Why wasn't three-engined bomber more popular?

[Nodeo-Franvier]

Tri-motor seem like a nice compromised between He-111 and B-17. And SM.79 was also quite successful.

 

[Greyman]

Wild speculation:

• nose better used for other goodies like bomb-aimer position and forward defence
• less complication
• cg considerations

 

[Shortround6]

Tri motors were great if your engines weren't powerful enough to get the job done with two engines.

SM.79s had engines between 750hp and 1000hp depending on version

Rumanian SM-79 with Jumo 211 engines.

twin 1200hp engine or three 800hp engines?

 

[GrauGeist]

Trimotor configurations were from a time where engines didn't produce enough horsepower and needed that third engine to get enough power for the aircraft to safely perform.

The B-17 and the He111 were in two different classes.
The He111 was designed as a medium bomber thinly disguised as a passenger aircraft.
The B-17 was designed as a heavy bomber from the start.
Both types had adequate engines available from the drawing board and as SR pointed out, the Romanian SM.79 only needed two engines of reasonable horsepower (and looked MUCH better that way, too) instead of three lesser HP output engines.

 

[Nodeo-Franvier]

But couldn't three motor heavy bomber with powerful engine be designed as well?

 

[pbehn]

Where do you put the fuel for the third engine? With the same fuel as a twin it is shorter ranged while building more fuel in the design means it is almost the size of a 4 engine plane.

 

[fastmongrel]

But couldn't three motor heavy bomber with powerful engine be designed as well?

Nothing stopping a design with 3 powerful engines but several points occur to me.
Where does the bomb aimer go, if you fit a gondola for him that's less aerodynamic.

View for the pilots is always going to be worse

No forward firing armament unless it's synchronised.

No space for nose mounted radar

It seems to me that 2 or 4 wing mounted engines always beat a Tri motor.

 

[yulzari]

I think that the Italians solved the practical problems. They were short of powerful engines and putting a third engine in the nose was easier than putting two more engines in the outer wings.

 

[pbehn]

Is a prop at the front of a bomber as efficient as one on the wings?

 

[GrauGeist]

I'm far from an expert when it comes to determining the values of drag versus power in a situation like this, but while an additional radial in the nose certainly provides more power, it is also introducing a drag penalty.
It would seem to me that the cost of adding an additional engine to each wing (making it a four engine) would be offset by better perfomance as well as providing additional defensive stations.
Messerschmitt put six G-R 14N engines on the Me323, which worked out well.

 

[MIflyer]

One of my high school teachers got to fly a Ford Trimotor in the 30's as part of his USN training. He said they used to leave the nose engine shut down until they were at the end of the runway and ready to take off. It caused too much noise and heat. Now, imagine enduring those same disadvantages for the whole mission. .

 

[pbehn]

If you put 1 wasp major on the front of a Savoia Marchetti 82 you have what the Fairey Battle always wanted to be.

 

[Admiral Beez]

I've read that the Ju-52 was very noisy inside. Was this due to the nose engine or the corrugated sides?

 

[swampyankee]

Putting an engine in the nose does complicate things like where to put the bomb aimer. It's also going to make the interior of the aircraft more uncomfortable, which is like to be more problematic for bombers than fighters (the vast majority of which have nose-mounted engines) as the bombers, especially those designed in the 1930s, are going to be on much longer missions than fighters.

A number of other issues are really not significant. C/G issues crop up with all sorts of aircraft, with 0 or more engines. Quite often, when you see a conventional piston-prop aircraft with swept wings, it's a pretty sure sign somebody screwed up the c/g calculations (this is also the case with the Me262. The wing was swept because the c/g was miscalculated in the design process, not because of airspeed). A trimotor will have a more complex fuel system and will have a greater workload for the crew than will a twin, but less than would a four-engine aircraft. A four-engine aircraft will likely have more drag than a trimotor, both because of increased wetted area (those nacelles) and because of interference between the nacelles and the wing. More engines means more maintenance and will likely mean lower availability.

 

[MIflyer]

Well, if you have a powerful enough engine, you don't need three of them....

 

[GrauGeist]

I've read that the Ju-52 was very noisy inside. Was this due to the nose engine or the corrugated sides?

The Ju52s used for passenger service were insulated, like the Fords and Fokkers.
The military versions didn't have that and were noisy.
I've ridden in a Ford trimotor and it was very comfortable inside. Matter of fact, it was one of the nicest rides I've had out of all the aircraft I've ever flown in.

 

[Dash119]

Well, if you have a powerful enough engine, you don't need three of them....
View attachment 591962

Pretty sure this is some photoshop magic... The turboprop testbed B-17's all retained the four original wing mount Wrights.

 

[MIflyer]

Yeah, I understand the image was manipulated. That B-17 was given to the Bradley CT Air Musuem and was badly damaged when a tornado hit and destroyed the waist section. The remains were given to the O'Reilly Air Museum in Kissimmee FL in return for some repair work they did at Bradley. They transported it to FL and found a "new" waist section that had been purchased in the late 40's, wrapped with tarpaper, and used for a children's playhouse. They rebuilt the B-17, which escaped being destroyed when the rest of the facility was badly damaged by Hurricane Charley in 2004.

 

[Acheron]

My guess would be that an engine in the front turned out to be less efficient than one in a wing. Of course, with fighter crafts, you were better of avoiding wing-mounted engines, thinking of the Do 335. Catch is, bombers (at least the bigger ones) tend to be larger, so I suspect with them, mounting a propeller in the front was less effective, as the air the propeller moves has to be shoved around the considerable hull.

Pure speculation on my part though, I have no clue about aerodynamics.

 

[Shortround6]

For air transports the tri motor had a few things going for it and a few things against.
Sticking a 3rd engine on the nose of a transport is almost free in terms of drag. No radial was as wide a pair of seats with an aisle in the middle so no increase in frontal area. local airflow in another story. Compared to a twin flying in an engine out situation is much easier. You have 2/3s power instead of 1/2 power so your controlled crash can be a lot further from the point of engine failure Asymmetrical thrust is reduced, if the nose engine fails there is no asymmetric thrust. If a wing engine fails 1/2 the thrust is still on the center line and 1/2 the reaming thrust out on one wing instead of 100% of the reaming thrust being out on a wing. If you are flying slow enough for prop blast over the tail to offer any benefit then you have that in a wing engine out situation.
Against it were the factors of noise and vibration. Both could be dealt with varying amounts of insulation and better engine mounts. Cabin insulation works but every pound of insulation is a pound of cargo or pound of fuel not taken into the air. Exhaust and "fumes" also have to dealt with, especially in an area when some of the cabin ventilation was from open windows
Also please note that Wright and P & W didn't offer enclosed lubricated valve trains until the mid 30s and a few other nations were several years behind. Nose engine leaked oil and/or grease from grease fitting supplied "joints" and hot oil/grease vapors/fumes were not an attraction to most customers.

The tri motor's hay day was before engines got really powerful and before constant speed , fully feathering propellers became common.