My new way of measuring on-road drag

[freebeard]

You must have read my mind...

Reads minds but can't read whole books?

Actually it was the pictures of the bandsaw and brake you already have that set my mind in motion.

[JulianEdgar]

Originally Posted by M_a_t_t

In "Throttle stop drag testing - including new design front air curtains" (a few posts above) you said a 8% decrease in drag with the edgarwit air curtain. What was the math behind that calculation? Is it only based on the speed change or did you need other factors to calculate that?

Have a look at the two videos shown in Post #1 of this thread. The first one is long (20 min) but it gives the needed background to understanding the technique. The second one shows the calculation you asked about.

[JulianEdgar]

Originally Posted by California98Civic

Those are great design and build standards in those arifoils, hey. Much respect. Enviable. Good luck testing.

Thank you.

The aerofoil is available off the shelf by the metre (yard). It's amazingly good value and all the aerodynamic specs of the GOE222 profile are also available.

From the point of view of those in the US, I've only been able to find the extrusion here - Oz Wind Engineering (Scroll down page.) Currently AUD$55 = US$35.

[JulianEdgar]

Originally Posted by freebeard

Reads minds but can't read whole books?

Actually it was the pictures of the bandsaw and brake you already have that set my mind in motion.

It's taken me all my life (I am now 56) to build up a home workshop, typically getting one major machine or tool every few years. It was made harder by moving through houses in three different Australian states (Australia is about as big as the continental US, so 'moving interstate' is similar here.) Most of the tools were bought secondhand and then I rebuilt them. The brake is a good example:



I was about to start building a new workshop, but unfortunately Coronavirus has knocked-out our training business (no face-to-face training now occurring) so the new workshop is now on hold. (But the upside is that I now get plenty of time to play with car aero.)

[M_a_t_t]

Answer: It is just a simplification of the aero drag formula. Since both runs should be in similar enough condition you can cancel all factors except Cd and Velocity (unless reason to suspect a change in area). The change in velocity then becomes the ratio that the Cd(or CdA) has changed.

After watching the first longer video I realize a few of my questions had been already answered in there.

[JulianEdgar]

Back to the topic of 'a new way of measuring on-road drag'.

Last night, the fourth and final professional aerodynamicist got back to me about the veracity of my technique. I don't want to name them because they haven't given me permission to do that, but if you look at who reviewed my book, or worked with me on it, you'll get a good idea. They are all top experts in the field.

All the aerodynamicists agreed that the throttle-stop approach to testing changes in drag was technically valid.

However, one aerodynamicist who gave feedback also made the point that any testing methodology where a change in speed is part of the test (as it is with my throttle stop technique), also has other factors introduced - eg a change in tyre diameter, so impacting ride height. However, I am happy that such impacts will be minor when we're talking only a small change in road speed.

He also said that my technique gives only a single data point - ie it gives a drag change at a certain average speed in specific wind conditions. He said what may give a drag reduction in one condition may not actually be the best for the conditions in which the car spends most of its life. I think that's a valid point, that can in part be addressed by doing the throttle-stop testing of a specific modification in lots of different atmospheric conditions before claiming that it works.

Based on my testing, the technique seems to me to be much more accurate than coastdown testing. It won't find tiny changes in drag (without very special equipment, I don't think they are actually measurable with any on-road technique) but those aerodynamic modifications that have the potential to make changes in mileage should be able to be seen. (And perhaps the combined impact of many small modifications may also be measurable.)

So, as far as I can see, based on the expert judgement, and on what I have found in my actual testing, we have a new way of testing for changes in on-road drag.

[JulianEdgar]

Originally Posted by M_a_t_t

Answer: It is just a simplification of the aero drag formula. Since both runs should be in similar enough condition you can cancel all factors except Cd and Velocity (unless reason to suspect a change in area). The change in velocity then becomes the ratio that the Cd(or CdA) has changed.

No, the ratio of the change in velocities squared becomes the proportional change in Cd.

(Maybe watch the second video?)

[M_a_t_t]

Originally Posted by JulianEdgar

No, the ratio of the change in velocities squared becomes the proportional change in Cd.

(Maybe watch the second video?)

I did that on paper figuring out what I said. It wasn't obvious re-reading what I posted.

104^2 * Cd original = 98^2 * Cd changed
Cd changed / Cd original = .8879 = -12.21%
(referencing windows up vs down in 2nd video)

I did watch the both videos (at 2x)

In case anyone was thinking like me at the beginning of this exercise, you can't use delta V because of the square function.

[JulianEdgar]

Edgarwit air curtains tested in final form. Throttle-stop drag testing and wool tuft testing. Also includes data on what proved to be the optimal inlet/outlet duct area ratio. My final figure on drag reduction (on still days, anyway) is 5 per cent.

https://www.youtube.com/watch?v=MaGg_8M8Us8

[freebeard]

The tuft testing suggests there probably isn't much point in adding air curtains to my Superbeetle.

An alternative to the aluminum extruded wing section might be sheet foamboard as used in model airplane wings.