Viper Modifications

By Joe Luxford

1       Introduction

The Great Planes Viper ARF is an excellent kit and good value.  Our experiences in Perth and others in NSW and the USA are that the Viper flies superbly when properly trimmed but can break easily.  This article describes the distilled knowledge on how to fix the trimming issues and strengthen the Viper without adding excessive weight.

2       Information sources

There are a number of modifications that will make the Viper much more durable and able to better withstand the rigors of competition.  The best information source on the web is the pylon universe forum at http://www.rcuniverse.com/forum/Pylon_Universe/forumid_323/tt.htm.  There are lot of discussion threads on Viper fixes.  I did a search on the forum under “Pylon Universe – RC Pylon Racing” with “Viper wheels” typed in.  You can experiment with the word Viper plus anything other word that takes your fancy and you will bring up a wealth of information on common Viper problems and fixes.  Other excellent sources of information on the Viper include:

http://www.pylonworld.com

http://www.rcpro.org/html/articles/quickies/quickies.htm

http://www.nmpra.org/phpBB2/index.php

http://www.nswpylon.org/

3       Fuselage

3.1      Firewall

The firewall is prone to falling out, either in a bad landing or over time due to vibration.  The fix is to drill 3mm diameter holes in each side of the fuselage and underneath into the firewall.  Bamboo skewers about 10mm long and glued in with epoxy work well.

The fiberglass inside the fuel compartment in the Viper fuselage can be on the light side and not well adhered.  It can be worth while running some strips of fiberglass across the back of the firewall and down the sides and bottom of the fuselage.

3.2      Wheels

Viper wheels rely on a friction fit to stay on the axles.  They almost inevitably vibrate off in the first few flights.  The fix is to either replace the wheels or solder or braze ¼” brass washers onto the outside of the axles.  Just make sure that no solder gets into the thread inside the axle.

3.3      Tank

The Viper tanks are prone to splitting.  If and when they do split, suitable replacements include various 4oz tanks such as Great Planes, Sullivan etc.  The best option is a bubble-less bladder tank, either Tetra or Jett, or home made using a baby bottle liner.  The bubble-less tank ensures no air bubbles because there is no air in the fuel bladder inside the tank. 

                            
                                                (Click for larger image)
            Jett  bubble-less tank located on centre of gravity

3.4      Landing gear mount

The ply plate to which the landing gear is bolted is prone to breaking out of the fuselage.  This problem is minimised by applying some 4oz cloth and resin the landing gear area block on the inside.  The cloth should cover at least 20mm of fuselage floor and sides around the block to properly key it in.  Its also worth applying some resin to the bottom of it for fuel proofing.

                                 
                                                (Click for larger image)
Reinforcing on landing gear plate and in fuel compartment behind firewall

3.5      Wing hold down plates

    3.5.1   Glue
 

The four ply plates holding the 8-32 blind nuts to hold the wing in place can come loose when not properly glued in the factory.  The front wing mounting plate can be seen in the photo above. The fix is to run a fillet of epoxy and micro balloons along the fuselage joint under each plate.

Its also worth running a bead of epoxy & microballoons around the edge of the fuselage formers to strengthen those joints as well.

    3.5.2   Room for aileron torque rods

The torque rods can hit the ply plate across the fuselage – which will limit aileron movement.  The fix is to grind some of the plate out (about a millimeter) with a Dremel sander.

3.6      Wing hold down bolts

The metal 8-32 bolts into the metal blind nuts are prone to coming loose.  One solution is to run a small amount of blue locktite on the wing bolts for the first time. This gives them some grip whilst still being easily removed. Make sure you do them up tight and check them.

A better solution is to flip the blind nuts out with a knife and replace them with 10 gauge blind nuts and use 10 gauge (either 32 or 24 pitch) nylon bolts.  They won’t crush the wing if over tightened and will shear off in a bad cartwheel that would otherwise tear the side out of the fuselage.  (Ask me how I know)

3.7      Fuse behind wing

The back of the fuse between the rear wing mount and the last bulkhead behind the servo tray is weak and will break easily here on a heavy landing.  Ways to strengthen this area include:

(i)      running 4 mm square spruce along the top of the servo tray between the last bulkhead and the rear wing mount bulkhead;

(ii)      fibreglassing the fuselage interior sides and floor between the bulkheads

                               
                            (Click for larger image)
Damaged Viper sans firewall and rear fuselage after a hard landing

 

3.8      Fuse sides at tail plane

The rear fuselage is prone to cracking at the bulkhead at the tail plane leading edge as shown here:

                             

            Cracked fuselage at rear bulkhead at front of tail plane LE

The fix is to reinforce fuse sides at tail plane LE by laminating some 1/32 ply or fiberglass.  Cut the Monokote covering away, apply the lamination and recover the area.

                             

            Reinforcing the fuselage at the tail plane LE

3.9      Tail plane incidence

Vipers are notorious for incorrect incidence angles on the tail plane and firewall.  The incidences should be 0-0-0.  Its vital to check.

The tail plane can then be shimmed up to the correct angle with a piece of popsicle stick cut and sanded to size.  That is glued in place to start with and then the whole tail plane can be glued to the fuselage with epoxy and micro balloons.  Its only necessary to leave the tail in its detachable form if you want to travel to contests and need to remove the tail to get the fuselage into a box for airfreight.

The firewall is often out too.  That can be cured with washers behind the engine mount to get zero up or down or side thrust.

4       Wing

If a bubbles tank is used, use 3mm ply for mounting the aileron servo (instead of the 6mm rails) to leave more room underneath the wing.

5       OS & TT 46 Mufflers

The TTPRO and OS 46 engines are notorious for losing the rear half of the muffler if the following precautions are not taken.  The TT46PRO and OS46 FX/AX are prone to breaking the bolt through the muffler which lets the rear half fall off in flight – never to be seen again.  The fix involes:

(i)      removing the long bolt through the muffler;

(ii)      JB welding the baffle and two muffler halves together;

(iii)     Putting a length of 3mm mild steel threaded rod through the muffler with a dab of JB Weld and lock nuts on each end.

JB Weld is currently available from K Mart.  Any of the metal epoxies will do but JB is the best.

Do not over tighten the nuts on the mild steel rod or it will break.  The aluminium muffler expands more than the steel rod and will over stress and break the rod if it is over tightened.  Wire or cable tie around the muffler is an extra option to ensure the rear half is never lost.