Japanese engine cowlings

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spicmart

Staff Sergeant
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May 11, 2008
How aerodynamic were the Japanese late-war radial engine cowlings compared to others?
They looked and were rather elaborately streamlined.
Could drag be reduced to the level of inline engines'?
 
spicmart said:
How aerodynamic were the Japanese late-war radial engine cowlings compared to others?
They looked and were rather elaborately streamlined.
Could drag be reduced to the level of inline engines'?
Click to expand...

I think it was virtually impossible to have designed a radial-engine aircraft with aerodynamics that would rival that of the most advanced inline-engine types. That said, of all the Japanese fighters to see service the J2M series probably came the closest to this goal, The set-up was somewhat complicated however with it's cooling fan arrangement and very long propeller shaft, which caused some headaches during development and in service (primarily in regards to shaft vibration).

The biggest drawback of radial engines has always been frontal area, but inline-types such as the BF 109G (which had multiple radiators and other airframe protuberances) could in some regards be considered aerodynamically inferior to the more advanced radial-engine fighters of the time. The saving grace for the fighter was it's overall small size in comparison to it's contemporaries. So although it's zero-lift drag coefficient was considerably high for the time (exceeding that of many radial powered types) it's equivalent flat plate area was very low which practically made up for any streamlining inefficiency found in the design. This is why the BF 109's overall performance remained very competitive right up to the very end of WWII.

Both engine types have their strengths and weaknesses; radial-engines were renowned for their immense durability, and they didn't require a heavy external cooling system which reduced weight some, while the inline-engine's main virtue was smaller frontal area (thus technically 'cleaner' aerodynamically), and to some extent the extra exhaust thrust produced. Both are a compromise of sorts and were selected after taking into account the various requirements the designers were facing at the time.
 
DarrenW said:
Both engine types have their strengths and weaknesses; some radial-engines were renowned for their immense durability
Click to expand...
Fixed it for you.

Some French Radials just before WW II acquired a reputation for shedding their propellers in flight, with no assistance from enemy aircraft.

Discrete vail (or not so discrete) pulled across durability of Armstrong Siddeley Tiger radial engine.
 
Air cooled engines also have their quirks. The fact that they rely on air from the outside (whose temperature can vary a lot depending on altitude) and on airflow generated by the propeller (could be a problem when you operate on constant speed with different loads) means that a pilot has to remain watchful and look at its operating temp very often. The cowlings had louvers that were controlled by the pilot to regulate the airflow and maintain the heads/oil temperature as constant as possible to avoid thermal stress to the hottest engine parts (and thus cracking).

Back on topic, Mitsubishi introduced a streamlined cowling for the J2M to obtain slightly higher speeds from the Kasei engine (which, at the time of design, was limited to 1560Hp). Later Mitsubishi used an upgraded engine with methanol / water boost (1850HP) anyway because the first prototype had climb performance below expectation so it couldn't meet the navy specification. Once the problem of vibrations was solved, they must have thought that the streamlined cowling was helping a bit nevertheless, since they also used it for the larger 18 cylinder Kasei derivative employed in the Ki-67/100.

Aside from these two Mitsubishi aircraft, the rest of the Japanese designer went on to use the tried and true ring cowling.
 
I seem to recall that the J2M Raiden had such a good engine cowling that it was less draggy than the A6M even though the latter's engine had 20 cm less diameter.Anybody know about that?
The later Japanese planes seem to have more rounded engine cowling.
 
Could be. The J2M also had a cooling fan that must have generated some small additional thrust; the intake looks a lot like the leading edge of a wing, just like in modern ducted fans.

You can appreciate the complex construction of the whole cowling in this old pic



And here is a rare picture of the J2M1, the first prototype with the 1530HP Kasei engine



You can appreciate the beautiful smooth lines of the nose. Also note the same three bladed propeller used in the early versions of the G4 'Betty' bomber (the engine was the same!) and the exhaust ports. These were later switched to individual exhaust stacks that served a pair of cylinders. The exhaust tubes in the subsequent J2M were also shaped like a nozzle to convert the residual heat/pressure of the exhaust gases into thrust.

Regarding the Mitsubishi Kasei engine, it was considered "large" by the Japanese, but it was all relative. Its diameter is 134cm, which is comparable to many radials used by other nations of similar power. It was also a very successful and reliable engine and, by 1942, it was able to reach 1850HP with methanol boost. Same as the Homare, though the latter was designed to reach this power with high octane fuel and had to resort to methanol only because such fuel was unavailable. One could argue that the Japanese obsession with making the smallest possible engine (Homare was 118cm in diameter, 16cm less thus only 77,5% of the frontal area of a Kasei) with an unproven design and lack of suitable material/alloys was a very short sighted decision.
 
