CHAPTER 4
The Airframe of the Flack

If the deck is the soul of the Flack, then the airframe is the first of many bodies. While the airframes are remarkably tough, they eventually get soft after around 30 or so hard crashes. When that happens, a new airframe can be created and the deck transferred in the Brooklyn Aerodrome’s version of reincarnation. We have decks that have been reincarnated at least 10 times. Whether there is a metaphysical lesson to be learned here is up to the reader.

This chapter covers how to create the airframe and attach the deck to it. The airframe consists of the wing, control surfaces, and stabilizers. Just a few steps require precision: (1) cutting the elevon hinge and attaching it, (2) getting correct trim on the control surfaces, and (3) getting the center of gravity (CG) right. This is also a good time to spend some time on a flight simulator. Enough said. Let’s get building! Behold the object of your efforts—the Flack—in Figure 4-1.

FIGURE 4-1 The Flack (“flying” + “hack”).

The Airframe

Be sure to check brooklynaerodrome.com for advancements on airframe design. However, any advancements will be close to what we do here. The construction material of choice is Dow fan-fold, but all materials will employ the same basic steps.

If the foam was shipped, it may have been cut to keep shipping costs reasonable. For example, Brooklyn Aerodrome kits need to have tape applied as shown to re-create the airframe pictured in Figure 4-2. If not, then skip to tracing the plans onto the foam/cardboard/wing material.

FIGURE 4-2 Airframe reconstituted from shipping triangles.

The steps to create a raw airframe from the kit include

1. Puzzle together the pieces of foam as shown in Figure 4-2.

2. Tape the pieces together using a tension-maintaining technique. This works as follows:

a. Align the foam edges.

b. Get a 2-inch length or so of packing tape.

c. Hold up one side of the joint, and apply the tape to the angle at one end of the joint.

d. Lay the foam flat, and the tape joint should be very tight.

e. Do the same for the other end of the joint, as shown in Figure 4-3.

f. Lay the foam flat, and apply tape along the length of the seam.

g. Flip the foam, and tape the length of the other side.

FIGURE 4-3 Taping foam triangles to create the wing. Hold the foam at an angle when applying the tape to ensure a tight seam.

This technique for rejoining foam has been used for years at the Brooklyn Aerodrome. It is how we get as many airplanes out of a block of foam as possible.

Now that there is a big enough piece of foam to trace the plans, let’s cut a wing.

1. Trace the plans from Figure 4-4 onto the foam. If the foam is a full 24- × 48-inch sheet, then place the triangle in the middle to allow creation of another airframe from the two remaining triangles.

2. Cut the outside shape from the foam using a sharp blade. Do not cut the elevons out yet. A dull blade will tear at the bottom side of the cut. You can use a ruler to keep the cuts straight, but the edges turn out just as well with long, smooth hand cuts.

3. Draw in the elevon hinge line on the surface you want to have on the top 2 inches back from the rear. It is recommend that you write “Top” on the wing.

4. Cut all the way through the foam at a consistent angle (around 30 degrees) along the entire length of the elevon, as shown in Figures 4-5 and 4-6. This cut needs to be straight and is easier to do with a ruler. It is worth practicing making this cut and hinge on some scrap to understand what is going on.

5. Flip the wing over so that the acute angles align on top, as shown in Figure 4-7.

6. Use 2-inch sections of tape to hinge the touching angles at the end, middle, and other end of the elevon, making sure that there is no space between the angles. Then tape the entire length of the hinge—several 1-foot sections are easier to work with than doing it all in one 42-inch length of tape. It is good to have a helper. Do watch the build video on the website to see how this is done. Tape only one side.

7. Flip the wing back to the top, and verify that the hinge allows up and down movement.

FIGURE 4-4 The Flack 1.3.

FIGURE 4-5 Keep straight and consistent.

FIGURE 4-6 Be sure to cut all the way through.

FIGURE 4-7 Taped hinge that allows both up and down movement.

Reinforce Nose

If you are a beginner, then reinforcing the nose is a good idea. Use packing tape to wrap the nose back to the rear of the prop hole on both sides, as shown in Figure 4-8. Cut out the prop hole.

FIGURE 4-8 Nose reinforcement for beginners.

Control Rods

The control rods are made from wire coat hangers. If you have only one hanger or you have the paper-tube kind, the hanger will have to be unwound and straightened. It is very difficult to get them perfectly straight, but do your best. Once there are two 14-inch sections of coat hanger, proceed as follows:

1. Grip the end of the hanger in needle-nose pliers, and wrap it around the tip in a U, as shown in Figure 4-9. This can take more hand strength than children have, so they may need assistance.

