Propellers: less blades vs. more blades
 

Hello!
I was wondering why some propeller airplanes have 2, 3, 4 or more prop blades. I've seen, for example, C210's with 3 and with 4 blades, this last one looks like a modification the owner did, and I don't know if it has something to do, but I have seen this mods more often in european planes. On the King Air 90 as well, seen them with 3, 4, or even 5 blades. Why is this?
My thoughts: :ugh:
More blades mean more drag, but the surface to move air is greater, so you'll need less speed to move the same amount of air as with a prob with less blades, so... is it the same thing in the end? also, more blades are more expensive, so what's the advantage on this?
It would be much appreciated if someone could technically explain this.
Thanks a lot! :}
ludo

You are more or less right in you assumptions- More blades of the same area at the same pitch produce More thrust but also require more power. It's probably analogous to the gear ratio of a car- more blades equals higher gear and is more efficient at higher speeds (Hence may modern turbo-props have multiple bales), fewer blades equals faster acceleration (Needed for small aircraft using Short strips)

The reason, however, that you see more 3 or 4 blade retrofits in Europe is not because of this, however. More blades allows the blade Diameter to be reduce, reducing tip-speed for a given RPM. Much of the noise associated with prop aircraft is due to the prop tips operating at a high percentage of Mach 1.

More blades=smaller diameter= less tip speed= less noise. Necessary in noise conscious European cities.

Another benefit...more blades allows the propellor to absorb increased engine power (on those aircraft so modified) while allowing better ground clearance, due to the propellor diameter being contained.

More blades helps to improve climb performance but also account for a reduced cruise speed due to the parasite drag it creates

You can google, or otherwise reserach the term propeller solidity :ok:

More blades, less diameter then put a casing round them all and you have a multi bladed shrouded propeller, i.e a modern high bypass jet.

Just to complicate things, I've seen photos of a single-blade prop (with a counterweight, of course). Presumably less interference drag (think monoplane vs biplane...), but there must be quite a vibratory condition resulting from unbalanced aero load. :uhoh:

bfisk: thanks for the hint, really interesting stuff.

Once more, questions solved!
Thanks for all your responses.

Flow interaction reduces prop efficiency, more blades less efficient. However, more blades will allow more power absorption within a smaller blade diameter reducing noise. With newer airfoil designs the efficiency loss can be compensated.

Get ready for the seven-bladed prop from MT-Propeller of Germany. Already being tested on a Piper Cheyenne.

Apparently, more blades allows lower rpm gearboxes reducing blade tip drag. The number of blades, diameter, twist, chord, and thickness distribution all determine propeller efficiency which is driven by the lift distribution along the blade radius and the airfoils. Most modern designs have been designed with reduced lift at the tip reducing the vortex.

An interesting statement in the article I am reading about the 7-bladed prop is a quote in the article by a spokesman...."Everybody told us it's not possible"......."They said it was dangerous;that fuel control units will go into resonance and King Air's will crash ".

Anybody have more details about FCU resonance and number of prop blades?

Oh wow yet another blast from the past!

or "more blades vs fewer blades".

Anyone tried prop tip winglets?

If you go from two blades to three, then for the same rpm and power, the diameter will be somewhere around 10% less. For some single engine aircraft this gives a bit more prop to ground clearance, same cruise speed, but less thrust for the climb.
There are several design programs for model aircraft props, where you can make alterations to get the performance you require. Maybe this also works with full size props..... ( edit.. Yes it does.. )
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http://www.godolloairport.hu/calc/strc_eng/index.htm
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Just some more observations on model airplane prop design that should carry over to the full size aircraft...


The diameter will determine the static thrust, and the Pitch will have to be selected to match the cruise speed. Mathematically, because the diameter squared determines the swept area, any delta increase of diameter will need twice that amount of reduction of pitch. So starting with an 11x7 for the usual model engine, If you want more speed go for a 10x9, or for a better climb ( vertical.?) try a 12x5.


There are snags to going oversquare though ( 9x11 for this example) in that at a standstill or slow taxi speeds, the propeller will be stalled. Some speed freaks, put up with this slow take-off, as it gives them speeds of over 200mph in a dive. Conversely some Fun-Fly pilots use 13x3 props to give prop-hanging performance, with colossal amounts of static thrust. Increase the diameter ad-infinitum and you will end up with a helicopter !

Another question I have about two bladed versus three. The question may be very simplistic but I have been out of prop aircraft for a long while and have delightfully gone back to them.

Does the P factor change when you switch from a two bladed propellor to a three bladed one? I have an aircraft which was designed for a two bladed prop (C177RG) and I bought it with a three bladed. Would you expect the three bladed prop to require more right pedal (or right trim) at high power settings than the two bladed one? I need an inordinate amount of right rudder trim in climb and cruise, and was wondering if this could be one of several reasons.

Thanks in advance

I guess that a big powerfull engine, with only two blades would get the speed of the tips so far out (to absorb the power), that compressability problems will occur... hence the bigger the engine, the more blades to absorb the power... without the tips spinning too fast.

I think?