Why wasn't three-engined bomber more popular?

[Elmas]

Any bomber can hit with precision, with the right support equipment, aircrew training, tactical and strategic use etc. That the He 111 was successful was no accident. In the early years of the war the Luftwaffe had an enormous advantage in technical use of aerial navigation aids, Knickerbein, X and Y-Gerat, and its crews had thorough training and experience in combat that neither the RAF nor other air forces had by the time WW2 broke out in 1939.

I do agree with you, but up to a point...

The photo shows the last moments of the HMS Juno, as she steamed with the Mediterranean Fleet against the German air-borne invasion of Crete.

HMS Juno was attacked by an Italian CANT Z.1007 (three engines!), 50° Stormo B.M., 30 nautical miles south-east of Crete on 21 May 1941, flown by Ten. Mario Morassutti and his crew.

Tenente Morassuti's Cant 1017 dopped nine bombs (eight are visible in the water) and one went directly to the destroyer funnel, causing his loss in a couple of minutes.

Was the "Morassuti's strike", as it was known in the Regia Aeronautica of those times, the proof that ships could be attacked from altitude with bombs? Quite the contrary, as from the photo can be clearly seen that some of the bombs are distant hundreds of meters from the target, giving a precise idea of the dispersion of bombs, even from a FL relatively low, I should say no more than 10.000', probably less.

My personal ideas about "precision bombing " have been expressed in this 3d:

Norden Bombsight

 

[nuuumannn]

Tenente Morassuti's Cant 1017 dopped nine bombs (eight are visible in the water) and one went directly to the destroyer funnel, causing his loss in a couple of minutes.

Interesting, but do you think the type of aircraft had anything to do with that strike? Nope. It was down to sheer skill and training. You don't just become a good accurate bomb aimer. I think we might be trying to prove the same point here - this just proves exactly what I've been saying all along. The type of aircraft alone has little to do with how accurate your bombing is - its training, doctrine, equipment, experience; all those things.

 

[Shortround6]

type of aircraft has something to do with accuracy, how much is certainly subject to question but a number of bombers of the 1930s went through several modifications of tails and/or changes in dehydrial of the wings to cure snaking or lateral instability which the purchaser's thought would affect accuracy

Of course bombing trials without the hazards of AA fire and with good weather are a whole lot different than operational accuracy.

 

[SaparotRob]

Found it.

 

[Elmas]

Interesting, but do you think the type of aircraft had anything to do with that strike? Nope. It was down to sheer skill and training.
omissis

I started to play golf in 1977, when I was on holiday in a village in Kent.
As I always wanted to play, I went to the Golf course and started taking golf lessons.
It sorted out that golf suited me well, and after four or five lessons, I did with a 5 iron a very good shot, of about 160 yards, that ended within two yards from the flag.
"Very good shot!" Graham, the golf Instructor, kindly told me " a professional could not have done better. But.." he added " a professional does it every time..."
Tenente pilota Morassuti and Puntatore Balboni had certainly an outstanding skill, training, cold blood and steel nerves, but I wouldn't bet that every time they dropped nine bombs onto a British destroyer that ship would go into Davy Jones' locker.

 

[xylstra]

Wild speculation:

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

Why is it you guys are fixated on full tractor configurations? No problem with the third engine at the top of the tail. Been done before. Use your imagination!

 

[GrauGeist]

The B-36, a huge favorite of mine, certainly wasn't a tractor configuration.

 

[Shortround6]

Some of the tri-motors just weren't that good.

 

[swampyankee]

Why is it you guys are fixated on full tractor configurations? No problem with the third engine at the top of the tail. Been done before. Use your imagination!

Was it done before WW2? (I know Britten-Norman did it after

Source: File:G-FTSE Trislander Aurigny Air Services (7003405361).jpg - Wikimedia Commons)

They could do a twin-boom aircraft, with the third engine as a pusher in the fuselage, as in the Caproni Ca.3.

Or a twin fuselage aircraft with the third engine in a nacelle on the center section.