2. The U should not be too wide (greater than ¼ inch) or it may interfere with the top of the servo. Use the pliers to narrow the U if needed.

3. Measure 11 inches from the back of the U, and mark on both control rods at the same time.

4. Place the pliers at the mark, and bend the coat hanger 90 degrees to make an L. Keep the bend in the same plane as the U bend. Note that the bend goes in the opposite direction from the U bend, as shown in Figure 4-9. Repeat for the other control rod (Figure 4-10).

5. Rarely does the L bend actually end up in the same plane as the U bend. Place the U bend flat on a level surface to test that the L bend also lies flat. In Figure 4-11, the L and U bends are not aligned. To fix, hold the U bend in the pliers, and twist the L bend so that the bends lie in the same plane (Figure 4-12).

6. Verify that the control rods are within inch of each other. If one is significantly shorter, then use the technique for establishing reflex trim discussed below to shorten the longer one or just try again.

7. Trim the L bend to 2 inches.

FIGURE 4-9 Control rods. Bend a piece of coat hanger around the end of needle-nose pliers.

FIGURE 4-10 A pair of matched control arms.

FIGURE 4-11 L bend out of the plane of the U bend.

FIGURE 4-12 L bend in the same plane as the U bend.

Attaching the Control Rods to the Elevons

I use Coroplast material for attaching the control rods to the elevons. The elevons are simple but do require some precision in placement. The goal is to match the left and right control-horn placements. This will ensure that up and down commands will not introduce roll because the elevons move different amounts for the same inputs. The end result of these steps for each control horn is shown in Figures 4-13 and 4-14. Note that the elevon horns are very close to being the same distance from the tape hinge line—the control-rod hinge point is what matters, not the Coroplast.

FIGURE 4-13 Left elevon hinge placement.

FIGURE 4-14 Right elevon hinge placement. Note how the coat hanger hinge point is the same distance from the V at the bottom of the bevel as on the left side.

Steps for the build include

1. Place the tape hinge side of the airframe down; the bevel is up.

2. Make a 2- × 2-inch piece of Coroplast scrap material, and cut it in half across the grain of the flutes.

3. Carefully use the end of the L bend to make a hole through the flutes of the Coroplast about inch from the edge. Try to keep the coat hanger parallel with the edge of the Coroplast. Wiggling helps, but be careful to not stab your hand when the wire comes out the other side (see Figure 4-15).

4. Placing the deck over the prop hole, follow straight back from the servo output shafts to the elevon. Mark this location—it is where the control horn needs to be left/right on the elevon.

5. Trial fit the control horn with the control rod parallel with the hinge line and about inch back from the bevel. The important consideration here is that the hinge line be parallel with the hinge point of the control rod, as shown in Figures 4-13 and 4-14.

6. Once you know the location of the control horn, apply double-stick tape to the elevon side of the Coroplast, and attach to the elevon.

7. Repeat the preceding steps with the other elevon/control rod, paying particular attention to mirroring the distance of the elevon hinge to control-rod hinge of the first elevon.

8. Secure both Coroplast control horns with zip ties to ensure that they do not come loose. If one control horn comes loose, it is a guaranteed crash.

FIGURE 4-15 Creating the hinge by forcing the coat hanger through the flutes.

Attaching the Deck

We are almost done. Next, we do the fundamental alignment step that sets up the Flack.

Drill and Attach the Servo Arms

The servo arms need to be drilled out to accommodate the diameter of the control rods. You need two drilled arms. Making an extra one is not a bad idea for a crash kit. A crash kit is a collection of spare parts and tools for taking to the flying field—see Chapter 6 for more information. Steps include

1. Select a drill bit that is the same or slightly smaller than the size of the coat hanger wire. In a tight spot, you can use a bit of coat hanger to drill the hole.

2. If you are a beginner, drill about ¼ inch out from the spline hole. This is usually the second hole out. If you know how to fly, you can drill farther out on the arm to get increased control throws or drill both and decide on the field.

3. The servo horns in Figure 4-16 are drilled for both beginner (closer to the servo-shaft hole) and advanced control throws (farthest from the servo-shaft hole). The servo-horn retaining screw is on the right. The two servo retaining screws on the left are not used, but keep them around because in a pinch they can be used to retain the servo horns when there has been slight violence to the servo output shaft. Double-sided horns can be used as well by cutting the extra arm off as shown.

