Do heavy planes really consume more fuel?

[maxkang]

Most people argue like so, more weight>more lift>more induced drag>more thrust required for cruise flight> more fuel burn, which I think is fair. However, to account for more weight you would fly at a higher speed giving rise to high lift so don't you get to the destination faster? If the optimum range of engine setting allows you to have a higher thrust setting I feel that heavier planes might rather consume less or about the same amount of fuel.

Can you really that simply conclude that heavy planes consume more fuel?

[skyking]

Originally Posted by maxkang

Most people argue like so, more weight>more lift>more induced drag>more thrust required for cruise flight> more fuel burn, which I think is fair. However, to account for more weight you would fly at a higher speed giving rise to high lift so don't you get to the destination faster? If the optimum range of engine setting allows you to have a higher thrust setting I feel that heavier planes might rather consume less or about the same amount of fuel.

Can you really that simply conclude that heavy planes consume more fuel?

yes you can. It really is that simple. To fly the same speed as a lighter plane, you'd need a higher angle of attack. This takes more thrust. More fuel.
My background is commercial pilot and flight instructor.

[maxkang]

Don't you need high AOA for heavier aircraft, not for lighter aircraft since high AOA at same speed will allow for more lift to be generated.

What I was curious about is whether or not you can increase the speed while reducing or keeping the same AOA, and by doing so you would get to the destination faster. Is this simply impossible to achieve due to limitations on the engine or..?

[skyking]

All those things you are talking about are variable to some extent. AOA applies to any wing, no matter how light.
all things equal, it takes a higher AOA and thus induced drag to fly a heaver plane vs a lighter one. Speeding it up won't change that fact, the heavier plane will have a higher AOA than the lighter one, more drag.
Now there are tradeoffs in operation. Sometimes it pays to go higher with tailwinds for example.
The airlines will often fly a heavily loaded plane at a lower altitude until some fuel burns off, then go on up to a better cruising altitude. This is because the AOA is going to be out of the efficient range a the higher altitude and weight.

[maxkang]

Thanks for the reply.

I can understand every point except that heavier planes HAVE TO fly at high AOA. Why can't they simply operate at faster speed with the same or a smaller AOA? Provided that you have increased a reasonable amount of thrust so that you still get an optimum efficiency, isn't it possible for you get to the destination faster if you flew faster? so you would just have to shut off the engine immediately and the required amount of lift will be still achieved as you have increased the speed.(Lift being almost proportional to the square of speed)

[skyking]

There is no such thing as a free lunch. To achieve the higher speed, it takes a lot more thrust.

[maxkang]

I agree with you. However, once you have reached the speed that you wish to cruise at, as long as you produce thrust that equals drag I think you can still maintain cruise flight. From my understanding, a bigger thrust is simply to match the total drag, posing no constraint on the speed(reasonable amount).

Sorry for keep bothering you, I just want to understand why I am wrong

[skyking]

Ok, look at it this way.
two planes, A and B
Each plane is the same but plane A weighs more.
At any given speed and altitude, in unaccelerated level flight, plane A must have a higher angle of attack.
Going faster makes no difference. Plane A must have a higher angle of attack to produce the needed lift, to overcome that added weight.
Speed up both planes and look again. Plane A still has the same problem, still has to fly at a higher angle of attack than plane B does.
Unfortunately speeding up has a big fuel penalty. Drag increases as approximately the square of speed.
Thrust increases as approximately the cube of speed.
To go 10% faster could require as much as 33% more thrust. Not a good tradeoff.

[maxkang]

Thank you so much for the reply!

I thought we might be able to consume a lot less fuel by arriving at the destination faster but I realize that it's not quite like that in reality.

Another issue was that I was merely thinking about the induced drag, ignoring other source of drag. I thought the penalty we get by increasing speed might be still be worthwhile as the induced drag coefficient is proportional to lift coefficient squared(my initial guess was that lowering AOA might compensate for there being higher speed)

Thank you so much for clearing up my confusion!

[ksa8907]

just to add my $.02, if you think of things of this nature in terms of energy it is much easier to understand. Sometimes the numbers get confusing and "lie" to you, but energy is simple. Faster? more energy. heavier? more energy. bigger? more energy.