oriol
Well-Known Member
Hi!
I started building an RC aircraft to be able to design an aircraft, and better understand the theory. As a sort of rule of thumb, I have noticed that many aircrafts with flat bottom airfoils, have the lower surface parallel to the bottom of the fuselage.
The wiki tells that a common angle of incidence in sport aircraft is 6º. In my case I picked the NACA 4418, because it is the most possible thick airfoil for optimal strength. I followed Pazmany's advice, and picked an Aspect Ratio of 7 as a good compromise.
According to Theory of wing sections. The optimal region for cruise for the NACA 4418, is between 0 and 0,6 CL, that is between -4 and +6 degrees.
My lift slope for a finite wing of Aspect ratio 7, is around 0,08 per degree of AoA. I picked 0 degrees angle of incidence, which results in a CL of around 0,32 for cruise. I did not mess much with reynold numbers for my model though.
A more experienced RC builder told me that a few more degrees, of wing incidence, lower surface parallel to the fuselage, would be better for taking off, but with flaps it would be OK.
By following my approach, the flat bottom of the airfoil is not parallel to the bottom of the fuselage. Am I doing something wrong, by trying to optimize the angle of incidence, instead of sticking to the tradition of the lower surface parallel to the bottom/6º wing incidence?
What I did copied from other airplanes, is a decalage for the horizontal tail, parallel to the center line/ bottom of the fuselage. The slipstream of the wing, gets to the horizontal stab with enough incidence to provide negative lift.
Below are pictures of some random airplanes, with the lower camber parallel to the bottom of the fuselage. The only exception is the Dornier 27, which is a STOL airplane, and my RC aircraft which is the opposite; the lower camber has a negative angle relative to the bottom of the fuselage.
Many thanks for sharing your thoughts about this issue!
Cheers
Oriol
I started building an RC aircraft to be able to design an aircraft, and better understand the theory. As a sort of rule of thumb, I have noticed that many aircrafts with flat bottom airfoils, have the lower surface parallel to the bottom of the fuselage.
The wiki tells that a common angle of incidence in sport aircraft is 6º. In my case I picked the NACA 4418, because it is the most possible thick airfoil for optimal strength. I followed Pazmany's advice, and picked an Aspect Ratio of 7 as a good compromise.
According to Theory of wing sections. The optimal region for cruise for the NACA 4418, is between 0 and 0,6 CL, that is between -4 and +6 degrees.
My lift slope for a finite wing of Aspect ratio 7, is around 0,08 per degree of AoA. I picked 0 degrees angle of incidence, which results in a CL of around 0,32 for cruise. I did not mess much with reynold numbers for my model though.
A more experienced RC builder told me that a few more degrees, of wing incidence, lower surface parallel to the fuselage, would be better for taking off, but with flaps it would be OK.
By following my approach, the flat bottom of the airfoil is not parallel to the bottom of the fuselage. Am I doing something wrong, by trying to optimize the angle of incidence, instead of sticking to the tradition of the lower surface parallel to the bottom/6º wing incidence?
What I did copied from other airplanes, is a decalage for the horizontal tail, parallel to the center line/ bottom of the fuselage. The slipstream of the wing, gets to the horizontal stab with enough incidence to provide negative lift.
Below are pictures of some random airplanes, with the lower camber parallel to the bottom of the fuselage. The only exception is the Dornier 27, which is a STOL airplane, and my RC aircraft which is the opposite; the lower camber has a negative angle relative to the bottom of the fuselage.
Many thanks for sharing your thoughts about this issue!
Cheers
Oriol
Attachments
Last edited:
