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Sokes
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Can engines be too big?

Mon Feb 15, 2021 5:40 pm

I thought of it when reading about A330 neo.
Even the -900 competes with new generation B787 on range. What I believe is needed is a -800 for lower ranges. Would a new, smaller engine help? Or is big beautiful, as the plane climbs faster and higher and can start at reduced power which saves on maintenance?

Similar would a smaller engine on the B737 Max 7 be better?
Why can't the world be a little bit more autistic?
 
VSMUT
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Re: Can engines be too big?

Mon Feb 15, 2021 6:22 pm

Bigger fans are more efficient.

Sokes wrote:
Similar would a smaller engine on the B737 Max 7 be better?


The way smaller Embraer E2-190 and -195 as well as the A220 actually have bigger fans than the MAX 7.

Ignoring the issues caused by the short landing gear and hence sub-optimal engines for the 737MAX, the MAX-7 is an overweight aircraft for its job. I can't find an estimated OEW for the MAX-7, but the 737-700 had an OEW of about 37.600 kg, and the MAX update added around 4 tons to the OEW of the 737-800/MAX-8. An OEW of around 40-41 tons probably isn't too far off the mark. The E2-195 by comparison has an OEW of 35.700 kg and the A220-300 lies at around 37.000 kg. That's the issue with the MAX-7 and A319neo.
 
RJMAZ
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Re: Can engines be too big?

Tue Feb 16, 2021 8:20 am

I actually think a full optimised lightweight version of an existing widebody is possible with smaller engines. The key is when you reduce engine size you must reduce the MTOW but you are still carrying all the heavy parts to support the original MTOW.

If you look at the Trent 700 and Trent 500 they both have the the same fan diameter. The Trent 500 has a 45% higher bypass ratio, is 15% lighter and has better 5% better fuel burn. The Trent 500 has 20% less thrust.

In theory the Trent 500 could have been fitted to the A330-200 and the empty weight would have reduced by 2t (1.5%) just from the engine weight and trip fuel burn improved by 5% However the MTOW would have had to be limited to around 200t due to the lower thrust. The lighter A300 landing gear and centre wingbox could have been fitted that supports a 188t MTOW this would save another 6t (5%). Range would take a big hit and definitely be below 5000nm. Fuel burn per passenger of such an A330-200 lightweight in the year 2000 would be equal to that of a decade newer cleansheet 787-8. Perfect for JAL as they use the 787-8 for domestic work. But the shorter range A330-200 lightweight would not have been as flexible as the 787-8 when performing long flights.

I think Airbus the A330 and 787-8 are perfect for a future lightweight shorter range version.

The 787-8 could easily remove 10t of empty weight providing the MTOW is reduced to around 200t. Range would drop below 6000nm but it would need a lighter cleansheet engine as there is nothing around 55,000lb of thrust and derated a bigger engine is a bad idea. The wingbox, fuselage barrel and landing gear would need to be lightened to support the lower weights/loads
 
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seahawk
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Re: Can engines be too big?

Tue Feb 16, 2021 11:45 am

The 787 can easily remove 10t empty weight? How?
 
VSMUT
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Re: Can engines be too big?

Tue Feb 16, 2021 12:08 pm

seahawk wrote:
The 787 can easily remove 10t empty weight? How?


By reducing the MTOM, making the aircraft less capable.
 
gloom
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Re: Can engines be too big?

Tue Feb 16, 2021 12:17 pm

RJMAZ wrote:
The 787-8 could easily remove 10t of empty weight


Not really.

And I do not mean it cannot be done. If it would be done, it's:
a) not 787 anymore, be it 8 or whatever else - not in the meaning we have now,
b) it would be awfully expensive, since range of work (and certification) is close to building new plane.

But, if we assume this is the way to design a plane similar to MOM in terms of capabilities, on existing 787-8, this is more-or-less the way.

Cheers,
Adam
 
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seahawk
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Re: Can engines be too big?

Tue Feb 16, 2021 1:09 pm

VSMUT wrote:
seahawk wrote:
The 787 can easily remove 10t empty weight? How?


By reducing the MTOM, making the aircraft less capable.


The reduction of the MTOM reduces the OEW - how?

You would need to change major structural components and that is far from easy.
 
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lightsaber
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Re: Can engines be too big?

