Very true. Not just the weight/fuel burn of the third engine but now you have an additional 30% in MX/engine cost. I have no idea what the big boys run but I do know that on private jets those puppies run around $300-$400k a piece. Not sure what a hot section on a jet eng cost but I'm sure it's high. Regardless if I won the lotto I'd have to go with the Hawker 900EX.

Keep in mind there's more than just cost associated with two vs three. Operational performance varies. I don't know specific numbers but I saw the 900EX do a short field takeoff the other day and and it made my jaw drop.

 

Perfect example of how a 3 engine aircraft can be useful. You can get better climb performace (since it is predicated on losing one engine) out of short fields. With your example of the 900EX, the aircraft can leave a short strip (for example HHH) and then it can probably fly inter-continental without much penalty. The 727 exploited some of that by being able to have good short field and decent high altitude performance. Now with large jets, it was economics before the twin-jets and ETOPS came into being since commercial inter-continental jets at the time we almost all 4-engine. The -10 and the Tristar eliminated one engine saving costs while still meeting the overwater EO requirements.

 

Three engines does not equal three pilots on relatively modern aircraft.

MD-11, Falcon 7X, etc.

 

Airlines are still buying plenty of 4 engine planes. How do the A330 and A340 compare? Again, as I said, I can't see how 3 engines would make sense with the way that larger airliners have evolved to be built. It would have to be something smaller that couldn't fit the engines on the wings.

Correct me if I'm wrong, but I believe that the reason larger aircraft mid-mounted wings is for ground clearance for the engines. Airplanes that have fuselage engines can have a 1 piece wing with the fuselage resting on top. Its supposed to be more structurally efficient. Also, I can't imagine why a trijet would necessarily hold more fuel in the fuselage.

 

The Falcon 50EX, with its three small TEF731s, takes more weight off a shorter runway a longer distance at higher speed with less fuel burn than many of its two-engine competitors. I have no hard data to back it up, but I'd bet a 50EX slowed way back to .78 burns less fuel over a 1000nm segment than a Hawker 800 at HSC does.

The 900EX also has more range than G450 on substantially less fuel burn...but it weighs a fair amount less than the Gulfstream too.

As far as engine costs go...a three-engine plane doesn't necessarily mean 33% higher costs. Three engines don't have to work as hard (produce as much thrust) as two engines to push the same weight, and because of this you can use smaller (and less expensive) engines. As an example, the three TEF731s on the Falcon 900EX only cost 10% more to operate vs. the 2 RR Tays on the G450, but burn 32% less fuel over a 1000nm segment.

 

For a better example of 3 small engines pushing equal weight vs. 2 larger engines, the Falcon 900EX and the Embraer Legacy 600 have max takeoff weights within 100lb of each other @ an average of 46,651lb.

The Legacy 600's AE3007-A1Es have 7953lb of thrust each, where the Falcon 900EX's TEF731-60s have 5000lb/each. That's 15,906lb of total thrust for the Legacy and 15,000lb thrust for the 900EX.

The Legacy's two engines cost 21.4% less per hour than the 900EX ($447.09 for the Legacy vs. $568.48 for the Falcon)...but over a 1000nm segment, the 900EX will burn 18.5% less fuel than the Legacy (4150lb for the Falcon vs. 5090lb for the Legacy).

The Falcon's 140 gallons in fuel savings over that 1000nm segment more than offsets the higher costs associated with its third engine.

Of course, that may not hold true when you consider a MD11 vs. a A330 or B777...

 

Yeah thats exactly where I'm saying that any possible efficiency benefits would come from. Say you have a twinjet with 2x5,000lb engines that has a certain runway requirement. Holding everything else constant, maybe the trijet would have 3x2,900lb engines. So it would have 5,800lb thrust with an engine out and should be able to have a shorter runway requirement.

What I was asking is, say the trijet has 3x2,500lb so it has the exact same runway requirement as the twinjet, how would performance and efficiency compare to the twinjet? It uses 3 engines, but they are much smaller and closer in power capability to what the actual cruise requirements are.

 

Are there any active forums that are more technical about airplanes, where there might be people that have some expert insight?

 

Exactly what kind of "expert insight" are you looking for?

I provided you an example of two modern aircraft, one with two engines and one with three, that are within 100lb (0.2%!) in max takeoff weight. Sure I'm biased, but I'd say the answers to your questions are right there for the taking.

That said, you might try PPrune or Pro Pilot World, or a copy of Aerodynamics for Naval Aviators...

 

Because I agree with you and from the example of the Dassaults it would seem that 3 engines is ideal when structurally practical. They can take off with less runway and then cruise using much less fuel than comparable aircraft.

But then people will say that its not the 3 engines, its the Dassault design. And its lighter. You compared the Legacy 600 and F900EX with the same MTOW but the L600 is BOW 30,000lb compared to BOW 25,000lb on the 900EX. So on your hypothetical 1000nm mission where the 900EX burns 18.5% less fuel, its probably on average ~18% lighter. That still seems to suggest an advantage for the trijet when you consider the runway advantage.

I think that Legacy 600 vs 900EX is a pretty good comparison. I think that another good comparison is 900LX vs 2000LX. They have the same fuselage cross-section, wing, and systems. 900LX cabin is about 5 or 6 feet longer, total 20% more volume. 20% more range. Weighs a little more, 25,000lb vs 23,500lb. Has much much better runway performance. And in a 1000nm mission, 900LX uses about 6% more fuel. Seems like a tradeoff that favors the bigger, longer range, more takeoff capable, trijet.