I see airplanes with two, three and four or more blades on the propellers. does this help much ?
A lot of factors would come into play for determining the number of blades. One reason for more blades would be a limit on the length of the propeller. If you can’t make it longer because of physical limitations more blades can be added but the propeller may not be as efficient. One reason for this limit is the speed of the engine, the tips of the blades can get near the speed or sound or dangerous shock waves would be produced. Slowing down the engine or adding gearing to slow it down may be less efficient than adding more blades. There are plenty of other considerations based on the type of engine, whether the plane is intended for high speed or low speed use, cost, whether the propeller blades are fixed or variable pitch. More blades means there is some advantage or they wouldn’t be used.
In 2011 I posted a thread asking about the number of propeller blades.
Each propeller blade disturbs the air for the blades that follow it, so with more blades you get more disturbed air and less efficiency. People have actually made single blade propellers. They generally have counter-weights on the other side so that it’s not unbalanced and doesn’t shake itself to death.
On the other hand, fewer blades means longer blades if you want more power. If the blades are too long and move too fast, the ends start to go supersonic, which makes your efficiency go to hell in a handbasket really quickly.
Longer blades also means that the plane needs to sit higher when it is on the ground so that the blades don’t scrape the runway. For longer blade lengths, that can mean impractically long landing gear. Putting the wing on the top side of the fuselage (your typical gull wing aircraft) allows you to use a longer bladed prop without the blades hitting the ground.
So everything is a trade-off. There are times when you want fewer and longer blades and there are times when you want more and shorter blades.
More blades, thinner slices.
Behold a fascinating bit of aviation history, the Republic XF-84H “Thunderscreech”.
Unfortunately that thread had a lot of speculation and relatively little real data.
The below is from Flight Testing of Fixed Wing Aircraft by Dr. Ralph D. Kimberlin:
“…noise may be reduced by increasing the number of blades. However as the number of blades increase the efficiency of the propeller decreases. A propellor with only one blade is the most efficient propeller”
The below is from the December 1982 issue of Flying Magazine, by J. Mac McClellan, editor-in-chief:
“…many other factors are involved in prop efficiency, but diameter is the largest single factor. For example Hartzell Propeller Senior Engineer Dick Edinger calculated that an 80-inch-diameter prop would be 29 percent more effective at absorbing horsepower than a 76-inch-diameter prop on the same engine…but ground clearance requirements usually limit prop designers to rather small-diameter propellers”
“Another limiting factor is that larger props must turn more slowly than shorter ones because increased blade radius increases the prop tip speed…Prop blades operating in the transonic or supersonic speed range lose efficiency and produce a great deal of noise.”
“Extra blades usually lower prop noise because each blade is performing less work. Extra blades can also lower vibration because the prop’s effort is spread more evenly through the prop arc. Four-blade props, however, can sometimes be noisier than props with an odd number of blades. With four blades, the blades operate in synchronization around the prop arc. The blade on the way down passes the nacelle, for example, at the same instant the blade comes up past the nacelle on the other side. This can cause a pulse of noise as the blades pass the airflow obstruction. Props with an odd number of blades never pass an airflow obstruction in synchronization and may be less noisy.”
In the March 1995 issue of Flying Magazine, aerodynamicist Peter Garrison said the following:
“By the same token that a wing can carry only so much weight at a given speed and no more, propeller blades can only be loaded up to a certain point. When a propeller has to absorb more and more power, it has to have either more blades or broader ones. In theory fewer blades are better from the standpoint of aerodynamic interference, but powerful airplanes have no choice but to use four, five or more blades, and the subtle association of blade number and speed has helped to make multibladed props popular even in general aviation, where they are rarely needed. I recently saw a press release from a company manufacturing five-blade composite props for general aviation airplanes, claiming they are superior in efficiency to ones with fewer blades. The effect of blade count on efficiency is hard to isolate from from effects of blade twist, planform and section, and it would be interesting to know how rigorous was the testing that supports that claim.”
Modern fan jet engines use fan blades to move extreme amounts of air around and through a jet engine. They appear to have upwards of 40 blades. They aren’t propellers in the classic sense, but they pretty much do what propellers do.
yes, but as I understand it with jets, the fan is forcing air through a duct, and the close spacing of the blades minimizes the air which “pushes back” forward through the fan.
Two blades seems pretty common.
How does this work? The counterweight can offset the mass of the blade just fine, but the thrust of the blade would produce an eccentric axial load, resulting in a moment about an axis perpendicular to both the blade and the shaft. This could not be offset by the counterweight, unless the counterweight is positioned somewhere aft of the center-of-mass of the blade to produce a countertorque. And even then, you could only optimize things for one single thrust setting.
Were these just lab prototypes with a short lifespan, or did someone actually take to the air in a plane powered by a monoblade prop?
It’s been attempted with helicopters. The scarcity of such machines shows that it has problems. Helicopters have to deal with load disparities already and the single blade makes it worse in any transition. I’m not sure the eccentricity you mention is that much of a problem under ideal conditions but given changing direction of the aircraft relative to the air it flies in the problem will worsen.
A recent design for a helicopter had a complex means of shifting the counter-weight as it rotates to deal with these issues. Another approach was even weirder based on the shape of a maple seed.
I don’t know how often they are actually used, but here’s a youtube video of a 1936 J-2 Cub that has been modified to use a single blade prop. Seems to work well enough.
The gull wing design of the famous F-4U Corsair isn’t just to make it look cool. It’s to give ground clearance to that huge propellor.
The TU-95 Bear has 4 sets of contra-rotating propellers on its engines, and the tips are supersonic. This BBC article calls it “the world’s noisiest spy plane.”
I liked this quote from your article.
I used to think that the mechanism to make contra-rotating blades was probably some complex and delicate gear arrangement, but then I looked up exactly how it worked and I was surprised to find that it is very simple and rugged.
You can hinge the prop (blade and counterweight) so it is free to teeter. Forces due to radial acceleration (I am avoiding the whole centrifugal force debate) will keep the blade from coning too far forward. There will then be only the thrust loads transmitted to the crankshaft. Stops are needed to limit the motion when starting the engine.