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Old Feb 22, 2015, 01:00 PM
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Portugal, Leiria District
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Flying Wing landing skids affect stability?

http://tapatalk.imageshack.com/v2/15...7bc96b1cca.jpg

I've been flying scratchbuild wings for almost 1 year, and started to realise that on landings, after touchdown they did dive nose down, obviously because the winglets make the wing be separated from the ground, but the nose didnt.

And because i've seen some people using landing skids on front ive now build two wings with that.

After maiden yesterday and everything looking fine, today i went flying with 15kmh wind and i noticed one really big yaw instability, sometimes the wing rotates like 45degrees to the sides. Do you think its maybe the skids that create the instability??

Thanks
Sorry for my bad english
Cheers, Pedro
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Old Feb 22, 2015, 02:02 PM
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You've reduced your yaw stability by adding more vertical surface area ahead of the CG without adding an appropriate amount of area to the fins behind the CG. How much exactly is needed depends upon the moment arm from the CG (distance from CG to centroid of fin).
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Old Feb 22, 2015, 03:47 PM
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Quote:
Originally Posted by nuteman View Post
You've reduced your yaw stability by adding more vertical surface area ahead of the CG without adding an appropriate amount of area to the fins behind the CG. How much exactly is needed depends upon the moment arm from the CG (distance from CG to centroid of fin).
So basically the best to do would be removing the skids right??
thanks
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Old Feb 22, 2015, 09:04 PM
Ochroma Pyramidale Tekton
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How about a 'wire' skid under the nose, like on the tail end of the pioneer birds (like the ever popular Eastbourne monoplane)? That might support the front without adding signifigant drag and messing up your yaw control.
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Last edited by Fly Wheel; Feb 24, 2015 at 05:34 PM. Reason: Added "under the nose"
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Old Feb 23, 2015, 12:13 AM
Fremont, CA
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Some suggestions...
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Old Feb 23, 2015, 02:57 AM
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I've been curious about this myself and how much of an impact those tiny dubro nylon skids would have.
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Old Feb 23, 2015, 05:44 AM
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Originally Posted by Mr.Aaron View Post
I've been curious about this myself and how much of an impact those tiny dubro nylon skids would have.
Yaw stability is almost exclusively about simple geometry (other factors play a tiny role in comparison).
"Forward Torque" = Vertical surface area ahead of CG multiplied by moment arm from CG.
"Aft Torque" = Vertical surface area behind CG multiplied by moment arm from GC.
Aft Torque must be greater than Forward Torque for positive yaw stability.
I usually shoot for at least 20% more.
The trick is to get the centroid of those areas calculated correctly. For non-trivial shapes (lots of curves), this can be a chore. Rather than get an exact figure, I "estimate" the areas involved by "giving" them angular lines that only approximate the actual curvy shape.
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Old Feb 23, 2015, 04:18 PM
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Thanks guys, i think ill remove the skids.. i want it as stable as possible
Cheers
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Old Feb 24, 2015, 12:57 AM
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Have you considered a single wire skid on the nose as used on some control line planes? It's similar to the tail skid depicted in Fly Wheel's post, except it's just a wire extending down from the forward ventral area of the wing. If you've not seen them, the single wire is installed so that it angles rearward to avoid grabbing obstacles upon landing. The lower end of the wire is twisted into a small loop to avoid injury.
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Old Feb 24, 2015, 05:33 PM
Ochroma Pyramidale Tekton
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Quote:
Originally Posted by DT56 View Post
Have you considered a single wire skid on the nose as used on some control line planes? It's similar to the tail skid depicted in Fly Wheel's post, except it's just a wire extending down from the forward ventral area of the wing. If you've not seen them, the single wire is installed so that it angles rearward to avoid grabbing obstacles upon landing. The lower end of the wire is twisted into a small loop to avoid injury.
Actually, that is what I was proposing (The photo was submitted by djacob) Although I should have specified in my post "...under the nose".
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Old Feb 25, 2015, 11:24 PM
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Originally Posted by Fly Wheel View Post
Actually, that is what I was proposing (The photo was submitted by djacob) Although I should have specified in my post "...under the nose".
Sorry about that! I'll share the royalties with you.
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Old Feb 26, 2015, 02:46 PM
Ochroma Pyramidale Tekton
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OK, Let's see, 15% of "free" equals...wait, let me get my calculator......
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Old Feb 26, 2015, 07:23 PM
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OK, Let's see, 15% of "free" equals...wait, let me get my calculator......
My spiritual adviser, Father Guido Sarducci, insists that 50 / 50 would be a deal I can't refuse.
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Old Mar 02, 2015, 04:05 PM
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Ive removed the skids and the stability got a lot better. Now i see when flying low that she doesnt fly very stable. Sometimes she drops a wingtip and that cant make me get confident when flying low. Do u think it can be the cg too far aft? Thanks
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Old Mar 02, 2015, 04:33 PM
Fremont, CA
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Sometimes it's good to provide a bit of washout on the wing tips. That means to have the wing tip twisted with the LE downwards. This forces any stalls to happen at the root first and prevents tip stall.
A CG a bit too forward is better than a CG a bit to aft.
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