Is there a primer on how to rework / repitch / modify / tweak wood props?

[Bill Johnson]

I've searched many threads on reworking props and have a pretty good idea and I have the tools, a balancer and a pitch gage. Just looking for a basic primer on how to take a basic Zinger prop and rework it for improved performance. Once I can do that without turning good props into junk, I'll work on the more exotic modifications like Phillips entry.

[Steve Helmick]

Here's one from Larry Renger, and if I can locate it, one from Nils Norling...  Steve

"Zingers are exellent prop kits. Good wood and lots of it to work with. The key changes are to add undercamber and Phillips entry (a bit of upcurve on the underside below the leading edge), a ROUNDED leading edge, a thin trailing edge, move the high-point of the airfoil to 33% from the 45 or 50% position that is provided, and thin the blades. Basically, you are taking a propellor blank and airfoiling it with a good, low Reynolds Number airfoil.

If you are using a big spinner, it is easier, as you have little work to do at the hub. Otherwise, you have a LOT of work to do at the hub.

First off, I work the underside of the prop. I draw a circle on top and bottom of the hub to give me reference for how far in I need to re-contour. Then, I rework the hub area to match a helical surface in as far as the circle with a nice smooth fillet to lead in from the hub to the blade bottom.

Next, I draw a line at about 10% chord from the leading edge, and then lightly bevel that part upwards about 5 degrees. Just enough to see. Now the magic part! If you angle a cutter (I use those Tungsten Carbide round rods you can get from Micro-Mark) outward from leading edge to trailing edge about 45 deg, it will automatically cut in undercamber. I actually start at 30 deg at the root and end at 60 deg at the tip, but who'se counting.

You should see the center of the blade get cut first, and then the cut area extend from your new leading edge contour to the extreme trailing edge. At this point, I will sand the bottom smooth and blend in the undercamber to the new leading edge upsweep. This gives you a good base for your next step.

Now it is time to thin the blade. I mark 1/2" stations, and on a 10x4 prop, start at about .200 and taper evenly to .060 at the tip. I suppose one should compensate for the increase of chord toward the middle of the blade, but I don't. The thickness must, of course match the bottom surface of the blade, I use a caliper which gives me a good flat reference to the bottom.

OK. Now, draw lines on the top at the 1/3 chord and 1/2 chord position all the way out. Start hogging away at the rear section of the blade to give you a very thin trailing edge (.015 or so) and a flat surface to the 50% chord line.

From here on, use a point-source light, and your eyeballs to shape a nice airfoil everywhere. Add your choice of tips (I like Hoerner {Flite-Streak} tips), sand smooth and balance the prop. I have the Top Flite Magnetic balancer, but tossed out that plastic junk hub. I got the nice machined balance hub from Great planes, and use it in the magnets. Superb!

Finally, I give a couple of coats of Urethane spray varnish, recheck the balance, and then I have a real gem of a prop.

Larry Renger
think S.M.A.L.L."

"NilsN
   Member

   Posts: 100
   From: Metolius, OR,
   USA
   Registered: Oct 2000
                       posted March 18, 2002 12:49 AM               

                    Reworking props can make an amazing difference in the performance of a stunt ship.
                    Due to easy availability I use Zinger "kits" for prop blanks too. I do have one *very
                    important* suggestion- if you are going to be reworking props, by all means get a pitch
                    gauge first so you can see what you're doing when you start sanding on your props!
                    I usually start with an oversize prop & trim it. I use a razor saw to get the blades close,
                    then use my disc sander to get the final length. The saw makes tiny rips in the wood &
                    the sander removes them.
                    I use a 1/2" X 1" stick about ten inches long with a quarter inch dowel glued through one
                    end to check length. I put the dowel through the hole in the prop & turn the prop end to
                    end to make sure the blades are the same length measured against marks I've made on
                    the stick. I have it marked for props from 10" out to 13" in quarter inch increments.
                    Next I set the pitch to what I want by sanding the back of the blades with 150 grit
                    Stickit paper on a home made styrofoam sanding block. I have found it to be true that an
                    extra quarter inch of pitch at the tips makes for more line tension up above. I don't know
                    why, but it does. For instance my ST.46 on my profile Cardinal likes 12-5 Zingers cut to
                    11 1/2" & repitched to 5.75 on the main blades with 6.0 at the outer 3 stations (about
                    3/4").
                    I only undercamber props when I want to load the engine a little more for a given pitch.
                    After I get the pitch I want I start sanding the front of the blades to move the high point
                    forward to about 30%. I count the strokes I make on one side & then give the other
                    blade the same until I get them thinned & shaped the way I want them. I like my props
                    fairly thin & the edges semi sharp. There is a lot of meat on a Zinger especially on the
                    trailing edge & I get rid of most of it. I take wood off of the high point of the factory
                    airfoil (which is about in the middle of the blade front to rear, usually) when I move the
                    high point forward, so the entire blade is thinner. Generally they come out very close to
                    the same when I put them on the balancer.
                    I should point out that I've been sanding bondo for 33 years, so I'm fairly handy with a
                    sanding block, your results may vary.
                    Something I have found after checking dozens & dozens of them, is that very few wood
                    props are actually pitched at what they are marked. Some are off a little & some are off
                    a lot! Some are off the same on each blade & some are off up to an inch (or more!) from
                    side to side. Therefore, just because your airplane worked great with an 11-5 Zinger (for
                    instance) once, doesn't necessarily mean it will work well with another 11-5 Zinger after
                    you bust the one you've been using. They could be quite different.

