Davew Wrote:Thats not the sound of the prop tips breaking the sound barrier. Its the doppler effect of the engine noise and exhaust noise, combined with the propeller spinning, and the airflow through and around the prop and airframe. All piston engined aircraft make that sound to a greater or lesser extent doing a flyby. The amount of noise will depend primarily on the speed of the aircraft. The noise also changes when you get multiple aircraft or multiple engines with aircraft in formation.
Having the prop tips go supersonic is extremely inefficient, and fighter designers avoided it by putting bigger or more props on the aircraft so that the aircraft could use the full power of the engine.
In the case of the black sheep aircraft, the Corsair had a 14' diameter prop on it to use the power of the engine fully. The Spit XIV used a 5 bladed prop for the same reason.
As an example of an aircraft that does have the prop tips going supersonic, check out youtube clips of the Harvard (or Texan for the americans) like this one http://www.youtube.com/watch?v=8GAx3Xu0Jis at 16/17 secs and 51 and 58 secs. That loud rasping noise is the prop tips breaking the sound barrier and exactly the same as with any other sonic boom, you dont hear it till just after the aircraft has gone past.
Here's an example of a NON-PISTON ENGINED aircraft making this noise. This is a turbine engine.
http://www.youtube.com/watch?v=v5ufzaoW ... re=related
Also, if you noticed even on your examples in the Texan that the sound from the PROP is heard when the listener is "edge-on" or nearly so, to the propeller arc whether he's turning, passing, or even doing a roll. In your examples, you can EASILY separate the sound of the engine from the sound of the prop. All that doppler does is change the pitch of the sound relative to the viewer.
Secondly, larger diameter props are MORE prone to supersonic tip inefficiency because the relative tip speed increases with diameter. Corsairs had large diameter props to take advantage of the greater low-end torque produced from the Pratt & Whitney necessary to carry heavy loads off of a short carrier deck. A larger narrow-blade prop is like having a shorter first gear in a vehicle. If you notice the Spitfire XIV has a wide-blade 5 prop (probably better utilized for medium to higher altitudes with respect to the Griffon 65). Probably for greater top-speed at those altitudes. Inline engines were better utilized for top-end (airframe streamlining) rather than radials which were better utilized for carrying heavier loads. Both had torque and lack of effective rudder issues at full-power and low airspeed.
Thirdly, if you do some research, you'll find that SINGLE blade props are more efficient than 2, 3, 4, or 5 bladed props, although more sensitive to out of balance conditions.
Please receive my comments with the knowlege that I'm experienced in air racing, I have a pilots license to fly airplanes, and I'm a certified flight instructor in helicopters.
-TSU
