Sep 29, 2015, 05:30 PM Alan Cambridge Joined Sep 2005 751 Posts Discussion Measuring dm2 on wing + v tail I'm curious about how one goes about measuring the area of the wing and tail to work out the dm2 of a model so you know the max weight per fai.regulations. I've been told that it is only the horizontal projection you measure hence swept up tips are effectively flattened so the measured area is very slightly smaller than the actual area. Is that correct? How does a vtail get measured?
Sep 30, 2015, 01:52 AM
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United Kingdom
Joined Jan 2007
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Hi Alan, I may not be the best person to answer this but at the world record attempts in Germany this year I had my plane measured: The tool they used for marking down the shape of the wing and tail was basically a small engineers square with a Biro pen in the corner, so that when you ran the square along the side of the wing, it marked down the shape of the wing onto a large piece of paper. I searched Google quickly but couldn't find a picture to show you.

The model's area in effect is only measured as you would see it in a plan view (from above).

The wing was left upright (as it would rock on the wing tips when upside down), it was held firmly down onto the paper by a few people while someone else took the engineers square and ran it around the whole outline of the wing. The same was done for my T tail.

So a V tail would be done in the same way, but be put upside down (for ease of measuring, unless you can clamp the model down without it moving) and someone would use the square to mark out its area. The area would not be measured as if the two V's were flattened out, but simply as you see it from above.

After this is done and you are pleased with the sketched outline, the guys in Germany used something called a 'Digital Planimeter tool' and this measures the perimeter of what you have just drawn. If you have a read of the below link, you may be able to get an idea of what one does and they describe it better than I ever could..... If not, watch one of the videos below to see how it works.

I am at work so cannot open the YouTube links, but they may be able to explain how they work.

 Planimeter Planix 7 Area Measuring Tools (0 min 37 sec)

 Digital planimeter demonstration (4 min 39 sec)

As far as I am aware, when you have finished, there are several different readings that are given on the screen. The officials did a small calculation with the reading given and the outcome was the area of my wing and tail.

I hope this helps a little, maybe someone else will see this and would be able to elaborate on what I have written.

Tom.
 Sep 30, 2015, 12:56 PM f5b-uk United Kingdom, Dorset Joined May 2002 3,198 Posts I think you've got the answer you want, and it is the projected area that is measured. However, it is interesting how the officials choose to work out this projected area. At the WC's I've been to they have used: 1996 Czech Republic - planimeter 1998 Germany - attempted to take a digital photo and count pixels to approximate wing area. The technology was not good enough and everyone was given a huge margin of error and no one failed due to wing loading. 2000 San Diego USA - planimeter 2004 York, UK - several chords across the span of the wing were measured and the area was mathematically approximated 2006 Romania - planimeter I don't know how expensive a planimeter is now, but my cheap method was to trace onto 80g/dm2 paper, cut out the pieces and weigh them. Area in dm2 is calculated as mass of paper / 0.8
 Sep 30, 2015, 12:58 PM f5b-uk United Kingdom, Dorset Joined May 2002 3,198 Posts BTW, the area of the wing that covers the fuselage IS included in the measurement but the tabs at the le and te of the wing are not.
 Sep 30, 2015, 01:05 PM Registered User UK Joined Mar 2005 202 Posts It is projected area and I have a very expensive and calibrated planometer that can be used to measure the area of irregular shapes. The problem comes when you have to trace around the wing or tail as the tool used must be guaranteed to have followed the true size and shape of what's being measured. It is probably as best to go with the manufacturer's figures as these will have come from the original production files.
 Sep 30, 2015, 03:25 PM Registered User Hemet California Joined Jul 2009 1,216 Posts A way I have used is kind of touched on above, I get a heavy paper/posterboard and trace the projected outline on then cut that out for wing and tail. Then cut a precise 1 dm square out of same paper and weigh it... Weigh your planes outline and divide the 1dm square weight into the planes outline weight...