Four blade props...

[Mike Mulligan]

The pics of Derek's Matrix got me wondering if there is still a source for four blade props. Anyone still making them? Anyone still using them? If I ever manage to finish my Legacy (ST .60) I think I'd like to try one.

Cheers,

Mike

[PJ Rowland]

Derek isnt actually using the 4B.

Brian Eather is making 4Bladers.

There is a reason they are not generally used........

[Garf]

APC makes a few of them. They don't work terribly good.

[Mike Mulligan]

Yeah, I figured they weren't the greatest or they wouldn't be hard to find. I just thought it might be fun to fiddle with, and, well, they DO look cool! 

[PerttiMe]

You could always make one for static display... I don't think I'd want to run a prop after I cut the hub part to join a couple of two blade props into a four blader.

[rustler]

...4Bladers. There is a reason they are not generally used........

O.K. - you have my attention - can you say why?

[Bill Little]

(snip)

There is a reason they are not generally used........

Yes, PJ,

What, pray tell........ what is the reason?
Bill

[Doug Moon]

Derek isnt actually using the 4B.

Brian Eather is making 4Bladers.

There is a reason they are not generally used........

Why?  I must have missed something...   

[Paul Wood]

I'm sure there are other reasons they are not generally used, but here are a few.  Inefficient due to leading blade distorting airflow of the following blade, expense of manufacturing, difficult to balance, impossible to hand start without catching a finger, always catching a blade during a nose over, etc.

Paul

[PJ Rowland]

Of the fliers I've spoken to whom have run 4B tell me they slow the model down too much.
As a matter of fact, I have read research results that stated that three turns out to be the optimum number of blades for best air moving performance and efficiency. The research found that adding more blades does not improve performance and may actually make it worse by creating more aerodynamic drag on the motor.

Perhaps I missed something ?

[Howard Rush]

Perhaps I missed something ?

Sounds like it. 

Doug Moon uses four-bladers quite successfully. Two-bladers fit into your toolbox better.  Eugene Larrabee himself told me that I don't understand propellers.

[Doug Moon]

Well, from what I have learned is stunt very much about personal preference more that anything. 

I started using them with good results in 2000-2001.

I have come to prefer 4 blade props.  They are just like any other prop out there. You have to set up your motor to run one with the best results.  Usually you can go from 2 to 3 or 3 to 2 without much adjustment if any.  But put the 4 on there and the motor most likely will not act the same. But once adjustments are made the gain in performance is extremely noticeable.

I tried Bollys and had no luck. 

Eather 4 blades are a thing of beauty.  Especially the old ones he used to finish out with the pitch carved into the blade.  No pitching needed.  It will always be the same no matter what. 

I did have one I pitched for high wind use.  12" diameter 3.6 down to 3.2 pitch.  Take off RPM 10,500.  The motor would rev up and down all during the pattern but there was no acceleration the prop along with the pipe was a nice brake.  It was like driving in your car in low gear at max REDLINE RPM. 

This is what it felt like in the high winds.  You pull the plane up for inside loops and you hear the motor rev up, ALOT, but the plane speeds up minimally and hits what feels like top speed.  At the same time the pipe is working hard to trap the motor back to its RPM setting.  At the handle and to your eyes you feel minimal speed up if any after the first half of the first loop is completed.  The next 2 loops are the exact same predicable speed.  You forget the wind is there.

I have never been able to replicate this much regulation using 2 and or 3 blade props.  Granted I can still get very good and workable regulation but nothing quite like that of the 4 blade descending pitch.  Plus the short diameter keeps the corners nice and tight.

[Paul Smith]

This one has worked well for me.

[Avaiojet]

Paul,

That's really a nice look. Kudos on that!

Well worth the effort.

Charles

[Paul Smith]

Thanks.  The mission of the prop was to get more static points than the next guy and it did that very well.  The actual flying is delegated to a Master 10-6 two blader.

I think the real reason for four-bladers is to harness the power of a huge engine without causing supersonic tip speeds and/or tall landging gears.  I believe that the two-blader is just right for control line stunt.  If anything, we'd be better of with more, not less, diameter.

[Derek Barry]

As Doug has proven the 4 blade works very well, I have had some fun with them myself and so has my dad. As a matter of fact Dad prefers 4 blade props to anything else. They do slow the turn down quite a bit but as Doug stated that can be trimmed out. The main advantage of the 4 blade is no wind up in the wind and it feels like you are flying in slow motion. My planes tend to fly best at a 5.2 to 5.3 lap timewith a 3 blade, that can be increased to 5.5 to 5.7 with a 4 blade.

Derek

[rustler]

I'm going to be new to 3 (and 4) blade props. I know rule of thumb says when switching to 3 blade knock 1" off the dia.
Is there a similar guide for 4 blade? - Knock off another inch?

[Steve Helmick]

Thanks.  The mission of the prop was to get more static points than the next guy and it did that very well.  The actual flying is delegated to a Master 10-6 two blader.

