How can I isolate altitude as a specific range variable on a Sporty's electronic e6b?

Question

So, I'm trying to understand the specific range calculation on my Sporty's E6B electronic calculator. The manual gives two examples.

In one, the assumption is 150 Ground Speed, 24 Gallons of fuel per hour and 140 gallons of fuel on board. It also assumes 12000 MSL. The answer is 875 miles.

Then there is another example: 160 GS, 27 G/Hr, 140 gallons on board, and MSL is 8000. The range changes to 855.6.

The problem is: the altitude is not an enterable variable (those others are). Entering it and adjusting in different parts of the calculator does nothing. Does anyone know how to isolate MSL as variable in this device?

Answer 1

TL;DR; You need to look up the burn rate in your POH.

I think you are misunderstanding the examples...

Your fuel burn rate is determined by your power setting and altitude from your charts in your POH. You need to look that up, so to calculate the range, here is what you do:

First, use your POH to look up your fuel burn for a specific power setting/manifold pressure/altitude, here is an example from a Cessna 172 POH:

You can then you can use that fuel burn rate in your calculations. A range calculation is simply:

$$ Range = {Fuel \above 1pt BurnRate} GS $$

So using the table above, if we are cruising at 6000 feet @ 2200 RPM on a standard day we are burning 7.7 GPH. The 172 has a capacity of 53 gallons. If we are indicating 100 knots ground speed on a GPS (1knot = 1 nautical mile per hour), we can calculate our rate as:

$$ Range = {53 \above 1pt 7.7} 100 = 688 $$

The result would be 688 nautical miles (this is burning every drop of usable fuel, not taking into account VFR or IFR reserves).