Hi Guys,

I made SU-47 out of biofoam, its really nice plane to fly, with HK 1806 2500kv motor it i not Jet but pretty good park flyer, can do high Alpha!!!!!
Details link is http://www.rcindia.org/electric-planes/scratch-built-biofoam-su-47-20-wingspan/

But while trying to do high alpha I noticed it is shaking badly  . Pls see my latest video. Initially I thought that servos might be getting overloaded and may be fluttering. Or may be push rods are not much stiff. But then when I increase speed to come out of alpha, or taking speed loops, I did not observe this. I started searching web and I came across something called “Wing Rock”. This is also observer in many fighter plane at high alpha at low speed!!!!! Somewhere I read that at high angle of attack wing tip stalls or creates turbulence at trailing edge and elevator get caught in this turbulence and many more things happens…. and plane starts oscillating like this!!!!! This is known as Wing Rock!!!! 

I tried to find solution for this, but then I started to like this when my SU47 does this trick!!!! 

In the following video you can see this at time 2.15 & 3.15!!!!

Any reason for this and some solution will be highly appreciated!!!

Was hoping to see more discussion around such an interesting topic like this

The plane looks great with the forward swept.. For your wing rock i will say as the wing span is 20" with forward swept try flying the model in a clam weather early morning. I think a 20" model will wing rock in windy conditions.

Check the CG & all the control surfaces to be without "play" & fly the model in a very calm weather & let us know.

Hi Ash

That is a dutch roll oscillation in my opinion. I too faced the same rolling when designing a micro airplane for the IIT competition.



Wings placed well above the center of mass, sweepback (swept wings) and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies.

Try to damp yaw...use a bigger vertical stabilizer.

Regards

thanks guys!!!

thanks ankurkaul17,

it seems like it is dutch roll!!!!!
vertical stabilizer,,, not sure if i can do it now!!!!!

sharlock,

thanks for suggestions.
when i fly this model, it was very little windy. abt CG, i read somewhere to shift it little back will help, havnt tried yet!! there is noplay in control surface, but i bent push rod in u shape so that some adjustment is possible, and it is where push rods becomes little softer and tends to bend.

i also thought that, this might be causing wing rock, but sometimes wing rock is so fast it wouldnt cause by push rods.

even i check some of the videos from rcpowers and some models tends to show this, but very gentle and very slow effect.

again if i increase speed plane becomes stable!!!!

Ashish

Can this be due to resonance? If it is rocking only at a particular speed/band seems probable, try changing the prop and see

Yes moving the CG slightly back can help the plane be stable, but it wont be as stable as a similar size delta plane, compare your plane with a similar size delta plane. Due the the forward swept in the wing there is less tendency for a plane to be stable in high alpha maneuver.

But still move the CG slightly back & let us know the out come..

yes u can say so, it does it at high angle of attack and at low speed, if i increase speed then it stabilizes!!! and if flying normal, without doing any high alpha then no issues at all.
i think changing prop wont help!!!
this oscillations are because of at low speed and high angle of attack, leading edge stall or create turbulence and elevator is caught in this turbulence and many many many .......thing happens and plane starts oscillating!!!!

sharlock,

i also started to have same feeling!!! but best part is i started to like this ROCK effect and keep on doing it while flying!!!
will try moving CG back and will let u know!!!

Do you have flaperons ? Have you tried doing the same move with them deployed as spoilers? In slow high alpha moves I have noticed that these spoilers help improve the stability