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Thread: Checking Wing Incidence?

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    Checking Wing Incidence?

    Hello All,

    I am in the process of building a Cygnet SF2a with a few other EAA members in our chapter. This was a free project donated to us by a widow where the builder passed away before finishing. Its become our club project. Anyhow we found many mistakes we have had to fix. We had to cut off all the wing attachment brackets as well as the gear brackets. I designed and made new ones that could be installed to a fuslage that was already welded with obstructions from other tubes. We have everything ready to weld and I made a fixture to emulate the wing using a 3D printer. With my fixture, I can align both sides to have the same incidence then tack weld the brackets for a the test fit of the wings. Should something not be right I grind off the tack welds and make an adjustment. My dilemna now is while I can gaurantee that my brackets holding the wing will hold them at the same angle, I cant gaurantee that the spars and incidence connection exiting each wing are correct or match each other. I was hoping to buy an incidence meter like the ones I use on my Model airplanes.
    Here is a link to what I want to find on a larger scale
    TowerHobbies.com | Robart Model Incidence Meter

    Does something like this exist for the full scale airplane world? If so where can I buy it? If not I may just have to make one. Or is there another way of doing this? The bottom of the airfoil is more or less flat and I was thinking I could put the electronic level there to get a measurement. Downside is it will only be a measuremnt I can use for comparison between wings. Doing it that way wont give me the actual incidence in relation to the harizontal stab.

    Any thoughts?

    Marc

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    Moderator Dana's Avatar
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    Re: Checking Wing Incidence?

    The drawings or builders manual, if there is one, should give rigging instructions, no? If it's a flat bottom airfoil the rigging information is often given from the flat bottom, regardless of the actual true incidence. I've never heard of a full size incidence meter like the model ones, though doubtless it would be easy to make one.

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    Re: Checking Wing Incidence?

    Hey Dana,

    Thanks for the reply. The Plans leave A Lot to the imagination. They are only average compared to other plans I have seen for a scratch built only plane. There is not any mention of Incidence or rigging in the plans. Closest they come to is they show a way to jig the spars on a table and you build around the jigged spars then all should be good when you attach them to the fuselage brackets welded in. Problem is the guy made a mistake on the fuselage and in doing so caused the brackets to be in the wrong place because his reference tube was installed wrong. We have gotten around this and put the brackets where they should be, but to double check I wanted to refer to the plans wing incidence in relation to the water line of the fuse and there is no mention. Only thing close is the Horiz Stab is at -2.5 degrees to the waterline, but no mention of the wing in relation to the waterline or even relation to the Stab. The Cygnet is a high wing cub like airplane and figured 2 to 3 degrees would be about right. Right now with my only way of measuring being the difference between the stab and wing I get 5 degrees using my wing fixture jig in the brackets we are about to weld. That seems a bit extreme which is why I am questioning it. I suppose if needed I can make something using some square tubing and some angle iron.

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    Registered User Jan Carlsson's Avatar
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    Re: Checking Wing Incidence?

    If the bottom isn't flat, you can make a templet for the top and use a libelle or electronic one.
    The info must be in the drawings, maybe the first one, do you have all sheets?

    this guys arouund?
    The Cygnet Model SF-2A is available from:
    Viking Aircraft
    P.O. Box 646
    Elkhorn, Wisconsin 55321
    Phone 262 949 3247
    viking02@charter.net
    Jan.

    A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away. Antoine de Saint-Exupery
    "Flight by machines heavier than air is unpractical and insignificant, if not utterly impossible." Simon Newcomb, 1902

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    Registered User Head in the clouds's Avatar
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    Re: Checking Wing Incidence?

    Quote Originally Posted by addicted2climbing View Post

    Any thoughts?

    Marc
    Marc, I don't know if you followed another thread but I described something very similar some time ago. A friend here built a CH601HD and it didn't fly well, he had to hold a lot of aileron in to keep it from rolling and as a result couldn't roll in that direction very fast at all due to running out of control motion. It was hard work flying it too because it had the 'flex the wing sheeting' type hinges, so he wasn't happy. I offered to help so he pulled the wings off and I laser measured them individually, they were perfect so I checked the wing-box. It had been built with a big twist in it, clearly they'd not jigged the fuselage properly during the build. We devised a simple fix by de-riveting the centre section stubs and using a pair of levers attached to the wing mounting points, hauled them parallel and held them there with chains to the hangar floor, then re-riveted with oversize rivets. It flies perfectly now.

