Sorry, haven't been reading in a couple weeks so I had a bit of catching up to do. I hope you'll bear with this long posting. Having been a CFI before/during/after the entire SFAR 73 debacle I think I have a few good things to add to the discussion.
[email protected]: What would be interesting is to know exactly why the flick roll to the right occurs.
The only explanation I can offer is that the TR is above the vertical centre of gravity in a robbie - most of the weight (engine, fuel, pax) is below the line of the tail boom/TR.
This is correct. When I first did the safety course in the mid 80-s Frank still taught some of it himself. He mentioned that R22 TR thrust could produce roll rates in excess of 100° per second, quote "faster than an F16 can roll".
Lonewolf_50 asked: In an event like that, is the correct response to load the head using collective before using cyclic to maneuver/roll? Never flown a Robinson, so I have no idea.
Robinson (and the Army's training video that was included as part of the SFAR-73 training) recommend gentle aft cyclic to regain gee forces, and then once gee forces are restored you can roll level. They have never advocated addition of collective pitch and I've never tried that - I've always used aft cyclic as recommended. It's worked so far.
[email protected] says: At zero G in a teetering head helicopter, you have no control power (ie how much you have to move the cyclic to affect the attitude of the fuselage). You can wave the cyclic around as much as you like but it won't affect the attitude of the fuselage.
At zero G you have no cyclic control power so therefore aft cyclic won't be effective and can make things worse (ie chopping off the tail) - the only thing that will save you is to restore positive G and that would have to be with the collective.
At 0.99 G you have a very small reduction (probably imperceptible) in control power so aft cyclic will immediately (if you have any forward speed) restore normal operation.
There is a big sliding scale between those bookends of performance - the lower the G, the lower the control power and somewhere (I don't know exactly) there will come a point where the control power is sufficiently low to render aft cyclic ineffective in restoring G (and therefore control power).
I'm not 100% certain, but I think this is likely incorrect. I don't disagree that at very low gee the fuselage will not respond because of the low thrust, but you are still in command of the rotor. Aft cyclic will load the rotor, and as it loads back up gee force will increase and you'll be back in command of the fuselage once again. My experience as an instructor teaching low gee right rolls for almost a decade before the FAA finally put a stop to it is that in a normal demonstration of a cyclic pushover resulting in a right roll, we would push until we would get a right rolling motion and then we would command aft cyclic. I never remember there being any delay, i.e. the aircraft would respond immediately to aft cyclic with gee force increasing. I suspect we were not very low gee but I have no way of knowing what percent of gee we would get the roll at.
I did have an experience with a student in an R22 where at the entry of autorotation he pushed the cyclic to the forward stop as hard as he could, i.e. it was a very quick application of full forward cyclic, along with the normal lowering of collective and application of right pedal. We ended up pretty close to 90° nose low (I know because we were above the end of the runway and the numbers of the runway were straight ahead in the windshield). Again, no way to know how low gee we actually achieved, but my guess would be pretty darn close to 0 gee.
I applied gentle aft cyclic and after what seemed like an extended time, the gee forces built up and the nose finally came up. The thing is, I'm not really sure that it
was an extended time, it's very possible that the adrenaline and time dilation just made it
seem like a long time. Certainly it worked fast enough that from 500 feet and 90° nose low, we avoided hitting the earth!
After we landed I spent some time thinking about why we were not dead and it occurred to me that it was because we pushed right pedal as we lowered the collective, i.e. there was no TR thrust to roll us (and indeed, there was no roll, just a violent nose pitch down). This is a long way of saying that even at extremely low gee the cyclic will in fact be effective. Whether the collective would work as you mention, I simply don't know.
Arrrj says: Second thing, the 66 is really powerful (3/4 fuel, 5 up, confined area, 30 degrees, up, up and away) and fast. 60% torque, 130 knot cruise.
This is an important point I believe. If you think about the arc you need to fly in order to reach 0.5 gee, it's dependent on speed. In an R22 it takes a pretty good push before you get the right roll, but R22s cruise at 85-90 knots. In an R44 doing 110 knots it takes a gentle application of forward stick to start feeling yourself get light in the seat. In an R66 at 130 knots, it would be even less of a push. So, the faster the aircraft you are flying, the easier it is to push enough forward cyclic to get low gee rolls. It's something I worry about soloing an R44 student versus an R22 student. In turbulence it pays to fly a little slow.
[email protected] says: As I said, it depends on how little G you have - at zero G aft cyclic will not have an effect other than to move the disc nearer to the tail boom - that will get people killed.
Please explain how using the collective will get people killed??
Again, I don't agree with you that at zero gee aft cyclic won't work. Perhaps it will take a little longer to work than at 0.5 gee, I'm not sure. As to whether collective would help or hurt, I don't know but my concern would be that adding power and adding pedal will just increase TR thrust and probably keep things at least as bad as they are, and that adding power and not adding pedal would put you out of trim and the resulting tuck probably isn't going to be helping the head->mast clearance situation. I'll be curious to have one of the test pilots comment on that.
This I very much agree with. I always train people that the first time you find yourself in a low gee right roll, you are very likely going to do the instinctive thing and push the cyclic to the left. Better is to associate the light feeling in the seat of your pants with the need to move the stick aft, i.e. load the rotor back up long before you get to a gee factor low enough to allow the right roll to develop. And also what everybody says: slow down.
Hughesy says: An addition to this. Neg G will not cause mast bumping. It's the incorrect pilot actions that causes it.
I don't think this is correct. I think that the TR alone will cause the mast to be bumped, pushing left cyclic probably just makes it happen more quickly. If Frank is right that the TR can cause roll rates of 100° per second, it won't take long to roll past the 12° of flapping clearance in the robby.
topradio asks: Has anybody survived mast bumping?
When I was training, about 20 years ago, there was a story about a chap who managed to land a 22 after a mast bump and survived. I don't know how true this was.
I seem to remember that a couple guys were out in an R22 Mariner (fixed float) when they did a practice autorotation at VNE and did a complete 360° tail-over-nose flip, i.e. Red Bull kinda move and that they bumped the mast but landed okay (and the VNE was then reduced to 95 knots).
Sorry for the long post, but obviously an interesting discussion for many of us. And, as to the design of the rotor... I've wondered for a long time why someone doesn't make a hingeless 2 bladed system? I like the idea of no low-gee restrictions but still being able to tuck a bunch of aircraft in a hangar skid-to-skid... (and I feel like the slow response of the fuselage in a 2 bladed system like the Robby or Bell makes it more difficult to teach hovering to a new student compared to an articulated system with some hinge offset).