Adventures With Air
Adventures with air
TDC
Kevin Cameron
WHEN AIR-COOLED ENGINES WERE FIRST used in airplanes and motorcycles, the method of cooling was the same: Let the cylinders stick out in the breeze. Aircraft-engine cooling advanced from there. For air-cooled motorcycle engines, the past remains the future.
Back in the late 1960s, my friends and I were trying to race 250cc air-cooled Yamaha TDl-Bs and Cs on road courses. We of course had to have fairings (where else to put the numbers and stickers?), which we ordered from strange men who breathed acetone and MEK peroxide. These fairings were vaguely streamlined, because they had roundish noses and big smooth windscreens. Their lower parts consisted of two flat side flaps that acted as a big air scoop, directing air disturbed by the front wheel over the suffering engine. The problem was that (a) this air scoop wasn’t streamlining at all, but the reverse-a kind of parachute, and (b) these side flaps were often mounted so as to form a wedge, narrower at the front, wider at the back. Counter-intuitively, a wedge is not a streamlined shape.
The essence of streamlining is partly to push air aside to let a vehicle pass through it, and mainly to then gently put that air back exactly as it was before you approached. This way, you give that air none of your vehicle’s precious energy in the form of a large and turbulent wake. Fish and zeppelins do it right. Our wedge-mounted fairings and tall windscreens, however, pushed the air aside with an outward kick that made the airflow bulge out even wider than the vehicle itself, creating an oversized wake and added drag.
Cut to the present. Vintage racer Todd Henning decided to mount new fairings on his Honda 350s and 450s for this year’s Daytona. These fairings had hemispherical rounded noses that guided airflow around the thick part of the machine, then brought the flow parallel to the machine’s motion. No part of the shape kicked air outwards, wedge-fashion, to make the wake bigger than necessary. The rider’s forearms were faired over, as permitted by FIM rules since 1958. With these fairings, Henning’s bikes over-revved badly on the tallest gearing he had previously pulled at Daytona-he had to gear up a full tooth at the front. Because horsepower requirement rises as the cube of speed. this fairing shape was the equivalent of a 20 percent horsepower increase. Not bad for a bolt-on.
There was the predictable political controversy over the fairings. Anything that works that well can’t be legal! This was solved by Henning’s agreeing not to run the fairing lowers-but the drag reduction remained. Was this a breakthrough design, an insight reached through computer fluid dynamics studies running on teraflop supercomputers? No, just a fair copy of the fairing that every 1950s hopeful at the Isle of Man knew he had to have: Peel Fairings’ “Mountain Mile.”
Fairing lowers help less than uppers because they must operate in disturbed air, and because they are usually just crude cooling-air scoops. Radial aircraft engines of the late 1920s were regarded as unsuitable for high-speed aircraft because of the drag of their exposed cylinders, projecting into the airstream like petals of a giant daisy. Then came the Townend ring, a kind of smooth band that encircled the engine. This helped. Then came the NACA cowl, which reduced drag tremendously. It curved inward over the front of the engine, limiting the intake airflow to a circular hole, and continued smoothly backward to join the fuselage. A narrow gap was left open at the rear, through which heated air could rejoin the airstream. Later, this gap was controlled by movable cowl flaps-open at low speed, closed down as speed rose.
When engines gained power through development, they rejected more heat. Engineers added cooling-fin area, but it didn’t work very well. Then someone had a bright idea: Just because we blow a lot of air at finned cylinders does not guarantee that any of that air actually passes between the cooling fins. The air naturally takes the path of least resi stance-around, over and between the cylinders-and avoids the narrow, highpressure-drop pathway through the finspace. The bright idea was to make metal baffles that forced all the air through the fins, and let none of it pass between the cylinders. Instant success. Further, because this again reduced the amount of air that had to be scooped in at the front of the cowl, it also reduced drag. It costs less in drag to flow air around a shape than to flow it through internal paths full of obstructions.
Back to the motorcycle case. Our 1960s fairings were the equivalent of the Townend ring-smooth on the outside, but functionally just a high-drag air scoop. The next step was seen in only a few motorcycle fairings-limiting the amount of air scooped in for cooling, and directing that air more or less at the cylinder(s). Certain single-cylinder fairings, Harley’s bulging but effective 1969 Wixom/Cal Tech fairing, and Peter Williams’ excellent 1972 Norton fairing did it this way. Like the NACA cowl, they reduced air flowing through the vehicle, thereby reducing drag.
The next stage I have never seen on an air-cooled motorcycle-forcing all the cooling air taken into the fairing to flow through the fin space. That requires making baffles that fit closely and which supply air to all the finning on cylinder(s) and head. On modern, liquid-cooled bikes, radiators fit fairings pretty well, but lots of air still leaks upward through fork-leg openings, or between radiators and oil coolers. Drag! Cooling air drag arises because high-speed free-stream air hits the rad or fins and slows down. If it could be sped up again before leaving out the back, this source of drag would disappear. This, as I have said before in this column, requires placing the radiator or fins in a duct, so air can both enter and exit at high speed.
For all these reasons, fairing lowers are less effective than uppers, and leaving them off entirely often has little effect on top speed. That’s why I like to call them “covers.” That’s about all they are.
