Why do high end racing teams such as formula one (with deeper pockets than RC Heli manufacturing) & many other racing classes use spur gears within the drive train (straight cut gears) instead of helical gears?
The main reason is paradoxical to helicopter design & that is basically Spur gears have the best strength, weight & efficiency ratio over many other forms.
Helical gears are quieter & presumed can handle higher torque loads but the strength issue is questionable if using the same mass gear of equal module. The full surface area of a helical cut tooth is not in complete contact with the next tooth during engagement (gradual engagement) whereas a spur gear has instant full surface contact during mesh (hence the additional noise). Helical gears introduce greater thrust loads (the pinion wants to twist the main gear off) & heat into the equation in the form of friction (due to greater surface contact & the gears wanting to pull away or ride on top of each other) , decreasing efficiency. There has to be greater mass introduced into the gear, housing & bearing design to compensate for the additional thrust loading (cork screw effect of gears onto shaft, bearings & supports).
For our application it is far simpler lighter and stronger to go with straight cut gears than to try & reduce gear noise that is overpowered by a screaming two stroke with exhaust noise. The harmonics of the spur gear has no adverse affect or resonance within the air frame, this has been tried & tested.
It makes no logical scense to reduce performance with a helical main gear of the same thickness, module & mass as the existing spur gear in the Trex 700 drive train.
Pictured below formula1 internal gearbox