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I have recently been reading about the issues with the Apache's (AH-64) strap packs. I've read that the strap pack holds the rotor blades on to the hub.

However, it appears the blade also connects to the pitch housing assembly. So I don't really understand the function of the strap pack and why it's failure would cause a catastrophic failure leading to loss of aircraft.

Questions:

  1. What is the Function of the Strap Pack?

  2. Why does its failure lead to loss of aircraft?

  3. Is this a common design on other helicopters?

Reference: Story

Apache Hub Assembly Pitch Housing and Strap Pack Image Source

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enter image description here

Picture above is from Helicopter Performance, Stability, and Control by Ray Prouty, and shows the fully articulated rotor such as installed in the Apache. The Fully Articulated term means that the blades can rotate in three Degrees of Freedom: fwd/aft, up/down, and nose up/down. The rotation axes are single points of failure: when they break the blade is launched, unbalancing the rotor and rendering flying and autorotation impossible.

The problem with the Apache setup is with eight nuts degrading due to corrosion, as from this link:

The order came after officials learned that bad weather and salt water in coastal regions could lead the eight “strap pack nuts” on each of the rotary blades to reach the end of their life expectancy sooner than projected.

Fully articulated hubs are the most common design in helicopters, the others being the teetering rotor such as in the Bell 212 and the hingeless rotor in for instance the Westland Lynx.

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  • $\begingroup$ So the "Strap Pack" is the 8 nuts that attach each blade to the 3DoF hing? I'm getting more understanding from your answer than I had from the question, but it's still leaving me at a loss. It doesn't feel like it really answers the question, but maybe I'm just not understanding the answer.... $\endgroup$ – FreeMan Feb 14 at 16:25
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    $\begingroup$ @FreeMan I don’t have a picture of the actual Apache hub, unfortunately, and had not heard of the term “Strap Pack” before. This article did seem to confirm though. $\endgroup$ – Koyovis Feb 14 at 22:49
  • $\begingroup$ The strap pack is the Y looking object in the bottom left of the figures in the question. It is a series of metal straps laid on top of each other. $\endgroup$ – OSUZorba Feb 21 at 3:56
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The strap pack is the structural member which transmits loads between the blades and the pitch housing assembly. Not shown in the drawing above is the shear pin that goes down through the hole in the pitch housing and through the holes in the strap pack members. The pitch housing ends at that junction and there is no other connection between the blades and the pitch housing.

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  • $\begingroup$ Could you re-word this a little bit or add a figure? I think I understand what you are saying, but not 100%. Is the strap pack what is transferring the radial load from the blade back to the hub and the pitch housing can't actually handle the radial load? $\endgroup$ – OSUZorba Feb 21 at 3:59
  • $\begingroup$ the strap pack is the go-between that transfers radial loads from the blades to the pitch housing through the shear pin.It's a way to mate the blades to the pitch housing; the pin joint is there to allow the blades to flap fore and aft. $\endgroup$ – niels nielsen Feb 21 at 16:39

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