Have questions for the aeronauts to answer pertaining to wings. I have seen more than one way to measure the below in publications and plans dating from the late 1920's to present date.


1. How is wing area determined?
a, measured vertically with wing laying flat
b, measured vertically with wing set at proper dihedral angle. (This can make quite a difference on a free flight model with large dihedrals).


2. How is chord length determined?

a, measured horizontally as overall length with wing laying flat
b, measured from farthest point of trailing edge to farthest curve of leading edge.
c, measured along b, but at flying angle for straight and level attitude.
d, measured from farthest point of trailing edge to farthest curve of leading edge when set into proper incidence angle in fuselage.


3. How is dihedral angle placed in degrees when not noted via offset on plans?

a, Along top surface of wing.
b, along bottom surface of wing
c, along centerline based on the taper of ribs from the average of top and bottom surfaces.
d, through center of rounded portion of wing tip, no matter high or low.



4. How is wing span determined.

a, it is the actual overall width of model at furthest point of wing tips.
2, it is the accumulation of all the portions of the total wing as measured flat on plans.


Now rotating angle of incidence, dihedral angles to get different distances may make for just a few thou in measurements on a model, but it does make for a couple square inches when calculating wing areas.


Wm.

How to measure depends on the use to which the measurement would be put.
Assuming that the use of the measurements will be data for aerodynamic functions:

1. Wing area projected onto a horizontal plane with the chord line at the wing root horizontal.
2., b.
3. The dihedral angle is measured from the horizontal to a line connecting the points 25% of the chord from the leading edges, with the root chord line horizontal.
4., a.

If, for example, you were using the wing area measurement to determine the weight of covering material, or the wetted surface of the wing, then the area would be the developed length of the airfoil surface times the panel length, summed across all panels.

As a practical matter, the differences in definitions, where angles of less than 5 or 6 degrees are involved, will only show up in the third significant figure and can be safely ignored for many purposes. That is the difference between a length and that length times the cosine of an angle less than about five or six degrees.

The percentage differences you're talking about are trivially small. It only ever matters for competition models with specified min/max values and in those cases the regulations will usually tell you exactly what they intend you to measure.

Steve

For example, all of the FAI rules for international freeflight competition that include area limits refer to "...measurement of the orthographic projection...". Whenever I have had models processed the actual panel dimensions are measured, not the vertical projection.

Yes, the differences are small. But as I found out to my cost, an error of 2mm can make a difference between complying and not.

Well, I mention all of this evasive stuff as I had a conversation with an old fellow who used to judge and grade models back in the 1930's. Seems there are differences in just how these numbers were figured between then and now. The models were in contests for performance, free flight in power and rubber catagories. He showed me a reprint of some model airplane magazine article which applied to these same questions. It seems the fine gentlemen competitors of then were pretty devious as to how the rules should be interpreted.

Most of the stuff had to do with maximum wing area for particular classes of competition. Things were loose for awhile, until some judge showed up with a set of venier calipers to do the measurements stated, and then things tightened up. If you built a model from some kit or plans with wing flat on a table, and used the cited measurements for wing area calculations, you were pretty well home free. However, if you used the interpretation of the rules to advantage, you could have a larger appearing wing and still be graded as meeting the same area rule.

The fellow's interpretation came as thus:

The chord for a wing was measured at flying attitude. If you had five degrees or ten degrees of incidence built in to the normal flying attitude, then your chord length was measured at the flying attitude. This 5-10 degrees could in affect shorten the numerical value for the chord by 0.020 to 0.050 inches. Not a lot, but if you are trying to be under the maximum area rule, it could add another inch of span. Then again, if you did not sand the trailing edge to knife like precision, the little 1/16" width edge may be considered to add back a few hundredths into your calculation.
Then being more creative, a few guys tried to reverse the semisymetrical and Clark-Y airfoils on a wing, by building a heavy incidence angle in and then claiming a shorter chord.

The wing span was also measured with calipers. These came from a machine shop which used to rebuild steam locomotive cylinders. They were big, heavy and accurate. They did not go up the chord, or direct to tip, but located whatever was the furthest out. As most people at that time built with rounded tips, the furthest point was pretty obvious. If you build a wing on a flat surface then add in some dihedral angle, it will take away about 1/2" from the span. Then with a chord measured the standard way, the calulated wing area could easily differ 2 square inches per 100. I don't know how much advantage a couple square inches may gain, but they were not leaving it up to doubt.

The former judge mentioned any new wing was strutinized as to area and placement. The builder was required to give hand written dimensions and a set of calculations for determining the area when he presented the model to the board. The model weighed, the wing measured as indicated, and then a quick run through the calculations. The judge mentioned most times they just approved the stuff due to time constraints. However one time, he had pocketed some of the entry material and had some office worker run through the calculations. He found that a few contestants when calculating area of curved surfaces had used the wrong Sine-Cosine values. And they always erred to the smaller side. He next time broght along a book of Sine-Cosine values, would check such on the contestant's calculation sheet, and have them refigure the new area right on the spot!



Wm.