Why wasn't three-engined bomber more popular?

[GreenKnight121]

Here is the un-adjusted photo of the Pratt-Whitney XT-34 testbed (B-17G-105-VE 44-85734).
The XT34 turboprop eventually went into production and ended up powering the Douglas C-133 Cargomaster long-range transport aircraft.
Following the completion of the tests in 1967, NX-5111N was donated to the Connecticut Aeronautical Historic Association based at Bradley International Airport. In 1979, it was heavily damaged in a tornado. In 1987, the damaged hulk was traded to Tom Reilly of Kissimmee, Florida, who plans to restore the aircraft to flying status in its original military configuration.

Note that the cockpit has been moved aft. :

And front view:

A Pratt & Whitney T64 turboprop was installed briefly to test different engine and propeller combinations.

 

[GreenKnight121]

Then there was B-17G-110-VE 44-85813 - it was used by Wright to test the XT-35 Typhoon turboprop.
This engine was more powerful than all four of the standard Wright Cyclone piston engines operating together. However, the Wright Typhoon was ultimately unsuccessful, and did not go into production. The aircraft was later used to test the Wright XJ65 turbojet, the engine being slung below a streamlined nose structure and the intake being covered with a cap for protection during ferrying.

The designation of this plane was changed to JB-17G in October of 1956, the J prefix having been introduced in 1955 to designate aircraft temporarily assigned to test work. In 1957, the plane was sold to Wright, which continued to use it as a five-engined testbed under the civil registration of N6694C. That year, it was used to test the R-3350 turbo-compound engine. The plane was later sold to an air tanker operator, and the missing nose was replaced by a hemispheric cap. N6694C crashed on takeoff in 1980 during a tanker mission and was damaged beyond repair. Its remains were purchased by warbird restorer Tom Reilly of Kissimmee, Florida for use in restorations of other B-17s.

Note that the cockpit has been moved aft.

 

[GreenKnight121]

A third conversion was in the form of B-17G-110-VE serial number 44-85747, which was employed by Allison for use as a five-engine testbed for the XT-38 turboprop.

Unlike the first two, this conversion did not require that the cockpit be moved aft.

 

[GreenKnight121]

Then there was the 4-engine B-17 turboprop conversion - I know it is off topic for the thread, but it goes with the other B-17 turboprop conversions.

B-17F-50-VE 42-6107 was converted to TB-17F, and sent to Department of Public Institutions, Clarkson, WA in 1946 for use as instructional airframe. Became derelict, sold on civil market to Columbia Airmotive of Troutdale, OR as N1340N Nov 18, 1953.
Used as tanker by Aero Enterprises and later by Aero Flite. Re-engined with Rolls-Royce Dart turboprops in 1969. Crashed at Dubois, WY while firebombing Aug 18, 1970.

 

[SaparotRob]

Keep the B-17 stuff coming friend.

 

[Elmas]

omissis
Cabin insulation works but every pound of insulation is a pound of cargo or pound of fuel not taken into the air.
omissis

The noise inside a modern C-130 is deafening: a crew member always gives you a pair of ear-plugs before boarding.

 

[tyrodtom]

The noise inside a modern C-130 is deafening: a crew member always gives you a pair of ear-plugs before boarding.

They may do that now, but in the late 60's and early 70 when I did some riding in C-130s, they certainly didn't hand out any earplugs.
It was loud, but not any great deal louder than any other military aircraft.

 

[Elmas]

They may do that now, but in the late 60's and early 70 when I did some riding in C-130s, they certainly didn't hand out any earplugs.
It was loud, but not any great deal louder than any other military aircraft.

Safety prescriptions in '60s and '70s were certainly not so strict as they are today.
And probably, if you did your ride as a military, you had to know to have at hand your own ear-plugs.

 

[davebender]

View for the pilots is always going to be worse

No forward firing armament unless it's synchronised.

No space for nose mounted radar
I suspect these were the most important reasons. Nose space is prime real estate on a combat aircraft. Wing space is not.

 

[stug3]

I saw this pic of Bob Hoover's model of the Allison EB-17G on fb. He was one of the test pilots.

 

[nuuumannn]

To bring this back to three-engine bombers again, the SM.79 at the Italian Air Force Museum. From this view you can appreciate how the centre engine changes the arrangement of equipment on the aircraft.

