quote from futaba USA-
The BLS451 is designed by Futaba for use in Nitro or Electric cars, they are not precise enough for the demands of heli flight.
The only brushless servo we offer at the moment for heli use is the BLS251.
Product Development & Support Specialist
Futaba Service Center USA
if it were me i would give these a wide berth since your buying new. if i had 451's then i would use them but i would certainly not buy them. futaba used to say use them in helis then when the glitching issues were reported they said the above and subsequently they removed helis as a suitable application from their website.
i run my 8915's at 6v and they are fine fyi.