3 D. .. .This post follows the 'air down the blade' theme of your thread but differs in the application of this air.. .. .Vibration is a problem to helicopters and a reasonable amount of research is currently being conducted in an attempt to reduce it. This effort is referred to as Higher Harmonic Control, Individual Blade Control and Active Blade Control. It appears that all or most approaches have a mechanical device move a tab or change the shape of the trailing edge.. .. .I, in ignorance, believe that the time required to activate the mechanical device, which will change the angle of attack, which will change the lift of the blade (and then return it all) Is TOO long for anything higher then the lowest harmonic(s). A pulsed discharge of air immediately changes the angle of attack without having to overcome the blade's inertia about it pitch axis. And in addition, nothing has to be done to return the angle of attack to is original attitude.. .. .rotorque has mentioned the probable limitation of being able to supply a sufficient amount of air.. .A tuned exhaust is used on two-stroke engines to increase the compression ratio. It is a simple concept and only requires the opening and closing of a single port. The frequency of the pulses is only dependent on the length of the tube. . .________. .. .I am very concerned about rotor-to-rotor vibration on my two intermeshing helicopter projects, so if you patent the idea, please give me rights of use. <img border="0" title="" alt="[Smile]" src="smile.gif" />