Questions on propmodifications

I've been seeing a lot comments on props spread across several threads. I'll try to consolidate my questions here.

As I've mentioned before I'm a "retread" returning to the sport after many years away. So, my questions may be old news and the current thinking may (will?) be quite different. I appreciate the responses and the education!!!

On props it seems as though a couple of simple treatments give a large return in performance and consistancy while other treatments are more complicated and produce only small further improvements more necessary to the very top fliers. So, for a bottom of the pack kind of guy (who should spend more time flying rather than reworking props..) what treatments do you feel are necessary (good?, best?, etc.)? In the past I have used Zinger wood props (that's what my local hobby store sells), sanded the LE smooth and rounded (should it be sharp rather than rounded?), and balanced them. Nothing more. If you look at a new of the shelf Zinger you can easily see the LE is still square from the manufacturing process. The available 1" increments in pitch and diameter were all I have ever used. Here on this forum I see lots of comments about repitching and/or recutting the diameter in .25 inch increments. How big is the change in airplane performance is there with a .25 inch change in either pitch or diameter? Is this something that an old retread would plainly see? Or, something the would only be seen by the top contenders at the Worlds?

What standard things should a beginner who is trying to improve do WRT props?

Hi Ron,
For the time being, balance first. For the off the shelf Zingers, I like to do a "Phillips entry" on the leading edge. Sand the bottom leading edge about 3/32-1/8th inch back to a point on the leading edge. This improves the eff. of the prop. My first investment would be a pitch gauge, and learning to use it. You would be surprised how big a difference there can be in wood from prop to prop (even from the same manu.) as to the pitch. I have 10 x 5 props that range from 4 1/2 to 5+ from the same maker. It will help especially if you happen to break one.
Bill <><

Seemingly small changes in diameter can make large changes in engine load. the load to diameter relationship is an exponential relation.

Example. Setting up a stock, hi rev OS 25 FP . Using a grish tornado prop, load was too large for a 10-4 but it was excellent running after clipping 3/16 from each tip. It is a load adjusting fine tuning tool.

Balance, smooth, pitch for speed, trim dia for loading.

FWIW

Here in a nutshell are the things we stunt flyers should be looking at when modifying props.
1 Balance - make sure the prop balances in both the vertical and horizontal positions. The blade can balance in the horizontal position but not in the vertical position due to a hub that is out of balance. After you balaance the prop, lift it from the balancer and give it a 180 so it faces the opposite direction and balance again.
Contrary to popular belief, multi blades are easier to balance than 2 blade propellers.
2 Length - make sure all blades are of equal length.
3 Blade tracking - verify that the hub face is square to the shaft hole so that the tip of each blade is in line with the rest of the blades. On full size aircraft the error cannot exceed 1/8" but they have the ability to shim the back of the hub to obtain that measurement.
4 Blade thickness - thin props are more efficient but beware of overdoing it and allowing the blade to flex too much. Blade flutter can be a result and can adversely affect performance.
5 Airfoils - most props that are available have an airfoil similar to a Clark Y. This is an efficient foil for our needs with a high point at somewhere between 30-40%. Some of the cheaper props on the market have far less effective airfoils with some of the Master Airscrews coming to mind. In our applications, a somewhat sharper leading edge treatment seems to be preferred. Phillips entry, which is a modification of the leading edge to prevent flow seperation and slightly increase the angle of attack of the blade may also be tried.
6 Pitch - use a pitch guage and measure the pitch of the last 1/3rd of the prop. Anything inboard of this area is really only material to fair the outer part to the hub and its measurement is not critical. Pitch can be progressive (increasing in pitch toward the tip), helical or constant (remaining the same toward the tip) or regressive (decreasing in pitch toward the tip). What ever pitch arrangement you decide on, the important thing is that the measurement is the same at corresponding locations on each blade. Blade thickness should also be the same at corresponding locations.
Some rules of thumb.
Increasing diameter or pitch will reduce RPM and will require more power from your engine. More blade area (Wider blades) will reduce RPM and need more power. Undercambered airfoils will increase static thrust and reduce RPM and require more power.

>Here in a nutshell are the things we stunt flyers should be
>looking at when modifying props.

Alan, although a lot of your comments to follow concerning props in general are good, I just want to go back to what Ron mentioned at the very beginning so we don't confuse the issues. Ron stated: "for a bottom of the pack kind of guy (who should spend more time flying rather than reworking props..) what treatments do you feel are necessary (good?, best?, etc.)? " I think we should distinguish what he should do (and other like him) from what the top flyers might do.

