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Old Aug 31, 2007, 10:50 PM
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Mini-HowTo
Scratch build a performance electric plane

Whether you’ve flown a model airplane, or just wanted to try to build one, building a model from scratch can be a truly rewarding experience. Not only do you have the pride of a scratch build, but you can change parameters and flight characteristics easily to suit your flying style. In addition, spare parts are easy to find after the inevitable crash! Also, it’s usually cheap.

I tried to make this a fairly complete tutorial. You will likely skip many of the sections in this in your build, but you might want to go back for reference. The sections are titled so it’s easy to determine which one to skip to break up the monotony of this guide.
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Old Aug 31, 2007, 10:52 PM
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In this tutorial, I will be building the “Scorcher” and the “Carnivore”. It is a very versatile and durable, yet simple airplane. They are nearly identical except the Carnivore will have two motors and a swept back wing. These models will demonstrate most of the basic principals of basic scratch-build modeling. The plane is capable of performing well at speeds as low as 20 mph up to as fast as 75! It can be built to fly docile enough for beginners or aggressive to suit the experienced pilot. The plane is very durable and minor wrecks will often leave the plane unharmed. Hard wrecks can often be repaired in under an hour!

So let’s get started. First a little bit of theory and plane size selection…..
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Old Aug 31, 2007, 10:55 PM
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Before you start building....

(Skip this section if you have an idea what type and size plane you wish to build)

First you must determine the approximate size of your model. Field size is a self explanatory factor. A 72” wing plane is likely a poor choice for flying in an average park. Another consideration is available motor power. For wingspans 24-36” motor power should be 80-200 Watts (motors from 370 size up to 480 size brushless or 400-540 brushed). For wingspans 36-48” 100-250 Watts works well (400-480 brushless, 480-540 brushed). These are just good starting numbers for motor power. More power is always fun. Go as high as 450 watts for wings between 36 and 54” for a real thrill ride, but hold on tight!

Also you should choose whether you want to go single or dual motor. The dual motor is nice as you have thrust vectoring capability rather than a rudder stick if you radio does mixing. Also the dual motor model may be easily set up for water as a boat bottom may be velcroed on the bottom for a flying boat!
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Old Aug 31, 2007, 10:57 PM
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Wing terminology and some theory

The wing is the most critical part of the airplane and will affect the flying characteristics more than anything other part. There are three main dimensions of a wing; wingspan (length), chord (width) and thickness.

Wingspan is the primary dimension. Rule of thumb: the greater the wingspan, the more stable the plane will be to rolls. Shorter wings will snap roll faster, larger wings roll slower.

Chord (or width) is the second dimension. Rule of thumb: Deeper wings are more docile and are capable of flying slower, but stall more easily. Shallow wings are more stable at higher speeds, but require more speed to fly.

Thickness is the third critical dimension. A thick wing will be stiffer and more resistant to flex, and in the case of asymettrical curved wings, they will lift more. On the downside they are heavier and will have higher drag.

Aspect ratio (ratio of wingspan to chord): Lower ratios are good for slower planes such as a 3-d or slow flyer. Larger ratios are better for gliders and pattern planes.

3.5:1-4.5:1 – Good for 3D aerobatics and slow flyers

4.5:1- 6:1 – Good for pattern planes

6:1- 9:1 – Best suited for gliders and hotliners

A good ratio for most planes is 5:1
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Old Aug 31, 2007, 10:58 PM
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Wing loading

Wing loading: If you multiply the wingspan by the chord (wingspan X chord), you get the wing area in square inches. Divide by 144 and you get square feet. Wing loading is calculated by dividing the expected weight of the plane (in oz) wing area in square. Light wing loadings (8-12 oz/sqft) fly slower and are suitable for beginners, but are harder to fly in windy conditions. Moderate wing loadings (12.5-16 oz/sqft) require a bit more speed and motor power, but are more stable in wind and are suitable for pattern planes. Heavy wing loadings (16-20 oz/sqft) requires a fair amount of speed to fly and will land fast, but perform well in windy conditions.

