Another one for the aircraft developers, I'm really scratching my head.
At the moment I'm playing around with an aircraft that goes far in excess of its max top speed at sea level yet is a real pig at high altitudes where it is supposed to excel, and won't accelerate much at all.
Now, I'm assuming this is far beyond the realms of simple aircraft.cfg tweaking so... what can be done? I've opened up the air file and I'm sat staring at a rather bewildering array of figures that all seem to effect something; is there a hard and fast way to go about this or is every .air file different? Anywhere I should start looking to fix my particular problem or... is this a case of trial and error? And finally... how likely am I to break something? :dance:
Thanks everyone.
Another one for you aircraft developers... .air file editing
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Quixoticish
- Concorde
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Chris,
Most things in fs2004 can be tweaked one way or another in the cfg file without venturing into the .air file, as the cfg file overrules settings in the air file.
I'm sure this is covered in a "how to play with air & cfg files" file by Bob Chilico on avsim. It's a while since I read it but I think the solution for this altitude speed problem that he suggested was in/decreasing the intake area in the cfg file - amongst other things. As long as you keep a copy of the cfg file from before starting to play with it you can always retrace your steps - which reduces the danger of breaking something like next door's cat in frustration. When I change a value I try to remember to put the old one after it with a couple of // to comment it out. It also helps remind me what I've been playing with when it all goes pear shaped! Trial and error rules ko.
Anyhow something along the lines of :
Doohickey = 1.23 // 1.000
where 1.23 is the setting that cures world poverty and 1.000 is the one left in by the original author.
(This project wouldn't be a Phantom by any chance? I gave up and learnt to love light speed+ at sea level on one.)
Most things in fs2004 can be tweaked one way or another in the cfg file without venturing into the .air file, as the cfg file overrules settings in the air file.
I'm sure this is covered in a "how to play with air & cfg files" file by Bob Chilico on avsim. It's a while since I read it but I think the solution for this altitude speed problem that he suggested was in/decreasing the intake area in the cfg file - amongst other things. As long as you keep a copy of the cfg file from before starting to play with it you can always retrace your steps - which reduces the danger of breaking something like next door's cat in frustration. When I change a value I try to remember to put the old one after it with a couple of // to comment it out. It also helps remind me what I've been playing with when it all goes pear shaped! Trial and error rules ko.
Anyhow something along the lines of :
Doohickey = 1.23 // 1.000
where 1.23 is the setting that cures world poverty and 1.000 is the one left in by the original author.
(This project wouldn't be a Phantom by any chance? I gave up and learnt to love light speed+ at sea level on one.)
Hi Chris,
It sounds as though you are working on a jet aircraft and the engine parameters in both the aircraft.cfg file and the .air file are a bit awry.
Getting jets to perform correctly at both high and low level in FS9 is not easy because it relies on several section of the .air file as well as the entries in the aircraft.cfg The easiest thing to try is to change the inlet_area parameter in the [TurbineEngineData] section of the aircraft.cfg. Reducing the area can reduce intake drag at high level and, hence, increase max speed.
I don't like to discourage you but I have been doing FDE for a number of years and still find jet performance tricky to get anywhere near correct.
Good luck,
Brian
It sounds as though you are working on a jet aircraft and the engine parameters in both the aircraft.cfg file and the .air file are a bit awry.
Getting jets to perform correctly at both high and low level in FS9 is not easy because it relies on several section of the .air file as well as the entries in the aircraft.cfg The easiest thing to try is to change the inlet_area parameter in the [TurbineEngineData] section of the aircraft.cfg. Reducing the area can reduce intake drag at high level and, hence, increase max speed.
I don't like to discourage you but I have been doing FDE for a number of years and still find jet performance tricky to get anywhere near correct.
Good luck,
Brian
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Quixoticish
- Concorde
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- Joined: 15 Oct 2005, 15:16
- Location: York
Hi Chris,
If you are dealing with performance, rather than handling issues with an aircraft, much of the .air file will not be of concern. However, what you will need to look at (in both the Aircraft.cfg file and .air file) are the sections dealing with both drag and thrust.