Wed Feb 17, 2021 12:59 am

VSMUT wrote:
Bigger fans are more efficient.

Sokes wrote:
Similar would a smaller engine on the B737 Max 7 be better?


The way smaller Embraer E2-190 and -195 as well as the A220 actually have bigger fans than the MAX 7.

Ignoring the issues caused by the short landing gear and hence sub-optimal engines for the 737MAX, the MAX-7 is an overweight aircraft for its job. I can't find an estimated OEW for the MAX-7, but the 737-700 had an OEW of about 37.600 kg, and the MAX update added around 4 tons to the OEW of the 737-800/MAX-8. An OEW of around 40-41 tons probably isn't too far off the mark. The E2-195 by comparison has an OEW of 35.700 kg and the A220-300 lies at around 37.000 kg. That's the issue with the MAX-7 and A319neo.

First, nice summary. To add the engines on the NEO are optimized for the A319. I notice people quoting pressure ratio without realizing it is a function of engine loading. The PW1100G runs at a lower pressure ratio, by my back of the envelope estimates, than the PW1500G on the A220-300. Pressure ratio is also efficiency.

A well optimized engine is always more efficient. But unless an aircraft is expected to sell extreamly well, truly optimized engines are rare. The MAX only received a more optimized engine as Boeing threatened to dual source as keeping with a NEO core would have been very poor for the MAX.

A new engine requires a new supply chain. No one does custom for small thrust differences. E.g., the A330NEO engine isn't optimal, but no one was going to offer optimal. The E2 would have done a little better with an engine optimized for it, but who spends billions unnecessarily?

Lightsaber
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lightsaber
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Re: Can engines be too big?

Wed Feb 17, 2021 1:05 am

RJMAZ wrote:
I actually think a full optimised lightweight version of an existing widebody is possible with smaller engines. The key is when you reduce engine size you must reduce the MTOW but you are still carrying all the heavy parts to support the original MTOW.

If you look at the Trent 700 and Trent 500 they both have the the same fan diameter. The Trent 500 has a 45% higher bypass ratio, is 15% lighter and has better 5% better fuel burn. The Trent 500 has 20% less thrust.

In theory the Trent 500 could have been fitted to the A330-200 and the empty weight would have reduced by 2t (1.5%) just from the engine weight and trip fuel burn improved by 5% However the MTOW would have had to be limited to around 200t due to the lower thrust. The lighter A300 landing gear and centre wingbox could have been fitted that supports a 188t MTOW this would save another 6t (5%). Range would take a big hit and definitely be below 5000nm. Fuel burn per passenger of such an A330-200 lightweight in the year 2000 would be equal to that of a decade newer cleansheet 787-8. Perfect for JAL as they use the 787-8 for domestic work. But the shorter range A330-200 lightweight would not have been as flexible as the 787-8 when performing long flights.

I think Airbus the A330 and 787-8 are perfect for a future lightweight shorter range version.

The 787-8 could easily remove 10t of empty weight providing the MTOW is reduced to around 200t. Range would drop below 6000nm but it would need a lighter cleansheet engine as there is nothing around 55,000lb of thrust and derated a bigger engine is a bad idea. The wingbox, fuselage barrel and landing gear would need to be lightened to support the lower weights/loads

The engines are under contract. GE and RR would legally block this idea.

Now what you propose is the NMA. Boeing was going to do a lighter aircraft, reduced range, optimized engines, for a shorter mission. Part of the weight reduction effort was reducing total weight of payload (cargo).

We can debate for years if that would have happened.

You neglect subsystems. New subsystems are heavier, but more efficient, and take far less maintenance.

Lightsaber
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Sokes
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Re: Can engines be too big?

Wed Feb 17, 2021 3:13 am

According to the Wiki page about Trent the -500 is only 3% more efficient than the -700.
It is only 1,2 t lighter. https://en.wikipedia.org/wiki/Rolls-Royce_Trent

However I tend to believe you more than Wiki. If bypass ratio is improved around 50% it's hard to imagine that fuel efficiency only improves 3%.

The A330-200 has 120,6 t OEW and is 58,8 m long.
The A300 has 88,6 t OEW and is 54,1 m long.

The PW400-94 of A300 had 7cm less fan diameter. Moreover the A330-200 is longer than A300. Is landing gear length of A330-200 and A300 really the same?