Front view

Bomb bay with bomb aimer's position to its rear.

Bomb bay

Bomb aimer's position.

Bomb aimer's position

The SM.79 was originally developed as an eight seat fast passenger aircraft intended for air races, which suited the propagandistic desires of the fascist regime, and first flew in September 1934. The second prototype was configured as a bomber and for its time had excellent performance. Through its development it went through a number of powerplant changes; it's first engines were licence built derivatives of British radials, the 590hp Piaggios in the first prototype were based on the Bristol Jupiter, then the 861hp Alfa Romeos fitted to the bomber versions were based on and derived from the Bristol Pegasus. There were plans to fit 800hp Issotta Fraschini Asso V-12s to the prototype, but the radials were fitted. It's worth noting that the majority of Italian bombers and larger aircraft were powered by these Alfa Romeo radials, which remained little changed throughout their various installations, but varied in horsepower from around 640hp to around 950hp.

Both the tri-motor types in this picture were powered by the same Alfa Romeo engines derived from the Pegasus in service, although this particular SM.82 was retrofitted with 1830 Twin Wasps. The SM.82 had three 949hp Alfa Romeo 128 RC.21s and the Z.506B Airone was powered by three 750hp Alfa Romeo 126 RC.34 engines.

SM.82

This G.212 was powered by Twin Wasps.

G.212

 

[GrauGeist]

Aside from terrible frontal view for the pilot, having a "dustpan" pod for the bombardier had to impose a considerable drag penalty on the aircraft.

 

[Elmas]

Yes, there was a certain increase of drag, but the worst feature of SM 79 was the vertical arrangement of the bombs in the bomb bay. After release the bombs were subject to such somersaults that any ambition for even a small precision was impossible.

 

[GrauGeist]

Thanks for that illustration, Elmas - I was always under the impression that the SM.79 carried her bombs nose down!

 

[fastmongrel]

Thanks for that illustration, Elmas - I was always under the impression that the SM.79 carried her bombs nose down!

I have always wondered why vertically mounted bombs were dropped tail first. I guess it could be to protect the delicate tail fins which could hit the sides of the bomb rack as the slipstream hit the bomb, dropped tail first the bomb nose is less likely to hit the sides and likely just slide rather than cause any damage.

 

[GrauGeist]

It appears that not all were dropped tail-first.
In this photo, you can see the bombs dropping from an SM.81 nose-first, so I'm not sure which carrying method was the norm.

 

[fastmongrel]

Thats odd I was under the impression the SM81 dropped its bombs horizontally.

 

[fastmongrel]

Mystery solved according to wiki 500 and 250 kg bombs were carried horizontally, 100 kg and smaller bombs stored vertically.
Savio-Marchetti SM81

 

[GregP]

The pic in post #15 is a photoshop fake. There were two B-17s used to test turboprops. One tested the P&W XT-34 and the other was used to test the Curtiss-Wright XT-35. You can tell them apart by the spinners easily. The XT-34 would up in the Douglas C-133 Cargomaster. Neither was EVER tested single-engine. Here's one of the real pics of an early XT-34 installation:

Below is the later XT-34 version:

Below is the Curtiss-Wright XT-35 version:

There was also a 4-engine turboprop B-17 built in Wenatchee, Washington to fight fires. It had four Rolls-Royce Dart engines from a Vickers Viscount in 1970:

With both outboard engines shut down and feathered, it was still faster than a stock B-17. It was short-lived as it crashed near Dubois, Wyoming due to power loss from ingestion of superheated air during a retardant drop in the same year it was built. Above, it is landing on only the two outer engines, which were generally used to cruise home after dropping the fire retardant. The nacelles are long due to the CG issues with the turbines being lighter.

The Dart was used on several freighters and airliners, including the Handley-Page Dart Herald:

which was likjely the best turboprop airliner never built in production. The prototypes did demos from unimproved dirt and even mud runways in Africa.

I surely like the lines of the photoshop pic of the single-engine B-17 turboprop, but it never actually existed and flew.

 

[fastmongrel]

The Herald was production built but only 50 of them. The Fokker F27 took most of the orders for the DC3 replacement market.