Curtiss P-36 with the Wright R-2600 Twin Cyclone (hypothetically)

[_PabloSniper_]

Hello everyone!
I believe everyone here should know the history of the P-36.
What if Curtiss had insisted a little more on its development?
When I think of the development of the P-36, I imagine this.
A P-40N with an R-2600 radial. (Same as the XF6F-1 Hellcat prototype, the B-25 and the TBF Avenger)

The R-2600 is about 600 pounds heavier than a V-1710, but I think with the right tuning it could have extended the life of the Curtiss fighters.
And since the P-40N had a larger tail than the P-36, perhaps it would be possible to balance the weight.
I'm not sure how much the V-1710's liquid cooling system weighs, but since it would be unnecessary, it would probably save about 200 pounds.
In short, we are talking about something around 400 pounds more weight, and 450 to 500 more Hp.

 

[tomo pauk]

When I think of the development of the P-36, I imagine this.
A P-40N with an R-2600 radial. (Same as the XF6F-1 Hellcat prototype, the B-25 and the TBF Avenger)
The R-2600 is about 600 pounds heavier than a V-1710, but I think with the right tuning it could have extended the life of the Curtiss fighters.

Not the 1st time such a contraption was suggested. On this forum several times (once by yours truly, too), eg. here, and there was also Curtiss' own thinkering (see here). The end result would've been very dependent on the engine version installed.
If the so-called 1700 HP version is in the nose, as indeed powered the B-25s and Avengers, we'd probably gotten a fighter than is great at low altitudes, still useful at medium altitudes, and bad at high altitudes, due to the inadequate supercharger of that engine.
The engine that powered the XF6F-1 was with the 2-stage supercharger (more or less the auxiliary S/C was added to the basic 1700 HP engine), that much improved the altitude performance. If that engine is used, with the assumption that it works as advertised and that there is a ready supply of them, we'd get a fighter that is no worse than the Fw 190, and probably better than it above 20000 ft - IOW, an excellent fighter.

Shortcoming of the R-2600 was the bulk, that meant the drag is increased vs. the V12 installation. It also took a while for the USA to implement the good layout of the exhaust stacks for the increased thrust and thus better speed. Another shortcoming is that fuel mileage will go down due to the heavier, bulkier and more thirsty engine, so the brave new fighter will be even more shorter ranged than the P-40.

FWIW, the early A-20 was still good for 350 mph with 1600 HP R-2600s, despite the bulky engines, the portly fuselage of a bomber, and despite the thick (18% at root) and big wing.

I'm not sure how much the V-1710's liquid cooling system weighs, but since it would be unnecessary, it would probably save about 200 pounds.
In short, we are talking about something around 400 pounds more weight, and 450 to 500 more Hp.

P-40's cooling system was at just under 300 lbs for the V-1710 engines. The R-2600 will have the heavier oil system (extra 50 lbs?), a bigger & heavier prop (another 50 lbs?), heavier engine bearers.
If the 2-stage engine is used, another 200 lbs weight vs. the 1-stage R-2600?

Engine power at 12000 ft is 1450 Hp (196 gal/hr fuel used) for the R-2600 as installed on the B-25, while in the same time the V-1710-39 from the P-40D/E was making 1150 HP there (130-140 gal/hr). Cruise fuel consumption was more modest, the R-2600 still using at least 50% more fuel per hour.

 

[_PabloSniper_]

I confess that I was happy to see Curtiss' drawing.


It's a sign that I'm not crazy.

 

[Shortround6]

Tomo has given a very good rundown on the problem/s.
The drawing is interesting as it shows the cockpit near where the YP-37 cockpit was which may indicate trying to get the CG in balance.

Many engines got 2 speed superchargers with little increase in bulk or weight (20-50lbs?) but two stage superchargers and especially turbos added bulk and weight.
Fitting a two stage supercharger with intercoolers intended for an F6F into a P-40 airframe is going to require a heavy high steel alloy shoehorn with a 3 meter handle.
As Tomo says the 2 stage engine was almost 200lbs heavier and that does not include the inter coolers and ducts.

C-W may have had problems with high altitude cooling. While the air is colder at 22,000ft you have 1/2 the mass airflow (weight of air) at the same speed that you do at sea level.

