What is the advantage of a tri-engine aircraft?

[ShVAK]

I noticed that a lot of Italian designs and some German designs of 1930's origin and later bore three engines, not sure why this configuration was so popular. More drag, more frontal area, more expensive to build, more resources and more complexity than a twin; less powerful or effective than (an admittedly larger and more complex) four-engined design on paper, etc.

Sure some were effective airplanes, I happen to like the SM.79 a lot (which for a while was the fastest medium bomber in the world) but they didn't really do anything a twin with more powerful engines couldn't do. I guess the redundancy of an extra engine is helpful but there's fuel consumption to worry about too and frankly in your average medium-sized bomber or transport if you're in a situation that knocks it down to one engine it seems like you'd have much bigger problems than just staying airborne.

Any thoughts on this? Do you think the Germans and Italians would've been better served in WWII by binning tri-engine designs and using the resources for more efficient twins? Discuss.

 

[Vincenzo]

commonly is reported that tri engined were used when they had not enough powerfull engined for did as a twin engined

 

[wuzak]

they didn't really do anything a twin with more powerful engines couldn't do.

That would be the crux of the matter - not having powerful enough engines to make a twin.

 

[Rick65]

Or when you had a good single that could be upgraded in capacity by more power eg Ju 52 started as a single.

 

[gumbyk]

Three engines gives you better redundancy than a twin, for the same frontal area.
Also, flight with an engine out would be easier, due to the reduced asymmetric thrust.

And, as Wuzak stated - the more powerful engines may not have been available.

 

[ShVAK]

Three engines gives you better redundancy than a twin, for the same frontal area.

How would a third engine on the nose not increase the frontal area? The fuselage itself has to be wider/larger in diameter to accommodate an engine and then you have the extra prop, cowling, etc. etc. That would seem to equal a lot more drag than a comparable twin.

 

[wuzak]

How would a third engine on the nose not increase the frontal area? The fuselage itself has to be wider/larger in diameter to accommodate an engine and then you have the extra prop, cowling, etc. etc. That would seem to equal a lot more drag than a comparable twin.

The fuselage on aircraft that were triples tended to be wider and deeper than the engine, since most were transport types.

I believe the spec that was given to Douglas for the DC-2 asked for 3 engines, but Douglas figured they could do it on two, since they had an engine sufficiently powerful.

 

[wuzak]

The other alternative if the engines aren't sufficiently powerful is to go to a 4 engined design. But that will increase the frontal area and drag, and may require extended wingspan and strengthened wing structure.

 

[GrauGeist]

When the tri-motor was in "vogue" as a design in production, the radials available at the time were not as powerful as they were in later years, thus the design for a tri-motor was adopted. You'll see that aircraft like the the Ford Trimotor (1926) and Junkers Ju52 (1932) transports were very reliable and produced in large numbers.

There were many others like Fokker, Boeing, Stinson, de Havilland, Armstrong, etc...

 

[gumbyk]

How would a third engine on the nose not increase the frontal area? The fuselage itself has to be wider/larger in diameter to accommodate an engine and then you have the extra prop, cowling, etc. etc. That would seem to equal a lot more drag than a comparable twin.

The engine and cowl in the nose fits within the frontal area of the fuselage. Props aren't considered to be part of the frontal area, as in normal flight, the prop is providing thrust.

 

[gumbyk]

Oops..

 

[davebender]

If mass produced it would have been one of the better WWII era transport aircraft. That third engine provided 1,340 additional hp which means greater range / payload.

 

[Rick65]

The SM 79 quoted by ShVAK in his opening post is interesting because it existed in both three and a twin engined versions.
The SM79-JR was built in Romania, served on the eastern front and used two Jumo 211 engines rather than three lower powered radials.
I haven't been able to find any comparative figures for the differing versions but the development of the plane tends to confirm that suspicion that the three engined option was a response to insufficient engine power for a twin.
From Wiki

Romania
In 1937, the Bucharest government ordered 24 twin-engined SM.79B bombers fitted with 746 kW/1,000 hp Gnome-Rhône Mistral Major 14K radial engines. These aircraft, however, proved to be underpowered. Consequently, in February 1940 Romania ordered from Italy eight machines equipped with two Junkers Jumo 211 inline engines of 1,200 hp (890 kW) each. These aircraft were designated JIS 79 (J for Jumo, I for Italy and S for Savoia) and were delivered in 1941-2. A further 72 SM.79s were built under licence by the Industria Aeronautică Română (IAR) and designated JRS 79B (J for Jumo, R for Romania, S for Savoia).[17][18] Another version was the JRS 79B1, armed with a 20 mm Ikaria cannon and with an enlarged cockpit for a fifth crew member. Due to its role in low-level attacks, it suffered heavy losses
Eight Italian built aircraft (designated JIS.79B by Romania), followed by 36 license built JRS 79B powered by the Jumo 211Da and 36 JRS 79B1 with 1,029 kW (1,380 hp) Jumo 211F engines. Production continued until 1946.

There was another twin engined version, again from Wiki
SM.79B Twin-engine export version powered by the less reliable Fiat A.80 engines and with a glazed nose for improved bomb-aiming. More economical but slower (420 km/h/260 mph and 21.45 minutes to 5,000 m/16,400 ft) than the standard SM.79, but weighing 6,600/10,100 kg (14,551/22,267 lb, around 500 kg/1,100 lb less than the basic SM.79), was longer (16.22 m/53.22 ft), and had the same armament. Iraq bought five, but this version achieved little success in Italy.

 

[model299]

Another advantage is that everyone sitting within the fuselage gets to enjoy the oil, gas and exhaust fumes of the #2 engine along with the pilot and co-piplot!

In May of 2010, I took a ride in the EAA's Ford trimoter. I sat in the front portside of the aircraft, right next to the #1 engine, and got a nice video. It's not the highest of quality, but it's fun to watch and includes a buzz of the sod field at Oshkosh. I'll try and get it uploaded.

 

[davebender]

I suspect that holds true for most aircraft designed prior to 1930. Not just tri-motor aircraft.

 

[Stephan Wilkinson]

How would a third engine on the nose not increase the frontal area? The fuselage itself has to be wider/larger in diameter to accommodate an engine and then you have the extra prop, cowling, etc. etc. That would seem to equal a lot more drag than a comparable twin.

Also substantial added cooling drag.

 

[parsifal]

I think a big problem for trimotor designs was working out effective forward firing armament. I suspect also that observer view forward might be restricted without some form of glazed nose

 

[davebender]

Most tri-motor aircraft were transports or medium bombers. Forward firing armament is not a high priority for such aircraft.

 

[tyrodtom]

Most tri-motor aircraft were transports or medium bombers. Forward firing armament is not a high priority for such aircraft.

I would agree with that on transports> But medium bombers ??

If a bomber got chosen for low level missions, it was more than usually medium bombers, and every bit of forward armament was wanted to suppress AA at the target arera.

 

[davebender]

A pair of machineguns or 20mm cannon can be sychronized to fire through the center engine prop if prop shafts aren't hollow.