Setting wings to fuselage (a.k.a. calculating wing sweep)

[dholly 718]

When I set my wings to the fuselage to locate and drill the front spar pin holes, I ended up with a very slight amount of forward sweep. Using (4) plumb lines on the leading edge, (1) at each wing tip and root, I aligned all (4) plumb lines horizontally and squared wings to the fuse by measuring from tailpost to each tip rib flaperon hinge. Using a homemade blind-hole locating tool (thanks Russ!), I noted that resulted in hole placement about 1/2" from the end of the spar. I felt that looked like insufficient spar material between the hole and the spar end, and a quick measure confirmed the pre-drilled rear spar pin holes had 3/4" clearance from spar ends. When I adjusted to keep the same minimum clearance for the front spar pin holes, I ended up with +1/4" at each wing tip vs. their respective root vertical plumb line. That is where I eventually drilled the front spars and now I'm wondering how to accurately calculate exactly what % that wing sweep actually is. Any clue?

For reference, all Avids, Avid+ and Kitfox I-IV used 0% sweep. The wing sweep came about with the Kitfox Series 5 and 6 aircraft when installing the heavier certified engines (O-200, IO-240 and Lycoming O-235). On a Super Sport Series 7 the FWF moved the engine forward about 5" and Kitfox recommends a 1% forward wing sweep only when utilizing the heavier engine choices… not with the Rotax 912uls power. The SS can be built in the 750 to 800lb empty weight range with the 912, and typically 900lb plus with the Continental or Lycoming…

I did find one potentially helpful comment while searching for info. One SS-7 builder with the forward swept wings said:

"the 1% sweep results in about a 1/2" difference between the leading edge and trailing edge of the butt ribs. My butt ribs aren't parallel to each other; they are slightly more than 1" closer together at their leading edges than their trailing edges. Since they are 27" long, the trigonometry works out to very close to 1 degree forward sweep for each wing."

A bit confusing but I presume everything is relative to the aircraft centerline. I don't have my root and butt ribs on yet (still need to install wing tanks) but, next time I have the wings folded out and pinned, I'll measure my rib location tape marks on the f/r spars to the center stringer over the cabin and see how it compares.

Anyways, since forward wing sweep does move the center of lift forward and give a different CG envelope, I'm wondering what the net effect will be with my teeny bit of sweep. If I understand correctly it may reduce the baggage compartment load limit ever-so-slightly and, with no baggage and pilot only, may put the CG forward or close to the forward limit if I want my battery up front. Just to make matters more interesting, my Avid+ material does not include a revised W&B, just the standard Mk-IV specs.

[akflyer 1427]

What engine are you putting on?  I have seen quite a few posts of guys sweeping the wings forward on Avids and various kitfoxes when they went to heavier engines such as the VW, 912 etc, so they did not have to put so much weight in the tail.

[dholly 718]

912ul or R.A.M. built EA-81, 912uls in my dreams 

[jackak 20]

Doug,

It doesn't look like you've installed your spar doublers yet, couldn't you use them to get your wings back to o deg sweep?  I'd bet the doublers are the items that provide the real strength at the spar attach points. I know the holes in the spars might no be round anymore but does it matter?  I wonder if Steve Winder is reading this, and if he or another of the original Avid crew could comment?

Jack 

[dholly 718]

Well, probably could do that, though I'm not so sure I want/need to. I'm not concerned about excessive forward sweep, there is only a +1/4" sweep from tip to root. I have help coming over this evening to remove the wings so I can get the doublers installed. Before we do that, I'll fold the wings out and measure the side-to-side diff between the front and rear spar ends. Since all spars are 144" LOA and I don't have my root ribs on yet, that should give me more accurate measure points than attempting to use my rib location tape marks. Then, try to do some fancy trigonometry like the KF-7 example.

  Ouch,  my head hurts already...

P.S. - if you look close, you an see the pre-drilled center rivet holes on the largest doublers do not correspond with the line drawing pattern. Not my doing!   

[jackak 20]

Yes, I doubt that moving your datum about 1/8" forward will make much difference.  If you go with the Subaru it will even help.  The actual weighing will give you the true story on the w&b.  I guess you'll have to make your own pattern for the doubler holes. No biggee, eh?

Are going to leave the spacer boards in place while you glass in your tanks, or what set-up did you end up with?  Were your carry-throughs and bushings actually the right distance apart?

Jack 

[dholly 718]

Well, I measured between the spar ends side-to-side last night and the difference was 1/4" exactly front vs. rear spars. I guess that makes sense as that was the diff between the tip and root plumb lines. IIRC, the measure between the f/r spar pin holes is 27-1/2", so with the spar length of 144" those three measures should allow figuring the sweep %. Now I just gotta ask one of my kids to do the math figuring! 

