When building a scale model of a flying aircraft, how do its parameters go with the scale?
I.e. a halfsize model will have all of its dimensions cut by half. Volume becomes 1/8th (1/2 ^ 3 for three dimensions) so weight ought to be 1/8th also, though I have gut feeling 1/5th or so would be more realistic.
But what happens to the required engine power? I think airspeed is a complicating factor there: if the original has a Vne of say 200 kts, does one think the half scale model's Vne as 100 kts?
Thanks for any pointer - I did search many sites of r/c modellers but must have used the wrong search terms - found only headaches.
PS those who wish to question the idea can do some web searching for the Shorts S.31, for just one example.
PPSS comparing the 4x90 hp of said S.31 to the 4x1375 hp of the Stirling's Perseuses, I come to a ratio of 1:15 , which seems quite hefty..?
Jan,
As I understand it, the scaling thing needs to also take into account that whilst scaling up or down dimensions will preserve proportion, the 'medium/fluid' that the a/c will fly though is not scaled in density - i.e. that 1:x scaled models effectively 'fly' in a less dense medium and therefore require less power to overcome drag as a function of their size.
A transcript: "Early work in fluid mechanics, or the study of how fluids and gases behave and their effect on objects in a flow, indicated that the airflow around a scale model would not correspond exactly to the flow around a full-scale aircraft. To ensure the correlation of model data to full-scale aircraft data, researchers also determine the Reynolds number of flow in a wind tunnel.
Mach Number: The ratio between the speed of a craft and the speed of sound in the surrounding medium (the atmosphere) is called a Mach number. The speed of air flowing through a wind tunnel is usually expressed in terms of the speed of sound, which is approximately 761 miles per hour at sea level. However, the speed of sound through the atmosphere varies with temperature. Sound travels more slowly through cooler air. Aircraft usually fly at higher Mach numbers in the upper atmosphere where the air is colder.
Reynolds Number: Reynolds number is a nondimensional parameter representing the ratio of the momentum forces to the viscous forces in fluid (gas or liquid) flow. Reynolds number expresses the relationship of the density of the fluid, velocity, the dimension of an object, and the coefficient of viscosity of the fluid relationship. Osborne Reynolds (1842-1912) demonstrated in experiments that the fluid flow over a scale model would be the same for the full-scale object if certain flow parameters, or the Reynolds number, were the same in both cases.
For example, the Reynolds number of 1/4-scale models tested at flight velocities at atmospheric pressure would be too low by a factor of 4. Because the Reynolds number is also proportional to air density, a solution to the problem could be to test 1/4-scale models at a pressure of 4 atmospheres. The Reynolds number would then be the same in the wind tunnel tests and actual full-scale flights."
I hope this helps . . .
