We have seen Airbus and Boeing going all the way for composites on their A350 and 787, but when it comes to smaller planes like the Embraer E-jets or the Bombardier C-series, they choose to use aluminium, I'd like to know what are the reasons for this to happen, why is more cost-effective?
There are some all-composite business jets, like the newest Bombardier Learjet. The design choice is about trade-offs in manufacturing: supply chain, number to be built, available tools and workers, legal certification (old methods are considered proven so require far less testing data), existing techniques, cost/percentage of build flaws, market price of the end product, needed performance and lifespan.
Then there is a question of available qualified and certified after market mechanics for repair, inspection and maintenance. Downtime is required to repair minor damage like bird strikes and examine internal damage from loose carts or cargo that is a visible dent on metal but only looks like a scuff on composite but could be internal delamination. (Boeing had to consider and develop solutions for all of these external demands. Some solutions only made sense for large carriers with major maintenance divisions.)
For example, if a significant mistake is made on a lamination layer, the whole composite part is pretty much landfill garbage, while an aluminum panel or rib can be recycled and replaced as fairly low cost and effort.
On the other side, business jets like Lear generally have far different flight profiles over their life compared to commercial jets, operate at different cabin pressure, are more concerned with altitude and speed, may consider small airports a constraint, and pennies per seat mile is a much lower consideration. Cost of downtime for repair or inspection is a very different priority as well.
Then there are the aspects of fire, repainting (paint stripping specification), effect of fuel any that may leak into the structure, and of course weather certifications for lightning and icing conditions. For de-icing and anti-icing, aluminum can be easily warmed from the inside while plastic is a poor conductor and may need another method. For lightning protection the 787 has a metal cloth for the outer layer and electrical bonding between all the parts, and this adds weight and thickness; as a percentage, a widebody jet is less affected by the extra material than a narrow body (the circumference doubles but the cross section area is quadrupled). Not all private aircraft are required to have full lightning protection. If they will not be flying near such conditions, maybe just the fuel tanks will have static and lightning protection. Also, useful payload is far less of a concern for business jets than for regional carriers who often sell extra baggage capacity to freight or parcel carriers like UPS/FedEx.
The C-Series has a carbon wing box. The fuselage is aluminum but is a newer fairly exotic aluminum lithium alloy that is a bit lighter than 2024. The decision to go with aluminum for the fuselage was mostly development and manufacturing cost (a fuselage requires a massive autoclave and it's development is fairly high risk the first time around) and ease of repair (the fuselage takes the brunt of vehicle damage on the ramp, especially on smaller aircraft) compared to carbon.
Cost is probably the number one driver. They stuck with things like bleed air services, and wisely decided to avoid lithium ion batteries (Boeing had to use L-ion with the level of electric services). The C-Series is overall a more conservative design than the 787, and it still almost broke the company, which was forced to give the airplane to Airbus for free or go under.
The E-jet is more or less the old 190 with a complete systems state of the art revamp (and I believe a new airfoil) that make it more or less a smaller C-series system wise without the risks of composites development. An even cheaper choice, and a wise one on Embraer's part.
The C Series has carbon in highly stressed wing skins and spars, so you have to restrict your question to fuselages if you want to be accurate.
While carbon is theoretically lighter for a given load, in fuselages there is a minimum skin thickness able to resist damage from things like runway debris and hail. It turns out that a narrowbody fuselage is way overbuilt when using carbon, so the potential of lighter weight is not realized. The higher cost of carbon rules it out.
A widebody has higher loads from pressurization of the larger diameter fuselage as well as bending from greater length. Such a fuselage can be made lighter than in aluminum as the required strength is well above the minimum gauge needed for damage tolerance.
Widebodies typically fly long segments, where every kilo of takeoff weight is multiplied by the fuel required to complete the flight. The fuel fraction (fuel/takeoff wt) can be as high as 45% so every kilo of airframe weight matters. Short haul regional flights are much less sensitive, with fuel fractions below 20%. Over the life of the aircraft a kilo of airframe weight is worth several thousand dollars to a widebody.
So widebodies are able to realize a weight savings from use of carbon, and have a flight profile that can turn the weight savings into a cost savings. Narrowbodies can't realize a weight savings, and couldn't reduce operating cost enough to defray carbon cost even it they were lighter.