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Cleaning up would pay off

23rd July 1971, Page 31
23rd July 1971
Page 31
Page 31, 23rd July 1971 — Cleaning up would pay off
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Which of the following most accurately describes the problem?

Keywords : Aerodynamics, Drag

TWO VEHICLES of very similar shape can have very different aerodynamic drag characteristics and relatively minor changes to the vehicle body can provide a substantial reduction in the power wasted in overcoming wind resistance. So inspired "tidying up" could well reduce the air drag of a van (or platform vehicle) by say 25 per cent or more, and if small changes were made to its shape at the front and rear the saving might amount to as much as 50 per cent in some cases.

Admittedly. this is guesswork but since the original draft of this article was prepared. a technician of the Motor Industry Research Association has stated (at a wind-tunnel seminar) that a big reduction in air drag could be made by simple modifications to the bodywork. A 50 per cent saving in drag at 70 mph could give a saving of 30 per cent in the power required to propel the vehicle.

In the absence of any quotable wind-tunnel data, it is enlightening to go back to an IMechE paper on "The design and operation of motorway coaches" presented at a meeting of the Institution in December in 1961 by Mr J. Pearson who was then development engineer of Midland Red. A graph in the article, showing theoretical drag losses in bhp of the Midland Red CM5 motorways coach at speeds up to 80 mph is reproduced here.

Mr Pearson said that the actual power required to operate the vehicle at 80 mph was 135 bhp whereas the calculated power, as shown, was around 189 bhp. No explanation could be given for the difference, and in the author's opinion the saving could be attributed to the relatively "clean lines" of the coach. The gross weight of the coach with 34 passengers was 9 tons 12 cwt. It is reasonable to assume that the drag figures of a 16-ton-gross van would in practice approximate to the calculated drag values of the CM5.

Reading from the graph it will be noted that air drag is 2 bhp at 10 mph and is only about 5 bhp at 30 mph. At 50 mph it is 26 bhp and increases to 48 bhp at 60 mph, 80 bhp at 70 mph and over 120 bhp at 80 mph.

Longer engine life Taking drag at 70 mph as a datum, a 25 per cent saving in air drag, would give a 15 per cent (20 bhp) saving in total drag. A higher overdrive top gear could then be used without sacrificing the vehicle's acceleration potential and this would augment the saving in fuel consumption provided and give a longer engine life.

Judging by wind-tunnel experiments with private cars, tidying-up and /or minor changes in front and rear end contours, can give a reduction in air drag of the order . mentioned and in some cases a lot more.

There is insufficient space to detail all the possible "minor ways" of reducing drag and, as mentioned, information is lacking on drag coefficients of typical commercial vehicles. But it is known that radiusing the corners of the cab and van body of an articulated vehicle can give a very worthwhile saving.

Number plates and mirrors that are virtually mounted in the air stream, bumpers that project forward from the main structure, name boards. that do not blend into the body and front ends with an irregular contour are bad offenders and even rain guttering can increase drag. Even blanking off a radiator can reduce drag.

The example was given some years ago in CM of an engine maker who was faced with the necessity of reducing the speed of a test vehicle of the platform type for evaluation purposes without increasing the load. A 6ft headboard was placed above the cab and -this reduced the speed of the vehicle from 62 mph to 48 mph. The saving in bhp is not known, but in the case of the CM5 it would have been around 45 bhp. Possibly, if the headboard had been blended into the cab contour, the air-drag loss would have been greatly reduced.

The impressionist sketches show a cleaned-up vehicle compared with a similar more conventional type with all the usual paraphernalia very much in evidence. In the author's opinion the indicated saving in bhp at 70 mph offered by the second vehicle (or something like it) is a conservative estimate. And there would be very little difference in cost, if the know-how were available.

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People: J. Pearson

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