I'm often asked about techniques I use to build these profile pusher jets, so I thought I'd start a thread that describes many of the techniques I use and tips I've discovered in the past few years in order to have all the information in a single place.
Keith Sparks has a wonderful book called "Building With Foam" that has many tricks and tips, and it is a highly recommended read. As well as all of the build threads that Steve Shumate has done for his wonderful parkjets. But these builds are typically for built-up foam planes, not the profile jets (although most of the techniques can be applied to the profiles as well).
First I'll talk about a list of materials that I typically use for these builds. If you are downloading the plans and building from scratch, then you will need to buy foam. Most of these are done with 6mm depron, and a good source is www.rcfoam.com. However, most hobby shops now carry sheets of depron as these planes have become more popular over the last few years.
There are other sources of foam that can be used, such as paper backed foam board that can be found at most craft stores (but you'll have to soak the board and peel the paper off), or fan-fold foam (or "FFF") which is normally found at home improvement stores. But personally, I like depron the best as it needs no prep.
Most builds typically have a carbon fiber tube for the wing spar. I usually buy Midwest Products Item #5821 as that is what my LHS sells, and it has an outside diameter of 4mm, giving me 2mm of working room for epoxy when using 6mm foam. There are also times when using CF strips or smaller diameter tubes may apply. The LHS may have a supply of various sizes of both tubes and strips.
The final material needed is lite ply for the motor mount. I use 1/8" lite ply (and I also have a source for laser cut ply mounts which are also 1/8"). If you buy a kit from 6mmFlyRC, Grayson Hobby or the like, you will most likely get the ply mount included (as well as a CF spar and control horns, etc).
For cutting the foam, I use a standard Xacto knife with a No 11 blade. I buy these in bulk as the blades go dull after a short period of time. And for anyone whose worked with covering, you know that when the blade goes dull it tends to tear the covering. The same goes for depron. Once the blade dulls it will tear the foam on the bottom side. So change that blade often. I typically use 2 blades per plane.
There are several types of glue that can be used. My favorite is UHU Creativ for Styrofoam (also called "UHU Por" in Europe). This stuff is very strong and holds up well over time. The only problem is that it's getting very hard to find in the US. BP Hobbies lists it on their site, but as of this writing they are out of stock. A Google search will reveal other possible sources. EDIT - It now seems that UHU Creativ is no longer available anywhere in the US (at least I can't find any).
My second favorite glue is Yardbird's Ultimate RC Foam Glue. I've received bottles of this the past two years at the EFExpo in Arizona, and it is a great glue for styrofoam. And if I'm out of both of these, I'll use the glue that GWS includes with their kits (I always have several tubes lying around). EDIT - I tried to order some more glue from Yardbird, but they are out of stock. And I come to find out that this stuff is exactly the same as Beacon 3-in-1 Adhesive. This stuff is available at most craft stores like Michael's and Jo-Ann's. It's now my number one glue for foam.
But the trick to ALL of these glues is that they must be used properly in order to work. These are all "contact cements", which means that you apply glue to the surfaces to be joined, press the parts together briefly and separate, allow the glue to get "tacky", then rejoin the parts. All of these cure within 24 hours, so I like to glue a plane up in a single sitting, then allow it to cure overnight before removing any tape or continuing with construction.
The first step to almost every build (once all the foam parts are cut out) is to install the wing spar. You will need:
Flat, level working surface
Straight edge (I use a 36" metal ruler)
Tape (low tack 3M painters tape is best)
Glue (I use 30-minute epoxy)
Scraper (any straight edge scraper will work, I use old credit cards and such)
First, determine the length of your CF spar and cut as needed. Then, using the straight edge, cut your spar slot in the foam. Be sure to cut the slot a few milimeters longer and wider than the CF spar so that the epoxy can work around the edges.
Next, take a length of tape and entirely cover the slot, then flip the wing over. Now use some tape to "frame out" the slot opening. This is to prevent excess epoxy from sticking to the foam and creates a nice clean install. Before mixing your epoxy, be sure you have a nice flat and level surface to work on, as well as something to weigh down the foam. I use a couple thick heavy books to make sure the foam stays flat while the epoxy sets up.
Now mix your epoxy and run a thick bead into the slot, making sure it fills the entire opening and is about 2mm deep (DO NOT fill the entire opening, just enough to fill the bottom). Insert the CF tube and press down firmly so that the epoxy runs all around the tube. This should eliminate any air bubbles on the bottom. Finally, fill in the top of the spar with epoxy so that the entire opening is completely filled with the spar and epoxy.
