Merlin 20 series power at altitude?

[tomo pauk]

There seem to be discrepancies at what altitude the Merlin 20 series developed it's 1150-1175 HP in second gear and 3000 rpm. Eg. data sheet for the Hurricane IIA shows that 1175 HP was to be attained at 20500 ft. Same value can be found on the Beaufighter IIC data sheet. Both data sheets are from second half of 1941, values are without ram. The provisional chart for Merlin XX shows 1175 HP at +9psi at 21000 ft.
(data sheet for Beau IIC and provisional chart are courtesy of Neil Stirling)
On the other hand, the graph for many Merlin 20 series engines shows the ~1150 HP at only ~18500 ft (at 21000 ft about ~1040 HP). The data sheet for the V-1650-1 shows 1120 HP at 18500 ft (boost 48.2 in, or just under +9 psi). This chart (for Merlin 22, 24 and T24) shows similar altitude power.


What I'm I missing?

 

[Jugman]

RAM

 

[tomo pauk]

RAM

All figures that I've quoted are without ram effect.

 

[tomo pauk]

This chart is with ram effect due to 400 mph forward speed: chart.
With ram, seems like perhaps 1170-1180 HP at 20000 ft?

 

[Deleted member 68059]

Not sure if this helps ?

 

[tomo pauk]

Thank you, Calum.
Unfortunately, that chart brings another set of questions. Like - what RPM are the lines for the Merlin XX, 45, and 46/47?

 

[Deleted member 68059]

Thank you, Calum.
Unfortunately, that chart brings another set of questions. Like - what RPM are the lines for the Merlin XX, 45, and 46/47?

Everything on the chart unless otherwise mentioned is 2850rpm (if you go to 3000rpm with the XX, basically the peaks move right about 1.6km and up about 30PS).

 

[tomo pauk]

Everything on the chart unless otherwise mentioned is 2850rpm (if you go to 3000rpm with the XX, basically the peaks move right about 1.6km and up about 30PS).

Thank you.

Looking at other two charts I've linked or posted, going from 2850 rpm to 3000 adds 1300-2000 ft (~400-600m) in high gear at around 18000-20000 ft.

 

[Deleted member 68059]

Thank you.

Looking at other two charts I've linked or posted, going from 2850 rpm to 3000 adds 1300-2000 ft (~400-600m) in high gear at around 18000-20000 ft.

Very... VERY difficult to be exact with any of these charts.... Rolls-Royce barely ever used a high-altitude test cell, the only one in Britain was at Farnborough and it spent
half the war not working properly.... however the chart above is almost certainly from one of the German high altitude test cells - I try not to get too wrapped up in the minuitia of these charts as unless you get a time machine and go back and test them all on exactly the same test-bed, I dont think anyone should lose sleep over not managing to get them to all overlap nicely.

I dont know if that makes you feel better or worse !!

 

[Shortround6]

This book has a lot of technical material and a lot of it has to do with the Merlin XX engine. The are a number of charts (not like the one posted by Calum, usually less detailed) and al ot of math.

However on page 22 (very confusing as there are 40 "figures/charts" which are full page but do not count as pages) there is a table of a test of a Hurricane II (P.3269) with a Merlin XX engine C.10225 with a variety of conditions/calculations/results while running at 3020rpm in F.S. ratio.

Altitudes and boost listed are.
15,000ft -48.24 in
20,000ft-48.24 in
20,000ft-50.67 in
25,000ft-42.12 in
30,000ft-34.30 in
35,000ft 26.56 in

such things as observed air speed are given,
5 different temperatures from observed atmospheric temperature to the charge temperature ( in the intake manifold?)
Back pressure
Atmospheric pressure
Charge flow in lbs/min
Shaft horsepower
S/C horsepower
Brake Horsepower
expansion ratio across the ejector exhaust.
Exhaust jet velocity
ejector horsepower
air intake momentum drag.

Max speed was 340mph at 20,000ft using 50.67in of boost with 1126 brake hp (supercharger needed 236hp to drive) engine was consuming 151.0lbs per minute and the ejectors were contributing 126.8hp. The RAM intake was costing 32.8hp.
some of the numbers are observed, some are calculated, some are the result of test bench.
However the two previous "figures" are charts for the same plane and engine, one was the engine performance calculated from observed flight results with curves/lines for both Hawker calculations and RR calculations. The other chart is the "deduced performance of the Merlin XX engine in flight plotted against boost pressure".

different aircraft with different intakes and speeds are going to vary slightly, from the table at 20,000ft using 48.24in of boost.

335mph with 1073 brake hp (supercharger needed 225hp to drive) engine was consuming 144.0lbs per minute and the ejectors were contributing 113.0hp. The RAM intake was costing 31.0hp.

 

[tomo pauk]

Thank you

Silly me, I have that book. After all is said and done, I'm more inclined to believe the more conservative figures that also seem to be of a later date, rather than to the more optimistic figures that seem to be, at least in British data sheets, a 'spin-off' from the provisional chart. The tests of Hurricane II and different Mosquito say that +9 psi boost is achieved under 20000 ft even with ram.
All of this being my take, FWIW.

 

[jmcalli2]

Thank you.

Looking at other two charts I've linked or posted, going from 2850 rpm to 3000 adds 1300-2000 ft (~400-600m) in high gear at around 18000-20000 ft.

I found this in a link at the bottom of this web site: Hurricane Mk II Performance

http://www.wwiiaircraftperformance.org/hurricane/merlin-xx-curve-c1.jpg

 

[Sid327]

Why,

Would ram air effect cost hp?

 

[Shortround6]

Why,

Would ram air effect cost hp?

Increased drag. Ram air is air that is compressed to higher than ambient air pressure in the intake duct before it reaches the carburetor/supercharger intake.

Why it is important is the supercharger multiplies the incoming air pressure. say our supercharger can multiply the incoming air pressure 3 times. If the static air pressure in 14in Hg then we would have 42 in Hg of manifold pressure. If, however our air scoop and ducting can deliver air at 16 in Hg to the carburetor/supercharger intake then we can get a manifold pressure of 48in Hg.

25-30hp in drag was a small price to pay for the extra boost.

 

[Sid327]

From about minute 10 talking about ram air.