Large Model Association Forum

Hi, i need help i am new to building from plans, and the model i am building states that it needs 2 degrees of washout. How do you calculate this, so you know what size of block to prop under the rear of the tip rib, ie 1/8" - 1/4" ?. Why can't the designer just say chock up the tip rib by 1/4" of an inch or whatever, as they must have done this when they built the model.
Please help, John Whyte.

Use this calculator...

http://www.carbidedepot.com/formulas-trigright.asp

Side b is the tip chord and angle a is 2 degrees.

HI John,
2 degrees is a rise of 3.4 mm for every 100mm of semi span.
Just divide half the span by 100, multiply the result by 3.2 and that is the packing in mm.

Hope this helps


Robin

If you're doing washout you won't use the semi-span, you'll use the chord at the tip. 3.4mm for every 100mm of chord.

Within reason, how much washout you build in doesn't matter, anywhere betwen 2 and 4 degrees will be ok, all you are trying to do is ensure that the wing root stalls before the tip. The important thing is to make the washout on both wing halves the same, so just chock up the trailing edge of each tip by the same amount. Less than 2 degrees may not provide the safety net you were trying to achieve, more than 4 degrees and the twist starts to look a bit odd. But for our models where we are not concerned with aerodynamic efficiency, it really doesn't matter.

Unless you are trying to faithfully follow the outline of the fullsize, I'd say just chock up each tip by whatever handy pieces of wood you have that will give 2 - 3 degrees!

To find out the size of chock you'll require use a schoolboy type protractor and draw two lines on a sheet of paper, one is the reference or zero degree line, the other is the angle of the washout, in your case 2 degrees. Measure the tip chord and apply that distance to the reference line. The distance between the two lines at that point is the height of the chock - that's simple enough!

Remember that the washout angle is measured from the leading edge, so when you chock up the trailing edge the main spar may be lifted off the board (it will certainly be angled) - don't try and chock the trailing edge leaving the spar pinned to the board or you'll end up with more washout than intended and a distorted tip shape.