FX79 flapping wings!?

KillerDave · May 31, 2016, 01:16:25 PM

Hi folks...

A friend suggested I join here and so I am very new to this group....

I have been flying the FX79 Buffalo for a few months now and have a bit of an issue....
A few weeks ago I found myself quite high so I pointed it down into a moderate dive and very quickly, she started to "flap her wings!" I did pull out of it ok but it ended up in a crash... not too bad but I would say it was 80% my fault and 20% due to damage caused my the extreme flutter...???? maybe???

Anyway, the plane is flying again and what I want to know is....

Has anyone else experienced this and how can I fix it?

I was planning on laminating the wing but it was suggested to insert extra carbon spares into the outer wing section to add rigidity???

Any ideas???

Miles · May 31, 2016, 01:47:55 PM

I get the same effect on the X8 - one of them can't sustain WOT on level flight without the wings starting to go.

I've reinforced one by fiberglassing it, will be experimenting to see how it's worked, but the overall fix is just to stiffen the airframe up.

BigT · May 31, 2016, 06:55:33 PM

Aileron and or elevon flutter is normally caused by either too much slop in the linkages, control surface hinges either breaking or loose or servo not man enough for the stress and bouncing between neutral point. The most likely is hinge failure. I am never 100% happy with the EPP foam type of hinge. I prefer to separate the surface in the build stage and use a different system for hinges. Robart or even fuzzy Mylar, any hinge method to take the slop out.

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KillerDave · May 31, 2016, 08:27:56 PM

Thanks for the replies guys....

The servos are HiTec HS 56HB's I'm sure they are way better than the standard???

This even happens when I turn into moderate wind and keep the ground speed up....
From the ground, it looks like the the wins "flap" up and down maybe a foot???

Here's a video... look at 2:30-ish...

https://www.youtube.com/watch?v=NL-SiqbBhWQ

It was a very gusty day so probably shouldn't have flown....

Billy_boy_2010 · June 01, 2016, 01:23:44 PM

dig some small grooves into the wing and hot glue some fibreglass rod in there. 3mm will be fine. I would put a couple on the top and a couple on the bottom of each wing. you will be surprised how much it stiffens them up. just make sure they are smooth to the wing surface.

if needs be you could insert a diagonal one as a bonus.

test this you will be fine. but if you want to laminate after you can.

but be aware you have the wrong plane if you want to do steep dives and lots of aerobatics and agility stuff. Any of the ritewing planes are much more suitable for this sort of stuff.

KillerDave · June 01, 2016, 01:35:22 PM

Quote from: Billy_boy_2010 on June 01, 2016, 01:23:44 PM
dig some small grooves into the wing and hot glue some fibreglass rod in there. 3mm will be fine. I would put a couple on the top and a couple on the bottom of each wing. you will be surprised how much it stiffens them up. just make sure they are smooth to the wing surface.

if needs be you could insert a diagonal one as a bonus.

test this you will be fine. but if you want to laminate after you can.

but be aware you have the wrong plane if you want to do steep dives and lots of aerobatics and agility stuff. Any of the ritewing planes are much more suitable for this sort of stuff.

Thanks BillyBoy...
The steep dive I did was just that I was thinking I was too high and wanted to scrub off some altitude...
I have some carbon strips so I think that's the way I'm going????

FPVSteve · June 01, 2016, 01:45:13 PM

Had this happen with an old wing - the wings warped so much that they themselves became elevons making the plane uncontrollable.

Now I fly grandma planes :)

KillerDave · June 01, 2016, 10:41:50 PM

Quote from: Steve W on June 01, 2016, 01:45:13 PM
Had this happen with an old wing - the wings warped so much that they themselves became elevons making the plane uncontrollable.

Now I fly grandma planes :)

I was surprised the wing didn't brake up in the air! I had a huge deflection and maybe that was the beginning....
I'd don't know???
It's flying again, I just wanna add some rigidity to the outer panels....

BigT · June 01, 2016, 10:48:45 PM

Quote from: KillerDave on June 01, 2016, 10:41:50 PM
I was surprised the wing didn't brake up in the air! I had a huge deflection and maybe that was the beginning....
I'd don't know???
It's flying again, I just wanna add some rigidity to the outer panels....

Basic principal of flight is that altitude is governed by the throttle not the elevator. You should have just closed the throttle a bit and descended by bleeding of height. A power dive like that could pull the wings off.

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KillerDave · June 01, 2016, 11:07:12 PM

Quote from: BigT on June 01, 2016, 10:48:45 PM
Basic principal of flight is that altitude is governed by the throttle not the elevator. You should have just closed the throttle a bit and descended by bleeding of height. A power dive like that could pull the wings off.

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Yeah, I just wasn't thinking.... :-[

drambuidhe · June 02, 2016, 10:09:09 AM

My FX61 did that on it's maiden. It was due to too higher gains on the elevon's.
I use an FY-41 lite fc and once the gains were dialed down it behaved much better.
I still get a little in a fierce dive which I think IS due to wing flex although it is laminated. :)

Chris in Kabul · June 02, 2016, 05:21:28 PM

Maybe a spin would have been a better idea to lose height.

Billy_boy_2010 · June 02, 2016, 08:11:50 PM

Quote from: KillerDave on June 01, 2016, 01:35:22 PM
Thanks BillyBoy...
The steep dive I did was just that I was thinking I was too high and wanted to scrub off some altitude...
I have some carbon strips so I think that's the way I'm going????

carbon strips will work. Probably better if anything. just be sure you insert them in such a way that their direction resists the flapping.

only reason I suggested fibreglass was because it doesn't block rf signal. carbon does. so if it's an ultra long range plane fibreglass will still be better.

it will never be an agile plane though. buy a ZII for that:) Or a mini drak in a year when they are avaliable.

KillerDave · June 03, 2016, 01:06:42 AM

Quote from: Chris in Kabul on June 02, 2016, 05:21:28 PM
Maybe a spin would have been a better idea to lose height.

A tight spiral works well.... ;D

KillerDave · June 03, 2016, 01:12:09 AM

Quote from: Billy_boy_2010 on June 02, 2016, 08:11:50 PM
carbon strips will work. Probably better if anything. just be sure you insert them in such a way that their direction resists the flapping.

only reason I suggested fibreglass was because it doesn't block rf signal. carbon does. so if it's an ultra long range plane fibreglass will still be better.

it will never be an agile plane though. buy a ZII for that:) Or a mini drak in a year when they are avaliable.

I realise it is a cruiser and it does fly real nice on a good day but this flapping happened at a speed I would never have expected... even turning into a strong-ish wind was enough to get it going...