One of my previous emails talked about the importance of weighing the impact of something with the actual effort you’d need to put into it.
Sometimes it’s just not worth it to put a bunch more energy and time into something, only to get a tiny improvement. It’s a skill to know when to do this cost-benefit analysis, and to do it well.
Today I have a counterpoint to my own advice: it can be worth it to keep digging at something that just doesn’t feel right. And it’s also a skill to know when to.
A few months ago I was working on a mission profile simulation code. There are plenty of ways to estimate endurance, range, and service ceiling (i.e. max altitude) for an aircraft, but they make a lot of assumptions. Super useful, but sometimes you want something better.
That’s what a mission profile sim is for: it iterates through time steps to calculate the change in aircraft state as the flight progresses.
As the airplane flies, it burns fuel,
which decreases the weight, and so lift required,
which reduces the drag generated,
and therefore the thrust needed from the propulsion system,
which decreases the fuel burn rate, and so fuel consumed per time step,
which reduces each decrease in weight,
And so on it goes.
You can also add segments in the code to simulate climbs and descents. This gets a somewhat more accurate estimate for the fuel burn and other impacts of those flight phases, since most general performance prediction calculations assume straight and level flight.
Initially I’d just chosen a set climb and descent rate for these entire segments, based on some estimates of excess power at different altitudes.
But something about that didn’t feel quite right: climb rate decreases as you go up in altitude. Why would I hamstring the bird at lower altitudes when it could zip on up much faster than my hard-coded value? I’m building this to have more accuracy.
Same thing for descent: we usually don’t set a specific descent rate anyways. Other aircraft I’ve worked on basically took the throttle to idle, and the resulting descent rate was what you got. Why would I guesstimate one now?
Sure, climb and descent are tiny segments of the entire flight, maybe 5% or so of the full duration. Following my previous advice, I should just ignore the problem and continue on.
But something about how I’d set this up didn’t sit right with me. It didn’t feel quite based in physics. I didn’t like it.
So, I went all the way back to the fundamentals.