Why no three engined heavy fighters in WW2?

[Elfnet]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why? Obviously there must have been serious downsides, but I couldn't see any fatal flaws (like the layout was well understood, not something like the coupled engines as in the German Heinkel He 177, the 3rd engine's added cost would have been offset by the significantly higher survivability, and so on).

Obviously, it would have been a wartime necessity (this is why the great powers simply built bigger engines), and I'm mainly a tank guy who doesn't know much about aircraft, but in WW2, multiple countries made two-engine tanks, like the United Kingdom (with two separate engines) and Hungary (with coupled engines), while Australia even built a three-engine tank called the AC3 "Thunderbolt" (it was only a prototype because soon after they got American Shermans).

Italy and the IMAM Ro.67 were only examples, but not a single country mass-produced any three-engine heavy fighters in WW2 (despite building other three-engine aircraft), so it is clear that this would have been a bad idea in practice, but I can't wrap my head around it (seeing the three-engined German Blohm und Voss BV 138 or the half dozen different Italian three-engine medium bombers).

 

[Justo Miranda]

- Upon learning the performance of the Polikarpov I-16 Type 6 (440 kph) in Spain, Latvia acquired thirteen Gloster Gladiator Mk.I (407 kph) fighters in 1937 and thirteen more in 1938, after the Polikarpov I-16 Type 10 (448 kph) entered into service. That same year they also acquired three Hawker Hind bombers. The Polikarpov I-16 Type 24 (470 kph) appeared in 1939 and Latvia tried to acquire thirty Hawker Hurricane Mk.I and twelve Westland Lysander, but the British cancelled the operation after the German invasion of Poland.



Between 1937 and 1940, the local industry Valsts Elektro-Fabrika (VEF) produced several all-wood monoplane trainers, with fixed landing gear, that proved to have exceptional flight performances and a great potential, susceptible of being used as ‘emergency fighters’. The VEF I-12 light trainer, built in a number of 12 units, reached 230 kph powered by a Cirrus engine of 90 hp only. In 1938 four aircraft were converted to single-seat fighters, armed with a light machine gun, to be used by the Latvian National Guard.



The VEF I-15, built in 1939, was a single seat training aircraft with two machine guns that could fly at 330 kph with a 210 hp Gipsy Six engine. The VEF I-16, that performed its first flight in 1940, with a maximum speed of 460 kph, a 520 hp Walter Sagitta ISR engine and four machine guns, was roughly the equal of Polikarpov I-16 Type 24. Construction began on a series of twelve aircraft that was interrupted by the Soviet occupation on 17 June.



After the purchase of Hurricane fighters was cancelled, the Latvian government ordered the design and construction of the air superiority fighter VEF-19 capable of competing with the MiG-3 and the Bf 109E. The complexity of the hydraulic equipment for the retractable landing gear was above the VEF capabilities and it did not seem possible to obtain engines with more than 500 hp.



These difficulties did not stop the resourceful designer Karlis Irbitis who proposed building a composite engine connecting three D.H. Gipsy Six to each other to make an inverted 'Y', 6 bank, 36-cylinder, 36 litres, air-cooled power plant, driving a three-bladed variable pitch airscrew. The monster, denominated MI-02, could possibly develop up to 1,800 hp at take-off and 1,470 hp at 3,670 m.



The VEF-19 would be built of wood/plywood and armed with four machine guns. It would have had 11 m wingspan, 9.2 m length and 650 km/h maximum speed. The plan was to use the American engine Allison V-1710 for the preliminary calculations and for the flight of the first prototype. It was also considered to obtain some Rolls-Royce Merlin and Hispano-Suiza H.S.12 engines.

 

[Nahida1645]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why?

Where do you put the guns?

By synchronization gears, fit for only one gun
Or maybe as a multi-place fighter such as the twin-engined Airacuda.

Trimotor fighters are impractical for the era.

 

[publiusr]

Now if the third engine was a jet or rocket, might that assist a plane with two props and keep the nose clear?

The two props for range…thrusters either side of the tail to turn faster?

 

[Scott Kenny]

Now if the third engine was a jet or rocket, might that assist a plane with two props and keep the nose clear?

Yes, mixed power would solve that issue.

 

[Arjen]

Jets were only developed successfully by nations that produced lots of high-power piston engines, largely removing the need to go down the trimotor way. Mixed-power aircraft mostly came about to compensate for high fuel consumption in early jets (XP-81, Fireball, various German projects), or, after WW2, to improve sprint- or takeoff-performance (Mirage III, Neptune, Savage, C-119).

 

[riggerrob]

A couple of thoughts...
Unless have enough through-hub guns and/or interrupter gear, three front engines would only suit a 'turret fighter'.
( 'Too few' front-firing guns are only good for dissuading fighters rather than culling bombers... )
Upside, perhaps thus fast enough to climb, chase, catch and kill bombers. Down-side, better ways to do such: Thee engines give you three single-engined fighters, or one and a twin...

