F1D prop Construction
An article by Steve Brown from Indoor News and Views (INAV),
issues 103 & 104
This article focuses on the practical aspects of prop construction, not theory. Some of the techniques may seem simplistic, but they work. For the theoretically minded reader, the author uses "pitch" and "blade angle" interchangeably and knows that they are not the same. The information in this article is applicable to both fixed pitch and variable pitch propellers.
Wood
Wood can be obtained either from Indoor Model Supply or from "hobby store" balsa. Using the right wood is critical to the success of the prop. All parts are best made of wood that is as close to pure "A grain" as possible.
I use a Harlan balsa stripper for almost all construction. If you obtain your wood from "hobby store" balsa then the Jim Jones stripper will probably be more appropriate.
Materials
4.5 - 5 lb. "A" grain, .023 - .025" thick X 18" long
5.5 - 6 lb. "A" grain , tapered from approximately 0.1" thick to .30" thick, at least 12" long
.013" straight music wire (not stainless steel wire or wire that has been coiled)
An empty aluminum cola or beer can
Glue: Ambroid or Duco thinned with acetone to the consistency of cream. Titebond or similar for one joint.
Prop block
Much has been written on making prop blocks and helical pitch vs. non-helical pitch. I won't repeat that information. I only use one prop block. A solid block carved from 3" X 3" X 12" balsa has proven to be the most useful. It's better to make the block longer and wider than you'll need for 55cm props since you can use the same block for microfilm models and props up to 24" in diameter.
The sketch below illustrates an adjustable prop hub mounting for the prop block. The prop spar is rotated in relation to the block to achieve various "pitches." This adjustable hub mounting makes setting the spar quick and simple. Once I have set up this block for a given "pitch" I make index marks on the block and the circular plate. The "fence" is just a small block of wood that is exactly parallel to the prop shaft. I shim the block to place the 45-degree point for a given pitch at the appropriate distance from the prop shaft. Then I loosen the screw and rotate the mounting so that the prop shaft is vertical when checked with a square. The "fence" provides a reference surface to place against a square. You only have to do this once for each pitch. Generally, I find I don't change pitches very often. For 55cm F1d I usually set the block at 26 for a variable pitch prop or 31 for fixed pitch props.
A helical pitch block is the simplest to construct. If you want to try a non-helical pitch distribution try 28" or 30" pitch and decrease the blade angle as you move toward the tip and hub, to about - 6 degrees.
The basic formula for locating the 45-degree angle station for a given pitch is:
For example, to locate the 45-degree angle station for a 30" pitch prop, the formula would be:
I have always used what is described in the literature as the "graphical" method to lay out the prop block. This method involves locating the 45-degree point for the block, then projecting lines from the hub through the 45-degree point to the tip. I then use a saw to cut down to lines drawn directly on the block and finish the form with a spokeshave and a sanding block with rounded edges.
Make sure that the centerline is straight from hub to tip and that the surface of the block is flat when measured at any point perpendicular to the centerline. The block surface should not be cambered. Resist the urge to dope or paint the block. Give it two coats of sanding sealer and sand down to the wood when the sealer is dry. The goal is a dull, matte finish that will not absorb water rapidly. Avoid glossy finishes, which will result in static attraction problems with either plastic or microfilm covering. Mark exact diameters at ½" intervals measured from the mounting of the prop shaft directly on the wood surface of the block, beginning at a radius of 9" (for an 18" diameter prop).
Bending the prop outline
Construct a balsa prop outline form as illustrated:
When bending the outline do not attempt to make the tip radius too small. A 7/16 - 1/2" radius is safest. A lot of balsa will not bend around small radius curves without kinking. Aside from the bending technique, the main determinant of bendability is the grain structure of the wood. Using "A" grain increases your odds of success, as will the use of densities of 4.5 - 5 lbs. Most wood will be "AB" grain at best. "B" grain wood will not bend.
Draw 4 or 5 lines perpendicular to the long dimension of the sheet across the wood using a waterproof felt tip pen before cutting the wood. This will identify the "top" or "A" grain sides of the wood and helps to assure that all the strips will be oriented the same. The best pen for writing on balsa is the Sakura Pigma Micron in .005mm. or .01mm. sizes. These pens are available at art or craft supply stores. The ink won't run on balsa and is impervious to water or acetone when dry.
Cut 18" strips of .023 - .025 thick 4.5 - 5 lb. "A" grain .024 or .025" tall. Don't make the outline out of larger wood thinking it will ease construction. Larger wood will be harder, or impossible, to bend and may add excessive stiffness to the structure. Soak the strips in a tray of water for 45 minutes.
Select two strips and, under strong light, check to make sure the orientation of both strips are the same. It's easy to get one with the "A" grain face up and the other with the "C" face up. Align the ink marks to check orientation.
