This article was written for the 25th anniversary issue of R&WI. It has been republished in celebration of our 50th anniversary.
The laws of physics that govern aerodynamics do not change. The complicated aerodynamics that applied to helicopters in 1967, when R&WI began publication, still apply. Our understanding of them is somewhat improved — but not yet perfect.
Three fields of endeavor have helped make us smarter since then. The first, a fairly new one, is computational fluid dynamics (CFD). The second is wind tunnel testing, and the third, that only standby, flight-testing.
Today’s computer power has given the theoretical aerodynamicist the tool he’d long dreamed of to use on problems that previously were too complicated to even contemplate. He can now program the very basic equations governing fluid dynamics almost down to the molecular level. He can also quickly sole the equations without making many of the assumptions that had previously been needed to make the old computational methods practical.
Currently this capability is used to design new rotor-blade airfoil sections that can go to high angles of attack before stalling and to high Mach numbers before suffering from compressibility effects. High-speed computers allow the designer to account for the complicated interactions between shock waves and boundary layers that affect these characteristics.
The computer has also found apace in studying one of the helicopter aerodynamicist’s biggest challenges: determining the velocity distribution in and round a rotor. This information is needed to calculate the local blade element angle of attack vis-à-vis the element’s position along the blade and around the azimuth. But knowing the local angle of attack, we can estimate blade loads, and the rotor’s performance at high forward speeds and/or when developing high load factors.
This information is also essential to make good noise estimates from a rotor in…