Pinewood Derby Stories and Photos from Maximum Velocity
When I first got involved in pinewood derby racing, I bought every speed tip booklet on the market (not very many at the time). One of the booklets I bought, "What Every Parent Should Know about Pinewood Derby Cars and More" by Thad Marvin, stood out because of its focus on aerodynamics (and its trivial treatment of other factors such as lubrication and weighting. Even though I was somewhat inexperienced, I was skeptical that aerodynamics was truly that significant.
One of the techniques touted in the booklet to improve aerodynamics was the implementation of turbulators. Probably like most of you, the word turbulator sounded like a made-up word to me. But I looked it up, and in a dictionary of technical terms, a turbulator is a device for improving air flow over a surface.
As air flows over a surface (such as an aircraft wing or propeller), the air wants to stick to the surface. As it leaves the rear of the wing, the air becomes turbulent, creating drag. However, if the air can be made turbulent before it reaches the rear of the wing, the drag is greatly reduced. So a ridge (a turbulator) is added to the wing to break up the airflow, creating turbulence before the rear of the wing. A similar concept is used on golf balls (the dimples).
Turbulator on a Model Helicopter Rotor Source: www.rcgroups.com (Note the ridge above the warning sticker)
Generally, turbulators are used where air speeds are high, and weight is low, such as on a glider wing or a propeller. So, given the lower speeds of a pinewood derby car, it seems questionable that tailing turbulence is strong enough to affect the speed of the car. But let's find out.
EXPERIMENT SETUP Instead of using a strip like on the helicopter rotor, I used the technique described in Thad Marvin's booklet. The turbulators are implemented as a row of pins along the high point of the car.
I used a more traditional car-shaped body, as it would likely have more aerodynamic drag than a flat design. So, if turbulators help, they should provide more benefit on a fully bodied design than on a thin design.
The cars features were: - Five ounces with a one inch balance point - standard wheelbase - three wheels on ground, set to rail ride - RA Racer Wheels (less aerodynamic drag than full width wheels - Awana Speed Axles - Krytox 100 Lube
Ten heats were run with the turbulators. The pins were then pulled, and (to compensate for the weight of the pin) 0.015 ounces of weight were put into the underbody pocket (at the same relative location as the pins). Ten more heats were then run.
With the turbulators, the car ran a 2.503 average time with a standard deviation of .003. Without the turbulators, the car ran 2.504 with a standard deviation of .002. The one thousandth second difference is less than the standard deviation, so it is not significant. The test was re-ran with a similar result.
Aerodynamics is a factor in pinewood derby racing, but it is a lesser factor. So make sure to take care of the "big ticket" items such as lubrication, weighting, wheel and axle prep, and alignment. Then minimize the aerodynamic affect, not by turbulators, but by building a low-profile car.(1)
(1) For more information on Aerodynamics, see: Volume 3 Issue 9, "The Big Debates - Aerodynamics", and Volume 5, Issue 9, "Wind Tunnel Testing".