It seems engine size might be set by one of three criteria

1) Thrust needed for takeoff
2) Thrust needed at top of climb
3) Thrust needed for engine out

Obviously different planes are different. But for 737-777 type aircraft, what is the most critical?

People on the forum seem to assume it’s #3, but I’ve not heard anyone with authority or a citation to authority actually say so.

I notice the a330 and the a340 have similar total installed Thrust, which makes me think that for this set of planes the determining criteria was not #3.

https://en.wikipedia.org/wiki/Boeing_737#Specifications
https://en.wikipedia.org/wiki/Airbus_A3 ... ifications

Engine thrust requirements are determined by aircraft weight. #3 requires the most amount of thrust per engine in an engine out emergency and it is always considered in the design. The A340 has 4 engines because it is heavier than a 737. Compare the METO (Maximum Take Off Weight) of the 2 aircraft.

Engine design is also highly effected by required cruise this profile which is a function of aircraft weight, wing area, wing aspect ratio, fraction of wing laminar flow, and aircraft wet area.

Engines are optimized for a mission. Usually one launch customer will distort the design to be most cost effective for them. All engines are designed for growth. With the MAX, the -8 and -9 drove the process with the -7 getting what it could. For the NEO, Pratt focused on the A321 and everything else got what they got.

Engines lose durability (increase cost per takeoff) at higher thrust. Optimizing for a longer mission increases cost for shorter as engine weight grows as well as wetted area (drag) in the attempt to minimize cruise fuel burn.

The MAX is variable cycle. The worst wear happens at end of climb. By means of a very hot valve, CFM (it's GE technology) cuts cruise turbine cooling which really helps cruise fuel burn (the air powers the turbine instead of cooling it).

Pratt broke the optimization by the GTF. By increasing turbine Mach # (think RPM if you want), climb above 10,000 feet was dramatically improved.

There is a lot more fuel burn during takeoff and climb than cruise. So optimizing for those conditions really helps the whole mission. Improving the cruise by variable cycle is even better. Pratt was going to have a variable fan nozzle on the PW1500G and PW1100G. But by exceeding promises, they cut out that weight. The concept for 1.5+ hour missions has merit and will return. In particular, I think 3D printing the parts out of titanium today shifts the cost advantage significantly from the manufacturing estimates when Pratt made the decision.

This brings up one final point, the optimal engine changes every year as new technology is developing. Every launched engine is a snapshot of the technology level about 8 years prior to entry into service. PIPs try to bring in new technology, but a new engine designed around a new technology gets about twice the benefit of a retrofitted engine.

Some technology, such as variable turbine cooling, multiple turbine cooling systems, variable fan nozzles, compressor Mach #, and turbine style (shrouded, unshrouded, large blade aspect ratio) must be designed in at the start. The PW1100G is doing a very unusual step of switching casings and thus part commonality for increased low turbine efficiency and I see improvements in low compressor feed into the high compressor. Thus allowing non-retrofittable technology perfected to a high enough TRL only about 6 years ago to be retrofitted.


The GE9x is insane at it's new technology. Thanks to electric actuators, much of the technology known since Whittle (I love reading through his patents) can now be implimented. For example, more turbine cooling valves, more precise oil cooling, variable turbine cooling (much more refined than the LEAP), shorter combustors, higher Mach # compressor (requires more compressor vane actuators that need electric motor precision as pneumatics and hydraulics stick so much more), and today's sensors enabled by long ago FADAC computers that take in digital data.

Much of this generation of engines improvements are on the maintenance side. Pratt and GE underpromised as it will take time to perfect as it did with 787 maintenance. But as with 787 maintenance, once it is done right, another cost advantage over prior generation engines.

Lightsaber

Wow. Lightsabre, I'm sure everything you said is true. However the poster, I believe, is just asking what thrust available requirements are the most stringent in a certified airliner.

To me and stratclub, that would be the requirement to have enough thrust available on its remaining engines at a T/O v1 engine out that can get the airplane in the air. I'm just talking about thrust to weight ratios not re-liabilities, fuel efficiencies, weight of the engines, etc which are all consideration factors in engine design and air- frame matching.

RetiredWeasel wrote:

Wow. Lightsabre, I'm sure everything you said is true. However the poster, I believe, is just asking what thrust available requirements are the most stringent in a certified airliner.

To me and stratclub, that would be the requirement to have enough thrust available on its remaining engines at a T/O v1 engine out that can get the airplane in the air. I'm just talking about thrust to weight ratios not re-liabilities, fuel efficiencies, weight of the engines, etc which are all consideration factors in engine design and air- frame matching.

Yes exactly.

But how do you know the required thrust is set by the engine out requirement?

At top of climb the aircraft is at nearly max weight and the engines are working in very thin air. I think I’ve read they are only capable of producing 10% of sea level thrust. They are literally run max output and the plane can barely climb. Additional altitude would give advantages like flying in gunner air. But the engines are maxed out. Maybe that’s the limiting factor???

I note that at top of climb the maximum climb rate is LOWER than the one engine out at sea level scenerio.

Not absolutely sure what you are asking, but this thread may be helpful. https://www.pprune.org/tech-log/433331- ... keoff.html

More good stuff. https://www.pprune.org/tech-log/282945- ... itude.html

stratclub wrote:

Not absolutely sure what you are asking, but this thread may be helpful. https://www.pprune.org/tech-log/433331- ... keoff.html

More good stuff. https://www.pprune.org/tech-log/282945- ... itude.html

That was helpful.

The pprune link says that if on is taking off using flex/reduced thrust, and an engine quits, the plan is to continue the takeoff with the flex/reduced power setting. That would imply that the engines are bigger than strictly needed for the engine out case (where flex takeoffs are possible).

The second link says at cruise the engine might be producing 20% of rated thrust (and be near full throttle).