Bigger Engines for Longer Range?

[grumpyoldgeek]

This was posted over in the BA038 thread:

"In 1998 Boeing proposed new longer range variants of the 777X; taking advantage of the Trent 800's growth capability, Rolls-Royce designed and built an improved engine designated Trent 8104 which was later scaled upwards to the even larger 8115. This development was the first engine to break through 100,000 lbf (440 kN) thrust and subsequently the the first to reach 110,000 lbf (490 kN). However, Boeing required that the participating engine developer assume a risk-sharing role on the overall 777X project. Rolls-Royce was unwilling to do so, and in July 1999 Boeing announced that it had chosen the development of the GE90, the GE90-110B and GE90-115B to be the sole engines on the long-range 777s. This resulted in the 8104 becoming just a demonstrator programme, despite setting further industry firsts for thrust levels achieved and the first to demonstrate the use of a fully swept wide chord fan."

I'm trying to understand why you need a bigger engine for longer range. I could understand a more fuel-efficient engine, but not a bigger one. How does that work?

[TheGorrilla]

Higher bypass ratio perhaps? Therefore the engine is physically bigger. It could also simply boil down to the power setting vs design operating conditions, ie, a more powerful engine derated is better than one that has to work harder at a higher power setting.

[411A]

First off, within the bounds of runway performance, it generally takes a bigger engine to haul the weight (mass, for you European folks) of all the aeroplane and fuel, off of said runway...for that longer route.
Twins, especially.

Three or four engine types are (usually) not so compromised.

Therefore, the engines on a twin are just slightly larger than they need to be for the range anticipated.
Enroute failures enter into the picture, as well.
Many compromises.

[boeing boeing.. gone]

also don't forget that the twin has to be able to fly on one engine... therefore if the plane is heavier because of the additional fuel the single engine has to be considerably bigger!!

[Mach Tuck]

It could be simply that with more thrust you can get higher quicker and as you burn less fuel the higher you go your overall economy improves.

I fly a type of which there are two models which are structurally identical and have the same fuel capacity. The later model's engines are also identical except that they have 500lbs more thrust. This model has a 200nm greater range than the older one.

MT

[NigelOnDraft]

I'm trying to understand why you need a bigger engine for longer range

Same Airframe, Longer Range = higher MTOW (more fuel). To lift a higher MTOW you need either a longer runway (if that is limiting), and/or more thrust (bigger engine) to climb on 1 engine at that weight...

QED?

NoD

[Graybeard]

My primary flight instructor said twins have two engines because they are too heavy to fly on one...

Really, aircraft design is governed by engine availability. They design the airplane around what can get it off the ground. The L-1011 was dead and bankrupting Lockheed in its final stages of development - until RR was rescued from bankruptcy.

Second design requirement is where to put the landing gear. If you don't want to put it in the fuselage like a BAe-146, then you gotta' make it a low wing, which limits the size of wing-mounted engines. See above.

"An airplane is a whole bunch of compromises flying in close formation."

GB

[The Real Slim Shady]

"An airplane is a whole bunch of compromises flying in close formation."

GB you forgot to add " And supplied by the lowest bidder"

[Old Smokey]

It's really pretty basic.

If you have a "standard" B777-300 with maximum payload, you cannot carry full fuel within Maximum Takeoff Weight limits, thus the range is limited.

If you want to carry the same payload in essentially the same aircraft for a greater distance you require to carry more fuel, and thus operate at a greater Maximum Takeoff Weight.

Apart from structural considerations, the only way to increase the Maximum Takeoff Weight is to add BIGGER engines.

The MTOW difference between a standard B777-300 and B777-300ER is approximately 30 tonnes, thus, for the same payload you may carry an additional 30 tonnes of fuel. This provides the longer range.

Regards,

Old Smokey

[chornedsnorkack]

If you have a "standard" B777-300 with maximum payload, you cannot carry full fuel within Maximum Takeoff Weight limits, thus the range is limited.

Ditto with 777-300ER. Most airliners cannot carry full fuel and full payload, because they exceed MTOW in that case.

Apart from structural considerations, the only way to increase the Maximum Takeoff Weight is to add BIGGER engines.

That - or accept longer takeoff runs. But since the original plane already used much of the runway, you often do need bigger engines.

The MTOW difference between a standard B777-300 and B777-300ER is approximately 30 tonnes, thus, for the same payload you may carry an additional 30 tonnes of fuel. This provides the longer range.

No.

Some of that 30 tonnes must go for the strengthened landing gear, strengthened wing and bigger engines to carry those 30 tonnes. Some, but not all, so there is some extra fuel lifted (how much?). Some of that extra fuel is burned to deal with the extra induced drag over the original range. And in the end, there may be some fuel left over to fly the extra range.