To turn, use a wing. To cruise, use an airbreather. To accelerate, use a
rocket.
On 2016-04-03 12:37, Henry Spencer wrote:
On Sun, 3 Apr 2016, Paul Mueller wrote:
So in a sense, a turbofan (moving away from turbojets now) is basically a ducted fan, driven by a smaller turbine engine. And in the case of big high-bypass turbofans, they are essentially big ducted fans.
Yes, pretty much so. The fan is essentially a specialized high-speed
propeller, powered by the turbojet-like core, running in a duct that
shapes the high-speed air flow. The duct is aerodynamically important
-- it's not just a housing for the fan -- because it lets you avoid
the big limitation of propellers: the dramatic drop in efficiency
when their blade tips go supersonic.
The resemblance to turboprops has recently gotten even stronger with
the first operational geared-fan engines, which have a gearbox between
the core and the fan so their rotational speeds can be optimized
separately. (The idea has come up before, but now they're actually
starting to enter airline service.)
Makes me wonder why smaller "traditional" ducted fans (like on airships or the E-fan aircraft) have much fewer blades, not a big blade disk like a turbofan.
Don't know for sure, but I would expect that it's because they're
optimized for much lower airspeeds.
At the opposite end of the speed spectrum, in engines optimized for
sustained supersonic speeds, the fan tends to disappear entirely.
(The prevalence of turbofans, albeit low-bypass ones, in fighters is
mainly because they mostly fly at subsonic speeds, with occasional
brief supersonic sprints.)
And if what you want is not efficient cruise, but rapid acceleration
-- people often don't appreciate that this is a radically different
class of mission -- then it becomes very appealing to add three or
four zeros to the density of the oxidizer by pouring LOX into a tank
in advance, rather than struggling to collect and compress passing
air. The cost is carrying the extra mass of the LOX. The benefits
are much easier engineering, and vastly lighter and more powerful
engines.
Henry
