# What is the advantage of a tri-engine aircraft?



## ShVAK (Aug 26, 2012)

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.


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## Vincenzo (Aug 26, 2012)

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

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## wuzak (Aug 26, 2012)

ShVAK said:


> 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.


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## Rick65 (Aug 26, 2012)

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


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## gumbyk (Aug 26, 2012)

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.


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## ShVAK (Aug 26, 2012)

gumbyk said:


> 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.


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## wuzak (Aug 26, 2012)

ShVAK said:


> 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.

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## wuzak (Aug 26, 2012)

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.


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## GrauGeist (Aug 26, 2012)

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...


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## gumbyk (Aug 27, 2012)

ShVAK said:


> 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.


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## gumbyk (Aug 27, 2012)

Oops..


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## davebender (Aug 27, 2012)

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.


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## Rick65 (Aug 27, 2012)

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.


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## model299 (Aug 27, 2012)

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.


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## davebender (Aug 28, 2012)

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


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## Stephan Wilkinson (Aug 28, 2012)

> 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.


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## parsifal (Aug 28, 2012)

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


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## davebender (Aug 29, 2012)

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


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## tyrodtom (Aug 29, 2012)

davebender said:


> 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.


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## davebender (Aug 29, 2012)

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


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## tyrodtom (Aug 29, 2012)

davebender said:


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



Did anyone do that on a trimotor ?


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## parsifal (Aug 29, 2012)

No they didnt, moreover having a fixed forward firing armament is quite limited. What cn this arrangement do to combat an attack by fighters from the forward quarter. A flexible forward firing mount at least gives the gunner the aibility to train his guns on the oncoming attacker. If the twin is manouverable, like a Mossie or a Ju88, ther is some hope of aerobatting the a/c to achieve a firing solution. For the older style and (usuay) unprotected trimotor configurations, this was not really a viable option.

The conclusion....trimotors tend to be more vulnerable to certin types of attack


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## Shortround6 (Aug 29, 2012)

By the time you hang 3 engines on an airplane the ability to maneuver like a fighter (even a poor one) is pretty well gone. Which rather limits the use of fixed forward firing guns even if the SM 79 had one. The SM 79 was only about 25,000lb and had three 9 cylinder engines ( 27 total) compared to most twins two 12s or 14s ( 24 cylinders or 28). Three 12s or 14s, let alone three of most peoples 18s make for rather large planes. 

Many early twins, especially with fixed pitch propellers could not maintain height on one engine, The triples offered a measure of security in case of engine failure. As planes got cleaner (less drag) and got controllable pitch or constant speed propellers their ability to maintain height and make it to an airfield instead of putting down in a field or meadow got a lot better. 

Tri-planes usually had a worse view over the nose, had more vibration in the cockpit (if not the entire fuselage) had higher maintenance costs than twins of equivalent power.


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## davebender (Aug 29, 2012)

> By the time you hang 3 engines on an airplane the ability to maneuver like a fighter (even a poor one) is pretty well gone.


I agree.

German Ju-252 and Ju-352 cargo aircraft carried some defensive weapons but I suspect they wouldn't have been terribly effective. Fighter escort is the only way to protect transport aircraft from enemy aircraft.


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## R Pope (Aug 31, 2012)

Still seen as a viable concept in some respects. There were a number of DC3's with three turboprops installed for use in the north country, don't know if any are still being used. One version was called the Tri-Turbo Three.


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## R Pope (Aug 31, 2012)

Still seen as a viable concept in some respects. There was a DC3 with three turboprops installed for use in the Arctic, I believe it was called the Conway or Conroy Tri Turbo Three. It's now a derelict hulk, I think.


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## R Pope (Aug 31, 2012)

Still seen as a viable concept in some respects. There was a DC3 with three turboprops installed for use in the Arctic, I believe it was called the Conway or Conroy Tri Turbo Three. It's now a derelict hulk, I think. Then there was the post-war Northrup Raider, 13 built.


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## davebender (Aug 31, 2012)

DC-10 passenger liner has three engines. One under each wing and one in the tail.


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## wuzak (Aug 31, 2012)

davebender said:


> DC-10 passenger liner has three engines. One under each wing and one in the tail.



As did the Boeing 727 and Lockheed Tristar


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## davebender (Aug 31, 2012)

de Havilland Australia DHA-3 Drover
de Havilland Australia DHA-3 Drover - Wikipedia, the free encyclopedia







Britten-Norman Trislander
Britten-Norman Trislander - Wikipedia, the free encyclopedia


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## vikingBerserker (Aug 31, 2012)

Now that's a pretty interesting bird.


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## ShVAK (Aug 31, 2012)

The Trislander looks really odd. Never seen a trimotor prop aircraft with the centerline engine in the tail rather than the nose.


