Tri-motors - any thoughts?

Planes with 3 engines were tried used in some countries - Germany, Italy, Netherlands, even the US - but were never looked upon as a mainstream of pre-1945 era. If you were to design some for WW2, for what role that would be suitable and how would it looked like?

The tri-motor went away because engines of the day became more reliable and were more powerful. Although the Italians probably had the best performing tri motors, I see the configuration as obsolete.

Like FlyboyJ says, the tri motor was a good idea when the required power could not be gotten from 2 engines. Some early twins could not maintain altitude on a single engine and so the 3rd engine offered some insurance, it also increased the risk of an engine failure on any single flight by 50%

In the late 20s and early 30s "cruise" power was often 80-90% of full throttle. there was little reserve power. you also had fixed pitch props. By the late 30s many commercial planes (and bombers) could "cruise" on 65% power or even lower. This left enough power to keep a twin in the air if an engine went out even if at a much reduced altitude.

FLYBOYJ said:

The tri-motor went away because engines of the day became more reliable and were more powerful. Although the Italians probably had the best performing tri motors, I see the configuration as obsolete.

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J,

Even today? for the same reason? There hasn't been a new one in a while (I think since the Dassault Falcon in 1984? Not counting the last update in 2005). Your comment made me wonder whether the tri-engine combination may find (or has already found a revival) in the space booster industry? The new Delta IV Heavy is a tri-motor as is the planned Atlas V HLV. Of course the Space shuttle itself was also a trimotor. Does it all come down to one engine out performance played against engine reliability?

The DC-10 comes to mind, too.

A little different for a jet. The concept there was jumbo jet size with range. Additionally tri jets allowed manufacturers to get around a "60 minute rule" for commercial engine out operations.

Fair points re. were they worth it.

Of course, I'd like to throw in some proposals, discussion sake
For Germans, I'd like to see Do-17 with 3 Bramos, then next-gen Ju-88 Do-217 conceived with 3 Jumo-211s. Maybe even the He-111.
For RAF, a Manchester with 3 Merlins, Wellington with 3 Pegasus engines.
For USAAF, a 3 x R-2800 heavy bomber,then a 3 x V-1710 'schenellbomber'.

All with a minimum to none armament

A problem with bombers is that if you stick an engine in the nose the bombardier has to go somewhere else. Like in an under fuselage gondola or bathtub.

Not a bad idea on something like a Fw Condor which didn't have an internal bombbay to begin with and could use a little extra support for the fuselage. For an existing bomber it is just more frontal area and drag. Nose mounted engines tended to transmit more vibration to the fuselage than the wing mounted engines. Another reason for moving the bombardier in back of the bomb bay on some planes.

For the Wellington it's a "why bother"??? three 900-1000hp Pegasus engines or two 1400-1600hp Hercules.

The Manchester is a "no go". Literally, At the time the Manchester is being built your choice of Merlin is pretty much the Merlin X of about 1130hp down low. If you are talking about tacking more aircraft onto the production run instead of the Lancaster you can get Merlin XXs but it took a while before they were rated at 1480hp. For quite a while you rae limited to around 1280hp for take off or just a bit more than the two Vultures except that you have added to the drag, added to the weight and screwed up the center of gravity. Of course on a large plane this can be fixed easier than on a small plane, Maybe you can use a lead tail cone when you pull the rear turret

For the Do-17 just steal some Jumo 211s or DB 601s (DO 215) and have done with it. Sticking a third 9 cylinder radial on the nose would really increase the drag, killing a fair amount of the extra power.

The Cant Z.1007 seem like a pretty good bomber
The Wellington can have his Hercules engines, but not before BoB? By that time RAF has a bomber that can make, say, 270 mph instead of 235.
The Vultures, while on paper able to make 1750 HP, were restricted to 1400-1500 in reality? So that's 3400 HP for Merlin X instead of 2800-3000 for Vultures. The added engine adds drag, so we're to delete the dorsal turret. When the Merlin XX is around, we can be as fast as the Z.1007*, 285 mph? The rear turret can stay
As for CoG changes, we have deleted the front turret too (not the very streamlined item itself). The Merlins are lighter than Vultures.

