Naked Science Forum
General Science => General Science => Topic started by: neilep on 02/09/2007 19:11:01
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Why doe some helipcotermobiles have two and or four blades ?
FOUR BLADES
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TWO BLADES
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why not 6 or 8 ?
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It is basically about what you are trying to lift, for a certain radius each blade will give a certain amount of lift, so if you have a small light helecopter 2 blades will be fine, the heavier the helecopter the more blades are useful. the more blades you have the more complex and expensive the rotor is so you probably want as few as possible. Really big helecopters have more blades eg the CH53 has 6 blades
http://en.wikipedia.org/wiki/CH-53K
Some more modern helecopters have odd numbers of blades which I think is harder to build but quieter
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I went in one that had 5! Can't remember the make -could have been a Harris(?)
There is an issue of tip speed; it mustn't approach Mach 1 or you get turbulence, damage and inefficiency.
Several short blades give you similar lift and you can rotate them faster.
Why doesn't anyone make a modern Chinook?
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It is basically about what you are trying to lift, for a certain radius each blade will give a certain amount of lift, so if you have a small light helecopter 2 blades will be fine, the heavier the helecopter the more blades are useful. the more blades you have the more complex and expensive the rotor is so you probably want as few as possible. Really big helecopters have more blades eg the CH53 has 6 blades
http://en.wikipedia.org/wiki/CH-53K
Some more modern helecopters have odd numbers of blades which I think is harder to build but quieter
THANK YOU DAVE !
I see...lighter the chopper...less blade joy required.....
But would a four blader on a lighter chopper need spin so fast as a two blader ?..I'm thinking fuel efficiency here ...though..I see your point regarding complexity....
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I went in one that had 5! Can't remember the make -could have been a Harris(?)
There is an issue of tip speed; it mustn't approach Mach 1 or you get turbulence, damage and inefficiency.
Several short blades give you similar lift and you can rotate them faster.
Why doesn't anyone make a modern Chinook?
Me want to come for a ride too !!
THANK YOU Sophiecentaur.......I did wonder about odd numbers of blades but felt I might be silly in mentioning it.....
I see what you mean about turbulence at speed of tip revolution....
...do you know what kind of speeds they do achieve ?..I gather it must vary quite dramatically but is Mach 1 the big no no then ?
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is Mach 1 the big no no then ?
I think it introduces lots of problems which are best avoided.
It's not possible (I'm pretty sure) to make a propeller aircraft aircraft go at Mach 1. This is probably for the same reason. Several small airscrews are better than one - one reason for having multi-engines.
I guess the tip speed of a helicopter blade is made as high as reasonable., whilst avoiding supersonic speeds.
If the blade radius is 5m, the rotation speed could be up to about 10 revs per second before Mach 1 is reached at the tip - sounds about right. "tupatupatupa....."remember?
A fixed wing aircraft airscrew sounds much faster - 0.5m radius could do about 100 revs per second -"brrrrrrrrrrrr" - ten times the speed.
How do you like the multimedia demonstration folks?
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This is REAL multimedia...
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbestsmileys.com%2Farmy%2F1.gif&hash=2a8fdc32e1c82e0e3e243bd93dce9597) tupatupatupa
[:D]
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Let's have a brrrrrr too.
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Let's have a brrrrrr too.
THANK YOU sophiecentaur.......for the above post.......what about jet engine blades...they spin well fast don't they ?
Here's a real helicopter...!!
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brrrrrrrrrrrrrrrrrrrrrrrrrr !!
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If you had like 8 blades on a light chopper.. What difference would it make than the same light chopper with 2 blades?
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If you had like 8 blades on a light chopper.. What difference would it make than the same light chopper with 2 blades?
It wouldn't be the same chopper as the engine and drive systems would have to be changed. The number of blades a helipcopter has is all to do with what you want your helicopter to do. If you want heavy lifting capabilities then you go with a very powerful engine with lots of blades. However with that comes lots of extra weight as the drive components will then need beefing up to cope with the extra torque and the stresses and strains that come with it.
And also you then end up with a very high tech piece of kit with a high level of complexity which requires lots of extra components to monitor everything increasing the cost of purchase and servicing.
