To: Mr. Frank Robinson
I welcome the opportunity to exchange Ideas with you.
The following paragraphs are keyed to the main paragraphs in you posting.
SIDESLIP WITH R22
You stated in your post that the inclusion of the restriction against sideslipping was in part due to a misunderstanding on the part of the FAA. This misunderstanding was based on the FAA’s misinterpretation of a US Army training film on mast bumping. You then went on by stating that helicopters in general have an adverse roll characteristic but not the Robinson unless it is equipped with floats
My comments regarding those statements are,
1) If the inclusion of the restrictions against sideslipping (and, possibly out of trim flight) were based on a mistake on the part of the FAA why hasn’t the Airworthiness Directive (AD 95-26-04) been modified to reflect a change in the attitude of the FAA by them admitting their mistake.
2) What does your explanation have to do with the high flapping angles and attendant loads? The restrictions were implemented to minimize the flapping angles and loads that could precipitate mast bumping and loss of the rotor or, rotor incursion.
3) You stated that you had no objection to the caution against extreme sideslip during forward flight. No where in the AD or in the respective POHs does it say anything about extreme sideslip. The POHs and the AD state,” Avoid sideslip during flight. Maintain in trim flight at all times”. Surely you must have objected to them not using the word extreme.
R22 FAA CERTIFICATION
Sooner than holding up this posting I will fill this section in when I get some information regards the certification.
R22 ROTOR SYSTEM
Your explanation places the cart in front of the horse. You stated that feathering occurs while the rotor disc is being tilted. The rotor disc is being tilted due to gyroscopic precession, which occurs due to cyclic input, which minimizes the pitch on the right blade and maximizes pitch on the left blade. Simply put, feathering causes tilt. Tilt does not cause feathering. The only feathering caused by tilt is that pitch change that results from pitch coupling. Or, the pitch coupling that results from flapping about the cone hinge and in the case of flapping about the cone hinge the pitch coupling will only occur when the pitch horn/pitch link attach point is not aligned with the cone hinge.
You further stated, “aerodynamic feathering is necessary to compensate for the difference in airspeed between the advancing blade and the retreating blades". Otherwise, you stated, the advancing blade would climb and the retreating blade would dive, and the rotor disc would tilt aft.
This would be true if you placed the helicopter on a flat bed truck and propelled the helicopter forward while maintaining the cyclic stick in the rigged neutral position. However I believe if the disc tended to flap back the pitch coupling effect would cause the disc to tip to the left. At least I think so.
NOTE: What I am about to say is more for the edification of the participants in this thread so that they can better understand my argument.
This how it happens. (All comments assume a no wind condition and we discount the effects of induced flow and transverse flow effects.
1) The pilot moves the cyclic stick forward from the rigged neutral position tilting the swashplate and changing the pitch on the left and right blades. The right blade decreases pitch and the left blade increases pitch. For the want of a better word, the blades have feathered.
2) At point 1) above the helicopter is in a hover and standing still. The feathering action described above causes an imbalance in the lift forces across the disc and this results in gyroscopic precession tilting the disc down over the nose
3) With the tilting of the disc there is a change in the thrust vector which causes the helicopter to move forward and now and only now do we have an advancing and retreating blade
4) It is true that the feathering compensates for the difference in airspeed across the disc, but it is the feathering and the resulting tilting of the disc that causes the lift differential. But, the advancing blade has decreased pitch decreasing its’ effective lift relative to the retreating blade which has increased pitch to reflect the decreased airflow due to forward flight thus equalizing the lift across the disc. As a result the helicopter is flying forward and there is no adverse roll effect.
You further stated that as the helicopter started to move forward that if the pilot did not correct the helicopter would roll to the right. What you described is the effect of transverse flow and this is common to all single rotor helicopters. Regarding the term “wee-wa”, I have been associated with helicopters since 1949 and have attended twenty helicopter factory schools and familiarization courses and I never heard the expression. That is not to say it doesn’t exist.
Elsewhere in you posting you indicated that the delta 3 angle of 18-degrees was selected as the best compromise angle to reduce the two undesirable characteristics described above which would have been present in the R22 had a 90-degree pitch link been used.
First of all, a 90-degree pitch link could never be used as to do so; you would have to eliminate the cone hinges. The fact that you have cone hinges dictates the requirement for a 72-degree pitch horn. If you had a 90-degree pitch horn the pitch coupling would be so dramatic as to render the helicopter uncontrollable.
The cone hinges were designed into the rotorhead to minimize flapping loads and to eliminate spanwise blade bending. Incorporation of these hinges dictated the offset of the rotating swashplate relative to the stationary swashplate. I can accept the fact that your designers selected the 18-degree offset and designed the rotorhead to reflect that selection. The 18-degree offset you indicated was selected to eliminate the effect of transverse flow effect.
If this is true why have so many of the Robinson pilots on this thread indicated that they roll in left cyclic to counter the effects of transverse flow?
You have stated that there is an 18-degree offset between the fixed and rotating elements of the swashplate. In the rigging procedure the blades are disposed laterally and longitudinally in respect to these two major axes just like on a Bell helicopter. The Bell has a 90-degree pitch horn so the pitch horn/pitch link are directly over the points of maximum deflection (Fore & aft and laterally). On the R22 with the blades in the same positions the pitch horn/pitch link are 18-degrees away from the points of maximum deflection. This raises several questions:
1) Because the blade must rotate the additional 18-degrees to get the pitch horn/pitch link over the points of maximum swashplate deflection the resultant gyroscopic precession would tilt the disc forward and to the left. Please explain in layman’s terms why the blades should tip down over the nose as opposed to tilting to the left when the cyclic stick is pushed forward.
2) When the blade angles are set with the blades disposed as indicated in 1) above is it possible that the pitch angles will change when the blades have traveled the additional 18-degrees on the tilted swashplate? Is it also possible that this increase or decrease in pitch angle could have an effect on blade stall under certain atmospheric conditions or at high-density altitudes? The rigging procedure requires that certain basic low collective and high cyclic angles be measured, then the collective range is checked and after that, autorotation checks are made which under most condition requires the addition of pitch. Now we add in the added pitch that results from that 18-degrees of travel on the swashplate. Is there a possibility that the rotor has too much pitch under the above stated conditions and the result is blade stall and low rotor RPM? In the rigging procedure the pitch angle is taken at the 75% span position. Have your designers ever considered what the reflected pitch would be at the root of the blade where on most helicopters the pitch readings are made. What with the negative twist of the blades and the station where the reading is taken the reflected pitch could be significantly higher. With the higher reflected pitch at the root where very little lift is created the drag can be quite significant. Possibly significant enough with the added pitch from the 18-degree offset to cause the blades to slow down.
3) In the rigging procedure the cyclic is rigged in its’ neutral position which is slightly to the right of the longitudinal center line and midway in its’ Fore & Aft travel range as dictated by the stop plate. If the movement Fore and Aft is the same, please explain how you can get different pitch angles Fore & Aft?
There are many other points I would like to raise but first I’ll give you time to digest my statements and return with your response.
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The Cat
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]