By: Jack Jella
An AMA newsletter recently published this interesting chart prepared by Mr. Jack Jella, displaying chemical compatibility of some finishing materials, commonly found in our hobby.
Example:
C indicates compatibility. N indicates not compatible.
| OVER Polyurethane
|
C |
By: Dick Tonan
Recently I found a technique for determining the most aft CG (center of gravity) location for a model airplane that will provide an “adequate” stability margin. I thought I would put this information together for a wider audience than those that just read Replica.
Before I go any further, a disclaimer! The word adequate is pretty subjective. An r/c plane with an adequate margin of stability for the r/c pilot with several hundred hours on the sticks is going to be a pile of balsa at the bottom of a smoking hole for the r/c pilot that soloed a week ago. Therefore temper the results with your experience! The fewer hours of stick time, the further forward the CG should be from the MOST AFT location determined by this method.
I did not come up with these formulas, therefore I cannot claim credit for them. What caught my interest was that this method was the only one that I have seen that takes into account the aircraft aft of the wing leading edge. All other methods treat tail feathers as if they didn’t exist. The following formula determines the “CG” location expressed in percent of the Mean Aerodynamic Chord as measured from the M.A.C.’s leading edge:
CG = [0.17 + 0.3 x ((TMA) x (SH) x HTE))] x 100
CG = [0.17 + 0.3 x ((MAC) x (SW
Definitions:
H.T.E.: Horizontal Tail Efficiency. Value ranges from 0.5 for a flat tail located in its normal position in the down wash of the wing to 0.9 for a “T-Tail” at the top of the vertical stabilizer. Before we start banging numbers into the calculator, we need to determine 2 critical factors in the formula. They are the M.A.C. and the T.M.A. We will start with the M.A.C. because we need it to determine the T.M.A.
There are two ways I know to find the M.A.C. The first is mathematical, the second is graphic. The graphic method has the advantage of not only determining the M.A.C., but also indicates its location on the wing. If you have a straight rectangular wing, the M.A.C. is the actual wing chord.
Let’s go over the mathematical method first. To use the following formula, you need to know only two things: the wing’s chord at the root (Cr) and the tip (Ct). Plug these numbers into the formula below.
M.A.C.= 2/3 [Cr + (Ct -((Cr x Ct)
M.A.C.= 2/3 [Cr + (Ct -(((Cr + CT)))]
Using the example of a wing with a 15″ root chord (Cr) and a tip chord (Ct) of 10″, the formula cranks out a M.A.C. of 12.7″. This is very close to being the average chord of the wing.
Now. let’s take a look at a graphic method. If you have either full size or scale drawing of your wing, the hard part is already done as the first step is to make a scale drawing of “your” wing. Figure 1 is a scale computer drawing of the wing in the above example. On your drawing, draw two lines; the first extends forward (perpendicular to the wing span) from the leading edge at the tip and it’s length is equal to the length of the root chord (Cr) on your drawing. Next, draw a line extending back (again perpendicular to the wingspan) from the trailing edge at the root that is as long as the tip chord (Cr) on your drawing.
Actually, you can reverse the direction of these lines and the results will be the same but one must go forward and the other back. From the forward end of the line at the tip, draw a line that connects to the aft end of the line at the root. This line will cut diagonally across the wing.
The next step is to draw a line that connects the midpoint of the root chord (Cr) to the midpoint of the tip chord (Ct). The location of the M.A.C. is at the intersection of the lines drawn in these two steps. The length of the M.A.C. is the chord of the wing at that point. Measure your scale or full size drawing.
Now, that we know what the M.A.C. is, we can determine the T.M.A.
To do this, measure the distance from a point at the root that is 25% of the M.A.C. to the midpoint of the horizontal stab root chord. You can do this either on the model itself, the full size plans or your scale drawing.
At this point, we now know the M.A.C. (12.7″) and the T.M.A. (33″). Now, we need to know the areas of the wing and the horizontal stabilizer. For non-rectangular surfaces, the easiest way to calculate the area is to divide the wing into rectangles and triangles. The area of a triangle is equal to 1/2 the length times the width. For example, the wing area is 875 sq.” (SW) and the horizontal stab area (SH) is 165 sq.”.
HTE (Horizontal Tail Efficiency)
For the horizontal tail efficiency, let’s pick a number…we’ll assume that the tail is in the normal location, so we’ll use 0.5 in our formula. Here’s what our formula will look like.