    What you could do is much the same principle, and the simile is what every builder does to erect a building. You need a self-leveling laser or a dumpy level, you can rent/hire them from any tool and plant rentals place. If you use a laser make sure it is calibrated, that's easy to do, set it up on a tripod and mark where the laser dot hits a wall, then turn it around (most of them project four horizontal beams and vertical ones as well) and check that the other beams also point to the mark on the wall, if not it's been bumped hard and needs re-calibrating.

    Set the laser on a tripod with a swiveling head (or rent a spinning laser) and with the beam(s) higher (or lower) than the airplane. Set the plane on stands with the 'waterline' (datum line) level by measuring vertically from the laser beam to the datum line at nose and tail and each side and adjusting the plane as necessary.

    Check the HS incidence by measuring from the laser beam to the front and rear HS chordline points. Simple calc to determine the HS incidence angle. Another simple calc to determine what the height difference between the front and rear of the main-plane chordline should be. And another calc to determine what the height difference should be between root and tip (to account for dihedral). BTW 5* incidence sounds too much for the mainplane, usual is more like 2-3*. Then all you need to do is measure from the laser beam to the front and rear chordline points of the mainplane, at tip and root, and make them match at port and starboard.

    The method is the same using a dumpy level and make sure it is self-leveling or very accurately set-up using the circular spirit level incorporated in it.

    Hope it helps.
    rtfm likes this.

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    Re: Checking Wing Incidence?

    Hey Head in the clouds,

    Thank you for the reply. I was going to do soemthing similar using some masonry string lined up with the poins on the fuselage, but your methods with the spinning level is much better. I may see if I can rent one at Home Depot. I can add a chord line emulator to my fixtures so I can then make the measurement before I weld but adding it will be a pain in th ebutt since our 3d printer is down for over a month now... I may have ot do th eold fashioned method and actualy cut some metal.

    On another note, my Incidence meters were essential when drilling the incidence pins on my R/C sailplanes and I am curious why there is not something similar in a much larger package for homebuilders?

    Marc

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    TFF
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    Re: Checking Wing Incidence?

    Search the biplane forum for rigging boards. On the site, someone has been drawing plans up for various rig boards. It might give you an idea. Kind of similar to the model ones in idea.

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    Moderator Dana's Avatar
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    Re: Checking Wing Incidence?

    Quote Originally Posted by addicted2climbing View Post
    On another note, my Incidence meters were essential when drilling the incidence pins on my R/C sailplanes and I am curious why there is not something similar in a much larger package for homebuilders?
    Several reasons. R/Cers typically build dozens of airplanes so a shortcut like an incidence meter is a useful thing... but it's hardly "essential", I built R/C models for years and never owned one. R/C models are smaller, so it's harder to set up the plane by making measurements from the floor, and harder to get your fingers in between the wing and fuselage to set things. What's a ten dollar gadget that an R/Cer would use many times would, in a larger size, be a multi hundred dollar device that a homebuilder would use once. And most homebuilt plans do give rigging information in a manner that makes such a device unnecessary (if the aircraft is built correctly).

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    Re: Checking Wing Incidence?

    I can't really picture your situation in my head, but...

    Can you position the brackets with undersize attachment bolt holes, then drill the holes out to full size with the wings physically in place to ensure correct alignment? I realise that may not apply in your case.
    Flying Bearhawk kit no.125

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    Re: Checking Wing Incidence?

    Quote Originally Posted by Battson View Post
    I can't really picture your situation in my head, but...

    Can you position the brackets with undersize attachment bolt holes, then drill the holes out to full size with the wings physically in place to ensure correct alignment? I realise that may not apply in your case.
    Hey Battson,

    I have recently read a bearhawk build manual and know they use the undersize hole method . Had I read it before I started the retrofit, I may have done it that way. As it is, now that I have the bearhawk info I plan to follow its procedure to get everything ready to go. Using a drywall hanger as shown in the bearhawk manual is a good idea and I plan to do the same. My problem is there is absolutely no mention of incidence reference between the wings and the waterline. There is a refference for the Horizontal Stab and the waterline. As the plane is built from scratch by way of assembly and jigging the wings and brackets should come out with correct alignment My problem is that a few members of my EAA chapter made the decision to cut off all the brackets on the fuselage that had been installed before the Fuselage was welded. I and one other guy vehemently opposed this as I told them we would lose our datum's and getting everything back would be a huge Pain in the arse... I suggested we just reinforced or back the existing brackets to add strength to the welds that were not so well done. It has been a challenge to say the least and those who made that call were there to help cut the brackets off but have never been around to work on the plane to solve the problems to put them back on. Now there are just 2 of us left working to get this done.. We have solved all the problems and it is just a matter of getting the incidence info from the plans distributor who I have been in contact with. Hopefully he will come through when he speaks with the actual designer.

