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mac pinches
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speed and scale, all the time iv been in or around model cars this topic comes up every now and then and i have never got an answer, what happens to speed as the scale gets smaller? it came up again in the NSCC mag this month, a Mr Raynor stated that theFROUDE system said " the time scale is increased by a factor of the square root of the scale" so 1/32 scale would be something over 5 times as fast. can anyone educate me as i would love to get to the bottom of this one mac p
 

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Premium Member
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Hi mac p,

I just times my slot car speed by 32.

For example if my 1/32 scale slot car can lap a track 60 ft long in 5 sec, this equates to my slot car lapping at an average speed of 8.18 mph so in 1:1 8.18 x 32 = 261 mph,

60ft x 32 = 1920ft = 0.36 miles

1 divide by 0.36 = 2.75

2.75 x 5 seconds = 13.75 seconds per mile

1 hour = 3600 seconds divde by 13.75 = 261 mph

I think that's right but don't know if that answers the question
.

PS for scale realism fifties/sixties cars averaging say 100mph should be going round a 60ft track at an average 3.125mph.

Marlon.
 

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Scott Brownlee
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4,275 Posts
I don't see how Time can be scaled so speed can only be scaled up by the scale of the model (i.e. multiply by 32 for a 1/32 slot car).

No?
 

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I think the answer to this question depends on what you really mean by speed. The answer based on scale...(32 times the actual velocity)..makes sense in those terms. It is simply ratioing up the speed to allow for the smaller size. It's how fast a 1/32 scale observer would think the 1/32 scale car is going.

When you get into Froude numbers and such, something different is happening. There are ratios that are used in scale prototyping such that the actual EFFECT on the scale car is the same. So if you are using a wind tunnel to test a miniature flight surface, and you want the model to actually behave like the real 1:1, then what you do is pick the appropriate Standard Number and keep it constant for both the 1:1 and scale model. The Froude Number is a standard number which relates to Gravity and Inertia (it is actually Velocity**2/Length * gravitational constant). But in keeping this constant you are only modeling the Gravity and Inertial effects. If aerodynamic response is what you want to model then you might keep the Mach number constant for kinematic similarity and the Reynolds number for dynamic similarity and pressure loss.

One of the obvious problems with this approach, it that whereas it is a useful predictor of behavior it is impossible in most cases to model for all the various effects. You pretty much have to pick what you want to emulate and build a model that conforms to those criteria.

I hope this helps. I know this question comes up every once in a while. Since you mentioned the Froude number I thought I'd try to clarify it. If you have real interest in this stuff, look up Standard Numbers in a general engineering text like Mark's Standard Handbook.

cheers,
John
 

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I think Froude also appears in 'The Lord of the Rings', which is marginally more fun to read than an engineering handbook.
Or did I miss something here?
Does it mean my little 1/32 drivers are experiencing life at 32 times the speed I am? Some of them should have had their telegrams from the queen a long time ago in that case.
 
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