Spitfire V/Merlin XX vs Hurricane I/Merlin 45

Historically the first major improvement to the Spitfire was the Mk III, which involved many airframe modifications and the addition of the 2 speed Merlin XX. This development was discontinued in favour of the "stop gap" Mk V - basically a Mk II with the Merlin 45 fitted, and requiring far less modification.

Over at Camm Land, the Hurricane MkI was used as a test bed for the Merlin 45 (thanks RCAFson for sharing that info), but that was not proceeded with, the Hurricane becomeing the Mk II with the Merlin XX.

In several threads a theory has been expressed that the Hurricane was short-changed by using the XX instead of the 45, including in the thread
http://www.ww2aircraft.net/forum/av...carrier-fighter-other-than-seafire-39549.html.


So was the option the RAF chose (ie Spitfire/Merlin 45 Hurricane/Merlin XX) the best they could have had.

Or would it have been better to have them all with Merlin 45s, all with Merlin XXs (bearing in mind that supply of XXs may hve been limited and bombers got first dibs), or Spitfire/Merlin XXs Hurricane/Merlin 45s.

I am aware that the Spitfire with the Merlin XX would have required more mods than was historically the case with the Mk V and may have taken longer to get into service.

But would a Spitfire V/Merlin XX have been a better match for the Bf 109F and Fw 190?

Would the Hurricane/Merlin 45 have been more competitive in 1941 than the Hurricane II?

Some info on the climb rate of the Hurricane I/Merlin 45 from RCAFson

RCAFson said:

The Spitfire I at 2800fpm (6050lb)was about 5% lighter than the Hurricane 1 (6317lb) . At similar weights (as per the Sea Hurricane 1B/ Spitfire Vc with HSH 1% heavier) there is very little difference in climb rates:

Hurricane 1/merlin 45 @ 6645lb = 7.1min to 20k ft and 2940fps at 14400ft. (Secret years, p303)
Spitfire Vc/Merlin 45 @ 6965lb = 7.9min to 20k ft and 2650fps at 14900ft. ( Spitfire Mk V AA.878 Report )
The other climb trials reported by Williams and in The Secret Years show some variation by weight but probably average to a similar result as for the Hurricane 1/merlin 45/

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Not sure why the Hurricane I with Merlin 45 would be lighter than the Spitfire V, since the Hurricane I was heavier than the similarly powered and armed Spitfire I. Other than to note that the comparison was with the Spitfire Vc, which had 2 20mm cannon and 4 0.303" mgs.

Also, no indication of how fast that hybrid was compared to the Spitfire V or the Hurricane II.

I believe that the Hurricane II was being introduced about the time of the BoB and is a closer equivalent, timewise, to the Spitfire MK II. It may have been thought that the Hurricane needed the Merlin XX engine in order to remain a competitive (1st line) fighter at the time. A Hurricane IIA posting numbers (climb in particular) vs a 109E that certainly suggest it could at least hold it's own.
6-9 months later things had changed. Spring of 1941 sees the 109F showing up and nothing short of a rocket engine stuffed in the Hurricane is going to make it competitive.
Given the production numbers during and just after the BoB sticking Merlin XIIs in the Hurricane and Merlin XXs in the Spitfire (with the disruption in production) would have meant fewer effective fighters in the fall/early winter of 1940.
The Better climbing Spitfire had less need of the low gear in the Merlin XX engine than the Hurricane (at least until the 190 shows up).

According to RAE Spitfire charts:

Merlin XX aircraft would have the edge 0 ft - 11,000 ft
Merlin 45 aircraft would have the edge 11,000 ft - 22,000 ft

above that height - little to choose.

In terms of climb performance, the Merlin 45 Spitfire holds a slight advantage at all heights, especially in the 12,000 to 18,000 ft range where the two-speed performance dips.

Roughly similar comparisons can be seen in A&AEE tests of the Hurricane '45' and the Hurricane II.

The Merlin 45 was approved for 16lb boost in Dec 1941, so after that it could probably outperform the XX under 11k ft as well, until the XX got approval for 16 lb boost in late 1942.

Hurricane 1 at 6645 lb/Merlin 45 Vmax = 324mph at 19400ft.

The Merlin XX seems to have added about 200lb to the Hurricane II but some of this was probably added structure weight.

wuzak said:

Some info on the climb rate of the Hurricane I/Merlin 45 from RCAFson

Originally Posted by RCAFson View Post
The Spitfire I at 2800fpm (6050lb)was about 5% lighter than the Hurricane 1 (6317lb) . At similar weights (as per the Sea Hurricane 1B/ Spitfire Vc with HSH 1% heavier) there is very little difference in climb rates:

Hurricane 1/merlin 45 @ 6645lb = 7.1min to 20k ft and 2940fps at 14400ft. (Secret years, p303)
Spitfire Vc/Merlin 45 @ 6965lb = 7.9min to 20k ft and 2650fps at 14900ft. ( Spitfire Mk V AA.878 Report )
The other climb trials reported by Williams and in The Secret Years show some variation by weight but probably average to a similar result as for the Hurricane 1/merlin 45/

Click to expand...