4. If the control arms are double-sided or round, just trim them so as not to interfere with the deck.

5. Insert the control rods into the holes. Make sure that the arm can move freely—the U can be too narrow and bind, preventing free movement. Adjust as necessary.

6. Make sure that there is no slop in the connection between the servo horn and the U bend. During a test build, one builder ran the correctly sized drill bit backwards, which made the hole too big, resulting in a loose fit.

7. Attach the servo arm to the splined shaft of the servo. Do not put the servo-horn retaining screw in yet.

8. Adjust the servos by moving the arms gently with your hands. Be sure that the servo arms are at 90 degrees to the deck, as shown in Figure 4-17. Be gentle in moving the servos. Do not do this with servos powered.

FIGURE 4-16 Drilled and trimmed servo horns—only two needed.

FIGURE 4-17 Servo arm at 90 degrees to deck surface.

Aligning and Attaching the Deck

This is the moment when the whole airplane comes together. Check everything twice, make sure that the servos have not moved from 90 degrees, and be patient. The steps include

1. Put some weight on the back of the airframe between the two control horns and the rear of the wing to keep the elevons flat to the surface.

2. Adjust the deck to be centered left-right on prop hole. The important thing is that the servos be at 90 degrees and the rear of the wing and elevons be flat. If the deck prop hole and wing prop hole do not match exactly, that is okay. If the prop is a little skewed in the prop hole, that is also okay. This is not a crucial dimension, believe it or not.

3. Flip the deck up, and apply double-stick tape to the underside (Figure 4-18).

4. Flip the deck back, and place on pens or pencils so that the tape does not make contact with the airframe. The pens or pencils allow for a final alignment check.

5. There are two things to check—this is important. The elevons and rear of the wing should be flat on the surface, and both servos should be at 90 degrees to the deck. Check again. If the prop hole of the deck doesn’t quite match the prop hole on the wing, don’t worry about it. You can cut out more foam to give extra clearance.

6. Remove the pens or pencils, and press the deck into place on the wing gently, starting from front to back. Starting at the back can result in the elevons being raised. Check the elevon trim—if either is more than ¼ inch up from the tabletop, then gently pry up the deck and redo the placement. If the elevons are a little down, this is not a problem because there are ways to make the control horns shorter. Once satisfied, press down firmly.

7. Insert zip ties at the front corners by the battery pack and at the rear edge of the deck. You also can add zip ties around the servos if you think the tape might not hold the deck well enough.

FIGURE 4-18 Deck with double-stick tape attached. Servos are still attached to the control rods.

Centering Controls

The next step is to center the servos, attach them, and verify control movements. Steps include

1. Establish elevon mixing with the transmitter. Consult your user manual.

2. Cut ¼ inch between the elevons in the center so that they move freely.

3. Move the aileron and elevator trim tabs to the neutral position on the right-hand stick, as shown in Figure 4-19. Neutral is in the middle of the tab’s range. If you have digital trims (rocker switches that control trim), consult your radio’s manual for how to know that you have neutral trim.

4. Remove the servo output horns from the servo shafts, but keep them on the control rods.

5. Remove the prop.

6. Power up the transmitter with the throttle down.

7. Power up the airframe.

8. Attach the output horns to the closest spline to being vertical. If one arm is slightly forward, then let the other arm be vertical or slightly forward as well. Try to match the positions of the servo arms. Do not force the arms when the servos are powered—they will resist, and you may damage the gears.

9. Power down the airframe.

10. Apply the servo-horn retaining screws with care. You can strip the servo gears if too much force is used. Snug is good.

FIGURE 4-19 Trim tabs at center on right stick.

Establish Elevon Trim

1. Power up the transmitter and Flack.

2. Verify that the controls work as expected. Figures 4-21 through 4-24 show control stick and corresponding elevon deflections. Check this twice. You may have to revisit your transmitter manual to reverse channels or mixing functions.

3. Use only the trim tabs (not the control stick) to create the ¼ Hnch up reflex with both elevons, as shown in Figure 4-20. The foam is ¼ inch thick, so use that as a measure if you have it. You may need to make adjustments to aileron and elevon trim tabs.

4. If the trim tabs have insufficient throw to establish correct trim, then the control rods will need to be physically adjusted. See the section on physically adjusting trim below. It is a good idea to have the correct flight trim done physically anyway with the transmitter trim tabs in a neutral position.

5. Power down the Flack and transmitter.

FIGURE 4-20 The ¼-inch up reflex.

FIGURE 4-21 Neutral elevator, neutral aileron.

FIGURE 4-22 Down elevator, neutral aileron.

FIGURE 4-23 Up elevator, neutral aileron.

