Hi guys, it's my first time to post here. Forgive me if i'm asking questions that have been asked before.Thanks to the avsim community, i've been flying with many fantastic freeware add-ons. But i've found that in general, the fuel flow always decreases with higher altitude, which is contradictory to real world datas. After passing optimum level, the fuel flow should increase.I looked into the air file but it seems too many parameters are linked to such a behaviour. Are there any guidelines or tips on modifying? Thank you very much.

Hi guys, it's my first time to post here. Forgive me if i'm asking questions that have been asked before.Thanks to the avsim community, i've been flying with many fantastic freeware add-ons. But i've found that in general, the fuel flow always decreases with higher altitude, which is contradictory to real world datas. After passing optimum level, the fuel flow should increase.I looked into the air file but it seems too many parameters are linked to such a behaviour. Are there any guidelines or tips on modifying? Thank you very much.

I can't help much, except by saying I noticed the same problem. I'm not sure it can be solved by tweaking the air file, but I'm not an expert.In fact, I noticed this when I was searching for a way to determine when the engines are on IDLE. I thought I could read the fuel flow for this, but it appears the idle fuel flow varies with altitude and airspeed...Eric

Hi guys, it's my first time to post here. Forgive me if i'm asking questions that have been asked before.Thanks to the avsim community, i've been flying with many fantastic freeware add-ons. But i've found that in general, the fuel flow always decreases with higher altitude, which is contradictory to real world datas. After passing optimum level, the fuel flow should increase.I looked into the air file but it seems too many parameters are linked to such a behaviour. Are there any guidelines or tips on modifying? Thank you very much.

I think your premise of fuel flow increasing with altitude may be in error. That would be odd behaviour under any condition I can think of. Fuel flow will always DECREASE with power as altitude increases, jets and props both. This holds true even in the case of a simple carburetor piston engine which gets richer with altitude, if that is what you are basing your assumption on. In this case, the fuel flow does not DECREASE ENOUGH with increasing altitude to keep the proper fuel/air ratio as the air thins with altitude. But again, flow decreases.In the case of a supercharged piston engine flat rated to some (critical) altitude, the flow will be nearly constant to that critical altitude. Above that altitude, the fuel flow will drop along with the power just as a non-supercharged engine would do from sea level on up.OK, that is engines. Now airplanes. If you fly an aircraft level at the same indicated airspeed, the power REQUIRED increases with altitude, because the aircraft is going faster. ( power= force X distance/time). Maybe this is what you have in mind. And finally, re-reading your post, optimum altitude is not some magic altitude of minimum fuel flow. Maximum fuel flow always decreases with altitude. It means that above opt alt you burn more fuel climbing that you gain in descent and/or you L/D, slower TAS or whatever will cause more fuel to be burned for the trip at that higher cruise altitude.

Thank you for your reply, Ron.And do you know a way of knowing if the engine is idle?Thanks,Eric

I think your premise of fuel flow increasing with altitude may be in error. That would be odd behaviour under any condition I can think of. Fuel flow will always DECREASE with power as altitude increases. This holds true even in the case of a simple carburetor piston engine which gets richer with altitude. This is because the the fuel flow does not DECREASE ENOUGH with increasing altitude to keep the proper fuel/air ratio as the air thins with altitude. But again, flow decreases.In the case of a supercharged piston engine flat rated to some (critical) altitude, the flow will be nearly constant to that critical altitude. Above that altitude, the fuel flow will drop along with the power just as a non-supercharged engine would do from sea level on up.

Thx for your reply, you're absolutely right about piston. But the case becomes different to jets.After passing the optimum altitude, the lift generated from the wings is not sufficient to keep the plane flying efficiently. Induced drag will increase more significantly. Thus the jet engines give extra power to overcome this and hence more fuel burn. Until the engines run to maximum rpm to yield the ceiling level, fuel burn surges when compared to that at optimum level.Taking PMDG 747 and you'll see the difference.Thank you anyway for your reply.

