Fuel Burn as a Function of Weight

[Njacko]

This is an interesting question, which Mike Busch touches on but does not answer in this EAA article:
https://www.savvyaviation.com/wp-conten ... iently.pdf
I’ll have a go at the math, based on a couple of assumptions:

Let’s assume that we’re going to cruise somewhere close to max Lift/Drag ratio speed, which is the “best glideâ€￾ speed in our POH. We may want to fly a bit faster, say the “Carson speedâ€￾ in the above article, but it won’t make a heap of difference to the assumption that we’re at, or close to, the speed at which parasitic drag and induced drag are equal. Since the whole point of our engine is to convert fuel into thrust equal and opposite to the total drag, we can say that roughly half of our fuel burn is used to overcome induced drag.

The next assumption is that propeller thrust is directly proportional to fuel consumption. That’s probably not exactly true, but any finer assumption gets way too complicated.

Applying this to my MX-7-180, burning 34 litres/hour at 95 KIAS and weighing 885 kg with just me on board, plus 20 kg of clutter in the back and full main tanks, we can say that 17 litres of fuel is being used to produce the required 885 kg of lift.

If I fill the aux tanks with 110 litres of fuel (or add an 80 kg passenger), the total aircraft weight will be about 965 kg and (first approximation) I will need to burn (17 x 965^2 / 885^2) = 20 litres per hour to provide the required lift. So my overall fuel consumption goes up about 3 l/hr or 9% of 34 l/hr for a 9% increase in weight.

Put another way, it “costsâ€￾ about 9 litres of fuel (about US$ 20 at my home bulk supplier’s price) to carry those extra 110 litres during the 3 hours which it takes to use them. With airport pump prices varying up to 50 cents above my home fuel price, it’s usually well worth filling up at home.

[PCH]

L/D increases with wing loading. Gliders improve their glide by loading water ballast in the wings on good soaring days (the increased wing loading does reduce the climb however).

I'm not sure how to compute the improved L/D with full fuel for a Maule.

[gdflys]

L/D increases with wing loading. Gliders improve their glide by loading water ballast in the wings on good soaring days (the increased wing loading does reduce the climb however).

I'm not sure how to compute the improved L/D with full fuel for a Maule.

Just to be clear the actual L/D ratio doesn’t change with weight but the speed at which best L/D occurs does. The heavier you are the faster you can fly at the same best L/D ratio.