Why wings flap, and the best fix...

[Kitsu]

Hey everyone,

I've known about the dreaded "Zagi flap" for a while, tho I had never flapped my own wing (FMA Razor). Then one day I told a friend of mine to dive his Zagi so he could see the flap, as he had never heard of it.... Fortunately we saw where the battery landed and eventually stopped laughing about it.

Not long after I flapped my Razor when using a 400-X (heavy) pack and getting into a sticky situation trying inverted flight too high and far away. Now my Razor seems to flap with not much speed.

From an aerodynamic standpoint but in simple terms, why does a wing flap?

Also, to fix it, my friend installed a spar on his Zagi to prevent further flaps, but installed it on the top of the wing. I just saw a Razor 600 with the spar installed on the bottom of the wing. Is there any difference if I put my spar on the top or bottom of the wing?

Thanks.
~A

[Sparky Paul]

Foam has little structural strength. When it is used for a wing, with no other structure except the covering, the loads at speed are sufficient to create the "flappng", which weakens what resistance the foam had so the next time it flaps at a lower speed.
A spar top or (and) bottom will add a lot of resistance to flapping.
Or covering with sheet balsa.
The spar(s) is(are) lighter and easier.

[Kitsu]

If I had to choose top or bottom only, which will be more beneficial? Or do whichever is easiest?

~A

[Sparky Paul]

Most Zagi style flying involves positive g. The foam stretches on the bottom, and loses structural integrity..so keeping the wing from flexing upwards is more important.
A spruce spar inlaid in the bottom surface would be optimum, if CF isn't available. CF top and bottom is best overall though.

[Andy W]

It's a lot easier to install in the bottom of an already build Zagi, and the bottom of the wing tends to be flatter, so it's easier to embed it here without leaving a large hole at each end. It's also easy to install on top when you're building a new one, however.. Make sure it's fairly long - my first spars were 12" long, and the model started to flex at the ends of the spar. Now I make them at least 2' long..
..a

[Kitsu]

Thanks for the tips. I think since the canopy and motor tray unit on the Razor is epoxied into place, I'll install on the bottom of the wing (depsite the fact that I'll have to cuz a bit chunk from the fuse/pod thing.)
I'm still a pretty active sport kiter, so I'll dig around and see if I have an extra wrapped carbon spar, weeee.

~A

[steve262]

With out going into vortex shedding and aeroelasticity theory etc.

From an aerodynamic standpoint what happens during the flapping is that the wing deforms and twists under load which creates a strong vortex on one side of the airfoil. This creates a low pressure region that reverses the loading on the wing which pulls the wing sharply back the opposite direction which causes the same condition on the opposite side..... this back and forth oscillation caused by formation of alternating vortices at a frequency that is similar to the natural frequency of the structure will continue until the driving force is removed or the structure fails. It is a form of flutter that on much stiffer structures than a ZAGI can quickly cause the loss of a control surface, stabilizer or a wing. In certain conditions for models and full scale aircraft it can occur so quickly that the structure is driven to failure before the flutter is observed.

It is the same phenomenon that took down the Tacoma Narrows Bridge and causes the buzzing noise we here from tie down straps on the roofs of our cars. Obviously the forces, frequencies, material strengths and ultimate results of the vibrations are different for all these cases!

Putting a spar on the Zagi stiffens the structure which prevents it from deforming under the loads imposed by high airspeeds.

Hope this answers your question.

Steve

[Kitsu]

Originally posted by steve262
Hope this answers your question.

Steve

Yes, thanks for that explination. For whatever reason I understand it now whereas I didn't in the past.

Here's the bad news..... In a display of disregard for my plane's safety, I decided "one more flap can't be too bad, especially if its for the camera!" So I flapped it on video (I'll post it somewhere later) but while I did it, heard a crack. Landed the plane, and saw actual cracks in the lexan tape, plus some fairly pronounced fractures in the foam.

Got it back to my room and took off the motor tray and canopy, and am ready to epoxy a 20+ inch graphite spar top and bottom.

One more thing before I do this, for the bottom spar, I'm going to put it at the CG. On top I was planning on putting it much closer to the trailing edge as the Razor is particularly weak at the corner of the motor cutout and I thought this will strengthen this area. Is that OK? or should both be at theCG for the best flutter resistance?

Thanks again,
Andrew

[avie]

for the bottom, you can use fiberglass tape and simply stick it to the bottom of the zagi. it is strong enough to handle the stresses, and much more comfortable:)

Avi