Late Do-17s were pretty much messed up with the bloated crew compartment. Hence the front part receives the rework, 2-3 crew members max + engine. So we are at 3000 HP, less wing area weight than the Z.1007= 300 mph instead of 265 for the Do-17Z?
The DBs and Jumos, while fine engines, are needed for other planes.

The Junkers Ju 252 airliner/transport seemed to have excellent payload/range/fuel consumption figures. I would have thought that the fueselage would make the central propeller relatively inefficient, but it doesn't seem to.

I agree. If not for Milch's decision to derail the project the Ju-252 would probably have been a great success and it would have been an excellent use for Jumo 211 engines which Germany had plenty of by 1942.

Might have made a good maritime patrol aircraft for the submarine force too. Replace the cargo payload with additional fuel tanks and I think a Ju-252 could reach a long way into the Atlantic.

tomo pauk said:

The Cant Z.1007 seem like a pretty good bomber

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It was but it was also a little lacking in bomb load. The Italians were forced into the tri-motor configuration by a lack of suitable engines. The Cant Z.1007 cannot fly fast enough to avoid interception. Top speed is achieved (in almost all bombers) at the 5 minute military rating of the engines which cannot be maintained and also sucks fuel like a bullet hole in the tank. A Hurricane I is about 30mph faster which means it can gain 2 1/2 miles on the Cant in 5 minutes. Ditching the guns won't gain much.

tomo pauk said:

The Wellington can have his Hercules engines, but not before BoB? By that time RAF has a bomber that can make, say, 270 mph instead of 235.

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Not sure where the 270mph comes from, some sources only give 255mph for the MK III with the Hercules engines. Granted they were heavier aircraft but adding darn near a ton to the nose of Wellington (minus the weight of the turret) isn't going to help. You also need more fuel to feed the 3rd engine.

tomo pauk said:

The Vultures, while on paper able to make 1750 HP, were restricted to 1400-1500 in reality? So that's 3400 HP for Merlin X instead of 2800-3000 for Vultures. The added engine adds drag, so we're to delete the dorsal turret. When the Merlin XX is around, we can be as fast as the Z.1007*, 285 mph? The rear turret can stay

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True the Vulture was derated but a 285mph TOP speed is no guarantee of immunity from interception by 330-360mph fighters. If it was the Bristol Blenheim would have made out a lot better

A Merlin in the nose may add drag but at least it isn't the barn door of engine you are trying to foist off on the poor Wellington. The Pegasus having the diameter and frontal area of an R-3350 only without anywhere near as low drag a cowling.

tomo pauk said:

As for CoG changes, we have deleted the front turret too (not the very streamlined item itself). The Merlins are lighter than Vultures.

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A Merlin installation is going to go something over one ton,assuming you are going to use a propeller and put coolant in the radiators and such. Ditching a 500-600lb turret doesn't quite cover it. While the Merlins are lighter than the Vulture 1500lbs 15 ft from the CG is worth 3200lbs 7ft from the CG. You just might make it. and that extra 20mph advantage over the Blenheim is going to make all the difference when all the guns but the tail turret are gone

tomo pauk said:

Late Do-17s were pretty much messed up with the bloated crew compartment. Hence the front part receives the rework, 2-3 crew members max + engine. So we are at 3000 HP, less wing area weight than the Z.1007= 300 mph instead of 265 for the Do-17Z?
The DBs and Jumos, while fine engines, are needed for other planes.

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Messed up as in giving the crew actual room to function? With a whacking big radial stuck in the nose where does the bombardier go? or are we giving up on bomb sights?

Messed up as adding to drag, so the plane can be easily intercepted even with pre-BoB Hurricane. Since my idea is to make it once more a fast bomber, we are ditching the armament, along with gunners.

For RAF bombers, mainly operating in night, the extra 20 mph means that LW night fighters will have much tougher time to shot them down, while the bombers will be less within the Flak envelope. For bombers operating in MTO, that extra speed means that early Italian fighters are in further disadvantage.
As for extra fuel for the 3rd engine, the 3000 HP in 3 engines need about the same fuel as in 2?

