I would say that Density Altitude (Pressure Alt corrected for nonstandard temps) is what counts.
dirkdj wrote:
At a QNH of 1030 the air will be denser than at 970.
It will not if you fly at the same pressure altitude.
At a QNH of 1030 the air will be denser than at 970. About 1800 ft worth. More air weight means more fuel required to get same Peak mixture. More fuel means more power at same MAP/RPM. For normally aspirated engines the formula LOP is Fuel Flow x 15 = HP.
Peter wrote:
In a QNH of say 1030 the peak-EGT fuel flow will be 6% bigger than it would be at say 970.
I don’t get that. For the same MAP/RPM the peak EGT FF should be the same regardless of QNH if everything else remain constant (particularly temperature). The difference is that your true altitude will differ depending on QNH at the same pressure altitude.
Peter wrote:
I think Dirk must have the explanation. In a QNH of say 1030 the peak-EGT fuel flow will be 6% bigger than it would be at say 970.This is why setting cruise power using the flowmeter is not the way to do it. One really should use the engine (EGT) monitor.
But with 6% more power the plane should also fly some 2-3% faster, no?
I think if you look at it in terms of pressure altitude you will see no difference: other than vertical wind effects, High Press or Low Press is the same as flying a bit higher or lower, (err, the other way around) but in the FL’s you won’t tell the difference: just pick your FL regardless of HP/LP area and the result will be equal for the same temp.
Dirk, of course the impact is related to whether your engine is NA or TC, but is the same as changing altitude: it also depends on what parameters you ‘set’ .
If NA and you ‘set’ WOT with peak EGT FF, then the higher you fly (LP) the slower you go.
If TC and you ‘set’ LOP FF which equates to almost constant power below critical altitude then it is faster the higher you go (LP)
The air will be more dense, helping the engine produce more power, but the airframe drag will be greater too. I don’t know which one trumps which one, power or drag. Depends on the airframe I guess. I wouldn’t loose sleep over it.
I think Dirk must have the explanation. In a QNH of say 1030 the peak-EGT fuel flow will be 6% bigger than it would be at say 970.
This is why setting cruise power using the flowmeter is not the way to do it. One really should use the engine (EGT) monitor.
But with 6% more power the plane should also fly some 2-3% faster, no?
It’s the opposite that happens IMO. A clear winter day with high pressure, and the aircraft skyrockets. In cruise you hardly need to apply throttle. It’s all temperature and pressure. The whole reason for the high pressure is because the air resist moving, it’s in the pressure gradients the air moves.
Peter wrote:
That’s what I don’t know but I recall a very old post somewhere by @bookworm that it is of the order of a few feet per second which, if true, would be enough to produce a very significant effect on IAS.
That’s easy to disprove. If the air descended at that rate, it would be a very well documented and quantified effect as it would have a significant effect on aircraft performance – even at sea level. If we define “a few” as 6, that’s 360 feet per minute and it would be extremely noticeable and it would have to be in performance tables. It would mean high pressure days would be unflyable for some low power types – for instance the C140 I had in the US had a cruise prop and a C85 engine, and it would only climb at 400 fpm on a good day. If high pressure meant air sinking at 360 fpm, then high pressure days would be non-flying days. Even if we define “a few” as 3 fps, then this would still need to be in aircraft performance tables.
@Maroaigh
Googling this in German, I found a value of 90 m per hour. If I am not mistaken that means it is just 25 mm/second, not even an inch/s. Seems insignificant to me.