Need all three front engines be same ? Logically, the wing-engines should match, but nose engine may be smaller / larger, perhaps with prop-feathering. Perhaps turning opposite to the wing engines. Perhaps shut down nose engine for loiter / cruise...

Regarding an in-fuselage engine, I'm reminded sub-hunting and mag-mine sweeping fighter-bombers had a rather big generator in the bomb-bay to drive Leigh-light or sweep coil.

IMHO, a third 'mid-engine' driving nose and/or tail prop would introduce ghastly 'long drive-shaft' issues that thwarted umpteen designs...

A twin-boom configuration with two 'pullers' and a central pusher might work, sorta-kinda, leaving nose free for armament...

Good point.
A tri-motor P-38 could mount its third engine close to the center-of-gravity, eliminating the need to a long drive shaft. Drive shafts were problematic until the introduction of smooth-turning turbo-shaft engines (see Alouette III helicopter) during the 1950s.

Changing the direction of rotation on the center engine would have little affect on engine-out handling.
“Handed” propellers are most valuable after an outboard engine quits. To illustrate this concept we look at the example set by the asymmetric Blount & Voss 141. The key concept is that - in climb configuration - the descending propeller blade takes a deeper “bite” of air and tried to turn the airplane. BV-141 solved this problem by installing its clockwise-rotating (as seen from the tail) engine to the left of the center-line so that it’s descending blade overlapped the center-line.
The farther outboard the descending blade, the worse the turning effort. So if all props turn clockwise (as seen from the tail) the worst (aka. critical) engine to lose is the left engine because the right engine’s descending blade is outboard … er … farthest from the center-line and creates the worse yaw problem … in turn requiring the pilot to press harder on the right rudder pedal the fly straight.
Note that most light twins revert to climb configuration after losing half of their power.

 

[riggerrob]

- Upon learning the performance of the Polikarpov I-16 Type 6 (440 kph) in Spain, Latvia acquired thirteen Gloster Gladiator Mk.I (407 kph) fighters in 1937 and thirteen more in 1938, after the Polikarpov I-16 Type 10 (448 kph) entered into service. That same year they also acquired three Hawker Hind bombers. The Polikarpov I-16 Type 24 (470 kph) appeared in 1939 and Latvia tried to acquire thirty Hawker Hurricane Mk.I and twelve Westland Lysander, but the British cancelled the operation after the German invasion of Poland.



Between 1937 and 1940, the local industry Valsts Elektro-Fabrika (VEF) produced several all-wood monoplane trainers, with fixed landing gear, that proved to have exceptional flight performances and a great potential, susceptible of being used as ‘emergency fighters’. The VEF I-12 light trainer, built in a number of 12 units, reached 230 kph powered by a Cirrus engine of 90 hp only. In 1938 four aircraft were converted to single-seat fighters, armed with a light machine gun, to be used by the Latvian National Guard.



The VEF I-15, built in 1939, was a single seat training aircraft with two machine guns that could fly at 330 kph with a 210 hp Gipsy Six engine. The VEF I-16, that performed its first flight in 1940, with a maximum speed of 460 kph, a 520 hp Walter Sagitta ISR engine and four machine guns, was roughly the equal of Polikarpov I-16 Type 24. Construction began on a series of twelve aircraft that was interrupted by the Soviet occupation on 17 June.



After the purchase of Hurricane fighters was cancelled, the Latvian government ordered the design and construction of the air superiority fighter VEF-19 capable of competing with the MiG-3 and the Bf 109E. The complexity of the hydraulic equipment for the retractable landing gear was above the VEF capabilities and it did not seem possible to obtain engines with more than 500 hp.



These difficulties did not stop the resourceful designer Karlis Irbitis who proposed building a composite engine connecting three D.H. Gipsy Six to each other to make an inverted 'Y', 6 bank, 36-cylinder, 36 litres, air-cooled power plant, driving a three-bladed variable pitch airscrew. The monster, denominated MI-02, could possibly develop up to 1,800 hp at take-off and 1,470 hp at 3,670 m.



The VEF-19 would be built of wood/plywood and armed with four machine guns. It would have had 11 m wingspan, 9.2 m length and 650 km/h maximum speed. The plan was to use the American engine Allison V-1710 for the preliminary calculations and for the flight of the first prototype. It was also considered to obtain some Rolls-Royce Merlin and Hispano-Suiza H.S.12 engines.

Cute!
That combining gearbox could have been designed to fire a 20mm, 30mm or maybe even a 37mm auto-cannon through the prop hub. 20mm was the minimum to kill bombers by 1940.

 

[publiusr]

In the past, interruptor gear was used to protect blades from bullets.

Might they be used to prevent them from hitting each other if they were to intermesh?

