To: Nick Lappos
The gamma rigging change of 18 degrees is there to KEEP the cyclic in proper phase so that forward stick is forward tilt. That is what happens on the Robinson, and the S-76. Period. The disk tilts forward when the stick goes forward. Got that? Do you? Have you learned ANYTHING in the interminable posts that you have sparked??
Response:
Are you saying that when Frank Robinson determined that in order to keep the blades from breaking off and to get the helicopter to fly straight with forward cyclic he placed the cone hinges where he did? By placing the cone hinges in their present place he ended up with an 18-degree offset. That is one hell of an engineering job especially when you consider the helicopter was designed in his living room. Which came first, the chicken or the egg. Did he select a blade design and build the rotorhead to accommodate that design or, did he design the blade to accommodate the rotorhead with its’ 18-degree offset in order to get the disc to move down over the nose when pushing forward cyclic?
The stick does travel at an angle as you advance in speed, so when you get to cruise, it is a bit to the right. That is the STATIC TRIM position of the stick. Repeat that slowly, Lu. STATIC TRIM. That has NOTHING to do with which way the disk tilts when the stick is pushed forward. NOTHING TO DO WITH STICK PHASING. NOTHING. repeat that, Lu.
Response:
If I remember correctly the certification document for normal category rotorcraft states that disc movement must be in the same sense as cyclic input. They do allow a few degrees mismatch due to pitch coupling. That means that what you refer to as static trim means that if the stick is displaced to the right then the rotor disc is following suit. Now this may reflect the aerodynamic loading on the disc but in any case it ends up that the disc is moved in the same direction as cyclic displacement.
Maybe this is the case for the S-76 but I still believe until the test tells me different that the right displacement on the Robinson is to compensate for the 18-degree offset in the rotor system. Please don’t blow a gasket.
The delta 3 coupling of the rotor is on almost every tail rotor in existence. EVERY TAIL ROTOR. Repeat that, Lu. It is not some mysterious problem of the Robinson, Lu. It is not unusual, Lu. It does not make rotors fly off, or aircraft crash, Lu. All main rotors that have elastomeric bearings have some delta 3, Lu. ALL.
Response:
I am well aware of the mechanics and the reason behind the delta hinge effect on both tail rotors and main rotors. However I don’t believe I mentioned anything about pitch flap coupling on this thread. I would strongly suggest that you acquaint yourself with the design of the Robinson rotorhead both on the outside and the inside. My comments about rotor loss are based both on the tendency for the Robinson rotor system to demonstrate severe flapping loads with certain maneuvers and how those loads are reacted by the internal droop stops inside of the rotorhead. If a flapping blade contacts one of the stops the kinetic energy of the blade will turn the rotorhead into a lever forcing it down pivoting on the teeter hinge. This in itself can cause mast bumping or rotor incursion however, if in turning the head into a lever it causes the other blade to contact the static stop it will cause severe bending loads in that blade as well as the heavy bending loads in the first blade. All of this can cause mast bumping, rotor incursion or, loss of a blade all of, which will cause the helicopter to crash.
OK, what did we learn? 1) The static stick position does not relate to the control phasing. 2) The S-76 and the Robinson fly nicely with forward stick making the rotor tilt forward. 3) Lu is fishing around for some way to prove he is "right" about some crackpot theory that he discovered some biblical truth about Robinsons. 4) I rose to the bait.
Response:
Let’s say that you eventually get promoted to be the head of flight test. You are aware that the engineers have been developing a new design. I know that you would be involved in that design but for the sake of argument, you are not. You have a great deal of faith and respect for the engineering staff but one day they come to you and tell you that they have just run the prototype out into the hangar and the techs are correcting all of the manufacturing crabs. Then they tell you that it is not necessary to flight test the helicopter because engineering has thought of everything and that the helicopter will perform to spec. With your background and education will you accept the fact that your department will not be able to proof the helicopter to determine that in fact it will meet all of the specs?
I have a great deal of respect for you for your experience and education and maybe the engineers were correct in stating that no tests were required or that according to you my theories are incorrect. Well I want to prove that I am correct but I will also accept that I am wrong and the only way I can make that determination is to perform the test.
Please bear with me.