function [v_best,res_eff_best] = eff_calc(interp_power,v_a, TC, v_w, WD )
%eff_calc calculates the best speed and best efficiency of the plane for a given
%efficiency curve, track, wind speed and wind direction

WA = (WD-TC)*pi/180;
WA_grad = 180/pi*WA;
if WA==0
    WA=0.0000001;          %durch Null teilen verhindern
end
if WA==pi || WA==-pi
    WA=pi*1.00001;         %Singularität verhindern
end

v_a(v_w>v_a) = v_w;            %complexe Zahlen verhindern durch |asin_argument| < 1

v_g = v_a.*(sin(WA+asin(v_w./v_a.*sin(WA)))) ./ sin(WA);
   
eff=v_g./v_a;
for idx=1:numel(v_a)
    if eff(idx)<0 || v_w>=v_a(idx)
        eff(idx) = 0;
    end
end

res_eff= v_a./interp_power.*eff.*100./max(v_a./interp_power);     %Effizienz des Fliegers (Formel hat ggf. Optimierungspotenzial^^)   
[~,idx_best] = max(res_eff);
v_best = v_a(idx_best);
res_eff_best = res_eff(idx_best);

%Ergebnis soll Strecke/Energie in % sein (Bezug auf Windstille)
figure(1)
plot(v_a, res_eff);
hold on
plot(v_a(idx_best), res_eff_best, 'marker','x');
xlabel('airspeed/ km/h')
ylabel('efficiency / %')
end