Measurements of drag force are made on a model automobile in a towing tank filled with fresh water. The model length scale is 1/5 that of the prototype. State the conditions required to ensure dynamic similarity between the model and the prototype. Determine the fraction of the prototype speed in air at which the model test should be made in water to ensure dynamically similar conditions. Measurements made at various speeds show that the dimensionless force ratio becomes constant at model test speeds above 4 m/s. The drag force measured during a test at this speed is 182 N. Calculate the drag force expected on the prototype vehicle operating at 90 km/hr in air.