Hi im quite sure u have this covered but i think i dont like in many fsx addons is that the fuel compsumtion is lower the higher u fly no matter the weight and optimal level.

Jet engines are more efficient at higher altitudes. So this is mimicking real life.

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Jet engines are more efficient at higher altitudes. So this is mimicking real life.

hmmmmm don`t they work better with more are and lower altitude has more air than higher

hmmmmm don`t they work better with more are and lower altitude has more air than higher

The less air density there is the less fuel you need to run the engine. Thats the whole idea to fly higher, thats also the reason of Step Climb on Transatlantic Flights.

Jet engines are more efficient at higher altitudes. So this is mimicking real life.

But there is also weight in it so if u get to your max fl it doesnt mean ut mof economic than you optimal that is why there is optimal level and not just nax level. Jiri Sekerka

hmmmmm don`t they work better with more are and lower altitude has more air than higher

Yes they do. You'll get a lot more performance at low altitude than at high because the air is more dense. But you really don't need TOGA performance at FL410, so at the dispence of performance, you'll get better fuel efficiency in return.

And with less air density, there is less drag, so the reduced performance is not too big a deal. And Jiri, yes, that is why there is an optimum altitude. But the optimum altitude changes based on weight, the lighter you get the higher it is. Once the OA has passed a certain point, you initiate a step climb.In real life it is much more complicated, as a higher altitude usually means larger winds. If you are going against them, it could mean a lot of spent fuel, enough so as to make a lower altitude (more burn but less wind) more desirable.

And with less air density, there is less drag, so the reduced performance is not too big a deal. And Jiri, yes, that is why there is an optimum altitude. But the optimum altitude changes based on weight, the lighter you get the higher it is. Once the OA has passed a certain point, you initiate a step climb.In real life it is much more complicated, as a higher altitude usually means larger winds. If you are going against them, it could mean a lot of spent fuel, enough so as to make a lower altitude (more burn but less wind) more desirable.

i know all of this my q is: In fsx normaly the higher u go hmthe less fuel engines will take no matter the weight. Is this problem solved in ngx because in 747 is not.Jiri Sekerka

i know all of this my q is: In fsx normaly the higher u go hmthe less fuel engines will take no matter the weight. Is this problem solved in ngx because in 747 is not.Jiri Sekerka

Jiri,that is how it is in the real world.The engines don't consume more fuel when the aircraft is heavier. A higher weight will result in a longer climb and a lower ceiling, but the overall trend of higher=less fuel consumption remains the same.

Jiri,that is how it is in the real world.The engines don't consume more fuel when the aircraft is heavier. A higher weight will result in a longer climb and a lower ceiling, but the overall trend of higher=less fuel consumption remains the same.

so tell me why they fly on optimum fl and not on max pilots dont care about rate of climb (if they would a343 would be flying at fl200 :-) ). If u r heavy the engines have to work harder to maintain the speed thus more fuel is burned but in fsx even when engines r working harder the fuel compsumtion is always less.

so tell me why they fly on optimum fl and not on max pilots dont care about rate of climb (if they would a343 would be flying at fl200 :-) ). If u r heavy the engines have to work harder to maintain the speed thus more fuel is burned but in fsx even when engines r working harder the fuel compsumtion is always less.

It's true that a heavy aircraft will burn more fuel than a light one of the same type, but both will burn less at cruise level than at sea level. It's all about air density.The Optimum Flight Level for a near empty aircraft will be near or at the maximum Flight Level, while a heavy aircraft can not even reach the same maximum Flight Level. In addition to that, climb performance deteriorates rapidly near the max FL, which results in a very long climb, increased fuel burn and engine wear. So, while the engines burn less fuel at max FL, to get there is not always economically feasible. Optimum Flight Level calculation is a complex subject and fuel isn't the only factor to consider.So again, what you see in the 747 and most other aircraft is realistic.

Even though it is difficult to parse what this guy is saying I think I figured it out.Planes flying through the air are not subject to friction the same a heavy load on the ground would be. sekos appears to be asserting his idea based on the fact that a 10 ton truck's engine works a lot harder to push/drive the 10 tons on the ground than a 1 ton truck. Just throw this idea out of your head because airplanes rely on different principles. The fuel burn the higher you go will always be less, regardless of where you are inside the aircraft's operating envelope. The only difference that will significantly change as you get heavier is the maximum altitude. The heavier you are the less able you are to climb to the service ceiling. sekos: Think a little bit more clearly about what you're trying to say. Then proofread your submissions. The lack of coherent statements make it seem like everybody in this thread is wasting their time trying to answer your question because you may never grasp it. Please fix this.Drew

