Why electric generators in turbojet engine always need reduction gearing? The rotor of the turbojet engine is rotating with average speeds from 15 000 to 150 000 rpms, depending on size. 
Why reduction gearing is used to drive the generator? Why it is not common, or probably inpractical, to use permament magnets mounted on the shaft and rotating with such speed, and stationary coils? This would probably save some lossed in reduction gears?
I think there is some fundamental obstacle to gearless solution, e.g. back-EMF rising sharply at higher RPMs. Correct me if I'm wrong. 
Thank you!
 A: The reduction gearing is necessary because electric generators have, in general, much lower revolutions per sec than turbojet engines. Direct generation of electric power by spinning magnets mounted on the shaft has at least 3 problems: (1) Heavy rotating magnets are an additional impediment to the air (gas) flow in the turbine and might also have some material reliability problems  (2) Permanent magnets lose their magnetism above a certain (Curie) temperature (3) The electric coils with insulating wires in (hot) walls have to be close to the rotating magnets which also might cause reliability problems.
A: A generator's AC output frequency is a small-integer multiple of the
rotation rate.  High rotation rates for generators are avoided because efficient magnetic materials (iron, basically) have AC
losses related to electrical conductivity (eddy currents).   At frequencies of
50 Hz-1 kHz they are efficient and make lightweight high-power-output
generators and motors.  From 1 kHz up to 20 kHz they are usable if
some care is taken in construction (iron in thin laminations
with insulation between sheets).   
At frequencies below 50 Hz, the usual issue is the effectiveness of the
copper wiring, which has to be made thin (but with insulation around it).
Thin wires are prone to breakage, and the insulation thickness starts
to crowd out the copper, which means the electrical resistance of the
copper (in a given winding throat) becomes too high for efficiency.
Traditionally, 400 Hz was deemed
the 'sweet spot' for aircraft transformers, motors, generators.
Permanent magnet rotors can be OK at higher frequency, but only in
conjunction with a soft magnetic material for the stator; the magnetic
field in the output coils MUST change, or no generation of current takes
place.  Iron-based magnetic materials are still the limiting factor.
Ferrite (magnetic oxides) can go to higher frequency without losses
due to electrical conduction, being nonconductive.   The magnetization
of ferrites is, however, inferior to that of iron.   Generators are also possible without soft magnetic materials, and with superconducting coils are efficient
and lightweight, but there are reliability issues with cooling.
