A question about muliple bladed props...

[jim gilmore]

Does a multi-bladed propeller deliver more thrust than a propeller with less blades and why don't we use even more if it does

[Dan McEntee]

  The simple answer is what can the engine "use." What does it take to make best use of the power that the engine makes? What will the load be on the engine in the aircraft in flight? The speed guys sometimes use single blade props. The theory is that the single blade is basically always in undisturbed, clean air. To the other end of the spectrun, the rubber powered free flight guys sometimes use single blade props, more or less for the very same reason. This is something that you canstay up all night and drink alot of beer discussing and argueing about, and is a very interesting subject. To get some other insight, go to the web site for APC props. They used to have a very good section on prop theory.
   I sometimes use three bladed props when I think it makes the airplane look cool!
   Type at you later,
    Dan McEntee

[Motorman]

The more blades you have the more frontal and surface area drag you have so there is less power available for thrust.

MM

[Howard Rush]

This is a rather esoteric question.  You will get some BS answers.  You can get the lowdown from Martin Hepperle's site, http://www.mh-aerotools.de/airfoils/javaprop.htm , or from Adkins, C.N. and Liebeck, R.H., Design of optimum propellers, AIAA Paper 1983-190, January 1983, https://www.aiaa.org/IframeTwoColumn.aspx?id=4745 .

Also, http://www.supercoolprops.com/

Maximizing thrust so you can go as fast as possible with a given engine is one thing, but if you are flying stunt, you are probably interested in having the prop thrust change as much as possible when speed changes: adding thrust when the airplane slows down, reducing thrust when the airplane speeds up.  The above sources can help with either.  

In general, for model airplanes, the fewer blades the better.  That allows each blade to have more chord and operate at a higher Reynolds number.  In our speed range, a higher Reynolds number is better.  There are other considerations: wood makes better two-blade props than three-blade props.  Full-scale airplanes have other constraints like landing gear clearance and prop tip Mach number.  

For stunt, you want to keep blade diameter down so the airplane can turn better than it would with the optimal (for efficiency) prop diameter.  You also want to keep blade diameter down to minimize prop moment of inertia.  Reducing prop moment of inertia reduces the airplane's tendency to yaw in corners.  This may lead you to more blades.  Another reason to favor a three-blade prop over a two-blader for electric stunt is that prop inertia opposing a turn changes less with the three-blader.  When a two-blade prop is straight up and down, it has a lot of inertia opposing the airplane's turn; when it's sideways, it has very little.  You can hear and feel a shudder from a two-blader in a corner.  So can your motor bearings.  For this effect, a three-blader is better than a two blader, and even better than a four-blader.

[Howard Rush]

Before Igor beats me to it, I should add that reducing prop diameter also reduces the "p" effect in corners.

[jim gilmore]

My questing does in some ways does have a side to it  that might reflect more on real airplanes than the average.
I tend to fly smaller models than most. And would prefer to be able to be stooge launched. So for me I want to maximize ground clearance. I also wonder if a smaller multi blade prop might be a better thing for the 1/2a magician since it might reduce waggle in the corners??
I have not yet bough or started to build my model since a I'm waiting till next month. Its a little cool to be sitting in the desert at night where I tend to spend my time.
Hope to build it and fly it in less than 3 weeks time from start to finish....we shall see....

[Dan McEntee]

  Depending on what you use for an engine, and three blade prop of smaller diameter may be beneficial. A Cox 3 blade was one of the "hop ups" for the foam wing stunters. APC makes some nice props that will work fine in the 5 inch diameter range, I think if you are using a Norvel, TD or Medallion .049. Larry Renger did the construction article in Flying Models for that airplane and I think he covered engine and prop combinations in it. If not I'm sure he'll pop in with some insight into the issue.
  Good luck and have fun,
   Dan McEntee

[jim gilmore]

Unfortunately this will be an electric setup.
But as I have said it seems the most common thing is to run larger diameter props on 1/2a from what I have seen.
But I want to make sure ground clearance is not a problem on take-off.

[Igor Burger]

Less blades have better efficiency especially because of higher RE number on tips of larger blade, because they move at higher speed and they have longer chord (as the prop is proprtionally larger). So yes single blade has advantage on speed models (that is why they use single blades), however they and also two blades make troubles with uneven mass distribution as Howard wrote (before I had chance to beat him :- PPP :- )))) ) ... three blades are lowest number of blades wich soves that problem, so that is reason why we use just that.

I also heard resoning that "more tips makes more tip loses". Well ... if one rotating blade has loses 40% two blades will have also loses 40% as well as 3 blades IF they have the same parameters. The "tip loses" reasoning is true in case that blades are cut on tip, so the blade aspect ratio is lower on 3 blade prop then on 2 blade prop.

If I had to choose prop for small model I will use 2 blade if possible. Just because reynodls number. Shaking, precession, P effect does not play large role on small models, but you will know only if you try ... so ... :- ))) 

[Motorman]

You can get the lowdown from Martin Hepperle's site, http://www.mh-aerotools.de/airfoils/javaprop.htm , or from Adkins, C.N. and Liebeck, R.H., Design of optimum propellers, AIAA Paper 1983-190, January 1983, https://www.aiaa.org/IframeTwoColumn.aspx?id=4745.

Also, http://www.supercoolprops.com/

If you are flying stunt, you are probably interested in having the prop thrust change as much as possible when speed changes: adding thrust when the airplane slows down, reducing thrust when the airplane speeds up.  The above sources can help with either.

Howard, you've read through the above rhetoric, how do you do that impossible task?