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FT Viggen Crash-Causing Problem

Nsims4400

Junior Member
#1

So I recently put together one of my first speed-build kits from flite test, an FT Viggen. I was super excited to get it in the air using the 70mm edf setup from Grayson hobbies. When I finally got out to a field by my house to fly it, I ended up crashing and wrecking the airframe on takeoff. Shortly after takeoff, my Viggen rolled twice to the right, then spiked into the ground. When I reached it, I found that the edf had seperated form the mounts, and I assumed that this was what caused the problem. So I rebuilt the plane from scratch (with the same electronics) and got back out to the field, only to have it perform another uncommanded roll and crash on takeoff, however this time the mounts and airframe were still intact. I was wondering if anyone else has had this kind of problem and how I might be able to fix it (I couldn't find anything like this anywhere not the forums). By the way, my transmitter is the fs-t6 from fly-sky.
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#4
Sounds like too much throws or reversed elevons. I just built and maidened a speed build version myself and even while testing it on 3s, which was way underpowered (1kg auw) and want enough to get airborne, it was still really stable and controllable.
 

Nsims4400

Junior Member
#5
That's what I thought as well, but after inspection my elevons move the way they should and my throws are the recommended 9 and 12mm.
 

AkimboGlueGuns

Biplane Guy
Mentor
#6
If the throws are correct then I don't know what could have cause the problem. The torque from the fan shouldn't really be enough to roll the airplane that fast. Were your trims neutral when you launched the plane?
 

earthsciteach

Moderator
Moderator
#8
I have a 90 mm edf in mine and have never had any torque issues. Make sure the wings aren't warped and quadruple check your control directions. The Viggen is a very docile flyer. Since you've been flying for 2 years, I'm sure its not a "newbie" issue. Sounds like something is definitely amiss!
 

makattack

Winter is coming
Moderator
Mentor
#10
How did you connect the aileron servos? Y cable or dual aileron setup? Did you double check the mixing on the TX if you setup for dual aileron to ensure you have the directions correct -- no reversing needed, etc? When you give full left roll, how much does the left aileron travel up vs the right aileron travel down? What about right full roll?
 
#11
The servos were connected individually, and the rx was setup with a preprogrammed elevon mix, and the elevons move as they should, with the elevons moving the same amount down as they move up on the other side.
 

makattack

Winter is coming
Moderator
Mentor
#12
Oops... I forgot that the Viggen is a elevon plane! My only question next is if the plane is balanced on it's long/roll axis? Seems odd for an EDF to have such a roll tendency if all the surfaces are neutral/straight, and you've verified the elevon control directions and mixing is correct. I've had fixed wing planes behave oddly, acting out of trim on the roll axis, when the CG between the left and right was off enough and I had to land and shift the battery from one side to the other a bit more.
 
#13
I just checked the roll axis and it seems all is in order... I'm beginning to think maybe this was pilot error, although to have the same thing happen twice baffles me.
 

nerdnic

nerdnic.com
Mentor
#14
I've used an even more powerful EDF than that grayson hobby set up and have had no issues with torque roll on the Viggen, and on 4s too. Can you post a video of your control surfaces with coordinated info on what your stick movement looks like?

The only things that would explain the behavior you're seeing is:

1. Setting the trim way off
2. Tail heavy
3. Control surfaces set wrong
4. Your build came out drastically disproportional
 
#15
Unfortunately I cannot uploa a video of the control surface due to the limitations of the device I'm using, and I have already ruled this out as a problem. However, I think the cg might be the issue. How far off would the cg have to be to cause the plane to roll drastically?
 

earthsciteach

Moderator
Moderator
#16
That's a hard question to answer, but a cg that is too far aft will make a plane do very squirrelly things. The recommended cg is 5.25" (133 mm) forward of the leading edge of the rudder. Check to see if that matches your plane's cg.
 

nerdnic

nerdnic.com
Mentor
#18
If your CG was an inch off, it doesn't matter how bad the trim was, the plane would be unflyable.

I'm glad this was identified as the issue.