This model has been around for seven or eight years now, so there's plenty of collective experience with it. Reading through some of the threads on RCU it seems a wide range of engine sizes have been used, from 7kg Wren Super Sports and P-70's at one end, up to K-140's at the other.

This says a lot about the versatility of the airframe, but here's my question: I have a VT80 and a P120se.
Which is the more appropriate engine to use?

In an early post, Ali Machinchy said the P80se was a great combo, and that many of the jet's virtues come from being kept light. But other posters have installed bigger engines, arguing better performance off grass and better verticals. But bigger means heavier means higher stall/safe landing speed, which is a minus.

Not quite sure which way to go given what I've got: a VT80 or a P120. The strips I use will mostly be rough-ish grass, 100m long. I don't really need unlimited vertical, but nice fairly large loops would be good to achieve. Any advice?

i'm building one now.. go with the 120. the perfect engine. i'm putting a RX140 in mine WB_1

I would go with the 120.... Grass operations and such. I have a Cheetah SP ( 140+ ) and it is way more than needed but it is nice to have a get out of jail card if needed... That being said, remember the manual, 180mph max..

Dan

Go with the 120, I have a P120SX in mine, perfect off grass.

https://youtu.be/r2p8FOmJ2AQ

Hello GRVW
Heres a link of a display flight at an air show I did recently with my P80 powered Viper. Plenty of power for everything right from takeoff as you'll see. I did build it super light though, only because I already had a P80. Only one fuel tank, no cockpit or ballast.
Starting from scratch though as others have said a 120 would be good also. Don't worry about stall/landing speed. It's a floater.
Levi

Thanks for the advice everyone. Nice footage, Levi.
Ok, the P120 it is then, noting also that the later P80s are closer to 10kg thrust than 8kg.

ok.. i'm into the build finally and i'm running into things that don't add up... On the ailerons they say to put the pivot point of the control horn on the hinge line. well if i do that there will be nothing for the control horn to anchor as the ail. spar is so far back from the LE of the ail. it's got to be at least 1/2" back from the hinge line. i looked at the pictures in the manual and they are showing they did move the horns back. so why tell me to put them on the hinge line? so i did finally decide on my servo setup . because i have a 28X i'm going all high voltage " 7.2" through out the plane using Xbus exclusively. all control servos besides the elevators will have 8921's and the elevators will have 3421's the 3421's fit perfect with no mods and produce over 100oz. using HV. for power i opted to go for a p140RX as it was only a few ounces if not the same weight as a p80se. so i was wondering how you guys installed your ail. and flap horns in , thanks, WB_1

I ignored the instructions and moved them to a spot where they would tie into structure, they work just fine there.

i agree .. the other option i have is to add a block to the front of the aileron spar which would not only move the horn forward but give the horn a better anchor point as that spar is only like 1/8 - 3/16 wide and the horn mount surface is 1/2"

bill, did you use Velcro to secure the fuel tank?

Hey All,
This is my 1st jet, and I'm pretty psyched. The ARF itself is fair, kind of heavy, but I'm used to Compare IMAC planes, ect...
Anyway, 1 question, and one complaint.

1: What is the true diameter of the wing-tube. The supplied aluminum one is too large and will not go in with out damaging the model. It is also very soft, ect... So, I want to replace with a carbon wing tube. Been using them for years with no issues. Thoughts?? Dimensions?

2: Elevator servo mount is not perpendicular to elevator hinge. This really surprised me. I ended up installing 2 horns on all surfaces to accept a ball-link in between, and hope this will be OK with the side force of the servo on the elevator. Has anyone noticed this, or had issues?? I see videos of turbines crashing because of flutter and this concerns me.

Anyway, looking forward completing this assembly, and becoming a turbine pilot.

Cheers,
Wayne

ok . i know this is an old thread but i would like to get a question that's been on my mind. everyone keeps talking about the CG and it being forward of the spar/wing tube . my question is.. when SM is referring to the 25mm CG point being ahead of the spar ,are they meaning 25mm from the front edge of the spar/ wing tube or from the center of the spar/wing tube? my first 8 flights were 15mm in front of the forward edge of the wing tube. if you look at my pic .. 40mm from the centerline of the wing tube would be 25mm a head of the wing spar. . i put mine 15mm from the front edge of the wing tube. too much? mine seems nose heavy meaning i have a little trouble getting the nose to come up on flare.

The CG you are referring to is merely s starting point for first flights.

If you work the CG back a little at a time you can check flor in flight stability with the gyro off. Fly upright, trim for level pitch. Fly the next pass inverted. Hands off inverted should have the nose dropping slowly for a pitch neutral airplane.

Once the CG is set in the neutral zone the pitch will be much more sensitive requiring caution to. It over control the aircraft at first. It will fly faster using less fuel and land slower with less elevator deflection.

i agree but you have to know where to start first . my question was basically that. do i use the center of the wing tube as a starting point or the front edge ? then you can adjust to your liking ,WB_1

balance it on the leading edge of the wing tube and you will be fine ,,,can move slightly back to your liking . I just hang mine from the wing tube with 2 loops of string and make sure it hangs slightly nose down

ok, i checked the CG this morning and it balanced perfectly right on the forward edge of the wing tube. makes sense because when i balanced it for the first 8 flights , it was 15mm in front of the wingtube thus nose heavy. after i sold my 28x , i had to remove all the heat blanket and ,wood , Aluminum tape and Xbus cables from the rear any then redid it all ,i'm sure i add some weight to the tail. so it looks like it worked out. now to test
fly

Hi,

Some time ago I acquired a half built Viperjet and finished it. Since then I made many, many flights and I love it a lot.

I am using the original landing gear, including the original inflatable tyres. Those are starting to wear out and don't hold the air very well. Does anybody know of replacement tyres of the solid rubber kind ? I am not such a fan of having to pump the tyres every flying day. I would like to retain the original hubs and brakes as they work well.

Thanks in advance for any help


Ro

you could try BVM or inairco wheels . hub diameter is critical as you don't want a loose fitting tire

Dear Viper Jet's fans
Good afternoon,
I know that the thread is old, I wanted to ask you some questions:
- The tail rudder hinges, are they attached to the rudder fin?
- If so, what glue do you use to glue the hinges?

Thank you

spider2323

I read all of this thread, most people have pass on, and some are flying bigger jets now, I thank all the ones that posted on this Viper thread.