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Design => Stunt design => Topic started by: Motorman on May 27, 2015, 12:23:48 PM
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I see some planes with more ribs than others. Is this a trade off between better stiffness and airfoil shape vs weight? Is anything less than 3-1/2" spacing really necessary on a 600 sq. in. wing?
MM
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I see some planes with more ribs than others. Is this a trade off between better stiffness and airfoil shape vs weight? Is anything less than 3-1/2" spacing really necessary on a 600 sq. in. wing?
3 and a half inches? I don't know about that, most are more like 2" and I have considered adding half-ribs every inch to avoid "starved horse" problems.
Brett
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.............. I have considered adding half-ribs every inch to avoid "starved horse" problems.
Brett
Which brings up the next question "why avoid the starved horsy thing?" (Or as we say here in Oz "built like a roasted greyhound.")
Purely appearance?
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The sagged part is less than the intended airfoil thickness. If you scientifically selected the airfoil, don't you want it everywhere?
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3 and a half inches? I don't know about that, most are more like 2" and I have considered adding half-ribs every inch to avoid "starved horse" problems.
Brett
3.5 inches isn't too bad behind the spar, although if you want more I'm not going to prevent you. If I'm building something without sheeting ahead of the spar I like to put in half-ribs (my latest RC plane is an Ugly Stick with two half ribs for every full rib, with about 3" spacing between full ribs but 1" spacing between half ribs).
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The sagged part is less than the intended airfoil thickness. If you scientifically selected the airfoil, don't you want it everywhere?
Hi Paul,
I thought that may be the obvious answer but I have yet to see a model suffer from the lack of scientific airfoil in the bays.
Perhaps the "starved horse" ribbing helps to prevent span wise flow? Or an average between the minimum and maximum sections allows better for varying conditions?
Thanks.
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Some of the Seilig(sp?) airfoils are designed with covering sag in mind; the published airfoil data is actually more a rib profile rather than what you expect the wing itself to be.
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The sagged part is less than the intended airfoil thickness. If you scientifically selected the airfoil, don't you want it everywhere?
I would not claim that my airfoil was carefully selected based on science! However, I want the things that are supposed to be straight to be straight, the curved parts curved and no "accidental" shapes. My turtledeck is a bit "starved horse" and I have often wanted to cut it off and make a new, correct replacement. So far I have resisted.
Brett
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Brett,
replacing the turtledeck would be faster than building a new plane!
heck you have a week before the nats,, get to it LOL
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There are plenty of real airplanes and lake freighters with the covering shrink over the ribs on the body (or hull). But airplane wings are generally built to keep the airfoil true. Stunters and combat planes have so much excess lift, you wouldn't know or care if it's efficient or not. The only way to tell would be to build another plane with the wing sheeted to keep the airfoil true.
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Brett,
replacing the turtledeck would be faster than building a new plane!
heck you have a week before the nats,, get to it LOL
I would be better off finishing the almost-finished clone (that has been sitting in that state for about a year...).
Brett
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The sagged part is less than the intended airfoil thickness. If you scientifically selected the airfoil, don't you want it everywhere?
You can select an airfoil scientifically. But as Mike Selig at UIUC proved, building a wing with close enough tolerances to get realistic performance measurements is VERY difficult. And when you do the differences are only apparent at the very extremes of performance.