|Sep 18, 2009, 11:08 PM|
How to scratchbuild a wicked fast electric airplane
So you’ve been bitten by the speed bug and want to build a bird that can rip up the skies. Oh, yes that’s fun and hard to handle, but it doesn’t have to be expensive. This tutorial will demonstrates how to build a screaming fast aiplane without breaking the bank. If there are any questions on this tutorial, feel free to Email of PM me. I will try to update this if necessary.
In this tutorial I have built three 100+ mph airplanes:
The Diablo pylon racer – Clocked at 120.14 mph!
Bird of Prey – Estimated 115 mph
Projectile – Estimated 110-115 mph
|Sep 18, 2009, 11:10 PM|
Choosing an airplane style
There are two ways most people get their planes going fast:
1. Increase motor power
2. Reduce drag
Increasing motor power can get very expensive very quick and can add a lot of weight. I’m going to focus on reducing drag. Drag can be reduced two ways:
1. Reduce the airplane’s frontal area
2. Slicken and smooth the airplanes lines
Using a combination of both gets the best results. Afterall, many 3D flat foamies have little frontal area, but their lines are anything but streamlined which makes them slower. In context many flying wings are quite slick, but they are often too thick to get over 100 mph without extreme motor power.
First you must choose a style. There are two common styles that are used for high speed: The flying wing and the pylon racer.
The Flying Wing is very often noted to be fast and highly stable. They are easy to design and build, and are often quite durable. Their naturally low profile shpe keeps the frontal area down to a minimum, thus allowing higher speeds with minimal power. These planes however can be difficult to launch.
The pylon racer is based more on the conventional airplane. They take a standard airplane and strip off as much drag as possible. These planes are highly maneuverable and are often more quiet than a flying wing. They also tend to be easier to launch.
It’s up to you to decide on a style. Both are really fast.
|Sep 18, 2009, 11:16 PM|
Making the wing
First you must build a wing. Your wing should have an area of approximately 1 square inch for every 100 mWH (milliWatt hours) of battery capacity. If using a 3 cell lipo this means 1 square inch per 10 mAh of battery capacity. For a 4 cell, it is 1 square inch for ever 7 mAh of capacity. So for a 2200 3S Lipo, the wing should be 200-240 square inches in area. This should keep your wing loading acceptable.
The wing should be a low profile airfoil. The thinner the wing, the lower the drag. Ideally the wing should be as thick as your servos are which is about ½ inch. Two common wings are made from fiberglassed foam and balsa wood. If making the flying wing, you will likely opt for the fiberglassed foam. The pylon racer can be built just as well with balsa as with glassed foam.
Fiberglassed foam wing:
In order to make the wing a “wing jig” and a hot wire bow are used. The jig is simply wooden (hardwood) wing templates secured to a flat board. You’ll need one template for the root and one for the tip. The wing cross sections can be found all over the internet by searching for "air foil". Or you can click on this Airfoil database
A few good arifoils for high speed planes are the USA 49 and the MH30. To make the cross sections edit the picture of the airfoil so that it prints out to the dimensions of your desired wing. I used MS paint and the the stretch command. Print out the template and glue it to a piece of hardwood then cut it out with a saw. Sand the airfoil template smooth. You might also add a piece of aluminum insulating tape to make a low friction surface.
Now you need your foam. I use DOW blue EPS foam. It is very dense and easy to find. The lack of large pores also allows me to use less epoxy. Pink foam is also excellent. I have also had success with white foam. EPP foam should not be used for unless using the “Ritewing Method”, which will not be covered in this tutorial.
Once you have the templates and foam, you need a bow. My bow is 28” of .032” stainless steel fishing leader (tension on this wire is kept by the spring) connected to an automotive battery charger. .025 welding wire for a MIG welder works well too. Tension must be kept on the wire for an even cut. Use the selector switch in the battery charger to adjust the heat of the bow. Some people even use a light dimmer switch to fine tune the bow heat. 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. You will want to use as little heat as possible for the best cut. Also do not force the bow through the foam or the wire will curve and the cut will not come out straight. Make two halves and glue the halves together.
