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Setting Wing Dihedral Using a Digital Level

KJC

KJC

FRIEND
Clear Lake, MN
My PA-12 is slightly left wing heavy and I would like to start the entire rigging process over to make sure it's correct. I'm familiar with both the L-21 Erection Manual and ac 43-16. The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed. Because the aircraft has extended wings with booster tips, I'm not sure this method will produce entirely accurate results.

I understand my heavy wing is probably a a washout issue and a turn in on the left rear lift strut will probably fix it. What I'm asking about is DIHEDRAL not washout (or twist if you prefer).

With the the aircraft leveled, is there a proper number (or at least agreed to by most) for DIHEDRAL when measured under the front spar using a digital level? Both wings are currently set at .81 degrees with an equal number of threads showing on each fork.
 
KJC said:
My PA-12 is slightly left wing heavy and I would like to start the entire rigging process over to make sure it's correct. I'm familiar with both the L-21 Erection Manual and ac 43-16. The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed. Because the aircraft has extended wings with booster tips, I'm not sure this method will produce entirely accurate results.

The 3 1/8 " number is for stock wings.
If you are running the string from the top of the last full rib just out board of the original aileron then you would get good results.

If going from the extended tip then you will not have enough dihedral .
Click to expand...
 
you need to TAPE the string DOWN to top of wing above front spar, at ORIGINAL end of aileron rib location, then measure.... simple
 
As long as the dihedral is symmetrical, 1 or 2-10ths of a degree above or below spec will have no perceptible effect on how it flys. Arguments can and have been made for adding it, and reducing it. I advocate setting dihedral with the string method because it eliminates the guesswork of determining the true and correct level reference. Just be sure to measure very precisely. Once you've done that, twist can be easily set with the smart level. Twist usually has to be tweaked a little anyway, no matter how theoretically perfect it is. Take a little twist out of the heavy wing.
 
PerryB said:
As long as the dihedral is symmetrical, 1 or 2-10ths of a degree above or below spec will have no perceptible effect on how it flys. Arguments can and have been made for adding it, and reducing it. I advocate setting dihedral with the string method because it eliminates the guesswork of determining the true and correct level reference. Just be sure to measure very precisely. Once you've done that, twist can be easily set with the smart level. Twist usually has to be tweaked a little anyway, no matter how theoretically perfect it is. Take a little twist out of the heavy wing.
Click to expand...

Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
 
On floats I'd want enough dihedral to add to longitudinal stability. I suggest 1* at least and test from there. Extended wings require some more than stock in my experience especially when floats require added rudder springs or ventral fins. Dihedral is our friend.

Gary
 
Do the digital level three different places on each wing. You will be surprised at the variance.

Before you do that, take your bird aloft, trim it for stable flight, let go of the stick, and take a good look at both ailerons. Both should look the same - if one is slightly up and the other slightly down you have an aileron mis-match. It takes a lot of differential wash to fix that. Better way is matched ailerons.
 
J5Ron said:
Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
Click to expand...

Are you holding the ball centered with your feet when the wing rolls right? If so then, yes the wing is heavy.
 
KJC said:
The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed.
Click to expand...
It is important to measure each wing spar (not just one side) to ensure that the fuselage is symmetrical to the wings. The plane can fly straight with the fuselage cocked at an angle. If the fuselage is not symmetrical the ball will not be centered and you will be pulling your hair out wondering why you are having difficulty with the rigging. Be sure both wings are the same with the fuselage first.
 
In my notes I have .7 but I can tell you when you start putting a digital level along the wing your measurements will be all over the place. I use a long straight edge and measure it inboard of the strut attach fittings.
 
J5Ron said:
Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
Click to expand...

Its a figure of speech. If you let go of the stick and the aircraft begins to bank right, the right wing is said to be "heavy". Either way, shortening the rear strut on the heavy wing cures the problem, assuming everything else (such as dihedral, and flap/aileron rigging) is true and symmetrical.
 
J5Ron said:
Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
Click to expand...

Ron,

People tend to fixate on fixing the so called heavy wing. My approach is to ask the question "do I want more or less washout after the rigging". The reason this is important is if you look at cub rigging in general the amount of washout is all over the place ( this is just my observation over time). I have seen cubs with flat wings and cubs twisted to 2.5 degrees at the outboard measuring point. Both of these scenarios are incorrect (per the instructions) but sometimes this done intentionally to make tweeks in flight characteristics. In the case of the flat wing you would do the opposite of what is being suggested and you would remove lift(add twist) from the "light wing". In the case of the over twisted wing you add lift(remove twist) to the "heavy wing".

With regards to dihedral - I have never been able to warm up the digital level and pull out the string.

