Drag can also be reduced by making the blades thinner, which increases the speed that the blades can attain before the air ahead of them becomes compressible and causes shock waves. For example, the blades of the Hamilton Standard test propfan had a thickness-to-chord ratio that tapered from less than 20% at the spinner junction to 2% at the tips, and 4% at mid-span.[75] Propfan blades had approximately half the thickness-to-chord ratio of the best conventional propeller blades of the era,[78] thinned to razor-like sharpness at their edges,[12][79] and weighed as little as 20 pounds (9.1 kg).[80] (The GE36 UDF engine that was tested on the Boeing 727 had front and back blades that weighed 22.5 and 21.5 lb (10.2 and 9.8 kg) each.)[81]