I told Brett to make me up one pair each of long ribs w/ flaperon hangers, short ribs, and the special transitional root ribs for speed wings without the oem f/g wrapped fuel tanks. About my wing tank decision... I know I'm not using the oem f/g tanks and 99% certain I'll use the poly tanks, but still not exactly sure how I will install. Last night one set of eyes belonged to a EAA Tech Counselor who brought a few new suggestions to ponder. I've been reluctant to use (2) of the poly tanks in a wing, if for nothing more than the slooow fuel transfer between tanks when filling. I really don't want to spend 1/2 hour while the fuel burbles thru the tiny connecting lines. Obviously, increasing dia. of those lines is an option, as are two filler necks. I don't like the look/drag of using two raised necks/caps per wing, let alone the leak potential of the lower cap. I could use a flush cap on the outboard tank and raised on the inboard for aesthetics, hoping the dihedral and fuel equilibrium level with both tanks full would not exceed the level of a raised filler neck on the lower inboard tank. Personally, I don't think it would and therefore the cap seal become critical. Another alternative suggested would be to use (2) tanks per wing but plumb each separately. I had planned on fuel shutoffs for each wing before the header for fuel management, maintenance etc., so instead of a simple on/off valve I could use a L/R/B/O selector on each side and re-label inboard/outboard/both/off. Some fuel line spaghetti to deal with but flush fillers in all (4) tanks would make for a really good looking install, IMHO. So, still lots to think about... Comments welcome!

[jackak 20]

Doug,

I noticed that there were messages on the yahoo site that Steve Winder is back in the country.  I'd try to get in touch with him and see what he might have to offer.  Everyone would be interested in how to use larger lines between the tanks to speed up refueling, especially if the mod could be done with the tanks installed.  I figured that I'd put 5 gal. in one wing, go to the other side, put 5 gal. in, then back to the first tank.  Overall, the time would be the same, but you'd be doing something rather than just waiting.

Glad your carry-throughs were properly spaced. 

A good solid bottom is going to be needed in the tank bays, supporting them could prove to be interesting.  Can't imagine the consequences of a bulging wing bottom after a rough landing or pulling a couple of g's.

Let us know what you come up with or if Steve suggests something unique.

Jack

[jliltd 0]

Doug,

I have really been enjoying your builder's web site and just joined this site after a tip from your resident C5 flier.  It brought back lots of memories of when I built an Avid years ago.

I just read your posts about your wing sweep.  The 27.5" distance between front and rear spar doesn't enter into the equation since the whole wing swings as one fixed unit.  With your plumb bob points projected on the level shop floor we are left with a long skinny right triangle.  The far (opposite) side from the sweep angle would be the 1/4" forward your measured at the tip of the spar.  Per your description the hypotenuse of the triangle is 144", if in fact 144" is the distance along the spar from the fuselage pin point out to the area you measured at the tip.  Otherwise if the overall spar length is 144" from cut end to cut end then the hypotenuse of the triangle shall be shorter (by the distance of material between the new pin hole and inboard end of spar - "edge margin" in engineering parlance).  I am assuming you had the aircraft completely level on pitch and roll datums with the tail in the air over a level slab when you did the plumb-bob projections.

Mathematically the sine of the sweep angle equals the opposite leg's distance (1/4") over the hypotenuse (144").  So the sweep angle itself is the arcsine of the opposite over the hypotenuse, or asine(.25/144) which equals a smidgen under 1 tenth of a degree (.0994712 degrees) which is probably negligible.  If you want to change the pin joint to tip distance from 144" to another number you can recalculate the angle with the above formula.  If you want to use Microsoft Excel the formula is "=ASIN(.25/144)*180/PI()" where you can see the 1/4" and 144" numbers in between the first set of parenthesis.  Leave the 180/PI() alone since Excel gives the answer in radians by default and what we want is degrees.  If the 144" dimension is shortened then the sweep angle becomes even more negligible.

I went one step further to check this out by laying down the geometry in a CAD program showing the sweep of a 144" spar resulting in a 1/4" tip movement.  It came out the same as the math.  I make mistakes sometimes so I like to cross-check things when I can and didn't want to post wrong info (been there and done that and won't let that happen again [).

I'm not sure how important the value of the sweep angle is compared to the tip movement distance (again one tenth of a degree taint much).  The Kitfox recommendation to use a 1 degree forward sweep angle for heavier engines would yield a 2.5" forward measurement at the tip (assuming the same 144" spar distance)!  2.5" sounds like a lot to me but it's all based on a right triangle with the same formula (144*sine(angle)) where you just plug in any sweep angle you want.

If wings were lengthened this tip deflection distance will increase.  Shorter wings would have a lower deflection distance for the same sweep angle (i.e. a wing clipped 3/4 length would have a forward tip movement of 3/4 of 1/4" inch or 3/16").

What is important that your sweep change is negligible and now you have good edge margin in your spar root fitting joints.  That's not a bad thing since the design of the Avid/Kitfox plane spar to fuselage joints are 90 degrees out of common.  Usually strut-based wing spars are pinned horizontally and don't transfer bending loads to the fuselage; but our planes are pinned vertically to facilitate wing fold pivot and do transfer some bending lift loads across the joint.  Not a big deal with Dean Wilson's design due to good safety margins and the fact that the wing struts take the majority of the lifting loads from the wing spars.

I'm a windy bastard at times and a lot of the above is probably overkilling an answer to a simple question  .

I most want you to know I enjoy your builder's installments and documentation.  You're doing a great job.

[dholly 718]

Jim-

THANK YOU for registering and taking time to provide that great explanation! I'll need to read through it a couple more times to get the calculation method straight then adjust for the 3/4" 'edge margin' and additional 18" of wing tip per side, but the important details sunk in. Bottom line is, in my instance the sweep is so minimal as to be a non-issue both functionally AND structurally. Lots of good info in your post, thanks for confirming and putting my mind at ease. Windy is good when combined with knowledge and experience. I very much like to understand how things work, particularly those that I will trust my life too!

Doug