Tweet
I have been wondering about the design angle of attack of the Taylorcraft's wing. We all know about the procedure of using a level across the spar bottoms out at the last full rib on the wing in order to adjust the wash-out and rig the wings. That is a nice pragmatic procedure for rigging, but really doesn't tell us much about the actual angle of attack designed into the airplane. The Type Certificate doesn't go to this level of engineering detail, and none of the drawings that I have run across have the information. So I got curious.
My fuselage is sitting in the hanger without wings waiting for its wings to go back together, so I took a rib, ran a line of tape from nose to trailing edge to mark the chord-line, and taped it in place against the attach fittings and along the fuselage top in (what I hope) is good alignment. The fuselage is levelled with a level on the horizontal stab. Now it is fairly simple to measure the angle of the chord-line. It seems easiest to measure from spar to spar and I find that the level-line from the rear spar to the front spar drops 2.5 inches in the 31 inches from spar-center to spar-center. If I remember my high-school trig... lets see 2.5/31 should be the sin of alpha... I get an angle of attack relative to the level on the stabilizer of 4.625 degrees.
Now one big surprise is that I notice that the placement of the spars in the ribs is such that the bottom of the front and rear spars are level with the chord line!!! On most wings, laying a level on the top or bottom of the spars tells one little or nothing about the actual chord line. I have seen a number of home-builts mis-built due to this mistake. But on our beloved Tcrafts, the bottom of the spars gives us a very convenient place to measure the actual chord. Now the book procedure for rigging the wash-out when installing the wings makes a lot more sense. The book uses a 30 inch level rather than the 31-inches centre to centre of the spars, but using 1 5/16 inches over 30 inches and take the inverse sin... gives exactly 2.5 degrees angle of attack at the tip. So there seems to be 2.1 degrees of washout in the wing.
With that bit of analysis, a number of questions come to mind.
That's probably enough to get started. There is little we can do to the the existing airframes, but if we want to daydream about replacements, this is (possibly) an interesting question.
Skip Egdorf
TF #895
BC12-D-4-85 by way of STC-SA1-210
N34237 SN 7700
My fuselage is sitting in the hanger without wings waiting for its wings to go back together, so I took a rib, ran a line of tape from nose to trailing edge to mark the chord-line, and taped it in place against the attach fittings and along the fuselage top in (what I hope) is good alignment. The fuselage is levelled with a level on the horizontal stab. Now it is fairly simple to measure the angle of the chord-line. It seems easiest to measure from spar to spar and I find that the level-line from the rear spar to the front spar drops 2.5 inches in the 31 inches from spar-center to spar-center. If I remember my high-school trig... lets see 2.5/31 should be the sin of alpha... I get an angle of attack relative to the level on the stabilizer of 4.625 degrees.
Now one big surprise is that I notice that the placement of the spars in the ribs is such that the bottom of the front and rear spars are level with the chord line!!! On most wings, laying a level on the top or bottom of the spars tells one little or nothing about the actual chord line. I have seen a number of home-builts mis-built due to this mistake. But on our beloved Tcrafts, the bottom of the spars gives us a very convenient place to measure the actual chord. Now the book procedure for rigging the wash-out when installing the wings makes a lot more sense. The book uses a 30 inch level rather than the 31-inches centre to centre of the spars, but using 1 5/16 inches over 30 inches and take the inverse sin... gives exactly 2.5 degrees angle of attack at the tip. So there seems to be 2.1 degrees of washout in the wing.
With that bit of analysis, a number of questions come to mind.
- Did I do it right?
I am not an aeronautical engineer, and I usually screw this sort of thing up mightily when I try to expose my level of ignorance on the net. So my first question is "Did I get the basics right here?" Please be gentle when you point out the mistakes. - How typical?
How typical is 4.6 degrees AoA with 2 degrees washout? Does anyone know the numbers for Champs, Cubs, Monocoupes... - Spar-bottom alignment
I wonder if CG Taylor designed the spar placement in the ribs so that the spar bottoms were parallel to the chord. Or was it just a happy coincidence. Everything that I read about CG's skills lead me to believe that he probably did it on purpose. - How much does it matter?
I don't have sufficient aeronautical engineering background to find the formulas, but my intuition tells me that there are a few knots rattling around here somewhere. How much difference would a degree of AoA make in the performance of our planes? - Different models
The fuselage wing fittings are non-adjustable. Did CG Taylor re-calculate the angle of attack from the 50 HP to the A65 of the BC12? As the power and weight go up, the angle of attack should decrease in order to keep the fuselage and tail drag at optimum values. Did the newer Tcrafts, F19 and F22, have a different wing attach geometry than the earlier? It seems that CG did the design properly as our planes seem to be well optimized, but how much did they change over time as power and speed increased? - STC SA1-210
When we upgrade an A65 BC12D to a C-85, power goes up about a third. should a properly optimized airframe have a slightly lower angle of attack to keep the fuselage and tail at optimum lowest drag for the higher power?
That's probably enough to get started. There is little we can do to the the existing airframes, but if we want to daydream about replacements, this is (possibly) an interesting question.
Skip Egdorf
TF #895
BC12-D-4-85 by way of STC-SA1-210
N34237 SN 7700
Attached Files
Comment