Viper Conversion


CoxViper.jpg (8183 bytes)

Specifications for COX Viper (P-51) Conversion:

bulletCox Sky Cruiser Viper from Tower Hobbies or Mike's Bargain Page
bulletWingspan = 20.5"
bulletWing Area = xx sq. in.
bulletLength = 16"
bulletRTF @ 4.2oz. 
bulletElectric Goldmine G9330 motor for $1.25
bulletor GWS direct drive DX series motor
bulletGWS ICS50 2-amp ESC from Balsa Products
bulletGWS BP 101 Sub-Micro Servo from Balsa Products
bulletGWS R4-P micro-Receiver from Balsa Products
bulletstock Viper 4.2x2 red prop
bullet1.0 amp @ xxxxx RPMs with 8-cells of 120mAh NiMH (1/3 AAA)

Here some pictures of my COX Viper. The conversion was mostly copied from Mike Blott's Viper Conversion article. His article can also be seen in the May 2001 edition of RC MicroFlight magazine.

I will comment only on my deviations from Mike's article approach.

Initially, I have tried a $1.25 G9330 motor instead of the $10 GWS DX-A motor. I am using 8-cells of 120mAh (1/3 AAA size) NiMH and only use the inner half of the ailerons instead of the entire wing length. The motor did not appear to have enough power to make the Viper climb with either a U80 (3x1) prop or GWS 3x2 prop. I decided to then try a GWS motor like Mike Blott used.

My tiny 20.5" wingspan ship is RTF at 4.2oz.

Viper&Radio_thumb.JPG (5169 bytes) ViperSide_thumb.JPG (6135 bytes) ViperBottom1_thumb.JPG (3655 bytes) ViperAilerons_thumb.JPG (3657 bytes)

Next to my new Futaba 8UAFS transmitter, you can really get a perspective of just how small this ship is!

This pictures show the aileron servo, linkage, and bottom access hatch.

Flight Report:

So far, I have successfully flown it around the field a few times. After several rough landings and repairs, my Viper now weighs 4.7oz.

The best combination to date has been what Mike Blott used; a GWS motor and the stock Viper prop (4x2).

I have tried a GWS 4.5x4 prop and it drew way to much current. I have recently ordered a new GWS size prop (4x2.5) for additional testing.

Since turning was a bit "twitchy", I have since modified my ailerons for full lenght of the wing but I have not test flown it yet.


The best controlled flights with my 4.7oz. Viper came with an Astro Flight (AF) 4:1 Firefly and a GWS 5x4.3 prop. At the time, I was using an 8-cell, 120mAh NiMH pack from my GWS motor testing. The power system was an efficient 0.5amps but could not sustain flight due to the overall weight.


Round Two (My Second Attempt):

After suggestion by fellow E-Zoner, Roy (rkissin), I tried fitting a 5x4 Gunther "spoon" prop on my Astro Flight Firefly 4:1 coreless motor. Not only did it fit perfectly without an adaptor but it spun very well at 0.5amp draw.

This inspired me to try cutting a second $15 COX Viper model with a renewed emphasis on reducing weight. My new Viper attempt weighs 3.7oz but without any rudder or elevator control. This is using a GWS R4-P receiver and an AF Firefly 4:1 on 6-cells of 120mAh NiMH. This setup draws 0.5amps and will power the efficient coreless motor for a long time.

I am currently waiting for some 1.0gram BIRDs (Built Into Rudder Device) from John Worth at Cloud9. The required Digital to Pulse Converter board weighs an additional 0.6grams. This gives a rudder or elevator control for only 1.6grams each or a total of 3.2grams for both!

If this still isn't enough power to fly the COX Viper then I'll use the parts on a lighter plane like the Plinker, White Baron, or micro-Cub.

ViperFirstCut_thumb.JPG (5122 bytes) ViperReceiver_thumb.JPG (3840 bytes) ViperStockWing_thumb.JPG (2632 bytes)

ViperFireflyWired_thumb.JPG (3565 bytes) ViperMotorMounted_thumb.JPG (4693 bytes) ViperNew2_thumb.JPG (4055 bytes)

  I recently measured 5500 RPMs using the Gunther "spoon" prop on 6-cells of 120mAh NiMH. According to a chart by Gordon Johnson, he has measured a steady 35grams of thrust using the 4:1 firefly with a GWS 6x5 prop on only 5-cells of 120mAh NiMH.

Two new versions of the Viper ready to test fly.

After deciding that my biggest mistake on my previous attempts was to use the 120mAh NiMH cells on the GWS motor, I decided to give it another go.

This time I used a GWS S1 motor instead of the DX-A. The 4.14:1 ratio is a better choice for the small 6x5 prop. The DX-A ratio is 5.9:1. So basically, we are looking at a 4:1 and 6:1 ratio difference.

My favorite prop with the 6:1 DX-A motor on slowflyers like the Cannonshot, Tilde, and soon to be published "Ultra-Lite Stik" is a GWS 7x6 prop. This prop gives an incredible thrust/speed combination.

The faster spinning 4:1 S1 motor should work well with the smaller 6x5 prop and use a similar current to my DX-A setup with the 7x6 prop.

I feel that some folks have been "burned" by the S1 motor for being advertised as "more power" for the Lite Stik when in reality they are simply reducing the gear ratio but still using large 10x4.7 props. This merely draws even more current and stresses the motor even more. I don't feel that the carbon brush version is the culprit in this scenario.

I'm RTF at 5.7oz using 7-cells of 110mAh NiCd. I now have plenty of power. My 300mAh NiMH flat packs didn't fit so I decided to initially use the narrow 110mAh NiCd pack.

This time I also borrowed Mike Blott's pin technique for securing the wing to the fuselage. The front of the detachable wing has a wooden peg and the rear has a wooden post that gets held by the pin. It is very secure.

Here are some pictures of my 5.7oz version with the DX-A motor:
Viper_S1_Cut_thumb.JPG (3372 bytes) Viper_S1_Mounted_thumb.JPG (4613 bytes) Viper_S1_Mounted2_thumb.JPG (2665 bytes) Viper_S1_Finished_thumb.JPG (2878 bytes) Viper_S1_Pin_thumb.JPG (2690 bytes)

While I wait for my test flight on my DX-A powered Viper that weighs 5.7oz, I decided to deviate a bit on my 3oz project and put my brick and AF Firefly into my older worn Viper hull.

I modified a few other things like removing the switch and circuit board to keep it light and created 2 new foam wings that have more wingarea than the stock Viper. My all up weight is 3.2oz using:

bulletAF Firefly and ESC
bulletHS-50 and R4P brick
bullet6-cells of 120mAh NiMH
bulletGunther "spoon" prop

The 3.2oz version is a rudder/elevator plane. I really expect my DX-A version with ailerons to be the performer but I am curious to see if the lighter Firefly version even flies. If not, i'll continue on my path to build a modified Flying Aces Stick.

Here are some pictures of my 3.2oz version with the AF Firefly motor, Gunther 5x4 "spoon" prop, and an alternate orange "Light Wing" set from Takeoff 2000.:

Viper_3oz_wing1_thumb.JPG (3597 bytes) Viper_3oz_wing2_thumb.JPG (3576 bytes) Brick_thumb.jpg (3255 bytes)