Newsletter October 2011

Transcription

1 RC BEES of Santa Cruz County, Inc. Newsletter October 2011 Next Meeting Editor: Alan Brown, 388 Aptos Ridge Circle, Watsonville, CA Phone: (831) Web site: secrecy! My apologies to any members who brought Show-and-Tell subjects, or who made brilliant, earth-shattering remarks! Thursday, October 20th, 2011, at the EAA building, Aviation Way, Watsonville Airport, 7:30 PM. Treasurer s Report Beginning Balance $ Income Fuel Sale $45.00 Dave Seiler s Raffled Waco $ Dues $ Subtotal $ Expenses Field toilet service $64.80 Fun Fly Expenses $75.78 Subtotal $ Ending Balance $ Note: $0.60 discrepancy between beginning balance and last month s ending balance is because field toilet service was reduced by that amount starting in August. September Meeting Unfortunately, your secretary/editor was out of town for the October meeting, and no notes were taken. So the meeting will be forever shrouded in Scale Fun-Fly Our first fun-fly devoted solely to scale models was held on September 18 th, and was, I thought, a great success. Each of the eight competitors had to submit their airplane to a static review which included points for overall shape, finish and details, prior to making their flight. As all but one entry were ARF s, one might think that the static awards should go to the manufacturers rather than the individuals, but our judges (Steve Jones and Alan Brown) didn t get too hung up on that issue. Here s a group of some of the airplanes.

2 Marcelo Montoreano is about to put his F4U in line, while his F-18 is in the foreground next to Don Good s Pitts Special. Don Good both competed and took photographs while Nick Tannaci took videos from a camera strapped to his bicycle helmet.. Don hasn t yet mastered the art of taking pictures while flying, as Nick has! Here Nick is coming in to land having just nipped Paul Weir with his Cessna 182 by one point, 80 to 79 out of a possible hundred. Marcelo was a close third with 74 points. Alan and Steve check out Nick Tannaci s gorgeous Ryan STA, which took joint first place in the static category with Marcelo s F4U. Kevin Crews brought along an ARF De Havilland Beaver, to which he had added both floats and a four-wheeled amphibian gear. Here it is. A unique airplane, very nicely done. Unfortunately, Kevin didn t think it was up to flying on the day, so it only collected static points. Meanwhile, Don Good s Pitts sits by the river awaiting its turn to be judged. So what was the flying program? Each competitor had to make a scored take-off followed by a horizontal figure eight directly in front of the judges. Then he had to perform a maneuver appropriate to the full scale airplane s capabilities, which had to be detailed to the judges prior to takeoff, and then make a judged landing. The judges decided that they may have to have a written free-style maneuver in future, as it was often difficult to tell what all the arm waving was really about. ( I m going to go straight up, followed by a sort of a twist at the top, and then turn back before doing an inside or maybe that should be an outside loop all this accompanied by wild body gyrations). I think you get the general idea! However, we all had a lot of fun, and the final ARF placings went like this. First, second and third went to Nick, Paul and Marcelo as noted above, with Paul s second entry, a P-51, coming in fourth. Then came Don Good with his Pitts, Joe Parisie with his T-28, and Benno and his very colorful F-5, shown here in static judging.

