TresCrows
Elite Member
Re: Why do helicopters \"chop\"?
"As an aside, a friend of mine used to say "Helicopters *always* mean bad news!" Someone is hurt, being rushed to a hospital, some kind of miltary action, some criminal on the run, some natural disaster, and so on. I think he exaggerated, but it was an amusing thought. I'm certain that to a person stranded inside enemy lines, the sound of the chopper is the sweetest in the world!"
I recently purchased a 14 lb. 10,000,000 candlepower spotlight from Northern Freight. While shining up my backyard to see what critters are currently eating my trees I heard a distant helicopter and saw the strobes from it--it was way off---several miles. I, instinctively, innocently, pointed the mega light toward the chopper and lit him up like daylight /forums/images/graemlins/crazy.gif. He was going east to west as I said several miles north of me, he stopped, turned and headed my way. I said--"oh ##@$^^&*" and ran into the house. My house has a large skylight and I ran for the basement. Hiding in the basement with my dogs I peeked /forums/images/graemlins/blush.gif up the stairs and could see the chopper flying around over my house, the reflection of the strobes off my livingroom walls. I darted back downstairs and hid 'till it flew away. I don't think I will do that anymore /forums/images/graemlins/shocked.gif /forums/images/graemlins/tongue.gif.
OK, the rated RPM of a typical Lycoming engine is 2700 RPM and direct drive, the prop turns at the same speed as the crankshaft to which it is affixed. Propeller diameters for a 180 horse Lyc (360 cubes) are usually in the 72-80 inch range. Tip speeds without taking forward airspeed into effect can be from 550 to 650 MPH. If you consider that the aircraft, for example one like I am slowly building, can fly at 230 MPH in level flight at approximately 8,000 feet (200 hp at 2700 RPM and a 74 inch prop) one can see that the tip speed of the propeller gets very close to sonic speed when you vector in the velocity of the aircraft. Yes, the props become increasingly inefficient at these tip speeds but propeller diameter is a compromise between efficiency at low and high speeds and lots of other things as well. Therefore, tips speeds going transonic at max aircraft design speed is not as much of a concern as is having a propeller diameter sufficient to provide thrust for climb and acceleration and efficiency at cruise speeds. Any number of civilain aircraft can push their prop speeds into transonic and even supersonic speeds when flown at max performance. The AT6 WWII trainer, radial powered, with a large diameter prop is one in particular that comes to mind, that annoying blat-blat-blat as it roars by after a power dive with the prop set for max RPM and the throttle to max manifold pressure is definitly in the high transonic range. J
"As an aside, a friend of mine used to say "Helicopters *always* mean bad news!" Someone is hurt, being rushed to a hospital, some kind of miltary action, some criminal on the run, some natural disaster, and so on. I think he exaggerated, but it was an amusing thought. I'm certain that to a person stranded inside enemy lines, the sound of the chopper is the sweetest in the world!"
I recently purchased a 14 lb. 10,000,000 candlepower spotlight from Northern Freight. While shining up my backyard to see what critters are currently eating my trees I heard a distant helicopter and saw the strobes from it--it was way off---several miles. I, instinctively, innocently, pointed the mega light toward the chopper and lit him up like daylight /forums/images/graemlins/crazy.gif. He was going east to west as I said several miles north of me, he stopped, turned and headed my way. I said--"oh ##@$^^&*" and ran into the house. My house has a large skylight and I ran for the basement. Hiding in the basement with my dogs I peeked /forums/images/graemlins/blush.gif up the stairs and could see the chopper flying around over my house, the reflection of the strobes off my livingroom walls. I darted back downstairs and hid 'till it flew away. I don't think I will do that anymore /forums/images/graemlins/shocked.gif /forums/images/graemlins/tongue.gif.
OK, the rated RPM of a typical Lycoming engine is 2700 RPM and direct drive, the prop turns at the same speed as the crankshaft to which it is affixed. Propeller diameters for a 180 horse Lyc (360 cubes) are usually in the 72-80 inch range. Tip speeds without taking forward airspeed into effect can be from 550 to 650 MPH. If you consider that the aircraft, for example one like I am slowly building, can fly at 230 MPH in level flight at approximately 8,000 feet (200 hp at 2700 RPM and a 74 inch prop) one can see that the tip speed of the propeller gets very close to sonic speed when you vector in the velocity of the aircraft. Yes, the props become increasingly inefficient at these tip speeds but propeller diameter is a compromise between efficiency at low and high speeds and lots of other things as well. Therefore, tips speeds going transonic at max aircraft design speed is not as much of a concern as is having a propeller diameter sufficient to provide thrust for climb and acceleration and efficiency at cruise speeds. Any number of civilain aircraft can push their prop speeds into transonic and even supersonic speeds when flown at max performance. The AT6 WWII trainer, radial powered, with a large diameter prop is one in particular that comes to mind, that annoying blat-blat-blat as it roars by after a power dive with the prop set for max RPM and the throttle to max manifold pressure is definitly in the high transonic range. J
