0 200 100 300 Length of arm 3.18 ft Collar Friction adjusting wheel Arm Hinge Drum Propeller shaft Scale 200 lb Figure 1-40. Typical prony brake. load is then increased, the tendency of the propeller shaft to  carry the collar and arm about with it becomes greater, thus  imposing a greater force upon the scales. As long as the torque  increase is proportional to the rpm decrease, the horsepower  delivered at the shaft remains unchanged. This can be seen  from the equation in which 2πr and 33,000 are constants and  torque and rpm are variables. If the change in rpm is inversely  proportional to the change in torque, their product remains  unchanged, and bhp remains unchanged. This is important. It  shows that horsepower is the function of both torque and rpm,  and can be changed by changing either torque, rpm, or both. Friction horsepower is the indicated horsepower minus  brake horsepower. It is the horsepower used by an engine  in overcoming the friction of moving parts, drawing in fuel,  expelling exhaust, driving oil and fuel pumps, and other  engine accessories. On modern aircraft engines, this power  loss through friction may be as high as 10 to 15 percent of  the indicated horsepower. The indicated mean effective pressure (IMEP), discussed  previously, is the average pressure produced in the combustion  chamber during the operating cycle and is an expression of the  theoretical, frictionless power known as indicated horsepower.  In addition to completely disregarding power lost to friction,  As long as the friction between the brake collar and propeller  shaft drum is great enough to impose an appreciable load on  the engine, but is not great enough to stop the engine, it is  not necessary to know the amount of friction between the  collar and drum to compute the bhp. If there were no load  imposed, there would be no torque to measure, and the engine  would “run away.” If the imposed load is so great that the  engine stalls, there may be considerable torque to measure,  but there is no rpm. In either case, it is impossible to measure  the bhp of the engine. However, if a reasonable amount of  friction exists between the brake drum and the collar and the  1-33 Friction and Brake Mean Effective Pressures Friction Horsepower Example Find bhp substituting in equation: bhp = 6.2832 × 200 × 3.18 × 3,000 33,000 = 363.2 = 363 Given: Force on scales = 200 lb Length of arm = 3.18 ft rpm  = 3,000 π  = 3.1416