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## parsifal (Aug 31, 2012)

very unusual pics dave. well done


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## MiTasol (Feb 21, 2015)

The Drover was a nightmare - did not perform at all well with one engine out, especially in tropics.
Qantas lost one in the water off Lae in PNG when an engine failed. No survivors
This is one of the hundreds of accidents that Qantas claim they never had.

Mi


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## dedalos (Feb 21, 2015)

I wonder for years ,if the tri Motor configuration could be a solution for germany. Since germany failed miserably to provide powerful emgines for its medium bombers, another approach could be interesting
A JU 188 version with three Jumo 211 J/N, no ventral gondola and the bulged bomb bay of the Ju 88A 15, the tail remote control turret, a smaller cocpit with lower drag canopy. I believe that even with the bigger and stronger airframe ,three jumo s 211 N could deliver better power to weight ratio than 2 BMW 801s and lower specific fuel consuption, especially if the BMW use B4 fuel, as it was the case in bomber use


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## tomo pauk (Feb 21, 2015)

Three Jumo 211 will provide ~4000 HP for take off in 1941, that being equivalent of 2 x Jumo 222 (1st versions) or two R-2800 from 1942-44. At altitude it is about 3100 HP - not peak power, but 30 min rating. The 3 Jumos will be draggy as about 2 R-2800? 
I'd go maybe for two engines on wings, 3rd behind the wing as a pusher? Granted, a new airframe is needed. Sorta cross between Mosquito and Mixmaster. Should work for the Americans, too.


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## Shortround6 (Feb 21, 2015)

Adding a 3rd engine to an existing airframe may be possible, it is a lot easier to take one away. 






By the time all is said and done, adding a 3rd Jumo 211 engine is going add around 2000lbs (1 ton) to the _empty_ weight of an aircraft. Ju 88/188 had a fuel problem as it was. Feeding a 3rd engine is going to call for being really creative in regards to fuel storage. 

What was teh fuel consumption of the Jumo 211 and BMW 801s per hp/hr at _cruise_ settings? not full power.


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## tomo pauk (Feb 21, 2015)

The BMW 801A and -C will provide 1170 PS @ 4.5 km on max. continuous, while consuming 280 g/PSh of fuel, or 327.6 kilograms of fuel per hour. On max cruise, 990 PS at 225 g/PSh -> 222.7 kg/h at 4.2 km.
BMW 801D makes 1180 PS on max cont @ 5.5 km, 265 g/PSh -> 312.7 kg/h. On max cruise, it is 985 PS @ 5.4 km, 210 g/PSh -> 206.9 kg/h.
Jumo 211F on max continuous will give 900 PS at 5.3 km, for 218 g/PSh, or 196.2 kg/h. Alternatively can use over-revving to 2400 RPM at 5.9 km, making 920 PS and consuming 224 g/PSh, or 206.1 kg/h. 
The Jumo 211J will do 1020 PS at 5.1 km, or 1030 PS at 5.6 km, same specific consumption as 211F, total consumption 222.4 kg/h or 230.7 kg/h, respectively.

I don't have the data about the consumption of the Jumo 211 on max cruise at the moment. If it is 2/3rds of the ratio, like we observed for the BMW, it will be some 140-150 kg/h, for powers of 80% of max continuous power, or something along 720-820 PS. 
Thus three Jumo 211 will consume 420-450 kg/h for 2200-2500 PS, two BMW 801A/C will consume 445 kg/h for 1980 PS, two BMW 801D will consume 414 kg/h for 1970 PS. The BMW 801A or C will be forced to cruise at lower altitudes than other engines, thus making lower mileage. Three Jumo powerplants (211F was at 720 kg dry weight) will also weight perhaps 1000 kg more than two BMW 801, that is quite a weight. Bigger wing is needed, that will negate many of benefits.


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## dedalos (Feb 21, 2015)

tomo pauk said:


> The BMW 801A and -C will provide 1170 PS @ 4.5 km on max. continuous, while consuming 280 g/PSh of fuel, or 327.6 kilograms of fuel per hour. On max cruise, 990 PS at 225 g/PSh -> 222.7 kg/h at 4.2 km.
> BMW 801D makes 1180 PS on max cont @ 5.5 km, 265 g/PSh -> 312.7 kg/h. On max cruise, it is 985 PS @ 5.4 km, 210 g/PSh -> 206.9 kg/h.
> Jumo 211F on max continuous will give 900 PS at 5.3 km, for 218 g/PSh, or 196.2 kg/h. Alternatively can use over-revving to 2400 RPM at 5.9 km, making 920 PS and consuming 224 g/PSh, or 206.1 kg/h.
> The Jumo 211J will do 1020 PS at 5.1 km, or 1030 PS at 5.6 km, same specific consumption as 211F, total consumption 222.4 kg/h or 230.7 kg/h, respectively.
> ...