For any high-performance aircraft, there are both dynamic pressure limitations (in other words, limiting ias) and Mach number limits: usually, at low altitude it will be IAS, while at high altitude it will be Mach number.
Therefore, in addition to the issues of thrust (mostly in the .cfg file), you need to look at the drag coefficients (profile drag mostly at high speed) in the .air file, section 1101 - 'zero lift drag' and the Mach drag effects table in section 430.
As an aside, unfortunately, it's not uncommon for designers to 'pinch' a .air file from another aircraft (often one of their own, I hasten to add) which leads to some of their aircraft having bizzare performance anomalies. However, you could try something like this yourself, in order to see whether the .air file really is at fault: take a .air file from another aircraft of similar performance and geometry and try it as a subsitute. Mind you, while the Hunter could be a good analogue for the Sabre, the Gnat would not be very representative of the Lightning, so you would need to select carefully.
What I do with jet aircraft is first to balance thrust and drag at sea level: by the way, do all the following at the correct aircraft weights for the performance figures you have. Usually, it's fairly easy to find performance figures for max ias at sea level, the thrust can be set (or the drag) and the other parameter changed to match the speed to within a knot or two. Then check rate of climb @ sl : this will depend mostly on the excess thrust available, not the drag, so it will serve to show if you had 'got it right' earlier when you varied the drag and thrust. You then go round this iteration loop until you've got something close to the published figures.
Then check the aircraft's service ceiling (the usual definition is when the rate of climb reduces to 500 ft/min) at the optimum climb speed. If this matches reasonably (within 1000 ft or so at around 50 000 ft) then make a flat-out run at the altitude for which you have the performance figures (eg 'M1.9 @ 47 000 ft'). Adjust the Mach drag in the .air file to scale back any excess.
BTW, the main reasons for checking the service ceiling are (i) as a sensitive check on the excess thrust as well as the intake drag, etc and (ii) as a further confirmation of the other relevant parameters, lift and weight. Weight should be a known value (see above) but lift may not match (too much/little). Again, we go around an iteration loop until it matches.
There's more to it, if you want to get in further, but this, and the intake area mentioned by Brian, will get you most of the way there.
Sorry for the long-winded discussion, but I'm a high-speed aerodynamicist by training irl, and it's hard to break the habit.
I hope that this is of some help, and good luck to you.
Kevin
If you are dealing with performance, rather than handling issues with an aircraft, much of the .air file will not be of concern. However, what you will need to look at (in both the Aircraft.cfg file and .air file) are the sections dealing with both drag and thrust.
For any high-performance aircraft, there are both dynamic pressure limitations (in other words, limiting ias) and Mach number limits: usually, at low altitude it will be IAS, while at high altitude it will be Mach number.
Therefore, in addition to the issues of thrust (mostly in the .cfg file), you need to look at the drag coefficients (profile drag mostly at high speed) in the .air file, section 1101 - 'zero lift drag' and the Mach drag effects table in section 430.
As an aside, unfortunately, it's not uncommon for designers to 'pinch' a .air file from another aircraft (often one of their own, I hasten to add) which leads to some of their aircraft having bizzare performance anomalies. However, you could try something like this yourself, in order to see whether the .air file really is at fault: take a .air file from another aircraft of similar performance and geometry and try it as a subsitute. Mind you, while the Hunter could be a good analogue for the Sabre, the Gnat would not be very representative of the Lightning, so you would need to select carefully.
What I do with jet aircraft is first to balance thrust and drag at sea level: by the way, do all the following at the correct aircraft weights for the performance figures you have. Usually, it's fairly easy to find performance figures for max ias at sea level, the thrust can be set (or the drag) and the other parameter changed to match the speed to within a knot or two. Then check rate of climb @ sl : this will depend mostly on the excess thrust available, not the drag, so it will serve to show if you had 'got it right' earlier when you varied the drag and thrust. You then go round this iteration loop until you've got something close to the published figures.
Then check the aircraft's service ceiling (the usual definition is when the rate of climb reduces to 500 ft/min) at the optimum climb speed. If this matches reasonably (within 1000 ft or so at around 50 000 ft) then make a flat-out run at the altitude for which you have the performance figures (eg 'M1.9 @ 47 000 ft'). Adjust the Mach drag in the .air file to scale back any excess.
