Specifications for COX Viper (P-51) Conversion:
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.
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.
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.
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.
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
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.: