AIR PERFORMANCE 127 



Take a sheet of celluloid a little bigger than 1 2 inches by 

 8 inches. It should be about - mm. to I mm. thick, since a 

 moderate degree of flexibility is an advantage. Celluloid gener- 

 ally has a pronounced camber : it is very rarely quite flat. Lay 

 it down then hollow side downwards and scribe a line 1 2 inches 

 long near the bottom : this is the speed axis ; also a line 

 8 inches long, at right angles to the first, near the right-hand 

 side : this is the horse-power axis. 



Now lay the sheet (still hollow side downwards) on top of 

 the curves of page 132, axis lying on axis, and having weighted 

 it to hold it still, trace the curves (the squares are not wanted, 

 of course) on to the celluloid with french curves and a scriber. 

 A compass point (a coarse one, not a needle point), or better the 

 point belonging to a set of trammels, will make a good scriber : 

 the point should be quite sharp but not at a fine taper, and round, 

 not like a knife blade. No difficulty will be found in making 

 clear scratches on the celluloid with a light pressure. 



The curves of page 132 will cover the right-hand bottom 

 quarter of the celluloid. 



Then trace the curves of page 133 continuously with those 

 already traced : they are the left-hand bottom quarter. 



Then trace page 134 (top right-hand corner) and page 135 

 (top left-hand corner). 



The celluloid throttling curves are now ready for use. By 

 the way, do not attempt to rub down the burr left by the scriber 

 with any abrasive, as that would make the celluloid difficult to 

 see through. 



In use, of course, the celluloid sheet is turned hollow side up 

 so that the scribed lines are right down on the paper, the curva- 

 ture of the celluloid makes it easier to press it flat on the paper, 

 the origin is at the bottom left-hand corner, the axis of speed is 

 along the bottom and the axis of horse-power is along the left- 

 hand side as all our performance curves are plotted. 



Most designers plot performance curves, whether on inch or 

 millimetre squared paper, on a moderate area, and so the 12 inch 

 by 8 inch size of throttling curves will be all right (they can be 

 used as they stand for performance curves plotted in any units, 

 or any scales, and on paper divided on the English, Metric, or 

 any system, by the way). 



If a designer is in the habit of plotting performance curves 

 on a larger area of paper than 1 2 inches by 8 inches, however, 

 and intends to continue his practice, he should first plot a family 



