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03-14-2013


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ARF for F2B
arf gerry-93

RC 3D ARF for F2B ?

Those commercially available „almost ready to fly“ ( ARF) models for RC-3D flying show very similar construction features as our F2B airplanes. Thick symmetrical airfoils, large flaps, and long moment arms allow for high manoeuvrability at stable flight characteristic. Light construction together with powerful engines support execution of demanding manoeuvres in a very small space. Since in America stunt flyer Richard Oliver has modified such a model quite successfully for control line aerobatics some time ago, I have repeated his experiment under similar conditions. These are my experiences:

Original airplane
A large number of RC 3D kits are commercially available. Some of them are copies of the Giles aerobatic airplanes. With a fuselage length between 1,5 and 2 meters most of them are too big for our needs. Therefore just like Richard Oliver, I’ve chosen a smaller version, namely the 3D Giles from the Chinese manufacturer “China Model Productions” (www.cmairplane.com). This is a foil covered ARF model with a span of 124 cm ( 49”). Despite the short span, the  wing area is still the same as on the Cardinal ( 42 square decimeter = 65 sqin ). Except for the control line components the kit contains all necessary parts in high quality. Building instructions are illustrated very well ( even in German language for those needing this ).grafik 

Rebuilding
Since the Giles 202 parts are offered fully covered, rebuilding for F2B is not an easy task. Mainly because it wasn’t desired to remove respectively open the covering. While this is generally possible, it requires quite some effort. Rebuilding the ARF kit into an F2B airplane is a demanding work and should be accomplished by an experienced stunt flyer/ builder only. It took me about 50 hours. I’ve used commercially available F2 components  ( Brodak ) exclusively.

Important: foil is removed from all glue areas. These areas are carefully cleaned and sealed with 2 component lacquer after assembly.

Control system
Because of the very large flaps and the long moment arm of the tailplane it is reasonable to adapt the ratio of flap to elevator deflection accordingly.

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With a control system as detailed in the sketch the control ratio flap/ elevator is 0,8:1. At the same time, with the additional hole in the bellcrank at shorter distance, very large deflections of the bellcrank are required for sufficient elevator deflection. Together with appropriate Center of Gravity position this combination makes for smooth flying characteristics. Components used are:

Bellcrank  Brodak  BH-392. Additional bore at 16 mm radius for flap control
Flaphorn  Brodak   BH-749. Connect flap pushrod on flaphorn at position 32 mm. Connect elevator pushrod  at position  25,4 mm.
Elevator horn Brodak BH-746. Connect pushrod at position 25,4 mm

Tailplane
Those hinges supplied with the kit are replaced by solid barrel hinges. Since the fin will be installed in a fixed position later, the cutoff of the elevators is not required, so it’s possible to enlarge their area. This is easily done by adding two triangular parts to the inside edges of the elevators. They are covered with foil. The control horn ( Brodak BH-746 ) is bent asymmetrically, so the horn will be placed outside of the fuselage.

Wing
On the RC version both wing halves are connected and glued together with the aluminium tube. The supports for these tubes are completely removed. One of the glass fibre tubes will later serve as tip weight box in the right wing tip. The servo trays are removed, too. On the inboard wing half we need openings for the leadouts. To make these we prepare special made sanding tools ( sandpaper glued around rods ).

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Replacing the support for the aluminium tube we install a ply bridge including the bellcrank. This bridge must connect the main spars.

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An additional hole for the flap pushrod is drilled in the Brodak bellcrank  ( BH-392 ) at 16 mm from the pivot point. After installation of the bellcrank pivot point is positioned at a distance of 165 mm forward from the trailing edge of the wing ( = hinge line ).

An adjustable line guide is mounted to the innermost rib. The center between both guides in its most forward location is at 182 mm measured from the hinge line. After the flap pushrod is mounted the wing halves are glued together.

The finished wing fits exactly into the fuselage cutout. Following the width of the fuselage the covering on the wing is VERY carefully cut and removed. Flaps are mounted with rigid hinges and connected with the appropriately bent flaphorn ( Brodak BH-749 ).

Fuselage
In the original RC model the engine is mounted to the firewall. Consequently a clunk tank would be required. To be able to use a standard uniflo metal tank, I preferred to mount the engine on wooden bearers and to build a suitable closed tank compartment. This is inserted from the front into the formers, which have to be cut out accordingly.bild4

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The Super Tiger ST 60 with a big venturi needs a lot of fuel. So the dimensions of the former cutouts have to allow a 170 ccm tank ( Brodak BH-563  6 oz, length 152 mm ) to be put in from the front. With a fuselage nose length of 230 mm ( spinner rear face to leading edge ) the tank compartment will reach about 35 mm into the wing
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If a Super Tiger ST 51 is used, the fuselage nose should be 25 mm longer. For this engine a 155 ccm tank ( Brodak BH-562  5,5 oz, length 140 mm ) will do. A cutout in the wing is not required.

On the fuselage there’s a weak point around the trailing edge area of the wing cutout. After installing the wing it’s unalterable to add two fuselage joiners. These are  securely glued to the former ( cockpit rear wall ), wing top, and front former.

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The elevator pushrod runs outside of the fuselage. For this a U- shaped channel is built into the fuselage side.

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Specifications after rebuilding:
3 D Giles-202 / F2B
span :  124 cm
wing area ( including flaps ) : 42,6 qdm
tailplane, modified : 8,35 qdm
airfoil symmetrical : 18 %
Center of gravity position : 17 % MAC
Engine : ST 60
Weight, trimmed : 1755 gram

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Flight characteristics
The wing with constant thickness and deep chord, the bulky fuselage, and the long undercarriage create lots of drag. Therefore a lot of power is needed to achieve lap times of 5,1 sec ( on 21,5m X 0,45 mm lines  =  70 x .018 ). On lower speeds line tension will suffer. On my ST 60 a venturi diameter of 9 mm ( 0,354” ), 10 % Nitro, and a TopFlite 13 X 6 ( 2 blade, old version ) was necessary to have safe control over the model in all attitudes. Tests with the new Chinese ST 51 have proved that this engine will suffice, too. However with the 51 it will also be necessary to increase the venturi bore to about 8,2 mm ( 0,323” ) to have sufficient power ( propeller 2 blade 12 X 6 ). With a CG position at 17% MAC ( = 192 mm measured from the hinge line forward ) the model flies softly and pleasantly. Some strong deflections are required to fly corners, and it’s almost impossible to stall. Weak points are a slightly asymmetrical turning between insides and outsides. Probably because of the somewhat flimsy tailplane pitch stability is not quite perfect under changing loads. The model is suitable to practice F2B until contest level performance. It flies definitely better than most trainer airplanes. With the sum of its properties, flight performance of the Giles  reaches pretty close  that of a usual F2B airplane.

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