Lu: I agree with WNR - you just can't be a helicopter pilot..
OutsideLoop: Yup .. spot on, agree with most everything you say there! Same here 'uncountable' experience (25yrs+) .. some VERY extreme flight regimes (60kts+ side and back .. no bump yet but don't want to try any harder than I have (have taken paint but not undercoat off tail boom with blade) .. sustained 90kt+ cliff turbulence survived) still massive respect tho (you only die once).
But don't you agree that the Robbie machines rarely break by themselves - much more reliable than any other helicopter I can think of. (how often does an S76 fly without something not working? How much pain does a Bell 206 owner have to go through before he saves himself $400,000 dollars and flys more reliably and faster in an R44 (and the guys in the back get to join in too))
ArkRoyal: Should know better... really!
Lu: Pilots are amazingly sophiticated feedback processing units which many engineers have a hard time to understand. You just can't fly one of these machines by moving the controls in the theoretically required direction - it just does not work like that.
(you'd probably feel tempted to design a 'system' which sat in between pilot and machine to make fore and aft stick movement command an exactly fore and aft response to eliminate the (many) effects which make this 'not necessarily the case' in all sorts of machines)
Even if you were right about 72deg being wrong which you are NOT (see explanation below) 18deg mismatch is child's play - part of the process of learning is to calibrate and 'map' control input with response.
WHY 90 DEGREES PHASE ADVANCE IS WRONG:
In a gyroscope situation if you take the 'vector cross product' of an angular momentum vector and a couple perpendicular to the angular momentum vector you'll get a result which is perpendicular to both vectors. ie 90 DEGREES to where you thought you were pushing it. People think this is very strange and so they have great respect for this mystical gyroscopic effect - inventing all sorts of jargon (Gyroscopic Precession (90deg) or talk of 90deg 'phase lag' etc.)
The reality is that it is not at all strange nothing is 'lagging' anything and anyone can UNDERSTAND this, I think. Picture if you can (ie if my prose is capable of conveying it clearly enough) a bicycle wheel on a short axle arranged horizontally in front of you, spinning very fast (anti-clockwise(when viewed from above(never did understand what clockwise meant without specifying viewer position))) - no gravity so just 'hanging' in space. You take one finger and push the top axle stub away and another finger and pull the bottom axle stub towards you simultaneously - same magnitude of force. The rotating wheel does not change its plane of rotation in the direction which one might on casual inspection think intuitive (ie top axle stub away from you) but it moves with top axle stub going left. THIS IS THE POINT at which you must not run and hide behind a 'jargon term'. It makes sense like this: picture a point mass on the end of a spoke (part of the wheel) - think about how your axle pushing is affecting it as it rotates - when it is on the extreme left your action is neither pushing it up nor down - only when it is in the half closest to you are you pushing it up - when it gets to the extreme right then you are again not pushing it up any more - during the half cycle when it furthest from you, you are pushing it down. So during the WHOLE of the half cycle which is closest to you you are pushing it such that it will be higher than it was - since this is true for the whole of this half then it will be at its highest point at the extreme right (ie where you have finally stopped pushing it higher) So it LOOKS like it has moved 90 degrees after you thought it should have done if you had not actually thought about what you were doing! Well ... instead of saying all that, people just feel more comfortable saying "yea .. that's gyroscopic precession"
Well I hope that was clear because you kinda need it for this bit:
A helicopter is a bit like that... hence the '90 degree error' which Bell and Lu made.
FOR A HELICOPTER (EGGS GRANNYS SUCK TEACH ....SORRY etc):
The blades (spokes) go round and round and flap up and down however they want to - which results in the plane of rotation being almost exactly whatever it wants to be (regardless of rotor-masts and tail booms).
So we have to look at where blades WANT to flap to in order to work out what the plane of rotation is going to be. If you increase the Pitch on the half closest to you (same layout as the bicycle wheel) blades when in that half(ISH)will want to flap up to the equilibrium position for that degree of pitch - how fast they try and get their will depend on their difference between actual flapped position and equilibrium flapped position. (still with me anybody?)HERE'S THE KEY BIT. The problem is that near (BUT BEFORE) the end of this near half the blade will have reached an actual flapped position which is equal to the equilibrium flapped position ... so it won't really want to go up any more. ie. It will have got as high as it is going to go some number of degrees BEFORE it gets to the extreme right hand side. If you actually do want to have the blade be at its highest point at the extreme right hand side you are going to have to delay (by a number of DEGREES (... SAY 18! for a robbie)) where in the cycle the blade experiences extra pitch. THAT'S WHY 90 DEGREES IS WRONG!
Lu: The subtlety of a further aspect of this - which you entirely miss the point on is progress which Frank has made in the Robbie.
Lu writes:
Regarding the differences between theory and practice, the THEORY regarding gyroscopic precession and its’ relationship with rotor head design is not theory but has been reduced to engineering practice ever since the VS 300 took flight. In two blade rotor systems like Bell the pitch link leads the blade by 90 degrees [JOE: does not really matter that they have made this error]. On some other helicopters the swash plate command leads the desired reaction by 45 degrees and the pitch link leads by 45 degrees [JOE: now this is a positively BAD thing to do] which totals 90 degrees which is the phase angle of the blades. On the Robinson the blade phase angle is 90 degrees but the pitch link leads the blades by approximately 72 degrees and, the swash plate movement leads by 90 degrees which requires the pilot to adjust his cyclic by just a tad to compensate for the 18 degree difference.
JOE: This is a tragedy of missing the point.
The swashplate changes attitude with the (pendulous) helicopter, so, sure, you have to have the swash plate rigged 90deg phase 'advance' so that changes in aircraft attitude have the same directional effect as stick inputs (which would not happen with the 45 + 45 deg option) Since 90deg is not the phase advance you want for pitch inputs to the blades you MUST arrange for the correct advance angle to be above the swash plate - to do that you must arrange your pitch horn at the required angle back from 90 degrees. (having the disc respond to a/c attitude changes is important for stability/responsiveness - which you can influence by changing the length of arms to change the amount that the blade's pitch is changed for an amount of a/c attitude change)
So if you think Frank has not responded to you because you are 'getting close' think again ... He gets thousands of well intentioned sincere inputs like yours. He is not heartless at all - it is an awful burden and great personal sadness when someone dies in one of his helicopters - he would and does do anything he can to stop it happening - including building the most reliable helicopter and instilling in everyone he can a fear of screwing around with his 'envelope' - hence the flight manual warnings at the very least. If you go looking for the edge - don't be surprised if you find it!
(and as for the money thing - you really think that's what it's about for him? - He leads the most amazingly modest life you can believe ... under the circumstances)
Sorry to bore everyone
JOE
