Airfoil Sections

The choice of an airfoil—or a series of airfoils—for the main rotor is another exercise in compromise. The ideal airfoil should simultaneously have a high maximum lift coefficient and a high drag divergence Mach number. A study of airfoil characteristics show that these two characteristics do not go together in any one airfoil. If the information in Chapter 6 does not provide enough guidance for this difficult decision, the blade designer will have to rely on the results of later analytical and experimental studies.

Tip Shape

Tip shapes other than square may be selected for a variety of reasons. The most common is the reduction of compressibility effects on the advancing tip at high forward speeds through the use of leading edge sweep. This, of course, is the same reason that jet transports use swept-back wings. There are two compressibility effects that have proved to be significant: the generation of high blade torsion and control loads through Mach tuck (discussed in Chapter 6) and the generation of noise by propagated shock waves. By sweeping the leading edge of the tip, both of these disturbing phenomena can be delayed to forward speeds above the helicopter’s normal speed range.

Another motivation for non-square blade tips is to reduce the blade loads and noise generated when a blade passes through, or close to, the concentrated tip vortex left by a preceding blade. If the tip vortex could be spread out by using a special tip shape, the argument goes, the subsequent interaction should be less violent. At this writing, this remains a reasonable but as yet unverified hypothesis.

Swept-back tips have yet another potential advantage: dynamic twist. At high forward speeds, most blades with twist carry a nonproductive downward load on

the advancing tip. If the tip is swept back, this download acts behind the structural axis and tends to twist the blade nose up, thus reducing the download and its aerodynamic penalty. On the retreating side, the upload on the swept tip twists the blade nose down and alleviates retreating blade stall. The effect also works in hover, where the upload increases the twist, which is beneficial for hover performance.

As with most concepts in helicopter aerodynamics, the use of a swept blade tip is not as straightforward as it might seem. Tests reported in reference 10.6 show a lag in the compressibility effects such that they peak after the blade enters the front half of the disc. This is where a straight blade naturally takes on a swept characteristic from the combination of rotational and forward speed—whereas the swept blade is being aerodynamically unswept by the same effects. Thus it is ■ possible that the swept tip could suffer more from compressibility than the straight one in this region. This possibility has not prevented designers from using swept tips. Figure 10.6 shows several used on contemporary helicopters.

Unfortunately, nothing comes free. Swept tips complicate the structural design of the blade, doubly so if they must be replaceable in the Field when damaged. The cost of designing, testing, and building these blades is significantly higher than for straight blades.