I had an interesting email (off-net) from one of the fellow netters - thank you sir.
I have been around since 1949 so I am aware of most broad strains of rationale regarding wing lift (and then some). When we get down to brass tacks and sort through the 10-100, most of the various hypotheses actually support reach other, but I hate it when the simple facts (truth) become distorted and difficult to comprehend because of an over supply of words, theories and ego. There is no reason why the regular builder /pilot should not have a reasonable understanding of how these things actually work (and let the academics go debate the higher sciences to their hearts content) The demonstration that placed everything into perspective for me was really brilliant and would not be that difficult to replicate. This is a highly visual demonstration that really settles all arguments. Maybe someone teaches the subject or would like to make one for the next hangar meet. There is much to be learnt from this, we can even clearly see how the centre of lift (pressure) moves around as the angle of attack varies. One simple drawing would tell it all, but let's see how far we get with words. The demo comprised of a wing section with a chord of about 30" and a similar span - no more than a short piece of typical wing with a simple 15% section and not much else - no spars etc. There was a series of holes down the centre of the wing, all along the "rib line" from front to back, top and bottom. Each such aperture was connected, via a flexible tube, to a glass U tube that was half full with colored water. By subjecting the wing to an air flow, it was possible to visibly display the actual degree of pressure (positive or negative) that the wing surface was experiencing at any point along the chord. The glass tubes were side-by-side, so the resultant image was almost like a graph. The lift profile (water height in each tube) formed a parabola starting just behind the leading edge, progressively increasing to a peak at about the 30% aft point - then tapering of to zero at the TE. This was a vivid and visual, live view of the lift "sucking" away at the surface of the top skin. It was also possible to vary the angle of attack and observe (in real time) how the peak moved fwd (and higher) as the AofA increased. The area of the curve encompassed by the points on the parabola sort of gave a pictorial representation of the quantity of lift. Similar results were displayed in the second set of glass tubes displaying the pressure from each point under the wing - except that the parabola was inverted, flatter and approx 1/2 the area generated on the top of the wing - really. I have seen illustrations of something very similar in a book - "Flight Without Theory" - I think the author is AC Kermode (sp??). This publication has a similar illustration of the span-wise lift profile at the Clmax - very useful for someone trying to figure out some wing tips to actually see how little lift is being generated in this zone (and how useful some winglets may be) A similar, simpler model of smaller proportions could be made out of some scrap 015" Alclad, ABS plastic, heavy card, 1/32" ply, etc. With some fish tank tubing, a dish of water (with food coloring) and a leaf blower - we could have a model that would serve well to illustrate the point. Please remember - this is all about the importance of the bond between the top skin and the spar(s). One last plug at this comes from my RC experience - I am sure many netters will relate to that. It was a sport model with solid foam /balsa wings that were covered with one of the glossy heat shrink materials. This airplane flew really well, but every so often it would suddenly enter a diving turn - always to the same side. Obviously everything in the radio dept. was suspect and gradually swapped out until it was painfully obvious that I had a strange aerodynamic problem. During yet another test session (with high pucker factor) one eagle eyed observer said something about one wing puffing up just before the airplane went berserk. The penny dropped - I recovered the wings making sure that the covering was well bonded all over. Thereafter the Mongrel was a treat to fly. The lift was sucking the covering away from the wing on one side only and acting like a massive airbrake, spoiler and aileron all in one. Take care Steve J
