I got the following from Bell Engineering. Included below is the question that percipitated the answer below.
The nodal beam system was originally developed by Bell Helicopter in the
early 1970s. It was designed specifically to reduce/eliminate the inherent 2
per rev "bounce" (vibration) found in the Bell two bladed main rotor
helicopter designs. Most of the models designed and brought to production by
Bell during the late 1970s and early 1980s (206L, 214, 222) incorporated
nodalization of the main transmission to isolate the main rotor vibration
from the airframe.
For a good technical description of the nodalized pylon concept and its
theory, we refer you to the American Helicopter Society for a copy of a
paper entitled "Fuselage Nodalization" which was presented by several Bell
engineers at the 28th annual forum of the Society in 1972. Unfortunately a
copy of this paper is not available from Bell Helicopter.
-----Original Message-----
From: Lu Zuckerman [mailto:
[email protected]]
Sent: Monday, June 18, 2001 3:19 AM
To: Rodriguez, Leticia
Subject: Question
Dear Ms. Rodriguez,
I am a consulting engineer and when on assignment I sometimes
arrange with the local A&P school to teach a course in helicopter
aerodynamics. I once worked for Bell Helicopter International in Iran and it
was during my tenure as manager of technical assistance that the 214 was
introduced. This was the first time I had encountered the Nodamatic
suspension system. Technical literature on the system was limited but I was
able to figure out how it worked. However, I was never told why the system
was designed and why it was incorporated on the 214. I came to the
conclusion that the downward vertical movement of the transmission was due
to the decreased lift from the rotor system when the blades came into
alignment with the centerline of the helicopter in flight. As the blades
departed this position the lift increased and the blade began to lift
raising the transmission.
I have been laboring under this conclusion since 1976 and I really
don't know if I am being truthful when I discuss the flight characteristics
of a two blade rotor system with my students. Can you please have someone
contact me via email and set me straight.
With warm regards,
S L Zuckerman
RMS Engineering
[email protected] <mailto:
[email protected]>
Since I received the answer I submitted another question about the blades losing lift when aligned with the AC centerline. It does however seem that the so called 2 per rev was caused by this phenomenon. When I get the answer, I will post it.
To: Nick Lappos
You said,"The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue".
I have never been in a 206L or a 222 but I have flown in the 214. I had the opportunity to observe the transmission movement through an open inspection panel. In flight the ride was the smoothest I had ever had in a helicopter but with all this smoothness the transmission was going crazy.
It was moving up and down at least 2 inches or more at two times the rotor speed. To me the reason for this up and down movement had nothing to do with a vibration or coupled forces. The trannie was moving up and down due to variations in the lift of the rotor system. This is the reason for the 2 per rev.
If you have ever seen the blades on a 214 they look like barn doors when they spin up. They are capable of generating a lot of lift and at the same time any variation in airflow over the blades will result in the loss of lift resulting in the two per rev.
Regarding pylon rock, I made my comments on that in a post above. Check it out.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 June 2001).]