To: Try_Cyclic
Pitch coupling, and correcting for the 18-degree offset are two separate things. The rotorhead design requires the pilot to compensate for the offset and the pitch coupling results when the cyclic stick is displaced. If you read the Certification requirements for normal category rotorcraft it states that the movement of the aircraft must be in the same sense as the movement of the cyclic stick. This must be proven through the use of a stick plotting board. The question I raised in my report was, “did they use the plotting board in certification”. The regulations state that a certain amount of pitch coupling is allowed and intimates that it should not exceed a degree or two. This is normal and I’m sure that the Robinson complies with the pitch coupling aspect but it does not comply with the aircraft and cyclic stick moving in the same sense.
The delta-3 effect is normal for every helicopter and was not put in the Robinson design for any special purpose. The delta effect happens when the blade pitch horn rises above the pivotal axis of the teeter hinge on a Bell and when it rises above the “coning hinge” of the Robinson. It also happens on Sikorsky, Boeing and any other helicopter when the pitch horn/pitch link attachment point is above the actual or imagined flapping hinge. I don’t know if the effect is as pronounced on a rigid or flex beam type rotor.
Regarding Robinson putting the correction in the controls this is not true. Aside from the differences in the actual control systems the swash plate on the Robinson moves in exactly the same way as on a Bell 206.
As far as divergence is concerned ask Robinson. As I stated, there have been at least 32 accidents on R22s and R44s where the rotor diverged from the plane of rotation and struck the fuselage. If you read FAA AD 95-26-O4 you will see that the R22s and R44s have been restricted from sideslipping and out of trim flight among other restrictions. It was proved that in these two flight regimes the flapping loads are so severe as to cause mast bumping. To do this the rotor blade(s) must diverge from the controlled position selected by the pilot and where he had his controls at the time.
Regarding the blade coupling effecting the phase angle I believe that you are wrong. Blade flapping (coning) around the hinge will result in pitch coupling and have some effect in which direction the helicopter flies but it has no effect in correcting for the offset. If in altering the direction of flight pitch coupling must be restricted to 2 degrees or less (approximately).
You ask if there are any accidents that would point to blade divergence read NTSB Report PB96-917003. It completely describes 31 rotor incursion accidents on the 22 and 44. Jim Hall says that there may be more. There was another in California in August where a 20,000-hour pilot lost his rotor.
The Robinson is a semi rigid rotor with individual flapping hinges. You can call them coning hinges but the blades do not flap about the teeter hinge they flap at the coning/flapping hinge.
To: Grey Area
The so-called delta hinge effect is…..(Read above, as I don’t want to repeat myself). Naturally there is no pitch couple in a hover, as it does no occur until the pilot introduces cyclic input. If there is pitch coupling in hover the result might be a slight drift of the helicopter.
The comparison I made between the Bell and Robinson heads was the positioning of the pitch horns in relation to the tilting of the swash plate. On a Bell with forward input on the cyclic the swash plate tilts down (on most Bells). The same is true for the Robinson. On the Bell the advancing blade is at its’ lowest pitch as the pitch horn is at the forward and lowest point on the swashplate. In the same situation the Robinson pitch horn has not reached the lowest position of the swash plate because it only leads the blade by 72 degrees (approximately) and as such it must travel an additional 18 degrees (approximately) until the blade reaches the lowest pitch. Aerodynamics and the laws of physics state that a rotating body will move 90 degrees in rotation as a result of an input or perturbing force that was applied 90 degrees previously. On the Bell the perturbing force was at its’ maximum when the blades were disposed laterally over the helicopter. On the Robinson the blades are disposed 18 degrees against rotation when the maximum perturbing force was applied. That is why the helicopter will fly to the left unless cyclic position is corrected.
If you read my report I ask several question relating to what happened during the testing phase prior to certification and to what extent Frank Robinson who was the DER had in approving the tests.
To: Helo Teacher
Your first statement you stated that it doesn’t matter what you do with the cyclic in a Zero-G situation, the cyclic controls nothing but the disc. I am not a pilot but I certainly know that what you do with the cyclic in a Zero-G situation can kill you if you don’t move it correctly and at what speed you move it. I know that the right roll is caused by the tail rotor. In the Robinson POH it states that in pulling back on the cyclic you do not want to add to the right roll and you can lose control of the helicopter and you don’t want to input any left roll as it will induce high flapping loads and result in mast bumping and subsequent loss of the rotor.
Regarding rigging and its’ effects, the report lays it all out. The rigging procedures are vague, misdirecting and totally wrong. Here is an example. The cyclic is rigged in its’ neutral position. A pitch reading is taken with the blade over the nose. The mechanic must adjust his controls to get a specific pitch setting. Or, is it specific, as he is given a range. He then moves the cyclic to the forward stop and he reads the pitch setting. He then moves the cyclic to the rear stop and measures the blade angle. If it is not correct he must adjust the same controls. He is told to check if the original setting has changed (and it will) but he is not told what to do to correct it.
He then reads the pitch settings on the rear blade and goes through the same procedure. Check for changes but not told what to do if there are changes. He is then told to average the pitch setting to come up with a final figure. Here is another point. The cyclic is rigged in the center of its’ F&A travel and slightly to the right of that line. If the cyclic is in the middle of F&A travel how come the pitch readings are different from the pitch on the forward and rear blades. With a fixed amount of travel the readings have to be the same. Another point, the certification requirements state the all helicopters have adjustable control stops. In this case the stops are adjusted to the control range. On the Robinson the stops are fixed and the control range is adjusted to the stops.
Why is this design different?
In the rigging procedure there are instruction that can lead to binding of the uni-ball in the swashplate and there is a possibility of rigging in too much pitch. This is compounded by the fact that the pitch angles are measured when the blades are over the lateral and F&A axes. In this position the pitch horns have not reached the maximum displacement of the swashplate and must travel an additional 18 degrees (approximately) before they reach maximum pitch reading. In the application of cyclic the pilot under these conditions can input an excessive amount of pitch resulting in rotor stall (not retreating blade stall)
I hope this covers it all.
[This message has been edited by Lu Zuckerman (edited 21 November 2000).]