08 Jun 2025, 13:14 [ UTC - 5; DST ]
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Post subject: Re: Mitsubishi for first twin  Posted: 24 Jan 2015, 12:15 |
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Joined: 12/03/14
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Username Protected wrote: However, max endurance for turbines is also achieved at any altitude. Nothing is gained by climbing. You are in for one heck of an education when you start flying one. The altimeter and the fuel flow gauge will conspire against you. Mike C.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:23 |
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Username Protected wrote: For turbine powered planes the range extension at altitude is generally a function of getting the powerplant(s) up to an altitude where they are efficient. The airframe seems to only require proper IAS for efficiency. Turboprop engine is more efficient higher, airframe is more efficient higher. Mike C.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:24 |
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Joined: 08/18/13
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Aircraft: 737
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I have an Aerostar 700SS. I fly it at 1000' sometimes because it's fun. I indicate 165 at 17.5gal/side. Sometimes, I go to 17.5k'. I indicate 165, and I burn a touch more because the wastegates close and the %#$@ go up some. It's 18.5gals/side for cooling, although I could (and have) run the same indicated speed and fuel flow albeit with a higher (but still within POH limit) TIT. It takes no more fuel to get a higher TAS in my experience, although there is extra fuel burned in the climb.
Here's an equation that discusses your ground effect flying:
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:26 |
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Joined: 08/18/13
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But as I'm not interested in re-teaching myself partial differentials today, here's my take-away:
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:29 |
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Joined: 08/18/13
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If that's correct, you really need to be quite high to get TAS to give all that back, but nobody is operating aircraft at a half a wingspan from the deck in cruise unless the ship is a purpose built watercraft or you're coming back from a bombing run circa 1944 and you need to conserve fuel.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:31 |
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Joined: 12/17/13
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Location: Hollywood, Los Angeles, CA
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Username Protected wrote:
Very few turbine flights start with the premise, "I want to fly for 3 hours".
Many turbine flights start with the premise, "I want to fly N-hundred miles".
This is true. I can't see much use for that unless maybe it's a coast guard plane on a patrol or perhaps you're waiting for weather to get better below and have no alternates or something.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:38 |
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Username Protected wrote: At Sea Level, you go 140 kias on 20gph. Your TAS is 140kts. You have a 100 gallon tank so your range is 700NM.
At FL230, you go 140kias on 40gph. Your TAS is 280kts. You have a 100 gallon tank so your range is now 700NM. At FL230, 140 KIAS is 202 KTAS, not 280 KTAS. It doesn't take twice the fuel flow to fly 140 KIAS at FL230 for any normal airplane. Mike C.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:38 |
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Joined: 10/27/10
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Username Protected wrote: Turboprop engine is more efficient higher, airframe is more efficient higher. Agreed on the first. I haven't seen anything to suggest the second is correct.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:38 |
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Joined: 05/29/09
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Username Protected wrote: For turbine powered planes the range extension at altitude is generally a function of getting the powerplant(s) up to an altitude where they are efficient. The airframe seems to only require proper IAS for efficiency. Turboprop engine is more efficient higher, airframe is more efficient higher. Mike C.
I don't think this is true. The turbine engine is most efficient at max ITT. The airframe is most efficient at L/D max.
The airframe portion of the equation doesn't care at what altitude you are. The IAS is what matters.
The engine portion is dependent on the specific engine. Turbine engines can be built to be most efficient at any altitude that the engineer wishes. It just so happens that they target higher altitudes because that is where people want to fly because of weather.
The efficiency of the entire package of a turbine powered plane should be maximum at an altitude where the airframe is at L/D max at the same time the engine is tapped out. If you wanted the most efficient turbine aircraft possible, you would need the most aerodynamic airframe possible equipped with engines that exhibit the lowest SFC and size those engines so that they max out on ITT at the altitude and temps where you intend to fly when the airframe is at L/D max. This aircraft would never sell though because it would be seen as underpowered and unacceptably slow.
Just for fun, think about the U2. It operates just above stall speed with the engine at redline temp. Of course it is at a ridiculously high altitude but this is because the engine is custom built to max out at the aircraft's maximum altitude. If you took the same airframe and operated it at the same IAS and fitted it with an engine of identical SFC that maxed out at 10,000', then the MPG should be the same.
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Last edited on 24 Jan 2015, 12:44, edited 1 time in total.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:41 |
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Joined: 08/18/13
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Ok, but the same little power at altitude cost the same gas down, with the only difference being the extra fuel burned in the climb. At altitude, you go a lot faster. If the distance traveled is insufficient to make the juice worth the squeeze, stay low. Otherwise, higher is better.
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Post subject: Re: Mitsubishi for first twin Posted: 24 Jan 2015, 12:48 |
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Username Protected wrote: At Sea Level, you go 140 kias on 20gph. Your TAS is 140kts. You have a 100 gallon tank so your range is 700NM.
At FL230, you go 140kias on 40gph. Your TAS is 280kts. You have a 100 gallon tank so your range is now 700NM. At FL230, 140 KIAS is 202 KTAS, not 280 KTAS. It doesn't take twice the fuel flow to fly 140 KIAS at FL230 for any normal airplane. Mike C.
I did not think I needed to say it was an illustration.
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