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Pitch Gauge Drawngs

Links to two drawings are provided above. Although they are not a large download they are quite large in sise to print so it may require sectioning the drawing and printing parts, depending on your printer and printer dirver. The drawings are only approximately to scale so that a reference dimension has to be used to test and enlarge or shrink the drawing to suit the properties of the output device you have.

Be aware that all pitch gages currently in use overestimate the pitch near the hub of the propeller. The larger the blade chord the larger the error.

In the drawing I have placed a scale because the size of the printed drawing will vary according to your equipment. If you want to print the document you may have to cut the imgage in four parts to get it on an standard "A" size piece of paper.


Your Propeller and You

Taken from Model Aviation August 1975

Most Competitive events have reached a state of sophistication that requires complete control of every function. The one function that has been very slow to receive much attention has been the propeller. Luckily, the prop manufacturers have done a good job in providing the competitive modeller with not only a wide selection, but also some very well designed propellers.

Wood propellers are generally the most popular props for competition, although in some events cast fibreglass props are quite popular. Nylon propellers are usually reserved for the sport flyer. The selection of a propeller material is usually determined by two things; namely, the application and economics. In events where high engine performance is essential, nylon props are rarely used. This is due to the fact that the nylon prop may fail at high rpms and can do severe bodily injury or damage to equipment. In addition, the nylon propeller tends to flatten out and lose pitch in time, where as the fibreglass or wood prop will not. On the other hand, it is impractical for the sport flyer to use a fibreglass prop or to break a wood prop on each landing. The nylon prop is by far his best bet.

Ironically, the molded nylon prop, including those that are fibreglass filled, because of their manufacturing techniques as the most consistent performers. The weights, thickness, balance and pitch of all similar nylon props are essentially identical. This is, however, not true of most wood or cast fibreglass props. The wood props are machined and the dimensions will vary with the different pieces of wood used as blanks and, also, with machining tolerances. Fibreglass props are usually cast by hand in a very time consuming process. Because of this, the dimension of these props will vary all over the map from one prop to another.

It is the intent of this article to describe the processes involved to standardize cast fibreglass and wood propellers. I do not intend to tell you what dimensions you should use for your individual application. This is up to you to determine by trial and error. There are five critical factors to standardizing props: diameter, pitch, blade width, blade thickness, and balance. In addition, other factors include blade design and airfoil.

Most of the factors can be quickly reduced to a ballpark range through either availability or trial and error. For instance. If you fly FAI free flight you know you need about a 7?3 prop, and you want to eliminate possible breakage, so you choose a fibreglass prop. There are only a couple available brands of fibreglass 7?3 props, so you are limited pretty much right there. Through trial and error on a couple of props you try various blade pitches (say 7/2, 7/2-1/2, 7/3, 7/3-1/2) and various blade thickness. You decide on a set of dimensions and henceforth rework all future props to these previously determined dimensions.

On the other hand, if you fly CL. B Speed you know that you need about a 7/10 prop. You don't want fibreglass because you usually ruin the prop on each landing and, if nothing else, the cost is prohibitive. You select several props of each available brand of basic 7?10 wood prop (say 7?9, 7?91/2, 7?10, 7?101/2) and, again through trial and error, determine the best basic design for your application. You then determine the set of dimensions you want on future props and either inspect and reject enough production props to supply you with your requirements or rework them to your own individual specifications. The tools required to properly inspect a propeller are a pitch gauge, callipers or micrometer, and a prop balancer. These three basic tools combined with various files and sandpaper, plus plenty of time, will give you consistent performance from your propeller. First you need a pitch gauge. You can build one for less than $2.00 and about two hours of your time. Either way you must have a means of determining propeller pitch

With a pitch gauge you are measuring the theoretical pitch. The actual pitch of the prop varies with blade velocity (rpm) and airplane velocity (forward speed), plus other factors. To operate the pitch gauge, mount the prop in the mounting block and align it so that it is perpendicular to the slots and tighten the locking screw or nut. Next, put the mounting block in the first station, usually the shortest radius, or slot "A." Then, bring the pointer up so that it contacts the underside of the propeller. By sliding the block forward or backward, adjust it so that it is in contact over the entire width of the blade. Mark off this location with a pencil or pen on the prop blade for each slot or station on each blade. Then, at each station, check and record the pitch on each blade. The pitch is merely the angle of the blade converted to linear motion per revolution for that particular radius. The pitch gauge is merely a protractor used for measuring the angle, with the scale converted to pitch at each station. If you encounter a situation where the bottom of the prop blade is not exactly flat, the safest bet is to file it flat before going any further. If the pitch is not what you want, file or sand that station to the desired value.

