The ‘lift’ funciton is symbolic, so no function handle is necessary to plot it with fplot. (If you want to create a funciton handle from it, use the matlabFunction function.)
% Lift load
syms q_l ka L x
q_l = ka.*sqrt(L.^2 - x.^2) % Expression for load distribution function
% Integrate expression over the length of wing to calc total load acting on
% wing due to lift
lift = int(q_l,0,L)
lift = subs(lift,ka,150)
% Plot lift as a function of wing length
fplot(lift,[10 20])
.
