Number of rotor blades?
 

Im wondering why some helicopter have two baldes and others have six. Is it to do with the amount fo lift being needed to be produced? could someone please give me an answer which utilses physics...


Thanks

Each blade acts as a wing and depending upon it's shape and size the amount of lift it can generate will vary. The number of blades is a fundamental starting point for the design of a rotor system. Depending upon your level of maths and engineering knowledge a good book to look at might be "principles of helicopter aerodynamics" by J.G. Leisham.

The surface area of the wing is directly proportional to the amount of lift produced (google "lift equation" if you don't know this one already). Increasing the number of blades is one way to increase the surface area.

With every design change comes a compromise. More blades means a more complex head, potentially more components, storage can be more difficult (than a 2 blade), etc.

Another way of increasing the area are to increase the length of each blade, but that means you have a larger rotor diameter so you need bigger landing areas, more strength requirements for the blade, and you will experience compressibility effects at a lower forward airspeed.

You could also increase the blade chord to increase the surface area, I think the biggest issue here is you get more blade twisting concerns, and center of lift changes (becomes a structural issue). I'm sure other ppruners have better detail on the pros and cons of increasing blade chord.

Matthew.

Oh great stuff, thanks.

If you've ever had a ride in a Bell 206/204/205/212 with 2 blades versus an
AS350 or a machine with 3 or more blades you'll definitely notice a difference.

Number of blades
 

I've been flying helicopters for a few years now and have always wondered and could never answer the question 'Why are helicopters designed with a differant number of blades??? whats the reasoning behind a 2,3,4 or 5 bladed aircraft?:ugh:

Tradeoffs. 2 Bladed is cheap to make and maintain but not as manoeverable and are slower.

Multibladed have a smaller diameter disk so may have lower tip speed hence less issues with retreating blade stall, disymmetry of lift and supersonic tip speeds and hence allow faster flight speed. Multibladed ships take up a lot more hangar space.

I remember hearing that 2 bladed is good for hover performance and 4 bladed is good for cruise performance but I can't remember the source or how true that is.

...until they start getting big (and heavy). Then you have to reduce RRPM to keep tip speed under control... and once you do that, you start running into disk loading issues and total blade surface area which increases chord width (for a given diameter) and so on. Imagine a heavy helicopter with two blades and you can see where da Vinci got his ideas...

http://farm4.static.flickr.com/3062/...c1666f02e2.jpg

Solidity ratio maybe.

I always thought it was a Ford vs. Chevy kind of thing. :8

Which gives rise to another important question...
 

I took the first student I ever had onto the ramp to talk about safety etc and he pretty much immediately asked me: "so why does the 2 bladed helicopter go faster than the one with 3 blades?". That question never came up in the CFI!

In general, the higher the number of blades the less vibration will be transmitted through the rotor hub. This is judged against minimising the number for cost. You can't stop each blade from flapping & twisting at specific frequencies, but more blades means they all vibrate in phase at a higher frequency.

There is a lot more to this, but for a full understanding you need to look up Campbell plots and symmetric and antisymmetric resonance modes. Just accept that designers try to fit as few blades as the dynamics will allow...

You have to know that lift is pricipaly function of the wing surface. In the case of helicopters, the device offering lift is the blades. Increasing the number of the blades increase the lift surface, which means, depending on your helicopter wight and shape, the number of blades is determined to produce enough lift to maintain the airplane in the air.

I'd say that 2 blade helicopters exist for two reasons only:

1. Rotor system is easy and cheap to manufacture.
2. Ease of storing helicopter.

In all other aspects they're worse than a multi-blade rotor, I'd say. They mast bump, they make more noise, they fly rough, they get weird gravity centers (long masts) and so on.

Just ask Captain Hollywood
 

I contacted a representative from the manufacturer of the R66 Goat Mk eleventy three?....Captain Hollywood.

As it turns out it seems he will make a helicopter with any number of rotors you want.

I based my choice purely on how it sounded.

Yes maintanance costs are a little high but you should hear my 17 bladed main rotor and 6 bladed rear rotor's..(thats right twin tail boomed) when its purring along on final.

Sweet.

Thanks Captain Hollywood.

Fizics is for dummies.

HF

So that would be an R22 and a Schweizer 300? Every model of helicopter goes faster than the Schweizer and that has more to do with its aerodynamic profile (such as it is!).

Cheers

Whirls

How about only one oar in the water?

http://www.eads.net/xml/content/OF00.../68/543682.jpg
Single-Bladed Rotor

# of Blades Vs Ballistic Tolerance
 

Ballistic tolerance is another consideration when selecting the number of rotor blades. Increasing the number of blades results in selecting reduced blade cord to retain an optimum solidity ratio. Wide cord blades can be made more ballistically tolerant, thus steering the designers to less blades. This was a prime consideration when going with only four main rotor blades on the Blackhawk and the Apache helicopters. Many arguments took place at Sikorsky in the early stages of the Comanche program. The ballistics group fought for a maximum of 4 main rotor blades while the pilots, performance and dynamics people fought for 5 or more. The latter group won out.

Ballistics Vs Dynamics
 

Jack,

Good information. So why does the S92 only have 4 blades? The only people sniping at us jockeys are our management!

TD