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Author Topic: 840 real world full fuel payloads  (Read 11498 times)

donv

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Re: 840 real world full fuel payloads
« Reply #15 on: March 08, 2020, 03:41:15 pm »
I have the tanks. I've seen them with my own eyes. It's been a mystery since I've owned the airplane, and even before that. We've filled it up, pumped all the fuel out and measured it twice in my ownership. We've also checked to make sure they have fuel in them, and they do.

Regarding comfort, the Cheyenne IIIA is actually the quietest turboprop I've ever ridden in, by far. It's sort of a long tube, and you can decide if you like that or not, but it's very comfortable for the passengers.

As for FL350, the question is whether or not there is an RVSM package available for the IIIa. A quick google search looks like there is one, but I don't know how expensive it is or how common it is.

I would be very concerned about product support, from what I have heard, on the Cheyenne. Twin Commander exists to sell parts, so they are highly motivated to keep the fleet flying. Piper exists to sell new airplanes, so they are much less interested in the Cheyenne series. They would rather sell you a new M600.

My airplane, for unknown reasons, won't hold more than about 2850 pounds of fuel (and yes, it has the long range tanks).

<scratches head>

Ummm...I'm not sure how to tell you this...but...

The factory planes with standard fuel hold 425 gallons of useable fuel. Long range planes hold 474 gallons useable fuel.

At 6.7 lbs/gallon, that's 2,847 lbs and 3,175 lbs, respectively. 2847 is very, very close to "about 2850."

It sure sounds like you don't have long range tanks. Maybe there's another explanation?
« Last Edit: March 08, 2020, 03:43:07 pm by donv »

donv

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Re: 840 real world full fuel payloads
« Reply #16 on: March 08, 2020, 03:48:29 pm »
Also, if hot and high is an issue for you, you really need -10s. It makes an enormous difference in that situation.

Furthermore, you will find that the -5s won't make it all the way to hot section if you are regularly running them hard, while the -10s will.

JMA

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Re: 840 real world full fuel payloads
« Reply #17 on: March 08, 2020, 06:03:58 pm »
Jim- if you ever are on the east coast and nearby, we are based at PHL can give you a demo flight in our 980.
I know you are focused on an 840, but still worth the look if you have the time.

JimC

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Re: 840 real world full fuel payloads
« Reply #18 on: March 08, 2020, 06:13:50 pm »
Hot & high is a new fact of life for me. I have the 840-5 manual, and if the plane makes book (or close to it) I'm satisfied with the h&h performance available. I'll clear all terrain at my local airport if I lose a fan with an 840-5. At 6000ft/30C, my reading of the POH shows just over 2% climb gradient with gear down and just over 4% gradient gear up. With the PA-42 III, there are real-world conditions where it won't clear the gently rising terrain straight ahead on one fan (and turning left or right leads to rapidly rising terrain.) I've plugged in all my required fields under hot conditions, and I'll make both climb gradients as well as runway lengths using the -5s. The -10s don't change much on the ground. Yes, I'd like -10s but they are a HUGE premium over -5s. I'd rather get an 840-5 than a 690B-10. (Tell me if you think this is wrong.)

re: running hard
What's running hard? I generally fly my engines by the book. I'm new to turbines and don't know how to hurt them (other than hot starts.)
500B, B200

JimC

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Re: 840 real world full fuel payloads
« Reply #19 on: March 08, 2020, 06:15:23 pm »
I used to fly in and out of PHL fairly often, but haven't been in at least a year (my last trip to the area was ILG.)

I'm assuming 980s are out of my price range. That being said, I won't turn down a demo flight if I get back that way.
500B, B200

donv

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Re: 840 real world full fuel payloads
« Reply #20 on: March 08, 2020, 06:46:09 pm »
This is going to be a fairly long post, so bear with me... and Bruce or others, please step in to correct my errors...

A turbine engine generates power through heat-- as the fuel in the combustion chamber burns, it is extracted by the turbine section. The more power required, the more heat it generates. And, just like a normally aspirated piston engine, the higher the density altitude, the less power the engine has.

Unlike most normally aspirated piston engines, however, most modern turbine engines have an excess of power available. So, for example, the -5 on the 840 has the thermodynamic capability to generate 840 horsepower, but the airframe is limited to 717 horsepower. That limitation could be due to the gearbox, Vmc, general airframe limitations...  but also it does provide better hot and high performance.

Thus, the turbine engine actually has two power indications-- horsepower (in the case of a -5), and Internal Turbine Temperature (ITT). A -10 uses different terminology, but the result is the same.

