Climb Rate Discrepancies

wuzak · Mar 16, 2020

ThomasP said:
Hey wuzak,

Sorry, I was not clear in my question. Moving the boost lever to +16 lbs (for example) at take-off, even if the system is not fitted with automatic boost, would (of course) maintain +16 lbs as long as the engine/supercharger rpms did not change and as long as the aircraft did not climb above FTH. Turning and such would not affect this.

If the engine does not have an automatic boost control system, the boost will fall as the aircraft climbs unless the pilot adjusts the throttle to maintain the boost.

ThomasP said:
The situation I was wondering about was for an engine/supercharger combination such as the Merlin III in the pre- and early-war period, I may be misremembering/misunderstanding what I have have read, but what I think I read in many of the pilot accounts was that they had to monitor their boost as they descended to prevent overboost. I got the impression that the early Merlins (for example) did not have any built in any bypass/vent ability for the compressed air coming from the supercharger. What I am thinking is that with the engine at Combat rating (say 3000 rpm and +6.75 lbs), if the aircraft descended to SL the effective boost could?/would? increase to somewhere around +19 lbs at SL if 3000 rpm was maintained and if the boost lever was not pulled back. So I am not referring to an overspeed condition, although the same problem could occur if I am thinking right. Yes/No? Clarification would be appreciated (from anyone) please.

Not sure if the early Merlins had automatic boost control.

ThomasP said:
Also, my understanding (which again could be wrong) is that pre- and early-war the engine/supercharger did not always have the ability to safely descend without the pilot pulling back on the boost in order to keep the IHP in the safe range. An example of this is the 2-speed Merlin XX in 1942, the pilot could set the boost to +16 lbs at altitudes above ~12,000 ft, but had to manually reduce the boost to a max of +14 lbs at lower altitudes in order to prevent detonation and/or overstressing the engine. I think the early 60 series engines had the same problem? Admittedly, this was due to the change in supercharger rpm, but still.

+16 was for FS (high gear) and +14 for MS (low gear). Not sure if there was a system which would restrict boost in low gear with the automatic boost controller.

Soon enough the 2 speed engines were rated the same in both gears.

I think maybe teh Merlin 61 and 63 had different boost allowance for MS and FS gears, but thereafter it was the same.

This may have been more to do with the increased power required to drive the supercharger at the same boost in FS, possibly too much for the drive shaft or gear sets.

Zipper730 · Mar 19, 2020

wuzak said:
And what the fuck is a "skosh"?

It means a tiny bit.

ThomasP said:
The sag I was talking about only occurs in a precisely measured or calculated line from FTH to SC

That's what I was talking about...

SL to FTH. It is not shown in most charts/graphs, as it is very small, and the lines were often drawn as straight lines from FTH to SC for simplicity.

Oh, so it's not a perfect diagonal line, but actually a curved line from left/right going up?

The thing I'm getting confused by, and I'm not that good at math, so it shouldn't be a big surprise: Why when I measured by interval (1000/0.38) I got around 2630 fpm, and when I averaged the numbers I ended up with around 2575 fpm, but an average time to climb of 0.3883 minutes? Ultimately I'd get around 2.76 minutes but I'd get a slower time to 1000 feet, and a slower time from 6500-7600 feet. Was this the result of a sag in the curve, or was this an error in the way they were timing things?

Yes, although you have a typo above, if 5200 should be 2600?

If we have the average ROC value, multiply it by 2, and then subtract the a known instantaneous ROC from one end or the other of the climb interval. In your example above, let us say we know the average ROC from SL to 2000' is 2600 ft/min, and that we do not know the instantaneous ROC at SL, but do know the instantaneous ROC at 2000' is 2650 ft/min. Then the instantaneous ROC as SL would be:

(2600 x 2) - 2650 = 2550 ft/min

But what If I know neither? I just have the average...

The reason for the required decrease in boost with increased altitude upto FTH, so as not to exceed max allowable HP

I thought boost was a far larger limitation on engines than the horsepower?

wuzak · Mar 19, 2020

Zipper730 said:
It means a tiny bit.

You made that up.

pbehn · Mar 19, 2020

wuzak said:
You made that up.

It is a gnat's whisker more than a little bit.

Zipper730 · Mar 19, 2020

wuzak said:
You made that up.

Definition: skosh

pbehn · Mar 19, 2020

Zipper730 said:
Definition: skosh

From the Japanese for a little "skoshi"

Zipper730 · Mar 19, 2020

pbehn said:
From the Japanese for a little "skoshi"

That's correct.

pbehn · Mar 19, 2020

Zipper730 said:
That's correct.

Appears in the 1950s probably brought back by US military who had been in Japan. There is a huge lexicon of words in English brought back from WW1 France especially in aviation because British aviation grew up there.

Climb Rate Discrepancies

More options

wuzak

Captain

Zipper730

Chief Master Sergeant

wuzak

Captain

pbehn

Colonel

Zipper730

Chief Master Sergeant

pbehn

Colonel

Zipper730

Chief Master Sergeant

pbehn

Colonel

Users who are viewing this thread

Similar threads