Engine weight savings seem neglectable.
I believe same is true for fuselage.
Can the empennage be smaller when MTOW is less?
The wing is designed for turbulence. So with same wing design I doubt one can take much weight out of the wing. The main weight savings may be from landing gear and empennage. Total weight savings without changed empennage may be only around 6-8t.

But if one keeps the powerful engine and only changes landing gear, one can climb higher in the beginning. So that should compensate some of the savings from the more efficient engines.

There isn't much interest in the A330-800.
Should Airbus take the A330 fuselage and design an empennage for 200t MTOW?
One version could be 9x fuselage diameter = 50,8 m, the other version A330-200 length = 58,8 m = 10,4 x fuselage length.
Would a new wing be necessary? Early A330s had not much MTOW, but then it's the A340 wing.
Use the same Trent 7000 engines?
Why can't the world be a little bit more autistic?
 
Sokes
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Re: Can engines be too big?

Wed Feb 17, 2021 3:37 am

lightsaber wrote:
To add the engines on the NEO are optimized for the A319. I notice people quoting pressure ratio without realizing it is a function of engine loading. The PW1100G runs at a lower pressure ratio, by my back of the envelope estimates, than the PW1500G on the A220-300. Pressure ratio is also efficiency.

Lightsaber

I don't understand. Can you expand?
When the air enters at the fan there is a certain cross section for the air going through the core. At the last compressor stage there is another cross section. I thought these two cross sections decide the pressure ratio.

And why is the Neo engine optimized for the A319? I would have rather assumed for the A321. I mean the fan is huge compared to the Max.

The Trent 500 for the A340 was adopted from the Trent 800 of B777. Is the Trent 500 an example of an engine that wasn't optimized?
3% more efficiency with about 50% more bypass ratio compared to the Trent 700 of the A330 doesn't sound much.
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RJMAZ
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Re: Can engines be too big?

Wed Feb 17, 2021 3:55 am

seahawk wrote:
The reduction of the MTOM reduces the OEW - how?

You would need to change major structural components and that is far from easy.

Browsing through Wikipedia it is easy to notice that the empty weight is approximately half of the MTOW for nearly all of the airliners.

A 250t MTOW aircraft full optimised weighs around 125T empty. A 100t MTOW aircraft weighs around 50t empty.

So if a fully optimised aircraft design such as the 787 has its MTOW reduced from say 250t to 200t then it's empty weight should reduce from 125t to 100t. This 25t reduction in empty weight is only if it is fully optimised for the lighter MTOW. I am suggesting that a reduced MTOW 787 would only lose 10t in empty weight instead of 25t as not everything would get fully optimised. I am fully aware many systems will not get any lighter which is why it would still be 15t heavier than the fully optimised design.

Empty weight per square metre of cabin area is usually the best metric for short haul efficiency. Such a lightweight 787 gains significantly here as it is reducing empty weight while maintaining cabin area.

Fuel burn per kg of payload is the best metric for long haul efficiency. A lightweight reduced MTOW 787 becomes worse here. The A350-900 for example did the opposite. It gained 12t of MTOW with no increase in empty weight. This improved fuel burn per kg without any engine or aero improvements. It allows 15+% more payload on a long flight with less than 5% increased trip fuel burn. So fuel burn per kg of payload improved by 10% just from the MTOW increase.
 
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seahawk
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Re: Can engines be too big?

Wed Feb 17, 2021 7:36 am

Consiering how the 787 is designed, relatively easy would be the engines and the MLG. The rest would probably be much more difficult.
 
RJMAZ
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Re: Can engines be too big?

Wed Feb 17, 2021 10:48 am

seahawk wrote:
Consiering how the 787 is designed, relatively easy would be the engines and the MLG. The rest would probably be much more difficult.

Actually the carbon parts can have more or less layers to make it stronger, heavier, lighter etc. It is probably easier to change layers than to modify a metal part. The 787-10 gained extra strength in the wing, the opposite could be easily done. The thickness and strength mostly depends on the loads. Reducing the MTOW reduced the loads on the wing and wingbox so these can safely have weight and layers removed. Reducing the payload and landing weight allows the landing gear to be lighter.