So on takeoff on a cool day at sea level, in a -5 840, you will easily be able to get 717 horsepower, and the ITT will be well below the limit (923). As the density altitude goes up, you will need more and more ITT to generate the same 717 horsepower, and eventually you won't be able to generate 717 horsepower at 923 degrees ITT. I don't have any -5 performance materials handy, but I would guess that happens around 6-7000 feet density altitude or so (it's been a long time since I flew a -5).

The more time you spend at high ITTs, the higher the wear on the turbine blades, which results in a little bit less power... which results in spending even more time at high ITTs... and so on. Eventually, they degrade enough that you really start to notice the decrease in available performance. It's a very slow process, but it does happen.

Now let's take a look at the -10. It uses "percentage of torque" instead of "horsepower" and "EGT" instead of "ITT" but the meaning is basically the same, though the actual numbers are not comparable.

The -10 on the 980 can generate 1,000 horsepower thermodynamically. However, you are still limited to basically 717 (733, to be specific). As a result, you are virtually never limited by the EGT on takeoff. In fact, you typically aren't limited by EGT on the 980 until FL200 or higher-- you are still getting the full 733 horsepower.

But wait, there's more! The -10's thermodynamic limit is actually much more conservative than the limit on the -5 in terms of wear on the turbine section. So running a -10 right on it's thermodynamic limit does not cause the wear on the turbine blades that running a -5 on it's limit does. And, in a virtuous circle, since the thermodynamic limits are so much higher on the -10 anyway, you spend much less time there.

The result is that -10s typically don't see the power degradation than -5s do over time.

If you are generally operating near sea level, then the extra power of the -10, while nice, may not be that important. However, in the case of hot and high operations, it makes a huge difference, both in terms of safety and long term cost.

donv

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Re: 840 real world full fuel payloads
« Reply #21 on: March 08, 2020, 06:49:30 pm »
I will also extend the same offer as JMA if you end up in the pacific northwest.

JimC

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Re: 840 real world full fuel payloads
« Reply #22 on: March 08, 2020, 07:05:14 pm »
Thanks for the long post on the engines. I was wondering if there was anything I could do with -5s that another operator wouldn't do with -5s to avoid "running them hard." Basically, any summer flight deep into the FLs will run them hard - I was wondering if there was anything I could do operationally to mitigate this.

I find it interesting that everyone always says "Get -10s" when it's not *their* wallet.  ;) For the average owner operator, it seems that purchase premium for a -10 plane over a -5 plane has become so large that you may not recoup the savings over the duration of ownership of the plane.

How far into the Pacific NW? I'm headed up to 3S8 later this summer.
500B, B200

donv

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Re: 840 real world full fuel payloads
« Reply #23 on: March 08, 2020, 07:17:49 pm »
3S8... I was there last summer, going on a raft trip. Probably not this year, though. Come up from there to HIO!

As for running them hard, you need to keep the temperatures down. 900 for takeoff and 885 or less at cruise, even though the book says you can go higher (I used 850 for cruise). And hot and high takeoffs just chew them up. When I flew -5 690s in air ambulance operations, we did a lot of hot and high ops, and even though I think we were reasonably conservative (although with a lot of pilots, who really knows?), we had regular issues.

JimC

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Re: 840 real world full fuel payloads
« Reply #24 on: March 08, 2020, 07:28:45 pm »
we had regular issues.
That's what I'm hoping to avoid as I move from piston to turbine. What kind of issues? Do you feel as though you would have avoided the issues with -10s?
« Last Edit: March 08, 2020, 08:13:30 pm by JimC »
500B, B200

JimC

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Re: 840 real world full fuel payloads
« Reply #25 on: March 08, 2020, 09:14:37 pm »
Jim, what are some of the legs you envision flying? Maybe we could figure out what the fuel requirements might be, and how the weights would work.
Here's an example: SAF to ROC.
Straight line is 1389nm - easy enough, especially eastbound. But...

Out of SAF, you'll fly the POAKE2/TAS transition. Coming into ROC, you'll get vectored around a bit at low altitude to join the conga line and usually do the ILS 22. If I put both of those into Foreflight, the no-wind fuel requirement jumps to 2,520 as calculated by Foreflight. That mostly matches up with what I get from the POH. That means a standard range 840 can't do that flight with a 400lb reserve.
Yes, you'll often have a tailwind, but sometimes the jetstream is blowing NW->SE across the route of flight - or often the turbulence in the mid 20s makes the lower 20s much more appealing.

Are those calcs about right?
500B, B200

donv

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Re: 840 real world full fuel payloads
« Reply #26 on: March 08, 2020, 09:43:36 pm »
Sorry, I should have been more clear. 331s generally either run or they don't-- they are very, very trouble free.

