Tail Boom Strike with Main Rotor

[DiscoNomad]

Hello all,

 

As a noob in training I have read lots about tail boom strikes and the risks in certain situtations. I know it's possible in an R22 and R44 but it is possible in all helicopters with even ridgid and semi rigid rotor systems? How about an EC-130 for example? For example I have obviously never attempted this but is it possible to strike the tail with the main rotor while on the ground at operating rpm with collective all the way down and cyclic at full rear? I've heard horror stories of students panicing during a shallow decent with a running landing on the skids and getting the sensation of tipping forward and pulling the cyclic backwards too far and boom! Is the risk due to flapping of the rotor system or do the manufactures actually allow the rotor disc to tilt that far back?

 

Thanks and sorry for the ameture questions....

Christian

[Gomer Pylot]

Generally there should be no contact at operating rpm and normal disk loading. Tailboom strikes usually happen under one of two conditions: very low rotor rpm, where the blades can flex under heavy g-force changes, either from wind or sudden change in descent rate, or from unloading the rotor system, so that the rotor and/or the fuselage can change planes very quickly. Under normal flight conditions, it's not a problem, but rapid control movements can quickly get you out of normal flight conditions.

[iChris]

Generally there should be no contact at operating rpm and normal disk loading. Tailboom strikes usually happen under one of two conditions: very low rotor rpm, where the blades can flex under heavy g-force changes, either from wind or sudden change in descent rate, or from unloading the rotor system, so that the rotor and/or the fuselage can change planes very quickly. Under normal flight conditions, it's not a problem, but rapid control movements can quickly get you out of normal flight conditions.

 

Excellent analysis, those are the main factors. When you hear Tail Boom Strike these were likely involved. Broken-down:

 

1. Low rotor rpm

 

2. Unloading of rotor system

 

3. Abrupt cyclic inputs

 

Aside from the R22 Low-G thing, one of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract the nose-down pitch. This action in combination with the fact that the blades are slowing down and moving downward as the rotor unloads can result in the blades contacting the tail boom. An abrupt in-flight entry to an autorotation will also cause a nose-down pitch moment. A pilot overreacts with sudden abrupt aft cyclic (as the rotor unloads) will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades contacting the tail boom.

 

Edited January 3, 2011 by iChris

[C of G]

Another area of operation to consider is start up and shut down, again resulting from low centrifugal blade loading. It is not too uncommon for a blade strike to occur due to winds, turbulence or down wash effecting the slowly spinning disc. Offshore it has a great deal to do with the structure surrounding the helideck.

[DiscoNomad]

Thank you guys. Makes lots of sense now.