Thx for your reply, you're absolutely right about piston. But the case becomes different to jets.After passing the optimum altitude, the lift generated from the wings is not sufficient to keep the plane flying efficiently. Induced drag will increase more significantly. Thus the jet engines give extra power to overcome this and hence more fuel burn. Until the engines run to maximum rpm to yield the ceiling level, fuel burn surges when compared to that at optimum level.Taking PMDG 747 and you'll see the difference.Thank you anyway for your reply.

I was editing in some more stuff as you replied. Optimum altitude, etc. Maybe that is what you were thinking of. ron

Missing the clue "optimum altitude" in the original post, I had to guess what realm of flying we were in. I realize it is heavy jets at LRC cruise and tweaking which specific params to get things closer to real.With a 747 say , Mach is in the picture and its drag rise reduces L/D with increasing high altitude at a given AOA. Additionally, Mach drag rise is also dependent on AOA which increases with M constant. If there is a param that will increase mach drag rise with AOA, increasing that value might help get the effect you need. If there is no such param, there should be.Along the same lines, there IS a delta lift/AOA increase at higher mach values. If the modelled induced drag is derived from these resulting lower AOA's at high M, then there may be an unrealistic reduction in induced drag at altitudes above opt. ( and too low fuel flow as well)Yeah, the more I think about it, the more I think the prob is in this area. ( assuming that the L/d curve is ok at low alt and low Mach)I think what has to be reproduced in the modelling is the double hit increase from normal induced and enhanced mach high AOA drag and reduce any mach AOA reduction drag credit so the drag increases realistically at high AOA and high M .ron

Thx for your reply, you're absolutely right about piston. But the case becomes different to jets.After passing the optimum altitude, the lift generated from the wings is not sufficient to keep the plane flying efficiently. Induced drag will increase more significantly. Thus the jet engines give extra power to overcome this and hence more fuel burn. Until the engines run to maximum rpm to yield the ceiling level, fuel burn surges when compared to that at optimum level.Taking PMDG 747 and you'll see the difference.Thank you anyway for your reply.

BTW, Here's a great link that illustrates how fuel flow relates to altitude for jets.http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htmlIf you consider this an aerodynamics problem these may help...tbl 401 CL0 due to mach Scalar on Lift (affects -> Induced drag)tbl 430 CD0 due to mach Scalar on Drag (affects -> Parasitic drag)But I suspect an aerodynamics solution will only lower your absolute ceiling Alt.Danny

BTW, Here's a great link that illustrates how fuel flow relates to altitude for jets.http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htmlIf you consider this an aerodynamics problem these may help...tbl 401 CL0 due to mach Scalar on Lift (affects -> Induced drag)tbl 430 CD0 due to mach Scalar on Drag (affects -> Parasitic drag)But I suspect an aerodynamics solution will only lower your absolute ceiling Alt.Danny

Thank you guys for advices. I tried to experiment on CD0 and CL0 in these few days and found that the modification led to dramatic change in the overall aerodynamics. Seems I have to give up in this complicated maths...

For a jet, it is essential to ensure your engines are running at the correct level.- Jet engines run at a given thrust specific fuel consumption. TSFC. This is controlled in the aircraft.cfg file.- You have to adjust TBL 1502, TBL 1503 and TBL 1504 to obtain the correct engine performance. TBL 1503 and TBL 1504 are relatively the same across jet turbines (for FS purposes), leaving TBL 1502 as the important one to get the correct N1 to N2 ratios.- You must next set the values in TBL 1506 so the engines have the proper thrust level to generate the correct fuel flows at a given N1 level.- You adjust the flight dynamics to have the aircraft actually stall at the correct clean stall speed.That should put your aircraft's performance relatively close.TBL 401 will affect induced drag increases due to mach speed levels, which affects maximum velocity.TBL 404 will affect amount of pitch for a given amount of lift. It's a linear ramp from mininum angle of attack to maximum and it's min/max values are primarily affected by wing shape/type.These are just a few 'general notes'... and isn't even close to a complete FDE design explanation. Making an accurate FDE for FS is very, very complex.