The model in this tutorial works well with wingspans of 24-56”. I will be using a wing 36X7” in this tutorial as it is a very common wing size and good for most applications. Therefore my aspect ratio is approximately 5:1 and wing area is: 36X7/144 or 1.75 sqft. My target weight for this model is 25 oz with battery. So wing loading is 25/1.75 or about 14 oz/sqft. It should be a good pattern plane.

Ok, enough theory. On to building the plane
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Old Aug 31, 2007, 11:08 PM
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Making the wing

In order to make the wing I used Home Depot white box foam, a “wing jig”, and a hot wire bow. The jig is simply wooden (hardwood) wing cross sections secured to a flat board. The wing cross sections can be found all over the internet by searching for "air foil". Or you can click on this link for a huge selection of airfoils: http://www.ae.uiuc.edu/m-selig/ads/c...atabase.html#U

A very good (and common) airfoil for high lift is the Clark "Y". Another excellent (and perhaps the easiest) lifting airfoil is the Gottengen 438. For moderate lift and a little more speed try the USA 27 or the Gottengen 286. For really high speed planes go with a MH 30 or USA 49.

The bow is 32" wide with 28” of .032” stainless steel fishing leader (tension on this wire is kept by the spring) connected to an automotive battery charger. The battery charger is adjusted for best temperature. It should cut at a rate of about 2-5 seconds per inch. Slower is fine. Any faster and you can have a depression in the center of your wing. The heated wire is run over either end of the wing templates with the foam in the middle with a weight on it to keep the foam from moving. My jig is 24” so I’ll glue two 18" sections together for a 36" wing. Alternately I could use .048” wire and a 39” bow to make a 36” wing.

When cutting the wing DO NOT PULL HARD, but use gentle pressure. Do not stop on either side of the wing to catch up, but try to adjust speed as the wire travels through the foam.

You may want to put carbon fiber in the wing to stiffen it. To do this I drag a low wattage (15W) soldering iron down the foam on the bottom of the wing. I then mix 4 parts gorilla glue to 1 part regular white glue and place that in the crevice followed by the carbon rod and then tape to hold it. WARNING: this glue mixture expands to 6-9 times its size while curing. Use glue sparingly!

Covering the wing also adds strength. Covering can be anything form ordinary printer paper to MonoKote to Tyvek homewrap. I used Tyvek homewrap for its superior strength. To apply it I sprayed the wing an the wrap with a few light coats of super77 spray adhesive. I then started at the back of the wing and keeping the covering tight, rolled the wing over the covering.

Here's a little on Foams:

Box foam - light but brittle. Best used for shaping balsa to make a fuselage. Not recommended for wings greater than 36".

Blue Core - Medium weight but flexible. Compression resistant. Excellent for making wings with minor reinforcements (such as Carbon fiber or covering). It is also good for sheating with balsa for an extra strong easy to repair fuselage.

Pink Foam - Heavy and stiff. Excellent for wing construction with minor or no reinforcements. Also good for making a fuselage without any balsa sheeting.
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Old Aug 31, 2007, 11:13 PM
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Alternative wings

If the wing jig and bow is too difficult, try these:

You can use other materials such as blue fan fold foam insulation folded over a balsa spar. This makes a symmetrical lightweight airfoil that is extremely durable, but doesn’t provide much lift. 2mm coroplast can also be used, but it is heavy.

Another method for wings 36” and less is to use a flat sheet of blue fan fold foam with a carbon spar glued to it (use the gorilla glue method mentioned above). To add lift to this wing you can cut balsa spars and gorilla glue them to the bottom of the wing. You can use the same cross section types I used for the wing jig. 1/8” to 3/16” balsa is well suited for this.

Alternately, go to the local hobby store and buy a wing. The HobbyZone SuperCub wing works well and costs about $20.
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Old Aug 31, 2007, 11:15 PM
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Making the Fuselage

Now that you have your wing, it’s time to cut a fuselage. Fuselage length should be about 70-110% of the wingspan. Longer fuselages will help resist flat spins, but increase tail weight. I will make my fuselage 32” (or about 85% of wingspan) in this tutorial. This is a good reference for stability and tail moment.

