|Quoting SEPilot (Reply 70):|
it is cheaper to build and maintain one large engine than two smaller ones, and the physics of jet engines dictates that the larger an engine is, the more efficient it is.
You're likely right about the production cost of two versus one engine. Given the quad's lower overall thrust requirement, however, how can we know which effect dominates - total thrust or cost per lb-T?
Same with maintenance - which effect dominates? There's a recent thread in tech ops that contains a link showing the A340-300's mx cost per lb-T being only 3% higher than for a 77E. That's the only data point I've seen on the ckmparison. Going from twin to quad on a large (V2-limited) plane will save more than 3% of total thrust requirement easily.
You're definitely right about tip clearance and I cited that in my quad v twin reply. Tip clearance loss is proportional to (1-T^2), where T is tip clearance as a proportion of fan radius. T is so small for tightly-engineered modern turbofans, however, that I'd be surprised if going from 55k to 110k lbs-thrust made even a 1% difference. VLA engines will be big whether quad or twin so tip clearance is already a smaller issue than for e.g. a narrowbody. Meanwhile, a quad engine optimized to cruise at 85% max thrust versus 70% max thrust has a roughly proportional 20% increase in cruise OPR. That's probably worth 3-5% SFC. The numbers I've seen show the GP7200 and Trent 900 having slightly better cruise SFC than the GE
-90-115B. I'd send a link but I'm on my phone... Also Lee ham News has a good "deep analysis" of the A380 showing its fuel efficiency to slightly beat the 77W per seat despite being a terribly over built plane with a stubby wing.
Re heat loss - engine cores have to be cooled anyway so isn't this a non issue or even a slight advantage for the quad?
My exercise "designing" the A380X in tech ops has convinced me that the bending relief effect of four engines is very important for big planes. Recall that Airbus was able to use the same spar structure for A330 and 340 solely due to bending relief.
One other quad VLA advantage is that the smaller engines should reduce landing gear height - a 2-decker's rotation angle is less critical to MLG design because of the stubbier fuselage.
Twins definitely would have less complicated and lighter support systems (Pylons, fuel pumps, pneumatics for bleed).
I used to think twins were always better for exactly the reasons you cite. My first A380X post in tech ops last year was basically a twin for those reasons. The more I've researched the more I think a quad is best.for a big plane. I can't provide a lot of hard numbers on my side, just a judgment call that the countervailing trends intersect somewhere around VLA size. If you have more insights or better numbers I'd much appreciate your sharing.