Question about setting incidence.

[turbojoe]

I'm nearing completion on my first true scratch build from plans. Here is the model. (http://www.eastwindmodels.com/gemini/gemini.html) I have a question regarding incidence. I have a Great Planes laser incidence meter and am not completely certain that I'm going to end up with the correct incidence. The reason I'm concerned is that the airfoil is a Clark "Y" and the LE sits lower than the rib centerline. If I zero the meter on the table; then mount it to the wing and adjust the plane to zero; when I look at it from the side it appears to have negative incidence. When I measure with a ruler from the table to the spar and TE it indeed has negative incidence by about 5 mm. Because the LE is lower than the true centerline of the rib it appears to be the "problem". Should I go ahead and set the horizontal stab using the meter zero setting or should I block up the fuse/wing so that the spar and TE are level; zero the meter on the wing and then use that zero reference as a zero point for the horizontal stab? I hope this makes sense and someone can tell me the best way to go about this. It's coming out beautiful and I don't want this setting to possibly ruin what will probably be a very nice plane. Adjustment after finishing and flight test is pretty much out of the question. I need to make sure this is right BEFORE I permanently glue the tail in place.
Thanks for ANY help.


Joe

[Mike James]

Don't the instructions tell you what the incidences are supposed to be?

If not...
My instinct would be that the horizontal stabilizer is probably at 0 degrees, with the wing having a small amount, as you mentioned, of negative incidence. Being a Clark Y section, the small amount of negative incidence would probably keep the plane from climbing with power application, like many trainers do. (Engine thrust line, relative to the wing, and CG also will have an effect.)

At any rate, you could set it up, go fly the plane, and adjust afterward. Don't be spooked about trying it out. After all, it's not a 200 mph turbine model. Have some fun, and adjust, depending on what happens when you fly it.

[turbojoe]

Mike,

I should have added that the specs are zero-zero and that's what I was shooting for. The motors being mounted to the wing in nacelles will have whatever incidence the wing is set to.
I need to avoid cutting and adjusting/shimming at all costs after assembly is complete as the wing and tail are shaped into the fuse and would look terrible after modifications.


Joe

[Mike James]

I would set the wing first, then the tail.

Here's a plot of a Clark Y, which could appear to have negative incidence.

[Andy W]

In the example able (Mike's post with the clark Y), the displayed airfoil has 0 incidence in this diagram (the LE and TE points are horizontal).. that gets many people, because they look at the flat bottom and assume this wing is actually sloping forwards..
..a

[turbojoe]

Yep. That's what is throwing me. I see the flat bottom portion sloping down. So.......just set it up based on the LE and TE relationship and ignore the area in between, right?
Thanks for the help guys!


Joe

[Mike James]

No problemo...
Glad you got it the way you want it. (I'd be as finicky too, so I appreciate your efforts.) Let us know how she flies.

[James Frolik]

When I built my Shorts Skyvan some years ago, the plan depicted a 2.5-degree negative wing incidence. Since it was my first build from plans, I too was confused — and thought the plane would fly into the ground this way. I even wrote the plan service to inquire whether there was some error. (They said it was correct.)

Along the way some people tried explaining the matter to me, and I sort of developed a realization for it. And even though my Skyvan flew just fine, my mind never really saw the relationship.

But Mike's diagram with the Clack-Y really sets the image straight — particularly with the bowed line from leading to trailing edge.

Thanks.

[kdwzagi]

hi

profili shows the datum line thus so i think if you line the laser incidence meter up with the leading and trailing edge of your wing it will be correct

all the best

kev

[turbojoe]

I had previously e-mailed the designer Al Eastman also and just got a reply from him. He proves that all of you were right. His reply is below:


The clark Y airfoil used in the design actually looks like it has negative incidence when viewed from the side, but in fact it is set at zero. set the flat of the fuse bottom on the bench, and set in incidence meter on the wing. Note the reading. then make the stab have the same reading. the wing and stab will then be zero-zero to each other.

the incidence line on a clark Y runs from the trailing edge point to the foremost point on the leading edge. when you do this it actually looks like the airfoil is sitting negative, but in fact it is at zero. If you lay the flat part of the wind bottom on a zero line, the airfoil would then have positive incidence. that threw me a bit too when I first started to figure it out.


It looks like I'd better get busy and finish it up. Thanks again for the help guys!


Joe

[HELModels]

another approach to getting the incidence right is to take advantage of the Flat Bottom on the Clark-Y instead of estimating where in the leading edge curvature/radius is the point of intersection for the chord line. make a copy of the airfoil on your plans or Clark-Y pics in this thread and use a little trig to determine the angle from the trailing edge and the Flat Bottom, then adjust the incidence meter to compensate. Your eye already sees the line the bottom makes so why not make use of it. Trick is to determine the angle with the chord line.

Nice Model btw.

Good Luck,
Harry

[turbojoe]

use a little trig.

Good Luck,
Harry

Hehe. Way back when I was in school the highest number known to man was 7. "Trig" was a word someone came up with when they misspelled "twig". :D
Seriously though, I doubt I'll be using any complex math. I'll just take the lazy way and do it like Al says. The max that the incidence could be off is 1.5 degrees either way from zero. I'm sure it could be compensated for with TX trim adjustments. I didn't realize how minimal it really was until I cheked the difference in readings between what looked right and what IS right.


Joe

[kdwzagi]

hi

have just thrown the image of the clark y into turbocad and measured the angle of the lower surface from the datum

2.12 degrees

[vintage1]

That goes along with other info that I have that suggests that wing and flat plate stab at zero-zero geometry wise, is about 2 degrees positive on the wing in aerodynamic terms.

I'll set my old timer up that way I think.

[HELModels]

Hehe I doubt I'll be using any complex math. I'll just take the lazy way


Joe

Back in the days when pyramids were built using kites to lift heavy blocks, they realized that the square of the hypoteneuse of a right triangle equaled the sum of the squares of opposite and adjacent sides. It still works today.

What I suggested applies to any airfoil. All that is needed is to accurately identify a corresponding point on the built airfoil with the airfoil you Think you built.

If not, trim drag is the lazy way.

Good Flying,
Harry

[kdwzagi]

hi

if you lay the plane on a flat surface so that the flat underside of the clark y is down flat then you can measure the angle of your tail using the following method to get zero degrees incidence

measure the chord of your tail at the root in inches and then times this by .040 it will give you the amount that the front of the tail needs to be higher than the back

ie

if your tail measures 5 inches from front to back then
5 inch *.040 = .2 inch

so the tail must be .2 higher at the leading edge compared to the trailing edge to get zero degrees incidence

an easy way to look at this is that 1mm = 0.040" so you could say that if your tail has a 5 inch chord then the front of the tail needs to be 5mm higher

or 3 inch = 3mm
or 6 inch = 6mm

simple eh

good luck

[turbojoe]

I just can't get over this picture. :D :D Seriously though, I don't want to over complicate things as my feeble brain will go into a complete overload. I've already set it all up according to the zero point I acheived with the laser incidence meter. It looks like I'll be good to go now. Thanks for all the input guys.


Joe

[kdwzagi]

1mm per inch chord it can't be much simpler unless you want me to come round and do it for you !!!!! :D :D :D :D

i forgot you had a laser incidence meter you flash g~~ :D :D :D

all the best