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bhunter2112
01-20-2006, 11:37 AM
Just a quick question for anyone in the know. Some fighters have 3 blades and some 4. Any plus and minus for each? And if one was better why did they all not have that number?
Mahalo

bhunter2112
01-20-2006, 11:37 AM
Just a quick question for anyone in the know. Some fighters have 3 blades and some 4. Any plus and minus for each? And if one was better why did they all not have that number?
Mahalo

MLudner
01-20-2006, 11:46 AM
More blades give additional power by increasing the amount of air the props can move. Broader blades have the same basic effect.

Aircraft manufacturer / Airforce in question likely did not see it as necessary. Bf-109's, for example, have 3, but they're broad.

p-11.cAce
01-20-2006, 12:13 PM
Not that it matters in this case but many of the ultralights I've flown have 4 or even 6 blade props cause you can gear them down and keep your engine screaming at high rpm without losing thrust. In addition the more blades the quieter (in general) which is good for UL and regional propliners.
This is from the Hartzell Website:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> If two-blade propellers are the most efficient, then why don€t all propellers have two blades?
The short answer is because efficiency doesn€t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will be highest. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.
</div></BLOCKQUOTE>
http://www.hartzellprop.com/engineering/engineering_faqs.htm

jugent
01-20-2006, 02:26 PM
If the propeller-tip speed gets close to the soundbarrier, it will loose its effect. A twoblade-propeller must do more rpm compared to a fourblade, to get the same effect.

The more blades you got (same pitch and area), the slower rpm you need for the same effect.
It is difficult to set a largearea propeller with good performance. You have to have a total different shape of the propellerprofile on the tip, compared to half way in to the center.
So it better to make multi-propeller blade.

XyZspineZyX
01-20-2006, 02:34 PM
The reason for more than 2 blades is to absorb the enormous amount of HP coming from the engine. Also, the more blades you put on a prop, the more you can reduce its diameter to a useful range. A 2 blade prop on a 2000 HP engine would be absolutely huge and impractical.

The more HP you have the more blades you need to transfer the HP into thrust. This is why you see the early war fighters flying about with 3 blade props and the late-war higher HP engined fighters starting to have 4-5-6 blade props.

jimDG
01-20-2006, 02:51 PM
in theory, the most efficient prop has just one blade (and a little ball on the other side to mass balance it). Some model airplanes have this.

In practice, as already pointed out, all of the power of the engine has to go somewhere. A shorter blade would spin faster than a longer one, given the same power. But, as soon as the tips aproach the speed of sound the prop efficiency decreases (it starts moving air arround as opposed to backwards). The length of the blades is limited by the ground clearance (height of undercarriage). So, as you can't have too long legs, one has to add more blades (this also reduces efficiency unless its a counter-rotating prop like on the Tu-95 Bear).

Alternatively, you can make a kinky wing that first goes downwards, then upwards (like on the Corsair) and attach the wheels to the kink - this gives you some extra ground clearance, and allows a larger diameter 3 bladed prop, as opposed to a less effcient smaller 4 bladed prop like on the p-47 (they have the same engine)

Taylortony
01-20-2006, 05:00 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jimDG:
in theory, the most efficient prop has just one blade (and a little ball on the other side to mass balance it). Some model airplanes have this.

</div></BLOCKQUOTE>

some powered gliders have thie it throws it out of the nose cone