Number of rotor blades?

[FAAjon]

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

[Chopper Doc Junior]

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.

[Matthew Parsons]

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.

[FAAjon]

Oh great stuff, thanks.

[rotornut]

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.

[winchman]

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:

[krypton_john]

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.

[KrisRamJ]

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.

[Um... lifting...]

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.

...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/3062/2326055832_c1666f02e2.jpg

[Gordy]

Solidity ratio maybe.

[TheVelvetGlove]

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

[Chickenhawk1]

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!

[Graviman]

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...

[kaptene]

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.

[AdamFrisch]

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.

[HELOFAN]

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

[Whirlygig]

"so why does the 2 bladed helicopter go faster than the one with 3 blades?".
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

[Dave_Jackson]

How about only one oar in the water? (http://www.urbandictionary.com/define.php?term=one+oar+in+the+water)

http://www.eads.net/xml/content/OF00000000400004/2/68/543682.jpg
Single-Bladed Rotor (http://www.unicopter.com/B472.html)

[Jack Carson]

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.

[Tractor_Driver]

Jack,

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

TD

[Jack Carson]

I believe that the initial concept was to have all of the S-92"s dynamic components retrofittable on to the Blackhawk. As a result the S-92 was trapped into having just 4 main rotor blades. It was good in concept but was lost as the S-92 put on weight. The airframe out grew its rotor.

[Graviman]

Jack,

I'm curious how you guys durability stress the blades for passing through the resonance rpms. Do you have standards to allow a period of soak for maximum alternating strain or can coupled FEA/CFD analysis provide realistic design goals?

I'm interested because fixed rpm seems to be one of the biggest limitations in helicopter design. Variable rpm rotors would allow efficient operation across the speed/weight envelope.

[leading edge]

Jack

Doesn't an additional blade also create additional drag? 5 bladed would have greater drag than 4 bladed?

[Paul Cantrell]

I've always thought that more blades will develop more thrust, but at a lower efficiency. If you have a big engine(s) and you want to use all the power it can produce, use more blades. With a smaller powerplant, a two bladed (or a 1 bladed I guess) will be more efficient.

I'd have to go back through some books to say that more confidently, though.

[Jack Carson]

The only rotor resonance issues I have had to deal with were related to rotor to airframe resonances and ground resonances where the entire system reacts through the landing gear. Specific MRB span wise, cord wise or lead lag resonance design issues were handled long before the blade got onto a flight article.

In some instances the vibratory inputs from the main rotor blades were handled through absorbers or dampers. The S-76A incorporated 3/rev and 5/rev bi-filars installed on the main rotor hub to reduce vibrator inputs to the airframe. These were blade related vibrations.

In the case of the H-53E series, the natural frequency of the airframe was approximately 3.25 Hz. The rotor speed at 100% was 179 rpm (3.0 Hz). During high rotor rpm operations (heavy lifts, autorotations) the rotor’s natural 1/rev vibration level increased as the airframe’s natural frequency was approached. Prior to the incorporation of cyclic and collective flight control dampers and flight control desensitizers, pilot assisted inputs were also introduced as contributing elements to the total vibration levels of the system. This never seemed to be as issue with the smaller airframes because the natural frequency of the airframes were significantly different from that of the main rotor and at a higher frequency than any input a pilot would be capable of making.

[Jack Carson]

As the gross weight of a machine increases so will the number of main rotor blades. The Bell 214ST at 17500 lbs. is about as far as a 2 bladed system could grow. A look at some other machines may reveal a similar trend. The S-61 had a five blades and a 62 ft. diameter main rotor and grossed out at 19000 lbs. The EH-101 at 32000 lbs. incorporates a 61 ft. 5 bladed main rotor. This is probably an upper limit for both the diameter and the number of blades. The H-53A/D series began with a 6 bladed 72 foot diameter main rotor at a gross weight of 42000-50000 lbs. When the weight grew to 73500lbs the diameter was increased to 79 feet and a seventh main rotor blade was added. The S-92 may be approaching the upper weight limit for a 4 bladed main rotor.

[Graviman]

Jack, Thanks for the helpful response. I had forgotten about frame modes often being uncomfortably close to the rotor modes.

My original resonance question was probably unfair because by the time a rotor is fitted to an airframe it has already undergone significant whirl tower testing. I would imagine that this does as much to validate the blade service strains as it does to validate the rotor dynamics. I would equally imagine that it is not unheard of that a prototype rotor requires some redesign before being fitted to a prototype helicopter.

We live in interesting times, and i am certainly curious how the next generation of X2 derived machines will cope with the need to reduce Nr in flight at high speed. Probably numbers of blades will give way to wide chord blades, allowing more structure (which gets back to your ballistic tolerance comment).

[nikhilheb]

Hello fellow rotorheads!

I wanted to know what the maximum number of rotor blades a helo can have is. What are the overall differences in multi blade rotors? Considering lift required, drag, power available, aspect ratio, solidity ratio, tip speed and any other factors you feel important, and at the same time disregarding landing space available, parking space, maintenance etc. Want a purely aerodynamic opinion.

Any and all inputs are greatly appreciated!

[Gemini Twin]

I hate replies that begin "It Depends" but it really does depend on what the designer has in mind for operational capabilites. It used to be that high lift, low speed want with two blades ie Bell 47, 205, 214 and high speed went with multiblade systems ie MD369 family. Current thinking is leading to multi blade systems that are fast and quiet.

[AdamFrisch]

OK, amateur explanation here:

2 blades - least drag. And because they normally have to have a greater diameter to compensate for less blade area, they have more lift. Big rotor diameter = greater lift and/or less power to lift same weight. Take your pick.

Multiple blades have lower Reynolds numbers, higher solidity and seems to produce less noise and less vibrations, but also have a more complex damping and hub system.

It seems to me that whenever a manufacturer choses a multi blade system, they also reduce the diameter to reduce the drag. This makes for agility, but will use more power and more fuel to lift same amount of weight. It would be interesting to make a big diameter, multi blade helicopter with high aspect ratio blades to mimic the lesser drag of a 2-blade system. I'm sure it has been done and I'm sure there might be some other drawback to that. Maybe blade structural?

[Gemini Twin]

I believe the actual reason to reducing diameter by using multi blade systems is to allow greater maximum speed. For example imagine a 369 with a 206 rotor system it would lift more but have much slower Vne. Probably have better autorotation characteristics too!

[Jack Carson]

Another aspect considered in military aircraft design is ballistic tolerance. Less blades of greater robust design equals greater ballistic tolerance. The four bladed systems on the Blackhawk and Apache seem to suggest this was the best combination or compromise arrived at in the 1970’s. This may change with the improved composite materials of today’s rotor blades.

[Hughes500]

Hm
comparison between 206 and 369. 369 has a greater mauw at 3550 lbs ( 3000lbs if internal) v 3200lbs with the same engine. I know the 250 works harder in the 206 then the 369, bleed valve is almost fully open in 369 in cruise, not so in 206 according to the engineers.
Mind you you can only make a blade so long before it loses structual integrity or is so heavy that it is difficult to turn it !