Blade Slap...?

[Peter-RB]

Just of late where my office is based we seem to get a lot of 2 blade helis flying at a mid-ish sort of altitude say up to 1200ft, beyond that we see many of the three and four bladers scooting along at around 1600 to 2500 ft, all seems pretty normal but then I have started to realise the two bladers are responsible for my estimate of 70 /80% of the noise generated by Blade Slap.

I have looked into all my books and can only find comments about advancing blades colliding with the tip vortices of the previous blade tip, is that really all that cause this noise, for it only seems to be the two blader ie Jet R and the Robbies, these are alway a slight tad lower than the bigger helis, last week we had a Utility Heli checking the line and I heard it again very loud but it always seems that the slap is heard upon what appears to be a losing of height, or could air density have some affect on this problem?

Peter R-B .

[chinook240]

If you've got $35, you could read this

http://www.sciencedirect.com/science/article/pii/0022460X66901325?via%3Dihub

Or this is free:

http://www.businessinsider.com/scientists-why-helicopters-are-so-loud-2014-2?IR=T

Don't know if either helps

[Hawkeye0001]

Usually more pronounced on descent, sharp turns and generally low power settings, in normal cruise speed and climbs it's not noticeable. Using a rapid descent before decelerating the aircraft alleviates most of the noise problem, i.e. when on final establish a higher than normal descent rate at cruise speed (~1,000fpm) before slowing down. Helps keep the neighbours happy.

[mixing lever]

"If you've got $35, you could read this

http://www.sciencedirect.com/science/article/pii/0022460X66901325?via%3Dihub"


... or, you can try here cheaper....


https://www.scribd.com/doc/117935596/Helicopter-Noise-Blade-Slap

[John Eacott]

A search of Rotorheads can be useful:

What causes blade slap (http://www.pprune.org/rotorheads/87669-what-causes-blade-slap.html)

[Peter-RB]

Good morning to all,

Thanky for the input, and John, I tried looking into the history on Prune, but failed to spot or ran out of patience.
However I almost under stand it now, thanks to all of you, the Jet Rs that I see regularly are all in the descending turn slowing down maneuver when I hear it mainly, and so now I fully see the reasons.

Peter R-B
Lancashire..:ok:

[Mick Cullen]

The HAI Fly Neighborly Guide has some pretty charts that show combinations of ROD and airspeed that can maximise BVI/blade slap
http://new.rotor.com/portals/1/Fly%202009.pdf

[topendtorque]

as a manoeuvre to deliberately exert sound pressure upon cattle many miles distant, especially if they are downwind on a cool morning when wind movement is only about 2 or 3 knots; just at about 35 to 40 knots, ease down the lever roll right slowly 15 to 20 degrees, ease up the lever as moving cyclic slightly aft. with practice a loud whop can be cultivated from the '47 and the 22. The usual constant chopping can be found on a good approach descent at about 18", 45 to 50 knots one way to tell if the student is doing things right and you are on the ground listening. at a high speed descent the same control inputs, slightly down on lever rolling slowly right then up slowly on lever, can really startle whatever is close by. RRPM top of the green.

cheers tet

[[email protected]]

Peter - that link provided by John has the answer in the first couple of posts by Nick Lappos:ok: