Autogyros are a unique aviation development that make interesting and challenging designs for R/C planes. In short, autogyros lack fixed wings and depend on forward airspeed to auto-rotate helicopteresque blade rotors, which provide the necessary lift for flight. In this article modified from Rick Anderson's excellent online series about the topic, we'll look at the design and construction of blades for ``typical'' RC planes. As you'll see, many high-wing RC planes can have their wings removed and rotors added for a new and unusual RC experience!
This is the first of a series of articles on the design, construction and flying of model autogyros that are intended for the Novice builder and flyer. The first one will deal with rotor blade construction and balancing. Future articles will cover the most relevant aspects of the construction of modern model autogyros and how to fly them.
While there are many different types of airfoils the one that is most popular and offers the most positive results for model autogyro use is the Clark-Y type airfoil. Other airfoil types may or may not yeild positive results and should be avoided unless you are an accomplished builder and flyer.
Step 1: First, a suitable blade material must be selected. SIG Manufacturing Co. manufactures a Clark Y Airfoil shaped balsa sheet that comes in two different thickness's 3/16'' and 1/4''. If you have a SIG cataloque check it out. SIG's PN# is B111. Using the preshaped material will save you a tremendous amount of time.
Step 2: Do you know what your blades aspect ratio is going to be? The aspect ratio is the blade's length divided by its width, as in Figure 2:
Another important term you need to be familiar with is blade thickness, normalized as a percentage (%) of the blade chord length. How a blade will fly and or feel on a gyro can be roughly determined by looking at the blade's aspect ratio and thickness, as in Figure 3 below.
Now that you have a supply of SIG blade material let's cut the blade's basic shape from which we will add the airfoil leading edge shape. Assuming that you know who long your blades are going to be, start cutting the blank to the correct length, then width. Begin your measurements from the trailing edge of the blank and mark the blade at each end. Draw a line between the two marks and, with a good straight edge and sharp Xacto knife, carefully cut along the line. Keep the trailing edge piece and disgard the rest. The finished blank should look like Figure 4:
Step 3: Airfoil Detail: Now that you have the basic shape cut out, it's time to add the leading edge shape to the blade. Carefully shape the leading edge by using a razor plane or sanding bar until the finished shape of your blade resembles Figure 1 above.
Step 4: Blade Reinforcement: We will now mark the location of the blade's center of lift. Let's make the measurement for the blades center of lift or chordwise CG. Measure the blade's chord. 25 to 30% of the chord will mark the blade's CG, measured from the leading edge. At this location, make a mark on the top and bottom of the blade and at each end and carefully draw a line from one end to the other on both the top and bottom. Refer to Fig 5.
Now add some iron-on carbon fiber tape to the blades to reinforce them. I usually use .004'' to .007'' thick tape that is 1/4 inch wide and slice it down the middle to form two 1/8 in. wide strips. Take a strip of carbon and either CA or epoxy the carbon over the top of the CG line on both sides.
Now let's make a pair of blade grips. These are small rectangular pieces of thin ply that will be attached to the blades to reinforce where the retaining screws will pass through the blades. I use 1/32'' ply for these pieces. You can use other material but it is important that the blade grip that is on the curved portion of the blade be pliable enough that it does not deform the blade when you attach it to the blade. Too stiff a material will deform the airfoil... see Figure 6.
After you have made the blade grips, attach them to the blade with epoxy or CA. Be sure to align the horizontal center of the grip over the 1/8'' wide strip of carbon fibre because this is where you will want to drill the holes to attach your blades to the rotor hub. Refer to Figure 7.
The last thing we need to do here is add the final blade detail. We will simply remove some of the blade material from the trailing edge. What this will do is to help protect the blade from being damaged in the event of a blade strike. If the blade were to strike something hard and if you used a nylon screw to attach the blade to the rotor hub then the blade will pivot to the rear and fold up rather than shatter the entire blade. Refer to Figure 8.
Step 5: Weigh the Blades: Weighing the blades is a very important step. Blades that are not balanced will totally destroy your model by literally shaking themselves to death and we don't want that to happen. Find yourself a small inexpensive electronic scale or a triple-beam balance if you can find one. Measure the weight of each blade and record the weight somewhere.
Step 6: Finding the CG: There are many different ways to balance a set of blades, but here's one that works well. Refer to Figure 9. Find yourself a small diameter steel rod or even a wooden dowel rod but make sure that what ever you use is not bent. Now lay the rod down on a flat surface and place the blade, flat side down, on top of it at about the center point of the blade. Carefully roll the rod back and forth and find that point where the blade balances itself on the rod. At the point that the blade has balanced itself on the rod, the CG, place a small mark. Now repeat this procedure for the other blade and mark its CG. After having done this you will notice that there is a difference where the two CG marks are located.
Step 7: Adjusting the Spanwise CG: In step 6, you found the CG point for each blade. Now we need to make that point common to both blades. Using the heavier of the two blades as a master, transfer the heavy blade CG to the lighter blade and mark its position. Assuming that the new CG point is farther out towards the end of the blade, we need to add some weight to the end of the lighter blade somewhere to balance out the two CG points. To determine how much tip weight to use, place the lighter of the two blades back on the steel rod and adjust the blade so that the new CG mark is located just above the steel rod. Now add some tip weight to balance the blade at this new point. I use split shot fishing sinkers or solder to accomplish this task. To do this I use a rounded carbide Dremel tool cutter and make a small pocket on the flat side of the blade as close to the carbon fibre as I can get without cutting into it. I try to dig a small pocket just off to the side of it and then burrow underneath the carbon fibre. This is where we will hide whatever weight is necessary to bring the two CG points together after having done this glue the weight into the pocket and fill in the remaining area with some wood filler.
Step 8: Balancing the Blades: In Step 7 you found the blades Spanwise CG point. Now its time to balance the blades themselves. Using the heaviest blade as a master, we need to bring the weight of the lighter blade up to match the heavy one. Now weigh the lighter blade once more and record the numbers. Once again add some weight to the lighter blade so that the two blades now weigh the same. Cut another pocket in the lighter blade, on the flat side (just as we did in Step 7) but cut this pocket directly on the new CG point and underneath the carbon tape. Once again glue the weight into the pocket and fill in the hole.
Step 9: Adjusting the Chordwise CG: In Step 4 we added some blade grip reinforcements to the blades. It is now time to add the blade mounting holes to them. Hopefully you know where these have to be - and if you haven't already put these holes in, please do so now before we go on. Remember that the blade grip holes need to be placed on the blade's Center of Lift or Chordwise CG as shown in Fig 5 and 7.
Having completed that little step we need to adjust the Blades Cordwise CG point to do this we will need to hang the blades from one of the holes you have just added and find how far off from vertical they hang. Refer to Figure 10.
When you hang the blade it will swing to the right as shown. What has to be done is bring the chordwise CG back into balance. To do this we need to add some weight to the blade to get it to hang vertical. To do this the best thing I have found is to find some small diameter music wire of .032'' dia or what ever size is needed to make the blade hang straight. This weight must be added to the leading edge of the blade. It may be necessary to trim some of the leading edge to fit the wire. Alternatively, you can make a small pocket in the flat side of the blade and glue it in there. Getting the blade to hang straight is our main objective. Take care not to deform the leading edge airfoil significantly or the blade will not fly.
Step 10: Covering the Blade's CG: Believe it or not, we have finished the construction portion of the blades. Now its time to finish sanding the blades and prepare them for the covering of your choice. I prefer to use Ultracote but you can use what ever you like. Stay away from spray painting the blades as it adds too much weight. The bottom line is to produce a smooth finish that will fly and produce lift.