Today some good progress is made on testing the aircraft performance formulas, but understanding some of them becomes troubling.

## Testing progress

Good progress has been made today on the aircraft performance code. All tests for derivatives of functions are complete, and the only tests in regard to the formulas now needed are where we check that it makes sense in regard to what is already known. This has been done for half of them, and apart from a few difficult ones that will noted and skipped, progress on the rest is proceeding well.

One of the difficult ones right now is about the total horsepower available during level flight.

$THP_{a,L}=\frac{88}{33000}\left[\frac{\sigma A_D V^3}{391}+\frac{391 (W/b_e)^2}{\pi \sigma V}\right]$

which is supposed to be thrust times airspeed, which must be the following:

$THP_{a,L}=\frac{88}{33000}\left[\frac{\sigma A_D V^2}{391}+\frac{391 (W/b_e)^2}{\pi \sigma V^2}\right]V$

But how do we start with this formula? Could it be that during level flight thrust equals drag, and we can make use somehow of the following drag equation?

$D=\frac{\sigma C_D SV^2}{391}\text{ here V is feet/sec}$

I’ll carry on poking away at this and see what I can find.