BUT i believe the more blades you have the more responsive it becomes ,the blades can also be made relatively short allowing you to go to places with restricted access like overhanging trees etc and i believe the ride comfort increases due to less vibrations due to things being better balanced.
Disclaimer. Should any one die due to any of the above statements being incorrect then please remember, i generally have no idea what i talk about. [;D]
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Because aircraft (except balloons) derive their lift from the rate of change of momentum of air, which can be obtained by accelerating a little air greatly, or accelerating a great quantity of air slightly, it is energetically favorable to accelerate the largest practicable volume of air, because energy varies as the square of velocity whereas momentum varies as the first power.
Hence, large diameter rotors are better, in terms of fuel consumption. This does not necessarily translate into higher tip speed, because if the diameter increases, the angular velocity may decrease, to keep the same tip speed, which the volume move will have been increased.
In terms of blades, I don't know; if we can assume that a blade moves only a relatively small portion of the available air, adding blades while keeping speed and diameter the same would increase the volume of air moved and thereby increase lift. However, there could be interactions between the air movements, particularly if the number of blades becomes such that the distance between adjacent tips is significantly less than the radius, which might result in diminishing returns from adding blades beyond a certain number.
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In terms of blades, I don't know; if we can assume that a blade moves only a relatively small portion of the available air, adding blades while keeping speed and diameter the same would increase the volume of air moved and thereby increase lift. However, there could be interactions between the air movements, particularly if the number of blades becomes such that the distance between adjacent tips is significantly less than the radius, which might result in diminishing returns from adding blades beyond a certain number.
Yes, in order for the blades to push air down, they have to permit up air to replace the pushed one, and this can't be achieved efficiently with too many blades.
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if you want lots of blades you could always buy one of these
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Or if you cant afford the above Kamov Ka-52 "Alligator
You could go for the cheaper dragon fly option below
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if you want lots of blades you could always buy one of these
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Or if you cant afford the above Kamov Ka-52 "Alligator
Nice toy. Is it made that way to reduce noise or what?
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if you want lots of blades you could always buy one of these
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Or if you cant afford the above Kamov Ka-52 "Alligator
Nice toy. Is it made that way to reduce noise or what?
The two sets of blades are rotating in opposite directions so the craft is more stable and has less vibration.
With one set of blades you need a rear side stabilizer to counter the effects of rotation drag.
As in the photo there is no rear stabilizing side rotor as it don't need one, just change the speed of a rotor to rotate.
Bee
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Nice toy. Is it made that way to reduce noise or what?
The two sets of blades are rotating in opposite directions so the craft is more stable and has less vibration.
With one set of blades you need a rear side stabilizer to counter the effects of rotation drag.
As in the photo there is no rear stabilizing side rotor as it don't need one, just change the speed of a rotor to rotate.
Ah, yes, I know it, but I was wondering why in some cases they prefer this kind of solution instead of the other kind.
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Well how about this cofiguration The Kaman HH-43 uses two intermeshing rotors.
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http://www.centennialofflight.gov/essay/Dictionary/heli_rotor_config/DI54.htm
but if you reeeeeeeeeeeally want an anwser then you could read this.
http://www.kamov.ru/market/news/petr11.htm
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but if you reeeeeeeeeeeally want an anwser then you could read this.
http://www.kamov.ru/market/news/petr11.htm
It was so simple I should have thought to it. Thank you very much for the link.
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what about jet engine blades...they spin well fast don't they ?
I think the difference is that, in a jet engine, the turbine is inside a tube. The pressure is higher so the speed of sound would be correspondingly higher - allowing faster tip speeds before Mach 1 is reached..
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There is an efficiency trade off once the tip speed approaches Mach 1 and the blade material also becomes expensive but i believe propeller blades can and do exceed Mach 1 if the trade off is worth it . There was a Russian bomber i think it was called "The bear" TU something which did so..
However as Sopiecentar has said A propeller driven plane cant even exceed MACH 1 in level flight so a helicopter would have no chance . For even a jet powered plane to achieve supersonic speeds its fuselage has to be built obeying specific design criteria,criteria which a helicopter could never meet.