Use the scraper to "smooth out" the top layer of epoxy so that it is flush with the top of the opening, being careful not to get overflow on the foam. This is where the tape framework helps. Place your weights on the foam so that it remains flat during the set up process.
After the epoxy has set up (but BEFORE it fully cures), remove the tape from both sides of the foam. You will now have a smooth and solid install of your spar.
Most of the parts on these planes are tabbed or slotted for ease of contruction and alignment. However, some pieces such as intakes, dividers and vertical stabilizers may require installation at an angle (for example, the vertical tabs on an F-22). Since you want to have as much surface contact as possible between the two pieces of foam, it's a good idea to sand the necessary angle in the contact surface as needed.
Before I begin gluing parts together, I always do a dry fit of the parts to ensure I cut the tabs and notches correctly. Now is the time to make any adjustments needed. You don't want to put glue on a piece only to find out it doesn't fit properly.
These jets typically go together in the following manner:
- Glue in wing spar
- Glue on upper forward fuse
- Glue on lower forward fuse
- Glue on intake sides
- Glue on belly pan
- Glue on vertical stab(s)
- Install control surfaces
- Install electronics
Once you are satisfied with fit, apply glue to one of the parts to to be joined, then press the two parts together and immediately separate again. Rejoin the parts after the glue becomes tacky. But also read the directions for the type of glue used to make sure you are using it properly.
And of course, use tape where needed to ensure parts stay together until the glue cures. Scotch tape does not stick well to depron over long periods of time. Packing tape works better for this.
OK, mounting the motor has probably been one of the most challenging items that I've encountered on these builds, but I think I have it down to a science now. If you are cutting your own plane from scratch, DO NOT cut out any of the slots for the motor mount (just cut out the prop canyon). Instead, leave that foam in place until you get all parts cut out and can do some measurements.
My motor mounts consist of:
- 1/8" lite ply motor mount with 6mm notches cut in at each polar location
- 6mm depron disc cut to match the ply mount
Step one is to measure the dimensions of the notches in your mount so you will know how wide to cut the slots in your foam. Next you will need to take your motor and attach the cross mount and prop adapter/prop to it. Then take the depron disc, ply mount and assembled motor and determine the cut distance needed so that the prop hub winds up in the middle of the prop canyon when looking from front to back. Once you have all your measurements, cut the necessary foam from the parts.
After the plane is fully assembled, check to see that the area where the depron disc will glue to is completely straight and level. These jets do not require you to add any thrust angles on the motor due to the fact they are in a pusher configuration aft of the CG. Next, glue the ply mount to the depron disc with epoxy or hot glue. After the glue cures, place the motor cross mount on the ply and mark the screw holes, then tap the holes. Then mount the motor.
Normally you can glue the mount in with the motor already attached, but with the prop removed. If you can do this, then that is the preferred method. If not, you will need to mount the motor after gluing in the mount.
The biggest problem with the motor mounts is that they tend to break loose after a while due to prop strikes on the ground. It helps to inspect the mount on a regular basis. To help prevent breakage, I actually use high temp hot glue to secure the foam mount the the airframe. Hot glue stays slightly flexible, and is not as susceptible to vibration as epoxy is.
Most people hinge their control surfaces using packing tape. I used this method for quite a while, but finally got tired of the tape peeling up on me as I live in a very hot summertime climate (it can get to over 120° in my garage in the summer). But to use this method, simply cut a 45° angle on the leading edge of the control surface, then attach the control surface to the plane with a piece of packing tape. Flip the plane over, then fold the control surface back until the 45° angle is lined up even with the trailing edge of the body and add another piece of tape. This will cut down on slop in the control surface.
However, I now like to hinge my control surfaces in the traditional manner. I've found they hold up better, and look better to boot. I've used such things as left over GWS paper hinges, cut strips of those fake credit cards you get in the mail (the thinner the better), or thin plastic packaging material. Anything that's thin and flexible will work. First, cut the hinge slots in your control surface. Then, sand or cut beveled edges in the leading edge of the control surface and hinge them like you would normally do on any other model. The 6mm foam is thick enough to cut in hinge slots with ease.
For control horns, I've used everything from old GWS horns, to laser cut ply horns, to homemade horns from 1/32" lite ply, to plastic scrap. Any of these work fine. I will cut a slit in the foam for the horn, then cover the back side with scotch tape. I'll then use 5-minute epoxy and fill the slot and insert the horn. Once the epoxy has cured I remove the tape from the back side.
