The Voluntary Noise Abatement procedures were implemented in the summer of 2006 at Gabreski Airport. We realize that one of the most important aspects of the program is pilot education.. We hope the information below will be informative and helpful. Fly safe!
While this letter is pertaining to pilots flying aircraft with controllable pitch props, the logic is still the same for those of you with fixed pitch props. We were all taught in flight training that the before landing checklist we are to do is our "GUMPS check".
- Gas - correct fuel tanks selected
- Undercarriage - landing gear up or down as appropriate
- Mixture - full rich for sea level take-offs and landings, leaned in cruise
- Propellers - flat pitch, high rpm for landing, lower rpm for cruise
- Switches - fuel pumps, magnetos, landing lights, heater, seat belts, whatever
This full check normally would be started on downwind or on final for a straight in approach. The problem with doing the complete GUMPS check early is we may be carrying power at that point and that would not be the time to jam the prop control full forward. With some manifold pressure, bring the props to flat pitch, would bring them to max RPM in the pattern. Which will increase aircraft noise.
Lets talk about RPM and aircraft noise.
As the tip speed of your propeller approaches the transonic range, the noise level outside the aircraft takes a tremendous jump. The transonic range begins about 625 MPH, or Mach .85, and continues until the speed of sound or Mach 1 at sea level. This increase in noise is much more apparent outside the aircraft than inside. Beginning at approximately 625 MPH, the tip of the propeller is going fast enough to cause compression of the air and is causing at least two mini sonic booms per revolution. Also the propeller efficiency goes downhill very quickly as the tip speed gets near the speed of sound (contrary to popular belief, propellers almost never are designed with tip speeds near Mach 1.0). The next table put it all together and shows the tip speed of your prop as a function of the Propeller Diameter and RPMs and it indicates the combinations that produce more noise.
I use full throttle and maximum RPM which is 2700 as called for in my POH. With a positive rate of climb I will retract the gear. At 500 feet I will reduce the RPM back to 2500 RPM. Wait a minute! Weren't we told in training never to reduce the RPH without reducing the Manifold Pressure first. Yes! But the reduction of 200 RPM is not placing undue pressure on the engine. What I have done is bring the prop tips out of supersonic with only a loss of 15 horsepower with that reduction. That reduction of 200 RPM probably cut the noise of my departing aircraft in half. On takeoff plan on reducing RPM 100 to 200 as soon as you feel it is safe and the fellow neighbors will thank you. By the way, there is a Bonanza at FOK that has a two bladed prop which is probably 84" in diameter.If he has an IO 520 with 285 HP his take off RPM will be 2700. When he takes of you can really notice how loud he is. Just look at the chart to see where his 84" prop tips are. 675 Miles per hour. Those of you with fixed pitch propellers can also control the prop RPM noise by adjusting your climb speed with climb pitch. By lowering you climb speed the propeller will work harder and therefore the prop will slow down. Just climb out at Vx or Vy and you will be quieter.
Back to the GUMPS check. You can do the GUMPS check on downwind, Base, and Final but do not change the props to full until final when the power is way back. Just say to yourself on downwind and base when you get to Props: "Props set till final" There are many traffic patterns at FOK where you will be over noise sensitive areas on the base leg and shoving the prop to the High RPM will make your aircraft very noisy. I myself leave the Props at cruise setting until I am off the runway. By keeping the props at course setting they are biting the air already. It only takes small adjustments of the throttle to make the aircraft respond. It the prop were at its High RPM low pitch mechanical stops it would take more movement of my throttle to effect the aircraft. I also leave my mixture at the leaned setting I had at cruise to keep my plugs clean. I know that for some of you that is going to be a problem. You are saying to yourselves "What about a go-around?" Well If I have to go around I say. Mixture Props and Throttle and push them all forward. It takes me 1 1/2 seconds to do that. There is no emergency that requires less than 1 1/2 seconds to react.
Those of you who want to learn more about Prop and mixture and engine control read John Deakin's great articles on AVweb:
Pelican's Perch #16:
Those Marvelous Props
Pelican's Perch #15:
Manifold Pressure Sucks
Pelican's Perch #18:
Everything talked about here should be done based on your own experience level and you must follow your POH. I would be glad to hear from anyone on this first letter. Safe Flying!