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Thread: built up wood spars

  1. #31
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    Re: built up wood spars

    Quote Originally Posted by TFF View Post
    I have never seen anything close to aircraft grade lumber locally. I used some hobby shop spruce, undetermined species, to fix some wing ribs on my plane. And I look at wood every where I go that has it. Somebody is lucky to have something local but not around here, there is a big lumber manufacturing/ stocking area around here. Nothin.
    Yes, some of us are fortunate. There is a local high end wood molding factory in my area. They will let anyone drive into the plant and spend as long as you want to look for aircraft grade wood in the warehouse, and the the prices are wholesale. They have wood boards as long as 24'. Wish they had Spruce.
    I bought all the wood molding for my home at this plant when I was building my house. Beautiful wood.
    Pops

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    Re: built up wood spars

    I am using the Light Sport rules for my stall speeds. picked those 2 airfoils based on a spread sheet called Aircraft and its Airfoils from the UIUC Applied Aerodynamics Group, I looked at the most commonly used airfoils, and the Clark y, was the most popular, plus I could find the stall speeds on all of those so chose that one.

    Max. Gross Takeoff Weight 1,320 lbs (1,430 lbs for seaplanes)
    Max. Stall Speed 51 mph / 45 knots CAS
    Max. Speed in Level Flight (VH) 138 mph / 120 knots CAS

    Granted I may not hit the max weight, but want to make sure it will be safe if I design it to that limit plus 1.5 for safety.
    I can cherry pick through the local lumber yard, in the premium SPF pile for the caps I want, in Douglas Fir, it may take a few hours to go trough the pile, as I need at least 16 foot pieces for my wings. looking at 30 foot span over all or close to that.

    Another factor is the lower my stall if in the event I do have to hit the ground, slower is better and I could walk away depending on what goes wrong.


    Robert

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    Re: built up wood spars

    Make sure you are honest with your self with the picking at aircraft grade level. Ring and slope; slope usually kills it. Spars are not a close enough part. Fuselage stringers or longerons or cap strip, I would stretch it. Remember the ANC procurement doc are minimums for truck loads of material. In that truck load, there is suppose to be better material for the specific tasks.

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    Re: built up wood spars

    correct, want tight rings, and good slope, the ANC standard is what I will go by, if it does not fit the standard I wont use it.

    if I cant find what I want locally, may have to go with the aluminum spars, or make my own from scratch, similar to existing examples using web and cap riveted to make it.

    Robert

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    Registered User Jan Carlsson's Avatar
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    Re: built up wood spars

    There are other airfoils then Clark Y and USA 35b, The NACA 4412 will have a softer stall then Clark Y, even better is the GA-30 Riblett just note that the Riblett number means CL and not camber, the -412 is as an exempel 3.3% camber and close to NACA 3412

    Use a camber that suits your cruise speed, and stall speed, with flaps (or not)

    When doing the structural load design, the wing area inside the fuselage should not be used, apply all lift on the wing area outside the fuselage.
    Use the Schrenk method for load distribution, Use MTOW minus wing weight, G load and SF.

    http://www.uotechnology.edu.iq/dep-M...%20Design3.pdf

    http://www.seqair.com/TheGlider/TheGlider.pdf

    http://www.recreationalflying.com/tu.../contents.html
    Lot of info on wood and airplanes there
    Last edited by Jan Carlsson; February 20th, 2017 at 08:02 PM.
    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

  6. #36
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    Re: built up wood spars

    I am only using the loads outside the fuselage since they are single bolt / hinge points, with lift strut, the mount points are where the stresses are on the fuselage.
    and the breaking point of the main spar is at the strut / wing joint with the unsupported outer panel area having the most free stresses.

    the inner panels compression and tensions are at the fuse, attach points.

    Robert

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    Re: built up wood spars

    Is Aircraft and its Airfoils the exact name? A search on that only gives me your post! I would love to see the spread sheet.

    What's the relative importance of high cruise and low stall for you?

    If the original aircraft's reputation included good handling, I suggest you use at least as much span as it did. This will give you faster climb (or less power required) and a better glide ratio.

    I looked at the NACA 4415 using Profili/Xfoil, and it looked very close to the 4412, plus it gives you a deeper spar. I assumed that you'd either be at 125 mph cruise, or trying to fly slowly. I also assumed that you'd use a sheeted d-tube, with a trip at 30 percent to simulate the end of the sheeting. I used the type 2 polar, Re = 2,000,000/(sqrt Cl) That corresponds to the Reynolds numbers seen in level flight. Maybe I should have used a slightly higher Reynolds number; I guess I was thinking you'd go for more than the minimal span to get the stall speed you want. Or maybe you will?

    If you're willing to consider other airfoils, there's a Tsagi airfoil, which I think was actually used on an ultralight, that would give you a bit more lift and much less pitching moment. I think it's called the P-III, and I have coordinates. No guarantee it was actually used on an ultralight, but I guess that can be determined. If you're going to fly relatively slowly (above Cl of 0.6), I have coordinates for what is allegedly a Kolb airfoil that looks good. Above that Cl of 0.6, it has a little less drag than the others. And a similarly low pitching moment.

    If you're willing to think about airfoils that may not have been used in aircraft before, I found something called the Ara D 20 percent, with performance comparable to the Tsagi. Except for a little less drag in cruise. It was intended as a wind turbine airfoil. I narrowed the trailing edge down for purposes of comparison, it was originally pretty wide. As I recall, the Ara D does well at considerably lower Reynolds numbers too, so if you wanted to make a longer, tapered wing, this might be a good choice.

    All these airfoils are relatively simple shapes. The Tsagi and the Kolb, I think, are mostly flat bottomed. At the moment I can't compare because this window is monopolizing the computer!

    Must run while there's still a little light out.

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    Re: built up wood spars

    you can find the list by scrolling down this page, http://m-selig.ae.illinois.edu/ads/aircraft.html
    I copied it from the site and put into an excel spread sheet, it gives you root and tip airfoils.
    so far I have not settled on the airfoil to use, initially looking at the same length chord on the wings, original span was 32' 6", and could use up to 160 HP engines.

    the original build had some stability issues, you could not relax on the controls, example if you bumped the elevator back it would stay there, it would not return to stable flight.
    you had to fly it all the time, so it made you quite tired if you wanted a good cross country flight.

    I want to improve the performance, so it is comfortable to fly.

    Robert

  9. #39
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    Re: built up wood spars

    Was there a lot of friction in the controls? Or was it a c.g. problem? Something else?

    I knew about the list, it was the spreadsheet that threw me off.

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