Or eschew symmetry. A trimotor expression of the Rutan Boomerang. Blohm & Voss probably proposed one.

 

[GrauGeist]

Some of the tri-motors just weren't that good.

The BV138 would be on thw opposite end of that spectrum.
A rather homely looking machine, but very reliable.

 

[bruno_bis]

Beside all the previosly described considerations, the impact of engine reliability has a role in discussion "three vs two" engines architecture.

In order to clarify the point, suppose that:
A) the same total power can be achieved by a two-engine configuration and a three-engine configuration as well. This is to eliminate situations were the choice is forced by other than just reliability considerations.
B) the fault rate of each single engine is the same, that is the probability of a fault of one engine is the same for both configurations.

Then imagine two scenarios for the three-engine configuration:

S1) One engine is enough to keep the aircraft flying so to return home.
S2) One engine is not sufficient, that is, if two engines out of order, the kite digs into the water or earth.

Of course, with respect to the two engine configuration, the assumption is that the aircraft can still come home with just one engine working.

In the above mentioned frame, calculations(*) show that the probability of a fatal loss of engine power of the three-engine configuration is lower than the one of a two-engine configuration in the scenario S1 while it is higher in the scenario S2.

(*) A math demonstration is possible but boring. Anyway it should be apparent that as far as S1 is concerned, a fatal loss of power occurs if and only if all three engines fail, whereas a fatal loss for a two-engine configuration occurs if two engines fail. In the light of assumption B), a failure of three engine is less probable than a failure of two engines.
As to scenario S2, the explanation is more cumbersome but, considering that, given three engines, there are three possibile way of loosing two of them, it is possible to see that in the case of S2, the three engine formula is worse than the two-engine configuration.

 

[Shortround6]

From Wiki on the Boeing 247 airliner.

"The 247 was the first twin-engined passenger transport able to fly on one engine. With controllable pitch propellers (standard equipment on the 247D), the 247 could maintain 11,500 feet (3,500 m) at maximum gross takeoff weight"

With controllable pitch props and later constant speed/feathering props the need for a 3rd engine to make it to an emergency airfield was much diminished.
Likewise the better props made more efficient use of the take-off power so three engines turning props at poor efficiency in the take-off end of the performance spectrum weren't needed.

 

[GrumpyOldCrewChief]

Another point not mentioned yet was availability of resources. The Germans made the decision early on, given the resources at hand, to create a larger number of twin-engine bombers, rather than a smaller number of four-engined ones. Yes, operational requirements also played into it, but at the time most of Italy's airframes were being developed and deployed, Italy was fielding a larger number of airframes than the USAAC. Yet their available resources (both money and materials) were far less. To keep the Fascist propaganda machine running at the levels desired, and the number of airframes up, smaller, but less capable types were nearly always selected. The Italians could, and did, design and build four engine bombers, with the engines on hand. But, with many other considerations - reliability, power capacity, building capacity, and crewing capacity, three engines provided what twins or four engined airframes could not.

 

[nuuumannn]

type of aircraft has something to do with accuracy, how much is certainly subject to question but a number of bombers of the 1930s went through several modifications of tails and/or changes in dehydrial of the wings to cure snaking or lateral instability which the purchaser's thought would affect accuracy

Yeah, if you wanna be pedantic, but a better trained crew with better support, lots more radio nav aids, well thought out policy etc in a mediocre aircraft will probably do better in accuracy than a tyro crew in a good aircraft without all that stuff. For example; the He 111 in 1940 with KG 100 and fitted with X-Gerat was far more accurate than an Avro Lancaster I on its entry into RAF service with 44 Sqn in 1942. The Lancaster (max speed 287mph) was faster than an He 111H (max speed 270 mph), could carry a bigger bomb load and had a greater range and was better defended. The difference was the KG 100 crew were experienced in pathfinder operations, had the assistance of sophisticated nav aids that the RAF didn't adopt until late 1942/43.

 

[Shortround6]

There are a lot of different "accuracies".