Okay, the "technical" explanation:The drag force acting on an aircraft can be expressed via the equation Fd = .5 * rho * V² * A * Cd, where rho is air density (in kilograms per metre cubed), V is the velocity (in metres per second), A is the cross-sectional area the drag acts on (in square metres), and Cd is the coefficient of drag (varies according to aircraft and has no units). Keeping in mind the equations of steady, unaccelerated flight (Thrust = Drag, Lift = Weight), it follows that for cruising flight, thrust must equal the force of drag. Since T = D, and drag can also be expressed as D = .5 * rho * V² * A * Cd. Since lift must equal weight, and the equation of lift is .5 * rho * V² * S * Cl (rho = air density, V = velocity, S = wing area, and Cl is the lift coefficient), weight must also equal .5 * rho * V² * S * Cl. If you then divide thrust by weight, you get Cl/Cd, which is essentially lift over drag. If you continue the algebra, the thrust required is equal to weight times drag all divided by lift. This, however, only gives you thrust required as a function of velocity.So going further, and extrapolating to include the height: the power required for an aircraft to stay in straight and level flight is equal to the force of drag times a factor of veloctiy, thus P = .5 * rho * V³ * A * Cd. The velocity at sea level in terms of weight can be expressed as sqrt((2 * weight)/(rho0 * S * Cl) or the square root of two times weight divided by the density of air at sea level times the area of the wing time the coefficient of lift. If we're assuming the coefficients of lift and drag remain constant, and the only variables which change are the density and the velocity, power required for straight and level flight at altitude becomes power required at sea level times (in parentheses) the square root of density at sea level divded by density at altitude. If you were to plot this, you would see that at sea level, for a given altitude, the power required is generally higher than that at a given altitude. Because there is less power required because of decreased drag etc. the thrust to achieve the required power is less. Therefore, less fuel is needed to maintain the forces of flight.The "simple" explanation:Drag is equal to thrust in cruise flight, and since air density decreases as you get higher, and since drag is proportional to both velocity and air density, it means as the air density becomes lower, but the velocity becomes"slightly" higher as you increase altitude, the overal drag force decreases as you climb. Since you're not constantly accelerating as you climb higher (per se, at least), the assumption can be made that the velocity INCREASE is proportionally less than the air density DECREASE, which in turn leads to the above statement. Since the thrust is equal to the drag, and the net force must equal 0 (i.e., the drag equals the thrust, since the forces are applied in opposite directions), the thrust also decreases as the drag decreases. This means that your velocity may slightly increase, but that your drag still decreases, that less thrust is required and thus that your fuel consumption is lowered for the velocity at which you are flying.Quick case and point for the proportionality of velocity increase and air density decrease: you are flying at around 320ktas when you transition from <250KIAS to >250KIAS (i.e. the airspeed transition at 10,000 feet). Your TAS at cruise is generally around 480. The proportion here is 1.5. The air density at 10,000 feet is about .90 kg/m³, and at FL350 about .36 kg/m³ or a proportion of 2.5. Since D = T = constant * rho * V² * constant * constant and rho is decreasing with a linear factor -2.5 (or increasing at a linear factor 0.4), V² increase at a linear factor 2.25, which results in a net decrease (i.e., factor less than 1 -- 0.9 to be exact).To the other guys, jet engines aren't more efficient at higher altitudes, they just need to do less work to provide the same force to overcome drag.References: Introduction to Flight by John Anderson, and Elements of airplane performance by Ger Ruijghok.Also, this is the quick crash-course version. I could get a lot more technical, but I'll spare you. If you're interested, you can always PM me.[EDIT] That last line came out wrong... I don't mean to come over as arrogant, but that's exactly how I sound now that I reread it. Sorry about that. ;)

Uh, so the last post was way more than this guy wanted to know....Even though your fuel burn is less at higher altitudes, you probably aren't being "as efficient overall" if you are both heavy and higher in altitude. Lets say a light airplane and a heavy airplane are flying right next to each other. The heavier airplane will have a higher angle of attack to maintain altitude if they both have the same airspeed, therefore needing more thrust, and therefore more fuel burn, even though they are at the same altitude. SO, if you are heavy, and you climb above your "optimal" flight level, you may have lower fuel burn per engine, but you've probably lost a lot of true airspeed (at least that's how it works in real life). I'm not sure how accurately FSX simulates this, or if PMDG have to tweak numbers to make it correct, but that's the theory at least.Hope this helped,Mark Hager