Well, lets start with the Ju 388K airframe, essentialy a Ju 188 with the bulged bomb bay of the Ju 88A15. With two BMW 801 s had an empty weight of 10250kgr.
In the 1943 time frame they would be the -D version. If we accept that C3 would be available for the bombers,it would be 3400 PS
If we accept three Jumos 211N of 1943, would add 1000kgr thats about 10%. They would deliver 4350PS. Thats 30% more take off power than the 2 BMWs and even more importantly ON B4 FUEL.
Furthermore i would like to notice that BMW 801s were really not available for ju 88 production until late 1943. Also ,in service, often the fuel consuption of the 801 was higher than in the manuals
Drag wise, three jumos 211s with anular radiators should be very comparable with the wider BMWs
Additional fuel could be a problem.The additional power could permit the standart use of external 900 lt drop tanks , and/or reduce the Internal bomb load from 3000kgr to still respectable 2000kgr and use the space for fuel.
2 20mm cannons would be possible to be installed in the front of the bulged bay,synchronized to fire through the central propeller disc
Finally such an aircraft could recieve a further performance boost, by using MW50 in 1944 for 1600-1650 take off power.

Also i would expect such an aircraft to have much more chance to survive after losing an engine


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## tomo pauk (Feb 21, 2015)

The engine-out situation certainly favors a 3-engine aircraft much more than a 2-engined. My guess is that's why such a popularity between the wars, until the 4-engines aircraft were more widespread.

I've proposed before the 'Ju-88 airframe' (whether it's Ju 88, 188 or 388, does not matter) with a narrow, but long bomb panier, and DB 603A. The weight penalty should not be that big vs. Jumos, max cruise power was at ~1170 PS at 5 km (20% more than BMW 801D along with less drag and use of B4; maybe 30% greater power than of Jumo 211). The similarly heavy Jumo 213A was installed in some Ju 188s and even heavier engines were to be installed in the Ju 388 . 
Problems - availability reliability in 1943; not a tri-motor (ie. not for this thread).


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## nuuumannn (Feb 22, 2015)

The Trislander is much like other trimotors in that its engine out performance is not brilliant, so it will just take you quicker to the crash site. I heard an interesting anecdote from an ex-Trislander pilot with Loganair in Scotland once. Because of the isolation of some of the airfields in the islands surrounding the Scottish mainland, the captain and first officer loaded the bags aboard the aircraft and with the Trislander, because of the engine above the rear fuse, it is adviseable to load pax first, then bags, but this guy told me that he put the bags on first and then proceded to watch the aircraft sit on its tail!

This is a Great Barrier Islands (New Zealand) Trislander that was badly damaged in a tarmac fire a couple of years ago. Note the stay propping up the tail.







There was prejudice against trimotors in the USA following TWA 599, which led to enormous changes in the US civil aviation organisation and the development of airliner technology. Not to forget the death of Knute Rockne.

https://en.wikipedia.org/wiki/TWA_Flight_599

The engine out safety factor with the majority of trimotors was cancelled out by the fact that they still couldn't hold height without all three engines running - piston engine models, that is. The proliferation of twins subsequently shows that the Trimotor was a bit of a throw back and was adopted for the same reasons in the jet age - lack of power with size and weight increase. When the Tristar and DC-10 were introduced, Boeing trumped the lot of them and went for the 747. Another example is the MD-11 versus the 777. The stretched DC-10 should have been a world beater, but its wing was a bit draggy and it was slower than predicted - and it had three engines, when the 777 had two. Needless to say, the MD-11 has faded into history as a bit of an also-ran by comparison - although it makes a good freighter (!). ETOPS has changed twin operations dramatically, although the trijet was certainly ground breaking in the respect of range and distance flown from diversionary fields, back in the early 70s. The only trimotor jets around these days are Dassault's Falcon business jets, which are seriously quick, but maintenance intensive.


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## gumbyk (Feb 22, 2015)

nuuumannn said:


> The Trislander is much like other trimotors in that its engine out performance is not brilliant, so it will just take you quicker to the crash site. I heard an interesting anecdote from an ex-Trislander pilot with Loganair in Scotland once. Because of the isolation of some of the airfields in the islands surrounding the Scottish mainland, the captain and first officer loaded the bags aboard the aircraft and with the Trislander, because of the engine above the rear fuse, it is adviseable to load pax first, then bags, but this guy told me that he put the bags on first and then proceded to watch the aircraft sit on its tail!
> 
> This is a Great Barrier Islands (New Zealand) Trislander that was badly damaged in a tarmac fire a couple of years ago. Note the stay propping up the tail.



It's for sale if you want it


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## Just Schmidt (Feb 24, 2015)

Shortround6 said:


> Which rather limits the use of fixed forward firing guns even if the SM 79 had one.



The Sparviero did have a fixed forward firing gun. If I remember correctly it was a 12.7, and mounted above the cockpit I believed it fired over the propeller arch.


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