Always interesting to see how little the folks where that is approaching understand the turbulence that will be created. We had the same thing on a smaller scale when I flew rotary wing.

7 people taken to the hospital from that event.

Another video a lot closer to the action shows how violent it is:

https://www.military.com/video/aircraft ... 0706565001

I think the extreme downwash would not be suitable for a presidential transport.

Mike C.

That sure appears to be a very slow stabilized approach. In that situation, it might be better to pass over folks more quickly, get wheels on the ground quicker and unload those blades.

Actually, the sound and vibration will be minimized by having the rotors synced this way. The pressure from both sides will be equal and opposite so they will cancel in the middle.

If the props were out of sync, then things would be much worse.

Mike C.

I thought low-frequency beat comes from a slight difference in blade RPM. If the props are at the same RPM, what does phasing do? It seems phasing would address higher-frequency vibration and/or noise.

The blades also fold back for transport / fitting inside hangars. Maybe having the props offset would have been an issue when they went to fold them back.

My V22 story from Bell engineers: They pointed to the test stand out in the field they used for the engine tests. Once they started testing they found they were melting the paint off the building some distance away. The solution was to rotate the engine so the exhaust pointed at the highway instead. Looking at google earth right now, I can't identify where the test stand was. This was at GKY; it must be those structures in the north end of the airport, but that doesn't fit with my memory of seeing the highway right there.

There is a big difference between the C-130, where all props turn the same direction, and the Osprey, where they are counter rotating.

The C-130 can't make the props on one side produce the same sound pattern on the fuselage because the props move opposite to each other next to the fuselage. The prop on #2 is rising, the prop on #3 is falling.

In the case of the Osprey, the pressure wave of the prop moves the same along the fuselage, so it is best to have them in sync for noise and vibration cancelation.

If you took the C-130 picture at a slightly different time, say about 15 degrees of prop movement later, you'd see the #2 and #3 engine blades would be pointed at the middle of the fuselage which perhaps gives the best results you can get with props that are not counter rotating.

Mike C.

The beat from slightly different RPM comes from the two props alternately adding and subtracting from each other as they go into constructive and destructive interference. So it gets noisier, then quieter, then noiser.

If the RPM matches, then you want to phase the props so they are always in destructive interference, basically always be in the quieter zone.

Mike C.

Is this right?

Prop synchronizing - match prop RPMs
Prop synchrophasing - match prop RPMs and some constant relative angular blade position amongst all props

Yes, exactly right.

Mike C.

I read that in forward flight, the Osprey's proprotors are turning at 333 RPM.

Assuming there is some type of shock wave that comes off the proprotor tips, since there are three blades, there will be a pulse hitting the cabin at 16.65 Hz, simultaneously on both sides.

If the proprotors were out of phase (60 degrees), the pulsing would be at 33.3 Hz, alternating between sides.

The human ear is sensitive down to about 20 Hz. Not sure if this was a design consideration.

Note that the above is the disc loading, the weight supported per sq ft of total rotor circle.

If you look at the blade loading, it becomes a shocking number. I estimate the blade area to be about 216 sq ft (six 18 x 2 ft blades). You get about 200 lbs/sq ft. That is a very high "wing" loading, particularly when the inner part of the blade does very little lifting due to slow speed.

That is 1.4 PSI pressure difference average top to bottom surface. It would mean an average person can be lifted by less than 1 sq ft of rotor blade area. You would be "standing" on 1 sq ft of air.

In VTOL mode, rotor RPM is 412. That makes tip speed about 560 MPH.

Mike C.