Each corresponding station on both blades should be the same. Previous experience has shown me that if you can balance the pitch between blades at each station within + - " that will be close enough. You may also find that the blade pitch is not consistent over the entire blade length. It may vary from, say, a 4?in. pitch at the hub to a 6?in. pitch at the tips. This is perfectly O.K. as long as both blades are consistent with one another.

After the prop is pitched on both blades, next check and record the blade thickness and widths using a caliper. I use a dial caliper reading in .001 " and find that it is the most convenient. However, any other tool or technique accurate to .005" can be used with satisfactory results. If the thickness is larger than the desired value, you can reduce it by filing or sanding on the topside of the blade. If it is too thin, there is not much you can do. Similarly, the blade widths should be checked and similar corrective techniques employed.

After the pitch, thickness and width are established at each station, blend all the reworks to form a smooth contour in between the stations. Now verify the exact radius of each blade. This can be done simply and accurately by using a round pin or dowel the diameter of the hole in prop, and using the caliper to measure each blade from the pin to the tip. To determine the actual radius, you must take into account the diameter of the pin. Either add or subtract one half the diameter, dependent on the technique. Next, check the balance using any reliable technique. Several inexpensive prop balancers are available and all will produce satisfactory results. If all of the preceding steps have been followed, you should find that only a minor touch up is required to balance the prop. Do this by sanding lightly the topside of the heavy blade. Do not attempt to balance the prop by trimming the radius of only one blade.

The pitch gauge in this article evolved from a gauge that I made for my own use prior to any other suitable gauge being available. The original model used a machined 3/4 in. plexiglass base plate and a chemically etched stainless steel scale. It had a larger scale and was about a pound heavier. I then was besieged with requests from my friendly Rat Race competitors for gauges. I made a dozen of them using a cast steel? filled epoxy base made in an RTV rubber mould This project didn't go any further, since these gauges cost us about $75.00 apiece. I then made a couple with a cast plastic scale. This method, although much cheaper, was not too practical due to the time involved.

These prototype units were essentially "bullet-proof." The gauge presented here however, is every bit as accurate and possibly even more practical because of its lighter weight. With proper care, it should fast a lifetime. The materials required are quite basic and readily available. Substitutions can be made in most all of the suggested materials and techniques. However, the critical dimensions shown must be followed.

To begin construction, cut out the 3/8 in. plywood details. Particle board, hardwood, or other material can be used, but it must be a nominal 3/8 in. thick. Then cut out eleven (11) 114 x 3/8 x 3 inch spruce strips. This is available through Sig Mfg. Co. Next, cut out the paper scale from magazine page. A photocopy of it is perhaps better, as you won't ruin the magazine. Cut out a piece Of 3/32 in. plywood, and also a piece of .030* acetate or other clear plastic to the outline of the scale. Cut a piece of 3/32 " plywood to the outline the pointer and bevel the prop blade contact edge as shown. Cut out two pieces of 3/32" plywood 5/8 x " and a 11/2" piece of " 3/8 spruce.

To assemble the base, use white glue and glue the 3/8 x 3/4" x 10 " detail to the main plate as shown and clamp in place. Next, glue the 1/4" x 3/8 x 3" spruce details exactly 1/4 in. apart as shown. I cut out 11 extra pieces or the same size spruce and coat them with wax for use as spacers. Then you can glue and clamp all the pieces in place at the same time. When the glue has dried, the spacers can be easily removed, as the wax coating prevents the glue from adhering to them. Then, turn the base over and install two (2) #1 x 1/2" F.H. wood screws up through the bottom into each spruce detail as shown in the photo. Although this stop is not absolutely necessary, it will greatly strengthen the assembly.

Next, apply a contact adhesive, preferably the spray on type, to the plywood scale pattern. Carefully position and install the paper scale. Then, apply adhesive to the .030* acetate and again position and install it. Carefully dress all the edges using a fine file or sanding block. Drill and counter sink two holes as shown and install the scale to the base using #4 X 1/2 F. H. wood screws. Install the pointer using a #4 x 1/2 R. H. wood screw and a flat washer.