Secure the pair of strips to one leg of the former with a thin soft rubber band. I make the rubber bands by cutting them from ¼" X 6" toy balloons. (These are the balloons used to produce "twisted" animal shapes.) When you come to the tip, push the wood against the former firmly with your index finger and do not allow the wood to spring back once it's been bent. Don't attempt to "pull" the wood around the former, always push it. Cut off the excess wood, if any, with scissors. Secure the ends of the pair of wood strips to the other leg of the former with another rubber band and bake for 30 minutes at 175 degrees. Always use light rubber bands about 1/8" wide because the wood will shrink during baking. Soft rubber bands allow movement as the wood shrinks during baking.
Ribs
The ideal wood for ribs is wood that is similar to the outline. Look for 4.5 - 5 lb. "A" grain .022" - .024" thick. Make a rib template from .020" aluminum or from scrap Formica. The exact airfoil thickness is not important. What is important is that the arc of the ribs is appropriate to the height of the spar so that the ends of the ribs will touch the block on both sides when glued to the top of the spar on the block. Try a 4% arc. Using a Harlan stripper cut 10 - 12 ribs the same height as the outline.
Assembling the blade outline
The prop outline is built flat on a simple jig:
Place the bent outline wood into the jig. Cut through the two ends of the strips where they overlap at the hub end and glue them together. Glue the ribs to one side of the outline, then turn the jig and, using a new, sharp razor blade, cut them to the appropriate length. Double glue each rib.
When the outline is completely assembled place a small ink dot (using the Micron pen) to on top of each rib and at the tip to mark the centerline. You will use these marks later to position the blade on the spar.
Allow the glue to dry for a few minutes, then run a razor blade under each glue joint to separate it from the jig. Gently pry the finished outline free from the jig. I usually loosen the outline in the jig and then turn the jig upside down and shake it lightly to remove the outline. If you find the outline glued to the jig you are using too much glue.
Examine the completed outline under magnification to look for bad glue joints. Re-glue as needed.
Prop spar
Cut the spars in matched (side-by-side, similar sized) pairs from a pre-tapered sheet of 5.5 lb. "A" or "AB" grain balsa. The easiest way to do this is to obtain 12" tapered wood from IMS. Only a few of the sheets will be suitable. All are tapered too much for F1d prop spars, so you'll need to sand them to the appropriate dimensions. You can cut the spars with the Harlan stripper, or, as I do, by "eye" using a heavy metal straightedge.
Once you have several matched pairs of spars cut to roughly similar sizes, orient the spars with the narrow dimension front-to-back and the wide dimension on the sides. This orients the grain structure for maximum front-to-back stiffness. Cut a diagonal joint either by eye or using the Mini Miter Box described by Bruce Kimball in INAV #102. Glue the spar pairs together with unthinned aliphatic resin glue applied with a toothpick. Using aliphatic resin glue for this one joint will later allow you to soak out a bent prop shaft with acetone without running the risk of dissolving the joint.
Once the center splice has dried (2 to 3 hours) locate and mark the center of the splice with a felt tip pen. Then place ink marks at 2" intervals on both spars from the center to the tip. Begin sanding using 320 grit paper on a 1" X 5" sanding block. Sand the wood on all 4 sides until the cross sectional dimensions at corresponding 2" intervals on each side are the same when measured with a dial thickness gauge. Each spar is slightly different, but I typically start with a dimension of about .068 X .075 at the center and taper to .035 square at the tips.
Once you have the spars tapered equally on both sides place the spar with the narrow dimension oriented top / bottom into a test jig as shown.
This jig is described in detail in Larry Coslick's "Hobby Shopper EZB" article in INAV # 90. Hang a 0.5 gm. weight exactly 8.5" from the center. Look for deflection of about ½" at 8.5". Sand the spar lightly until you get exactly the same deflection on both sides. You may need to reduce the center section to induce more bending. Spars that bend excessively (more than 3/4) will probably be too soft.
For fixed pitch props drill a .012" hole through the exact center of the spliced spars. I intentionally drill this hole slightly undersized for a .013" prop shaft. Cut a piece of .013" music wire about 1.5" long and form a 90 degree bend at one end. Be certain the leg is bent 90 degrees and is at least 0.2" long.
[For VP props don't install a prop shaft. Proceed at this point to construct your VP mechanism with these spars. Cut and discard the spliced joint area. Once both spars are installed into the VP mechanism go to Assembly, below.]
Cut a large piece of aluminum from a soft drink can and sand both sides with 220 grit paper to remove the paint. Drill a .013" hole in the aluminum. Using scissors cut a rectangular plate from the aluminum with the hole in the center. The exact size of the plate isn't critical, but I typically make them about .060 wide X 0.1 long. Finish the plate by squeezing it lightly with flat-jawed pliers to flatten it and deburr the edges with a file or sanding block. The plate will greatly strengthen the joint between the shaft and spar and will also function as a bearing surface.
