Jeez guys - never wanted to start a big issue here, just felt it important to stress the importance of the skin /spar bond - it has killed a few folks and it will probably kill a few more. I would be happy if I can do something to prevent just one future accident.
I considered leaving it alone, but I have seen so many constructive contributors give up that I do feel compelled to stay with it a bit longer. Let me be clear - I do not feel compelled to comment on everything that is said - even when I know little about the topic. I can happily do without defending my earlier post with what is really basic stuff, but it is important to me that this message gets across - is that not what KRnet is all about? Some folks seem to feel obliged to have the last word and that would be great if they would build on the post and develop the point - add some value. In this case an important point may get lost in a meaningless exchange - so let us dump the conjecture and opinion and deal with hands on, seeing is believing, raw reality - OK. Please - take a desert spoon and go to the sink. Open the tap until you have a nice steady stream of water. Hold the handle (stem?) of the spoon gently between two fingers, with the scoop hanging vertically down - the bulge facing the running water. A bit like the top of a wing about to enter an air stream. Slowly move the spoon closer until the bulge contacts the running water. Get the message? I really need say no more. Orma, you sincerely said you were out to learn and I would be really pleased if I can contribute something to that. Do this little experiment and you will immediately understand where that 2/3 of lift comes from. The spoon will be aggressively "sucked" (tugged?) into the stream and you will have no doubt that 2/3 of the weight of your airplane is being suspended by the upper wing skin - sucked into the air. The spoon has no "bottom surface" that is being influenced by the flowing water, so the under-camber aspect is NOT at work here. We could leave it there, but for those who are now curious. The spoon has no "bottom" as such, so that blows any verbiage about: "the airflow being increased over the top creating a pressure differential from under to over causing the air on the bottom to push up into the now low pressure area above the wing" I read the above a few times and I have no clue what he is trying to say - with great respect, I honestly tried. In any case, there is no water moving under the "wing" in this demonstration. If the stream of water from the faucet was in fact a thick sheet of water, the spoon would continue to be "sucked" through the water until it came out the other side - the upper "camber" is developing "lift". >From airplane to airplane (depending on configuration, loading etc.) the horz stab may or may not be contributing to lift and some parts of the fuselage may sometimes contribute a bit (it is reported that the GP4 canopy generates 400lbs of lift??) but for the purpose of this discussion, the wing provides the lift. The following is not my opinion, speculation, assumption or a product of thumb sucking - it is well established, simple, basic old hat fact. Approx 1/3 of the lift produced by a wing is generated from pressure under the wing and the other 2/3 of lift is created by the top surface of the wing. You have just proved the 2/3 bit with the spoon. We have all stuck our hand out the car window at 50 mph, so we know all about the 1/3 bit. (Even then, we are actually also seeing some of the 2/3 bit). If you want, you can do a second test to really make sure that your airplane is being sucked into the air (by the top skin) even it there is NO BOTTOM SKIN. This is more difficult to explain. This time we deal with air flow, not water. Take a sheet of typing paper and hold it by the bottom corners, one corner in each hand, between two fingers - sort of like you wanted to read it without smudging it. Bottom corners OK - the page is above your fingers. Try to hold the sheet near vertical (like you were reading it), then without changing the position of your hands, allow the paper flop over backwards (away from you). The bit you are holding is nearly vertical and the rest of the sheet is sort of hanging backwards over your hands - right. Now blow over the curve of the paper. Amazing how it rises up. The harder you try to blow it down, the more it rises up. The flow of air is "sucking" the trailing paper up - even without any airflow around the "bottom". The lift created by the top surface of a wing is indeed a function of Bernoulli's Law - also known as the venturi effect. The vacuum created in a venturi (Bernoulli's Law) is substantial, venturis were developed in days gone by to power vacuum instruments. Call it negative pressure, call it vacuum, call it what you like - in simple terms Bernoulli's Law is all about "suction". We really could leave it alone now, but in the interest of completeness. I am not sure why negative G came into the original discussion or why the explanation became so convoluted. Negative G is no different from positive G, just goes the other way. G forces have little to do with which way up the airplane is - an inverted airplane can pull positive G's Load testing a wing - if we wish to establish if the wing structure as a whole is strong enough (were our calculations correct). To establish the positive G capability we turn the wing over and pile sand bags on the upward facing lower surface until we are satisfied that we have represented a load of how ever many G's we want to demonstrate. Or keep going until it fails (to establish the ultimate load capability). This has zip to do with establishing if the skin spar bond is Ok - the topic of this discussion. I will be happy to take heat on any statement above that is demonstrated to be blatant, unsubstantiated conjecture and I will be delighted to expand on anything that may be vague. I will be equally happy to leave it alone, knowing that at least one buddy is going to make darn sure his /her wing skins stay put. Anyone who distracts from the importance of this matter is in sad need of help. My Sunday is done - enjoy what is left of yours. Steve J