Formula :
CG = [ 0.17 + (0.30 x (( TMA ) x ( SH ) x HTE ))] x 100
MAC SW
Formula with example values :
CG = [ 0.17 + (0.30 x (( 33 ) x ( 165 ) x 0.5 ))] x 100
12 875
Formula results :
CG = 24.35% of the M.A.C. or 3.09″
It is important to remember that this is measured from the leading edge at the M.A.C., not at the wing root…or event worse, at the wing tip.
For those incline to utilize “T” tail designs, let’s look at what happens when you change to that configuration HTE will now be 0.9 instead of 0.5. All the variables appear below. We have changed the desired location of the CG by almost 1″ just by making changes to the tail feathers. Remember, this formula was championed because it includes consideration for the tail feathers.
| FACTOR | Old Value | New Value | Old CG | New CG |
| H.T.E. |
0.5 |
0.9 |
24.3% (3.1″) |
30.2% (3.8″) |
| SH |
165. sq.” |
200.sq. “ |
24.3% (3.1″) |
25.9% (3.3″) |
| T.M.A. |
33″ |
25″ |
24.3% (3.1″) |
22.6% (2.9″) |
By: Tom Noser (Portions copied from “Sheet Metal Workers Manual” by Broemel, 1942)
Tempering: The term “ temper ” as used by steel makers , refers to the percentage of carbon in the steel. It has a different meaning when used by
the hardener. In the steel mill it means the amount of carbon steel contains. The meanings have been tabulated by an authority as follows:
Very high temper……………………………..150-point carbon
High temper………………………….100 to 120-point carbon
Medium temper………………………..70 to 80-point carbon
Mid temper……………………………….40 to 60-point carbon
Low temper………………………………20 to 30-point carbon
Soft or dead soft temper……………………20-point carbon
A “point is 1/100 of 1% of any element that enters into the composition of steel, so a 150-point carbon steel contains 1 &Mac189; % carbon. In the steel mill such a steel is spoken of as 150 steel.
Tempering, on the other hand , also denotes the process by which steel is brought to a previously determined degree of hardness. A steel chisel can be made so hard that it will cut another piece of steel; or so soft that driving it into a piece of hardwood will dull its point. This property of steel enables the mechanic to make it into tools suitable for any kind of work.
Steel is tempered by various means, all of which depend upon a heating and subsequent cooling of the metal. For instance, a piece of tool steel which is heated to a cherry red and then plunged into cold water, becomes very hard. If allowed to cool slowly, it becomes soft. Between these two extremes all degrees of hardness can be obtained. Every tool is tempered to the degree of hardness that makes it most useful.
When a polished piece of steel, hardened or unhardened, is exposed to heat in the presence of air, (it’s surface) assumes different colors as the heat increases. First will be noted a faint straw color, which changes to a deeper straw, then to dark brown with purple spots, then to a dark blue, and finally to a light blue. These colors are due to a thin film of oxide that forms as the heat progresses. These colors are valueless, however, to the tool maker unless the metal has first been cooled in a bath of water, oil, or some other liquid, when at red hot. Drawing hardened steel to any of these colors is called tempering. The following list of colors applies to all of the tools commonly made:
Color Tool: Visual Temper Gauge
Pale or light straw…………………………………………………..Lathe tools
Dark straw………………………………..Taps, dies, milling cutters, etc.
Woodworking tools (cooled in oil)
Purple………………………………………………Center punch, stone drills
Dark blue………………………………………………..Cold or cape chisels
Light blue……………………………………………………………Screwdrivers
Tool Tempering.—Let us now consider the tempering of a tool, taking for example the cold chisel, a tool widely known and generally abused. To obtain a chisel, it must be properly forged at a comparatively low heat, and then hammered with light blows at the last until it has cooled considerably below the heat ordinarily used when metal is displaced. The object of the light blows on the cooling metal is to close the grain or refine the steel, making it tough. Tools of this character stand up better if they are heated to a cherry red heat and cooled before hardening(as mentioned above.)S This is not always possible but when it is make the hardening heat a separate operation.
To harden, heat two thirds of the part forged to a cherry red heat, using great care not to overheat the point, and then cool one half of the blade in cold water; always move the tool about or set the water in motion, avoiding any danger of making a water crack at the water edge.
The next operation is to brighten one broad surface with an emery stick. A piece of emery cloth tacked over a stick of wood makes a very good polisher. The heat remaining in the body of the piece will reheat the end just cooled, and the various colors will appear in order on the polished surface. The proper color for a cold chisel when correctly tempered is dark blue. When this color is attained at the point the entire tool is immersed in cold water and is not removed until cold. If the tool is not cooled off enough in the first operation, the colors will run down very rapidly and become compact, and if not watched very closely, they will be gone (back to cherry red)before the tool can be cooled.
