2 vs 3 blade props

As tip speed rises past mach 0.6, there is a very noticable increase in prop noise, due to the fact that local flow speeds around the prop foil approach the speed of sound. To keep this noise down in flight, it has been published in European Aerobatic circles to prop not higher than a tip speed of mach 0.5 with the plane static. This also increases prop efficiency a bit, and provides better braking in down figures.

Absolutely, and for a little more depth and a chart to help prop selection:

Yes this is a factor, but pretty small. Consider a stationary plane and air moving through the prop. Although the plane is stationary the air moves through the prop and behind the plane. Note many planes will twist the rudder back and forth when you rev the engine up and down. That is p factor the primary reason the plane wants to go left when you give it the go juice. It is caused by the wake. It travels back at slightly less than the prop pitch. Consider a bi-plane with a fairly wide spacing between the wings, say 18" for wings with a 12 inch chord, that is a ratio of 1.5 to 1. Now consider a prop with 6" pitch and 1 1/4" choard. Adjust for slip and say 5.5 inches that is a ratio of 4.4. So that loss is pretty friggin small. The main reason the prop is less efficient is the increased number of blade tips tearing up the air, not interferance between the seperation of blades. And they don't travel through each other's wake even when static. For our purpose and slower prop planes the ground clearance is the primary reason to go to multiblade props. High speed planes can gain from them because you can go to a smaller diameter and higher pitch with the same loading and also the effect of a smaller higher velocity stream of air thrusting the plane foward, but again not a factor for our use.

U'mm Paul said that the extra blades were for asorbing power in the IMAC planes due to ground clearance. The example you gave was improved high speed performance!

You are missing the point. Where is the air coming from when the prop pulls it in? From the front of the prop disc. What happens when we pull on air? It moves and its pressure and density drop. What happens when it moves? It has a shearing action on unmoving air all around it, and this creates turbulence. ALL of these things happen in FRONT of the prop, never minding all the stuff happening behind it. More prop blades means more blades that have to operate in this less-than-ideal environment. And that's ignoring the effect upon air of a cambered airfoil moving through it. Sure, it forces air down or back or whatever, but the curved upper surface, the camber, disturbs air ABOVE or IN FRONT of it, as applicable. These things are never as simple as they appear.

Dan

U'mm is right. Why do you think the WWII birds went to 3, 4, even 5 blade props and the Corsair had the bent wings??????? The ever increasing power had to be absorbed AND ground clearance limited the diameter of the prop.

David

For high speed performance, nothing beats a single blase prop! Look at what is used in CL speed.

U'mm so why did they change some of them from three to four blade with the same engine and power?

Single blade props are not used anymore, least not often. Besides even speed record C/L models do not approach the speeds and altitude of WW II fighters. That is in dense air and speeds around 200 MPH single blade props may well be the best solution.

The air in front is expanding and thinner and is rotating with the prop as well, the next blade even with 6 blade props is little efected by the preceding blade. When you get rid of the tip losses with a shroud or duct then it is effecient to put blades right behind each other, as is done with jet turbofans.

Because most of the prop development was going into the 4 blade series. Also, I can't think of any common plane that changed from three blade to four blade without some engine or airframe changes.

acceleration (more

Ditto the Republic P-47. The original had a 3 bladed prop which could not properly apply the hp put to it. They then went to a 4 blade, but the blades were still skinny. Finally, they went to the butter knife or paddle blade prop. With NO increase in hp, the P-47 went from a vertical dog to outrunning the Spitfire ( and German planes as well ) in a climb!!!!!

IIRC, it was in Robert Johnson's book that it was related that the way for a German plane to get away from a Jug was to go vertical. It was a major shock to Luftwaffe pilots when, suddenly, the Jug was able to chase after them when they went vertical, thanks to the new prop.

On an aside, why does the C-130J use a 6 blade prop when 4 blades has been the norm for umpteen years. And I think the A model had 3 blade props!

David

generate

Mostly because in a dive the blade tips wre goimg supersonioc or at least near mach 1. 4 blades slowed the engine down a it.

Single bladers were extensively used in rubber contest models, so their use is NOT restricetd to high speed flight.

Also remember that prior to computers, aerodynamics was very much a 'test and try' science. Even if you got the right theory, the maths of spirallng semi turbulent airflows was and is way beyond what most engineers can calculate in a sensible time with a slide rule.

In wartime, it was more or less a question of 'new airframe, new engine, try half a dozen props on it and see which one works best'

Pretty much the way I approach my electric models too today! :-)

As I ave said before, planes have flown successfully with between one and 6 blades on the prop, and more in ducted fans. If there was one uniquel best way to do it, thats would tehy woul ALL be using. it would therefore sem that the devil is in the detail, and is all about RPM, prop blade area, turbulent flow, pitch angles, camber and blade shape, aircraft shape, ground clearance etc etc ad nauseam.

I think the reason was not the slow down of the engine, but the reduction of the blade radius, when going to four blades.

If the prop is made smaller, the tip covers less distance per revolution, decreasing the tip speed with the same RPM.

If I'm not mistaken the reason was to keep the tips from going supersonic...

cheers Papa Sharptooth

Bouncing about the living room, my then three year old grandson declared himself "a kangaroo". "If you're a Kangaroo, what is Grampie", I ask. His reply:  "you a dinosaur"

That was the purpose, the method was to shorten the prop.