More questions about wing numbers....

[Dave Stroud]

More questions again...please.  The plans and the narrative indicate that the wing angle of incidence is set by having the front spar one inch higher than the rear spar and that is fixed by the wing mounts being built into the fuselage, thus attaining the required 2 degrees. This whole scenario seems to  be based on the idea that the bottom of the front spar should be one inch higher than the bottom of the rear spar. So...in order for that to be accurately done, could we assume that the front and rear spar wing attach fittings should have their fuselage mount holes the same distance from the bottom of each spar ?  I would think so, but my plans on page 35 seem to indicate otherwise.

My main spar is about 6 7/8" tall. My rear spar is about 5" tall. If you look at page 35 in the plans it says the wing attach fitting hole at the fuselage end should be 5" down from the top on the front spar and that would leave the hole about 1 7/8" up from the bottom. On the same page, if you look at the 3 3/4" drop from the top on the 5" tall rear spar, you are left with only 1 1/4" up from the bottom. That seems to be about 5/8" vertical difference between the two spar fitting holes where they meet the fuselage. 

Any ideas appreciated....thanks.

[ArcticDave]

I haven't built my wings yet...but I would space them the same from the bottom and disregard the top measurements...that was my plan anyway. 
I'm sure somebody else will chime in with some better info.

[Dan_]

Don't worry about the spars...  The bottom of the wing is flat.  Go off that.  Cut out a plywood (luan) airfoil from a copy of the rib drawing, and mock up the spar fittings to this.  With a level on the backbone longeron, get the 2 degrees at the root either by trig or cell phone app and build the front and rear wing mounts accordingly.  The bottom of the root is 2 degrees the tip is rigged level.  This sets your washout...

https://play.google.com/store/apps/details?id=com.plaincode.clinometer&hl=en

[Dave Stroud]

Thanks, Dan...but we're not talking about washout here. We're talking about the vertical measurement of the wing root spar fitting's bolt hole in relationship to the bottom of the wing spar, front spar vs. rear spar.  I think they should be equal....front and rear spar, yet the plans on page 35 say different.

[Dan_]

Thanks, Dan...but we're not talking about washout here. We're talking about the vertical measurement of the wing root spar fitting's bolt hole in relationship to the bottom of the wing spar, front spar vs. rear spar.  I think they should be equal....front and rear spar, yet the plans on page 35 say different.

What I am saying is the bottom of the wing is flat...  Take advantage of that fact to figure out how to proceed.  

You will be able to take advantage of it when you are ready to rig the lift struts as well...

Cheers.

[ArcticDave]

I see what Dan is saying Dave. If you like I can measure my fuselage. Mine should be the same angle as I didn't change wing position at all. I'll do it later after the sun comes up. If the fuse has the required 2° the wing fittings should be spaced the same.

[Dave Stroud]

No need to measure, thanks Dave. I get what Dan is saying. I guess I'm just checking more or less continuously as there's been a few things to wonder about. It throws me off when the numbers on the plans are dead wrong and left up for the builder to interpolate.

[ArcticDave]

Well, you got me curious...so I measured it anyways. My top tube is 1.8-1.9°
Thats with a smart phone app so no idea as to it's accuracy.

[Dave Stroud]

I cannot seem to cut and paste an URL lately so to see the site I use for distance / angle calculator you'd have to google search    angular size calculator   . It's easy and versatile.  It shows that 2 degrees over a distance of 29" is 1.0124".  Conversely, if Arctic Dave seems to have only about 1.85 degrees, that would show a vertical difference of .93645".   Another handy example is that the dihedral is supposed to be 3 degrees and the calculator shows that three degrees over 154" would be 8.0653 inches. Nice and easy.

[Dan_]

I cannot seem to cut and paste an URL lately so to see the site I use for distance / angle calculator you'd have to google search    angular size calculator   .

This it..?

http://www.1728.org/angsize.htm

[Dave Stroud]

Yep...nice one, Dan.

[ArcticDave]

Since the subject has been broached...why is 2° the standard?
I was just given a set of Air Camper blueprints that were purchased in the early 60's. The Air camper has 2° as target AOI as well. A PA-18 on the other hand is spec'd at around 4.5-5° bone stock. A Cub setup for STOL competition can have 7° or more. I've been pondering the benefit of increasing the root angle a bit for a couple of months, and am seriously considering dropping the front wing fitting an inch to double that to 4°. I think slower flight will be possible... at a cost of cruise speed

[Tom XL-7]

Ok, I think I get what your original question is or was.  Follow along there could be an error or two in this statement, and I would love to be corrected.
 I will agree that a 1" drop over 29 inches (length of top tube) is for practical purposes 2 degrees
I also see that the front spar bottom and the rib bottom is the same.
The rear spar sits over the bottom cap strip which is 1/4 inch. 
the bottom of the rear spar sits 3/8 inch higher than the front spar bottom by the differing mount locations.
 lower the bottom rib cap strip 1/4 inch and we are within 1/8 of an inch
As in 1/8 inch too high in the rear. 
If it is going to keep me awake at night I could fix it while drilling the rear spar mount holes, set the top tube to be a little heavy on the fall,
or work it out on the wing mounts as I fit and weld to the fuselage.
Or a little in all three places. Sort of sneak up on it.
There is also some slop in the rib to rear spar fit up.
 
I imagine many have been built without any concern and are all over the place. No two planes exactly alike yet they all fly well.
Thanks for pointing it out.
Tom XL-7

[Tom XL-7]

Since the subject has been broached...why is 2° the standard?
I was just given a set of Air Camper blueprints that were purchased in the early 60's. The Air camper has 2° as target AOI as well. A PA-18 on the other hand is spec'd at around 4.5-5° bone stock. A Cub setup for STOL competition can have 7° or more. I've been pondering the benefit of increasing the root angle a bit for a couple of months, and am seriously considering dropping the front wing fitting an inch to double that to 4°. I think slower flight will be possible... at a cost of cruise speed

The air camper and pipers are different airfoils and each may have its own preference. STOL competition is a specialty situation after one goal at much expense to other missions. ultralights are usually very draggy and I would not want to build in any extra.
Make the stabilizer mount posts a little long and experiment where it can be adjusted.
(I have no idea why the font just changed)
Tom XL-7

[ArcticDave]

Idk why it changes like that. Does it to me too. It sorta makes it look like I'm yelling...weird.

I agree with with you Tom. It depends on mission. A guy trying to shoehorn himself into short strips could find more angle helpful. I'm sure 2° was chosen as a best "all around" angle. 
The stabilizer suggestion is a good one...is there even room to get it down to -4°? Without going under the top longeron?