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Old May 11, 2011, 04:17 PM
or F, J, K, or even TD
FLY F3B's Avatar
Joined Jun 2007
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Sorry James, there is simply no difference once a trim setting is achieved whether it is a small (like 1/32" to 3/64") deflection in a control surface or an angle of attack change on the whole stab. I challenge anyone in a blind flight test to tell the difference of two identically trimmed models, one shimmed one not, to tell the difference.

Mike (I don't fly crappy handling gliders) Smith
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Old May 11, 2011, 04:19 PM
or F, J, K, or even TD
FLY F3B's Avatar
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John, there really isn't a need for too much discussion on the topic, it is simply that some people care about surface alignment more than others. I do to a certain degree, but not nearly as much as other dudes.

You should do some "blind" flight tests John

Mike
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Old May 11, 2011, 05:18 PM
usaf3kteam
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It is my opinion that the CX2 needs no shimming required. You might move CG around to your liking long before shimming. I have three CX2's and no shims in any of them. I here this alot and set up must be wrong as Mike and I have no problems with it at all. I commonly ask where is your CG and I get don't know not sure well you need to know that you have a proven starting point. The CX2 has a very large CG range and a pilot should be able to find a spot that works for them. The CX2 does not need lots of camber for thermal start with 1mm flat bottom run in low sink conditions and around 2 to 3 mm for sinking conditions and fly the tail for best angle. I will see John and help him out and let you know maybe it needs shim I don't think so from my own experience.
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Old May 11, 2011, 05:23 PM
Jim C
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Quote:
Originally Posted by FLY F3B View Post
John, there really isn't a need for too much discussion on the topic, it is simply that some people care about surface alignment more than others. I do to a certain degree, but not nearly as much as other dudes.

You should do some "blind" flight tests John

Mike
I launch into the sun sometimes, does that count?

BTW I agree with you on the difference. The issue would need to be large to feel. (three layers of duct tape)
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Old May 11, 2011, 06:05 PM
Where is the lift?
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The only argument that I have ever heard was related to a "deadband" around the neutral zone of the elevator. For the guys shimming I would think that would put them into the deadband area and not the opposite. This is one for GT to chime in on. I am not sensitive enough to know the difference. I agree with the Blind test to remove any mental bias.

Charlie
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Old May 11, 2011, 06:50 PM
excelling at mediocrity
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Joined Mar 2009
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The Path to Enlightenment

George Charlie and Mike I was hoping you'd chime in...your input is most appreciated. I want to start off by saying the only reason I have for shimming the stab was for launch behavior.

This is the first plane I've ever had that has under-camber. I didn't know that until I had been flying them both for a couple sessions. Yesterday was the first time I ever actually used reflex for launch/speed mode. I saw that it changed things a bit.

The day I maidened the planes I set the CG at 86. Both planes would balloon upward on launch even with a lot of down elevator. I saw that as drag and sought to eliminate the ballooning and the down elevator. After the shimming it would fly straight with very little down elevator.

Fast forward to yesterday...oops undercamber! Start over... my impression after yesterdays flights led me to think the shims were OK in fact I want to try another layer. I know my ignorance is shining brightly right now but it has been fun nonetheless getting these awesome birds dialed in.

I certainly see all your points concerning the blind test and my lack of experience prevails. When I see an elevator deflected down a couple mm. I think drag... When I first saw that I thought "tail heavy" but the CG double-checked at 86. Combine that with looping launch behavior and I thought "decelage". If you have to add down elevator during a launch doesn't that add drag? I wanted to "see" a neutral elevator when launching. After that, at all the slower speeds, not a big deal...

Luckily I have two planes to fiddle with. Thanks guys some day I might have some expertise to share! I invite you all to set me "straight" on this...please!

John
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Old May 11, 2011, 07:12 PM
Maurice
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Seeing as I started this mess let me make it worse and hope as Charlie said that Gerald or some else with his knowledge to jump in.

