I add further formula details from an aircraft book, resolve a puzzle with Reynolds numbers, and face a lonely vigil at the pub quiz.

Building on a good base

My plans were to do more with fancy charts today, but some larger fundamentals need to be taken care of first.

The sample code that I’ve been working from is supposed to demo equations from part 2 of the book, Theoretical Background. The code itself doesn’t do that good a job of showing this though, so it’s time to rework the code so that it clearly exhibits this origin, and uses a separate and clear set functions for each formula relationship.

While doing that I’ve also been incorporating much more information from other parts of the book too, so it’s time to progress things on to version 1.2 of the code, as I move as much as I can from the book in to code and try to understand the fundamentals of what’s going on.

Reynolds number

One of the challenges was in Appendix I about the Reynolds number, where it uses viscosity and density of air at sea level

μSL = 3.737 × 10-7 slug/ft-sec
ρSL = 0.002377 slug/ft3

and gave good formulas for how to derive those figures, but getting from there to the next part wasn’t so clear

If we express the airspeed, V, in mph and the characteristic length, ℓ, in feet, the Reynolds number is

ReSL = 9324 V (mph) ℓ (ft)

and working out how to get 3.737 × 10-7 times 0.002377 to equal more than 6378 becomes a problem.

Fortunately, this article about the Reynolds number helps to explain things. We need to convert from feet per second to miles per hour, so multiplying by a conversion factor of 5280/(60*60) helps to resolve that issue.

It’s been really interesting working through the Practical Guide to Airplane Performance and Design, and I aim to publish at some stage the code that I’ve been working on.

Pub quiz

Due to today being Labour Day virtually everyone from our pub quiz group were missing in action this week.

It was a fairly lonely vigil there tonight with only Felix for company, but we didn’t do too badly. We managed to wade our way through a morass of sports questions in amongst it all to end up with a score about 50% overall.

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