Have fun - Jerry
 
Am I the only one to still use a adjustable bubble protractor? I shy away from the digital levels.... yeah I know it's old school, but I've used the same Starrett protractor for 30+ years, and am still slow to get on board with the digital stuff. I certainly don't think they are better, just because they are digital. My technique when setting washout and other things that I want as accurate as possible, is to "crowd the line", I don't try to center the bubble in the middle of the lines, I go for the bubble just barely touching one line, the important thing being the opposite side is also set using the same technique but opposite. Digital users will laugh, users of the old school adjustable protractor will know what I mean, I hope. I get more consistent results using the whiskey bubble, the digital stuff seems to give different results a few minutes later, when nothing has moved.....
 
Lots of chatter here and elsewhere regarding the relationship between wing rigging and flying performance. Flying performance being takeoff, landing, and even flight between those events.

I suggest some experimentation if the pilot's curiosity is sufficient. If not then the manufacturer's design will suffice for most. But for some the improvement is worth the time to explore.

It has been for me at least with several planes and gear configurations.

Gary
 
courierguy said:
Am I the only one to still use a adjustable bubble protractor? I shy away from the digital levels.... yeah I know it's old school, but I've used the same Starrett protractor for 30+ years, and am still slow to get on board with the digital stuff. I certainly don't think they are better, just because they are digital. My technique when setting washout and other things that I want as accurate as possible, is to "crowd the line", I don't try to center the bubble in the middle of the lines, I go for the bubble just barely touching one line, the important thing being the opposite side is also set using the same technique but opposite. Digital users will laugh, users of the old school adjustable protractor will know what I mean, I hope. I get more consistent results using the whiskey bubble, the digital stuff seems to give different results a few minutes later, when nothing has moved.....
Click to expand...

I have found the rigging process is much faster after I switched to a quality digital level. The airplane either flies wings level the first flight or after a half turn of a rear strut.
 
FWIW ........... I find the digital level is "more convenient" but a good "spirit level" is more accurate if you need to split hairs. Any good level advertises accuracy withing .005 per foot. If you use that dimension, then a good bubble level is almost 4 times as accurate as a digital level that uses a tenth of a degree increments. One degree in one foot is about .210. .... so .1 of that would be .021. There are a few good levels that have both the bubble and digital scale in the same frame. That way you can have the convenience of the digital and accuracy of the bubble.............. but the accuracy of the bubble is also in the "reading" of the vial. Either way I have found, like Steve, that you are usually within a half turn or so of a rear fork to get it to fly level. One thing that did make me chase it once was a drooped tip that had been "repaired" at some point and was slightly different from the other side. That did throw a little sheeee-at in the game but finally found it......... doesn't take much difference in a tip shape to make it roll.

As for brand, I don't have any strong preference......... I use Husky, Johnson and Stanleys. If using a digital you just gotta make sure it's adjusted to zero. And check occasionally, both digital and bubble types. If a bubble level ever gets off for any reason, toss it.
 
Thanks for confirming my suspicion that the bubble splits the hair finer, too fine to bother with sounds like! I do like my digital center finder I use when pipe fitting, and my inclometer app on my phone to see how steep a LZ is. But, I'm not getting rid of the Starrett for setting my wing washout, my experimental flies too well to change anything! Thought of this thread while coming back from breakfast today, hands off/feet off for long periods of time, real nice air of course.
 
You guys are way better with a spirit level than me. I can zero my digital level out on the level part of the fuselage and go straight to the outboard aileron bay rib and set to the desired angle. Don't have to level the fuselage etc.
 
This is a great thread...thanks for getting it started KJC. So my experimental PA12 has had this nasty roll to the right...heavy wing right....???. Today went to the hanger and double checked things after stuying this thread. My dihedral was off about a quart inch...I changed that which in turn allowed the washout to match the other side perfectly....end result no roll, hands off. Thanks everyone...who new a quarter inch was so significant.
 
Smart Level. It does have a bubble. I can check an airplane in ten minutes flat. It takes me around an hour to find the jacks and level a J-3, and I cannot even lift the tail of an 18 without a helper. I am with Steve. I did my first J-3 in 1969, before Smart Levels existed.
 
I had a couple friends building a Glastar and getting down to the nitty gritty with the smart level when they set dihedral. I told them to use a string and a bubble and tape...but they wanted to use the "Smart level"....couple days later, I saw the UPS guy at lunch and he asked them if they got the new struts he'd dropped at the back of the shop. Come to find out, they'd gotten the level switched from degrees to %grade..... and drilled the strut holes. After that, they called it a "dumb level" hahaha!
John
 
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