twinjet vs trijet
 

In terms of performance characteristics, what are the tradeoffs between a 2-engine and 3-engine design?

Lets assume that we are comparing a twinjet with fuselage mounted engines and a trijet with an s-duct layout and all engines on the fuselage. I think its unfair to compare to a trijet that would have 2 wing-mounted engines. It seems structurally inefficient on its face.

I read all over the internet that 2 engines is always more efficient than 3. But is that necessarily true?

It seems that a main factor is the requirement that a plane be able to complete its takeoff run with one engine out. On a twinjet you're losing 50% of your power while on a trijet you are losing 33% of your power.

So take two hypothetical airplanes and hold pretty much everything equal except the number of engines at the back. If the plane requires 0.5X thrust to complete takeoff, then the twinjet needs total of 1X thrust while the trijet needs 0.75X thrust to takeoff with the same runway requirement.

At cruise, which plane is more efficient? The one that is uses the three smaller less efficient engines or the one whose 2 larger engines that are incredibly overpowered for cruise? By how much?

Yes.

The main factor actually was the requirement that you have three or more engines for extended over-water ops. When ETOPS relaxed that requirement to two engines, the trijet was done.


Two engines are more efficient...

- Less rotating-machinery drag (saves energy).

- Fewer support systems (saves weight).

- Less flow energy-loss (no long s-duct).

- Larger fans are actually more efficient anyway. The core is going to be a little bit heavier, but it can optimized for max fuel efficiency at cruise speeds vice T/O.

If the weight and fuel savings were not enough, the reduced Mx cost (by 33%) seals the deal.

I know thats the narrative for the life of the trijet airliner, but couldn't there be applications where the trijet made sense? Maybe the airplane is too small for wing-mounted engines and maybe its mission is to be able to do long ranges out of difficult runways.


Yes I understand all of this, that to produce X thrust is more efficient in every way out of 2 engines than 3. But there has to be some fraction of X where its more efficient to get it out of 3 engines than 2. For example, 3x5,000lb engines have to use less fuel than 2x50,000lb engines powering the same plane.

So I ask, whats that fraction of X? As I said above, if the plane needs .5X to take off then the twinjet has to have 1X total thrust. What total thrust could the trijet have to have the same cruise efficiency as the twinjet? The minimum that it could be is .75X because of the takeoff requirements

Even if you are required to add thrust to a twin jet that cost pales in comparison to the cost to carry the weight of that third engine.

I see what you're saying. I don't know the break-even point off-hand, but it's generally known in engineering circles that two engines is more efficient than three.

There may also be structural weight savings, since you have to support all that weight on the tail and also transmit the thrust to the CG. Wing-mounted engines are closer to the CG.

But how much extra weight is it when the 3 engines together only have to put out 75% of the thrust of the twinjet? Each individual engine has to put out 50% of the power of one the twinjet. So even say that it has to weigh 60% as much to make 50% of the power, all together the trijet engines would weigh 180% to the twinjet's 200%. There has to be a breaking point somewhere. In this example it would be that each of the trijets would weigh 67% of the twinjets to make 50% of the power.

Also, there is supposedly some aerodynamic benefit from the s-duct engine where the exhaust out the back of the fuselage effectively lengthens the fuselage more than would ever be practical to build. Even if the trijet has to weigh a little more, it might be evened out by this aerodynamic benefit.

I can easily see why large airliners have moved to 2 or 4 engines. If you're already building engines on the wings, then it must take a lot of extra structure to also install an engine at the centerline of the back of the plane. But if you assume an airplane thats already using fuselage mounted engines (say its not tall enough to put the engines under the wing) then I would think that its only marginal extra structure to put a third engine between and behind them.

There are definitely tradeoffs between putting the engines on the wings vs rear of the fuselage. But I'm trying to figure out 2 vs 3 engines so I want to control as many variables as possible. I'm not in aviation and I don't know what you mean about the "CG" (center of gravity?) but some advantages I've read about fuselage-mounted engines are:
-since it doesn't need the ground clearance below the wing, plane can be designed as the fuselage riding on a single wing, which can save weight and structural requirements
-can build more flex into the wings, absorb more turbulence in the wings
-possibly better for unpaved runways

I do not have the data, but the new engines have some very high end geometry to their blades. A lot of that is possible due to where they are located; on the wind in the wind stream. There would be some efficency lost due to the partial disturbance of the airflow on a S-duct type of configuration of an enter-mount engine.

That said the new engines burn a lot less, require less maintenance and because of this using two works better than three. There large engines that they are using today have a ton of added thrust for take off and being able to perform the 2-4 stage climb gradients required. What it also means is that they are pulled fairly far back while in cruise. Their fuel flows are way below what you would see with a three or four jet engine.

Trust me if it were more efficient to have three, you would see airlines buying them. They are not. Add it cost to carry, maintenance, added burn etc, and there is no comparison. remember that airlines are taking pillows off of airplanes to save weight. EFB's save weight. Every added pound costs a ton of money over the operational span of a jet.

On the subject of structure and wing root loading, you might also note that 3 engine planes typically carry as much fuel in the center tank as the wings to fuel the number 2 engine, whereas twin engine aircraft typically don't use the center tank until the fuel required exceeds the capacity of the wings. Again, more wing root loading means more structural considerations.

The Real Issue
 

I realize that the attempt in starting this thread was to discuss the "design" philosophy of two engines vs. three; however, there is one point that hasn't been made and it's the reason tri-jets are not being manufactured anywhere in the world today. There is no market for them because of the cost of the third seat; i.e., the third pilot.

Rickair7777 is correct about the overall efficiencies of today's fuel efficient fan engines. When one can manufacture a high bypass fan that produces 125,000 lbs. of thrust and is as fuel efficient as the new GE engines are, the discussion is over !

Have a bodacious day Mates:)

True, but nobody making modern jets puts the other two on the tail if they can help it.

Some regional jets are the only exception, because with a short wing span they don't need longer (heavier) landing gear to provide wingtip clearance during a xwind landing. Larger airplanes with longer wings need tall gear for tip clearance, so that weight is already a done deal.

But even the newest RJ's have gone to wing mounts.

Simpler structure, yes but I doubt it saves a great deal of weight.

Whatever flex is required, gets designed in. The exact design doesn't matter. Again I suspect not much weight savings by carrying the spars all the way through.

Modern commercial transports don't operate on unpaved runways. Besides I disagree...the gear would kick up rocks which would get sucked into a tail-mounted engine.

That's why military transports have their engines mounted on the wings, with the wings mounted high on the fuselage.