Wash out, where to pivot this angle?
 

Is there a proper location for the pivot point of the wash out angle along the cord of the tip airfoil -
LE, 25%, midpoint, TE?

Thanks,

ScaleMe

RE: Wash out, where to pivot this angle?
 

I like to build it in whenever possible by blocking the TE up while gluing, so it used the LE as the start point.

RE: Wash out, where to pivot this angle?
 

In my experience it is over the span so there is no sharp "pivot". Washout is usually a small angle (2-3 degrees) so it is not really visible unless you look at the wing from the trailing edge. There are 2 ways washout is generally applied. The first is built in where the foam cores are cut or the wing is built blocked up slightly to include the desired angle. The second is less scientific and involes manually twisting the wing while shrinking the covering.

RE: Wash out, where to pivot this angle?
 

Draw a line from the center of the leading edge to the trailing edge at the wing root and at the wingtip. The angular difference is washout if the angle at the tip is lesser angle than at the root. Washin is when the tip is at a higher angle.

RE: Wash out, where to pivot this angle?
 

put the pivot at the LE and it adds a bit of dihedral...pivot at the TE will add a bit of anhedral. but we're talking miniscule amounts here so it really doesn't matter.


dave

RE: Wash out, where to pivot this angle?
 

IMO, you should pivot the tip at the trailing edge. I always use the F-16 and other modern fighters as examples.

The F-16 has a noticable amount of washout, but when viewed from behind, the wing has no dihedral.

If you washout anywhere but the trailing edge, you induce a small amount of dihedral as ZAGNUT has pointed out.

Dan

RE: Wash out, where to pivot this angle?
 

Thanks for the input guys. Seems that most of the opinions come from past experience rather than an aerodynamic standard. I was contemplating making the pivot at mid cord to minimize effects of unintentional dihedral and anhedral while still achieiving the washout angle that moves the tip's LE down and TE up normal to the root.

Maybe it is an exercise in splitting hairs, it is only 3 degrees total after all. I am no expert on this subject.

ScaleMe

RE: Wash out, where to pivot this angle?
 

ScaleME, if you use the covering to twist in the washout by shrinking it in then it'll tend to pivot the wing along a midline based on all the springiness of each element of the wing. So the actual pivot point will be somewhere around the wing spar but probably not centered on it. A D tube type wing will tend to pivot around the very stiff D tube somewhere in the center of the shape but towards the spar location.

Of course if you jig build the wing with the washout built in then you can have complete control of where it goes but it's not really neccesary.

RE: Wash out, where to pivot this angle?
 

Bruce,

Actually my application is a foam wing - think of it as setting up the templates to wire cut the wing. Rather than a matter of adjusting the wing after it is finish I am trying to cut the wing with the washout already in it - this is well known, I am just looking for an aerodynamically correct way to establish the washout angle/pivot point. I am ok with the emperical guesswork mentioned here, just curious if a NASA type person could set the record straight.

ScaleMe

RE: Wash out, where to pivot this angle?
 

Ah, I see.

OK but it's still all relative. Depending on how you cut and join the center section joint the washout may appear to be relative to whatever points are in a straight line through the whole wing. And if you use more dihedral than that then there won't really be any firm reference line anyway. As long as your twist angle from root to tip is good then you're fine.

RE: Wash out, where to pivot this angle?
 

Tip plates make washout mostly unnecessary.