FIGURE 4-24 Neutral elevator, right aileron. Left aileron is reverse.

Physically Adjusting Trim

The Flack is built in a fairly loose fashion, so it is not unexpected that the correct trim of the plane is beyond the range of the trim tabs on the transmitter. In general, it is better not to use trim tabs at all to get flight trim for a few reasons:

1. Trim is meant to be adjustable for different flight modes. Flying fast will use less up trim; slow will need more; etc.

2. Trim is useful when cargo is being carried. If you need a bit more up trim because of the camera mounted on the nose, then it is annoying not to have any extra up trim left from just getting the plane flying normally.

3. If you fly more than one model with the transmitter, then you will be able to swap models without worrying about what the relevant trim is supposed to be with the cheaper radios. More sophisticated radios store trims with individual models in memory.

The process of physically adjusting trim is as follows:

1. Be sure that the transmitter mixing is correct; otherwise, your physical corrections may not be working from correct centers. On cheaper radios, trim centers may change between mixing settings.

2. Center trim tabs before making adjustments.

3. To make a control rod shorter, consult Figures 4-25 through 4-27. Practice on some spare coat hanger material before doing it to the actual control rods.

4. There is no easy way to make control rods longer, but you can remove the retaining screw and slip the servo control horns one indent of the spline shaft back to get down trim. Both servo arms should be matched, however, if this is done. Alternatively, just make a new control rod that is a bit longer.

FIGURE 4-25 Twist clockwise while keeping the coat hanger straight. Practice on an extra coat hanger.

FIGURE 4-26 Grip with pliers and fingers. This takes hand strength.

FIGURE 4-27 Control rod is now shorter.

Cutting Stabilizers

Draw out two stabilizers from the diagram in Figure 4-28. Both tabs at the bottom need a ¼-inch cutout, as shown on the right. The tabs will need reinforcement with packing tape on both sides. It is easiest to apply the tape first and cut after. The ¼-inch notch should be the width of the airframe material, as shown below. If you have fiber tape, it is a good idea to reinforce the stabilizer tabs.

FIGURE 4-28 Stabilizer template.

Cutting Stabilizer Slots

Next, we cut the slots in which to insert the stabilizers. Steps include

1. On the top side of the plane, put the stabilizer next to the deck and with its rear edge at the bevel of the hinge. The stabilizer should be next to the deck. Refer back to Figure 4-1 for their placement.

2. Move the stab ½ inch forward.

3. Mark the stabilizer tabs with a pen. The idea is to have the stabilizers be nearly flush with the hinge line once they are inserted and pulled back.

4. Cut out the slots, being careful not to make the slots too wide—there is no problem if they are a bit too long.

5. Don’t make the stabilizer slots so tight that it is hard to remove the stabilizers. When transporting the plane, you should remove the stabilizers to keep them from getting damaged.

Test insert a stabilizer without forcing the foam to deform. If the slot needs to be a little bigger, then make the adjustment. When finished, the rear of the stabilizer on the top side should be near or at the hinge line.

Final Cleanup

The airplane is done.

1. Remove extraneous zip tie ends. Take care not to cut wires by mistake.

2. Attach the prop in the correct direction (raised writing toward the front).

3. Advance the throttle a third, and verify that the motor spins in the right direction (clockwise looking from back to front unless you have a special prop).

4. Grasp the plane firmly (be careful here), and apply full throttle for a few seconds. There should be a lot of thrust—papers blown off the desk—about a pound of force at full throttle.

5. Write your name, phone number, Academy of Model Aeronautics (AMA) number, e-mail address, and any reward you are willing to pay for return on the airframe.

6. Draw a line 10½ inches from the nose the width of the prop hole. This is your center of gravity (CG).

7. Verify the CG. The CG is the point at which the airplane is balanced. The most important dimension is fore/aft, which can be tested by finding the balance point of the airplane with two fingers as shown in Figure 4-29—see Chapter 10 for more on what is going on. Adjust the CG by moving the battery fore or aft, and if this is insufficient, add nose or tail weight to achieve the CG. If you are building with heavier materials, expect to need to add nose weight; if you are building with lighter materials, then expect to add tail weight.

8. Verify that you have appropriate flight trim, which is both elevons up ¼ inch.

FIGURE 4-29 Verifying CG by balancing the aircraft on your fingers. Do this indoors.

Conclusion

Now you are ready to fly. Chapter 5 takes you through Brooklyn Aerodrome’s fun and proven approach to flight instruction. You are going to crash a lot—and have a great time doing it.