                    Another thing I've found is that it's easier to pitch a prop "up" than it is to pitch it
                    "down", i.e.: if you want to make a 5 pitch prop, it's much easier to start with a 4 & sand
                    the back of the leading edge than to start with a 6 & try to take all that meat of the
                    rear trailing edge. Generally taking that much off the trailing edge will change the overall
                    outline of the blade by the time you get what you're after as well.
                    Note to Currell- to make a Phillips entry on your prop you sand a bit of taper on the
                    "back" of the leading edge, in effect upping the pitch slightly. Say you have a prop that's
                    working pretty well but you want a little faster lap time, sand a little Phillips entry into it.
                    (Conversly, if you want to slow it down a tad, sand in some undercamber to load the
                    engine a bit.) The pitch is determined by a line from the very front of the blade to the
                    rear, sanding the back of the leading edge moves the effective leading edge increasing
                    pitch.
                    Hmm, I'm not sure I'm explaining this very well. Maybe it would help if you imagined
                    changing a flat bottom wing to a semi-symmetrical by removing some of the lower leading
                    edge. I hope that makes it a bit more clear.
                    Btw, I learned a lot of this prop twiddling business from watching several of Windy's
                    videos. The rest I learned by quizzing the guys flying Expert & trial & error.
                    I haven't tried CF props yet, but I'm looking forward to it.
                    Fooling with props is time very well spent, it can really make a huge difference in
                    performance.

                    I hope I've been of some help.

                    Nils"

[peabody]

Dennis Toth has been repitching them in pretty much the same manner as repitching carbon props.....

[Motorman]

When you re-carve the face you loose a little blade width.

MM

[Steve Helmick]

When you re-carve the face you loose a little blade width.

MM

Actually, you might lose a little blade width. But most guys around here start out with an inch or two larger diameter prop, so there's a lot of wood to mess with. I have done a few, but not a lot. The gummy finish they put on most wood props is a factor in that. I'd start by using a good paint remover to get that crap off. I wish they'd put no finish on them at all, as I'm not a fan of paint removers.  Steve

[Brett Buck]

When you re-carve the face you loose a little blade width.

MM

  Not necessarily. Most of the benefit comes from moving the high point forward, essentially, removing the excess material in the aft section of the airfoil. You don't have to narrow the blade. Of course, narrowing the blade is also a strong potential benefit, but there's no reason you have to remove material right at the TE or LE.

   BTW, it's not always to your benefit to add "phillips entry" and round off the LE. Many magical tricks can be done to the very LE to make it act the way you want. It's not entirely about making the prop more "efficient", you also want to make the engine do the right thing, which may be conflicting goals.

   Brett

[Dennis Toth]

Bill,
There are several things you can do with wood props. Some of the simple things are to make sure you have a smooth rounded leading edge (some props have a square edge). This can be a fairly small radius. The next thing is to thin the trailing edge to a smooth thin edge. Check the balance, sand the top face to bring the heavy blade into balance. Once balanced sand smooth with 400 sand paper. Now check the pitch. I am lazy and only check the 70% out from the hub point (most of the work the prop does comes from the last 30% of the blade). To correct the pitch (or change it to meet whatever pitch you want) I simply use a heat gun and heat the first 10% from the hub, top and bottom. You will get a feel for how much heat but (it's about like repitching an standard APC) it should feel pretty warm. Then hold the hub and twist the blade (give it a good twist) to move the pitch were you want it. Again, you get a feel for how much twist you need to move it. Once you get the pitch were you want it, clear coat and recheck the balance (you can tweak the balance with addition clear on the top of the light blade, just go easy). You can also retweak the pitch any time with the heat. I have heat pitched many wood props and none have lost the reset pitch (One guy said it will lose the pitch once it get in a hot car, I asked when was the last time you had a warp in a wing come out when you left the ship in the car - silence).

Best,      DennisT

[Bill Johnson]

Thank you gentlemen! Lots of great information. I retitled the thread name using the various key words I used initially in my search so, hopefully, this thread pops up easily for others in the future.

