AVSIG: Turbine P-Baron wwswsigarch.jpg (7236 bytes)

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Robert Mann [HPN-NY]
AVSIG Member


Reged: 05/15/04
Posts: 11777
Loc: NY
Re: Turbine P-Baron [Re: Dave Siciliano (ADS)]
      #213787 - 09/19/08 11:25 PM

Dave,

<< I should post my new trading positions on the board when I place them. Then, one could just take the opposite side >>

There were some advisors I knew that always made the worst decisions. I used to rib them and say that I'm going to launch a new mutual fund called The Joe Smith Contrafund, which would do just the opposite of whatever he did. I also thought about saying the same things to some ex-clients, but I kept those thoughts to myself.

Some clients are really good at telling you where the bottom is. It's when they say, "Maybe we should just go to cash and wait this thing out."

--------------------
Best,

Bob


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Robert Mann [HPN-NY]
AVSIG Member


Reged: 05/15/04
Posts: 11777
Loc: NY
Re: Turbine P-Baron [Re: Scott Dyer [HPN/NY]]
      #213790 - 09/19/08 11:41 PM

Scott,

<<and lots of McMansions too, around here>>

Wait till the demographics of later/no marriages and smaller families really sink in. Who's going to be buying those things in 10-20 years? Problem is, the economics of development tell you that you're going to lose money if you don't build at least a 3,000 sq ft house, and you'll make a lot more money if you build a 6,000 sq ft house. The land cost the same either way. I'm sure you've seen lots around here that sell for less money with a small house on them than they would if it were vacant land.

--------------------
Best,

Bob


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: Turbine P-Baron [Re: Robert Mann [HPN-NY]]
      #213792 - 09/19/08 11:45 PM

Quote:

Dave,

<< better TAS at lower fuel burn higher >>

Is it both? I always thought that it's more TAS on the same fuel burn, or in other words, more mpg.




I'll get you some C-90 performance data in the morning.

In general, fuel flows are quite high at lower altitude while TAS is lower; after one climbs with these engine and gets into the thinner air, TAS is higher while fuel burn is lower. I'll get you some specifics when I can access it. In the E-90 King air I was recently given a demo flight in, it burned over 700PPH in the climb; high speed cruise was just over 300PPH IIRC at FL270. Since we didn't level off at a middle altitude, I don't have those numbers, but can get them int he morning. Maybe someone else has them handy tonight.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: Turbine P-Baron [Re: Dave Siciliano (ADS)]
      #213810 - 09/20/08 12:24 PM

Robert:

I have the data for a C-90 with the -135s (Blackhawk STCed engines). That's what we have looked at and a friend of mine has one.

At 10,000 feet, he gets 265 knots at 700 PPH fuel flow.

At 20,000 feet, he gets 272 knots at 525 PPH fuel flow.

At 25,000 feet, he gets 269 knots at 440 PPH fuel flow.

So, real low, he's a bit slow and burns a lot more fuel. As you can see, his TAS is best around 20,000, but he doesn't lose much speed and really reduced fuel flow in the mid 20s. Of course, one must consider wind and weather, but, in general, with these engines, he's better off in the mid 20s.

Best,

Dave


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Robert Mann [HPN-NY]
AVSIG Member


Reged: 05/15/04
Posts: 11777
Loc: NY
Re: Turbine P-Baron [Re: Dave Siciliano (ADS)]
      #213815 - 09/20/08 12:45 PM

Dave,

That's a huge difference in FF. When I asked whether it was "both", I guess it's always a tradeoff between TAS and FF. For instance, at 10,000 ft, if FF was 440 PPH, I imagine that TAS would be in the 200-220 range. So, as one increases altitude, either TAS goes up, FF goes down, or both, depending on your power settings.

IIRC, the difference is much more severe for a turbofan engine vs a turboprop. I don't really understand the theory behind that, but is that correct?

