What Causes Blade Slap

[VeeAny]

A simple question, but lets have some opinions, happy to accept what the technical boys have to say (Nick & Shawn etc) but what do the rest of us think causes blade slap. I've had several ideas over years none of which seem to fit.

Sorry if this has been done to death before, but I couldn't find anything on it when searching.

:confused:

[Ascend Charlie]

Well, for openers we'll try this one:

Under certain configurations of forward speed. collective position, rate of descent, angle of bank / g being pulled, the blade tips fly through the vortices of the preceding blades and the interaction of the vortices causes the slap. Also known as "wockering" because of the wock-wock-wock noise. (Or for Italian helicopters, wop-wop-wop):eek:

[Heli-Ice]

Thank you AC.

I thought the slapping sound from the rotor was caused by my bad piloting skills. :D

[Hilico]

Minding my own business in the circuit (c. 700 ft, 70kt) on a sunny day, a thermal bubbles up beneath us and for that moment we get slap on the advancing side. This would implicate at least the angle of incidence and the tip path.

[NickLappos]

The slap is actually a slight case of Mach 1 at the tip, caused by the BVI previously mentioned. The venerable Huey has very high tip speed to extract lots of lift from its two blades. This means it needs less push to get mach 1 flow at the tip, so the increase when the preceeding blade's vortex hits it is enough.

The 48 foot diameter Huey at 326 rpm has a tip speed of 818 feet per second, vice the usual 725 feet per second for many rotors (and as low as 675 for some). Sound moves at 1117 feet per second at 15 degrees C, so that the tip is doing Mach .73 in a hover, Mach .9 at 110 knots. It doesn't take much to push that to .95 or so, where the top of the airfoil could see Mach 1, and the slap is then heard.

On a cold day, say -10 C, the speed of sound is 1067 ft/sec, so the Huey has tips at Mach .94 at 110 knots, ripe for continuous slap, even without BVI

Here is a speed of sound calculator:
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html

[VeeAny]

Thanks for the answers so far. i'm trying to find out why the slap is so prominent in the R22.

I've done the figures for the R22 as you have for the Huey Nick :ok: , and they come out quite a bit slower, is there any other cause other than the blade speed approaching Mach 1.

Diameter 25'2"
Speed @ 104% Nr = 698 Feet Per Second.

With a Vne of 102Kts = 172 Feet Per Second.

Comes up quite a bit short.

It has been suggested to me that the noise is "the blades flapping to equality very quickly' I can't see this myself, but maybe i've missed the point.

I know that not everyone has inside knowledge of the robbie but all replies gratefully received.

[NickLappos]

Veeany,

I guess its like trying to describe the taste of a bananna when we describe a slap. The horrendous blade slap of a Huey that can be heard miles away is the transonic sound I describe, the typical lower amplitude popping that many rotor experience is pure BVI, where the rushing change in angle of attack causes some increase in noise as the blade enters the wash of the prevous one. For low disk loading rotors, the vortex does not descend very quickly, and so is more likely to be hit by the next blade.

These pops are more likely to sound in descending flight at moderate speed, and in turns. The sound plots from most rotors have a ""fried egg" shape to them, where the speed and rate of descent conspire to make the high noise boundary, and any faster or slower rate of descent will make the noise disappear.

[jonnyg]

I used to get Blade Slap in the R22 when turning & descending (like from downwind to base) too quickly. I thought it was the tips reaching sonic speed. You have too remember about speed is the angular direction and the loading of the blade.

[elpirata]

shallow high speed descents cause blade slap dont they, thats all you need to know isnt it, or maybe i am just wrong as usual

[Hilico]

Certainly I remember getting it in the descent and the instructor saying to bring the cyclic back a little - which would slow us down.

[ShyTorque]

Slap occurs best in a slight descent, as already stated.

In a previous life my job required us to be somewhere, doing something and making as little noise as possible in the process. Blade slap is a big giveaway that a helicopter is around (even from a distance away when the aircraft is very high) and was therefore a big no-no.

We used to achieve a very high hover (FL 80 plus) by doing a gently climbing quickstop to avoid flying through our own downwash. It is very satisfying to crisply achieve the hover in this fashion with no blade slap.

[proxus]

Is the silent copter a myth ?

I you would on a 4 blade rotor, put two of the blades higher than the pair underneath, loose some of the slapping sound ?

[ShyTorque]

Proxus,

Not sure but a 2 blade rotor still slaps in certain conditions anyway.

[Lu Zuckerman]

To: proxus

Is the silent copter a myth ?

No it is not. Many years ago Hughes took an OH-6 and modified it to an extreme. The placed a silencer on the engine exhaust, they isolated and eliminated any noise that radiated from the structure and they incorporated a five blade main rotor and an X tail rotor. In this configuration it could fly 500 feet AGL and all you would hear is a "whoosh".

:cool: