For Kevin:
Thank you for your clear and concise explanation.
Another one for you aircraft developers... .air file editing
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petermcleland
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Re: Another one for you aircraft developers... .air file edi
Deleted...my post refered to AI performance :redface:
Regards,
http://www.petermcleland.com/
Updated 28/8/2007
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http://www.petermcleland.com/
Updated 28/8/2007
My Channel
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LoadMaster
- Chipmunk
- Posts: 13
- Joined: 06 May 2005, 06:09
High Altitude performance
As the problem which prompted this forum topic appears to be altitude related I would like to suggest you have a look at the Inverse Pressure Altitude (IPA) setting in Records 1503 and 1504 of your .air file before you start messing with drag of any kind.
In FlightSim the Throttle is linked (1503/1504) to CN2 which is linked (1502) to CN1 which is linked to both Thrust (1506) and Mass Flow (1507) and possibly Corrected Fuel Flow. As 1503/1504 is the only link in this chain of events that is altitude related, your problem could be too much CN2 depletion as altitude increases.
This might be the case if the IPA is set too high and the engines might not be achieving the CN2 figure necessary to achieve cruise speed at the cruise throttle setting at high altitudes.
To increase performance at high altitude (without affecting your sea-level performance) you could try decreasing the IPA in Records 1503/1504 of the .air file.
DON'T FORGET to write down the original IPA figure in case you need to go back to it.
As a substantial decrease in IPA may be required to obtain the desired result, if you find you need to reduce the IPA to an altitude below the aircraft's service ceiling/maximum altitude, then you should find another solution to your problem.
In my experience with Jet aircraft in FlightSim, CN2 decreases as altitude increases, and increases as speed increases. It would therefore be reasonable to assume that initial testing/adjusting of this theory for power/speed should be done in level flight at the aircraft's service ceiling.
I used Rick Piper's HS.748 Series 2B as a testbed for the theory.
The HS.748 2B is supposed to have a max cruise speed of 244 Ktas but it was doing 252 Ktas at AUW 45,000 lb in standard Atmosphere and zero wind at its service ceiling of 25,000 feet. Reluctant to mess with the drag settings, I increased the IPA from 4.237 (=just under 35,000 ft) to 5.402485 (=40,000 ft).
In identical conditions after increasing the IPA, airspeed was reduced from 252 Ktas to 246 Ktas and fuel consumption dropped from 1,491 PPH to 1,434 PPH. Increasing the IPA again to 6.242197 (=43,000 ft) should just about get the speed spot on.
If you're presented with several Series of the same aircraft, each with different engine performance, but you're only given one set of engine parameters (as in Rick's 748), the above theory seems to be a simple way of varying engine performance to suit each Series in the set.
There are side-effects to this theory, however (which I haven't fully explored yet).
Depending on whether you increase or decrease the IPA (and by how much), your rate-of-climb/climb-speed will be affected, as will fuel consumption, aircraft attitude, idle thrust at high altitude, rate-of-descent/descent-speed, the time it takes for the aircraft to achieve cruise speed, and possibly Vref/Vat speeds and takeoff run lengths if you're in the habit of popping into and out of places like SEQU and SLLP.
To obtain the altitude equivalent of the IPA in Records 1503/1504, divide (the figure) 1 by the IPA figure in 1503/1504 to obtain the Pressure Ratio, then consult a Standard Altitude Table to obtain the altitude the Pressure Ratio relates to/is closest to.
To obtain the IPA for a particular altitude, divide (the figure) 1 by the Pressure Ratio given in the Standard Altitude Table for that particular altitude.
If you don't have a Standard Altitude Table, I downloaded one from http://www.Boeing-727.com (Standardaltitude.pdf, single page) which provides Standard Altitude data from 0 ft up to 50,000 ft in 1,000 ft increments.
Hope you find the idea of interest.
Regards
David
In FlightSim the Throttle is linked (1503/1504) to CN2 which is linked (1502) to CN1 which is linked to both Thrust (1506) and Mass Flow (1507) and possibly Corrected Fuel Flow. As 1503/1504 is the only link in this chain of events that is altitude related, your problem could be too much CN2 depletion as altitude increases.
This might be the case if the IPA is set too high and the engines might not be achieving the CN2 figure necessary to achieve cruise speed at the cruise throttle setting at high altitudes.
To increase performance at high altitude (without affecting your sea-level performance) you could try decreasing the IPA in Records 1503/1504 of the .air file.
