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| Posted: May/15/2018 at 10:36pm |
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Wellllll, I can’t say I didn’t believe you as I never read any posts on when when I was building but, I do agree with you! It does need it. Anyhoo, ballooning on power is easier to control than going the other direction I guess! I wonder whether top wing adjustment is better anyway? I suppose I don’t have any other option now at any rate.
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kuhn
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| Posted: May/15/2018 at 11:52pm |
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Just for my own information and in an attempt to learn about this incidence stuff ... 
Why does one only add shimming to the top wing and not the bottom one too?
If one adds 1/6" to the top and 1/6" to the bottom ... Is that the same as adding 1/8" to the top wing only? You know ... because there are two wings  .
I have a small electric Tiger Moth that balloons and the tailplane is glued on so only wings are shimmable.
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If in doubt. Don't!
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Building: Getting my Mo Jo back slowly to finish a few projects.
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avipatable
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| Posted: May/16/2018 at 2:17am |
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That's a good question, Kuhn, and a lovely little Tiger Moth!
By adjusting one and not the other is the net effect the airflow trying and squash the 2 wings together when flying, which would also increase drag.
Adjusting the incidence for the stab would also be a drag inducing exercise - but a fix for a wing producing too much lift for the flight envelope.
The correct shaped aerofoil for the range of speeds should allow for everything to be pointing the right way - that is the fundamental issue isn't it?
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ERFR
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| Posted: May/16/2018 at 3:12am |
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Shimming both wings by the same amount or shimming one wing by double the amount may seem the same but remember the wings are not alone...
Since there is also the horizontal tail flying together with them, then both options are not the same as effectively the incidence of the horizontal tail relative to the wings will be different in each case.
Ballooning is easily fixed with a little positive incidence in the horizontal tail.
The aerodynamic explanation behind this is that in comparison with a monoplane, the biplane can be compared so to speak, to a monoplane with a low aspect ratio wing.
A low aspect ratio wing means the wing tip vortices are ‘closer’ to the aircraft center line hence the downwash they produce on the horizontal tail is stronger in comparison with an aircraft with a high aspect ratio wing.
This stronger downwash effect causes a stronger ‘nose up’ moment that becomes even stronger when speed increases (because the wing tip vortices become stronger).
To compensate for all of the above, we give the horizontal tail some built in positive incidence which, when speed increases, reduces the downward force/lift normally produced by the tail, and that reduces the ‘nose up’ response to the increased speed.
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Sparky
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| Posted: May/16/2018 at 8:11am |
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You guys are way over thinking this!
Why shim one wing over the other... the reason I did the top wing was it was not detectable to the casual observer......
Since the lower wing had a dowel in the front and bolts on the back I would have to carve the rear part of the wing saddle along with the hardwood blocks and I didn't have the skills and knowledge and tools to do this without making a mess of my scale airplane. I did OK with in the three local contests I flew in.
Back then it was a pattern contest all day Saturday then 3-4 rounds of Scale on Sunday and possibly another round of pattern after that.
In several contests I flew in the Novice pattern with the Mini plane then Saturday night I would put the scale rigging on the plane got judged on scale and flew in the scale portion and then if a last round of pattern was flown I would pull the rigging off the plane change props and pattern time.
I got lots of flying in that way and my landings always got better.
Sparky
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kuhn
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| Posted: May/16/2018 at 8:14am |
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Eerrmm ... If you say so  .
No seriously ...I do get it .
Thanks Erez  .
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Kuhn
If in doubt. Don't!
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Building: Getting my Mo Jo back slowly to finish a few projects.
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Sparky
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| Posted: May/16/2018 at 8:19am |
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On the Tiger Moth, very nice 
The speeds on the full scale tiger moth are pretty narrow. Small speed band so there is a lot less of the effects you describe. as a model it is likely we are trying to fly a much broader speed band and consequently this issues come up.
The typical fixes are more down thrust to reduce ballooning/pitch upon power addition.
Increase positive stab incidence which is more for the entire speed range.
Both will also have other effects but that is what you are trying to accomplish by doing this.
Sparky
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avipatable
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| Posted: May/16/2018 at 10:06am |
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Thanks very much Erez and Sparky, as long as I am learning I am happy!
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