difficult incidence
 

Hey all,

I'm on the home stretch to getting my Sig Astro Hog done and ready for covering with Monykote and I've run into a problem that I thought I would past the collective consciousness.

I can't seem to get the wing incidence below 2 degrees positive (the plans call for 1-1/2 degrees positive incidence). I've been sanding all day long and can't seem to make any improvement whatsoever. The guage (brand new) always reads 2-1/4 degrees on one side and 2 degrees on the other. Frankly I'm scared to sand any more until I understand more of what's going on.

So anyone with any helpful suggestions is most welcome to reply to poor old Dave. [&o]

I'd like to get the incidence within 1/4 degree rather than the 1/2 allowance in the guage's instructions. For what it's worth here is a link to some photos ...

http://home.comcast.net/~dave.combs/daplane.html

I'm so fed up that I'm about to throw the front dowels away and glue in some more wing bolt blocks.

RE: difficult incidence
 

Are you sanding the wing saddle at the TE?

RE: difficult incidence
 

Well, you can keep sanding, or file the Dowl holes to drop the LE - But that would require filling the gap that you leave in the wing saddle. All in all, I think sanding is the easier way to go.

RE: difficult incidence
 

Yup. I'm sanding the saddle at the trailing edges. One friend said to open the dowel holes up and make repairs for the elliptical holes. Looks like fun all the way no matter what I do.

RE: difficult incidence
 

To alter the incidence you are actually tipping the whole airfoil up at the trailing edge. Sanding just the trailing edge will not do it. If the saddle is a good fit to the wing then the whole saddle has to be sanded with more material being removed progressively towards the trailing edge.

.

I would do that anyway. Why risk all that you have in an airplane on a couple of wooden dowels. Use four screws. It is then easy to make adjustments to the wing after some flights.

Ed S

RE: difficult incidence
 

I agree with ED S on this one. If you continue to sand towards the TE until you get the correct incidence and wind up with wing saddle gaps; don't worry. These gaps are easily filled with micro balloons and epoxy, or RTV if the gaps are small.

RE: difficult incidence
 

I've decided to cut some new wing saddles to bring down the leading edge and install extra wing bolt retaining blocks near the dowel location by the F2 bulkhead.

Last night I used sheets of paper that were 0.004 thousandths thick to shim down the leading edge. I was able to achieve 1-/3/4 degrees of positive incidence after using a whopping 0.084" of shims on the starboard side and 0.028" of shims on the port side.

I like my idea since, although it's not less work, it negates having to sand down the rear saddles so deeply. I'll glue the lite-ply saddles on the inside and sand to achieve the correct incidence, then fill with either EpoxoLite or epoxy and microballoons. The dowels will remain for alignment.

RE: difficult incidence
 

If "Epoxolite" Is that SIG product then do not use it. That stuff was dug out of the mines of hell. Sanding concrete will be easier. Unless of course the product has changed recently.

Ed S

RE: difficult incidence
 

Okay, epoxy and balloons it is ...

RE: difficult incidence
 

decombs21

Sounds to me that you have a good handle on this. Best of luck on your build. ;)

RE: difficult incidence
 

Thank you, Ed. I don't mind telling you that I was discouraged for a while but I will "soldier on." I almost ordered a fuse kit from Sig today but changed my mind.

Here's what I think led to this situation:

After gluing in the first 2 bulkheads (as per instructions), I then moved to join the 2 halves. I remember now that the bulkheads did not align at the saddle as advertised even though I took great pains to ensure that both sides were even. I used 3 6/32 nuts and screws to hold them side by side as I sanded them to size. The builder is responsible for cutting the sides from 3/32 planks of balsa, they are not pre-cut.

I glued them together anyhow since there was nothing I could do - I couldn't figure out how it happened to correct it.

I went on building and forgot all about it until I got to the point where incidence was verified. Now I decided to glue in a slab of maple backed by 1/2" and 3/4" triangle balsa stock into which I will drill new holes to hold the wing to the fuse when assembled. The angle stock on top of the maple will be shaped to conform to the wing.

I'm off to get some lite-ply now. Wish me luck.

RE: difficult incidence
 

decombs21,

Looking at your pic's, I can tell you're a good craftsman, but I'll wish you good luck anyway.

RE: difficult incidence
 

I made lite-ply wing saddle doublers and attached them to the fuse with 6-32 screws to force the leading edge of the wing down. Wouldn't you know it, the reading didn't change one bit. They are still 1-3/4 degrees + on the port wing and 2 degrees + on the starboard wing.

This is driving nuts but not insane. I'm about to remove the dowels altogether to see if that helps and then sand down the dowel to get the correct incidence. You can see my efforts at my Comcast homepage -

[link=http://home.comcast.net/~dave.combs/daplane.html]Difficult Incidence on my Sig Astro Hog[/link]

If anyone can contribute some explanation as to why, why, why this kit is giving such trouble I'd appreciate hearing from you. I suppose the first thing I should is throw the incidence meter away, close my eyes to the problem and cover this thing.

Dave

RE: difficult incidence
 

I've been putting the wing on and taking it off all day - while fiddling with those adjustable saddles - and here's what I've finally come up with:

starboard wing: 1-3/4 degrees +

port wing: 1-1/4 degrees +

Sadly when I get to this point and I take the measurement from the wingtips to the floor I get a half inch difference.

I imagine there are lots and lots of happy Hog owners with inacurrate incidence like it and worse. What do you think will happen to this bird if I let it go as is?

If you'll look to the right, you can see how high the dowel hole is and, consequently, how low the wing had to go to get close to the correct incidence.



Dave

RE: difficult incidence
 

Dave,

The 1/2* difference might be the way the incidence meter is being positioned. An old-hand taught me a long time ago that the meter should be facing the same direction when measuring both port and starboard wings. In other words, if the meter is facing the fuse while on the port wing, then it should be facing the outboard wing while measuring the starboard. Most folks have the tendency to face the meter towards the fuse reguardless which wing it's on. You wouldn't think it should make a difference, but often times it does.

If you're not facing the meter in the same direction, you might want to try it to see what your readings pan out to.

Concerning your dowel rods and holes. You can over-size the holes to obtain the proper wing position. Once you have the wing where you want it, place a new piece of plywood, with the proper sized hole drilled, in front of the former. Align it so that the dowel is through the over-sized hole and inserted into the new ply at the correct position.

I hope this is of some help, cause I know these alignments can be very frustrating.

RE: difficult incidence
 

Thanks, Ed. I appreciate the advice.

I am facing the meter in the same direction at each measurement.

Yesterday, I FINALLY got the meter to read within 1/4 degree positive. The port wing was at 1-1/2 degree + and the starboard wing was at 1-1/4 degree +. I decided to take it and run.

I marked the adjustable wing saddles and glued them in with some slow set 'poxy. Now it's just a matter of fabbing a that chunk of maple across the front in order to attach new wing bolts. The original dowels were set in a piece of 1/2 balsa in the wing and I don't know if I'm that good of a shot to drill a hole through that.

See photo -

and ...



As you can see, I have decisions, decisions, decisions ...

But I haven't had enough coffee to make a good choice yet so it's off to the coffee pot.

RE: difficult incidence
 

I agree with ya Dave. With the incidences that close, I'd run with it too.