You may want to put carbon fiber in the wing to stiffen it and keep it from folding. 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!
Now the glassing. I use 2 oz fiberglass. 5 oz glass cloth is overkill. Use only a high quality glazing epoxy. Do not use an adhesive or furniture epoxy. It won’t work. I use West System Epoxy with fast hardener. Slow hardener is better in elevated temperatures. Mix the epoxy and spread a thin coating over one side of the wing. Let the epoxy set for a short time allowing it to get a little tacky, then drape you fiberglass cloth over the wing and smooth out all of the wrinkles and air bubbles while working the fiberglass into the epoxy. Let this cure and then trim off the edges. Placing the wing in a warm room will accelerate the cure time. Once cured and trimmed repeat with the other side of the wing. Once fully cured, mix one more batch of epoxy and spread a thin glaze coating over the entire wing to obtain a smooth finish. You now have a rigid, fast wing.
The balsa wing takes a little more work, but might be cheaper and easier for some modelers. The wing is simply a 3/8” thick sheet of balsa wood sanded down. It is best to round the front of the wing. The round form reduces drag and makes sanding a little easier. Once cut, take a belt sander and sand the wing down. You’ll want the thickest point to be 1/3 of the way back from the leading edge. You it should somewhat resemble an airfoil when you are done. For added strength, route a channel (using a router or a chisel) and embed a piece of flat carbon stock into the wing and glue it in with CA.
This wing can be fiberglassed for improved strength. You can use the above method as on the foam wing of you can drape your cloth over the wing and then wipe it down with MinWax PolyCrylic. Let the ploy crylic dry, the do the same to the other side of the wing.
|Sep 18, 2009, 11:20 PM|
Building the fuselage
The fuselage should be as small as possible to reduce frontal area. If making a flying wing, the fuselage may be skipped altogether. There are three fairly simple ways to build a good strong fuselage: balsa sheeted foam, Balsa box, and lost foam fiberglass.
The balsa sheeted foam is probably the easiest and is very strong. 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! To make the fuselage I cut two identical cross sections from 1/8” or 3/16” balsa and secure them to a piece of white box foam or Dow blue foam sanded to shape. 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. Once done, glue a sheel of balsa to the bottom of the fuselage and trim off the excess with a knife. I typically use CA at the balsa meeting points and try to avoid letting it touch the foam.
Perhaps the simplest fuselage is the balsa box. It is simply a 3 sided box of balsa wood. Make your cross sections of the side of the fuselage and then secure them to balsa bridges at various points along the fuselage. Once done, glue a sheet to the bottom and trim off the ecess balsa with a knife.
Lost foam fiberglass:
Perhaps the most difficult the lost foam fiberglass fuselage is glassed foam just like the wing except acetone or laquer thinner is used to melt away the foam. Simply sand a fuselage out of foam then fiberglass it (I’d used 5 oz glass cloth). Once the epoxy has hardened, drop in some acetone or laquer thinner to melt away all the foam.
Mounting the wing:
I usually secure my wings to the airplane using nylon bolts and blind nuts. I’d recommend a 10-32 bolt with a washer as they will likely shear off in the event of a crash and minimize damage to the airplane.
|Sep 18, 2009, 11:22 PM|
Building the tail
If you are building a pylon racer, you need to make a tail. The elevator will be the only moving part as a movable rudder on a pylon racer will serve almost no function due to the low profile. I use 1/8” balsa wood with the leading edge sanded to a point for drag reduction. The tail should be 1/3-1/5th as wide as the wing and about 1/3 as deep. Again I make the tail rounded to reduce drag. To make a movable surface, I sand a 45 degree radius on the elevator and use high strength tape. You can use CA hinges if you like.