Not sure why the Hurricane I with Merlin 45 would be lighter than the Spitfire V, since the Hurricane I was heavier than the similarly powered and armed Spitfire I. Other than to note that the comparison was with the Spitfire Vc, which had 2 20mm cannon and 4 0.303" mgs.

Also, no indication of how fast that hybrid was compared to the Spitfire V or the Hurricane II.

Click to expand...

More strangness with this:

Why are the Mk V's figures for normal power (1 hour limit of +9 lbs and 2850 rom), rather than combat power (5 minute limit)?
More appropriate figures for outright performance (+16 lbs, 3000 rpm) appear to be 6.15 minutes to 20,000 ft and 3710 ft/min at 8,800 ft.
At the very least, it should be 6.4 minutes to 20,000 ft and 3250 ft/min at 15,000 ft for an earlier Mk Vb with Merlin 45 at +9 lbs and 3,000 rpm.

Jabberwocky said:

More strangness with this:

Why are the Mk V's figures for normal power (1 hour limit of +9 lbs and 2850 rom), rather than combat power (5 minute limit)?
More appropriate figures for outright performance (+16 lbs, 3000 rpm) appear to be 6.15 minutes to 20,000 ft and 3710 ft/min at 8,800 ft.
At the very least, it should be 6.4 minutes to 20,000 ft and 3250 ft/min at 15,000 ft for an earlier Mk Vb with Merlin 45 at +9 lbs and 3,000 rpm.

Click to expand...

The Hurricane I/M45 climb is at the normal rating so it seems appropriate to compare it to a Spit V using it's normal rating.

Greyman said:

According to RAE Spitfire charts:

Merlin XX aircraft would have the edge 0 ft - 11,000 ft
Merlin 45 aircraft would have the edge 11,000 ft - 22,000 ft

above that height - little to choose.

In terms of climb performance, the Merlin 45 Spitfire holds a slight advantage at all heights, especially in the 12,000 to 18,000 ft range where the two-speed performance dips.

Roughly similar comparisons can be seen in A&AEE tests of the Hurricane '45' and the Hurricane II.

Click to expand...

Why would the Merlin 45 Spitfire hold the climb advantage at all altitudes? Is it simply because of teh extra weight of the XX?

I understand why the 45 would have better altitude performance - since its FTH was slightly higher, IIRC. But at low altitudes, when the XX is in MS gear it should hold the advantage - assuming the same limitations on boost.

As to boost, what delayed allowing the XX to use the 16psi boost, as per the 45? Was it something to do with the supercharger drive, or just priority work?

wuzak said:

Why would the Merlin 45 Spitfire hold the climb advantage at all altitudes? Is it simply because of teh extra weight of the XX?

I understand why the 45 would have better altitude performance - since its FTH was slightly higher, IIRC. But at low altitudes, when the XX is in MS gear it should hold the advantage - assuming the same limitations on boost.

As to boost, what delayed allowing the XX to use the 16psi boost, as per the 45? Was it something to do with the supercharger drive, or just priority work?

Click to expand...

Not a large difference in weight, the chart says the Merlin XX Spitfire is 94 pounds heavier.

I've seen that one paper on the Merlin 45 being allowed +16 boost but I have a feeling there's more to that story. I feel in my gut the +16 Merlin 45 wasn't until later.

Something I keep a lookout for ...

Greyman said:

Not a large difference in weight, the chart says the Merlin XX Spitfire is 94 pounds heavier.

I've seen that one paper on the Merlin 45 being allowed +16 boost but I have a feeling there's more to that story. I feel in my gut the +16 Merlin 45 wasn't until later.

Something I keep a lookout for ...

Click to expand...

Here's a memo from Jan 3 1942:

http://www.wwiiaircraftperformance.org/merlin-ratings_3jan42.jpg

but AFAIK, it was not until much later in 1942 that the XX was approved for 16lb boost.

wuzak said:

Why would the Merlin 45 Spitfire hold the climb advantage at all altitudes? Is it simply because of teh extra weight of the XX?

I understand why the 45 would have better altitude performance - since its FTH was slightly higher, IIRC. But at low altitudes, when the XX is in MS gear it should hold the advantage - assuming the same limitations on boost.

As to boost, what delayed allowing the XX to use the 16psi boost, as per the 45? Was it something to do with the supercharger drive, or just priority work?

Click to expand...

The Hurricane's thicker wing profile and an airframe that produced more drag didn't help.

Flight Feb 26 1942 article on Merlin XX:

View attachment Rolls-Royce Merlin XX 1942.pdf