To: Crab
I know that Ray Prouty states that the nose will tuck under in one turn and pitch up in the opposite turn. I’m sure it is also stated in the FAA Rotorcraft Handbook but I was unable to locate the specific paragraphs. I’m afraid that until I can find that passage in the FAA handbook we will have to hold this conversation in abeyance.
Regarding trying to demonstrate this phenomenon on the Gazelle, Wessex and the Lynx. I am familiar with the Wessex control system as it is the same as the S-58. I know that the US Navy version, and the later US Army versions, had an Automatic Stabilization System, but normally, when control inputs were being made, the pilot would disengage the ASE and then reconnect it, when the new attitude and direction were reached, so that it would have no effect on canceling the tuck and/or pitch.
The Gazelle I know nothing about. On the Lynx you stated that there was an automatic stabilization system that cancelled the tendency to roll left or right depending upon which way you moved the cyclic. Is there a possibility that this system is restoring the attitude of the helicopter when it tries to tuck or, pitch up?
To: 212man
What I stated that the term low inertia rotor system applied to the minimal stored energy in the rotor system to carry through when you lose power. I also stated that the high rotational speed of a lightweight rotor system created stiffness of the rotor disc or, as you indicated, the polar moment of inertia. I would hazard a guess that with the light weight blade rotating at 500 RPM would reflect a tensile load at the blade attachment of several thousand pounds or, maybe more.
The feed back forces are aerodynamic and I totally agree. You made a statement about why they use small control inputs. You indicated it is that a small change in pitch will result in a much greater force due to the rotational speed of the blade and I agree with that. What we disagree on is the resultant force that changes the disc position. You say it is the individual blades that fly to their positions aerodynamically and, I state that the aerodynamic forces that occur across the disc are not the same, and this resultant force differential causes a perturbing force on the disc and gyroscopic precession causes the tip path plane to tip in the commanded direction.
Regarding the flaps used on the Sea Sprite can you imagine trying to fly a helicopter that is somewhere between a Blackhawk and a Seaking with out hydraulic boost? That has been the patented system used by Kaman since they made their first helicopter. Not having first hand knowledge of the Kaman system I would hazard a guess that the Sea Sprite hydraulic system is used mainly to retract and extend the landing gear plus the powering of several utility system. It is not used as a boost in the flight control system as in other helicopters. The control flaps generate the only aerodynamic feed back that the pilot might feel.
I am not familiar with the term Jack Stall. However it appears to be some sort of an abnormal problem in the control boost system. Tell me more about how it manifests itself and we can go from there. You stated the same things that I did in that most of the forces are due to Centrifugal Twisting Moment. However the present day larger helicopters use blades that are cambered on their upper surface. A blade with this feature is very unstable in that when it pitches down it wants to tuck under. If it is pitched up it wants to keep pitching. These oscillatory forces take place on each blade in its’ rotation and result in oscillating loads being resisted by the servos. This is allowed because the blade has a fixed pitch axis, which anchors the blade in position, and the pitch horn is attached to the swashplate by the pitch link and the swashplate is immobilized by the servos. Now, on the Apache the blades are attached to the rotorhead by flexible metal straps and the blade is attached to the swashplate like the previously discussed system. The blades on the Apache are also cambered on the upper surface and they too are unstable. Since the Apache blades are not rigidly anchored the rear strap reacts to the oscillatory pitching and tucking tendencies and as a result the strap is constantly being hammered against the lower portion of the rotorhead and subsequently has a high rate or replacement.
Again, I can’t address the Gazelle or the Lynx but the Wessex; if Westland did not change the design, used two constant pressure variable delivery pumps to power the two servo systems. I demonstrated the described pressure drop hundreds of times on dynamic simulators and on the actual H-34 during run-up practice. In fact, it is the standard means on Sikorsky Helicopters to rotate the cyclic as a means of detecting if you had a defective damper on the rotorhead. If on the other helicopters, they had an accumulator in the system, you would not get the pressure drop. In doing the demonstration, the cyclic would move faster than the “GYROSCOPIC PRECESSION” would take full effect. The rotor disc would only wobble.
Regarding your statement about the differences in national cultures being the cause of this problem. Just change “national cultures” into “technical approaches” and you have hit the nail on the head. That’s what this whole argument has been about. If a helicopter qualified exchange pilot came over from the UK and was attached to a US Navy Helo squadron or possibly to the US Airforce or US Army and he had to learn a new helicopter and new operational procedures he would not have a single problem. That is, unless he had to learn the aerodynamics of the new helicopter I would venture based on my experience on PPRune that he would be pulling his hair out. It is not like US pilots sit around all day and discuss aerodynamic and I would imagine that UK military pilots are the same. It is only in places like these threads that the arguments come forth.
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The Cat