BTW, the main reasons for checking the service ceiling are (i) as a sensitive check on the excess thrust as well as the intake drag, etc and (ii) as a further confirmation of the other relevant parameters, lift and weight. Weight should be a known value (see above) but lift may not match (too much/little). Again, we go around an iteration loop until it matches.
There's more to it, if you want to get in further, but this, and the intake area mentioned by Brian, will get you most of the way there.
Sorry for the long-winded discussion, but I'm a high-speed aerodynamicist by training irl, and it's hard to break the habit.
I hope that this is of some help, and good luck to you.
Kevin
Last edited by Kevin on 05 Mar 2006, 11:11, edited 2 times in total.
AIR file editing
May I suggest that you also ensure that you convert IAS into TAS at whatever height you are flying and that during your tests you have nil wind conditions or at least make corrections for wind velocity.
Yes, there is often a confusion between tas & ias: performance figures are almost invaribly quoted as true air speed, but numbers relevant to aircraft structural limits as found in Pilot's Notes, for example, may well be quoted as indicated air speed, because that is what matters in that case. That's one reason to do the run at sea level ISA conditions, because they're the same at that altitude. In any case, check how your performance numbers are quoted.
Don't forget, if the aircraft is transonic/supersonic at sea level, there will also be a few extra iterations over the whole performance envelope in order to try to apportion the low-level drag between Mach and non-Mach zero lift drag.
Also, to amplify Hobby's second point: groundspeed is not relevant to aircraft performance in any way (wind shear excepted). The aircraft moves through the air, not over the ground, and knowledge of the groundspeed is solely a navigation issue.
A couple of other points I forgot to make in my post above are to check the Aircraft.cfg file for (i) the 'flight tuning' section; there are scalars for lift and drag which may have been set by the designer to other than 1.00 and (ii) 'airplane geometry' (also in the .air file but mostly overwritten by the .cfg) may have very incorrect values set; it's as well to start with good values here, even if later adjustments are needed, otherwise you don't know where you're starting from.
There's also a 'thrust scalar' available under [jet engine] in the .cfg file.
Kevin
Don't forget, if the aircraft is transonic/supersonic at sea level, there will also be a few extra iterations over the whole performance envelope in order to try to apportion the low-level drag between Mach and non-Mach zero lift drag.
Also, to amplify Hobby's second point: groundspeed is not relevant to aircraft performance in any way (wind shear excepted). The aircraft moves through the air, not over the ground, and knowledge of the groundspeed is solely a navigation issue.
A couple of other points I forgot to make in my post above are to check the Aircraft.cfg file for (i) the 'flight tuning' section; there are scalars for lift and drag which may have been set by the designer to other than 1.00 and (ii) 'airplane geometry' (also in the .air file but mostly overwritten by the .cfg) may have very incorrect values set; it's as well to start with good values here, even if later adjustments are needed, otherwise you don't know where you're starting from.
There's also a 'thrust scalar' available under [jet engine] in the .cfg file.
Kevin
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Quixoticish
- Concorde
- Posts: 1124
- Joined: 15 Oct 2005, 15:16
- Location: York
Thanks for all of the help everyone. I've passed a link to this thread on to someone who is infinitely more capable with flight dynamics than my good self and we'll puzzle through it together. I managed to increase the speed/acceleration at high altitude using the intake attribute as everyone has suggested, I'm just now looking for a variable or set of variables to tinker with that decrease low altitude performance without harming that up top. It's definately going to be a delicate balancing act.
AIR file editing
For Kevin and all who may be interested:
The reason I suggested that the testing of Fs9 modified AIR or acft.cfg files be carried out in nil wind conditions is that I believe, and I could be wrong, that most of the published reference material regarding real world aircraft performance - speeds at different altitudes, climb rates, fuel consumption etc. are calculated as if the real test had been carried out in still air conditions.
Clarification of this point would be helpful to all who care to tinker with the fs9 mathematical models which make our aircraft fly - or not!
"Beware all would be pilots. If the weather doesn't get you then Isaac will!!!" RAF Flt Sgt pilot AD 1954.
