Helicopter Rotors - Why are five blades preferable for stealthy helicopters?

[Sanguine]

This question has been bugging me for some time. On helicopters such as the night stalker, AH-66 Comanche, and the stealth MH-60 used on the bin laden raid, they utilize five rotor blades, as opposed to four. This apparently creates non-directional sound, but how?

Also, what is the maximum practical amount of rotors on a helicopter? Some have six blades, but what is the limit?

[tater]

I have never really thought about it, but I would assume it is to increase rotor area without increasing the tip-velocity of the blades more than anything else. They might then be able to create equal lift to the other versions with lower RPMs. I'd also assume that they could pitch the blades less for hovering, which seems to be a flight mode that makes a lot of noise (sometimes they scoop water near my house for small fires in the mountains).

[Camacha]

In general: more blades on a helicopter provide more lift, but each blade suffers more from turbulence and disturbances of the previous blade. This causes diminishing returns. In reverse, less blades means cleaner air for each blade, but less lift on a rotor.

Of course, other factors come into play, like blade length and such, but that is a very crude rule of thumb.

Edited July 29, 2015 by Camacha

[sgt_flyer]

There are several things creating a lot of noise on helicopter blades - 1 of them is having an helicopter blade hitting the vortices left by the trailing edge of the blade before it. Guess having five blades allows them to have the following blade hitting the vortices at a point it generates less noise.

Now, the thing really loud on helicopters is having the vortices from both tail rotor and main rotor colliding between themselves - which is why they used fenestrons (at least on the commanche)

Now, there's some advances in rotor blades that makes even less noise (one of those is blue edge) - they use tiny electric engines to make some flaps vibrate at the trailing edge of the blades - limiting the generated vortices a lot.

http://gizmodo.com/5481081/the-blade-that-would-make-helicopters-almost-silent

Now, for your question ablut the number of blades, guess the MI-26 Halo is the helicopter with the most blades on a single rotor, with 8 blades.

Now, if you can quiet the blades, you'll also need to quiet the turbines those things are also noisy

Edited July 29, 2015 by sgt_flyer

[J.Random]

It's not necessary to have 5 blades to make rotor less noisy. Take Mi28N tail rotor, for example:

 

Edit: also, I think Apache has the "silent mode" - doesn't it change the engine revs or something like that to be silent?

Edit2: oh, crap. Didn't check the date. Sorry.

Edited December 4, 2015 by J.Random

[WinkAllKerb'']

xDr copter ?

 

Spoiler

( 386 ; 486 drivers ? no matter what xDr )

 

kinda a voxel/vectoring stuff antologia ; )

Edited December 4, 2015 by WinkAllKerb''

[Elthy]

5 rotorblades avoid 90 degrees of angle, which reflect radar perfectly back to the radar station.

Edited December 4, 2015 by Elthy

[RainDreamer]

The concept of a stealthy helicopter is new to me.

[More Boosters]

38 minutes ago, RainDreamer said:

The concept of a stealthy helicopter is new to me.

It's a dated concept and the closest one came to existing was the Comanche.

Basically, you'd rather use a UAV.

Edited December 4, 2015 by More Boosters

[magnemoe]

2 hours ago, More Boosters said:

It's a dated concept and the closest one came to existing was the Comanche.

Basically, you'd rather use a UAV.

The helicopters used on the raid who killed bin landen was stealthy. 
Most helicopters are used to transport people. for both special opperations and resuce behind enemy lines stealthy helicopters makes sense.