Each time you run the plot command you're only plotting a single point, which doesn't work very well with plot.
I recommend you switch to scatter instead, or index C_p for the loop and run plot outside the loop once.
for a=(0:0.01:0.3333333)
C_T=4*a*(1-a)*etaTipLoss
C_P=4*a*(1-a)^2*etaTipLoss*etaDrag
figure(1); scatter(a,C_P, "-.r"); hold on
end
% or
idx = 0;
for a=(0:0.01:0.3333333)
idx = idx + 1;
C_T=4*a*(1-a)*etaTipLoss
C_P(idx) =4*a*(1-a)^2*etaTipLoss*etaDrag
end
figure(1); plot([0:0.01:0.3333333],C_P, "-.r")
