next up previous
Next: Conclusion Up: Attempt at a Very Previous: Design Methodology

Results

Making the spar strong enough was easy enough. For simplicity, I kept the spar ply layout and thickness constant everywhere in the blade. (However, the breadth of the spar does vary because the blade thickness varies.) I found that, for many ply layouts, a spar thickness of 1.6 inches was sufficient to avoid failure. Generally, at least 25% of the plies were oriented at 0$^\circ$.

Sadly, trying to force the blade to twist into the desired shapes was hopeless. In fact, for almost every design I tried, the blades seemed to twist in the opposite direction. I attribute this behavior to the ``flimsiness'' of the skin behind the spar. Even when there was no coupling behavior in any of the three pieces (all 0$^\circ$ plies), the blades twisted. It was apparently due to the blades deforming directly under the loads. Making the skin thicker decreased the twist. Unfortunately, this ``flimsiness'' twisting was in the wrong direction.

So, not only did the blade structure have to overcome its own stiffness, it also had to overcome the tendency of the blades to deform directly under loads. It seemed that this situation pointed to a thick skin that has highly-coupled behavior. Unfortunately, the thickness of the skin was well over an inch before the difference in twist even had the correct sign.

Thus, I regret to report that, using the beam model by Rehnfield, optimal twist is not possible, or, at the very least, not easily found.

Figures 4 and 5 show some blade twisting results. These figures represent twist of blades that do not vary along its length, excepting the thickness of the blade. (That is, ply layouts of the spar and skin are the same, but the blade thickness does.) Figure 4 shows the twist distribution of a blade which tries to at least twist relatively in the right direction. The helicopter twist angle is much less than the propeller twist angle, but it should be greater. Figure 5 shows a blade that finally has a helicopter twist greater than the propeller twist. The skin on this blade is four inches thick.


  
Figure: Blade Twist Distribution. Spar layup is [0/(-70)$_3$/(20)$_3$/0], with a thickness of 1.6 inches. Top skin is [(-45)$_4$], thickness 0.08 inches. Bottom skin is [(45)$_4$], thickness 0.08 inches. (Obviously, in a real blade, there would be more and thinner plies than this, but these represent the ratio.)
\includegraphics[width=\textwidth]{graph1.eps}


  
Figure: Blade Twist Distribution. Spar layup is [0/(-70)$_3$/(20)$_3$/0], with a thickness of 1.6 inches. Top skin is [(-45)$_4$], thickness 4 inches. Bottom skin is [(45)$_4$], thickness 4 inches.
\includegraphics[width=\textwidth]{graph2.eps}


next up previous
Next: Conclusion Up: Attempt at a Very Previous: Design Methodology
Carl Banks
2000-05-04