Cut out the paper slot identification strip and apply it to the base using the contact adhesive. Be careful to align the letters with the slots in the base. Cover this with Scotch tape. Make the prop mounting detail using the two pieces of 3/32 in. plywood 5/8 x 1112,, and the 114" x 3/8" x 11/2" piece of spruce. Glue the two pieces of plywood together, drill a hole in the center and install a 4?40 blind mounting nut on the underside. Drill a corresponding hole 1/W in diameter in the spruce, align this with the hole in the blind mounting nut, and glue the assembly together. Lightly sand the sides of 1/4" x 3/8" spruce slider for clearance. Next drill a 1/W hole in the center of a 112" wood dowel pin, Bevel this as shown to about a 30 degree angle (60 degree included angle) and cut it off to about a 112" in. overall length.

Take a 4?40 x 3/4" bolt and run a nut up to the head. Install a flat washer and insert the whole assembly through the prop and thread the bolt into the mounting block. Run the nut down to clamp the prop in place.

You are now ready to start measuring the pitch or your prop. I am sure that you will find that this project is truly rewarding in the form of consistent performance.

Using a Pitch Gauge

By Robin Hiern (Long time flyer and Australian F2A Record holder)

A pitch gauge is an essential tool for anyone who is after top performance and consistent results from their motors. Many people possess good horsepower engines but are not utilizing this power in the air, due to not having the correct propeller. The only way to help find the correct prop is to know exactly what pitch 'Your' prop is. Pitch Gauges can be bought. The most well known brand is the "Prather (USA) priced at around $120 Aust. If this cost is too much you can build your own copy for only a few dollars and a few hours work. The plan in this newsletter shows you how. Maybe it's not as accurate as a Prather but certainly better than none at all.

This unit is O.K. for comparing "Your' own props. If you were to compare your results with those of a Prather or other brand the pitch could be different, but only by as much as 1/2 inch pitch. A pitch gauge is essential for use in speed, racing and to a lesser degree stunt. Good quality props from the likes of Sherlock, Bolly and Russia etc. do vary slightly, even if they are the same designation, and this variation is enough to effect performance. By measuring and labeling every prop and writing down the information in a prop book you will have some idea what you are working with (Not guessing)! Then if you need more or less pitch (LOAD) you can select another.

In you book you should also write down the different prop diameters and any other changes such as blade thickness, area, section etc. as this also has a big effect on performance. On obtaining a new prop you should write down all the details in your prop book. This could be an example. Brand XYZ 7" x 71/4
stationBlade 1Blade 2
26 1/46 1/4
36 1/46 3/8
46 1/46 7/8
56 3/47 1/4
677 3/8
77 1/47 1/4
877

The above is a typical prop as bought. You could use as is or custom pitch to what you think it should be and even up the pitch to as below.
stationBlade 1Blade 2
26 1/46 1/4
36 3/86 3/8
46 3/46 3/8
57 1/47 1/4
67 1/47 1/4
77 7
877

The above is a hypothetical prop, not suggesting that this is the pitch you want. I would then label this prop. XYZ No4 63/4" x 71/4,, I usually call my prop pitch by what they are at about 2/3 out from the hub, not averaging all the pitches. Do not bother with station I as it does not do any work and is usually very low pitch anyway. By making a family of the same brand/model props with different pitches, areas and sections you can proceed to find out what works well and then duplicate it.

If you also write down in your motor log book how each prop performs, needle settings, fuels, plugs and all other variables you can, after a while, get an idea what works and what does not. When it all

People like to tell me they can remember what worked and what they used. All I can say is "Rubbish". You cannot remember all the combinations of all the motors and classes over the years. Pencils and paper still works despite the computer age. A computer could be used to store all this information but a logbook can be readily accessed at the field and should be updated after each flight while you still remember.

Wooden props usually vary the most and can change pitch in use so it pays to recheck. Even carbon fibre props can alter their pitch. Do not accept what is marked on the blade as being anywhere near what it actually is. After a while you will see this to be true and notice trends and tendencies emerging. When you go out with a new model and motor armed with this information you will have an educated guess what prop to fit and then you need to get out there and Practice! Practice! Practice!