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Whop Whop Whop!


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Okay guys and Girls, heres what I need to know.


-The 212 and similar Helis make a snapping sound when theyre flying in sevral different situations. Some schools of thought state that the advancing blade is actually breaking the speed of sound, however from other schools of thought I hear that it is the next rotor passing into the inhearent turbulance of the airfoil prior.


Let the games begin, I hear this is a "Noone knows for sure, many theories exist" topic.


Have at'er

Cole :punk:

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Guest Bullet Remington



The Bell machines you mentioned are NOT fully articulated heads. They are underslung - tee teer totter like system. (I suppose somebody somewhere has the correct terminology for it.)


Basically, when I say under slung, it means that the attachment point for the main rotor head assy, is actuallyhigher then the baldes themselves.


Something like an inverted Vee where the upper point of the Vee would be the retaining point of the cuff, and the lower points of the inverted VEE would be the attachment points for the blades.


Assuming the blades are lined up fore and aft(whil the head is turning and in flight) the aft blade would START to be the advancing blade, while the fore blade would be the retreating blade.


While the advancing blade moves forward it moves in an upward arc, reaching its highest point directly over the nose of the machine (the blade is flapping up). At the nose of the machine (its highest point) it becomes the retreating balde and starts to flap down. Basically, the advancing blade would have a slighly higher AOA and lower laminar airflow then the retreating balde, while the retreating blade would be flapping down with al ower angle of attack and Higher laminar airflow.


At the algined fore and aft positions, both blades lift factors SHOULD be even. For that matter, when the baldes are aligned left and right of the machine, despite the advancing blade having a higher AOA then the retreating blade, the laminar airflow should cause and equal amount of lift from both blades.


It is USUALLY at the "fore and aft alignment point you hear that "Bell Slap" It will be very noticeable when the pilot transitions from forward flight into a hover, and/or pulls the cyclic back to slow down for a landing.


It will also be very noticeable (more so then "normal") when the baldes are out of track. Also, the prevailing wind, relative to direction and speed of flight, will also have an impact on the noise.


I can usually tell by listening to the sound (with my hearing aids turned up!) if a 206 is out of track.


As for the "speed of sound", that's not possible. The actual speed of a rotor is relatively slow. I stand to be corrected but I believe ( and I may be wrong here) that the tip speed of a 204/205/212 is around 420 miles per hour.


As a rotor and/or propeller starts to reach in the vicinity of 600 miles per hour, the lift and/or thrust starts to move in board, along the rotor blades/propeller and the thrust/lift factor diminishes, drastically. Hence the reason for Reduction gear boxes on turbo-prop aircraft. Should the prop and/or rotor blade speed start approaching the speed of sound, it would be akin to having a piece of round metal spinning in the air. It would accomplish diddly squat.


As for the Bell Slap, it is one of the inherent design characteristics of the machine. Whether the baldes are in track or not, when flying in those machines you will get a hump, (bump) when the baldes line up fore and aft. Being a wrench bender with alot of time on fully articualted machines, that hump really drives me nuts. So when I work on them, I spend too much time trying to smooth it out.


Of course I am getting senile! It is impossible to remove it all on an underslung head system!


Feel free to correct me if I'm off base with this folks. There are a truckload of folks on here with a heck of alot more time workin on Bell Thumpers then I have!

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Is it not as simple as this? :)


When the heli's rotor blades cut through the air, they create a region of low pressure above them and high pressure below them. That differential creates lift, but maintaining it depends on the smooth flow of air over both surfaces. The spinning heli blades turn or churn the air beneath that high-pressure zone, creating the chop, chop, chop (or whop, whop, whop) sound that is heard.


Maybe so. Maybe not. :rolleyes: I got no proof.

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Here's my two cents worth........


The blades are not hitting the speed of sound..........that would be quite destructive to all concerned.


The slap comes from the different air pressures above and below the blades........when the air slides off the END of those big square blade tips the different pressures bang together.

Different pitch and/or speeds means different pressures and therefore different levels of WHOP WHOP.

That's partly why modern military blades are tapered, swept etc. etc. to avoid the slap/vibrations etc. etc.

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Ok. I'm just a dumb pilot, but I was once told that the slap is due in part from the pressure differential above/below, and from the confluence of the pressurised air flows from the main rotor and the tail rotor.


The blade slap from two-bladed "teetering-hinge" rotors tends to be more noticeable due to the fact that blade cord is greater, rotor rpm is slower, and the number of blades means a lower "slap count".


Just my 2¢... :)

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