Originally Posted by Red Goose
Second segment climb (from gear up to 400ft above ground, 2.4% climb gradient for 2 engine aircraft, 2.7% for 3 engine aircraft), a certification requirement, is more limiting on 2 than 3 engine aircraft. This is simply due to the fact that with a 3-engine aircraft, with one engine out, you only lose 1/3 of your thrust (more drag, true, but you're still doing better!), and that counts for take-off performance.
Yes but if you compare the Dassaults to their competitors they have engineered the planes to take advantage of this. They have significantly lower thrust/weight ratios to begin with. After losing an engine and 1/3 of their power their thrust/weight ratio is still within 10% of a gulfstream losing one of its engines since the gulfstream has so much more thrust to begin with. If Dassaults started with higher maximum thrust maybe that would change their efficiency and they wouldn't be as economical relative to the other planes.
My basic questions about 2 vs 3 have not really been answered.
-Suppose that you would engineer a trijet to have 80% of the total thrust you would have engineered a twinjet to have assuming you were making a plane with roughly the same characteristics. Is the plane heavier with 2 engines making X thrust or 3 engines making .8X thrust? Assuming an S-duct layout like the Dassaults
-Is there something about engines operating closer to their maximum rating being more efficient? If you are making a twinjet and a trijet with the same characteristics, is it going to get better cruise economy with 2 engines that produce Y cruise thrust out of X maximum thrust or the trijet that needs about the same Y thrust to cruise out of .8X maximum thrust?
-What are some of the performance tradeoffs between having 3 smaller turbines versus 2 larger ones? If you have a plane that needs 50 "thrust" for takeoff, then the twinjet needs 50 thrust per engine and the trijet needs 25. So the twinjet engines will be twice as powerful. I know for example that the twinjet engine will be more efficient in terms of energy used per lb of thrust produced. Will the smaller engines spool up to their higher thrust levels faster? Is it significant enough to effect takeoff performance? What are the other tradeoffs?