When a tool is to be hardened all over, it is first heated to a cherry red and then cooled. After brightening with the emery stick, put on a square or flat piece of hot iron. The tool will absorb the heat and the colors will soon commence to run. When the desired color is obtained, cool again in water or oil.
Spring Tempering.-The method employed in hardening a spring in oil is as follows : First, heat to a cherry red as in hardening in water ; cool all over in oil; hold over the fire until the oil upon the surface blazes. This is called “flashing.” Cool again in oil. This “flashing” is done three times before the process is complete. Another method of hardening a spring employs water instead of oil. Pass the spring over the fire or through a flame until it is hard enough to make a pine stick show sparks (???????) ; then cool in water and a spring “temper” results.
Annealing.-The process of softening a piece of steel is called “annealing.” A piece of steel is softened or “annealed” prior to being worked on in the lathe or otherwise machined, as this process brings about a uniform softening, relieving any strain that might have occurred in forging. To anneal a piece of steel, it should fist be heated to a cherry red heat, and then allowed to cool slowly.
The above commentary is presented partly as a matter of historical interest, but particularly because we often need to re-temper things like landing gear. You will find that you need two different torch heads when doing things like silver soldering, one for point heating with a fine, intense pencil-point flame, and one with a diffuse flame for better control of temperature for longer periods of time…
By: Tom Noser
Soldering is an easy matter once you have some experience at it, but can be very frustrating when you don’t know the basics, and don’t understand what makes soldering work.
It is very important to make good joints, because the joint is either conducting electricity for you, or supporting an aircraft at the landing gear or even holding your wings up, or down in your flying and landing wires.
The various jobs to be done will require solder of “different” tensile strengths and therefore different material composition.
Our soldering experience usually begins with the relatively easy task of joining two wires or joining a wire to a post or clip for electrical work. In this case we are working with low temperature material, melting at 340 degrees, made up of 60/40 tin/lead (60% tin and 40% lead) a very common and readily accessible material called “radio solder” having an internal core of rosin for a flux (the material which cleans the surfaces you intend to join.) This type is meant to serve as an electrical solder only and does not have sufficient strength to use it for structural purposes. It is applied with a soldering iron of any number of types, or a soldering gun of the instant heat type.
Radio work, for me, usually involves putting additional lead between the servo and the plug, or making up “y” harnesses. For this work I use a pencil iron with fifteen and thirty watt temperature settings set to thirty watts. Also I use a small wire stripper, a pair of needle nose pliers, a set of diagonal cutters and some shrink tubing.
The most important point in soldering is to “always” have a mechanical connection before attempting a solder connection. Solder has little strength of its own. Solder does immobilize closely related items giving a mechanical joint between wires the full strength of the smaller wire.
Another important item is the flux. All metals corrode, and this corrosion must be removed before a solder will bond to the surface of the metal. The flux is an acid which does this molecular cleaning, enabling a bond between the molecules of tin, lead and copper, or whatever material you are joining.
There are different chemicals used for different temperature solders and different metals. Radio solder uses pine rosin to do clean your wires, usually copper. If the solder is “beading” up on your base material, it is because the flux you are using isn’t cleaning that material. Passing a file, steel wool or sand paper (wet – or – dry) over the surface will sometimes solve the problem, enabling a flux to get to the base material and clean it. Other times you need to try a different flux. In high temperature work, such as silver soldering, borax is used for the flux.
To make a soldered joint between two wires, strip some of the insulation from the ends of the two wires, twist the wires together, apply a small amount of rosin core solder to the tip of your iron, heat the joint with the liquefied solder at the same time you apply solder to the wire joint on the side opposite your iron to be sure the wire is hot enough to melt the solder. If you find the solder won’t flow into the joint, remove the heat, apply a little flux to the joint, and then re-heat and re-apply your rosin core solder.
A cold joint can result from heating your wire solder with the iron instead of the wire which you are soldering. When the wire is hot enough to melt the solder, the solder will wick along the joint in a very fine coat and color the wire silver wherever it bonds to the wire. A good joint needs very little solder and need have no lumps of solder on it. This is especially true when you use tinning . Tinning is preparing one or both of the items to be joined by pre coating with solder. Use this method when working with small, delicate parts which may be mounted in plastic or may be subject to damage by the application of excess heat such as a battery tab.
To tin a tab, coat the area to be soldered with flux, apply a small amount of solder to the point of the iron and touch it to the item. The solder will quickly coat the fluxed area and the heat can be removed. To fix a wire to the tab, flux the wire, place the wire against the tab, touch the wire with the iron until the solder on the tab and the iron tip flow onto the wire, and remove the iron.