[Lauri Malila]

Hi.

Of course, most commercial props are quite crap when you look at the airfoil. In that case it's ok to carve and sand them to change pitch and improve efficiency, if that really matters.
But personally I like to have the starting point allways the same, that's why we carve the props with CNC. And then adjust the pitch by heating and tweaking. It works great with wood, at least with the maple that we use.
I heat only at the root of the blade, about at an inch or 1,5" width. When wood gets to about 100C temperature it turns really easy to to tweak. It is important to have a powerfull hot air gun but heat slowly, so that the blade heats up all the way to the core. If center is not hot enough, you leave tensions inside the blade and it won't stay well. When it's done well, the pitch stays 100% well. I have props I have used for years and the pitch is still the same.
When I tweak them, I pre-set my gauge to desired pitch, heat the blade and twist it a little "over". Then I put it into the gauge and fine tune by returning to opposite direction.
The pitch range I use is between 5" and 5,5", I like allways to tweak down from higher pitch, the raw props come as 5,9 or 6". That way I get automatically a slight wash-out to the blade, I like it.

Lauri

[Motorman]

  Not necessarily. Most of the benefit comes from moving the high point forward, essentially, removing the excess material in the aft section of the airfoil. You don't have to narrow the blade. Of course, narrowing the blade is also a strong potential benefit, but there's no reason you have to remove material right at the TE or LE.

     Brett

You're talking about changing the airfoil on the back of the prop. When you carve more/less pitch on the face of the prop and keep the same airfoil shape on the face you're going to lose blade width.

MM

[Lauri Malila]

You're talking about changing the airfoil on the back of the prop. When you carve more/less pitch on the face of the prop and keep the same airfoil shape on the face you're going to lose blade width.

MM

Look at a Zinger, for example. Or other similar crap. You can change the blade angle at either l.e. or t.e about a millimeter without changing the blade shape

L

[Brett Buck]

You're talking about changing the airfoil on the back of the prop. When you carve more/less pitch on the face of the prop and keep the same airfoil shape on the face you're going to lose blade width.

  I will not get drawn in to one of these "front/back" arguments.

    Brett

[Bill Johnson]

Look at a Zinger, for example. Or other similar crap. You can change the blade angle at either l.e. or t.e about a millimeter without changing the blade shape

L

That appears very likely. I'll be using crap or something similar    That's just the way things work right now: More time then money. I could buy a few more expensive props but I break too many, make the wrong pitch and diameter selections, etc. Plus I enjoy learning about / building / repairing / modifying just about anything on an airplane. That's been true my entire life since the age of about 6.

Looking at the typical props available to me at a reasonable cost (Zingers and TF PowerPoints), moving the thickest camber section forward would be easy and necessary along with thinning the blade, reworking certain stations to maintain the correct geometric pitch across the blade span, etc. Honestly, about the most valuable tool to have besides the balancer and pitch gage would seem to be, as suggested by a member in another thread, a cabinet scraper.

Changing wood blade pitch with heat is interesting. I'll definitely give that a try as it's the best way to work non-wood props as well.

I can see that there is some disparity in nomenclature concerning face and back of the blade but see no need to argue about it. Working from Steve's initial instructions and adding everyone's tips, there's no doubt in my mind what particular side someone's talking about. I've often found that even folks who should have propeller blade technical nomenclature down pat (A&Ps) call things differently then I would. 

More input is welcome but all in all, I believe we have a fine thread going on the basics. 

[Brett Buck]

That appears very likely. I'll be using crap or something similar    That's just the way things work right now: More time then money. I could buy a few more expensive props but I break too many, make the wrong pitch and diameter selections, etc. Plus I enjoy learning about / building / repairing / modifying just about anything on an airplane. That's been true my entire life since the age of about 6.

Looking at the typical props available to me at a reasonable cost (Zingers and TF PowerPoints), moving the thickest camber section forward would be easy and necessary along with thinning the blade, reworking certain stations to maintain the correct geometric pitch across the blade span, etc. Honestly, about the most valuable tool to have besides the balancer and pitch gage would seem to be, as suggested by a member in another thread, a cabinet scraper.

   I always used single-edge razor blades as the scraper. Any wood prop is about equally good to start with, although you want one close to the right pitch. If you want more, try this thread:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=342491&mesg_id=342491

  A Zinger is actually a decent starting point, assuming you are willing and able to remove a lot of wood, because the exceptionally crude LE and TE permit a large change in the pitch without running out of blade area.

   Everybody should get an example of a Y&O or Clarence Bull BY&O prop (not the Brodak version, which is substantially different) to see what the prop airfoil should look like. It's a really good HLG airfoil with an almost straight line from the high point to the TE.

    Brett