Determining Wing and Tail Incidence

[Utahrd]

Are there any rules, or at least principles, that should be followed when it comes to setting the correct wing and tail incidence for a fast slope plane? Actually, I’m not sure I’m using the correct terminology, so let me state the question another way…

First, how do you know what the wing angle should be relative to the fuselage?

Second, what should the angle of the horizontal tail plane(s) be relative to the wing? Should the wing angle and tail plane angle always be the same?

I’m interested in all tail configurations and especially v-tail. If this is too complicated a question to answer here, can anyone recommend books or other information sources?

Thanks in advance!

Charlie

[Sparky Paul]

A fast sloper will have minimal difference in incidence between the wing and tail.
It depends on which airfoil you use for the wing.. the less camber the better.
Having a generous tail moment with a minimally sized horizontal.. 2 to 3 chords, and maybe 15% of wing area, c.g. starting at the usual 30% point.
Fly and change to fit your preferences.

[hul]

I would do this.

Decide on a design speed. Calculate coefficient of lift for the wing for level flight at that speed.
CL=(mass x g)/ (1/2 x air_density x design_speed^2 x wing_area)
mass= mass of plane in kg
g= gravitational constant (9.8m/s^2)
air_density= 1.2kg/m^3 (approx)
design_speed in m/s
wing_area in square meters

example:
m=2kg, design_speed=20m/s, wing area=0.3m^2 -> CL=0.27

find the angle of attack your airfoil needs to generate that CL from its polars. Set wing incidence at that angle relative to fuse. Tail plane at zero relative to fuse.
This isn't exact but better than guessing.

Read this for tail sizes: http://www.rcgroups.com/forums/showpost.php?p=6248726&postcount=3

Hans

[Ollie]

Angles, angles, angles.

The purpose of incidence is to set up the plane in the shop, not in the air.
The many, many angles of attack are in the air.

The measurments of the airfoil refer two dimensional air flow. A real wing has three dimensional air flow so, you must account for induced angle of attack (mainly to aspect ratio), Cl (due inertial forces, speed, area and air density), sweep angle, twist angle.

It is better to use a different point of view. The incidence angle to an arbitrary line reference (like fuselage center line). What is important are other angles like decalage (tail incidence between wing incidence) thrust ( thrust line between wing incidence) pitch angle standing on the ground.

[yoyoML]

Angles, angles, angles.

The purpose of incidence is to set up the plane in the shop, not in the air.
The many, many angles of attack are in the air.



The purpose of incidence is not about the shop, but about cruise. You'd like the fuselage to produce the least drag during cruise, but the wings require certain angle of attack even when the fuselage stays level, pointing diretly into the airflow. That's why you want to set up the incidence the way hul said.

For an efficient cruise, all surfaces plus thrust line need their incidences set up with respect to the fuselage, or to the expected flight direction.

[Utahrd]

Wow! Thanks for all the great information. Not sure understand some of it, but at least I know more about what I need to learn. ;)

Thanks again.

Charlie

[Aceshigh84]

Unfortunately its not a simple answer as you can see by all of the different responses. You are lucky though because wing/tail incidence doesn't make a difference when it comes to stability of the aircraft. So if you can get your tail sizings correct in respect to your wing I would just make the wing and tail have 0 degrees incidence to the fuselage to start with. This will get you close. If you really feel the need to put some incidence in something put a couple degrees neg. incidence on the tail. You should be close enough to trim with minimal elevator. There may be more in depth rules of thumb for modeling however if you really want to learn how to pick incidences your going to need to get some college classes in stability and controls for aircraft or get the text book for the class and settle in for some long boaring reading. My undergrad text was called "Flight Stability and Automatic Control" by Robert Nelson. Its much quicker to go with the TLAR design method...

Matt

[Hangtight]

Everything said makes perfect (?!) sense, but as a bit of a newb to designing for the slope, what sort of cruise speeds can you expect/ demand for different sized planes? Apologies if I'm hijacking, but this is all interesting stuff! :)

As an example, the Shuriken in G3 seems to have trouble cruising around at anything less than 50mph, even in light (10mph) conditions, while the Moth will quite happily bimble around at 25mph. How do you make an approximation for a given WS, foil, wing loading etc.?

[Aceshigh84]

A good guess for the CL that a slope glider is flying at while on the slope is aroune .2
a thermal glider will be more like .8

[Ollie]

If the flying wing has no fuselage and no tail, what is the incidence? If the wing has sweep and twist (washout), what do you reference to a arbitrary line? Think about it.

[Aceshigh84]

In that case you drop incidence from your calculations because it is not needed. Longitudinal static stability characteristics of a flying wing are easy to calculate in the sense that you don't have to make any assumptions about wing/tail interaction.