If you read and obey the rules, you are not allowed to change the propeller from appearance judging. Below is what the rule book says about it, tho at the NATS and a few other big contests, there is some practise flying done after Appearance Judging. Still, that would not allow a static only prop be used for AP judging.  Steve

10. Appearance. Models shall be judged for appearance complete and ready to fly. After model has been judged, nothing will be removed from or added to the model which, in the judges’ opinion, changes in any manner the appearance of the model from the way it was when presented for appearance judging. However, during an attempt for official flight after the contestant has begun to crank the engine, if it becomes necessary to remove the propeller spinner for change of propeller, etc., then it is permissible to leave off the spinner for that particular flight. Any damage to the model after judging, or changes that may be made as a result of such damage, will not be cause for loss of appearance points. Appearance judging will take place just before contestant’s first flight. Judges shall exercise prudence in assigning points, and reserve excellent point values for those models which are decidedly above average.

[PJ Rowland]

Doug : Yeah I though about your model as I finished writing it, was hoping it was goign to slip by un-noticed... I didnt mean my comment to be a "blanket rule of 4B props for everyone "

Your spot on its about finding out what works for your own program and one's own personal preferance.

Obviously if one turned around and said " Im thinking about building a stunter - whats the difference between 4 engines and 1 " Everyone would tell you that 1 is the way to go, until you point out that PWB17 placed pretty darn high at the World champs and US Nats..

I think there are exceptions to any rule.  What I meant specifically was ; perhaps wording it, the concensus of opinion is to much blade for most people.

Like everything - I shall be trialing it later on the PA75
 



 

[Mark Scarborough]

If you read and obey the rules, you are not allowed to change the propeller from appearance judging. Below is what the rule book says about it, tho at the NATS and a few other big contests, there is some practise flying done after Appearance Judging. Still, that would not allow a static only prop be used for AP judging.  Steve

In looking at the pictures posted, I think that is likely a scale profile airplane, at least judging by the wing thickness, not a stunter, scale allows the use of static props for judging,
otherwise you are correct for stunt,,
I was questioned about the color coordinated prop on the Avenger in Tuscon,, however since its the prop I fly with it was ok,,

[Chris Wilson]

How is the back of your flicking finger surviving when being close followed a 4 bladed carbon meat slicer?

I always though that the key to the 3 blader was its uneven number of blades more than the fact that it had 1 more blade than normal.

http://www.mh-aerotools.de/airfoils/propuls2.htm
answers the four blade conversion problem Ian.
(You minus 10% diameter when going from 2 to 3 blades and minus 16% when going from 2 to 4.)

[Mark Scarborough]

while the rule of thumbs are a good starting point, I really dont think you can make any assumptions. I have , for example, tried at least 6 props on my Magnum , they are all pitched to 5.5 across the blade,, and all 12 inch diameter. They all exhibit significantly different loads on the engine. The blade area makes a huge difference too. I would not be surprised that some of the narrow three bladed carbon fiber props  would put the same load on the engine as something like a BY&O two blade of the same diameter. There are just way to many variables to make any kind of assumptions that way,, my two cents worth. I know there are a lot more educated, experienced guys out there who can probably speak to this more authoritatively than me.

[Steve Helmick]

For most (if not all), I think it's probably best to find a useful 2 blade prop, then look for a better 3 blade prop, and perhaps then look for a better 4 blade prop. I cannot see good things coming from starting out with an unfamiliar engine/prop combination. In other words, don't design your LG around the hope that a 4 blade prop will be the bee's knees. 

[Doug Moon]

How is the back of your flicking finger surviving when being close followed a 4 bladed carbon meat slicer?

I never ever use my fingers when starting a motor.  It doesnt matter what it is.  Tiger 61, LA40, Fp 20, PA 65, I use the trusted heater hose chicken stick.  It's plenty strong to pull the prop through but soft enough so kick backs don't damage the blade.  Usually I am flipping the backwards off the front of the blade anyway.  But again I am never using my fingers.

[rustler]

I see scope for a good discussion here - the merits/demerits of 1, 2, 3, and 4 blade props.

My first - and only - contribution - can't help noticing the FAI Speed boys favour 1 blade

[Howard Rush]

One could get a prop analysis program like Javaprop or program this: 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 and mess around with it.  If you consider Reynolds number and how well you can carve or mold a prop airfoil, prop blades get wider and fewer.  With electrics you should know exactly how much power is going into the prop, so you can calculate everything if you are willing to do the work. 

[Steve Helmick]

I see scope for a good discussion here - the merits/demerits of 1, 2, 3, and 4 blade props.

My first - and only - contribution - can't help noticing the FAI Speed boys favour 1 blade

The speed guys use one blade props partly because they're hoping that it won't be ground down on landing. One less blade cuts the chance by 50%, I reckon. Further, they adjust the propeller pitch by using tapered shims under the prop hub to adjust precisely for conditions (including the condition of their engine). Can't do that with more blades.

Dad told me once about some scientific type coming to Wright Field with the single blade prop idea. He said that it always vibrated like a gurl dog at most places on the tach, except for the one rpm that it was balanced for. Not very practical for variable rpm setups, or low rpm setups. You ever see a single bladed rubber model? The nose does a nice oscillation...maybe the prop is more efficient, but it makes the airframe not very efficient.  Steve

[john e. holliday]

Don't forget those single blade props are turning  in excess of 40,000 plus rpms.

[Avaiojet]

I'm shoping for a 4 blade prop, I'll let you know how I made out.

Charles