decalage, and a mushroom cloud of balsa

[Balsa Pilot]

Well, the story starts with a pretty little O1E- Bird Dog. I THOUGHT it would be great to do the test glide(s) from my 2nd floor balcony. Turns out I was dead wrong. I had balanced the aircraft in a jig-1/3rd of the distance from the tip to trailing edge, and, The only thing I could think of was the stab/elev combination was really out of whack. After picking up all the scattered pieces of tissue and balsa, I remembered the term decalage. This is I believe the angle of the chord line of the wing relative to the longitudinal axis of the fuselage(as well as the stab/elev).
The questions I have are as follows:
A. How does one set the angle of the tail and wing relative to each other-accurately? or is this the ol'Mk.1 eyeball?
B. Rudder/fin set relative to the fuselage- same idea?
C. Is the balanced aircraft supposed to sit in any relation to the angle of the fuselage/wing/stabilizer or do you just trust the manufacturer to be on the money? and is the aircraft supposed to sit very slightly nose low or at the same angle of the wing fuselage?

Any help is (as usual) greatly appreciated.

Thanks Henry

[heywooood]

Not sure if this helps or if the images will appear as intended...
But the idea is to make sure all aspects are symetrical as shown (if shown)

As to wing incidence- depending on the aircraft there can be a need for zero incidence or incremental positive incidence, on the order of a degree or two up to maybe a maximum of 5 degrees..

[heywooood]

I usually cut a balsa strip and use it to check the dimensions...you can use string or something else that is flexible but I like a balsa strip because I can make pencil marks on it.

Double check your datum points and make sure you work slowly. Once you have the surfaces where you want them - pin in place - then use a slow cure adhesive so you can make corrections before it sets - so you dont end up with a crooked kite.

[heywooood]

One other trick is to use a drafting triangle - they are lightweight, typically made of clear plastic. I use one on every build
to get the wing and fuselage ribs at right angles to the building board and to set the tail at right angle to the fuselage.
it can help during your build if you draw a large cross on your board - with a perfect 90 degree angle, so you can use it for reference throughout your build.
Like this

[tom arnold]

I think the first lesson to take away from this is not to make your first test glide from a second story window----way too far to fall. During the building phase you should make sure that your wing bottom and the horizontal stabilizer bottom are parallel. Then build in moveable elevators hinged with soft wire such that the elevator can be bent slightly up and that the wire is stiff enough to hold it in that position. In the case of a tail dragger like a Bird Dog, the CG is slightly behind where the main gear touches the ground when the plane is in a horizontal position (a tricycle gear aircraft has the CG slight AHEAD of the main gear). Now find some tall grass to test glide over just from a standing height. Aim the aircraft at a spot about 10-15 yds ahead and give a gentle push into a glide. With the stab and wing parallel, the model should dive into the ground (the reason for the tall grass). Bend the stab up a little bit and repeat until you get a nice straight flight path to your target on the ground. You can accept a little "float" but not much.

Never move the CG. Now you can start some low powered winds and as the winds increase the model will naturally go up into a stall with increasing power. Keep that from happening by putting in some downthrust in increments to keep the powered flight path from going up and down. Guillows does not make any provision for tweaking in adjustable downthrust so a mechanism for building it in needs to be done on the workbench during the build. Check the posts of Mitch and his WWII fighters for some great tutorials on how to do this. He really can make a Guillows model fly.

[Balsa Pilot]

thanks all for the input. How does one ensure that the wings and horizontal stab are parallel?

[tom arnold]

Take a couple of very dead straight sticks of balsa, lightly rubber band them to the bottoms of the stab and wing---they should be long enough to touch (or overlap) each other--and you can easily see the difference. In fact, should you ever want a particular angular difference between the two, draw a couple of lines on a sheet of manila card and line up the sticks with them. Of course, you will have to juggle things around regarding the lines on the manila depending on the placement of wing and stabilizer. The technique depends on the bottom of both the stab and wind being flat.

[tom arnold]

You will read that the wing should be 2-3 degrees nose higher than the stab and that is OK as long as you go through the trimming process and adjust the elevator to get the straight glide. The problem is that many times due to bad CG placement or poor estimating of the 2-3 degrees, you may find that the only way to get a straight glide is with down elevator which is no good and is basket full of problems later with power. By starting out with the wings/stab parallel, you will always get an "up" elevator move in trimming. This will make power trimming infinitely easier.

[slopemeno]

Remember that the zero-lift line of your wing runs from the center of your leading edge through the center of your trailing edge. You might be building in more decalage than you think.

I build mostly fast, heavy slope soaring gliders, but the technique is the same. For instance- I recently finished an F-20 sloper. I glued the wing in, and cut the slot for the tail. Then I set the plane up on the dining room table, set the wing at 0 degrees relative to the table, and used a stack of CD jewel cases with a sharpie taped to the top to draw a 0 degrees line for the tail on both sides of the fuselage. For rubber powered or small stuff, you can do things like set the plane in a banker box with the wings resting in the edge of the box, and the fuse inside the box. From there- check and adjust

All the hard work of making sure everything was zero-zero-ninety paid off- that F-20 needed one click of trim on the ailerons on the maiden.

[David Lewis]

In order of increasing slope:
1. On a flat bottom airfoil, the lower chord (i.e. the flat bottom) is the rigger's line, a reference line from which, for convenience, model builders and designers usually measure incidence angles.
2. The line from the center of the LE radius to the TE is the chord line (simplifies the geometric definition of the airfoil shape).
3. The zero lift line of an airfoil rarely has practical usefulness as far as model design and trimming. When the zero lift line is parallel to the relative wind, the airfoil doesn't develop any lift.

Decalage is the difference in incidence between two flying surfaces, most commonly between the wing and the stabilizer. The correct amount of decalage depends on CG location. The farther forward the CG, the more decalage you will need. Where should the CG be located? That's controversial. In my opinion (on small rubber FF models) it should be as far back as possible consistent with adequate longitudinal stability.