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Why do some helipcoters have two blades ?..and others have four ?

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ukmicky:
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. [:)]

peppercorn:
Why do all the contra-rotating helicopters shown (like the KA-52) still have a 'tail'? Don't they cancel each other?

LeeE:
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.

graham.d:
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).

LeeE:
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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