[LMP]

Here we go

Apology accepted

The bottom line is, despite what everyone will say - air particles do not scale down.

There is no (discernible), and never will be, aerodynamic influence on a scale slot car.

The 'wing cars' that use aero for performance are most definitely not scale slot cars, but rather wings with wheels. As the name suggests..

Even then, they aren't wings (aerofoils), but spoilers, relying on an air stream acting on a centre of pressure rearward of the centre of gravity, creating a moment of which the resultant force will press the rear of the car to the ground. This is the same way a 1:10 touring car wing works - not through pressure differential by the changing speed of air particles on the surface of a form, but by the air simply blatting into a blunt object and pushing it out of the way - or in this case, a little harder down on the road. They are the proverbial 'stick your hand out of a car window' aerodynamic mechanism, and nothing more.

I'm going to leave now before somebody else comes along and tells you that it really does make a difference..

 

[LMP]

MansionHouse, do you have a picture of your crude wind tunnel?

Note that you cannot 'see' drag. Drag is a force, caused by low air pressure, existing in the space immediately behind a moving object.

What you may be seeing (although hard to imagine unless you are streaming smoke) is turbulent airflow, which although related, is not really the same thing.

Greg, what you are (trying to) describe is a venturi. Google 'wind tunnel design' and the first two images exemplify your words - note that you'll need a larger inlet than you do exhaust.

 

[LMP]

I learned from the best.

ps...... Venturi is a noun in the same way google is now a verb

 

[LMP]

Beat me to it Tony

the reason I asked about MH's crude wind tunnel in photos was to potentially suggest placing the car on a balance (or scales). The notion you would see the car being sucked down is laughable, but if there were any downforce being produced the balance would show it in numbers. At least, to the accuracy of the balance

Which of course, it would not, in my opinion

This is, as you point out, exactly how real wind tunnels work whether they are testing planes, cars or scale models. One difference is that most hang from an aerodynamically neutral 'stay', rather than sit on the floor. This is because of the effect the floor has on the airflow. This is also why all kosher vehicle wind tunnel testing incorporates a rolling road, to simulate that condition accurately.

Most vehicle aerodynamicist a would argue any scale smaller than 1:5 is useless for tunnel testing, maybe they're on to something...

 

[LMP]

With due respect Greg, you in turn do not know what I have seen or not seen. Neither did I say always, but 'most'.

Outside of the group which encompasses 'most' would be the scale model Nissan P35 Group C sports prototype, that I modelled and performed tunnel testing on as a basis for my dissertation, some years ago. In this case the model was mounted on a ground balance because the ground effect Venturi tunnels (careful) present on cars of this era has significant influence on the cL and cD figures in question, and I therefore needed to take ground effect, into effect.





I couldn't test with a rolling road, but together with my two colleagues set out a series of viable experiments with the ultimate goal of assessing the downforce produced by different configurations of rear wing. Before we fiddled with it, we got a cD within 7% of a book figure, which wasn't too shabby.





The smoke was for illustrative purpose only.

If you like, I can also post pictures of a voluntary work placement I undertook at Aerodyn, development centre for the United States bobsleigh team, and frequent NASCAR port-of-call just outside of Moorseville NC.

So, the scale of this R/C model you witnessed. Large, you say? Larger than perhaps 1:10? Maybe even closer to 1:5, as I eluded to..

 

[LMP]

QUOTE (300SLR @ 16 Sep 2015, 08:17) <{POST_SNAPBACK}>I'll explain why

Yes, that's true. Forces on a slot car are the real thing, but the point is that nobody should expect their slot car to have the same 'kind' of downforce, drag properties etc that a full size car does.

Another thing I don't think anybody has yet mentioned is that the airspeed that comes into play with a (scale) slot car is going to be too low to have much influence anyway. When we talk BSCRA, Eurosport etc obviously speeds increase to the point where, evidently, the 'real force' is enough makes a difference.

The fact remains any 'real forces' are at best, crude aero, analogous to sticking your hand out of the window as mentioned in my first reply. Slotcars are not much difference in size to hands, but they sure don't do 70mph..

 

[LMP]

Tow? Sigh.. Somehow I don't think the message is getting through.

I'd confidently state that scale slot cars don't push much more than 30mph. I'd say they might just touch about 20 on a normal sized club circuit. I'm in the privelaged position of racing digital, once per year, on a giant replica of the Le Mans circuit,without chicanes. There aren't many longer straights than the 90ft mulsanne, and it would take one to reach those kind of speeds in the first place.

The record for a Blue King might be one second, but I'd bet my bottom dollar that is not what Mansion has in mind with his crude wind tunnel experiments. Yes, aerodynamic forces occur on all objects. To argue that they don't, and that a wind up balsa aircraft (about the size of a 1/24) isn't subject to aero forces, is clearly nonsense. But the pro-aero arguments would benefit from a sense of perspective.

Do you get a tow in your road car at 30mph? How about 20mph. And they are 32 times bigger than one of our toys.

The tow mechanism is created by the following car effectively driving in a vacuum, by the way, and the reversal of flow has nothing to do with this. Flow reversal occurs when the flow remains (semi) attached behind an object, and a void of low pressure (present behind all moving vehicles) 'sucks' the air back in the direction it just came from. With considerable turbulence, I should add. It doesn't act as a 'helping hand' to push the car from the rear, which is what Mansion seems to be inferring

This is all largely irrelevant when you consider the above post to this, well reasoned and pragmatically put. You can derive whatever absolute conclusions that you want about minimal forces on 1/32 models, but they just don't matter in the real world.