Most of these plans call for the control surfaces to be taileron (aileron/elevator) only. But I've found that cutting in ailerons separately gives you much better control response throughout the flight envelope, especially at slower speeds. Here is my method for hooking the ailerons and tailerons up on the same servo so that you still only need two servos total instead of four.
On the servo arm, use something like a DuBro E/Z Connector on the outermost hole of the arm. Use the same thing on the taileron control horn. Then run .047 gauge wire from each E/Z Connector to the control horn on the aileron. Use a z bend on the wire at the aileron horn. You can then adjust the wires on the ends so that both surfaces are trimmed level.
Because of the limited size of the prop canyon, the biggest prop you can use is 7", and in some cases 6" is as big as you can go. Unfortuntely, that limits the choice of motors you can use. However, you do have options.
My absolute favorite setup is a Welgard/Suppo 2212-6 motor, a 30amp ESC and an APC 6X4E prop. This setup will give you unlimited vertical on a typical profile jet, and when used with a 1300-1500 lipo will give a flight of about 5 minutes of very spirited flying. Here are some places to get these motors:
Grayson Hobby - This is the combo deal with the 30A ESC included. These V2 motors seem to hold up better than most other standard motors listed below.
Light Flight RC (search for the Suppo 2212-06) - You can get this with a 30A ESC combo for about $35.
BP Hobbies - You can get this with a 30A ESC for $36.
RC Hot Deals - Scroll down the page to the A2212/6 and the combo is on $30.
I have also found out that there is an eBay store out of China that is selling the 2212-6/30A ESC combo package for as low as $22 with free shipping! Go to eBay and search the term "2212-6" and look for entries by "RC Hobbies" or "New RC Hobbies". There are a lot of Buy It Now auctions as low as the $22 price. I'm sure this is direct from the Suppo factory that makes these motors.
Another motor of choice is the Little Screamers Park Jet motor that can be purchased at Hobby-Lobby or Model Aero.
If your model ends up heavier that the typical 15 to 20oz, you can step up to either the Grayson Super Mega Jet motor, the Little Screamers Super Park Jet motor, or the BP Hobbies Ultimate outrunner. These should give you about 32oz of thrust.
As far as servos are concerned, any regular old 9g servo will do. I personally prefer GWS Naro Standards. I've beat the tar out of these and they keep on trucking. I've also used the cheap TowerPro or HXT900 servos that you can get from Hobby City or RC Hot Deals, but they have a higher failure rate than GWS.
There are some people who seem to have trouble finding the right paints for foam. In the years I've been doing this, I've used Testors rattle can spray paints more than anything else. I've used it on everything from beer cooler foam (EPS, like on GWS planes) to depron, to Elapor (EPP or EPO, like on Multiplex planes). And it works on blue and pink foams as well. Some people also use Krylon H2O, but I've never seen a need to buy any myself (as I do mostly military scale models and need the colors Testors sells).
The trick with using spray paint is to spray at least 12" away from the surface, and to use light coats. It's not the paint itself that eats the foam per se, it's the propellant used in some spray paints that is the problem. By staying 12" away, most of the propellant will evaporate before it reaches the foam.
Another good paint is the water-based acrylic craft paints that you can find at places like Michael's (Apple Barrell brand), Jo-Ann's (DecoArt Americana brand), or the craft section at Wal-Mart. Any brand will work well, as long as it's water-based acrylic paint. And even though they don't carry true military colors, you can find just about any shade you require. These can be applied by hand with a brush, or thinned and used in an airbrush.
Most of my planes get water-slide decals applied to them for the scale markings. I use 8.5"x11" sheets of decal paper that I purchased from www.beldecal.com. You can get sheets made for both Laserjet and Inkjet printers, and clear paper as well as white paper (white paper is good for decals that have white in them, such as USAF stars and bars, as printers don't print "white" ). If you have access to a color laserjet, this is the preferred method for printing decals. Because with inkjet, you have to also apply a protective coating after printing to keep the ink from running. Laserjet printers are just much easier to work with.
Most of my decal sheets are made up of Word documents that I create by taking images from the web and then sizing them as needed in Paint and pasting on the Word doc. And since I do a lot of USAF builds, I downloaded a font called "AmarilloUSAF" from the internet. This font will allow you to type words and numbers that match the USAF block type font.