wing angle
11-13-2005, 12:38 PM
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wing angle
How much more life will a wing generate if it is given 1 degree of incidence than the same wing at 0 degrees? What will be the effects on top speed? For reference I'm thinking in terms of the homebuilt Dragonfly that has two wings roughly the same size just at different angles.
11-13-2005, 01:33 PM
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RE: wing angle
The wing will only generate enough lift to hold the plane up at any given airspeed. It does this through pitch stability and the pilot trimming in the correct Angle of Attack with elevator trim. The incidence angle the wing sits in the fuselage at is just a convienient way of setting the wing to some basic angle so that at one particular speed the model will fly with the fuselage level and more or less directly pointed into the airflow. The incidence angle on it's own has noting to do with the wing's lift.
This is why racing models will set the wing with a zero or very near zero incidence angle because the model will be flying fast enough so that the angle of attack is also close to zero and the fuselage will offer the least drag. On the other hand a trainer will usually be flown at a lower airspeed and thus require a higher angle of attack. So those wings are set with a higher incidence angle so the fuselage does not look like it's trying to fly uphill all the time. Oddly enough the second option is more for looks as some extra drag in a trainer is actually a good thing as it prevents the model from accelerating too fast when the nose drops..... but I digress.
This is why racing models will set the wing with a zero or very near zero incidence angle because the model will be flying fast enough so that the angle of attack is also close to zero and the fuselage will offer the least drag. On the other hand a trainer will usually be flown at a lower airspeed and thus require a higher angle of attack. So those wings are set with a higher incidence angle so the fuselage does not look like it's trying to fly uphill all the time. Oddly enough the second option is more for looks as some extra drag in a trainer is actually a good thing as it prevents the model from accelerating too fast when the nose drops..... but I digress.
11-13-2005, 02:01 PM
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RE: wing angle
So, if two of the exact same wings are spaced such that the center of gravity is exactly between the wings the incidence won't change the center of lift? Assume for a second that on a design such as the Rutan Quickie or the Dragonfly the canard and main wing have the same airfoil, the same chord, the same span, and the same area; the center of gravity is directly between the canard and the main wing. The canard has 1 degree of positive incidence and the main wing is at 0 degrees. Which wing would stall first. Would each wing create the same amount of lift at the same airspeed?
11-13-2005, 04:23 PM
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RE: wing angle
Ah, I see where you're going now.
What you're describing is a tandem wing. But tandem wings must pay heed to the same rules of stability as any other conventional or canard planform. Those rules state that there's a Neutral Point where pitch stability will be zero. But to achieve a level of hands off positive stability the CG must be slightly ahead of that point. For your identical tandem wing the NP will likely be at the point equidistant between the two 25% chord points.(or not, I haven't run the numbers through the NP calculator to find out one way or the other)
When you do this the design wants to dive so we set an angular other aerodynamic difference into the two surfaces with the forward surface being set to a slight positive angle or higher lift configuration WRT the rear surface. Such a setup can be done with different airfoils or with angular difference if the airfoils are identical. This sets up a nose up pitching moment that is used to counter the nose down tendency of the CG being ahead of the NP. The fact that the nose up pitching moment is speed sensitive means that the design will trim to a stable speed. The final speed being set by the point at which the lift difference moment matches the nose down moment.
In your scenario the wings will provide this difference by angling one to the other and that forces the leading wing to a slightly higher lift coefficient than the rear wing. It also means the rear wing is not working as hard as the front wing.
It's actually inaccurate to describe this balance as a "Center of Lift" as there truly is no stable center of lift. There is a point for any given airspeed where the lifts will resolve to but that point is variable depending on airspeed. It is far easier and more conventional to show this force as a speed dependent rotational torque about the NP of the craft. So for steady flight what you have is a set of balanced twisting pitch forces that operates around the NP of the craft. The effect of gravity on the CG being ahead of the NP countered by the nose up pitching action of the differing lift coefficients of the two surfaces.
Now with both these surfaces sharing a common airfoil it also stands to reason that the forward surface will stall first since it will always be flying at a degree or two more positive angle of attack than the rear wing.
That help?
What you're describing is a tandem wing. But tandem wings must pay heed to the same rules of stability as any other conventional or canard planform. Those rules state that there's a Neutral Point where pitch stability will be zero. But to achieve a level of hands off positive stability the CG must be slightly ahead of that point. For your identical tandem wing the NP will likely be at the point equidistant between the two 25% chord points.(or not, I haven't run the numbers through the NP calculator to find out one way or the other)
When you do this the design wants to dive so we set an angular other aerodynamic difference into the two surfaces with the forward surface being set to a slight positive angle or higher lift configuration WRT the rear surface. Such a setup can be done with different airfoils or with angular difference if the airfoils are identical. This sets up a nose up pitching moment that is used to counter the nose down tendency of the CG being ahead of the NP. The fact that the nose up pitching moment is speed sensitive means that the design will trim to a stable speed. The final speed being set by the point at which the lift difference moment matches the nose down moment.
In your scenario the wings will provide this difference by angling one to the other and that forces the leading wing to a slightly higher lift coefficient than the rear wing. It also means the rear wing is not working as hard as the front wing.
It's actually inaccurate to describe this balance as a "Center of Lift" as there truly is no stable center of lift. There is a point for any given airspeed where the lifts will resolve to but that point is variable depending on airspeed. It is far easier and more conventional to show this force as a speed dependent rotational torque about the NP of the craft. So for steady flight what you have is a set of balanced twisting pitch forces that operates around the NP of the craft. The effect of gravity on the CG being ahead of the NP countered by the nose up pitching action of the differing lift coefficients of the two surfaces.
Now with both these surfaces sharing a common airfoil it also stands to reason that the forward surface will stall first since it will always be flying at a degree or two more positive angle of attack than the rear wing.
That help?
