Etl vs transverse flow
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"apologise unreservedly" - thank you
Sure friends now 
- we are after all brothers of the order of helicopter pilots !
[ ... and anyway I am sure someone else (please) will correct you with that swashplate orientation issue:
"Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right."]
...and i really mean it you'd love a session of ground school on PoF - it would genuinely enrich your life - promise, money back guarantee - and you can teach me how to spell (again)!!
- we are after all brothers of the order of helicopter pilots ![ ... and anyway I am sure someone else (please) will correct you with that swashplate orientation issue:
"Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right."]
...and i really mean it you'd love a session of ground school on PoF - it would genuinely enrich your life - promise, money back guarantee - and you can teach me how to spell (again)!!
Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right.
In order to obtain a disc forward attitude, the minimum pitch must be on the right hand side - this gives maximum rate of flapping down and a low point 90 degrees or so later (at the front in this case) This is phase lag.
In order to obtain a disc forward attitude, the minimum pitch must be on the right hand side - this gives maximum rate of flapping down and a low point 90 degrees or so later (at the front in this case) This is phase lag.
To be lowest at the front of its orbit, a blade needs to be at its lowest angle of attack about 90 degrees before, where the rate of flapping down will be greatest. So, when the blade's out to your 3 o'clock, pitch needs to be lowest.
However, pitch control to the blade comes from a horn and pitch change link setup leading the blade by (not quite) 90 degrees. Therefore, to give that maximum 'pitch down' command in the right spot, the swash plate will need to tilt forward, telegraphing its punch, so to speak, so the blade flaps down fastest out to the right and reaches its lowest point at the front prior to starting to travel up again.
A good old simple setup like a Bell 205 has the pitch command coming through the stab bar, leading the blade by exactly 90 degrees, whereas the Robbie is offset by 18 degrees or so, I believe, to create a good compromise between dissymmetry of lift and the effects of hinge offset (or some such thing that kind of makes sense when I read Frank Robinson's discussion of it, but drops out of my brain ASAP afterwards
)There is a good discussion of it somewhere in the PPrune archives that should settle this issue if cups of tea and good lie downs are had ...
Here's an excerpt of Frank Robinson's explanation of various R22 features from a post on PPrune - very good of him to take the time, I thought. The last line of the first bit clears up the swashplate tilt bizzo pretty clearly:
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
Last edited by Arm out the window; 6th Sep 2013 at 22:59. Reason: To add the second bit of Frank Robinson's text.
Aotw - you are completely correct and I know where the confusion between myself and AnFI has occurred:
On all the helos I have flown, all (bar the R22) have hydraulic jacks and are considerably bigger and heavier. As I said in my last post, they all use a combination of jack position and advance angle to ensure that max or min pitch is achieved 90 degrees before the desired high or low point.
Often, half of the angle is taken out with the jack positioning and half with the advance angle such that pure forward cyclic would tilt the swashplate to the half-past one position and the 45 degree advance angle obtained by putting the pitch change arm ahead pf the feathering axis would complete the job.
What I forgot, in my hazy recollection of the R22, is that the pitch change horn is mounted pretty much 90 degrees ahead of the feathering axis so that the swashplate does tilt forward with the cyclic.
This is the problem with arguing about helicopters since there are so many different designs which achieve the same effect in different ways.
Hopefully AnFI will agree that if there was no advance angle on a rotor system and the pitch change arm was exactly on the feathering axis, then to achieve a blade low at the front would require the swash plate to be tilted to the right - this was the picture in my head of a basic control orbit.
On all the helos I have flown, all (bar the R22) have hydraulic jacks and are considerably bigger and heavier. As I said in my last post, they all use a combination of jack position and advance angle to ensure that max or min pitch is achieved 90 degrees before the desired high or low point.
Often, half of the angle is taken out with the jack positioning and half with the advance angle such that pure forward cyclic would tilt the swashplate to the half-past one position and the 45 degree advance angle obtained by putting the pitch change arm ahead pf the feathering axis would complete the job.
What I forgot, in my hazy recollection of the R22, is that the pitch change horn is mounted pretty much 90 degrees ahead of the feathering axis so that the swashplate does tilt forward with the cyclic.
This is the problem with arguing about helicopters since there are so many different designs which achieve the same effect in different ways.
Hopefully AnFI will agree that if there was no advance angle on a rotor system and the pitch change arm was exactly on the feathering axis, then to achieve a blade low at the front would require the swash plate to be tilted to the right - this was the picture in my head of a basic control orbit.