The fuselage tube could also be made lighter. It is designed to carry the high payload weight of the 787-10 and to transfer tail loads. A lower thrust and lower flying weight 787 could have a lighter tail as less force is needed to turn the lighter aircraft. Lower tail force means the fuselage is also transferring less load.
 
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seahawk
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Re: Can engines be too big?

Wed Feb 17, 2021 11:03 am

Those would just be totally new parts and in the end practically a new plane.
 
LH707330
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Re: Can engines be too big?

Thu Feb 18, 2021 4:32 am

What you're describing is basically the A330-100 idea: https://en.wikipedia.org/wiki/Airbus_A3 ... d_variants

Regarding the T500, they went up to 60k in thrust, which is similar to the highest-thrust CF6-80C2 engines on the 767, the heaviest of which is 204t at MTOW. Given takeoff drag is dominated by Di, a longer-winged A330 could be more efficient around 210t, but as we saw, the airlines figured that it made more sense to have a more flexible design with more range.
 
Sokes
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Re: Can engines be too big?

Thu Feb 18, 2021 5:42 am

RJMAZ wrote:
Actually the carbon parts can have more or less layers to make it stronger, heavier, lighter etc. It is probably easier to change layers than to modify a metal part. The 787-10 gained extra strength in the wing, the opposite could be easily done. The thickness and strength mostly depends on the loads. Reducing the MTOW reduced the loads on the wing and wingbox so these can safely have weight and layers removed.

A plane with a certain wing flies at a certain speed in a certain altitude. Suddenly there is turbulence.

How does the wing deal with the force?
-the wing can lift the plane
-the wing can bend

I believe the force is the same independent of weight. In which case there would be no need to strengthen the wing. What is the mistake in my thinking?
Why can't the world be a little bit more autistic?
 
Sokes
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Re: Can engines be too big?

Thu Feb 18, 2021 5:44 am

Do different MTOW versions have different landing gears?
Why can't the world be a little bit more autistic?
 
Sokes
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Re: Can engines be too big?

Thu Feb 18, 2021 5:49 am

LH707330 wrote:
...
a longer-winged A330 could be more efficient around 210t, but as we saw, the airlines figured that it made more sense to have a more flexible design with more range.

Or the T500 was simply not good enough.
Did the A320 neo improve only 3% in specific fuel consumption?

The A330-800 does suggest that an engine can be too big. Or maybe there are just too many, too cheap A330-200 around.
Why can't the world be a little bit more autistic?
 
RJMAZ
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Re: Can engines be too big?

Thu Feb 18, 2021 8:09 am

Sokes wrote:
A plane with a certain wing flies at a certain speed in a certain altitude. Suddenly there is turbulence.

How does the wing deal with the force?
-the wing can lift the plane
-the wing can bend

I believe the force is the same independent of weight. In which case there would be no need to strengthen the wing. What is the mistake in my thinking?

The wing is lifting up and the fuselage is pulling down. If the fuselage is heavier from extra structure or payload inside then more weight is pulling down. The wing has to provide more lift for it to fly level.

Turbulence is much harder to explain the loading. But an aircraft that is heavier will definitely have higher loading during turbulence. A heavier aircraft will also need more a powerful engine. The engine also has to pull the aircraft forward and the fuselage is pulling back due to drag. This force passing through the wing.

Sokes wrote:
Do different MTOW versions have different landing gears?

Yes in nearly every case. 777-200, 777-200ER and 777-200LR. And the 787-8 has a smaller landing gear than the 787-9. The A350-900 has a a four wheel MLG and the A350-1000 has a six wheel MLG.
 
LH707330
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Re: Can engines be too big?

Fri Feb 19, 2021 3:34 am

Sokes wrote:
LH707330 wrote:
...
a longer-winged A330 could be more efficient around 210t, but as we saw, the airlines figured that it made more sense to have a more flexible design with more range.

Or the T500 was simply not good enough.
Did the A320 neo improve only 3% in specific fuel consumption?

The A330-800 does suggest that an engine can be too big. Or maybe there are just too many, too cheap A330-200 around.

Back then fuel was cheap, so a 5% total gain at the platform level didn't pay for the dev cost and loss of flexibility.

Now, there's a mix of cheap 332 and more-capable 339 that make the 338 a non-starter. A cleansheet 338 engine would be smaller than the 339 engine, but there's not enough market to justify it.

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