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  #1  
Old 02-26-2013, 05:53 PM
J_Turner J_Turner is offline
 
Join Date: May 2011
Location: Richmond VA
Posts: 86
Default The quest for perfect elevator alignment! NOT!!!

Hello everyone,
Lets start this off with a little history....
While waiting on the quick build fuse and wings for my brother's -10, I ordered a RV-7 empennage... I assembled everything except the elevators. Shortly after that I found a good deal on a RV-7 quick build fuse, wings, and finished empennage, so I bought it knowing that I wouldn't get around to building until the -10 was complete. That was a few years ago.....
The -10 is complete and flying, and performs EXACTLY as Vans says per their specs. Top speed and climb etc. I am very impressed!
Now on to the -7..... been working on it for a few months. I wanted to finish up the tail rigging a few weeks ago and that is where the problems begin. I leveled the fuse and mounted the horizontal stab (built by the previous builder). Next I attached the elevators (also built by previous builder) and noticed that when the counter balance arms were clamped to the horizontal stab that the elevators were not in trail. I checked everything that I could and adjusted the pivot points in and out until I decided that something was wrong with the elevators... I spent the next 5 days building the elevators from my original kit making sure everything was perfect. (except for ONE thing...the tips!) I used a bending brake for the trailing edges and used huge blocks of machined aluminium to make sure nothing was twisted.
I mounted the newly constructed elevators friday and saw almost the same twist / mismatch with the "other" elevators although it was a smaller distance between the mismatch now the difference is 1/4" at the counter balance tips when the trailing edges are parallel. I removed the elevators to check them for flatness and they are perfectly flat but the counter balance arms aren't the same. Before I riveted the elevators together I tried to move the tip rib assembly around and if you move it either up or down it distorts the trailing edge so, there is only one way it goes together.
I spent some more time thinking it over and for the life of me I can't figure out why the CB's are not the same. So, I made the trailing edges parallel and drilled the horns. I kinda wished I would have asked this question earlier but oh well.....

How do you make perfect elevators?
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  #2  
Old 02-26-2013, 06:14 PM
J_Turner J_Turner is offline
 
Join Date: May 2011
Location: Richmond VA
Posts: 86
Default


Last edited by J_Turner : 02-26-2013 at 06:22 PM.
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  #3  
Old 02-26-2013, 06:43 PM
J_Turner J_Turner is offline
 
Join Date: May 2011
Location: Richmond VA
Posts: 86
Default

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Old 02-26-2013, 09:12 PM
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Arie Arie is offline
 
Join Date: Jul 2010
Location: Dubai
Posts: 137
Default Horizontal stab

Good day, are you sure it is not the horizontal stab that is twisted ,and not the elevators that are twisted? Just me thinking out loud.
Arie
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  #5  
Old 02-26-2013, 10:09 PM
DaAV8R DaAV8R is offline
 
Join Date: Jul 2009
Location: Lee's Summit, MO
Posts: 751
Default Modeler?

I see your Robart incidence meters so I assume you model. I think there has to be a bit of twist in the elevators or in the horizontal as already suggested, maybe a tiny bit in both. I think the likelihood of twist when building pre-punched surfaces is directly related to the geometry of the surface. The more acute the angle, the more likely to twist. That is just my opinion and others might disagree. In a conversation with Van, he stated that when pre-punching was suggested he did not think it could be done for this very reason.

If you are concerned with the esthetics, maybe you could spit the difference between the counterweights and the trailing edge alignment. How many models have you flown where the control surfaces didn't trail in perfect alignment. Unless you are flying pattern it doesn't make a bit of difference.
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Lee's Summit, MO
RV-8 - Empennage & Wings Done
Working on Fuse
O-360-A1A

1946 Cessna 120
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  #6  
Old 02-27-2013, 08:52 AM
J_Turner J_Turner is offline
 
Join Date: May 2011
Location: Richmond VA
Posts: 86
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The HS does have a small amount of twist in it. Barely measurable with my incidence meter. If you take the elevators off and lay them upside down on a table and measure the height to the center of the CB tip that is where the big difference is. Its right at 1/4" difference so I offset the CB's 1/8 on each side of the HS and drilled it.

As far as model planes.... yes, I have flown planes with totally mismatched elevators and they fly, well.... O.K. My 40% Carden has excellent alignment on every surface and it flies GREAT! Very little trim difference between high and low pattern speeds. Never flew a pattern plane but I did fly IMAC with the Carden Giles G-300 and the plane did great..... lets not talk about the pilot.
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  #7  
Old 02-27-2013, 09:43 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,842
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My theory- when the elevator skin was being bent (at the factory / or finish bending by builder) affects the final "twist" alignment between the two elevators. Even a pencil line width difference between the bend line and the upper & lower prepunched spar holes results in the mismatch of angles.
What to do- clamp the CB horns to the HS & drill. Elev. trim tab will compensate for the mismatch in flight & ground bound critics don't have fodder over a preceived error in elev horn alignment. Keep on building
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built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more
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  #8  
Old 02-27-2013, 10:34 AM
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longranger longranger is offline
 
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Location: 45G, Brighton, MI
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Quote:
Originally Posted by Ralph Inkster View Post
My theory- when the elevator skin was being bent (at the factory / or finish bending by builder) affects the final "twist" alignment between the two elevators. Even a pencil line width difference between the bend line and the upper & lower prepunched spar holes results in the mismatch of angles. ...
I totally buy into that theory. Here's a gross example. One of my aileron skins was misbent at the factory, causing a half hole diameter (3/64") misalignment between the top and bottom. Here's what happens when you force the holes into alignment:


(The good news is, Vans replaced the skin.)
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Miles (VAF# 1238, Paid up as of 12/20)
RV-7 TU 904KM (reserved)
Wings Fitted and Finish Kit on site

Construction Log

1955 Cessna 170B flying since 1982

'To get something you never had, you have to do something you never did.' -Unk.
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  #9  
Old 02-27-2013, 03:33 PM
J_Turner J_Turner is offline
 
Join Date: May 2011
Location: Richmond VA
Posts: 86
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That picture was exactly what my trailing edge would look like when I moved the CB rib assy. (with only a couple of clecos in) up and down before riveting. With all of the clecos in, the trailing edge was correct and straight.

Thanks for the input guys, I'm moving on..

Jamie
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