However, you are not burning fuel at a given rate. I think in colder (denser) air you can burn more fuel (i.e. running at the optimal stochiometric ratio) so you get more power. For me, in this is true, the Q becomes: does more power in denser air give you more TAS? Possibly not…
Peter wrote:
However, you are not burning fuel at a given rate. I think in colder (denser) air you can burn more fuel (i.e. running at the optimal stochiometric ratio) so you get more power. For me, in this is true, the Q becomes: does more power in denser air give you more TAS? Possibly not…
You talked about constant fuel flow before, now you do not. Do you mean that in colder air you can make a more optimal leaning?
Almost everybody without a turbo, once high enough to use wide open throttle, flies as fast as they can, and uses the best economy setting (peak EGT or slightly LOP). The only time you need more power is when approaching the ceiling and then you have to go ROP but that’s a different issue.
Peter wrote:
Almost everybody without a turbo, once high enough to use wide open throttle, flies as fast as they can, and uses the best economy setting (peak EGT or slightly LOP).
So is your question what effect temperature differences on a given level have on speed when you’re already using WOT with peak EGT?
Yes. See my replies to bookworm.
Nothing else makes practical sense because “everybody” adjusts the mixture for the desired effect. One doesn’t just set the fuel flow to X USG/hr regardless of what the air is doing.
I’m afraid I’m quite lost as to what you’re varying and what you’re considering constant. The question you started with was:
Does the ISA deviation affect cruise speed, for a given fuel flow?
Assuming that you are burning that fuel optimally (i.e. at best economy) I believe the answer is that your cruise speed for a given fuel flow will depend only on your density altitude. However, on a warm day, you’ll reach that density altitude at a lower pressure altitude, so you’ll have more manifold pressure available at WOT. But if you open up the throttle, you’ll burn more fuel, so that’s not “for a given fuel flow”.
IFR flight is done mostly at a constant pressure altitude (flight level). I adjust the fuel for peak EGT at each new pressure altitude. The Q is whether the TAS will be higher at (say FL100) if the OAT is -10C or +10C.
Throttle is wide open.
I have never used density altitude in my 16 years of flying. For runway performance I have a chart.
It is probably not hard to test this, comparing 2 flights. But I think the effect is very small.
Peter wrote:
I have never used density altitude in my 16 years of flying.
To each his own… I always use a predetermined power setting (which usually does not involve WOT) because then I have a predictable fuel flow. When density altitude changes, I change the engine controls to maintain that power setting. Of course, I don’t usually fly above FL100.
Peter wrote:
IFR flight is done mostly at a constant pressure altitude (flight level). I adjust the fuel for peak EGT at each new pressure altitude. The Q is whether the TAS will be higher at (say FL100) if the OAT is -10C or +10C.Throttle is wide open.
So at FL100 and -10C you’re at ISA-5, more like 9,500 density altitude. You will get more power, and therefore higher TAS, but a lower distance/fuel.
At FL100 and +10C you’re at ISA+15, more like 12,000 ft density altitude. You will get less power, and therefore lower TAS, but a higher distance/fuel.
So in warmer air you struggle more to climb but you travel further on the same fuel despite the TAS being higher. Amazing!
Can you quantify the effect on the distance/fuel?