The reason I suggested that the testing of Fs9 modified AIR or acft.cfg files be carried out in nil wind conditions is that I believe, and I could be wrong, that most of the published reference material regarding real world aircraft performance - speeds at different altitudes, climb rates, fuel consumption etc. are calculated as if the real test had been carried out in still air conditions.
Clarification of this point would be helpful to all who care to tinker with the fs9 mathematical models which make our aircraft fly - or not!
"Beware all would be pilots. If the weather doesn't get you then Isaac will!!!" RAF Flt Sgt pilot AD 1954.
Hobby,
Performance parameters which are affected by winds, such as range, take-off distances (TORA/TODA/ASDA when referred to the airfield), initial climb angle to clear a 50' obstacle, landing distances, etc ARE always referred to still-air conditions and (usually) sea-level ISA (International Standard Atmosphere) conditions. An aircraft ODM (Operating Data Manual - the ring-binder modern equivalent of the old blue Pilots Notes booklets) will then add lines to the relevant graphs or tables to show the effects of variations in:
- ISA (eg 'ISA+10' for 10 deg C higher temperature)
- Altitude
- Wind speed and direction
for those parameters. Range, for example, is generally quoted as 'ISA gross still-air range' as a reference condition. It is then easy to recompute for the conditions prevailing.
However, for all other gross performance parameters, including aircraft speeds at different altitudes, climb rates (though NOT climb angles, as above), the wind speed is not relevant and is not a part of any real-world performance testing, EXCEPT in the unusual case of the use of ground-based tracking instruments to perform that testing, where it is obviously necessary in order to eliminate error. This latter case would be, for example, in an FAI speed record attempt.
For normal real-world aircraft speed/mach number/climb performance testing, wind speed is not an issue (it's not even noted on test sheets), but outside air temperature (OAT) and pressure are, because we still need to refer these to the reference ISA condition.
Incidentally, the OAT issue is why Hawker, Supermarine, etc often sent their transonic speed-record-attempt aircraft to places such as the Libyan desert in mid-summer - if the air is much hotter than ISA at the same altitude you will achieve a higher airspeed for the same Mach number. Because it postponed the transonic drag rise, it could be worth another 20kt or so - good for a speed record, but not giving a speed which could be reached by exactly the same aircraft on a British winter's day!
I hope that this clarifies things a bit.
Best regards,
Kevin
Performance parameters which are affected by winds, such as range, take-off distances (TORA/TODA/ASDA when referred to the airfield), initial climb angle to clear a 50' obstacle, landing distances, etc ARE always referred to still-air conditions and (usually) sea-level ISA (International Standard Atmosphere) conditions. An aircraft ODM (Operating Data Manual - the ring-binder modern equivalent of the old blue Pilots Notes booklets) will then add lines to the relevant graphs or tables to show the effects of variations in:
- ISA (eg 'ISA+10' for 10 deg C higher temperature)
- Altitude
- Wind speed and direction
for those parameters. Range, for example, is generally quoted as 'ISA gross still-air range' as a reference condition. It is then easy to recompute for the conditions prevailing.
However, for all other gross performance parameters, including aircraft speeds at different altitudes, climb rates (though NOT climb angles, as above), the wind speed is not relevant and is not a part of any real-world performance testing, EXCEPT in the unusual case of the use of ground-based tracking instruments to perform that testing, where it is obviously necessary in order to eliminate error. This latter case would be, for example, in an FAI speed record attempt.
For normal real-world aircraft speed/mach number/climb performance testing, wind speed is not an issue (it's not even noted on test sheets), but outside air temperature (OAT) and pressure are, because we still need to refer these to the reference ISA condition.
Incidentally, the OAT issue is why Hawker, Supermarine, etc often sent their transonic speed-record-attempt aircraft to places such as the Libyan desert in mid-summer - if the air is much hotter than ISA at the same altitude you will achieve a higher airspeed for the same Mach number. Because it postponed the transonic drag rise, it could be worth another 20kt or so - good for a speed record, but not giving a speed which could be reached by exactly the same aircraft on a British winter's day!
I hope that this clarifies things a bit.
Best regards,
Kevin