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What the heck is wrong with my giant wing?

Old 06-07-2016, 01:19 PM
  #26  
soarrich
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Originally Posted by mclarkson View Post
Maybe I'm exceeding the wing's critical mach number!
In the last dive on the video, yea I think so. :^(

OH NO Mister Bill was running though my mind as I watched it start to tuck.
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Old 06-07-2016, 03:09 PM
  #27  
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Yeah, I'm pretty sure I heard a 'sonic boom'.. or was it more a 'sickening crunch'?
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Old 06-07-2016, 08:43 PM
  #28  
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I glued the bits back together and made a couple of small changes.

I upgraded the servos from 9gram EMax ESO8A's (stall torque: 1.5Kg/cm) to 25gram Power Up AS3513NG's (stall torque: 3.5 kg/cm).

And I pushed the CG yet farther back. So far, every time I push the CG back, the plane's handling seems to improve.

Among the many potential problems suggested was that, at speed, the load against the elevons was over-powering the servos. This makes a lot of sense considering the high about of up reflex the wing was running. If the wind started pushing the elevons back, the plane would start to dive, picking up speed, pushing the elevons farther back, etc.

By pushing the CG back again, I was able to take some of the up reflex off of the elevons. And the new, stronger servos do a better job of holding everything where it's supposed to be.

As a result, I actually got in a couple of flights yesterday that did not end in disaster. Here's one of them:



Despite my success, I'm planning on tearing the plane apart and building a new wing with a thinner airfoil better suited to a flying wing.
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Old 06-08-2016, 02:11 PM
  #29  
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I'm glade you got it flying well.

Originally Posted by mclarkson View Post

thinner airfoil better suited to a flying wing.
Looks good to me. The volume of wings help planes fly better. Dave Platt said that's why bigger models fly better, while the area goes up by the square, the volume goes up by the cube.
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Old 06-08-2016, 06:31 PM
  #30  
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That did look a lot better but i'm sure a more tailless friendly airfoil will be better still.. It will be interesting to hear how it goes.
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Old 06-13-2016, 08:10 AM
  #31  
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For those following along at home, here’s how this story comes out.

The plane was initially nose heavy, requiring a lot of up elevator (‘reflex’) to maintain level flight. In addition – and this is something I *should* have learned from my M-226 flying wing project – fat airfoils on a flying wing require a lot of up reflex to fly.

Next, the underpowered 8 gram servos – EMax ESO8A's. Personally, I think these – combined with the excessive amounts of up reflex required – were the prime villain in the plane’s fatal “dive when going fast” behavior. I suspect that, as the plane went faster, air pressure increased on the raised elevons, eventually overpowering the little EMax servos. As the wind forced the elevons down, the plane would begin to dive. As it dove, it would pick up speed. When it picked up speed, the pressure on the elevons would increase, pushing them further down and creating a terrible feedback loop of death.

Finally, the airfoil itself was too fat and entirely the wrong shape.

First, I fixed all the non-airfoil problems by upgrading the servos to 25gram Power Up AS3513NG's (stall torque: 3.5 kg/cm, about 2.3 times that of the original servos).

I also kept pushing the CG back, which helped balance the plane and remove some of the up reflex.

Eventually I got in some pretty decent flights.

Convinced that I understood the problem, I ripped the plane completely apart, pulling off all the electronics, the elevons, and a few bits of carbon fiber.

I built a new version of the wing using my best approximation of the MH-60 airfoil. (With my build method and lack of dexterity, it’ll be an approximation at best.) It’s visibly thinner than the original. I installed all the electronics and taped on the elevons and I was ready to go with the Mark II.


According to my notes, the new plane is 150 grams lighter than the original but I’m suspicious of this. I don’t see where I could have saved that much weight. Still, with an AUW of 682 grams (24 ounces), it has a wing loading of around 1.8 grams per square inch. And it flies pretty alright.

Thanks to everyone who helped me to puzzle out this mystery.
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Old 06-13-2016, 12:04 PM
  #32  
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Looks like she's flying really well. Did you notice a better glide and longer flight times?
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Old 06-13-2016, 01:17 PM
  #33  
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I've come out that much (% of total weight) lighter on some of my builds vs other people's of the same kit just from the way I use glue... Using JUST as much glue as needed and no more can be huge.
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Old 06-13-2016, 05:58 PM
  #34  
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Originally Posted by JetPlaneFlyer View Post
Looks like she's flying really well. Did you notice a better glide and longer flight times?
Definitely.
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Old 06-15-2016, 12:40 AM
  #35  
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One more:

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Old 06-15-2016, 11:41 AM
  #36  
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Originally Posted by mclarkson View Post
One more:

Nice video Mark and i like the music.
Ella was a great Jazz singer and i like Blue Skies.
The Wing flies good.
Crash
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Old 06-15-2016, 01:06 PM
  #37  
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Well first of all I noticed that your elevens move a LOT !! Even when you were about to launch ... I could see them moving with airflow buffeting them.

They are rather large !

Just thinking if you could 'tighten' up those elevens so they don't move with buffeting air ... you may stand a better chance.

Nigel

Edit ... just gone back and read rest of last entries ... honest I did not read about the elevens before my post here ... so I wasn't far wrong ...
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Old 06-15-2016, 01:11 PM
  #38  
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Looks like it's flying perfectly to me so based on the old 'if it isn't broke don't fix it' rule I'm not sure anything needs changing?
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Old 06-15-2016, 03:37 PM
  #39  
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Originally Posted by JetPlaneFlyer View Post
Looks like it's flying perfectly to me so based on the old 'if it isn't broke don't fix it' rule I'm not sure anything needs changing?
If that relates to my post ... I only read / viewed first vids ... so saw the original problem ... posted reply ... only later after did I see that in fact he did sort out the elevons ...

In that first post and video - you can see the elevons fluttering in the breeze ... no way are you going to have 100% control or steady flight ...

Nigel
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Old 06-15-2016, 06:49 PM
  #40  
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Yeah, between the under-powered servos and ridonculous amounts of up reflex, they were definitely a bit of a problem at first.
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Old 06-16-2016, 01:54 PM
  #41  
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Another fix for using the smaller servos is to use booster tabs on the elevons. I experimented and did a lot of research on them and they do work well.
Takes about75% of the load off the servos. I found that making them 10% of the area of the aileron works well.
I have them on a number of planes including a 1/4 scale Corby Starlet and the Visionair. Control response is much faster once you hit the magic spot for throw on the tab.
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Old 06-16-2016, 07:12 PM
  #42  
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Interesting. I'v only used something like that on scale planes where the full-sized planes had them.
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Old 06-29-2016, 07:48 AM
  #43  
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Here's another video from that same site. You can see how nicely it slows down at about 1:02.

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Old 11-15-2016, 04:58 AM
  #44  
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Old 05-30-2017, 04:50 AM
  #45  
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This is still one of my favorite builds of all time, floaty yet agile.

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