>1 Balance - (snip)

Most certainly. Although not absolutely necessary, (except in rare cases where they are really out of whack) it doesn't hurt and can make things run smoother. At least get a cheap prop balancer and check.

>2 Length - make sure all blades are of equal length.

This is something I omitted in my post but is also good to check. A prop may be balanced weight wise, but still be out of whack thrust wise. Good reminder, and something simple to do.

>3 Blade tracking - verify that the hub face is square to
>the shaft hole so that the tip of each blade is in line with
>the rest of the blades. (snip)

This is someting I also did not mention, but is good to note and relatively easy to check. Except in rare cases, I find this to be more of a problem with carbon fiber props that we have heated and repitched, than it would be with wooden or molded props. Probably not a big issue for Ron.

>4 Blade thickness - thin props are more efficient (snip)

Top guys may rework in this area, but I think it is a non-issue for the beginners. They need to simply find someting that works off the rack with but minor checks as in above.

>5 Airfoils - (snip)

Although airfoil affects our props, I don't think beginners should even concern themselves with changing airfoils on props. If a certain prop doesn't work, try something else. There are certainly a number of good props out there that do work. They need simply, quick, repeatability.

>(snip)Phillips entry, which is a
>modification of the leading edge to prevent flow seperation
>and slightly increase the angle of attack of the blade may
>also be tried.

For upper level guys who want to play with props, fine. For a beginner, leave it alone. The Philips entry will effectively increase the pitch of the prop, and, unless you get good at it, will tend to make each prop different. Leave it alone until and unless you reach the point where you can feel and see the difference.

>6 Pitch - (snip)

Certainly pitch affects our props, but again we can waste a lot of time checking every station (or every other station) on a prop, only to break it on the next flight. Once you found a brand and size you like, buy a batch, give each one a flight test, and if it works, forget it.

>(snip)use a pitch guage and measure the pitch of the
>last 1/3rd of the prop. Anything inboard of this area is
>really only material to fair the outer part to the hub and
>its measurement is not critical. (snip)

Here is where I must disagree. The outer 30% of the prop is equal to the inner 70%. And the spinner area will only consume about 3 o4 4% of the total. That means the inner 2/3 (approximately) will pull an equal amount as the outer 1/3. So the closer to the hub you get the less percentage it plays in the total picture, but the whole blade is still very important. If you are going to repitch it, repitch it all.

>(snip) Pitch can be progressive
>(increasing in pitch toward the tip), helical or constant
>(remaining the same toward the tip) or regressive
>(decreasing in pitch toward the tip). What ever pitch
>arrangement you decide on, the important thing is that the
>measurement is the same at corresponding locations on each
>blade. Blade thickness should also be the same at
>corresponding locations.(snip)

Agreed, but again this is an area that I don't think the beginner needs to play with. Most props that they get off the shelf are intended to be helical or constant pitch, and that is where they should be.

>Some rules of thumb.
>Increasing diameter or pitch will reduce RPM and will
>require more power from your engine. More blade area (Wider
>blades) will reduce RPM and need more power. Undercambered
>airfoils will increase static thrust and reduce RPM and
>require more power.

These are good rules to remember in choosing props. The underchambered or thicker blades will effectively increase the pitch (speed of the plane) as well as the load on the engine. Not all props are created equal, nor do pitch sizes from one brand equate with the pitch sizes of another brand. You may, for instance, use a 10-5 of one brand and a 10-6 of another with near equal results.

Otherwise good stuff.

>6 Pitch - use a pitch guage and measure the pitch of the
>last 1/3rd of the prop. Anything inboard of this area is
>really only material to fair the outer part to the hub and
>its measurement is not critical. Pitch can be progressive
>(increasing in pitch toward the tip), helical or constant
>(remaining the same toward the tip) or regressive
>(decreasing in pitch toward the tip).

This has come up several times lately. It's my firm belief that you really need to do the whole prop, rather than the tips. Granted that the lift distrobution is skewed tremendously towards the tips (due to velocity gradient). But If you just alter the tip pitch, then you are also randomly introducing wash-in or wash-out (progressive or regressive).

Take an extreme example. If you take a 12-6 and make it into a 12-4 by just washing out the tips, you willl get a severely regressive response, and it will act somewhat like you deployed a drag chute in the corners. If you take a 12-3.75, and make it into a 12-4 by just pitching the tips, now you have a progressive distribution. Additionally, in the first case, you will have to back the engine off and spin much slower to get the same level flight speed compared to the second.

