"The Diamond" FunFly Profile for .40 glow

[dmcquinn]

I designed and built this simple fun-fly plane as my winter project. Finished today and not flown yet. It is pretty straight-forward, except for the fuselage where I wanted to try something different. I used a diamond section (foam) covered with balsa (like a sailplane wing). It is quite strong, yet light.

Specs:

Length: 46 inches (1168 mm)
Span: 41 inches (1041 mm)
Weight: 3 lb, 6 ounces (1531 grams)
Chord: 17 inches (432 mm)
WingArea: 700 sq.in (.45 sq. meters)
WingLoad: 11 oz/sq. ft (3400 gm / sq.meter)
Fuse Area: 320 Sq. in (.206 sq meters)
Servos: 4 (3 standard, 1 mini)
Channels: 4
Engine: OS .46 LA
Tank: 4 oz

[dmcquinn]

Here's the fuselage build. I started with one inch thick blue foam and used a hot wire to but it into a diamond cross section. In other words, when viewed from the front, the cross section is a very tall diamond. It is one inch thick at the center and tapers to a feather cut at the top and bottom. The overall height is 12 inches.

Then I cut a profile out that looks a little bit like an aerobatic airplane. Left it very tall to give lots of side area.

[dmcquinn]

I cut three rails out of basswood. two of these will be used for engine beams and the third is used as a landing gear mount. These rails are inserted into the foam before sheeting it with balsa. The rails are spaced for a OS .46. I used a OS 46 LA because I had one on hand and it is light. Also used a light 10x5 wood prop.

[dmcquinn]

The moveable control surfaces are built out of 1/4 inch square sticks. Note there is no vertical stabilizer - this plane utilizes a full flying rudder.

[dmcquinn]

I used 6 ounce fiberglass to reinforce the front section. Took a single piece, cut two holes for the engine beaks to poke through, then wrapped it around the front and both sides. One picture shows both the left and right wing skins made up and the fuselage foam core just prior to sheeting.

[dmcquinn]

Here's the vacuum bagging process. I used 6 pounds per sq inch of vacuum. Balsa skins are from 1/16 contest balsa (7 lb density). Before bagging, I put on the front and rear caps (also 1/16 balsa). This process is much like the sailplane builders make a wing. Also used in large scale aerobatic planes. I like the polyurethane glues (e.g. gorilla glue or Elmers Ultimate) for connecting the balsa sheeting the foam core.

[dmcquinn]

The fuselage tapers to nothing at the highest and lowest. At these points the left and right sheeting are glued together, making for a one-eight inch thick section. But where the fuselage is narrower, there is some exposed foam -- mostly on the top in front of the canopy and behind the canopy. For this I capped the foam with more 1/16 balsa.

[dmcquinn]

The wing is pretty typical built-up "D-tube" construction. All 1/16 balsa. 1/4 inch square spars and 1/16 shear webs. Note: sheet the top center section, but not the bottom. All the R/C gear is going into the center section of the wing. I did use 1/8 balsa for the last ribs (wing tips). I drilled out lightening holes in every rib but the tip ribs. The airfoil is thick - this typically gives good slow-speed performance and a smooth transition from flying to hovering. To make the ribs, cut them all our roughly, pin together, then belt sand to final shape.

[dmcquinn]

Cover the parts before you hinge them and before you glue the wing to the fuselage. I use Ultracote. You could save a couple ounces if you used a very light covering. This plane will be completed as one-piece - nothing removable. But it is small enough to fit in most cars.

Note the huckfest sticker from this year's Huckfest in my home town (St. Louis).

[dmcquinn]

All the R/C gear goes inside the wing. So after you cover the wing (leave the center section uncovered), you put the R/C gear inside. Then finally you glue the wing inside the fuselage, add the ailerons, and cover up the center section. I just cover it up "permanently" with iron-on covering. You could make a hatch. If I ever need access at the field. I just cut open the covering and tape it temporarily. I can recover it at home.

Note: I am using a pull-pull for the rudder, one aileron servo, and a push-pull rod to the elevator. I use 9 lb Kevlar fishing line for the rudder pull-pull. You need to put a one-inch wide balsa "cap-strip" down the center rib to give a place to attach covering after the wing is installed.

These pictures show the underside of the wing. The switch is installed so that is on the top of the wing. Servos are (from left to right): Throttle (mini), rudder, aileron, elevator.

The first picture is before the wing is glued. You want to do as much as possible before these two pieces are glued together. I have hinged one aileron (the left) and installed the switch. The right wing will be inserted through the fuselage, so you cannot install the right aileron until after it is glued.

I glue the wing and horizontal stab to the fuselage using polyurethane glue.

[dmcquinn]

Here's a summary of the weight of the various pieces. You need to watch the weight of everything if you want to keep it under 3.5 pounds. For example, I used a 370 mAh 5-cell battery. I'll have to recharge at the field if I fly it a lot. Also I use a 4 ounce tank. I know from experience this will give me plenty of time (10 minutes) because this is not the type of plane that you fly at full throttle.


Fuselage
Blue Foam before cutting cross section 3.9
After cutting diamond section 2.1
W/ hard points installed 2.9
w/ sheeting 4.7
w/covering 4 11.6
LG Alum with 2 wheels 5.8
Electronics
Servo (A, E, R) 3 1.5 4.5
Servo (Throttle) 0.3
RX 0.8
Switch 0.5
Battery 1.3
Linkages 1

Tail Group
Horz Stab, uncovered 0.5
add covering 0.3
0.8
Elevator, uncovered 2 0.25
add covering 0.15 0.8
Rudder, uncovered 0.3
add covering 0.2 0.5

Engine and accessories
OS .46 LA 9.7
Muffler 3.2
12.9
Tank w/ tubing 1.4
Prop (wood) 0.5
Spinner, aluminum 0.7
Wing
Ailerons, uncovered 2 0.3
add covering 0.2 1
Wing, uncovered 7
add covering 3 10

lb oz
Total 54.4 3.4 3 6.4

[Boulder]

What are you doing sittin' at the computer???


Get out there and fly that thing!!!!!!