Scaled down LSR

[robfrey]

These guys are doing some remarkable things.
I used to be into RC cars very heavily through the early 90's.
I bet we could learn a thing or two about aero by participating in this stuff then applying what we learn to full scale.

http://www.youtube.com/watch?v=T31juxlQuGA&feature=related

http://www.youtube.com/watch?v=PLOGAvhyJ84&NR=1

[robfrey]

The benefits for doing stuff with RC is that it really speeds development time.
Many things can be tried within a few months of testing time. This is just not possible for Bonneville LSR as the season is so short not to mention 2000 miles away.

[Peter Jack]

Sure don't want to stand outside that circle track car!!!

Pete

[Rex Schimmer]

Rob,
Regretfully it is not easy to "scale" aero results from small models up to full size cars. Most of it has to do with the Reynolds number which is a function of air density, velocity and length. So on a small model the Rn can be very low due to its short length and therefore there is a good chance that the air flow over a scaled streamlined body could still be completely laminar where as on a streamliner that you are thinking about building only the very first part of the body will have laminar flow and the rest will be turbulent, and if you are lucky attached.

Most of the F1 teams have 50 and 60% wind tunnels and would go to 100% if they were allowed to. When you build a scale model, even a 50% model the details have to be extremely accurate. One of the reasons that CFD is gaining strength in F1. The JCB diesel car was completely designed with CFD and now tunnel work and I would think that that car probably has 400 mph potential.

Rex

[aircap]

What you're saying is that for best results in wind tunnel testing, use scale air.

[racergeo]

   Rob, just find a picture of Mickey Thompson's streamliner with the two Ford engines and model your car after it and you will have it!  Went over 400mph and had a lot more in it. I think son Danny has the car and I hope he does something with it. I doubt if anything is more effective then that shape for LSR.

[robfrey]

George,
What car was that? Challenger II?

[jimmy six]

Step up.$$$$$$$$$$$$$$$$..Danny is ready to go......................

[wobblywalrus]

What would happen if one of these little cars were dimpled and compared to a smooth little car?  This is an experiment that needs to be done. 

[RidgeRunner]

     Scale dimples required for valid results?

                                                          Ed

[Rex Schimmer]

Aircap,
What I am saying is that when using a scale model smaller models give less accurate information and need more scaling done to the results to be able to get anything that is meaningful. The bigger the better and full scale is probably best for LSR guys.

If you happen on a copy of the lates "Race Car Engineering" magazine, Feb, 2010,  there is an article by Simon McBeath about doing scale aero models and it looks like you could have $100,000+ into a "good" scale aero model before you ever put it in the tunnel.

Rex

[Viau Boys]

If you want to get accurate aerodynamic data from a scale model the reynolds number of the model must be the same as the reynolds number of the actual car ( or whatever funny shaped two wheeled thing you want to model).  To ensure this the model must be an exact scale replica of the original, even the smallest deviations can cause major disturbances in airflow. If you happen to have the rescources to make such a model you then must scale one of the other factors (velocity, viscosity, or density) of the renoylds number to make up for the smaller model.

For example: if you want know what happening at 200 mph and you build a 1/4 scale model you need to multiply one of the other factors by 4 or double 2 of them. This means that if you leave viscosity alone and had a tunnel capable of doubling the air density you would need to flow the air at 400 mph!!!

Of course there are ways around that, you could use a different fluid with a lower viscosity and lower the temperature which also lowers the viscosity. You could also take data at lower velocities which would not be as accurate but could still be useful.

Its just a question of time and money, kinda like everything else

[robfrey]

What I'm interested in test is:
1) Stability
2) Comparing "A" and "B" models with the same power plant and amp draw.


 Plan on getting accuracy of model body by getting them made from 3D printing. They should be very accurate.

Is this logical?

[Viau Boys]

IMHO yes it is. Stability is something that can often be determined at lower velocities, but that is not always the case. For instance this year at Speedweek our front end was a little squirrely from 100mph to 130ish but when we got into the 140 -145 range you could feel the air-dam start to work and from that point on the front end would be glued to the track. So if I were doing the test I would get the velocity as high as possible to test stability and also test it at a range of lower velocities to see if anything is changing. You can use the Reynolds number formula to see what the real life velocity of your test would be and how far short it is of your actual goal. The farther away it is the less confident you can be that your model will be stable at top speed, but you should at least be able to eliminate any major stability problems at a lower velocity.

As far as comparing two models that can defiantly be done at lower than ideal velocities, as Blue has mentioned in other threads you will see much of the separation on a car with a tuft test at 60 mph that you would at 200. So something like using smoke streams to see how the air flows around features on two different models can tell you with a high degree of confidence which is better.

In my only experience with 3D printing the machine was not capable of reproducing detail smaller than .040", I'm not real sure if that's normal for those machines or not, but either way it wouldn't be a problem unless you make the models small enough to where that error is leaving out important details.

[robfrey]

My intentions are to build the model 1/12 scale. This means it would be 48" long.





I think it to be too long.
I have another idea.