To make the fuselage I cut two identical cross sections from 1/8” balsa and secured them to a piece of white box foam. I used super 77 spray adhesive on either side of the foam and on one side of each balsa piece. I aligned the balsa parts and stuck them to the foam. I then used my bow (a knife works fine if you don’t have a hot wire bow) to trim off excess foam. If spray adhesive is unavailable, the above mentioned Gorilla glue/white glue combination can be used. Spread the mixture on VERY THIN and use tape or weights to hold the fuselage together while the glue hardens.

This creates a very rigid fuselage. The beauty of foam inside balsa is not only stiffness, but in a hard crash, the balsa and foam break cleanly and can be glued and ready to fly again in only a few minutes with minimal added weight! This has been the most successful fuselage I have made yet.
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Old Aug 31, 2007, 11:23 PM
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Alternate fuselages

There are many alternatives for a fuselage. One is to simply use the bare foam. You can add a carbon spar or sheet the fuselage with regular printer paper, monokote or Tyvek homewrap to stiffen it. Stick the covering to the fuselage with spray adhesive. Blue (or pink) foam is preferable for this as it is significantly sturdier, but white box foam works ok when covered.

Another readily available (especially after election day) material is coroplasat. Simply cut 2 flutes from one side fold up and apply some CA glue. This is a very simple fuse and is very crash resistant. Unfortunately it tends to flex under higher stresses in fast maneuvers, and therefore is not recommended for faster planes. It also tends to be a bit heavy.

Yet another way to make a fuse is blue fanfold foam or just balsa wood. Simply cut you side and bottom pieces and glue with gorilla glue. Add cross sections for added strength
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Old Aug 31, 2007, 11:26 PM
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Tail assembly Theory

This is perhaps the easiest part of the build. A good horizontal stabilizer (elevator assembly) will be between 1/3 and 1/10 the area of the wing. High speed planes will be around 1/8. Slow flyers and 3Ds will be closer to 1/5. The shape is up to you. I make about 1/3 of the horizontal stabilizer movable as an elevator. 3D planes use up to 2/3, and some planes rotate the entire thing. The more surface that moves, the more responsive the control will be. A good tail for most planes is about 1/5 to 1/6 the surface of the wing with ¼ to 1/3 of the area movable.

For example my wing is 36X7 or 252 sqin. My horizontal stabilizer should be 12 X 4.5 or 52 sqin. So my ratio is about 1/5. My movable surface will be 1.5” deep for just over 1/3 movement.

The vertical stabilizer has a bit of science to it. The easy way is to make it about half the size of the horizontal stabilizer (or use some judgment). The hard way is to measure the surface area of the side of the plane in front of the wing and multiply it by ½ the distance from nose to front of the wing. Then divide that number by the distance from the back of the wing to the middle of where the rudder will be. The resulting number will be the minimum surface area your rudder should be.

For this model let’s say I predict I will have 10 inches of nose in front of the wings leading edge (this is a fair guess that the leading edge should be ¼ to 1/3 of the way back from the nose). The fuselage is 2.5” high.

Therefore 10 X 2.5 X 5 = 125. Since my fuselage is 32” and the wing is 7” chord, from back of the wing to tail section is 15”

Now 125/15 ~ 8.5 So my rudder/vertical stabilizer should be a minimum of 8 sqin. For this model it will be 2.5 X 4 or 10 sqin
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Old Aug 31, 2007, 11:30 PM
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Tail assmbly construction

I am using 1/4” balsa for the tail assembly as it is very rigid and performs well at higher speeds. I also cover it with aircraft coating such as monokote or ultrakote to add to the stiffness and strength. On the movable joints I sand a 45 degree bevel on the movable surface and attach it with tape. I tend to like Tyvek as it adheres better than any other tape I have used, but strapping tape works well too. The tail is glued to the fuselage with the 4/1 mixture of gorilla glue to white glue.

Alternately, the tail can be made very well from blue fan fold foam the same way. Although lighter, it tends to flutter more at high speeds.