By "issues" what I meant was that they weren't making it to the scheduled hot section interval. You would notice that, hey, I used to be able to take off from XYZ without hitting the temp limit most of the time, and now I am. Or gee, it just doesn't seem to be quite as fast at altitude as it used to be.

Eventually, that gets to the point where you decide you need to do a hot section to restore the power.

Of course, we had 3 airplanes doing 3-400 hours per year each, and we often had rental engines and mid-time engines that they bought from somewhere, so it wasn't unusual for us to have an airplane with 12-1500 hours since hot section.

So I would say that, from a budgetary perspective, don't assume you'll get 1800 hours out of a hot section on a -5, especially hot and high. Whereas, you will get 2500 hours out of a hot section on a -10, pretty easily (and maybe 3500, depending on the mod status of the engine).

we had regular issues.
That's what I'm hoping to avoid as I move from piston to turbine. What kind of issues? Do you feel as though you would have avoided the issues with -10s?

donv

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Re: 840 real world full fuel payloads
« Reply #27 on: March 08, 2020, 09:56:55 pm »
I just ran a flight plan on fltplan.com for SAF-ROC. I used this as the routing (just based on what others have used):

FTI J19 STL J24 VHP ROD KLYNE Q29 JHW ROC

Standard winds were +44 (FL270), and I could do it in 4+29. I would be fairly comfortable that I could do that leg most of the time with my 2850 pounds, landing with at least 600 pounds, and probably more. I try not to go below 500 pounds, and always plan for 600 or more.

ROC-SAF shows average headwind of -51 at FL280 and -39 at FL220. Either way, I don't see it working for me... 5+29 at FL220 is too much. I can do that at FL280 if I really stretch, but then the winds are too much. So I would plan on a fuel stop westbound.

Generally, if fltplan.com shows 5 hours or less, I figure I can do it nonstop. Between 5 and 5+30, I really look hard at it, and more than 5+30 is a nonstarter for me.

With 3200 pounds, I would probably be willing to stretch it to 6 hours, in which case it might make it westbound, but even then I kind of doubt it... it shows 5+54 at FL280.

On my airplane, with -10s, there is a big efficiency gain at FL280 over FL220. Maybe less in a -5, I don't remember. I don't have a lot of -5 840 experience-- in the -5 690s, I rarely went above FL240.

donv

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Re: 840 real world full fuel payloads
« Reply #28 on: March 08, 2020, 11:10:33 pm »
I realized that I ran the flight plans for today, rather than standard winds.

Using standard winds, SAF-ROC would be 4+25, with a +49 tailwind.

ROC-SAF would be 5+57 with a -49 headwind at FL280.

schrambow

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Re: 840 real world full fuel payloads
« Reply #29 on: March 09, 2020, 11:12:33 am »
I like these conversations and number crunching real life on our birds.

Jim, we have a 840-5 and love it, I told Bruce to keep looking for a 1000 with about only 500 TT on it for us in the future for an upgrade in a few years.

We just did a trip- 1025 Nautical miles one way back on forth this weekend and will share with you the numbers.  Granted this trip is less distance than your mission you stated earlier.
 
Our empty weight is : 6919 lbs
I cruise our ITT's at 890 degrees now and get the following fuel burns this past weekend.  Was fairly close to ISA a few degrees here and there on the trip down and back.

At FL27, we trued out about 273 knots and a fuel burn of 480 lbs/hour.  We had a very nice tail wind finally though and made this trip in 3:20!  love it - finally-= jet speed baby!
Return trip at FL20 was close to "no-wind" in reality, we trued out about 280 knots and a fuel burn of 573 lbs/hour and made it back at 3:50.   That 7 to 8 thousand feet higher makes a huge difference on efficiency i see also in the -5' on long trips.  At FL28 it is even more efficient on the fuel burn obviously- without losing much TAS.

we have our first hot section inspection in about 300 hours and are on the frightfully expensive MSP until this first hot section i think, but it will be about 3 years before we see how this inspection will turn out.

I like to drool over RVSM 1000's when i think about Commanders, but a wise Scott told me in his sales presentation, "You get 85% of the performance at 50% of the price for this plane:--840-5".  This plane fits our missions nicely. The 1400 mile trips it would do non-stop some of the time i believe, but not every time without a stop i feel.
Last September, we did a 1422 mile trip from RNO to FLD at FL27, and i believe we made that trip in 5 hours around 2313 lbs of fuel used if i remember correctly,  ISA +10 i think and trued out at 265 knots only, however, we did have a slight tail wind and great weather though.   

I hope this kind of gives you some numbers to go on for the 840-5's.  Good hunting.

Corey