Distortion

Spanwise Distortion


When a ram air canopy inflates, air pressure causes the canopy surfaces to bulge between the ribs. This both effects the airfoil shape and draws the ribs closer together reducing the span of the canopy (and therefore the surface area). The more ribs we have, the less distortion and shrinkage occurs, but bulk and line drag increase. On a conventional (non-crossbraced) canopy, not only do the cells bulge they also zigzag up and down between load bearing and non-load bearing ribs, further distorting the canopy and further reducing its span/area.

To quantify this, bulge distortion alone reduces a 9-cell canopy area by about 9% and zig zag distortion by a further 4%. If you are jumping a 100sqft canopy, you are actually flying with 87sqft of wing area above your head.

With a crossbraced canopy, you still get the bulge distortion (reduced slightly through 21 chambers instead of 18) but zig-zag distortion is eliminated completely. On a 100sqft crossbraced canopy, bulge distortion will reduce your area by a mere 8%. So you still have 92sqft of wing above your head (compared to 87sqft), that's 5% more lifting surface.

Dynamic Distortion
When you look at a photo of a conventional canopy on full drive the zig-zag appearance is obvious, but on a landing photo, the zig zag appears much more pronounced - and it is! During flare, your canopy is both slowing down and pulling more load, which reduces the supporting pressure within the canopy and pulls it further out of shape. In fact, during flare, zig-zag distortion will increase to around 12%. Add bulge distortion and a 100sqft canopy is now providing 79sqft of lifting area on landing.

Source
This article is coming from NZ Aerosports Ltd. website and has been originally written about the EXT-treme FX. Its commercial content has been edited. NZ Aerosports gave its permission for reproducing and editing this article.