3 Benno s F-18 and Kevin s Beaver rounded out the group, neither of them competing in the flying part of the event. There was a separate event for kit and scratch-built airplanes which was won by Stefan Warnke with his foam-built original L-19 jet airplane. He didn t have to fly to take first place as his only potential competitor, Alan Brown, arrived too late to both compete with his Westland-Hill Pterodactyl and judge the events. Wait til next year! Well, we don t have to wait til next year for the next fun-fly event. On Sunday, November 20th, we ll have our final event of the year, pylon racing. This will be broken down into classes, perhaps one for foam T-28 s, of which we seem to have a bunch in the club, one for miniature airplanes, say less than 24 span, and one for mid-size airplanes, say 40 to 70 wingspan. Depending on what shows up, we could have a WW I and/or a WW II warbirds race. I m even thinking about a slow pylon race, last plane to cross the finishing line wins, helicopters not included, and perhaps a helicopters only race. So think about the possibilities, and me with any suggestions. Down by the River A little more excitement than usual for the retirees this month with two rafting forays. Dick Muir put his Corsair in the Pajaro River at a time when the current was flowing inland. Out came the trusty rubber boat and Dick, who is fortunately not too heavy, bravely paddled upstream to retrieve it. He still commented on rubbing against the riverbed if he sat too far aft. The top hatch, which included the cockpit, unfortunately was lost, but none of the electronics got wet, and so a few days later, Dick reappeared with a new home-made hatch complete with windshield. Nice job, Dick. Photo by Allen Ginzburg. Not to be outdone, John Nohrden, making one of his rarer appearances since giving up driving, decided to pop one of his specials into the river, this time with the current going seaward. Dick Muir, now seen as the expert by the admiring throng, was conscripted into being the raft captain, and this time had to row across the river to rescue John s airplane from the Monterey County shore. All done with great aplomb, as one might expect. The newest airplane seen recently is Stefan Warnke s P-38. Apparently he s had it for a while, but only recently put the finishing touches to it. It looks very authentic, and Stefan flies it well. Here it is coming in to land. This and the following photos are by Don Good. Kevin Crews now flies his Beaver regularly, and here we see it airborne on a low pass. Kevin obviously likes De Havilland airplanes, and has bought a kit for a Dragon Rapide, a twin-engined 1930 s biplane, incidentally the first airplane in which my wife flew, on our honeymoon. Jacob Boracca is progressing by leaps and bounds in pattern competition. Those of you who read Model Aviation from cover to cover will have seen in the District X news in the September issue that Jacob took first place in the Advanced section at the Oakdale Annual Pattern Shootout. Here is his plane, a PassPort, in practice at our field. Well done, Jacob!

4 Marcelo has put a new fan and motor in his F-35, and it now has unlimited vertical performance very impressive! Aero 101 How wings really work On a more sedate note, here s Alan Brown s Lockheed ER-2, a NASA Earth Resources derivative of a TR-1, itself a derivative of the more well-known U-2. I believe that the original flew regularly from Moffett Field in Mountain View. A number of articles have been written in model aviation magazines, supposedly illustrating how wings work and how they generate lift. Most of them have had serious errors in them, and so this article will attempt to correct them. The first thing to note is that there is no dispute among professional aerodynamicists on this subject over the last 350 years! Here is a picture of flow patterns around a symmetrical airfoil at several angles of attack. In other news, Emmett White has put his foam P-51 back together after encountering a mid-air with Benno s Stryker. It now looks suitably battlescarred! Alan Brown put his F-117 in the field after running out of power from the very small battery supplied with the ARF kit. His record is a bit like that of Walter Matthau s Ferrari in a movie that Allen Ginzburg reminded him of, where his trips to the shop exactly equaled the number of times he took it out for a drive. Scott Steiner is still regularly flying his Extra 300, but now often joined by Steve Lock with his electric-powered Valiant pattern aircraft, fueled by a Himac cell LiPo battery. John Williams bought Gene Martin s own design Firebird, an electric-powered glider, and flies regularly, as does Johnny Skoch, with a greatlooking OS 120-powered Super Stearman, as well as his Big Stick and his TopFlite P-51. On the training side, Steve Jones continues to tutor Brian Lacey, flying the latter s Cessna, while new member John Di Sibio is getting lessons from Jacob, flying the club Tiger Trainer. Steve is also helping out new junior member, Zane Martinez. 0 deg. 10 deg. 90 deg. The sketches at 0 degrees and 90 degrees seem quite reasonable. Clearly, the flow over a symmetrical airfoil at 0 degrees will divide symmetrically at the nose and continue symmetrically to the trailing edge with no lift generated. At 90 degrees, the airfoil is like a flat plate at right angles to the stream direction, and air will flow around both sides, probably separating as it tries to go round the edges. However, it s the intermediate 10 degree position that seems to get people into trouble. There is a common assumption that the airflow divides right at the leading edge of the airfoil despite the obvious fact that it couldn t be true at 90 degrees. In fact, an article was written some time ago allegedly proving that modern aerodynamic theory must be all wrong, because if one measured the distance round the top and bottom of a Cessna 172 s airfoil, and used that information to calculate the pressures on the wing from Bernoulli s Theorem (more on that later), there wouldn t be enough differential pressure to sustain the weight of the airplane. The answer is, of course, that the writer s initial assumption is

