I normally use a 12x6 Evo prop... I want to get a wooden prop for my P51 Blue nose plane... as wood is lighter and more efficient think I must get a 13x6? engine is a evo 61

Well, the point here is thrust and RPM. You will get the same thrust out of the wooden prop as you will with the other one of the same size. That's just the physical aspect of the prop dimensions.

If the engine can turn the larger prop at a working RPM that is agreeable with you, well, it will certainly be a tad bit quieter.

CGr.

The problem is far, far more complicated than just diameter and pitch and it really gets convuluted if you start changing materials.

A propellor is an amazinginly simple wooden club, yes? Well, not even close its a horribly complex intertwining of many different engineering parameters that goes well beyond the two most basics of length and pitch. Just a few of which but certainly not all factors that will have a direct affect also on any combination of airframe engine and propellor are: The structural material and its aerodymamic stiffness (flexing), The infinate variety of airfoils, The blades chord (width), the pitch distribution (The progression of pitch from tip to root), The blades planform (outline shape), The number of blades and on and on and on.

This is the reason The "rules of thumb" and old engine size/propellor charts are not all that accurate and the real solution for any modeler is to slowly develop his own personal "Propwall".

Switch, If I were to respond to your speculation that based just on material that the wooden prop of the same pitch and length would perform better and I really try to avoid responding to stuff like this then I beleve your assumption wound be wrong more often than not. That based on my own assumption that the thinner airfoil in the root regions will be more efficient.

Yup Its a lot of speculation and kinda pointless. Work on your propwall and experiment. Its the only practical way to really know for any given engine /airframe.

John

It depends on which wood prop you are talking about.
Wooden props like a Vess or Xoar are quite efficient...most others are not as efficient as an Evolution or APC plastic-composite.

Because of it's strength, the blades of the "plastic" props like APC can be made thinner (airfoil ) and put less drag on the engine, while at the same time generate more thrust.

There are many factors involved in propellor design...
Yes, a wood prop might be lighter and thus give quicker throttle response, but at the same time, there will be less flywheel effect, and that can effect idle reliability.

To make a long story short, I'm thinking you might find that a wood 13 x 6 (let's say Zinger? ) will load down the engine quite a bit more than the 12 x 6 Evo. The thicker blades and lower r.p.m. will generate less thrust than the Evo prop. Just a SEWAG here... (semi educated wild ***** guess )

Edit: I see John got here before me...
I too have accumulated a lot of props over the years. Actual flight testing is the best way to find out for yourself which ones you like best.
Most of the guys I know who have been at this for a while have a fairly large "collection" of props.

thanks guys... I know that its more complicated than that... but this is what has worked for me normally... going up one diameter... I don't really tach and all that stuff... but I just based my assumption on weight itself and nothing else... guess I would get both... a 12x6 and 13x6 wood and see how it goes...

Agree with testing.

At the same time, I consistently use wood props. Safety factor and I like the way they perform.

Tom

Safety factor??? Please explain.

Also reinforce resin ( plastic ) props typically have more mass per volume than Wooden props.

This tends to help out engine idle A LOT.

Wood props are more likely to break when they hit a solid object (such as, say, a portion of your anatomy) than plastic or nylon props. Either is bad news if you get hit.

I can attest that "likely to break" tends to be an exception to the rule... I have the scars to prove it.

However wooden props ( particulary on large gassers ) tend to be more crankshaft friendly during nose overs.

As always, John B and Proptop have covered the subject pretty well. But since part of the purpose of these forums is to share information, here goes:

A really long time ago, I worked part time for a major prop maker that had made props since 1949. All of his props were machined from maple, which is a very hard wood with generally good grain structure. But because wood is a natural product, there is some variation in the grain. So when you get a nice shiny wood prop, you want to do more than check the balance and reaming the hub to fit the crankshaft (which is important, because forcing the prop on when it almost fits can cause it to fail at the hub).

You want to hold the prop at the hub and one blade, while flexing the other blade. On a 10" or 11" prop, you should be able to flex the tip about 1/4". Check both blades the same way. 99 percent of the time, the blades will just flex and you are good to go. But every once in a while a blade will break, and breaking it in your hands is far better than throwing a blade when you are running it up on your engine.

Yes, and when it breakes on the engine at a high RPM, which is where the centrifugal force is, well, it happens quite quickly and can do some damage if something is nearby or in the path of direction that the prop takes when it breaks.

Oddly enough, this is one check that I do kind of automatically, without thinking about it. My instructor, a devout pattern guy, taught that to me when I had my trainer. It was a good time to learn this practice because that's when we break the most props.. during training. We break them for various reasons, usually hitting the ground with them with our sloppy landings, but when replacing the broken prop with one from our "prop wall" that is under development at the time, well, grabbing one out of the box, and flexing it just as you suggested sort of came automatically after a while. If fact, when I buy a prop, I usually buy two of the same size and test them at that time, unless I mail order them, that is.

Good point, Highplanes

CGr.

After reading the above posts about prop safety, I'd like to add one more item while we're at it...

Wooden prop hubs "take a set" after the prop nut has been tightened down...the wood fibers compress after a while, so the prop nut is basically not providing as much clamping load on the prop washer / prop hub.
You need to check the prop nut from time to time, to re-torque it a bit, and this needs to be a part of your regular maintainence ritual. I've seen props loosen while starting the engine, and just slip on the shaft, which is annoying. I've also seen them go flying off into "who knows where" territory...not safe at all, especially if someone gets in it's way.

All prop hubs will take a set, or compress a little bit, even the much harder plastic or composite props. Wooden props are MUCH more prone to do so...especially the softer woods. Some of the newer Top Flite props seem to be a rather light wood...maybe Beech? (I don't recall exactly right off hand )

Good point and something that newbies often overlook.

It is also responsible for many a "thrown prop" problem.