OWT's to bed cause in the end, there is less risk when flying with turbine power. That was proven a long time ago.

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