THE AMAZING FORCES THAT MAKE HELICOPTERS FLY
EC135 rigid rotor system showing blades attached to mast flange, pitch change rods and driving link.
I have been working on helicopters for a long time.
Somewhere along the line, the wonder of this machine got pushed back into my subconscious. As a helicopter mechanic, I was trained by many manufacturers, in the technologies and physics of their helicopters. From Archimedes’ principle on fluid pressures to Bernoulli’s principle, I have been bestowed the secrets of our industry so that I could become the helicopter guru that I am today. OK, I guess I spread that on a little thick, but seriously, when I think back on it, I can see why many laymen are so amazed when they see a helicopter hovering for the first time — it is truly amazing! Many of us have heard these old helicopter adages:
1) If you pull the collective stick up, the houses get smaller, push it down and they get bigger.
2) Helicopters don’t fly, they beat the air into submission.
3) Helicopters don’t fly, they just vibrate against the earth and the earth rejects them into the air.
4) And the best one: Helicopters are really a bunch of parts flying in relatively close formation; all rotating around a different axis. Things work well until one of these parts break formation.
In this article, I want to talk about helicopter parts
and probably (in my opinion) the most important part of a helicopter that makes it a helicopter — the main
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HelicopterMaintenanceMagazine.com December 2019 | January 2020
TERRY PEED | CONTRIBUTING AUTHOR Bell 206 main rotor assembly.
rotor head and mast. From a mechanics point of view, there are other parts that are very important and can be very critical to the airworthiness of a helicopter. Sometimes the smallest part’s failure can be devastating, as adage #4 above suggests. While the very fact that a helicopter can lift itself up off of the ground and hover is amazing, I am totally in awe of the forces that the rotor head and mast are able to withstand and not disintegrate like a grenade. Keeping in mind that helicopter designers are trying to shave weight off of every component installed, engineers must design more strength into these components. Let’s just look at some of these forces at work on a helicopter rotor system.
THE MAIN ROTOR HEAD There are numerous variations of main rotor heads that are installed on helicopters but there are basically two designs: hinged and rigid. Bell has mostly designed its helicopters, such as the Huey, 206 Series, 212’s and 222 series around a hinged (or semi-rigid) head design. It later designed the laminated strap pack head for the Bell 407 Series which is more like a rigid rotor design. American Eurocopter (now Airbus) and Hughes Heli- copters (now MD Helicopters) opted for a rigid rotor system. They are all very efficient and handle the various forces they are exposed to in different ways. There is also a “fully articulating rotor head” system where the indi-
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