Hi Everyone,
I hear "trim an 11x6 to 10 inches" mentioned frequently in this column & I have done it myself; but I searched & couldn't find any information on how prop trimming should be done correctly. I assume that it matters how the tips are shaped, the blade thinned.....etc. Are there rules of thumb on cause & effects of prop trimming ?
Also what effects on line tension, in both level flight & overhead, does diameter & pitch have ? 
I'm just trying to learn, thanks for any replies,
Gil Mc Millan

Thanks for your reply Floyd, from your answer I assume that your telling me that the shape of a propeller tip isn't important, & that I dont need to be concerned with that.
Should I expect the same performance from a prop that was manufactured as a 10x6 as I would from an 11x6 that had 1/2 in removed from each tip ? ( assuming that both props were the same brand & style, & properly balanced)
As you didn't answer, do you not have any knowledge about line tension changes with prop diameter &/or p

Thanks again, Gil

I think a lot depends on where the motor wants to run. I have found on my McCoy 40 Red Heads that a 12-6 APC cut to 11 works really well on them. I cut them on my band saw with a homemade jig. I leave the tips square except for just a slight rounding of the corners. The McCoy's like to be loaded, and don't like to run very fast, so the 6 pitch props work well. Most of the older baffle piston motors like 5 and a half to 6 pitch. Later more modern engines like less pitch. The diameter will also effect turn rate. More diameter will slow the turn rate, and less will increase it. But there are exceptions to every rule.

Leaving the tips square will give the maximum area for thrust  for any given diameter and still keep the low ground clearance that modern stunters seem to love.

Its not the most efficient shape for drag but then again at the revs we run at ( and tip speed) its not that critical.

I have always thought that trimming down the diameter of any prop must go hand in hand with thinning it also.

Milton,
     it sounds like your average pitch (measured at 70% of the props length) could be changing at the tips when trimmed.

 I would expect a trimmed down prop of same diameter will exhibit a greater pitch at its tips that one that is not trimmed - so your stock 12X6 will be less of a load that a 13X6 that is trimmed down to 12X6 because it is no longer a 6" pitch at its 70% station.

That station or percentage will have moved inwards towards the normally coarser pitch that is found at the hub.

Milton,
     it sounds like your average pitch (measured at 70% of the props length) could be changing at the tips when trimmed.

 I would expect a trimmed down prop of same diameter will exhibit a greater pitch at its tips that one that is not trimmed - so your stock 12X6 will be less of a load that a 13X6 that is trimmed down to 12X6 because it is no longer a 6" pitch at its 70% station.

That station or percentage will have moved inwards towards the normally coarser pitch that is found at the hub.

Chris, I see where you are coming from, but I beleive you are mistaken, Pitch at each station is relative to the hub, not to the tip. therefore, 6 inches of pitch at station 10 will always be 6 inches of pitch.
The thing about cutting APC props down is that you loose the blade profile shape, and I believe that the unique shape of the APC is what makes them so solid. Therefore cutting down the tip eliminates a goodly portion of the efficiency of the prop.
I rework a fair number of props, wood mostly, I seldom if ever leave the tip square. In my opinion, its the least effecient shape available for a prop tip,,

Chris,
yes in fact most pitch gauges ( actually all of them I have seen for model use anyway) are not percentages, but instead they are measured relative to the hub center. hence at station 5 or 6 or,,,,,, 6inches pitch will always be 6 inches of pitch regarless. i do not know how that would affect it even if you did figure at percentages though, because the pitch is a relationship of the radius ( from the hub center) to the theoretical forward travel for one revolution, so by cutting it down, you move the 75% inboard indeed, but it will still pitch the same measurment because its related to the actual radius, and theoretical forward movement. What you call that station has no bearing on the physical measurment of pitch at that station.
In fact there is a thread on here somewhere which illustrates this using a degree measuring device instead of a pitch gauge, it gives the relationship of the distance from the hub, to the angle measured giving you the pitch.
of course this ( as has been stated before) ignores the actual camber of the blade airfoil.

as an aside, to prevent thread drift, the tip shape does make a difference, the same way as the shape on the tip of a wing makes a difference in the performance of the wing. as to quantifying that in exacting terms, I am not capable, but I have my pet theories which may or may not be accurate since my observations are of the "feel" variety as opposed to any specific measurement methods.

I used to work on props all the time.  Using wood props at the time, it was sometimes necessary to get the right pitch, etc.

I have a ton of Govt. Surplus 12-5 Rev Ups.  I made a bunch of props for McCoy .40s, etc., out of these props.  I generally traced a Top Flite paddle blade on the 12-5 once it was shortened to what I wanted.  Example like making a 10-6 and using a Top Flite 10-6 to shape the new blades.  I also almost always added Phillips entry to the LE of the prop.  I thinned the tips, sometimes even added under camber to the wood blades.  Of course I would correct the pitch.

The purpose of all this was to get a range of props to try on the model to see what worked best.
For my old .40FP, the best diameter of these reworked props seemed to be 10-1/2".  And the pitch liked to be around 5. So I would make a 10-1/2 X 4 1/2, 4 3/4, 5, 5 1/4, 5 1/2.  And the best performer on the plane was duplicated as closely as possible.

Now I have built up a good supply of CF props and using heat to repitch or correct pitch is about all I do!

BIG Bear
RNMM/AMM

Chris,
I think I see where we are missing each other here,, in most all of the MODEL props I have measured , the pitch is constant from hub to tip, I beleive based upon my experience with full scale props,, there is indeed some washout towards the tip.
In my experiments, I have played with some washout towards the tip,, ( less measured pitch) As I said, my observations are from the handle so they are pretty limited in overall scope.

Now given that you are assuming there to be in fact washout towards the tip, your comments would be correct. And given that I am basing my comment upon the pitch being constant from hub to tip, My comments are correct,,
so long story short, apples to oranges, we are both right, but in the model world, most times I have observed ( and target) constant pitch from hub to tip, therefore, changing percentages will not affect the pitch of the prop.

Agreed (I like agreeing!)

Strange thing is that all the props I have show tip washout - go figure.

Strange thing is that all the props I have show tip washout - go figure.

Are we just talking about the blade angle reducing towards the tips here, or something else?

Thanks Mark, that's what I thought.

HI Randy,

I would love to have the practice time to trim my model AND pick the three props you mention..

Unfortunately I have to settle with the prop that works best on that day and fly it when ever!

Of course Randy Smith might walk up and say "You need some more pitch", so I put on a prop that matches but has a little more pitch.......

BIG Bear
RNMM/AMM

Bill,

With me it's Paul Walker that comes over and looks at what I'm doing. But his response is usually just to shake his head and walk away.

  I don't ever get away that easy!

Bill