Insert the prop shaft through the hole in the spar and place the plate on the opposite end. Apply Ambroid or Duco liberally to the shaft, the bent leg of the shaft and to the back face of the plate before pressing everything together. Apply moderate pressure to the bent leg area to slightly imbed the leg into the wood of the spar. Immediately check the shaft with a triangle or square to assure it is at exactly 90 degrees to the prop spar. Reorient and apply extra glue as needed. Allow to dry for 30 minutes. Recheck for squareness, then re-apply more glue to the shaft leg and around the edges of the plate. The strength of this joint is critical - it's not the place to save glue weight.
Allow this structure to dry for an hour. Then bend the prop hook shape and cut off the excess wire. Before bending the hook I put a thrust bearing (Harlan) on the shaft and mark the point of the first bend on the wire 1/16" behind the pigtail of the bearing. Be careful when "eyeballing" the position of the first bend, since it's all too easy to make the bend in the wrong place and later find that it's not long enough to fit in the thrust bearing.
I use the classic "Richmond" hook shape, but a "S" shape will work as well. If you use a "S" hook it may be easier to form the "S" hook first and then insert the shaft into the spar and bend the 90 degree "leg." I've experimented with many hook shapes and have never seen any advantages to any particular shape beyond ease of rubber hook-up.
Assembly
Shim the block under the front or back edges to place the 45-degree point for the desired "pitch" (measured with a triangle and a square) and then set the adjustable shaft stop to 90 degrees with a square. Place the spar on the block and secure the shaft against the stop with masking tape if you don't have a spring retainer on the stop.
Lightly tape the spar to the block with 2 or 3 pieces of 0.1 X .75 strips of low-tack masking tape (blue painter's tape or drafting tape). Make sure it is straight. The spar should lie flat against the block for its entire length. You may need to reposition the pivot of the rotating prop shaft mounting to assure the spar will lay flat against the block. Cut the spar to the desired length using the reference ink marks on the block face.
Place the blade outline on the spar. Beginning at the tip, align the centerline ink dot on the outline with the "line" formed by one edge of the spar and double glue the tip and each rib station working from the tip to the hub. Press the outline to the block face as you glue. This will form the helix.
Allow 30 minutes for the glue to dry once all the rib stations have been glued. Then, using a ¼" round soft bristled brush loaded with water, paint water over the outline, but not the ribs. Soak the wood thoroughly. Caution: Blot any excess water from the tip with a Kleenex. Do not allow water to puddle at the tip or it may cause the outline to soften and kink. Draw the threads over the rib stations and secure the structure to the block face with gentle downward pressure. Dry at least 3 - 4 hours. The longer the structure is allowed to dry while it is "strapped" to the block by the threads, the better. Then complete the second blade.
Covering
I use a "wet" covering method since I am used to working with microfilm. Larry Coslick has described a "dry" covering method using spray glue in previous issues of INAV.
I use a prop covering jig as illustrated. The jig can be constructed of any available material, such as foam core, balsa or corrugated cardboard. Make the frame out of 1/8" or 5/32" square balsa. Reinforce the corners with balsa gussets. I usually make several of these frames since they are easily broken and having more than one speeds the covering process.
Unroll Y2K or Y2K2 plastic film on a flat sheet of the yellow foam rubber used in furniture. This will minimize the strong static attraction that characterizes the thin plastic films. The plastic should be as flat and taut as possible.
Spray one side of the wood frame with 3M 77 glue and place it on the plastic film. Cut with a 15 - 25 watt soldering iron or cautery. Place the plastic covered frame in the jig. Twisting the frame will create a lot of slack.
It is critical that the wood outline of the prop, the tops of all the ribs, and the tip and hub points of the outline be very wet. Using a ¼" soft bristle brush, wet the outline with saliva or "sticky" water (water mixed 50/50 with white wine or water with some egg white mixed in). Plain water does not secure plastic well. Move rapidly - all of the top and sides of the outline must be wet simultaneously. Immediately place the wet outline face down on the plastic film. Press it down lightly and make sure that the entire outline is in contact with the film.
Wait 2 to 3 minutes then cut the covered outline free of the frame with a soldering iron or cautery. Cut from tip to hub, cutting one side, then the other. Leave a tiny portion of the tip uncut to prevent the outline from flipping up out of the jig. Make certain the film is cut at the hub end (bump it lightly with the iron to be sure) then finally melt the uncut portion at the tip.
Seal the edges of the plastic to the outline sides by working saliva under the edges with a loaded brush. Then place the wet, covered prop blade back on the prop block. Draw the threads over the rib stations and dry for at least 3-4 hours. Repeat for the other blade.
Excess slack can be removed by drawing a small brush loaded with "sticky" water or saliva over the rib stations. Do this after everything has dried thoroughly.
Finally, place the new prop on a pitch gauge and tweak the spar as needed to assure that the blade angles are identical for both sides. Check the prop in the field when flying. Props always move as weather conditions change and you should check pitch at least once every flying session. Add one .050" diameter Teflon washer to the shaft before flying.
This article includes techniques developed by Cezar Banks and Bob Randolph.
Write or email me at the addresses below with any questions:
Steve Brown
297 Hartman Ct.
San Dimas, CA 91773
stbrown@hotmail.com
