its also because of the friction in the bearings. when the prop is at extreme angles, the bearing in the swashplate will rub against the plastic / CNC casing. this doesnt cause a loss in power. however, it does require more power to keep the same blade speed. if this is the problem, putting a drop of grease or oil on the bearings can help... if its a headspeed problem, consider adjusting the trim on your radio and checking that your servos are properly working and making a complete, steady sweep.

in reply to calcop's post, u are right on what u said. but in order for the heli to loose that much lift, the blade has to be at an angle that overcomes the energy from the lift. there must be at least a 49:51 % ratio of energy for one to counteract the other... in otherwords, just because u move forward doesnt mean u loose lift, it just means the direction of the energy (in this case, moving air) is being transfered, creating forward thrust.