One is the "mechanical accuracy" or perhaps system accuracy?
as in what is the deviation of the bomb from ideal/theoretical trajectory+ the ability of the bomber (the aircraft) to drop/launch/start the bomb on this "ideal" trajectory (planes that snake or have poor lateral stability are going to have problems) + the accuracy of the bomb sight and the reliability/speed of the bomb release mechanism.

However, much like the mechanical accuracy of a pistol, this is quite different from the "practical accuracy". This is where human error is introduced and even this can be split into different groups/areas. Human error in operating and/or maintaining the equipment. Human error in identifying the target and even human error in identifying the city (or country) over which the bomber is flying. However navigation errors may figure into "operational" accuracy that may be a different catagory than practical accuracy.

Much like the pistol may have a very good accuracy if fastened to a bench with a vise, when fired by a human we introduce sighting errors. Differences in holding/supporting the pistol and finally executing the shot (squeezing the trigger and following through) which can lead to a mechanical accuracy standard achieved at 100yds not being achievable by an untrained shooter at 5 yds.

Between the wars it seems more than a few nations spent more time on improving the mechanical accuracy of the bomb dropping sub set and not enough time working on navigation and problems of trying to operate in less than 0-1/10s cloud cover on summer days.

Doesn't matter what kind of tail fins you have, what kind of bomb sight and how steady the plane is if you are over the wrong city or no city at all (wheat fields?)

 

[Snautzer01]

Yeah, if you wanna be pedantic, but a better trained crew with better support, lots more radio nav aids, well thought out policy etc in a mediocre aircraft will probably do better in accuracy than a tyro crew in a good aircraft without all that stuff. For example; the He 111 in 1940 with KG 100 and fitted with X-Gerat was far more accurate than an Avro Lancaster I on its entry into RAF service with 44 Sqn in 1942. The Lancaster (max speed 287mph) was faster than an He 111H (max speed 270 mph), could carry a bigger bomb load and had a greater range and was better defended. The difference was the KG 100 crew were experienced in pathfinder operations, had the assistance of sophisticated nav aids that the RAF didn't adopt until late 1942/43.

You know that the beams where lets say tempered with by the Brits?

 

[nuuumannn]

One is the "mechanical accuracy" or perhaps system accuracy?

I do suspect we are getting a little too pedantic and off-topic, to be honest with you.

You know that the beams where lets say tempered with by the Brits?

Speaking of off-topic. Yup very much. R.V. Jones. Again, not really relevant to either the three engine or the accuracy debate.

 

[swampyankee]

Three-engine aircraft were a popular design solution in Europe between the wars because the engines were not that powerful and not that reliable, so having three decreased the chances of an engine failure leading to a crash. Westland, Fokker and Ford all made very good commercial tri-motors. But then engines in the late '30s got more powerful and more reliable, and two engines makes for simpler design, simpler instrumentation and control, better pilot view and just cheaper to build and service. When two engines weren't enough, most companies went to four engines. The problem with big bombers was to go with the simpler twin-engine design meant really big and powerful engines, and there were a lot of problems developing engines big enough. Rolls-Royce failed with the Vulture, and Daimler-Benz with the DB606 and DB610. Italy didn't have a powerful radial engine so the SM79 had to have three.

Not just Europe. See aircraft by Ford (the Trimotor), Boeing (Model 80), Consolidated (the XPY and XP2Y), Curtiss (Eagle and the prototypes of the NC and Model H), Fokker (F-10), the quite unsuccessful Johns Multiplane ), two prototypes by Keystone, airliners from Stinson (the Model A, with about 30 built, SM-6000, and Model U, with about 53 for the former and 24 for the latter).

The trimotor was a very common configuration during the 1920s and early 1930s. I think it began to fall out of favor when Douglas (DC-2) and Boeing (247) showed that profit-making airliners could be built with two engines, if the country had an adequate engine and propeller industry.

 

[maxmwill]

Is this restricted to pre-jets? Because there almost was a tri-engine bomber that the USAF had considered, the Martin XB-51, and it was even in a movie(forget the title)

 

[Graeme]

the Martin XB-51, and it was even in a movie(forget the title)

Toward the Unknown....