The scale speed is the clue of the problem.

Everything is explained here:

http://www.charlesriverrc.org/articl...scalespeed.htm
To summarize:

Rule 1 z2 = z1/r Wingloading, z, resulting from simple downscaling.

Rule 2 v2 = v1·sqrt(1/r) Velocity, v, resulting from simple downscaling.

Rule 2a v2 = v1/r True scale velocity.

Rule 3 w2 = w1/r4 Mass, w, required to fly at true scale speed.

Rule 4 svz = z1/r2 Scale velocity wing loading, svz, resulting from Rule 3

Plus:

If you want a plane to fly a the same scale speed than a plane that you take as a reference (mass w1, length L1, wing area S1), then the mass of your plane should be equal to:

Rule 5: w2 = w1 x (L2/L1)² x (S2/S1)

Note: If the plane is the plane of reference at the r scale reduction , then L2 = L1/r and S2 = S1/r², and we can verify that the mass is:

w2 = w1 x (L1/(L1 x r))² x (S1/S1 x r²)) = w1/r4 This is the Rule 3

Hope this helps.