 

[_PabloSniper_]

Not the 1st time such a contraption was suggested. On this forum several times (once by yours truly, too), eg. here, and there was also Curtiss' own thinkering (see here). The end result would've been very dependent on the engine version installed.
If the so-called 1700 HP version is in the nose, as indeed powered the B-25s and Avengers, we'd probably gotten a fighter than is great at low altitudes, still useful at medium altitudes, and bad at high altitudes, due to the inadequate supercharger of that engine.
The engine that powered the XF6F-1 was with the 2-stage supercharger (more or less the auxiliary S/C was added to the basic 1700 HP engine), that much improved the altitude performance. If that engine is used, with the assumption that it works as advertised and that there is a ready supply of them, we'd get a fighter that is no worse than the Fw 190, and probably better than it above 20000 ft - IOW, an excellent fighter.

Shortcoming of the R-2600 was the bulk, that meant the drag is increased vs. the V12 installation. It also took a while for the USA to implement the good layout of the exhaust stacks for the increased thrust and thus better speed. Another shortcoming is that fuel mileage will go down due to the heavier, bulkier and more thirsty engine, so the brave new fighter will be even more shorter ranged than the P-40.

FWIW, the early A-20 was still good for 350 mph with 1600 HP R-2600s, despite the bulky engines, the portly fuselage of a bomber, and despite the thick (18% at root) and big wing.



P-40's cooling system was at just under 300 lbs for the V-1710 engines. The R-2600 will have the heavier oil system (extra 50 lbs?), a bigger & heavier prop (another 50 lbs?), heavier engine bearers.
If the 2-stage engine is used, another 200 lbs weight vs. the 1-stage R-2600?

Engine power at 12000 ft is 1450 Hp (196 gal/hr fuel used) for the R-2600 as installed on the B-25, while in the same time the V-1710-39 from the P-40D/E was making 1150 HP there (130-140 gal/hr). Cruise fuel consumption was more modest, the R-2600 still using at least 50% more fuel per hour.

tomo pauk

Have more modest options been considered for the XP-42, such as the R-2000?

 

[tomo pauk]

tomo pauk
Have more modest options been considered for the XP-42, such as the R-2000?

Good catch wrt. the XP-42 with the much improved exhausts. Unfortunately, it took inexplicably long time before that improvement 'percolated' into the mainstream.
By the time R-2000 was around - the monthly production went into triple digits as late as early 1943 - USAAF was fully commited towards the V-1710 (both turboed and non-turboed), R-2800 (turbo mostly, but also 1- and 2-stage S/C), as well as towards the Packard Merlin. R-2000 was not offering anything over these engines (it was the other way around), and it will require the turbo to satisfy the altitude requirements. All of these engines were being manufactured in hundreds per month already by 1942; eg. the R-2800 was made in some 12000 copies in 1942, with new factories making the R-2800 entering the stage in 1943.
Some 7200+ of the Packard Merlins was made in 1942.

By that time, USAAF saw the P-40 as the fighter behind the curve, too.

 

[Shortround6]

the R-2000 didn't offer the improvement at altitude that the take-off figures suggest.

Engine.....................................take-off...................Mil power low....................Mil power high
R-1830-61...........................1200hp........................1200hp/4900ft.....................1050hp/13,100ft
R-2000-3..............................1350hp........................1350hp/2000ft....................not listed
R-2000-9..............................1450hp........................1450hp/1000ft.....................1100hp/16,000ft

It does not appear that they sized the supercharger to flow enough air at high altitudes.
They were interested in high take-off power for transports.

 

[_PabloSniper_]

I was looking for information about the XP-42, and found this version. Did it have a larger engine than the R-1830?

 

[Shortround6]

No.
They did test something like 14 different nose cowls on the XP-42 but it was much more of an aerodynamic test bed than anything else. They used a few different engines with different length prop-shafts but the basic engine didn't change much, if at all.

 

[_PabloSniper_]

No.
They did test something like 14 different nose cowls on the XP-42 but it was much more of an aerodynamic test bed than anything else. They used a few different engines with different length prop-shafts but the basic engine didn't change much, if at all.

Continuing the search for an aircraft that could accommodate the R-2600, I found the XP-44 Rocket.
Which was planned for the R-2180, but is already much closer to the R-2600 than the R-1830.