DON'T FORGET to write down the original IPA figure in case you need to go back to it.
As a substantial decrease in IPA may be required to obtain the desired result, if you find you need to reduce the IPA to an altitude below the aircraft's service ceiling/maximum altitude, then you should find another solution to your problem.
In my experience with Jet aircraft in FlightSim, CN2 decreases as altitude increases, and increases as speed increases. It would therefore be reasonable to assume that initial testing/adjusting of this theory for power/speed should be done in level flight at the aircraft's service ceiling.
I used Rick Piper's HS.748 Series 2B as a testbed for the theory.
The HS.748 2B is supposed to have a max cruise speed of 244 Ktas but it was doing 252 Ktas at AUW 45,000 lb in standard Atmosphere and zero wind at its service ceiling of 25,000 feet. Reluctant to mess with the drag settings, I increased the IPA from 4.237 (=just under 35,000 ft) to 5.402485 (=40,000 ft).
In identical conditions after increasing the IPA, airspeed was reduced from 252 Ktas to 246 Ktas and fuel consumption dropped from 1,491 PPH to 1,434 PPH. Increasing the IPA again to 6.242197 (=43,000 ft) should just about get the speed spot on.
If you're presented with several Series of the same aircraft, each with different engine performance, but you're only given one set of engine parameters (as in Rick's 748), the above theory seems to be a simple way of varying engine performance to suit each Series in the set.
There are side-effects to this theory, however (which I haven't fully explored yet).
Depending on whether you increase or decrease the IPA (and by how much), your rate-of-climb/climb-speed will be affected, as will fuel consumption, aircraft attitude, idle thrust at high altitude, rate-of-descent/descent-speed, the time it takes for the aircraft to achieve cruise speed, and possibly Vref/Vat speeds and takeoff run lengths if you're in the habit of popping into and out of places like SEQU and SLLP.
To obtain the altitude equivalent of the IPA in Records 1503/1504, divide (the figure) 1 by the IPA figure in 1503/1504 to obtain the Pressure Ratio, then consult a Standard Altitude Table to obtain the altitude the Pressure Ratio relates to/is closest to.
To obtain the IPA for a particular altitude, divide (the figure) 1 by the Pressure Ratio given in the Standard Altitude Table for that particular altitude.
If you don't have a Standard Altitude Table, I downloaded one from http://www.Boeing-727.com (Standardaltitude.pdf, single page) which provides Standard Altitude data from 0 ft up to 50,000 ft in 1,000 ft increments.
Hope you find the idea of interest.
Regards
David
David,
A very interesting line of thought - this sounds very plausible for the case of Rick's 748.
I think though that there is a big difference between (i) looking for 8 kt in cruise speed, plus 4% in sfc on a Rick Piper aeroplane with very well-thought-out FDEs and (ii) taking an aircraft from one of flight sim's more prolific designers with 'one-size-fits-all' flight dynamics, where speed errors of 100 kt+ are not unusual.
The very first thing the user has to look at is whether the parameters in the two FDE files reflect physical reality: for example, there's not a lot of point in adjusting the full-throttle height of the engine if the wing area is only half what it should be!
I only mention this because we are approaching the problem from two different directions and none of us appears to know which aircraft Chris refers to.
In any case, I agree that your approach is a necessary refinement once the basics are dealt with.
Cheers,
Kevin
A very interesting line of thought - this sounds very plausible for the case of Rick's 748.
I think though that there is a big difference between (i) looking for 8 kt in cruise speed, plus 4% in sfc on a Rick Piper aeroplane with very well-thought-out FDEs and (ii) taking an aircraft from one of flight sim's more prolific designers with 'one-size-fits-all' flight dynamics, where speed errors of 100 kt+ are not unusual.
The very first thing the user has to look at is whether the parameters in the two FDE files reflect physical reality: for example, there's not a lot of point in adjusting the full-throttle height of the engine if the wing area is only half what it should be!
I only mention this because we are approaching the problem from two different directions and none of us appears to know which aircraft Chris refers to.
In any case, I agree that your approach is a necessary refinement once the basics are dealt with.
Cheers,
Kevin
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LoadMaster
- Chipmunk
- Posts: 13
- Joined: 06 May 2005, 06:09
Kevin
I raised the IPA to =43,000 ft and the speed was back to 242 Ktas and fuel 1,404 PPH. 244 Ktas was achieved by the time the AUW got down to 41,000 lb.
