PPRuNe Forums - View Single Post - Certification of Robinson Helicopters (incl post by Frank Robinson)
Old 29th Nov 2000, 06:02
  #175 (permalink)  
Lu Zuckerman
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To: Joe Pilot (Post 1)

The tendency to "flap back" is compensated for by the individual blade flapping up and the delta hinge effect on the pitchlink takes pitch out of the blade compensating for the increased lift on the advancing side.

This is very true if you are flying an autogyro and not a helicopter. The entire theory of flapping up of the advancing blade is based on autogyro aerodynamics. What causes flapback is the increased lift on the advancing side of the blade causing a lift differential across the disc and gyroscopic precession causes the so called flap back. It is a very mild case of retreating blade stall. Only in this case the retreating blade is not in the stall range it is just generating less lift. The pilot counters this condition by restoring the position of the disc by pushing forward cyclic or, if the swash plate remains in the original position and the pilot does not correct, the delta hinge effect will pull pitch out of the blade on one side and increase the pitch on the other side and the gyroscopic precession (if I worked this out in my head correctly) will cause the helicopter to roll left.

Try to picture this. The pilot pushes forward cyclic, the swash plate tips in the required direction. The advancing blade is a part of a rotating disc and as such has the characteristics of a gyroscope. When the pilot pushes forward cyclic the disc due to precession will tip down over the nose and the other side of the disc will rise over the tail. To use the term flapping the forward part of the disc has flapped down and the rear part of the disc has flapped up. Now, the aerodynamicists tell you that due to the relative wind the advancing blade flaps up and the retreating blade flaps down. In a scenario like that, the helicopter would be flying backwards.

Using my description, the disc flaps up over the nose and down over the tail but when it starts the pilot corrects for the condition by pushing forward cyclic. The flapping is minimal but, if you read the text books you would be lead to believe that the flapping is extensive.

Once again they have taken a movie film that shows the entire process and have selected one of thousands of frames and tell you that this is what is happening.

In your first post you indicated that pulling the knob up will cause the elastic cords to increase the right force. The elastic bands are on the F&A cyclic. This was corrected in your second post.

To: Joe Pilot (post 2)

You stated that pulling up on the non anchored end of the spring assembly basically gives it an "arm" in the lateral direction, whereas in the lower position it has no "arm" therefore ,no effect.

The operative word(s) are non anchored end. Form that you can assume that there is an anchored end. From the pictures in the maintenance manual it is difficult to see how the end attached to the knob can move upward if the other end is anchored. The shaft that is attached to the knob is a first class lever which pivots in a bearing that allows shaft movement. I believe what you said about applyinf a force or removing a force but, my question is how it does it if one end of the first class lever arm is capable of beinfg displaced upward when the opposite end of the shaft is anchored. That is why I asked if some sharp mechanic could explain how it works.

NOTE: DISREGARD MY COMMENTS ABOUT THE RIGHT TRIM ASSEMBLY. IN LOOKING AT THE PICTURES IN THE MM I CAN NOW SEE THAT IT IS NOT A FIRST CLASS LEVER. IT IS A THIRD CLASS LEVER. WHEN THE PILOT PULLS UP ON THE KNOB IT RAISES THE A 177 PIVOT, WHICH PIVOTS AT THE A 176-1 YOKE. THIS RAISES THE CYCLIC STICK. THIS ALSO CHANGES THE ANGULAR RELATIONSHIP BETWEEN THE A581-1 ARM AND THE SPRING PACK SO THAT WHEN THE PILOT MOVES THE STICK TO THE LEFT IT COMPRESSES THE SPRING AND PROVIDES A RESISTANCE. WHEN HE MOVES THE CYCLIC TO THE RIGHT THE SPRING RELAXES THUS REDUCING THE RESISTANCE.

NOW, WE GET BACK TO THE EXPLANATION PROVIDED IN THE MM ABOUT THE PURPOSE OF THE RIGHT TRIM ASSEMBLY. IT STATES THAT IT REMOVES THE LEFT STICK FORCES IN CRUISE BUT, IF I GUESSED RIGHT ABOUT HOW THE UNIT WORKS, THE SPRING INCREASES THE LEFT STICK FORCE THAT THE PILOT MUST OVERCOME TO DISPLACE THE STICK TO THE LEFT. AM I RIGHT OR WRONG?

To:212man

Not all helicopters require compensating cyclic to overcome lateral drift caused by the propeller effect. Many Sikorsky helicopters, at least those that I am most familiar with have this built into the mixing unit. Sikorsky also tilts their transmissions forward by three degrees (again those that I am familiar with). That is why a Sikorsky hangs down at the tail and leans to the left. If I remember correctly they land left, tail right. Or, maybe tail, left and then right.

Regarding blade flap please see above.

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The Cat

[This message has been edited by Lu Zuckerman (edited 29 November 2000).]