ONE MANS OPINION OF A HELICOPTER
AUTHOR UNKNOWN/MAYBE JIM BURNS
Suffolk County AFB, NY. from the 1959-60 time frame
The helicopter is an amazing assortment of nuts, bolts, rotors, push-pull rods, irreversible, longitudinal collectives, differential quadrants, swash plates, wobble plates, gimble rings, cuff and trunion assemblies, and other gadgets to numerous to mention. All of these are welded, riveted, bolted or sewed together to make a single machine capable of flight. In fact, it is capable of flight in any direction, backwards, forwards, and sideways, up, down and even standing still. Standing still is known as ‘hovering’. This comes in handy for those who like to fly, but have no place to go.
One of the necessary components is the engine. This unit is expected to start with ordinary fuel, change it to BTU’s, the BTU’s to Brake Mean Effective Pressure (BMEP)
(SEE DEFINITIONS BELOW), the BEMP to RPM’s. The RPM is then transmitted through a series of shafts and gears to the main rotor blades which are responsible for the fanatic egg beater motion.
The engine has several important parts. Among these are the cylinders. A cylinder is a long hole covered on one end with a plate full of holes containing valves. The holes admit air, fuel and sometimes water and carelessly misplaced tools. The other end is closed with a plug called a piston. This is free to move up and down and would come out altogether if it were not fastened to a connecting rod. The connecting rod, too, is important because it is responsible for converting your BMEP to RPM. Without it you would be left with BMEP, which now one knows how to use up to now.
The power of the engine is measured in horsepower. Why? Who knows! It is often difficult to get a self respecting horse in close proximity with one of these machines. It is better to rely on instruments the electrical men have invented. They indicate power in Amps, volts, or kilowatts, depending on the individual whims of the designer. With a little imagination these values can be converted to horsepower.
Starting the “thing” requires some knowledge, steady nerves, and a certain amount of bravery. First make a careful check of all your instruments. This gives you a little self confidence and adds prestige in the eyes of the onlookers.
After everything has been checked, it is safe to start the engine. If everything is as it should be, there will be considerable noise and you will start to shake and tremble. That means the engine has started. When your audience has returned, synchronize your eyeballs and look at the instrument panel, noting pressures, RPM, and before you forget it, check the flight controls. This is important, even though they quite often do not perform the function for which designed. It is very embarrassing to get into the air and find those items not working properly, or just not working, period.
Once airborne, you are on your own; you have willfully and knowingly placed yourself in the most horrible of all predicaments. May the Lord have mercy on your soul. You have earned it. (COURTESY OF JIM BURNS)
(The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output). (COURTESY OF RON SMITHAM)
BMEP = Brake Mean Effective Pressure. Pulling high horsepower at low engine speeds (talking about recips) makes for a high BMEP. Same HP at a higher engine RPM means a lower BMEP. High BMEP means high efficiency for the engine, but also may mean a blown engine if the high BMEP lasts too long. (COURTESY OF JERRY KIBBY)
"The term BMEP is an engineering term that means Brake Mean Effective Pressure (psi). Mean is another word for average, which in this case means average effective pressure of all stroke cycles. This is used to evaluate all engines whether they are Two or Four Cycle.
BMEP is a function of temperature of the gases in the cylinder. To increase the temperature you need to burn more fuel, thus making more heat. Or another way is to make better use of the existing fuel.
Torque is a function of BMEP and displacement only. HP is a function of torque and rpm.
It can be said a high BMEP and a low rpm, or a low BMEP and a high rpm, can equal the same power. Larger valves, ports, pipes, compression, etc. all come into play to increase the volumetric efficiency of the engine. The most effective is to increase the number of cylinders. The more efficient it is, the higher the average pressure or BMEP.
Pressure increases by compression alone can do wonders to a stock engine; it is, by factory choice, usually a low number. Note that after compression gets very high it starts to work against you in pumping losses, and in the amount of heat lost to the surrounding parts." (INTERNET)