Some further info in response to some posts:
Engine power: Remember that one of the basic laws of flight is that thrust = drag, lift equals weight for steady level flight. Thus, for the same aircraft type (hence same drag), thrust must also remain constant regardless of engine type. This gets complicated as their are different types of drag which are not linearly proportional to speed. Hence by increasing or decreasing speed there can be a disproportionate change in drag, and hence thrust required. Thus, while engine type fitted to an aircraft will not change its design speed (ie, a 747 design speed is a factor of its drag, and wing design, regardless of PW
engines), its economical cruising speed may vary. This will be to do with the most economical thrust output of the engines fitted. This is however, an operator function and not to do with aircraft design.
As an aside, different airlines operate not only different engines on the 747, but also prefer to fly at different speeds suject to ATC requirements. For example, SIA try to fly a 747 at between M0.84-85, while in the same airspace, BA
will fly M0.83-84. This is because the higher speed of SIA burns more fuel, but over time (years etc) allows higher utilisation of the airframe and less hours. BA
bean counters conversly have decided that fuel is proportionately more expensive than airframe hours/utilisation, so choose to fly slower where possible. It may also be a function of the engines fitted. I have witnessed this first had from the flight deck of a QF
744, flying SIN
. We were vertically separeated from a SIA and BA
744. The SIA plane flew faster than QF
, and the BA
slower. It is just the policy of the airline, and all calculations are worked out as "programs" for the pilots in the FMC. Obviosuly speed subject to other operational constraints such as ATC, weather etc.
Tavve: Regarding your question about aircraft length as a function of speed. Speed (Drag) is more a function of width. Simply put the greater the width of an aircraft the greater the drag, and hence the more powerful the engines required and less economical; resulting in a efficiency/practicality driven lowering in speed. To illustrate: The B767 and 757 are a common type, yet the 767 is wider than the 757. The reason that Beoing did not just shorten the 767 to reduce capacity is more to do with the fact that a narrow body (and hence lower capacity) was more efficient. This is why most aircraft manufactuers when developing new a/c prefer to lenghten the aircraft rather than widen it.
Hope this helps everyone.