A 1:10-scale wind tunnel development program was undertaken by
the National Research Council of Canada and Airshield Inc. in 1994
to develop trailer side skirts that would reduce the aerodynamic
drag of single and tandem trailers. Additionally, a second wind
tunnel program was performed by the NRC to evaluate the fuel-saving
performance of boat-tail panels when used in conjunction with the
skirt-equipped single and tandem trailers.
Side skirts on tandem, 8.2-m-long trailers (all model dimensions
converted to full scale) were found to reduce the wind-averaged
drag coefficient at 105 km/h (65 mi/h) by 0.0758. The front pair of
skirts alone produced 75% of the total drag reduction from both
sets of skirts and the rear pair alone produced 40% of that from
both pairs. The sum of the drag reductions from front and rear
skirts separately was 115% of that when both sets were fitted,
suggesting an interaction between both. The reductions in
wind-averaged drag coefficient for single trailers with skirts only
were 0.0524 for the 14.6-m trailer and 0.0406 for the 16.2-m single
trailer. The data are quoted for skirt ground clearances of 305 mm
(12 in.).
The 0.50-m-long boat-tail panels combined with the
tandem-trailer skirts to increase the wind-averaged drag
coefficient reduction to a total of 0.1177. Boat-tails combined
with skirts on the single trailers also increased the wind-averaged
drag coefficient reductions to totals of 0.1051 and 0.0979 for the
14.6 m and 16.2 m trailers, respectively. Each boat-tail assembly
consisted of angled panels attached to the top and the side rear
edges of a trailer. The boat-tail panels on the single trailers and
the rear trailer of the tandem pair were set at the optimum angle
of 15°. The boat-tail panels on the front trailer of the tandem
pair were set at 0°.
Skirts were predicted to provide fuel savings for the tandem
trailers at 105 km/h of 3.79 liters per hundred kilometers (L/(100
km)) (1.61 US gallons per 100 miles - US gal/(100 mi)) and 2.18
L/(100 km) (0.93 US gal/(100 mi)) and of 1.95 L/(100 km) (0.83 US
gal/(100 mi)) for the 14.6-m and 16.2-m single trailers,
respectively. It is not known why the longer trailer had the
smaller fuel saving.
The addition of boat-tail panels increased the predicted fuel
savings for the skirt-equipped tandem trailers to 5.88 L/(100 km)
(2.50 US gal/(100 mi)) and increased the fuel savings of the 14.6 m
and 16.2 m single, skirt-equipped trailers to 5.25 L/(100 km) (2.23
US gal/(100 mi)) and to 4.89 L/(100 km) (2.08 US gal/(100 mi)),
respectively.
Track tests were undertaken using 8.2-m-long, skirt-equipped
tandem trailers to verify the fuel-saving predictions based on the
wind tunnel drag data. The boat-tails were not track tested. The
track tests were performed by the staff at the Goodyear Proving
Grounds in San Angelo, Texas. They demonstrated that skirts
provided fuel savings of from 0.86 L/(100 km) to 3.54 L/(100 km),
with the largest savings in the strongest side winds. The average
track-test fuel saving measured over all runs was 2.29 L/(100 km)
at an average road speed of 88 km/h. The average wind-tunnel-based
prediction for these track runs was 2.25 L/(100 km), using a
wind-averaged drag coefficient calculated for the wind conditions
of each run. This was within one percent of the average of the
track measurements. The wind speeds during the track tests were
generally lower than the national average 11.3 km/h wind at truck
mid height, averaging only 8 km/h. This indicated that the average
fuel saving on the track was less than would be expected annually
in North America.
The track-test results confirmed the fuel-saving potential of
skirts and demonstrated the reliability of fuel-saving predictions
made from high-quality wind tunnel data. This correlation was only
possible because the track-side winds were measured during each
run.