 

[Shortround6]

The R-2180 use 14 cylinders that were the same size as the cylinders used in the R-2800 and the engines were the same diameter.
P&W cancelled development of the R-2180.
The R-2800 was smaller in diameter than the R-2600, it was only about 200-250lbs heavier, it made more power. It pretty much killed the R-2600 as a fighter engine.
They did leap frog each other a bit. Each went through about 3 power levels.

 

[_PabloSniper_]

The R-2180 use 14 cylinders that were the same size as the cylinders used in the R-2800 and the engines were the same diameter.
P&W cancelled development of the R-2180.
The R-2800 was smaller in diameter than the R-2600, it was only about 200-250lbs heavier, it made more power. It pretty much killed the R-2600 as a fighter engine.
They did leap frog each other a bit. Each went through about 3 power levels.

I completely agree with your statement.
My point is that we will not have R-2800s fully ready for use until 1943.
A fighter using the R-2600 in 1941 could have allocated more investment to optimize the R-2600.
Remember that the Hellcat prototype used a modified R-2600 to maintain good high-altitude performance.

 

[tomo pauk]

My point is that we will not have R-2800s fully ready for use until 1943.

Care to elaborate?

A fighter using the R-2600 in 1941 could have allocated more investment to optimize the R-2600.

FWIW and IMO, just using a bigger supercharger, with the perhaps 12 to 13 in impeller (the 11 in impeller was used on the S/C of the R-2600s) would've netted a very useful high altitude performance.
Wright was one of rare Western radial engines' makers that stuck with 'good' impellers between the late 1930s and the mid ww2, rest of the radial engines' makers used impellers with straight blades instead of these with curved/parabolic frontal section.

 

[_PabloSniper_]

Care to elaborate?

I meant that the R-2800 was ready well after the R-2600.

 

[Shortround6]

I meant that the R-2800 was ready well after the R-2600.

Yes and no,
The 1600hp R-2600 was ready well before the 1850hp R-2800.
But the 1700hp R-2600 didn't show up until 3-4 months before the 2000hp R-2800s, at least in numbers more than a few dozen a month.
The 1900hp R-2600 didn't show up until late summer of 1943. The 2100hp R-2800s showed up in the spring/summer of 1944.
These are for either single speed single stage or two speed single stage. Production two stage superchargers took a while longer, either turbo or mechanical except....
Wright never had a production two stage supercharged R-2600, ever, of either type.

 

[_PabloSniper_]

Yes and no,
The 1600hp R-2600 was ready well before the 1850hp R-2800.
But the 1700hp R-2600 didn't show up until 3-4 months before the 2000hp R-2800s, at least in numbers more than a few dozen a month.
The 1900hp R-2600 didn't show up until late summer of 1943. The 2100hp R-2800s showed up in the spring/summer of 1944.
These are for either single speed single stage or two speed single stage. Production two stage superchargers took a while longer, either turbo or mechanical except....
Wright never had a production two stage supercharged R-2600, ever, of either type.

I found this page with the respective speeds of the R-2600 and R-2800.

 

[Shortround6]

What is interesting is the difference in climb.

Without knowing the altitudes the max speeds were at the information not really that helpful.
The Zero did not climb at 4500fpm. It's climb was deceptive. It best climb speed was slower than many other fighters so it climbed at a steeper angle.

 

[tomo pauk]

I meant that the R-2800 was ready well after the R-2600.

As noted above, that really depends on the version.
The erstwhile 1500 and 1600 HP types were with the low-altitude S/C, meaning that their suitability for the USAAC fighters was simply not there (at least not without the support by turbo system).
The 2-speed supercharged R-2600s, as installed on the A-20s, were the 1600 HP versions (giving also 1400 HP at 10800 ft). These were indeed made already in 1940, with 1925 pcs manufactured in 1940. So a scenario where the 1st versions of the P-36/-37/-40 are 1st flying in 1938/39, and are in series production might provide the AAc with a workable fighter of the era, with the some shortcomings (need to reinforce the wings and fuselage even more than what was the case historically; small radius of action).
The 1700 HP version, that was much better at altitude, was being series produced from mid-1941, and was made in 443 pcs in 1941. More or less a drop-in upgrade vs. the 1600 HP version, with much better altitude performance.
It will be 1942 until the 2-stage supercharged R-2600 is being made, and in a handful of engines.

