PDA

View Full Version : Let's talk props, two or three blades



Papee
07-01-2016, 08:24 PM
I've never had a preference either way. I've seen airboaters push the three blades as being more efficient. Efficient at what? I've never seen any real explanation. Of course design has a big part in it but I'm thinking apples to apples here. We're not talking about pitch or blade design here, just two or three blades.I'm figuring the airplane engineers may be a bit smarter than a bunch of airboat hillbillies on this subject. From what I'm seeing all or most of the lower powered airplanes run a two blade prop, you don't see the three blade props until larger planes and power plants. For this reason I've always leaned in the two blade direction if any.

Thoughts?

kent
07-01-2016, 08:37 PM
I use a three blade on the boat because I can have a small diameter which equals slower tip speed which equals less noise

Papee
07-01-2016, 09:02 PM
I'm not sure I even heard your prop Kent. That thing sure is a monster taking off, very impressive.

Smaller prop is a good point though.

aerokirk
07-02-2016, 09:04 AM
As far as aircraft use, there are basically three considerations when it comes to number of blades, first would be the horsepower that needs to be transmitted and then the amount of ground clearance to the blade tips (more blades allows for smaller blade lengths) and finally, a three bladed prop is generally better for damping vibration. In my research for my build I was most concerned with noise so I wanted to get the most thrust with the slowest prop rpm which is why I went with the re-drive and longer/wider whirlwind prop.

Papee
07-02-2016, 09:21 AM
Good points also but lets take tip speed and noise out of this. The discussion is about the number of blades and which performs better on a small power plant. Like I mentioned, apples to apples. If you have a two blade prop with as much surface area as a three blade it should be about the same thrust. As far as rotating mass it should be about the same weight as well with both versions being made of the same material. The only benefit I am seeing is with three blades you have the blades closer together cutting the air more efficiently. What that does as far as gains or performance I'm not clear.

This is not really a right or wrong deal, the purpose of the thread is to get more information on why we make the choices on props that we do. I guess we should also keep reduction drives out of this since they are just a "horsepower multiplier".

Same motor, same HP in both instances, two or three blades.

Doozit20
07-03-2016, 11:15 PM
Hmm this is a tricky subject as there many different ways of answering. I would say more blades for more push but at the same time a wide blade may push better. I've heard a short multi blade will spin up snappier then a longer 2 blade prop. And yet again take a helicopter for example...long blades slow turning huge thrust (lift)..

Corky
08-11-2016, 10:49 PM
Papee, I ran across this guy's comments on airboat props and wanted to post this here for some reading material.....


When you are moving, dynamic thrust takes over and you need more pitch to maintain the efficiency of the prop to convert HP to thrust. This is why planes have variable pitch props and this concept has been demonstrated by the Water Walker Concept boat. As you get faster (MPH), you need more pitch to maintain this efficiency. That is why it appears that boats run better with more pitch.

More prop diameter creates more thrust as long as you can put enough pitch in the prop. The larger disc area generates more thrust. However, the increased prop diameter causes you to give up snap. This is why you see all of the race boats with smaller props. They are trading thrust for snap when racing 400 feet. More prop diameter also lowers your top end speed (MPH). This is because there is less pitch and more blade area to cause wind resistance.

Wide and super wide blades make more static thrust when you are "stuck" than skinny blades. The skinny blades shine at higher speeds (MPH) and create less vibration.

A 4 blade prop will tighten the thrust curve and produce thrust sooner than a 3 blade prop. So an engine running 5000 RPMs with a 4 blade prop will make more thrust than the same engine running 5000 RPMs with a 3 blade prop (I am assuming the same gear ratio and the pitch in each prop is reasonable). The same is true when comparing a 3 blade and 2 blade. The 3 blade prop will produce thrust sooner than a 2 blade prop.

Horsepower is what determines how much prop you can turn and an engine will turn the maximum amount of prop where the maximum HP is made. You don't even need to look at the torque curve. All the torque curve tells you is how much the prop will accelerate. Torque isn't useful until you know how much torque over distance and time (BTW this is HP).

So here is a couple of cheap things to look at. Have you verified that your tach is reading correctly ? Most people don't realize how much error is in a tach even new from the factory. Have you experimented with adding more pitch to the prop and reducing WOT RPMs? This will lower your cruise RPM and improve fuel mileage. More flat bottom will help you plane better... Also what size of cage wire do you have? Less wire reduces air resistance and gets more air to the prop. Ultimately everything is about trade offs....

Papee
08-12-2016, 12:48 PM
So what I am getting from that is that a two blade is really a better all around prop. It won't make the quick thrust but will be better at cruise and higher rpms. The cage wire I am familiar with and is the reason I run the fence wire with the openings cut to 4x4 rather than the 2x4 they come as.

TLL
08-25-2016, 07:32 AM
Experience tells me there's a bit of sweet spot concerning blade pitch for slow moving machines (paraflyers @30mph).
If one has selected HP and a reduction ratio, all that is left for practical tweaking is the prop. Prop pitch at .75r seems happiest at 10-16 degrees at low travel speed (UltraProp is a little different).
Also adding pitch to absorb HP is much less thrust effective than adding blade length or blade speed to accomplish HP absorption.
Given constraints of prop RPM, fixed HP, prop diameter, adding a blade or two is better than pitching a 2 blade out of the sweet spot.

yamahaulerG1
09-10-2016, 10:22 PM
I have found out it's hard to compare anything on these mini's to each other but here's what I've found out for one TLL is very right about sweet spot in the pitch ultra props seem to be about 12deg. I went from a 53" 3bld ultra to the 4bld but 51" and set to run same rpm and it didn't make a real noticeable difference just little less top end and reved up slower went back to the 3bld. Later bought a wood prop 54" from Culver thinking I would get more push in weeds it was hard to get on plane in deep water and lost about 5mph. So I'm leaning towards more blades are better.

TLL
09-11-2016, 06:53 AM
I have found out it's hard to compare anything on these mini's to each other but here's what I've found out for one TLL is very right about sweet spot in the pitch ultra props seem to be about 12deg. I went from a 53" 3bld ultra to the 4bld but 51" and set to run same rpm and it didn't make a real noticeable difference just little less top end and reved up slower went back to the 3bld. Later bought a wood prop 54" from Culver thinking I would get more push in weeds it was hard to get on plane in deep water and lost about 5mph. So I'm leaning towards more blades are better.



Descending order of importance for increasing prop thrust (low speed vehicles)
more HP:-D
more prop diameter
more prop rpm
more blades
more pitch

Multi bld props can involve some thrust tail-chasing because more blades absorb HP through unproductive drag. Figuring out a good prop setup is important, sharing info with builders saves money and frustration.