KEITH, Glad to hear that someone agrees my original conjecture is true. (Pardon to BOOKWORM, he may have already hinted so too).
I’ve been away, or would have posted earlier.
While I have to agree with you that "generally" aircraft performance is better at low temps, it seems this conjecture supports the opposite for operations within engine flat rating and at V md.
If I follow you Keith, then using your example of a 5% increase in TAS due to a higher temp (a reasonable situation), and maintaining V md CAS, seems to result in a 5 % increase in ROC, assuming the SAME thrust output of the engine.
** 5 % increase in RATE of CLIMB when the OAT is warmer (flat rated, Vmd). **
And as BOOKWORM has repeatedly pointed out, an engine can be expected to put out the SAME static thrust over its flat rated temperature range.
Which leaves only the decrease in thrust output as TAS increases to further affect the conjecture.
So…..I’d like to take your computations a bit further…
The graph I have for a 731 shows approximately a 1.2% decrease in TA over a 10 KTAS speed increase at a typical V md. See previous post. If we assume that typically an aircraft requires 33% of its max thrust at V md, then a 1.2% decrease in TA becomes a 1.8% decrease in excess thrust. This 1.8% decrease in excess thrust will decrease Keith’s 5% increase in ROC.
But by how much?
Well, I believe that sin(gradient) = excess thrust/weight
Thus sin(gradient) decreases by 1.8%
At the low angles we’re considering, sin(x) is proportional to x, so climb gradient itself will decrease 1.8%.
Thus the ROC will decrease 1.8%
Note that the ROC decrease is the same as the gradient decrease since TAS is the same, at the 5% increase Keith proposed earlier.
So, going further with your conjecture, Keith (5% ROC increase), I further calculate that the decrease in thrust as TAS increases will lower this 5% ROC by 1.8%, to get 3.2%.
The conjecture of better climb rate at higher OAT’s (within engine flat rating) at V md seems substantiated!! A 3.2% climb rate increase due to temperature increase
Now I’ve flown jets enough to shake my head and say it just aint so, but mathematically, what’s wrong with this argument?
As TINSTAAFL put it, "Hawk has added 2+2 to equal 5. I just can’t put my finger on it"