Page 1 of 1

Clean sheet designs versus major upgrades of existing designs

Posted: Tue Mar 26, 2019 7:46 am
by SuperAzusa
Hi Airliners,net community!

Lately there's a lot of news about the 737 family and the A320..

and I was wondering how major upgrades of these classic aircraft (like 737 Max or A320 Neo)
compare to clean sheet designs like the MC-21, A220/C series, or the Embraer E models (which have some significant overlap in seating capacity). Not sure of the C919 is a clean sheet design or an A320 evolution

What are the pros and cons?

I would imagine the pros of an evolutionary step from a classic design are:
- existing support, relationships
- common parts
- less risk/cheaper

but pros of a clean sheet design
- not constrained by older influences from decades past?

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Tue Mar 26, 2019 8:27 am
by Starlionblue
I would add that there is a difference between "evolving" a type originally designed for modern operations, e.g. jetbridges and large engines, vs a type originally designed for small engines and no jetbridges. Furthermore, an FBW design is immensely easier to "evolve" since the control system can be adapted with a minimum of cutting metal.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Thu Mar 28, 2019 7:49 am
by JHwk
If it is still a very similar tube with wings, the clean sheet will always have business case challenges. The gains from evolution generally outpace revolution until you have a major step-change.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Thu Mar 28, 2019 6:49 pm
by WIederling
JHwk wrote:
If it is still a very similar tube with wings, the clean sheet will always have business case challenges. The gains from evolution generally outpace revolution until you have a major step-change.


Still at some point it is better to take your knowledge gained and design a better mouse trap.
Instead of layering on various new ideas that (un?)predictably show strong allergic reactions
to the old underpinnings.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Thu Mar 28, 2019 10:01 pm
by stratclub
Starlionblue wrote:
I would add that there is a difference between "evolving" a type originally designed for modern operations, e.g. jetbridges and large engines, vs a type originally designed for small engines and no jetbridges. Furthermore, an FBW design is immensely easier to "evolve" since the control system can be adapted with a minimum of cutting metal.

Very true. The 737 is becoming outdated, now all Boeing has to do is get customers to quite buying it. :biggrin:
But why fix something that isn't broken? The MCAS fiasco or many other problems can happen to a clean sheet design just as easily. Deffinatly, the MAX will be the last of the 737's because of some of it's technology that goes back to the 707/720 and the no jetbridges which made it low to the ground when it was first built.

Now to answer your question, the main reason of a derivative versus a clean sheet design is cost the downside is that sometimes part of the legacy design doesn't play well with the derivitive redesign elements.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Thu Mar 28, 2019 10:37 pm
by Starlionblue
The exact MCAS fiasco would not have happened on a clean sheet airliner because the engines wouldn't have been placed that far forward.

A clean sheet design can definitely have issues, but since aerodynamics are a primary driver one would assume the flight controls are proper from the start.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Fri Mar 29, 2019 12:24 am
by dakota123
Starlionblue wrote:
The exact MCAS fiasco would not have happened on a clean sheet airliner because the engines wouldn't have been placed that far forward.

A clean sheet design can definitely have issues, but since aerodynamics are a primary driver one would assume the flight controls are proper from the start.


Didn’t I read somewhere that tucking the engine up that tightly (necessary to get the maximum possible clearance of course) resulted in a drag reduction? Or am i misremembering?

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Fri Mar 29, 2019 12:53 am
by 1989worstyear
JHwk wrote:
If it is still a very similar tube with wings, the clean sheet will always have business case challenges. The gains from evolution generally outpace revolution until you have a major step-change.


Correct - and the last step change in the single aisle market was the A320 during 1987/88. I have my doubts we'll ever see another one.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Fri Mar 29, 2019 8:15 am
by WIederling
dakota123 wrote:
Didn’t I read somewhere that tucking the engine up that tightly (necessary to get the maximum possible clearance of course) resulted in a drag reduction? Or am i misremembering?


... In cruise flight attitude. ( i.e. lowish AoA ).
MCAS is there for HIGH AoA and the engine nacelles sticking out above the wing. ( as "viewed" by airflow :-)

Then this could just be corporate folklore.:-)

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Fri Mar 29, 2019 8:08 pm
by dakota123
WIederling wrote:
dakota123 wrote:
Didn’t I read somewhere that tucking the engine up that tightly (necessary to get the maximum possible clearance of course) resulted in a drag reduction? Or am i misremembering?


... In cruise flight attitude. ( i.e. lowish AoA ).
MCAS is there for HIGH AoA and the engine nacelles sticking out above the wing. ( as "viewed" by airflow :-)

Then this could just be corporate folklore.:-)


Yes, good point.

Re: Clean sheet designs versus major upgrades of existing designs

Posted: Sat Mar 30, 2019 4:18 pm
by LH707330
dakota123 wrote:
Starlionblue wrote:
The exact MCAS fiasco would not have happened on a clean sheet airliner because the engines wouldn't have been placed that far forward.

A clean sheet design can definitely have issues, but since aerodynamics are a primary driver one would assume the flight controls are proper from the start.


Didn’t I read somewhere that tucking the engine up that tightly (necessary to get the maximum possible clearance of course) resulted in a drag reduction? Or am i misremembering?

That was true for the switch from Jurassic to Classic, not sure about NG to Max.