When the USA first tried to build a plane in order to break the sound barrier they found no matter how much power they gave the jet it just couldn't exceed Mach 1.They also found that as they approached Mach 1 the plane would become uncontrollable.
The British at the time had a plane called the M.52 which was capable of breaking the sound barrier and the pilot was waiting for the day to officially become the first to go for the record when for some reason ( which became classified under the official secrets act) the government at the time ordered that all test planes to be dismantled and the parts destroyed.
The British government then ordered that the scientists hand over their designs which included a few necessary secrets to the Americans government and the Bell aircraft company. If it weren't for them secrets being handed over chuck Yeager and America would not have become the first to break the sound barrier. (Had to add that bit)
In other words breaking the sound barrier is hard very hard and helicopters are just not hard enough. [:)]
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Why do all the contra-rotating helicopters shown (like the KA-52) still have a 'tail'? Don't they cancel each other?
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The number of blades on a helicopter rotor can be even or odd. Although not used on helicopters, single bladed propellers have been used on light aircraft - these obviously require a counter-weight.
The Russian Mil-26, able to lift 44000lb, has an eight-bladed rotor.
An intrinsic problem with rotorcraft, with regard to directional speed, is that on one side of the aircraft the rotor is travelling forward while on the other side it is travelling backwards. This not only produces uneven lift, which get's worse as speed increases, but it also means that the tip of the rotor that is moving forward is moving at a much higher speed than the heli itself and so limits the max speed to well below the speed of sound.
Helicopters with dual contra-rotating rotors, such as the Kamovs or Kamans, while not needing a tail rotor to counter the torque produced by a single rotor, still use tail control surfaces because they are effective for directional control when the heli as any airspeed i.e. when moving or in winds.
The Russian TU-95 (Bear) bomber, and the TU-114 airliner that was developed from it, are(were) turbo-prop powered (and remain the fastest propeller driven aircraft ever produced). Their propellers are 6.4m in diameter and operate at a max of 850 rpm during takeoff (when airspeed is relatively low) but cruise at 750 rpm. At their highest speeds, the blade tips are supersonic and NATO jet fighters that regularly intercept TU-95 (these days TU-142) have reported that they are painfully noisey to escort.
Propfan engines incorporate an un-ducted fan, typically with an eight bladed single or contra-rotating prop, either as a suppliment to the jet thrust or as a replacement for conventional propellers. Google for General Electric GE-36 UDF (unducted fan) or Antonov AN-70 for some pics.
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I wonder whether adding more blades can cause problems because of each blade having to move through the turbulance created by the blade in front of it. There seems to be a jump from a few blades to ducted fans or turbine-like structures where the principles are slightly different. Is this related to why we do not see biplanes made nowadays? Structurally these are much easier to make than monoplanes so obviously have problems.
Boat/ship propellers are also rarely more than 3 blades, though some higher speed craft use a ducted scheme like a jetski nowadays. The issues of complexity do not come into play here as they are mostly fixed pitch (though yachts may have a simple folding scheme to reduce drag under sail).
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Turbulance is less of a problem than you'd expect - once a rotor or propeller is running you have airflow through it, so in effect the rotor or propeller blade has 'new' air to operate in. A propeller aircraft travelling forward through the air is constantly moving into undisturbed air.
An important factor in wings, propellers and rotor blades is the aspect ratio - this is the ratio between the span of the wing/blade and it's chord. A wing or blade with a high aspect ratio, i.e. one that is long and thin, is more efficient than a low aspect ratio one, which is why gliders have long, thin wings. However, because of their long, thin shape, high aspect ratio wings not suitable for very high speeds, where the wing would twist, or for high angles of attack, where they would stall too easily.
The main reason that biplanes are not made these days, at least on a large scale, is that they are inefficient - the high pressure region beneath the upper wing interferes with the low pressure region above the lower wing. Staggering the wings was one attempt to reduce the effects from this.
Old boat and ship propellers were usually two, three or four bladed affairs and these are still used on many small boats and those with outboard motors where their simplicity and ruggedness has obvious benefits, but most modern larger vessels, including submarines, now use 'fan' type props with eight or more long curved blades.
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Thanks, Lee. That is very clear.