In fact, unless you really, really know what you are doing, on TP airplanes, I recommend setting so it *measures* helical and if you need to change it, change it all over so it stays helical. It's not that it doesn't work with TP props - in fact, my experiments with props that *measure* progressive or regressive have suggested that it's works FAR TOO WELL for reliable and repeatable results. You can easily tell the difference in even .1" of pitch, and on a 12.5" prop, that's a pretty teeny angle.

For 4-2 break engines, adding or subtracting a 1/2" of pitch at the tips has detectable but not extreme results.

You don't have to do every single station, but I think it's very wise to at least to every other one. I usually do the odd-numbered stations on the Prather gauge, which is something like 3/4" or so.

I make a note to emphasize the *measured* pitch, since it looks pretty obvious that the airfoil and "correction angle" (as I spewed about at length in the other thread) actually results in, effectively, wash-in/progressive distribution that I am certain is another big factor in the difference between different types and brands of props.

I agree that beginners should just balance the props, and then pick one that gets a decent level flight speed when the engine is happy. If the engine is too slow or too rich, and/or the airplane is too fast, use one notch less pitch, and if the engine is too fast or too lean and/or the airplane is too slow, use more pitch. Actually, that's not way off the plan for experienced modelers, either!

Brett,
I really wasn't trying to say that you shouldn't check the pitch on the inner part of the blade, I just meant that the outer 1/3rd of the prop is the most effective. I've got a 68 X 66 prop on my bench right now that measures at 66 inches from the 50%R station out to the tip but about 40 inches near the hub. The prop was made for an RV-6 with a 150 HP Lycoming but it wasn't enough to hold the engine to 2700 rpm. Went to a 70 X 70 and it did the trick.

>Brett,
>I really wasn't trying to say that you shouldn't check the
>pitch on the inner part of the blade, I just meant that the
>outer 1/3rd of the prop is the most effective. I've got a
>68 X 66 prop on my bench right now that measures at 66
>inches from the 50%R station out to the tip but about 40
>inches near the hub. The prop was made for an RV-6 with a
>150 HP Lycoming but it wasn't enough to hold the engine to
>2700 rpm. Went to a 70 X 70 and it did the trick.

I am still thinking they are equal, Alan. The outer 30% and the inner 70% do approximately equal work. I know that Windy has made the comment that all you need to repitch is the outer inch or inch and a half, but that is not true. When you are close and working to change maybe a tenth of an inch of pitch, maybe it doesn't make a lot of difference, but when you reach even half an inch there is a very noticeable difference.

Now, how do we know? We did an experiment once just to prove it to ourselves. We took two Bolly props, both were 12 inch, both were 3-blade, but one was molded in 6.5 inch pitch and one was molded in 4.25 inch pitch. We then repitched the last inch and a half of each blade on both props (actually the 75% point, but should be close enough) to 5.5 pitch and ran them on our Magnum 53. the plane wanted to fly too fast on the one that started at 6.5, but launching at the same rpm with the one originally at 4.25 left us with lap speeds that were way too slow. There was a tremendous difference between those two props.

Now we repitched both props starting at the hub and set the whole blade at 5.5 pitch. The result was two props that were virtually interchangeable. They now worked beautifully, when before neither prop was satisfactory for this engine (it likes the 5.5 pitch).

Moral of the story: Check the whole prop. The outer 30% (approximately) and the inner 70% (approximately) do equal work. You will short change yourself if you think that moving the tips only is enough.

>The outer 30%
>(approximately) and the inner 70% (approximately) do equal
>work.

I agree on your general point, but I'm not so sure about the 70/30 split. Note that the velocity gradient along the radius results in a greatly increasing Q towards the tips. I'll figure it out later, but I would guess the swept area of the outer 30% probably does contain roughy half the disk area, but the inner is running at lower velocity, so equal areas != equal "work", or thrust. At least not necessarily.

>Brett,
>I really wasn't trying to say that you shouldn't check the
>pitch on the inner part of the blade, I just meant that the
>outer 1/3rd of the prop is the most effective. I've got a
>68 X 66 prop on my bench right now that measures at 66
>inches from the 50%R station out to the tip but about 40
>inches near the hub. The prop was made for an RV-6 with a
>150 HP Lycoming but it wasn't enough to hold the engine to
>2700 rpm. Went to a 70 X 70 and it did the trick.

I'm not discounting the observation, but I don't think that full-scale practice, and what we do, necessarily make good analogs of one another.

I've done most variations on the "pitch the tips" thing an awful lot of times, and I will merely state that you get drastically different results depending on whether you do the entire prop, or just the tips, and I generally liked what I got a lot better when I did the whole prop, and then varied it from that baseline.