My understanding is that we have spent a good deal of time developing airfoils for the horizontal and vertical stab. If the angle between the wing and horizontal stab is incorrect then elevator trim is required to compensate. If the required compensation is down elevator that gives you a highly cambered, very draggy airfoil and defiantly not what was originally designed. Shimming gives correct angle of attack and allows you to again have the stab airfoil as designed.
When you are a week launcher it's pretty obvious. If you already launch to the moon you probably wouldn't care any way :-)

Now the hope is for some one to rip me a new no I mean straighten me out.
Maurice
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Old May 11, 2011, 07:53 PM
excelling at mediocrity
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It has been my understanding as well that these airfoiled tailfeathers have been designed to optimize high-speed flight behavior, i.e. launch. It has been my experience that when flying slower any difference is small.
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Old May 11, 2011, 08:49 PM
a.k.a. Matt Nelson
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Quote:
Originally Posted by cptsnoopy View Post
The only argument that I have ever heard was related to a "deadband" around the neutral zone of the elevator. For the guys shimming I would think that would put them into the deadband area and not the opposite.
My understanding is that it was the old plank stabs and fins that exhibited deadband behavior. One of the big advantages of modern sections is that they don't do that.

As far as "decalage trim" vs. "elevator trim" and their effect on drag - seems to me that "lift"="drag" no matter how you get there, so the two methods have got to be essentially the same (think full-flying stab vs hinged elevator) - should especially be true with the small adjustments we are talking about.

-m@
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Old May 11, 2011, 08:57 PM
Tom Knudtson
Redlands, CA
Joined Aug 2004
574 Posts
Normally Phil Barnes responds emphatically to claims of decalage affecting performance. But in his absence I offer up this old post from Mark Drela on elevator trim (which I got from a Phil link). I think decalage is angle between the lines through the leading and trailing edges of the wing and elevator.

"The importance of decalage is way overblown.

First of all, if you have an all-moving tail, then obviously "setting
the decalage" has no meaning. You just set the elevator trim to
whatever it has to be for your CG position and desired trimmed speed.

It turns out that for a hinged elevator, decalage should also have
little effect. You can change pitch trim either by moving the
elevator or moving the entire tail. There should be little or no
difference in pitch stability or elevator response between the two
types of changes. This assumes that the tail airfoil is reasonably
good, and the elevator deflection is modest so that the tail's
cl(alpha) and cl(elevator) curves are still linear. The only reason
to change the decalage (angle of the front stabilizer) in this case is
to null out the elevator trim deflection. This will give more
symmetric elevator range, reduce elevator servo load at high speed,
and perhaps reduce the tail's profile drag very slightly.

If there IS a difference in handling from a change of decalage, then
you surely have poor airfoil flow over the tail, with some combination
of dead band or hysteresis. This also indicates the presence of large
separation bubbles and high profile drag. A simple slab airfoil with
a semi-circular LE is very common, and is prone to these types of
problems. Making the LE more elliptical rather than round should give
a significant improvement."
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Old May 11, 2011, 09:42 PM
excelling at mediocrity
gopherhunter's Avatar
Franktown, CO. USA
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Quote:
Originally Posted by TomK View Post
...(Mark Drela) "The only reason
to change the decalage (angle of the front stabilizer) in this case is
to null out the elevator trim deflection. This will give more
symmetric elevator range, reduce elevator servo load at high speed,
and perhaps reduce the tail's profile drag very slightly..."
Thanks, Tom... This quote describes what I expected to happen by shimming the stab (leave out the part about servo load), with the effect being a launch with a straight trajectory. The CX2s handling is superb and I didn't notice an obvious handling change by shimming the stab.

John
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Old May 11, 2011, 11:01 PM
Maurice
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Quote:
Originally Posted by TomK View Post
If there IS a difference in handling from a change of decalage, then you surely have poor airfoil flow over the tail, with some combination of dead band or hysteresis. This also indicates the presence of large separation bubbles and high profile drag. "
So us wimpy launchers who can't throw hard enough to push past the drag being created by the down trim do notice an improvement.

Nany Nany Na Na and thats the trute!

I won! I know I won! I really really won! Ya

Maurice
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Old May 11, 2011, 11:16 PM
Where is the lift?
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USA, AZ, Phoenix
Joined Nov 2005
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Except for the statement "push past the drag". Lol

Looking forward to flying and partying with you guys real soon!

Charlie
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Old May 12, 2011, 12:24 AM
Maurice
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Bakersfield, Cal.
Joined Jul 2004
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Me to Charlie. Wait untill you see how deep the finely cut grass is in the launch area at Visalia. You almost feel guilty about walking on it :-)

Maurice
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Old May 12, 2011, 12:39 AM View Post
amelie
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