What bothers me now is that I have an aircraft with a service ceiling of 25,000 that has an IPA of 43,000 ft which seems to indicate that the CN2 figures themselves are not quite right.
I'd like to get the IPA back to about 30,000 ft by editing the CN2 figures on the high altitude line. Very easy to do in 1504 but if I have to edit the same line of figures in 1503 as well I could get myself into all sorts of problems.
Another reason for getting the IPA back to a more reasonable figure for the 2B aircraft is that I was hoping to use this idea to reduce the max cruise even further back to 232 Ktas for the Series 2 which at present would require an IPA in excess of 50,000 ft which is getting a bit ridiculous.
I take exception to your comment that implies that Rick's 748 FD's are well-thought-out or that they resemble physical reality. In the context of this topic I will only say that the 2B I am flying has 2B wingspan and 2B wing area and has been given the wing sweep it deserves.
Regards
David
I raised the IPA to =43,000 ft and the speed was back to 242 Ktas and fuel 1,404 PPH. 244 Ktas was achieved by the time the AUW got down to 41,000 lb.
What bothers me now is that I have an aircraft with a service ceiling of 25,000 that has an IPA of 43,000 ft which seems to indicate that the CN2 figures themselves are not quite right.
I'd like to get the IPA back to about 30,000 ft by editing the CN2 figures on the high altitude line. Very easy to do in 1504 but if I have to edit the same line of figures in 1503 as well I could get myself into all sorts of problems.
Another reason for getting the IPA back to a more reasonable figure for the 2B aircraft is that I was hoping to use this idea to reduce the max cruise even further back to 232 Ktas for the Series 2 which at present would require an IPA in excess of 50,000 ft which is getting a bit ridiculous.
I take exception to your comment that implies that Rick's 748 FD's are well-thought-out or that they resemble physical reality. In the context of this topic I will only say that the 2B I am flying has 2B wingspan and 2B wing area and has been given the wing sweep it deserves.
Regards
David
David,
One of the problems of working with Microsoft's FS FDE's versus those in the real world is that there are many parameters which are oversimplified, merged, wrong or simply absent from the coding.
I have no doubt that the simple code of MS's basic gas turbine model represents SOMETHING reasonably well, but it has to stand duty for triple-spool turbofans, free-shaft turbines, simple pure jets as well as a whole range of turboprops. How well can it model the Dart, for example, as well as the Allison T-56, which is essentially a one-speed engine?
Your methodology assumes that the underlying MS simulation model is capable of responding accurately to valid parameter variation: however, your results seem to indicate that in this case, perhaps it can't.
There is one possibility, though: just because the 748 flies no higher than 25 000 ft doesn't necessarily say anything about the Dart's inherent altitude capabilities (simply as a gas generator set). While 50 000 ft might be 'over the top', 40 000 might well be plausible. Please note that I am not saying that the airframe/engine/prop combination is capable of operation at this altitude, only that the 'gg core' would like to operate as high as possible.
True, accurate flight simulation isn't yet possible on a PC: simulation FDEs are a compromise - go as far as you can with real-world numbers and then adjust judiciously to get as close as you can to reality.
Cheers,
Kevin
One of the problems of working with Microsoft's FS FDE's versus those in the real world is that there are many parameters which are oversimplified, merged, wrong or simply absent from the coding.
I have no doubt that the simple code of MS's basic gas turbine model represents SOMETHING reasonably well, but it has to stand duty for triple-spool turbofans, free-shaft turbines, simple pure jets as well as a whole range of turboprops. How well can it model the Dart, for example, as well as the Allison T-56, which is essentially a one-speed engine?
Your methodology assumes that the underlying MS simulation model is capable of responding accurately to valid parameter variation: however, your results seem to indicate that in this case, perhaps it can't.
There is one possibility, though: just because the 748 flies no higher than 25 000 ft doesn't necessarily say anything about the Dart's inherent altitude capabilities (simply as a gas generator set). While 50 000 ft might be 'over the top', 40 000 might well be plausible. Please note that I am not saying that the airframe/engine/prop combination is capable of operation at this altitude, only that the 'gg core' would like to operate as high as possible.
True, accurate flight simulation isn't yet possible on a PC: simulation FDEs are a compromise - go as far as you can with real-world numbers and then adjust judiciously to get as close as you can to reality.
Cheers,
Kevin