To Lu:
Welcome back Lu. Where do I start with all of your statements? How about this:
1. The Bell 2 bladed system DOES flap, lead and lag, despite the absence of cone hinges. It is designed to. Same as most tail rotors including Bell and Robinson.
2. ALL teetering head helicopters are subject to mast bumping when handled incorrectly – in utter isolation from the 18 degree offset.
3. The 18 degree offset is part of the ROUGHLY 90 degree phase lag that ALL helicopters have to contend with in designing their control system. It is IRRELEVANT in the context of mast bumping (but I know I am not going to convince you here!)
4. If you tried to roll hard left in a Bell 2 bladed system during a zero G situation you would also mast bump and adopt the glide path of a brick!! And it doesn’t have cone hinges or an 18 degree offset.
5. A good understanding of flapping to equality would help you understand these concepts (I sound like a broken record!)
6. Perhaps Frank was saying that your formal qualifications as a licensed aircraft (helicopter) engineer were discredited when you admitted to not having a licence, rather than discrediting you as an engineer per se. Depends how you read the grammar.
7. Lu Said: >>They tell you about low rotor RPM conditions and how to avoid them and they tell you to avoid zero G conditions and how to recover from them yet they do not address the basic design deficiencies of the helicopter that will allow you to get into these hazardous conditions. <<
Lu, you avoid zero G through piloting technique and avoiding turbulence if you can. Nothing to do with design. Nothing.
8. Lu said: >>Zero G is a given in a single rotor design so there is no problem there.<<
What are you talking about????
9. Here is how I believe the zero G situation works on a 2 bladed Bell. I don’t fly the R22/R44 so I cannot say for sure, but I cannot imagine how the Robinson system would behave any differently, cone hinges or not. As the aircraft approaches zero G, the aircraft can effectively be seen as weightless in relation to the rotor disc, i.e: the aircraft is no longer “hanging” from the rotor disc. This brings the tail rotor drift/roll into play because the fuselage is now free to roll, and so it does – it rolls right (due to the direction of tail rotor thrust) on American direction helicopters. The rotor disc, however, does not know the fuselage has rolled right, and stays (relatively) in the previous plane. Therefore, if we freeze the action at this point, the disc is displaced to the left of the fuselage, i.e.: the state that exists during the initiation of a fast left turn, only because of the zero G situation the fuselage does not “swing” into plane under the displaced disc. Accordingly, if you introduce left cyclic to counter the right fuselage roll (as would be natural) you are actually further displacing the disc to the left rather than countering the roll, the eventual result of which is to induce mast bumping and funerals.
10. Lu, you advocate the introduction of aft cyclic in the zero G situation. I wouldn’t in a Bell, and I suspect the same is true for the Robinson. Let’s revisit the zero G situation above in (9). If the zero G situation was induced by a rapid lowering of the collective or encountering a severe downdraught, it is likely that the fuselage will also pitch nose down due to the sudden lack of rotor downwash on the horizontal stabilizer. Again, as per above, whilst the fuselage pitches nose down, the disc remains in relatively the same plane so if we freeze the action, it resembles the initiation of a fast pitch up where the disc is displaced to the rear. Same as for the roll, an introduction of aft cyclic at this point will displace the disc even further aft rather than counter the pitch down, thus the eventual extreme result of which is to induce mast bumping and funerals.
This effect is not as noticeable as the rolling moment.
Due to the above, I would be very careful before advising the use of left or aft cyclic in zero G. One technique I have been taught is to “fly the disc” not the aircraft in this situation. During normal level flight, look up and note the position of the tip path plane in the windscreen, i.e. its “normal” position relative to the airframe. When encountering zero G, position the cyclic to move the disc to the “normal” position in the windscreen, thus maintaining the alignment between fuselage and rotor disc and avoiding mast bump. From here you can very gently reintroduce G and recover. I reiterate – I don’t fly Robbos so I do not know if this technique is valid for them.
Lu might like to know that the Australian accident rates with Robinson’s is reputedly much better than the USA’s, and mast bump accidents are virtually unknown. Frank once put this down to the higher minimum hours requirement of the Australian Instructor course (about 400ish I am told).
In reply to Vortex: Every professional (as opposed to private or recreational) helicopter pilot (as opposed to engineer) I know who has done the Robinson course has said it is excellent, and most agreed it is a “must”. Enjoy it as part of your professional development.
Edited due to some poor spelling and crap arguements (I mean discussions of course!
).
Edited again to include a bit about turbulence.
[ 27 October 2001: Message edited by: helmet fire ]
[ 27 October 2001: Message edited by: helmet fire ]