    Marc

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    Re: Checking Wing Incidence?

    You guys work too hard.

    You do not want to know the incidence, as interesting as that seems. You want to know that the incidence of the two wings is the same. To do that, you want to know that the wing mounts on the fuselage are the same places left and right and have about the right incidence, and then you want to know that both wings are the same. If the wings are mirror images of each other and so are the mount positions, it will fly fine. Remember you fly the wings and tail, the fuselage is basically along for the ride.

    As mentioned in other posts, you jig and make the wings correct (whatever that is) including mounting points on a flat table, using supports and templates as appropriate.

    You check your mount fittings with a pair of water levels. Use the water levels first to level the fuselage left to right, then to check that your forward fittings are the same height, that the aft fittings are the same height, and that the differences between forward and aft are the same, and about right according to plans.

    Once you have done these things, well, let's hope that you have enough adjustment to fix whatever is left. I am building a fiberglass bird. I have no opportunity to adjust twist in the wing, but I can put in small bias in flaps, and one aileron will have a trim tab... Bonanza's are built with one wing adjustable for incidence and they make that adjustment based on flight test before delivery. Bonanza drivers know that no two fly the same...wonder why?

    Billski

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    Re: Checking Wing Incidence?

    Might I ask a stupid question? If you want to know the incidence is the same, couldn't you establish a central point under the fuselage and measure the distance to the ground (assuming you have the aircraft jacked up and leveled) and then measure the distance from a point under each wing type. If you could do that, theoretically (if not practically) you could calculate the angle between the points....sort of a "rise:run" diagram.
    "Unfortunately in science what you believe is irrelevant."- Orion

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    Re: Checking Wing Incidence?

    SVSU,

    That works fine if the floor is flat and level. Generally, hangars and garages and shops have floors that slope.

    Bill

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    Re: Checking Wing Incidence?

    Quote Originally Posted by wsimpso1 View Post
    Remember you fly the wings and tail, the fuselage is basically along for the ride.
    Billski is spot on here. The wing flies at whatever alpha and that is what determines the angle that the fuselage is at. During climb the wing is at a high alpha, so the fuselage is pointed up and so on. The incidence is set according to what angle you want the fuselage to be in during cruise. Early in the thread you mention incidence of 5* but since most benign sections fly at about 2-3* in cruise it would mean that your fuselage would be flying nose down by 2-3* if the wing was set at 5*. Some people like it that way. The Drifter is a good example of that, some people trim their rigging and ballast to have the fuse fly nose low, some do it for a higher nose condition. I favored the latter because I'm the sort that flies for the journey, some are in it for the destination... In fact mine could fly faster than most because the drag was lower and the fuse added to the lift rather than detracting from it.

    Some aircraft do benefit from a bit of nose down in cruise if the forward visibility is restricted otherwise, but they are probably paying the penalty in terms of drag.

    Quote Originally Posted by wsimpso1 View Post
    You check your mount fittings with a pair of water levels.
    If this refers to a long polythene tube partially filled with colored water I would be very wary of using it for anything except building a rough fishing camp or landscaping your garden, as they can be very inaccurate unless they are made exactly correctly. Here is a link to making a water level which describes its good points but also note the mentions of inaccuracies due to temperature gradients and air bubbles, and entrained or microscopic bubbles of air can have similar effects. Making and Using a Water Level - Tools And Equipment - Professional Deck Builder Magazine

    What is not mentioned is the need to have the correct ratio of tube diameter, length, and height between the water level and the lowest point in the tube, because of the flow resistance within the tube. I had a very costly event where I used a 3/8" tube about 40ft long with colored water and a height of around 6ft. After building a structure and finding it to be out of level by some 2" I found that it took the water more than a minute to level itself at each end, you can test this yourself by holding the two ends of the tube next to each other and raising one then seeing how long it takes to re-stabilize. Using a spirit instead of water improves the situation considerably.

    If you use this method for your wings I'd suggest a tube of minimum 1" diameter and keep the tube as short as practicable. But since you can rent a laser for about $5 a day I'd suggest it would be a better proposition.

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    Re: Checking Wing Incidence?

    Quote Originally Posted by wsimpso1 View Post
    SVSU,

    That works fine if the floor is flat and level. Generally, hangars and garages and shops have floors that slope.

    Bill
    Well....****. I didn't think about that. LOL
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