> I've done most variations on the "pitch the tips" thing
>an awful lot of times, and I will merely state that you get
>drastically different results depending on whether you do
>the entire prop, or just the tips, and I generally liked
>what I got a lot better when I did the whole prop, and then
>varied it from that baseline.

I think depitching the tips can be a good solution for reducing wind up. Especially in slippery airplanes.

On your advice, I have, however, stopped doing it. I now pitch the whole blade. Sure makes it less complicated, even though I would never hesitate to try the former if I needed some increased braking.

>
>> I've done most variations on the "pitch the tips" thing
>>an awful lot of times, and I will merely state that you get
>>drastically different results depending on whether you do
>>the entire prop, or just the tips, and I generally liked
>>what I got a lot better when I did the whole prop, and then
>>varied it from that baseline.
>
>I think depitching the tips can be a good solution for
>reducing wind up. Especially in slippery airplanes.
>
>On your advice, I have, however, stopped doing it. I now
>pitch the whole blade. Sure makes it less complicated, even
>though I would never hesitate to try the former if I needed
>some increased braking.

Don't get me wrong - wash-in/wash-out is a very valuable trimming tool, assuming you have everything else the way you want it, too. But re-pitching a prop by simply raising or lowering the tips without regard to the rest of it is not a way I would recommend starting out.

If I am getting the reference, in fact, in the case Jim tried, I did in fact think helical worked a lot better than washed out. In fact, it wasn't even close. That's not to say it wouldn't work in other situations, just didn't seem to work in that particular situation.

This fine tuning for pitch is more a problem with engines that are set at a certain rpm (like a pipe engine). Then the prop is pitched to match the engine. Of course, one could go the other way around as well. One could take a prop that is in range and adjust the pipe, etc., to try to match the engine to the prop. However,

You are a beginner. Tou don't need to be doing all this.

You didn't say what engine(s) you are using, but most muffled engines will have a range in which they will work. Find a prop that works for you with your engine and plane. Thus if a 10-6 gives too fast of a lap speed, you can richen the engine a bit, or you can go to a 10-5 and may find that the engine works better with that prop in the speed range you are seeking. don't be afraid to shorten props, if needed, and also, don't be afraid to lengthen or shorten lines once you find the prop that works best with your engine.

None of this answered your question, but in a way it did (I sound like one of these candidates running for office, don't I?) What I am trying to do is help you to find the right prop without having to rework it. I would not repitch (add Philips entry, whatever) the props. You are probably going to break a few and you want repeatability. If you like the 10-5, get half a dozen and fly with one. When you get the engine setting you like, etc, don't change a thing except the prop. Then fly with the next and the next. One at a time change props so you know that they are all equals. If one is substantially off, you will know it. And it is best to find out now.

Having done that, you will now have a batch of props for this plane that you have already tested. Now, balance them. (Yes, we put our first flight on without balancing it, but it is not the end of the world. The sun will still come out tomorrow. Usually.) You can also clean up the square edges, if you want. Just do the same thing to every one, and all your spare props will be the same.

Some square edges won't hurt. Some out of balance won't hurt. They will work better if you do get rid of the imperfections. Just don't go overboard. Right now you need to fly, and messing with props isn't going to make the pull-out on the outside square corner any easier. And you don't need to spend five hours reworking a prop that you are going to break the next day in a nose over landing.

Oh, and it has been mentioned that wood props can vary. Yes, they can. That is the reason to test them, one at a time. These are all cut on machinery that will cut them the same. But wood will sometimes warp, or spring in another direction when material is removed. You can take an absolutely straight piece of 3 inch wide 1/16 balsa, for instance, slice it down the middle, and suddenly find that you have two pieces that each bow away in the opposite direction. That's what sometimes happens to our props when they are manufactured. It doesn't happen often, but it can happen. And unless you know that this is what happened, it can drive you nuts. So test the props one at a time. Then balance them and dress them up a bit if they are known to work. But, for now, don't spend a lot more time on them.

Fly.

At your stage of the game, the best thing you can do is get an assortment of brands at around the size you think you need and give them a try. I would balance them and let it go at that.

What Jim T. said. I would add: 1) Don't follow ANY pre-conceived notions you have about any prop, looks are deceiving. 2) Test props, test more props, and keep testing props at every opportunity...lots of magic in the prop. 3) APC always seems to have a prop that works quite well for me. A size that seems worth trying gets tested. Usually, it seems that 1 to 3 flights will tell me if the prop is worth further testing and trimming the diameter. 4) Don't forget that your entire setup determines what's the best prop for your combo and flying style. Steve