The R-2800 was manufactured in about 1720 pcs in 1941, and it made even better power than the 1700 HP R-2600s. The 1st 2-stage supercharged R-2800 powered the XF4U-1 from late May of 1940, ie. some 2 years before the XF6F-1 took flight, powered by the 2-stage R-2600.

 

[Shortround6]

A fighter using the R-2600 in 1941 could have allocated more investment to optimize the R-2600.
Remember that the Hellcat prototype used a modified R-2600 to maintain good high-altitude performance.

Going back to this.
The R-2600, for some reason, doesn't get a lot written about it.
Wright did try to optimize it but is seems like it was operating near it's limits in each "version/series".
The early engines were the A series that reached 1600hp for take-off.
The second series was the B or BA series that made 1700hp.
Wright had started working on this series in Nov 1938 and they ran the first experimental engine in Nov 1939.
Among other improvements it used a steel crankcase instead of aluminum, basically it was a new engine that just looked a lot like the old one.
list of other changes doesn't show up in most books/articles.
The A series had been designed to run on 91 octane fuel then upgraded a bit or re-rated to run on 100 octane not 100/130 fuel. It's max rpm was 2400.
They did NOT increase either the take-off power or the power at altitude with 100 octane fuel. What they did was increase the altitude at which the engine was allowed to make it's rated power. The A series engine using 91 octane fuel was allowed to make 1600hp for take-off and in military power it was allowed to make 1600hp up to 1000ft in low gear and in high gear it was allowed to make 1400hp at 10,000ft. With 100 octane fuel they raised the altitude it could make 1600hp to 1500ft and the altitude to make 1400hp was raised to 11,500ft.

The B engine with it's stronger construction was allowed to run at 2500rpm. It may have been allowed to use more boost. In any case the 1700hp rating was good to 4100ft in low gear and the 1450hp rating was good to 14,100 as Tomo has stated earlier.
Air cooled engines have a lot of limits and they are running a lot closer to the detonation limits than liquid cooled engines. They do not cool as well. Cooling is pretty much the amount of air (mass/weight) of the air flowing over the fins so you can either increase the size/area of the fins or change the total airflow at a given heat load (power level).
The problem with 2 stage superchargers or high capacity superchargers is that they take power to run. The two stage supercharger in the R-2800 cost about 350hp to run. Some was the actual power from the crankshaft and some was the hotter charge going into the engine was not quite as dense at the same pressure as the same pressure would be at lower altitude which used less supercharging (multiplication of existing pressure). Big reason for intercoolers.
Optimizing the R-2600 A for higher altitudes may not work as well as hoped. You have several choices but some of them run into the problem that the engine may simply not cool well enough to survive 1400hp at around 20,000ft instead of 11-12,000ft without some serious work to the baffles and cowls.
I don't know what they did to the B engines but Wright had been almost tripling the fin area on the R-1820 during the 1930s, they knew what it took to cool cylinders making over 100hp each. Which lead to next level. The BB series of engines which were almost entirely new engines again. New crankcase and crank allowed 2800rpm and entirely new method putting fins on cylinders (and large fins on heads) allowed for the change to 1900hp for take-off. Unfortunately something was just a little bit off and military power was 2600rpm and power was 1750hp at 3200ft and 1450hp at 15,000. (?)
Wright was trying to optimize the R-2600, they tried several paths. Some of the paths were used on other Wright engines. Maybe Wright was spread too thin? It does seem that minor tweaking was not going to work.

 

[Shortround6]

A lot of the liquid cooled engines got bigger radiators when they put bigger superchargers on them or upgraded the boost levels.
Like Merlin XX from Merlin III. The extra power to run the supercharger is not shown in the power to the prop numbers but it had to be taken into account for cooling.
The R-1830 in the Wildcat is a puzzle. It was allowed to run at 2700rpm for take-off and in both neutral and low gear in the P & W two stage system. But it was only allowed to run at 2550rpm in high gear according to most manuals/books. A way of limiting the power/heat generated???? I don't know but something was going on.