Another method is to use coroplast and remove one flute. This provides the best joint, but is the heaviest option.
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Old Aug 31, 2007, 11:39 PM
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The ailerons should be between ¼ to 1/18 of the area of the wing. The further out from the fuselage you mount them, the more responsive the plane will roll. 3D planes will be closer to ¼. High speed planes will be closer to 1/16. Pattern planes will be around 1/12 or so. A good starting point is 1/10 mounted in the middle of the wing or 1/12 mounted at the tips.

I like my planes very responsive, so I am using ailerons 1.5” X 11” . Since my wing is 252 and my total aileron surface is 33 this is about a 1/8 of the wing surface. I will also mount these near the wing tips. This should give the plane an impressive roll rate, but might not be suitable for beginning flyers.

The ailerons can be constructed from either blue fanfold foam, balsa wood or poster board. Use the same beveling technique used in the elevator and sand a 45 degree radius off of the joining surface and tape both sides to the plane. For the Scorcher I am using blue foam. For the carnivore I am cutting a "V" in the bottom of the wing and using the covering as the hinge.
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Old Aug 31, 2007, 11:46 PM
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Assembly

Now that the parts have been made it’s time to assemble the plane. I start with the wing by installing my ailerons and servos. The install the servos I trace out a box roughly the size of the servo, then heat up a flat head screwdriver with a torch or lighter and melt away a chamber with the heated tip. I then use double sided tape to hold the servo in and then place another piece over it for best hold. Hot glue works well in place of the double stick tape, but be careful not to overheat and melt the foam any more!

The next part is the tail. The tail should be glued in so it is square with the fuselage. Once glued, I place my servo wherever is most convenient. For the models in this tutorial, the tail is the elevated type and the risers act as the vertical stabilizers. A standard T ( __|__ ) type tail works well with this model also. Alternately a V-tail may be used. I used 1/4” balsa for stiffness. Again blue foam or coroplast may be used.

Next I obtain my needed COG. To do this I mount my motor to the assembled fuselage and then tape my wing in the approximate location where I want it to be. I then flip the plane upside down and attempt to balance the plane 1/3 of the way back from the wing leading edge (on this model about 2-1/4”). I balance by placing the battery on the bottom of the plane and sliding it forward or backward until I get the plane balanced. Wherever the battery balances the plane, this is where you cut the foam for the chamber. If you cannot balance the plane then you must move the wing until the plane balances. THIS IS CRITICAL! A tail heavy plane will be nearly impossible to fly. A nose heavy plane will be hard to glide. It is best to err toward nose heavy. Secure the battery in however you like. I prefer using a Velcro strap. These can be found at the hardware store and most electronic suppliers.
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Old Aug 31, 2007, 11:50 PM
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Now you can install the wing. There are many ways to do this including rubber bands, glue or bolts. I prefer ¼” nylon bolts. For the bolts there are three ways: drive the bolts through the wing and thread them into the fuselage, drive the bolts through the wing and fuse and secure with nuts, or secure the bolts to the fuselage, pass them through the wing and secure with nuts. For this model I’m going with the latter.

Drill two holes ½” holes in the fuselage about 1” deep. Drill two holes in a piece of balsa wood the same diameter as the bolts with the same spacing as the holes in the fuselage. Fill the holes in the fuselage about ¼” with the 4: gorilla glue/white glue mix. Place your bolts in the holes head first and place the balsa cover over the bolts. You my use super 77 or tape on the balsa to keep in place while the gorilla glue expands and dries. Next drill two holes in the wing where the bolts will come through.

If you are using the contoured wing made from the wing jig you will need to give it some down incidence (the wing should be pointed down slightly compared to the fuselage) or the plane will want to constantly loop or stall. This can be done by adding a piece of 3/16 balsa or blue fan fold foam around the rear bolt underneath the wing. I simply cut a small channel in the fuselage.
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Old Aug 31, 2007, 11:58 PM
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Flight time!

After connecting all the electronics, this plane is ready for flight. Final weight 24 oz! An ounce lighter than expected. This is unusual. usually my planes come out 25% heavier than planned.

I find it best to launch at full throttle to be sure the plane doesn't stall on take off. Bleow are some pictures of some other scratch builds I have made. I have made more, but I don't feel like taking anymore pictures

Good luck and happy building. If you need help, send me a PM

-Alex
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