Hi,

I had a discussion with a friend about speed in scale. We both agreed to get the linear speed in scale the actual speed (lets say for a 1:35 vehicle) of the model should be 35 times slower than the real vehicle. So the real vehicle travels at 70 km/h, the model should travel at 2 km/h.

But here comes the problem, what about rotational speed of e.g. a radar dish? He said that you also have to make the radar turn slower (less rpm) than the real thing. I think not. If you take the path the outside tip the radar travels in 1 rotation you basically calculate the circumference of a circle. A circle with radius of 2,5 meters has a circumference of 15,7 meters. In scale the circle has a radius of 0,17 meters and a circumference of 0,44 meters which is exactly 1:35th of 15,7 meters. So if the real radar travels at 1 rpm the outside travels at 15,7 m/s, the scale radar should travel 0,44m/s which is also 1 rpm

Am I right here?

Matthijs

I made a small mistake, 1rpm should be ofcourse 60 rpm (1 rotation per second)

RPM's would be constant regradless of scale.The center of the axle will rotate at the same speed, the outer edge of the dish will appear to move faster, however. Showing my age...remember vinyl record? The outer edge of the groove moved at teh same 33 1/3 rpm as the inner edge. A larger wheel, however, if you were to put it on the edge, will move you faster over the ground, but the wheel will still be rotating at the same speed.

Hi Mat,
You are correct in saying that the rotational speed stays the same. The key is that time is constant (doesn't scale). In your example, the radar rotates once per second. That means it will paint a target once a second. The target needs to be painted once a second no matter what the size.

Using the circumference to find the distance over time (speed) is also correct. What it also points out is the linear speed of a wheel scales but the rotational speed is fixed just like it would be for a radar. The rate of rotation for a road wheel is generally higher than for a radar, but the math for figuring it out is exactly the same. A stone stuck to the tread of a tire follows the circumference and is the linear distance the wheel travels. Your example shows the rotational speed is fixed but the circumference follows the scale of your radar (or wheel).


David