--------------------
Best,

Bob


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: Turbine P-Baron [Re: Robert Mann [HPN-NY]]
      #213820 - 09/20/08 01:50 PM

Quote:

Dave,

That's a huge difference in FF. When I asked whether it was "both", I guess it's always a tradeoff between TAS and FF. For instance, at 10,000 ft, if FF was 440 PPH, I imagine that TAS would be in the 200-220 range. So, as one increases altitude, either TAS goes up, FF goes down, or both, depending on your power settings.

IIRC, the difference is much more severe for a turbofan engine vs a turboprop. I don't really understand the theory behind that, but is that correct?




There certainly are many folks out there that can explain this better than me, but I'll give it a general go and be happy for anyone else to chime in. The turbines are a bit different in the way they are run; unlike a prop, they seem to do better at recommended power settings; that is you don't run them too hot or too cold. So, you are correct in that if one ran 440PPH at 10,000 feet, TAS would be down, but one generally wouldn't run a low power setting like that a lot. The small amount of fuel savings is offset by the cycle cost on the plane.

While turbines could be more efficient lower, most are set up to get maximum efficiency higher. The compressors (which could be compared to the turbo on a recip) produce full power up to an altitude, then, power drops off. So, the old C-90 King air engines made full power up to about 14,000 on a standard day and power dropped off above that. One would still benefit from being higher as the air was thinner and TAS would still be high with lower fuel burn as in the expamle I provided. The -135s on the Blackhawk conversion plane, maintain full power to a higher altitude; so, the sweet spot is mid 20s as opposed to low 20s on earlier King Airs.

I yield to the next speaker that can explain turbine efficiency better <g>

Best,

Dave

Edited by Dave Siciliano (ADS) (09/20/08 01:52 PM)


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Mase Taylor
Top Gun


Reged: 04/29/04
Posts: 9446
Loc: SOCAL
Re: Turbine P-Baron [Re: Dave Siciliano (ADS)]
      #213821 - 09/20/08 02:09 PM

When Hawker/Beech re-engined the C90 King Air to make the C90GT, they replaced 550shp PT6A-21 engines with 550shp PT6A-135A engines. Why bother, you say? Because the -135A engines are derated; they are thermodynamically capable of 750shp. So they can maintain full rated power of 550shp to a much higher altitude without overtemping. This make the C90GT almost 30 kt faster, and much better climb performance.

--------------------
Fly The Airplane As Far Into The Crash As Possible. - Bob Hoover 1922-2016 R.I.P.


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: Turbine P-Baron [Re: Mase Taylor]
      #213831 - 09/20/08 03:42 PM

Quote:

When Hawker/Beech re-engined the C90 King Air to make the C90GT, they replaced 550shp PT6A-21 engines with 550shp PT6A-135A engines. Why bother, you say? Because the -135A engines are derated; they are thermodynamically capable of 750shp. So they can maintain full rated power of 550shp to a much higher altitude without overtemping. This make the C90GT almost 30 kt faster, and much better climb performance.




Thanks Mase! Robert, if you have the same amount of power; you'll true out faster higher up.
According to Conklin & de Decker:

Up to about 14,000 feet at ISA, the PT6A-21 delivers its full take-off torque. At recommended cruise, this is 1,315 Ft.- Lbs per engine. Above this altitude, the engine loses torque and at 26,000 feet produces only 862 Ft.-Lbs of torque. This yields 235 KTAS for a mid-weight King Air C90A.

Just to compare the -135s: and I quote again:
At normal cruise, the PT6A-135A produced 1,520 Ft-Lbs. up to 18,000 feet. At 26,000 feet is still produces 1,170 Ft-Lbs. torque - 36% more power than the PT6A-21.

So, constant torque on the -21 up to 14,000, then, it declines. For the -135s, torque remains constant up to 18,000 feet; then, declines. This is primarily do to a better compressor.

Did this answer your question? Sorry to get into the new engine, but it shows how the same power is made to a higher altitude.

Best,

Dave


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