For the flying wing, you’ll need stabilizers. Typically I use 4mm coroplast and cut holes in the side and secure them to the airplane with tape. Balsa or foam can be used as well. Once mounted, I run two toothpicks or bamboo skewers cut 1” long through the stabilizers and into the wing and place a drop of CA on the stabilizer where the toothpick is stuck through. This keeps the stabilizer from shifting.
|Sep 18, 2009, 11:26 PM|
For fast airplanes, ailerons do not need to be large. 1/20th of the wing surface is usually sufficient. Elevons should be about 1/10th of the wing total area. For exapmle if your wing area is 200 square inches, then your ailerons could be 5X1” or the elevons would be 10X1” or 1.5X7” ect. Cutting out the ailerons or elevons on a fiberglased wing is a simple task. First cut the sides of the ailerons all the way through the wing using a saw such as a band saw or even a hacksaw. This leaves a small gap allowing the aileron to move freely. Make a “live hinge” by cutting a 1/8-3/16” channel on the bottom of the wing and remove the foam and fiberglass leaving the fiberglass on the top as a hinge.
The servos should be mounted inside the wing. I place my servos near the middle of the elevons to minimize flex and flutter. Cut away a rectangle in thewing the size of the servo in the wing and remove the foam. Glue the servo directly to the fiberglass using Goop, Shoe Goo, or heavy duty double sided tape. For control horns I use Du Bro or Great planes 1/2A control horns. My connectors are simply 2-56” fully threaded rods with snap connectors on either end. I try to limit them to 5” in length or less.
|Sep 18, 2009, 11:32 PM|
Selecting a motor
For a high speed plane you need a high speed motor. Select a motor and proepller in which the tip speed is at least 150% of the speed of the speed you wish the plane to go. Usually this equates to a motor in the 2500-3500 kv range and a 4-6” propeller.
For example my Diablo pylon racer uses a 2625kv motor and spins a 5.5”X6.25” prop on an 11.1V Lipo. The peak tip speed on this is computed as follows:
2625kV X 11.1 V = 29138 rpm
29138 rpm * 6.25” pitch = 182109”/min
182109/12 = 15175 ft/min
15175 ft/min * 60/5280 = 172 mph
Thus I expect my Diablo to achieve a top speed around 110-120 mph. (It was clocked at 120.14 mph)
Similarly, my Bird of Prey and Projectile use a 3300 kV motor and a 4.75”X 4.75” prop.
Below are a few good motors that won’t break the bank:
Go Brushless GBX triple kit motor – A 6 turn Delta with 2 srands of 24 AWG turns about 2500 kV
Dons RC Wicked 450 – An legendary $30 motor with a 4.75X4.75 APC prop.
Suppo 2217/4 – 3300 kv screamer that goes well with a 4.5X4.1 Prop
Just Go Fly 400 DF – Good for 1300 mAh Lipos and 4”-5” props
Each of my planes in the tutorial use a GBX triple motor kit I bought from Dan Sny (Micro Dan) at GoBrushless.com. The motor builds are unconventional but work very well. You may reference my motor build tutorial for information on how to build one of these. The Bird of Prey and Projectile use a 9N8P 4 turn WYE (3300kV), and the Diablo uses a 9N12P 10/3 turn parallel WYE.
Mounting the motor:
To mount the motor, I make a custom motor mount out of aluminum. I simply take a flat stock of aluminum and drill out my motor holes then bend it around the fuselage and glue it on. Another method is to glue a piece of plywood into the airplane and use the motor’s mounting plate.
|Sep 18, 2009, 11:35 PM|
Center of Gravity
A good starting point for CG on the airplane is about ¼ of MAC (Mean aerodynamic chord). There are many CG calculators out there. This one is fairly easy to use: http://www.geistware.com/rcmodeling/cg_calc.htm
The 25% does NOT work for the Bird of PRey. CG for the bird of prey is right at the wing leading edge where it meets the fuselage.
|Sep 18, 2009, 11:42 PM|
Now that the plane is built you are ready for flight. Before you fly PLEASE use a long range receiver and an external BEC (or at least an ESC with a switching BEC on board such as Dons RC Pentium ). The high speeds in conjunction with the stiff fiberglass make for high loads on the BEC and will cause most cheap ESCs to fail.
These are NOT parkflyers! Impact with a person could be lethal. If in a public park, it might be best to wait until everyone leaves before you launch one of these models. Also be sure you have plenty of runway. These planes take a long time to slow down to land.
One more note: paint the planes bright colors so you can see them. A small airplane at 100 + mph grays out really fast!
Let me know how your builds go and have fun.
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