Nacelles could be closer…each has four engines…and guns in the nose further interrupted?

 

[Justo Miranda]

https://www.secretprojects.co.uk/threads/dornier-canard-design.5013/

 

[Scott Kenny]

In the past, interruptor gear was used to protect blades from bullets.

Might they be used to prevent them from hitting each other if they were to intermesh?

Nacelles could be closer…each has four engines…and guns in the nose further interrupted?

I would not want to be the engineer responsible for designing and testing an interrupting gear for overlapping prop arcs...

Honestly, if your engines are low performing, using a third engine of some size just to drive a supercharger for each engine is certainly an option. Run the intercoolers in the leading edge of the wing sections between fuselage and nacelles where they can get all the cold air to bump air pressure up even more. Then the engines physically turning the props can get away with little to no supercharger boost directly, and put that power into the props.

Superchargers eat a LOT of horsepower, the running comment about a Top Fuel car is that a stock 426 hemi engine doesn't make enough power to turn the supercharger that feeds the Fuel engine. That means that the Fuel supercharger takes over 500 horsepower to turn!

 

[paralay]

One engine on an airplane is one problem, three engines are three problems

 

[Scott Kenny]

One engine on an airplane is one problem, three engines are three problems

Yes, and?

If your engines are not capable of making the power necessary, one option is adding more engines.

 

[publiusr]

Now, has anyone worked on combining rotary/Wankel engines?

Less involved than my earlier suggestion..though zipline propellers may help with that.

Perhaps three rotaries could feed into one shaft?

For electrics, I could see radial engines in a row inside a fuselage with power given to much smaller electric motors in the wing.

A person could work on a disabled engine in flight perhaps. You have two back ups.

 

[paralay]

Yes, and?

If your engines are not capable of making the power necessary, one option is adding more engines.

To prevent this from happening, the ministry orders scientific work on the selection of the required range of engines. Planes with more engines are less efficient. And more difficult to maintain. Therefore, even large-capacity passenger planes have become twin-engine. Less is not possible for security reasons

 

[paralay]

Now, has anyone worked on combining rotary/Wankel engines?

Rotary engines have a higher fuel consumption, since the shape of the combustion chamber is not optimal.As one of my friends, an aviation designer, used to say: - a rotary engine requires a fire system of the same mass as the engine itself

 

[Nicknick]

The Wankel engine wasn't even invented in WW2, so we are getting really offtopic...

Why would anyone try to syncronize three intermeshing propeller blades, if a small axial gap could do the same thing? Despite that, intermeshing props are unefficient, because the air in the intermeshing area will not be significant more accelerated by two props, than the rest of the air in the propeller section.

 

[Nahida1645]

Technically it is practical on jets, but suffers the same fate (higher fuel consumption, problems on CoM or CoL or CoG, higher dead weight and maintenance overcomplication) as what the propeller-powered does.

I agree on what Scott Kenny said that a mixed-power tri-engined fighter (that composes of two airscrews and one turbojet) could be plausible, however turbojets were only reaching its maturity in the late 1940 timeframe, or unless some aircraft designer would think of a Wunderwaffe or something else. Worth it for a design practice.

Using a drive shaft for the third engine is also plausible in order to give armament an unobscured clearance during dogfights and interception, but it could lengthen aircraft maintenance and the airscrew being placed on the back of the pod or the centre fuselage.

I am thinking of a concept of using the turbojet as a speed boost during takeoff like what a jet-assisted takeoff bottle functions, high-altitude flight and interception and keeping the then-nascent turbojet off during normal flight.

 

[Scott Kenny]

Now, has anyone worked on combining rotary/Wankel engines?

Less involved than my earlier suggestion..though zipline propellers may help with that.

Perhaps three rotaries could feed into one shaft?

Yes, there's a monster of a 12-rotor engine that a guy put together as 3x 4-rotors in a V arrangement. 1500 ish horsepower without revving the hell out of it. Also, the combining gears needed make it trivial to add some kind of Prop Speed Reduction Unit so you have the engine running at 6000rpm while the prop spins at 2000rpm.

But Wankel engines were not invented until the 1950s. Late 1950s at that.

For electrics, I could see radial engines in a row inside a fuselage with power given to much smaller electric motors in the wing.

A person could work on a disabled engine in flight perhaps. You have two back ups.

Ew, no. Not radials. Those were egregious maintenance hogs compared to inline engines. Keep those oil leaks outside the fuselage!

Plus, electric motors in the 1940s were heavy for the required power levels.

 

[Nahida1645]

Now, has anyone worked on combining rotary/Wankel engines?

Less involved than my earlier suggestion..though zipline propellers may help with that.

Perhaps three rotaries could feed into one shaft?

