|
This article was first published in 2008.
|
In previous AutoSpeed stories we have covered in
detail the direct measurement of aerodynamic pressures (see the series starting
at
Undertrays, Spoilers & Bonnet Vents, Part 1 and we’ve also covered
airflow visualisation with wool tufting (see the series starting at
Aero Testing - Part 1).
But there are also other down to earth aero
testing techniques that can be used.
Airflow Meter Output
At higher
speeds, the power developed by the engine is used almost entirely to overcome
aerodynamic drag. If engine power is then measured in different aerodynamic
configurations, changes to drag can be seen.
An indication of the amount of engine power
required to overcome drag can be found by measuring the engine’s air consumption
at a constant speed on a level surface. This can most easily be carried out by
measuring the airflow meter output voltage or frequency by means of a multimeter
mounted in the cabin. (Note that while it may at first appear that a more
accurate result would be gained from measuring injector duty cycle, in practice
this figure jumps around a lot, even at a constant speed!)
A test was carried out on a Volkswagen Golf
cabriolet, measuring the airflow meter voltage output. Soft top cars have much
higher drag when the top is down, due to the separation (and so turbulence) that
then occurs. This graph shows the results of the tests, which were all done in
fifth gear at a constant 100 km/h.
As can be seen from the graph, the drag is lowest
with the roof and windows closed. The next lowest drag occurs with the roof open
but the windows up. Having the windows down but the roof up is next poorest and
the worse results are gained (as expected) with the roof open and the windows
down.
This testing
must obviously be done on a flat road on a windless day and with the speed held
absolutely constant. Higher speeds will show aerodynamic drag variations more
clearly.
Measuring
Lift and Downforce
The following
technique can be used to measure the lift or downforce acting on the car.
A linear potentiometer (“pot”) is attached to the
car such that the wiper arm of the pot is moved with the suspension travel. This
can be done using a lever pivoting from a front or rear sway bar, or with the
pot mounted parallel to a strut.
A multimeter is placed inside the cabin of the
car, reading off the pot’s resistance. As the car passes over bumps, the value
being read on the multimeter will constantly change but on a smooth road driven
at speed, a change in average ride height can be seen. For example, at high
speed, the average resistance of the pot might change from 500 to 600 ohms. To
calculate an accurate average reading, an averaging data logger is required.
(See
Real World Spoiler Development for an example of this
technique being employed.)
With an
average figure gained, and the car again stationary, an assistant can then be
used to press down the front or rear of the car’s body (or lift it) until a
similar reading is measured inside the car. The force applied by the assistant
will then show the magnitude of the lift (or downforce) that is occurring on
that axle. This type of test will be most effective on a car with soft
suspension.
If a wing is
placed at the rear end of a boot or fastback car’s hatch, the downforce
developed by it can be approximated by the following test. For the sake of
clarity, the test procedure is described only for a three-box sedan with a wing
mounted on the trailing edge of the boot.
Firstly, a spring is temporarily attached to the
boot hinge assembly so that the boot is held slightly open. This means that
closing the boot the last few centimetres extends the stiff spring. An assistant
is placed inside the car. He or she accurately describes the height of the wing
by lining it up with a mark made on the rear window. Obviously, the passenger’s
eye level must be held constant throughout the testing. Before testing
commences, the boot lid is closed against the spring tension, being shut as far
as it will go before it starts to compress the boot’s rubber seal. The
assistant notes this position, and a piece of masking tape is applied to the
rear window to show this ‘closed’ level.
The car is
then driven with the boot latched shut. At about 50 km/h, the boot is opened
using the remote release, allowing it to spring up a little. As the car is then
driven faster, the wing pushes down on the boot lid, closing it against the
spring tension. The minimum speed that holds the wing in its previously marked
‘boot closed’ position is then noted.
For example,
with a certain spring pre-load, the car might need to travel at 110 km/h before
the boot lid is pushed down to its ‘closed’ position. With the car then stopped,
the wing is loaded with weights until the boot lid reaches this same closed
position. The minimum weight placed on the wing that closes the boot lid to this
position is the downforce being exerted by the wing at that speed.
A test was
carried out in this way on a R32 Nissan Skyline GT-R. At 100 km/h the standard
wing exerted a measured downwards force of just under 6kg.
Top Speed
and Drag
The
calculation of a car’s approximate drag co-efficient can also be carried out. If
you know the amount of power that the car has available at the wheels in watts
(kilowatts x 1000), the car’s frontal area in square metres (~ height x width),
and the car’s top speed in metres per second (km/h x 0.278),
| Power at wheels |
| the equation used is: Cd = |
-------------------------------------------------------- |
| 0.6 x Area x Speed x Speed x
Speed |
For example, for an R32 Skyline GT-R
with a top speed of 255 km/h that becomes:
| 200,000 |
| Cd =
------------------------------------------------ = 0.40 |
| 0.6 x 2.35 x (70.9 x 70.9 x
70.9) |
The drag coefficient of the R32 GTR at the time of
its release was quoted at about 0.38.
Alternatively, if you know the power it takes to
maintain a certain speed, you can do the calculation without having to drive
very fast.
For a slightly modified EF Falcon, the power at
the wheels at 105 km/h is 13kW. This was ascertained by noting the instantaneous
fuel consumption shown on the trip computer and then dyno testing the car at the
same speed, measuring the power at the wheels when the instantaneous fuel
consumption read out was as occurred on the road at that speed. With a frontal
area of 2.7 square metres, the calculated Cd is 0.32. At the time of release,
the claimed Cd of the EF Falcon was 0.31.
While this equation is good fun to
play with, it has several limitations. It does not take into account rolling
resistance (only a small proportion of the total resistance at full speed) and
you also soon discover that altering a variable like engine power or speed makes
a dramatic difference to the calculated Cd value. So you cannot use this
equation to give a definitive figure - but it can give some interesting
results.
Conclusion
It’s easy to
assume that directly measuring aero data is impossible without a wind tunnel and
sophisticated gear. However, that is simply not the case.
|