FT Goblin porpoising when accelerating - have built many before.

[Captain Video]

Hello Everyone:
My two adult sons and I have been following Flite Test for years. We have built Goblins numerous times. We almost have the build time almost down to less than 90 minutes. We are quite experienced with this model. One son purchased a Speed Build kit and assembled it. We flew it yesterday. When accelerating the Goblin would flutter or porpoise. Probably about a 2-3 inch deflection in the flutter. It did not flutter much when gliding.The control horns are the wooden variety. The push rod is bent at the proper locations. We use Micro Servos SG90 9 gram servos and have used them in the past. The receiver is a Lemon 6 channel and a yellow yellow Amazon ESC. This is the first time we encountered this. We have adjusted the battery placement with little change. Suggestions would be appreciated.
Thank You!

 

[Dean is the name]

Have you checked if the motor/prop are mounted correctly? If they arent that could be your issue

 

[Captain Video]

Initially the prop was backwards. It sounded wrong. That was fixed. Thanks for the suggestion.

 

[Captain Video]

What are the chances the BEC is not supplying enough amperage for the Servos?

 

[JasonK]

my first thought would be a thrust angle issue - were the thrust angle is causing some problematic pitch coupling.

other thoughts (less likely)

• slop in the control links
• throttle -> pitch mix in TX causing Issues

questions

1. Does the porpoising stop once it is up to speed or does it happen when ever the throttle is on at all?
2. Is the CG in the correct location?
3. Is there any slop in the control links?
4. Take the prop off and throttle up on the bench, do you see any twitching/other in the control surfaces?

 

[HVB79]

Because you have built a number of these I am going to suggest a more obscure potential cause.

You could have a bad set of servos. Sometimes with low cost servos the quality control is not great and they will have a large dead band at the center that will allow the arm to move a bit before the servo starts resisting the movement or the servo will not center up exactly the same when returning to center from a deflected in each direction. These issues can cause a flying wing with tight control links and sensitive pitch control to fly strangely. Swap the servos out for known good servos from an old plane and see if the issue goes away.

 

[Piotrsko]

If it's not servo jitter, could be too much motor up thrust or for this particular plane: needs the CG moved back about 1/4 inch.

 

[L Edge]

Another possibility is the flutter can be caused by loose surface hinges or the control wire oscillating(try thicker dia).
Anytime you get flutter, get it stopped or you will find the control surface missing. In old days, we had to put counterweights on the control surface to stop flutter..

 

[Piotrsko]

If only just the control surface came off. One plane I had explosively fluttered but YMMV