In the interest of neatness and preventing corrosion, you should always keep a damp sponge at hand to wipe newly soldered connections free of flux, and on which you should often wipe your iron to keep the tip from deteriorating due to the action of the flux.
The soldering gun is useful for working with wire of No. 16 Ga. and larger for household and audio speaker work, but is too large and too hot to be used for radio and servo work.
For high strength work you will need silver solder. You can buy a silver solder at the hobby shop, but you will find that it has a relatively low silver content and therefore a low melting temperature and questionable structural strength. The flux that comes with it is extremely useful as a flux for radio soldering. I apply a drop of this flux to wire joints to make the solder take to the joint easier and thereby reducing the heat I have to use to finish the joint. I used this sliver solder to make landing gear when I first started scratch building. The joints held up well except in high stress situations, but I never lost a plane because of solder joint failure.
Silver solder referred to on “plans” is not a low temperature material. The good stuff is available from Lathrop’s Jeweler’s Supply, 6702 Ferris, off Bellaire Blvd. In Houston, TX.
Lathrop’s has the solder in three different temperature grades so that you can make joint assemblies which can be soldered at high temp and then assembled to each other at lower temp.
Lathrop’s have solder in small and large quantities, and I use small quantities of the high temp, and larger quantities of the low temp material. They tell me that “Easy” melts at 1240° F and flows at 1325° F. “Medium” melts at 1275° F and flows at 1360°F. “Hard” melts at 1365° F and flows at 1450° F. All of these temperatures are attainable with a Propane torch, but not on large items.
Also at Lathrop’s you will find jeweler’s saw blades in all sizes. I buy a dozen each of their three smallest sizes, I think they are 4-0, 5-0 and 6-0. The 6-0 does a job on brass tubing with a 1/64 wall thickness. If you don’t have a jewelers saw frame, get one. It is what you cut metal with. Always set up the blade to cut on the pulling stroke. Lathrop’s sells tools.
Back to my landing gear. The plans always tell you to “bind and solder.” I think a better way is to use a brass tube to hold the parts together. Just find a size that will fit snugly around the group of wires you are working with. A &Mac189;” length should be sufficient. Put a liberal amount of flux on each piece of the wire set and re-assemble with the brass tube. Heat the set with a pencil point flame on the largest piece of wire and move the flame to heat everything evenly. When the flux turns to clear liquid on the metal, the temperature is approaching a melting temp of the “easy” solder, about 1240°F.
Touch the solder to the work without exposing the solder to direct flame and let the heat of the metal melt the solder. As the solder begins to melt, move the flame away from the work slightly to reduce the temperature rise in the work, and move the flame about to distribute the solder evenly. Be sure to put plenty of solder on the work, being careful not to over heat it. When finished, quench the work in cold water.
Wash the joint well in cold water to remove as much flux as possible and steel brush the joint clean. Use a piece of wet – or – dry sandpaper to clean the wires to a bright finish, and then heat the joint until the wires go through the surface colors of light straw, dark straw then dark brown with purple spots. Do this slowly and carefully or the work will turn light blue and you will have gone too far. When a dark blue stage is reached, quench the work in cold water and you have spring tempered gear. Again clean the work with a steel brush and spray with a military flat of any color, all of which are primers and hot fuel proof.
The main difference between radio and silver soldering, other than temperature is silver solder adds strength to the joint by filling in between the various wire parts. Whatever you use to bind the joint, put plenty of solder into the joint. And keep in mind that if you are working with large diameter axle wire, you will need a larger size brass binder wire to hold up to the high temperature you will be using and the long period of time it will be applied to the work.
Those are the main advantages of using tubing as a binder for axle joints. It is less liable to melt away since the heat will be conducted through the tubing to other parts of the joint. You
should concentrate the heat of the flame on the largest wire until all the joint is at a temperature that will melt the solder. Keep in mind that it helps to use plenty of flux in high temp soldering. The work is easier to clean and will stay cleaner through the heating process.
I hope this information will get you started on a project you have been avoiding for lack of confidence in your soldering ability. Just remember, the more you practice, the luckier you get.
By: Bill Miles
There is no doubt that more than ever, that Pro-bond glue is probably the most controversial none-R/C product to come along in awhile (at least on the I.M.A.C. mailing list). It seems everyone has their own technique for using this glue and everyone has their theories. I will give you my method and opinion on the glue and also go through some simple tests between odorless C.A., epoxy, 3m 77 spray, and contact cement. I will cover odorless C. A. because any of these glues can also be used for capping stabs and wings.
