To: Helmet Fire
In response to your comments;
1) It is true that the bell blades flap but unlike blades on other helicopters which flap up and down as individual elements the Bell blades flap in opposition to each other. One moving upward and its’ opposite number moving downward. The Bell blades contrary to your statement do not lead and lag as the underslinging of the rotor head minimizes if not totally eliminates the tendency to lead and lag. Bell and Robinson tail rotors do flap just like the Bell main rotor with this flapping action working in concert with the delta hinge equalizes the lift across the disc. On some larger tail rotors there is leading and lagging and it is allowed by the inplane flexing of the rotor blade or in some cases the tail rotor includes a lag hinge and the blade is restrained by a damper. The Bell and Robinson tail rotor blades do not lead and lag as you have indicated. On the early tail rotor used on the S-58 helicopter the blades literally tore themselves apart because of the tendency to lead and lag. The blade root was redesigned to allow some flexing thus reducing the flexing loads.
2) The 18-degree offset as I indicated is yet to be proven to have an effect in zero G recovery. Regarding mast bumping resulting from recovery from zero g on the Robinson the effect of left aft cyclic in the recovery will introduce very high flapping loads resulting in either mast bumping or rotor incursion or both.
3) See 2) above.
4) I agree, however on the Robinson it is strongly suggested that you do not move the cyclic left in recovering from a zero G situation because of the high flapping loads. See 2) above.
5) Flapping to equality does not enter into this discussion as it is mainly encountered during forward flight and can be countered with cyclic input.
6) Regarding Frank Robinson’s’ comment, I believe he was referring to me being discredited as an engineer and not as a mechanic (engineer). I never stated that I was not a licensed mechanic (engineer) as I obtained my Powerplant license in 1962 and I got my Airframe ticket in 1976 and have carried that combined license (A&P) since 1976.
7) Again I agree with you in your statement about avoidance of the situation. What I was alluding to was that the course tells you about these hazardous situations but they do not tell the students that these situations are the result of the rotorhead design. The rotorhead comprises a coning facility unlike the bell design. This was included to minimize the bending loads when the blades cone. This allows the blades to flap. In a recovery from zero G, the free flapping blades in a sense go wild which can have several effects. 1) They can allow rotor incursion. Or, 2) If the blade flaps down to the point of contacting the static stops (tusk) the energy of the blade can force the rotorhead down and it can make contact with the mast causing mast bumping. It is true that this can be avoided by proper piloting but as I pointed out there is a discrepancy between the teachings of the course instructors and there is a discrepancy between the teachings of one instructor and the POH. Another point is that the Robinson has low inertia blades, which can slow down under maneuvering conditions or mis-application of power. The lightness of the blades dictated the inclusion of cone hinges and all of this effects the way the Robinson helicopter is flown placing a higher burden on the pilot.
8) What I meant was that single rotor designs are susceptible to Zero G where multiple blades with offset are not. I was addressing the recovery from zero G and the resultant effects of application of left rear cyclic in the recovery.
9) Sounds reasonable to me.
10) I can’t fault you for some of the statements you made as you prefaced them with the fact that your experience is on Bell and not Robinson. If you get an opportunity read the POH for either the R22 or, the R44 (section 4) as it specifically states that in recovering from a zero G condition you carefully move the cyclic aft (TO LOAD THE ROTOR) and not to move it to the right which will add to the right roll induced by the tail rotor thrust nor, to move it to the left to counter the tail rotor induced roll which would result in high flapping loads resulting mast bumping.
Regarding the high safety record of Oz Robbies I would refer to the rotorhead used as a training device in another post. I would suggest that this head was removed because of the unbearable ride quality due to excessive lead lag loads at twice the rotational speed of the disc. If the pilots had not complained about the excessive inplane vibration they would eventually have been killed when they lost a blade or if the mast fatigued. A comparable situation is compressor stall on a turbine helicopter. It requires that the entire dynamic system and the driveline be inspected and in some cases all of it being replaced. If they only removed the rotor head and kept the blades and the mast there can be another catastrophe down the line.
Regarding the value of the course I agree but I go back to another post in which I stated that it is not what they do teach you it is what they don’t teach you that can cause problems.