Synchronized props

PRB

PRB

Administrator
Staff member
A question about sound file design. Those of you who have been inside twin prop airplanes know the distinctive sound associated with the propellers not being synchronized. Wouldn't it be possible to duplicate this effect in FS planes? All we would need was a set of sound files going “whum-whum-whum-whum” at varying rates which would be played depending on the difference between the RPMs of the two propellers, no? And the difference between the "whum" and the "-" could be simply volume. WBS P-51s have sound of varying volume, when you jam on the power, so we know that can be done. The effect would only be present when the two RPMs are different but within a certain range. If the two props are at very large RPM differences, and if they were at the exact same RPM, you would get a steady (but different) sound. I think the biggest problem would be determining which steady sound to play when the props are at the same RPM, the “loud” one or the “soft” one. In real life what determines this? Is it the props being “in-phase” or “out-of-phase”? It has to be phasing. That might be difficult to get out of FS. Wouldn't it be great fun fussing with the RPM levers to “tune out” the “whum-whum-whum”? Of course it would!

Example:

 
IRL, it's phase. The props can have the same RPM but still be out of phase, giving you the "whum, whum, whum.." sound in the cockpit, highly annoying on long XC flights :)
 
000rick000 said:
IRL, it's phase. The props can have the same RPM but still be out of phase, giving you the "whum, whum, whum.." sound in the cockpit, highly annoying on long XC flights :)
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Rick has it right. When he says "phase," he's referring (in simple terms) to the physical position of the propellers relative to each other. Interestingly enough, a prop sync actually "unsyncs" the props. The "whum whum whum" sound occurs when a blade on the left and right prop pass the cabin at the same time. A prop sync phases the props so that propeller blades do not pass the cabin at the same time.

Our Cessna 414AW (and the 310R we owned before buying the 414 14 years ago) actually has a phasing knob that allows you to precisely adjust the phase, whereas other aircraft such as King Airs only have an on/off switch. Makes for a much quieter, more harmonious cabin.
 
PRB said:
...Is it the props being “in-phase” or “out-of-phase”? ... It has to be phasing....
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So, in other words, what Paul said… :icon_lol:

If the props are at exactly the same RPM, but out of phase, the sound will be “quiet”, and if they’re in phase, the sound will be “loud”. If the props are turning at slightly different RPMs, then they will go into and out of phase repeatedly, thereby causing the sound I described as “whum-whum-whum”, etc., right? So RPM plays a role in this phenomenon. In “olden times”, say DC-2s over Western New York, how did they “tune out” the annoying affect? All they had was the RPM controls, no? Sounds like a fun "FS science project"...
 
Some moons ago, flying various piston and turboprop multi's my remembrance was that the audible sound was due to RPM and the phase synchronization (if installed) was to reduce airframe vibration and resultant metal fatigue. The four engine birds I flew had sycrophasers, but sometimes they could be inop. Even getting the sound to go away in a four holer is a bit interesting. If it was day and the sun was out you could sometimes sync each side by looking at the strobeing effect of the props and then attempt to match the two sides.

Rick, you might remember the lights used by the Wash State Patrol which had two lights which were out of sync, which gave a similar visual effect, flash flash separately then one together
as they synced. Similar to the combinations of audio frequencies.

T
 
Or you could just get the wife to sit behind you going “whum-whum-whum-whum” ;-)
Sorry
 
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