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Good catch...shouldn't paint with a broad brush when talking about anything that's related to aviation!!!
 
Admiral Beez said:
I think the cowl on the Kyushu J7W Shinden had good aerodynamics. Is there any other air cooled radial in a pusher configuration that's smoother?
Click to expand...
Meet the Piaggio P.119. Also this one didn't get past prototyping stage. Not a pusher though
The radial engine (a Piaggio P XII of 1500Hp) was placed behind the pilot and cooled by an air intake below the cockpit. Such an arrangement was tried because the radial was 141cm in diameter and Italian designers weren't generally very fond of radials (even if between 1933-38 they were told by the airforce to use these engines for any new design)



 
Interesting that the Tempest II is a cleaner design than the Tempest V, if that is true.

Let me add the American's odd effort in engine cowling design. Which none of them seem to care for any potential benefits. Even post war, the F8F-2 and F4U-5 still have the traditional blunt nose. Meanwhile everyone else has moved on with clearly more streamlined designs, La-9 and Sea Fury.

The P-47 was pitted up against a Fw-190 in a climb comparison, and the P-47 overheated slightly where as the Fw-190 did not. Its hard to tell what engine settings the P-47 was tested at. ei 2300hp or 2600hp, but its clear with later models producing 2800hp that this overheating would probably be worse.

However the P-47 did receive an aerodynamic upgrade with the XP-47J. The AAF tested the XP-47J's top speed to be limited to the amount of power it could produce at higher altitudes, but non the less, it was still extremely fast at altitudes where it produced maximum power. Yes there were other design changes to the XP-47J, but its a question of why the American's kept with their "if it ain't broke, don't fix it" philosophy.

484 mph (779 km/h) at 25,350 feet (7,727 m)

This is 31mph faster then the fastest allied fighter, the P-47M. Which used a similar engine and turbo configuration

This is faster than any piston engine fighter plane of WW2. Republic claimed a higher top speed at higher altitudes exceeding 500mph at around 33,000ft. This is now encroaching on the Me-262 top speed. Could the Me-262 have been a large enough threat could the P-47J been enough to counter it at high altitudes?

The engine cooling fan and improved cowling design benefits:

Engine Cooling
Top Speed
Cursing speed(which than grants you better range also shown with the XP-47J)

What would be the downside? Obviously its not power related as the offset in powering the cooling fan is clearly surpassed by the more streamlined design. So what's the deal? Why did the Americans not care for the cooling fan like everyone else?
 

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Evaluation air cooled radials can be full of pitfalls.

For instance the R-2800s used in the P-47s M and N used considerably cooling fins than the P-47D engines.

The engines needed about 10 % less cooling flow at the same power settings than the P-47D engines.
I don't know it if the baffles are the different, I would suggest they are different for the cylinder baffles and cylinder heads.

The 2800 HP versions can get along pretty good using the cooling flows in that the 2500hp older engines did.

Other engines tended to have small very differences with prop spinners.
 

Are you referring to the propeller cuffs?
 
DSR T-888 said:
Are you referring to the propeller cuffs?
Click to expand...
Not what you are referring to ?

The R-2800 C series engines shared about nothing except the bore and stroke with the R-2800 B series engines.

The C series engines had a lot more cooling fins on the cylinder barrels and the more cooling fins on the cylinder heads.
The C series engines had a more efficient transmission of heat than the B series when using the same amount of a mass of air.


There were allowed lot of detailed differences between the the P-47J and the P-47M.

The P-47J was a light weight version of the P-47 with a lighter airframe. Some accounts say less was the same 30% of the airframe was the same.
Perhaps that was a bit exaggeration. But the guns were not quite the same configuration. Less ammo was used, the fuselage tank was smaller.
NO underwing stores (or under fuselage) where use used, more importantly there were changes in the intercooler and turbo charger ducts.


There were changes in the duct work leading to more exhaust thrust and the intercooler was on the level just in front if of the tail gear.
The doors on the left on fuselage on all P-47s were mixing.

The P-47J was a about 1000lbs lighter than a normal P-47 razorback.

Yes the P-47J had all those changes to the nose but the changes are not all responsible for the changes in performance.

Trying to sort the fan cooled differences from the other changes is going to take a a bit of work.
 
It does look like it. When it comes to overall, I think the XF-12 would have been the winner of best cowl.
 
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