First, Probond is a product made by Elmers. It is found in most do-it-yourself super stores like Lowes, Home Depot, and even Wal-Mart. It comes in a black, white and gold bottle. It comes in two sizes, oz. and the large bottle oz. (in the long run-cheaper if you do a lot of wings and I do). It has the consistency of pancake syrup. It is a little thicker than some of the finishing resins out there, so it doesn’t soak into the wood or foam quite as quickly. It is water-soluble so it’s easier to work with than epoxy. With epoxy I usually have to have a bottle of alcohol, latex gloves and baby powder standing by (that’s right-baby powder). Along with epoxy, Probond is easy to take off of your hands with baby powder and if you just don’t want glue to ever touch your hands, I guess you may want to still use the gloves. But at any rate, it’s easier to wash off. As with epoxy however, you never want to let this stuff spill on your wood building surface. You will never scrap it off or pull it up without pulling up some of your building board.
Everybody who has used it out there has his or her own theory for using it. First of all, the directions on the bottle say to wet the surface before you use it. I have found that this is not necessary. The humidity here in S.C. is usually enough to negate the use of water. I first start with the usual for sheeting wings. A large (depending on the size wing panel) flat clean building surface, wax paper, the glue, masking tape, and whatever form of weight that you want to use. I use several sheets of particleboard leftover from jigs that I built for fuselages.
It is heavy in several sheets and it disperses the weight evenly over the entire wing surface. I usually take some form of sticky backed sandpaper and stick it to one part of my building surface in a long sheet. Great Planes makes great sandpaper for they’re sanding bars. I use this to edge sand the balsa so that all the sheets are even. The best method that I have heard of is to use T-pins to hold all of the sheets together in one stack and then run the stack up and down the sandpaper to make an even edge. After edge sanding (if I have to) I then tape the sheet together using masking tape, with the edges of the sheets butted together.
You then have an option with Probond (and this is probably the neatest part). To glue or not to glue, that is the question! What I mean by this is either you can edge glue the sheets or leave them taped together (believe it or not)! Do all wing skins in this fashion. Lay down the first saddle. You now have an option of putting wax paper down or not. If you do not glue the sheet together, you should probably wax paper the entire saddle. There is a chance of the glue coming out between the sheets. It is a very slim chance and it wouldn’t ruin the panel if it did, but it just adds time to the building when you have to pick or sand foam off of your wing.
Now lay in the first wing skin. You can put glue on the skin before or after you do this. I do it after. I usually start at one corner of the skin and then go back and forth diagonally about 2 inches apart at a 45 deg. Angle all the way lengthwise down the wing. Then I go back in the opposite direction. This creates a waffle pattern on the wing. I then take a squeegee or similar item and smooth out the glue.
Keep in mine that the more weight that you can put on the wing, the less glue you have to use. I have done tests to prove this theory. I then lay the panel onto the wing skin. Put glue on the other skin in the same fashion and lay on top. Put more wax paper on and lay the other saddle on top.
I then place the particleboard on top of the wing. I usually use 4 sheets the same exact size of the panel. I then place everything heavy that I can find in my shop on top of the panel. I even have old plastic soda bottles that I have filled with water to place on top of the particle board. Now, because of the wax paper you can actually move around the panel in the saddle to get it lined up properly (another advantage of fully sheeting the saddle with wax paper). I have only let the panel dry for about 3-5 hours before pulling up and cutting the sheeting, but I usually let it dry overnight.
I have also used this glue for capping the wing off. There is one word of caution however, excess. Don’t put a lot of glue on your caps. I usually put the glue on and then use my finger to squeegee most of the glue back off. If you don’t do this, the glue has a natural tendency to foam-up and may push the caps off of the wing panel to far; creating a wing panel that may not be square on the tip, etc.
Let’s talk about what tests have proven. First, Probond has a tendency to seep down into the foam about &Mac189; inch into the foam. You will not get this with contact cement. Epoxy seeps down into the foam but not as far and is heavier. Spray 77 is easy to use and light but tests have shown that in the sun and on larger planes it has a tendency to delaminate. Obviously because of time-to-work reasons, odorless C.A. is not good for sheeting large surfaces such as wings but may be better for wing tip capping and small areas.
Keep in mind that the object here is to use as little glue as possible and as much weight as possible. Remember that for a 1000 sq. in. wing panel, that 1000 lbs. is only 1 lb. per sq. in.!!!! Remember also that you must put in your phenolic tube support before sheeting your wing! Yep, that’s right I have done it. Not on a customers plane of course. Another tip, have all of your items that you will need ready and be totally prepared before starting your wing sheeting project. There is nothing worse than starting and not realizing that you don’t have enough weight ready or that you don’t have enough glue. I hope that your next project goes well and remember, “We build, You fly!” If you have any questions send us an email.
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