5 incorrect, and the dividing point for air that goes over the wing, from the air that goes under the wing, is underneath the wing, and not at the leading edge. Many general aviation aircraft have mechanical stall warning devices which are triggered by a small floating lever on the underside of the wing which is pushed backwards at low angles of attack (angle of attack is the angle that the airfoil sees from the incoming airflow, and which varies with the airplane s attitude). As the dividing airflow point moves backwards, there will come a point where it is behind the lever, and will force the lever forward, thus alerting the pilot to his critical angle of attack. Now we can see that a symmetrical airfoil can experience lift at an angle of attack because the air has to accelerate in going round the corner, thus reducing the pressure on the top side of the airfoil. We ll get to the equations that describe this in a few minutes. Now let s go back into history. In the 1670 s, Isaac Newton, a very bright physicist and mathematician, applied his postulated laws of motion to the flow over a wing, using his momentum theory, as shown below. It was in fact left to another brilliant man, Daniel Bernoulli, to come up with the mathematics that explained the dilemma. In 1738 he published his classic book Hydrodynamica which explained how mass and momentum conservation in fluids could account for the different pressures and velocities associated with the streamlines through pipes and around bodies. This is an illustration from his book, and here are his two fundamental equations as adapted to airflow. As he was mainly concerned with water, the weight of the water was part of the total head or total pressure in his equations, not present here. H = p + 1/2. ρ. V 2 where H = total pressure p = static pressure (measured on surface) ρ = air density V = velocity Mass Flow Continuity ρ. A. V = constant He assumed that the air could be thought of as a bunch of little particles striking the underside of the inclined surface, and giving lift via the change of particle momentum. He tested this, and I don t know exactly how, perhaps by hanging his wing outside his coach window while his coachman drove at a known speed (England had milestones on all major roads in his day). Surprisingly, he found that the lift measured was almost three times what his momentum theory calculated, and so he realized that he was missing something in his analysis. where A = area of stream tube The first one says that what we call the total pressure, nowadays we measure it with a pitot tube, equals the sum of the static pressure, as measured nowadays by holes drilled in the surface of the airfoil or body, and the dynamic pressure generated by the local velocity of the airflow. Furthermore, for an airplane operating at a given speed and altitude, the value of H is given by free stream conditions, and remains constant over the surface of the airplane. The equations are completed by noting

6 that along what we call a stream tube, the mass flow must be invariant, and so as the area gets reduced, the velocity must increase, which means that the static pressure must go down. Here is a graph of pressure over a wing with various flap deflections. Let s look only at plain wing first, and come back to the effects of flap angle later. predominantly affects drag due to the width of the wake, but has little effect on lift. You can see that as we move backwards over both the top and bottom surfaces, the pressures move steadily towards the atmospheric value, being almost, but not necessarily exactly, equal to it at the trailing edge. Here we can dispense with another common fallacy. The molecules on the top and bottom surface do not necessarily rejoin their original free stream mates at the trailing edge. They are not smart enough to do that! The reason why we balance our models typically at the 25% point on the wing is that simple theory shows that this is where the overall lift acts, based on the dominant pressures being well forward on the wing. This lifting point is independent of angle of attack until the flow breaks down due to incipient stall. The wing, NACA 23012, is what we generally would call a semi-symmetric airfoil, 12% thick, at 8 degrees angle of attack. The flow round it will be similar to that shown for the symmetrical airfoil at 10 degrees in the first figure of this article. Just to confuse the reader, we generally plot pressure upside down, so negative pressure is upward and positive pressure is downward, corresponding to the upper and lower parts of the airfoil. The first very striking thing is that within the first one or two percent of the distance back from the leading edge, the pressure goes from a maximum value on the lower surface representing the total pressure where the air has come to rest, and rapidly as it moves round the leading edge at very high speed drops to a minimum value on the top surface, well below atmospheric pressure, p 0, (the zero line on the graph). So the smallest pressure, and therefore the greatest lifting suction, is very close to the leading edge on the top surface. This is why good shaping of the leading edge is of paramount importance in obtaining an efficient lifting surface, much more so than tapering the trailing edge, which So to summarize up to this point, Bernoulli s equations give a very good result to explain lift on an airfoil as long as viscous effects, like flow separation and boundary layer, are not significant. Fortunately, that is generally true for most of our flow regimes. Newton s original application of his momentum theory does not explain lift at all, and so it is completely erroneous to talk about Bernoulli versus Newton in explaining how lift works. So how did the theory come about? We can work with a thing called a Stream Function to produce a simple picture of the flow round a circular cylinder, which looks something like this. Note that we can t, at this stage, represent flow separation behind the body. It can be transformed into an airfoil shape, and we ll get into that in our next issue. Upcoming Auction The Bayside RC Club is holding an auction Saturday, October 29 th, at the Holy Spirit Church, Fremont Boulevard, Fremont. Check-in at 6:30 a.m. Auction starts at 10 a.m. This is usually well attended.