I did not mention another advantage of 3 engines over 2 - when one engine is damaged, or disabled in another way, there are still 2 engines to go home; 2000 HP instead of 1500 HP, full asymmetric thrust for the later.

tomo pauk said:

Messed up as adding to drag, so the plane can be easily intercepted even with pre-BoB Hurricane. Since my idea is to make it once more a fast bomber, we are ditching the armament, along with gunners.

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The way the Mosquito "worked" was that in addition to ditching the armament, along with gunners it ditched a larger, heavier fuselage which enabled it to ditch a larger heavier wing. It was roughly the size of a Blenheim with 60% more power even in the early versions. I doubt sticking a third Mercury into the nose of a Blenheim is going to give you a 350mph bomber. Pulling guns and gunners from an existing airframe doesn't do a whole lot although some turrets did create a lot of drag. Those egg shaped dorsal turrets being among the worst. For confirmation of this please compare a Lancaster bomber to a Lancasterian Transport;

http://www.flywiththestars.co.uk/Pictures/Miscellaneous/Paintings/Star Light.jpg

Top speed was 310mph at 12,000ft at 53,000lbs. it weighed about 34,500lbs empty equipped. max take-off was 65,000lbs.

tomo pauk said:

For RAF bombers, mainly operating in night, the extra 20 mph means that LW night fighters will have much tougher time to shot them down, while the bombers will be less within the Flak envelope. For bombers operating in MTO, that extra speed means that early Italian fighters are in further disadvantage.
As for extra fuel for the 3rd engine, the 3000 HP in 3 engines need about the same fuel as in 2?

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20mph top speed doesn't make a whole lot of difference as the planes were flying at cruising speeds and low ones at that. The RAF often used bad flying techniques at this time. using slow long range cruise settings rather than running closer to the maximum lean mixture cruise settings. From the pilots notes on the Wellington MK III it could have run at 150 indicated instead of 135Knots indicated for around a 10% increase in fuel burn and still been short of max lean cruise. Cruise settings go from 73 gallons an hour to 117 gallons an hour, going to max Rich cruise goes to 222 gallons an hour and max throttle (needed for that blistering 255mph top speed) sucks up 290 gallons an hour. Obviously anything near top speed is pretty much useless for anything but the shortest raids.

tomo pauk said:

I did not mention another advantage of 3 engines over 2 - when one engine is damaged, or disabled in another way, there are still 2 engines to go home; 2000 HP instead of 1500 HP, full asymmetric thrust for the later.

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That is pretty much a given considering the earlier discussion but the flip side is 33% fewer bombers for the same engine production.

Acknowledged re. Mossie; I do not think that anything short of two Merlin 60 series would've turned Blenheim into a 350 mph bomber

The 3-engined Wellington cruising at, say, 75% of it's 1000 HP engines can use the 2250 HP, unlike the twin that makes 1500 HP. So I'd say it cruises faster.

That is pretty much a given considering the earlier discussion but the flip side is 33% fewer bombers for the same engine production.

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That did not prevented people making 4 engined planes

tomo pauk said:

The 3-engined Wellington cruising at, say, 75% of it's 1000 HP engines can use the 2250 HP, unlike the twin that makes 1500 HP. So I'd say it cruises faster.

That did not prevented people making 4 engined planes

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The British problem was that they were not cruising at 75% power but at under 50% power. The MK III Wellington could hit some of the suggested cruise speeds using 2000rpm and manifold pressures in negative numbers. As a rough guess some of the cruise conditions could be meet using 600hp. This leaves room for improvement even with the Pegasus engines, especially as the MK I was operating 4,000-6,000lbs lighter than a MK III.
That points out part of the problem with trying to upgrade an existing twin airframe with a third engine. Without restressing the airframe, changing landing gear and a bunch of other details ( al of which are possible but perhaps not practical) stuffing a ton worth of engine in the nose just cuts into the bomb/fuel load. You get a faster plane but it's actual utility as a bomber may not be so good.

