Let me add a bit to the discussion.

At McDonnell Douglas we called the FRL a Water Line (WL). The nomenclature was stolden from the boat builders. That is a line that you wanted the hull of the boat to be level when at rest. In airplanes it is a line that we might want to always be level when the airplane is flying at 1g level flight.

To make something like a transport in full scale a designer and aero engineer would huddle over his drawing board. The designer says the cargo floor must be level in flight and have a certain volume. He draws a horizontal line and calls it zero WL. Things above it are positive and things drawn lower are negative. It's just bookkeeping as far as the designer goes.

The designer draws a fuselage around the zero WL. The wheels would be negative and if it is a high wing the wing will have a positive WL. The aero engineer says OK, now based on our calculations for the lowest drag when attached to a fuselage and to get the needed lift at the speed we are flying the wing should be at an angle of attack of 4 degrees. The designer draws in the wing at 4 degrees with respect to the WL and calls it the WRA (Wing Reference Angle). So if the fuselage is flying level the wing will be at 4 degrees angle of attack.

Based on some more calculations (after all he has gone through 4 years of college and 10 years experience and is supposed to know this stuff) the aero guy says for stability and trim he has determined that the tail needs to be at an Incidence Angle (IA) with respect to the wing of 6 degrees. He is thinking of the wing and tail connected with an invisible fuselage but with the whole assembly flying level.

The designer says, hummm, a 6 degree Incidence Angle and I have the wing at 4 degrees already and I don't want to change that then the tail must be at a -2 degrees. The Incidence Angle is a total angle between the two surfaces.

The model is made for a wind tunnel tests, the test ran and the data tells the aero guy that he was close (I am an aero guy and writing this so of course he will be close) but the wing needs a little more lift and the tail is a little off. The designer makes the WRA a degree more, the TRA a degree more in the negative direction. Always keeping the balance of wing and tail forces so that the fuselage will fly level. So we have the WRA = 5, TRA = -3 and Incidence Angle = to the difference between them = 8 degrees.

Again it is bookkeeping. The need to discuss the same thing no matter who is in the discussion. The designer thinks in terms of WRA and TRA and the aero type in terms of angle of attack and Incidence Angle. They are all the same thing just looking from a different point.

The ideal design would have a varying wing and tail angle so that they could be individually adjusted in flight for the lowest drag while always keeping the fuselage cargo hold at zero WL.

Looking at our RC models we can note some trends. Always keep in mind that the designer wants the fuselage to look right in the air. When he draws that look on the paper and draws a horizontal line through it that is called the WL. Then the angles are measured with respect to that WL.

Pattern airplane WRA = 0 TRA = 0 Incidence = 0
Sport airplane WRA = 2 TRA = 0 Incidence = 2
Trainer airplane WRA = 3 TRA = -2 Incidence = 5

If you know the WL and any two of the above the third one can be determined.

Keep in mind that the best way to design an airplane if you don't have experience or knowledge is to copy a successful design. It is the equivalent of the professionals performing wind tunnel and flight tests. Just someone else has done it for you! Don't start from scratch each time. The pros don't.

After you have spent a lifetime of messing with models it becomes automatic but the brain is still going through the process. The infamous TLAR (that looks about right) is actually a very complex design process that the brain carries out. The only difference between an engineer and a normal guy using TLAR is that the engineer knows why, the TLAR works by feedback from previous models and what works. It is nicer to know why in my experience.

Did this help any?

Merry Christmas to all including RAPPTOR!

Ben