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"Squirrelly" SPAD Gnat

Old 01-15-2005, 07:27 PM
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-pkh-
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Default "Squirrelly" SPAD Gnat

My Gnat is acting a little "squirrelly"... with changes in air speed, or very small changes in elevator controls, it wants to dart up or down. I've read that "reflexing" the ailerons up a little helps alleviate this problem. What bothers me about doing this is my elevator trim for level flight is already a few degrees down, so I'm afraid I already have the ailerons up a little too much.

I'm wondering if this "squirrelliness" (and down elevator trim) is characteristic of a tail heavy plane... I put the CG exactly where the plans spec'd it, 4-1/4" back from the LE of the wing, but maybe it needs to be brought forward a little...

Any help/suggestions appreciated...

Thanks!
Old 01-16-2005, 12:25 AM
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ChrisSpad
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Default RE: "Squirrelly" SPAD Gnat

reflexing the ailerons up will (to a degree) alleviate the need for down trim - your in essence making the ailerons into spoilers and killing lift. I've read that some people like the cg at 3 3/4 - but, i've never built one so i can't say for sure.
Old 01-16-2005, 06:43 PM
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Default RE: "Squirrelly" SPAD Gnat

I talked to one of the more experienced guys at our club, and he said the "porpoising" flight characteristics of my plane indicate it is tail heavy. I have a weighted brass prop nut that I can bolt on to shift the CG up w/o modifying anything, so I'll give that a shot and see if it handles better... if so, I'll remount my batt pack to bring the CG up permanently.

This darting up/down or "porpoising" seem to get worse when the nitro tank runs down... further supporting the tail-heavy theory. I'll post again when I get a chance to experiment with moving the CG up.

Correct me if I'm wrong, but reflexing the ailerons up would require more down elevator trim for elevator flight, wouldn't it? The raised ailerons will provide down force behind the CG causing the plane to go up... so some down elevator trim would be required to counter it, right? I think that's why flaperons usually require some up elevator trim/mixing when the ailerons are lowered to act as flaps... down ailerons cause the plane to go down, and up elevator is required to level it off.
Old 01-18-2005, 08:13 PM
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MrGreenSpeed
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Default RE: "Squirrelly" SPAD Gnat

Some up reflex is the cure for a "Squirrelly" SPAD .
As for triming in a little down, it may be needid too.
Let us know how it go's
Jeff
Old 01-18-2005, 09:47 PM
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Default RE: "Squirrelly" SPAD Gnat

Just wondering.....Did you balance the Gnat with the tank empty or full. Another thing....is all the hardware on the fuse rail lashed down and not able to move about (like the batt. tucked away in the wing fold)?
Fred
Old 01-18-2005, 10:45 PM
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Default RE: "Squirrelly" SPAD Gnat

I always balance them with empty tanks. Everything is lashed down tightly. I have two of these Gnat-like SPADs (one per the plans, and one that's scaled up to handle a .35 engine), and I'm getting this high elevator sensitivity and "porpoising" behavior even with the ailerons reflexed up a good amount. Everything seems to be adding up to them being tail heavy...

I modified my small Gnat by upsizing from a .15 to a .25, and I noticed the original CG I had was about a 1/4" forward of the spec'd CG (I put the CG at spec when I added the new engine). It flew better with that forward CG, but still had a pretty sensitive elevator. It also flew better on a full tank.

As soon as the weather permits... I'll just bolt my brass prop nut/weight on my small Gnat and see what happens. If it flies much better, I'll put my aluminum spinner/nut back on and just move my batt pack to get the same CG. I'll let you guys know what I find with my CG experiments.

BTW, the Gnat (built per the plans) with an OS .25 FX is a blast (even though it was squirrelly!)... my Enya .15 barely kept it in the air, but this .25 really moves it around. After I sort out the CG issue with the small one, I can't wait to fly my larger Gnat with it's upgraded engine... an OS .50 SX ringed engine! I scaled the Gnat plans up to give about the same wing loading with a .35 engine as the .15 plans had. I figured if the .15 Gnat can handle a .25, then my .35 version can handle a .50! I'll let you know how it flies . I really like this SPAD design because it requires no wing gluing, assemble much quicker than PVC fuse SPADs, and is much more rugged than PVC fuse SPADs... I have only broken props and engine mounts in crashes. Plus, I expect to be able to land these planes on snow, since they have no gear and are belly landed.

Tattoo rules! What more can I say!

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