FAA mandates replacement of R22 & R44 main rotorblades

[blakmax]

Interesting!

Thanks Chopjock

I am intrigued. The previous designs had a relatively uniform chord with the additional TE section added by inserting a metal strip behind the uniform core profile to support the face sheets.

To change the chord length would either require a much larger and thicker strip to support the face sheets (which would add significant weight and centrifugal loading for that weight) or the core profile would have to step at the change in chord. There are two ways to change the core profile: 1. Machine the core block with a step and then expand the core and hope that the machining process provided a good match with the location of the chord change, or they could machine the core in two profiles and join them with core splice adhesive. Alternatively they could make the core in three sections, one for the smaller chord, one for the larger chord and a third block machined with the trailing edge angle incorporated so that the skin is supported in the transition zone. The three segments would then be joined by core splice adhesive.

Now if they didn't get the correlation of the change in core profile correctly aligned with the change in the actual chord change, OR if they didn't actually splice the core at all at the change in chord length, then either condition would lead to undesirable local stresses which could lead to cracking of the skins. In particular if the core splice adhesive was not applied, then this is a real problem. The core transfers shear loads, caused by the blade aerodynamic loading being distributed along the blade. If there is a disconnection such as caused by an absence of core splice or poor design of the transition in core shape then all the shear loads are dumped into the skins, and that would possibly cause a rapid increase in stress in the skins and could possibly initiate fatigue problems.

Alternatively if the core plan profile doesn't match the skin profile, then there will be some unsupported skin material which could experience local buffeting and again initiate fatigue. From a production perspective it would be very difficult to machine the core to the shape of the transition zone between the two different chord lengths.

I hope our Kiwi friends can enlighten us on the presence or absence of core splice adhesive at the step in the chord and the configuration of the core at the change in chord.

Regards

Blakmax