Yes, there's a monster of a 12-rotor engine that a guy put together as 3x 4-rotors in a V arrangement. 1500 ish horsepower without revving the hell out of it. Also, the combining gears needed make it trivial to add some kind of Prop Speed Reduction Unit so you have the engine running at 6000rpm while the prop spins at 2000rpm.

But Wankel engines were not invented until the 1950s. Late 1950s at that.

For electrics, I could see radial engines in a row inside a fuselage with power given to much smaller electric motors in the wing.

A person could work on a disabled engine in flight perhaps. You have two back ups.

Ew, no. Not radials. Those were egregious maintenance hogs compared to inline engines. Keep those oil leaks outside the fuselage!

Plus, electric motors in the 1940s were heavy for the required power levels.

How about broad-arrows and H-engines in place of radials?

 

[Scott Kenny]

How about broad-arrows and H-engines in place of radials?

Better.

You're better off having the engines directly drive a prop, though. You'd have heavy, heavy generators and equally heavy motors for electrical transmissions.

 

[publiusr]

True...but having engines in the fuselage allows repairs in flight.
Better insulated.

Electrics work better in the cold...to a point.

 

[Arjen]

Imagine an aircraft fuselage spacious enough for a mechanic to reach servicable engine parts. Then, consider the title of this thread. 'Why no three engined heavy fighters in WW2?'
Finally, picture a fighter of elephantine proportions lumbering after its prey.
The upside: Dumbo has plenty of room for its embarrassed crew (pilot, engineer, janitor) to hide in.
In the thirties, Fokker proudly presented this, the portly T.V as a Luchtkruiser - aircruiser.
Sixteen were built, when war broke out they served purely as bombers, because, at the behest of the Dutch air force, the space for an engine room had been wasted on a bomb bay. Sadly, the forward 20mm Solothurn was never used in any dogfight - a missed opportunity, no?

 

[GTX]

True...but having engines in the fuselage allows repairs in flight.

Not if you are a fighter...

 

[Avimimus]

I don't think the synchroniser issue is that serious - there would be an area where props do not overlap etc.

However, there are issues:

- Cost: One three engined fighter, instead of three one engined fighters, more fuel consumption, more maintenance issues.

- Manoeuvrability: Having wing mounted engines will spread out the mass, change the moment of inertia, and significantly reduce roll rates. A three engine design is likely to have the wing engines mounted even further apart. Furthermore, added weight will make the controls heavier.

- Cockpit view: Twin engine designs compensated for the loss of view from the wing mounted engines by having the pilot mounted far forward. By having a long nose with an engine in it, the view becomes severely impaired. However, if the centre engine is a pusher or has the pilot between the engine and the prop (P-39 style) this is less of an issue.

- Vulnerability: The engines produce redundancy, but the overall increase in wing area and fuel-system complexity should make the aircraft easier to hit.

It is likely that the most feasible design would be a twin boom pusher (with an engine at the front of each boom as well). Such an aircraft might make a decent interceptor... but the overall issues with cost and manoeuvrability are likely insurmountable.

What I don't understand is why we don't see modern designs (e.g. in the 'fleet interceptor' role) with a couple of lower-bypass turbines instead of afterburners... it would seem that having a booster low-bypass engine core for supersonic flight would be ideal, and that it could be shut down while on patrol. The fact that modern interceptors have power-boosted controls (or even fly-by-wire) and are already very large would seem to remove most of the existing objections.

Btw. I asked exactly that question some years ago: https://www.secretprojects.co.uk/threads/so-why-the-lack-of-three-engined-fighters.21491/

 

[Scott Kenny]

What I don't understand is why we don't see modern designs (e.g. in the 'fleet interceptor' role) with a couple of lower-bypass turbines instead of afterburners... it would seem that having a booster low-bypass engine core for supersonic flight would be ideal, and that it could be shut down while on patrol. The fact that modern interceptors have power-boosted controls (or even fly-by-wire) and are already very large would seem to remove most of the existing objections.


Btw. I asked exactly that question some years ago: https://www.secretprojects.co.uk/threads/so-why-the-lack-of-three-engined-fighters.21491/

Will read through that thread before I add my own opinion there...

 

[publiusr]

The reason I mention intermeshing props is that the nacelles can be moved closer together. The loss of one engine doesn't skew things if power is also shared.

I was skeptical of Osprey at first... but complex systems can be made reliable. Musk proves that every week or two...even if I still have to pinch myself to see if I'm dreaming.

 

[Scott Kenny]

The reason I mention intermeshing props is that the nacelles can be moved closer together. The loss of one engine doesn't skew things if power is also shared.

I was skeptical of Osprey at first... but complex systems can be made reliable. Musk proves that every week or two...even if I still have to pinch myself to see if I'm dreaming.

But power sharing across long distances is a complex piece of work... (my preference is to have the engines close together, the combining gearbox right there, and then send power to where it needs to go, like on a CH-47)