I made an error in Math, tri engine planes cut the number of planes by 50%. 600 engines=300 twins or 200 triples.

Most people's 4 engine planes carried double the bomb load of a twin further and used fewer crew. If you use the third engine just to increase speed, you haven't increased the bomb load or range capability of the bomber force. Increasing the bomb load and range using the 3rd engine cuts into the speed and the often overlooked field capability of the aircraft.

If you cut from 300 planes (100%) to 200 (67%), you do cut 33% If the planes are designed from ground up for 3 engines, you might need 33% less crew per tonnage of bombs dropped.

The figure of 75% of max power was just to illustrate my point; with the 50% we are at 1500 HP total in lieu of 1000 (for 3 vs. 2 Pegasus). A 50% increase anyway, and rather comparable with what was feasible with Hercules. Of course, we can have a bomber of this kind available in 1939, instead of waiting for Hercules production to ramp up some time after BoB.
All of this 'Wellington' saga serves for the purpose that 3-engined plane can put a good performance redundancy vs. a twin engined plane of similar HP, sure enough the plane designed from ground-up is a better choice than something made quick dirty.

If the goal was a simple 20mph increase in cruising speed a simple adjustment of rpm and boost would have done the trick with a slight loss in range without any fooling around with the aircrafts construction. Of course an increase in cruising speed from 170mph to 190 mph really isn't going to affect the interceptors a whole lot.

And that is the problem with most of these schemes. A high speed bomber needs to almost as fast as the fighters, not just 30-40 mph faster than a slow bomber. Something like an A-20 that had a top speed close to 340mph but could cruise close to 300mph (at least until the gas runs out) may work but a plane that tops out at 290-300 and cruises at 250-260 is just too slow to use speed as a defense even if it is 60mph faster than somebody else's bomber.

Lets say the intercepting fighter can cruise at 330mph and is 10 miles behind the 300mph bomber. It would take 15 minutes to catch it and cover 82 miles doing it. Going to top speed helps but may not work. even a top speed of 375mph is going to need 8 minutes to overhaul the bomber and few, if any, fighters can use top speed for 8 minutes. Against the 260 mph bomber the fighter cruising at 330 can catch it in about 8.6 minutes or just over 5 minutes at 375mph. And that is the trick. The bomber has to be able maintain these hi speeds for a number minutes at a time (not a few minute sprint) AND not run out of fuel while doing it. Granted the intercept distance improve for the fighter if the target goes even slower, like 220 mph, but you get the idea. Ditching the guns is really playing the odds unless your bomber can really fly at speeds close to what the fighter can do. Even 80% as fast will not guarantee freedom from interception. The A-20 in the above example can kick it's speed up to 330-340mph to try to stretch out the interception and run the fighter out of fuel/time at military power as long as it does't run itself out of fuel. A 275mph Wellington has no hope of out running the fighter if it starts within a certain distance and without guns it's ONLY hope of making it home is that the fighter pilot is such a bad shot He cannot inflict enough damage to the Wellington to bring it down. The attacking fighter isn't even being distracted by defensive fire. This does not sound like a good plan

Well, my idea is to make a bomber that is better than most common bomber that use the same type of engine. My bomber would have 3 engines, contrary to 2 that other have. The only guns to be carried are those at tail. The bomber can be tailored towards greater speed (carrying same payload as the twin), or towards greater payload (for same speed). My bomber will need less crew for same combat capability, and will be able to make it home even with one of it's engines out of order.

As for where an when, I've already proposed 3 x turboed R-2800 for the USA, 3 Pegasus for RAF etc, but can add 3 x Hercules for RAF and 3 x R-2600 for USA. Granted, the one with 3000 HP total will be subject to compromises (but less so than those with 2000 HP total), unlike the others that have from 4500 to 6000 HP.

BTW, I did not proposed deletion of Wellington's rear turret anywhere in this thread

Take Mosquito, put P-39 engine, shaft extension and propellor in bombay. (About 900kg) drive the prop through the nose. Ultra fast trimotor recon or interceptor.