Bees Breeze. RC BEES of Santa Cruz County, Inc.

Transcription

1 Bees Breeze RC BEES of Santa Cruz County, Inc. Newsletter April 2016 Editor: Alan Brown, 388 Aptos Ridge Circle, Watsonville, CA Phone: (831) Web site: Thanks were given to Dan Morris and Max Next Meeting Trescott for their organization of the night fun-fly, Thursday, April 21st, at the EAA building, which all agreed was a great success. Over twenty Aviation Way, Watsonville Airport, 7:30 airplanes were involved. A full description appeared in the March newsletter. PM. Treasurer s Report Keith Wigley has restored the club trainer in Beginning Balance $12, addition to his new airplane described here in Income 'Show and Tell'. Thank you, Keith! Apparel Sale $30.00 Donations/Rebates $5.00 Latest news from the Santa Cruz County Science Dues $ Fair, mentioned last month, is that Benno not only won his class and will go to the California finals Subtotal $ in Los Angeles, but also won top place overall, and so will represent our area in the national Expenses finals. Well done, Benno! Meeting expenses - food $12.48 D & G Sanitation $64.95 Steven Boracca and Dan Morris will go to the Field expense equipment $10.28 Cabrillo College engineering class which our new Field Expense mowing, etc. $ member, Melissa Pardo, attends, to talk to Fun-fly expenses $74.36 students about the forthcoming teaching program to be held at the club field on March 26th. Club trainers $ Subtotal $ It was noted that, in addition to training personnel with their airplanes, master and student Ending Balance $12, transmitters and umbilical cords, we should be sure that we have enough batteries and chargers March 17th Meeting on tap to see us through the day. The charging The meeting opened at 7:30 p.m. with seventeen system will be set up outside for ease of operation. members present. The previous minutes and treasurer's Dan Morris suggested that we put on a pylon race report were approved. as our next fun-fly event on April 24th, the Sunday

2 after our next meeting. It had been pointed out to him, I believe by Marcelo Montoreano, that the flying wing airplanes used recently for the combat competition, may not be strong enough for the high-g flying expected in pylon racing. Something a little more specialized may be more appropriate. Steve Boracca posted the recently updated flight rules at the club shed at the flying field for all to examine. They will be voted on after the required three months for everyone's reading, so be sure to take a good look! Each motor can be rotated individually to control conversion from vertical to horizontal flight, and aid in pitch, yaw and and roll control. Next came Don Edwards' airplane. He has made several of these, and would like to sell this one. Anyone interested, get in touch with Don at The club's non-profit status has lapsed, and will have to be restored. This will not be easy, as some important past documents will have to be found or replaced. The annual 'Wings over Watsonville' event for fullscale aircraft will be held this year on one day only, Saturday, September 3rd, with a similar format to last year, i.e. no full-size exhibition flying. We may have a small area allowed to us for demonstration flying, probably between a couple of hangars, as for last year. It was suggested in the meeting that we may advertise a fun-fly at our field for the following day to allow visitors to round out their weekend. Parking would have to be well thought out, of course. Richard Tacklind unearthed an old Balsa USA kit of a WW I Bristol Monoplane Scout, and put it together recently. Always one of the editor's favorites, (he worked at the Bristol Aeroplane Company many years ago), we're delighted to see this fine model. Show and Tell First up was Dan Morris's latest convertiplane, with three motors arranged as you can see in the picture. Keith Wigley has always liked models from Precision Aerobatics, particularly the Addiction. So he set to to make his own design 3D airplane of similar design, but intended to be even lighter than Precision's version. He spent a year building it, and feels that it flies better than the original, although without all the carbon fiber. Nice job, Keith!

3 Finally, another of Don Good's ubiquitous Gee Bees made it to the meeting. If anyone would like to make an offer for it, contact our president, Steve Boracca. With that, the meeting was formally closed at 9:15 p.m. Down by the River On March 26th, we had Cabrillo College students over to learn how to fly R/C airplanes. This was organized by recent member Melissa Pardo who thought that her fellow students would enjoy it. And I'm sure that they did! Dan Morris gave a good summary of the day to you all in his the following day, and although we only had four students at our field together with their teacher, Dr. Carl Ewald, probably because of coinciding with both spring break and the Easter weekend, we all had a very good time. Here are a few pictures from the event. Jacob discusses some of the intimacies of R/C with Professor Ewald. Mike Evans buddy-boxes with one of the students while Dick Muir gives helpful advice! We now have a much classier entrance to our field courtesy of Max Trescott. Here's the sign that he had made. Looks great, doesn't it? And Steve buddy-boxes Melissa.

4 As Dan mentioned in his note, club members started regular flying about 10 a.m., and a number of models were airborne in short order. Highlights were, at Laurie Trescott's persuasion, Marlene leaving her 'audience' chair to become a student participant, Nickolai De Malvinsky's drone patrolling the skies keeping a watchful eye out for all of us, and Benno launching his recoverable rocket which was part of his Science Fair winning entry.

5 The following day was Easter Sunday. My family was visiting and son-in-law Bob had a few goes at R/C flying tutored by Steve Boracca. Thank you, Steve! He thoroughly enjoyed it. Slight change of pace. Some years ago, I watched as Joe Sluga made an unusually poor landing for him, and promptly, in those non-p.c. days, set fire to his airplane on the runway. A few days ago, I had the rare opportunity to outdo him. After shopping briefly at CVS on Main Street in Watsonville, I came out of the store to see flames coming out of the hood of my 1993 BMW and washing over the windscreen! Police and Fire Department were there in short order, together with a photographer who worked for the Register Pajaronian, with the result shown below from last Tuesday's edition.

6 the other way round, as happens in real flight, Trust me, it doesn t affect the results at all. As the air moves past the flat plate (the simplest kind of body) it gets slowed down by friction and right at the plate surface the velocity is zero. The same sort of argument accounts for why we don t all get blown off the earth when it s spinning round at pretty high speeds (about 1000 miles an hour at the equator!!). Best guess is that mice had chewed through instrumentation wires behind the dash, and shorted them out. Ah well, probably time for a new car anyway! Aero 101 Drag and Reynolds Number A comment about laminar flow at our last meeting prompts this article. If we start with uniform flow, U, over a flat plate at zero degrees angle of incidence to the flow, then because of friction the flow right next to the plate will have zero velocity. The air sticks to the plate as it goes past it, and loses some energy, which translates into friction drag. As we go downstream, there is a continuous increase of drag, and so the slower moving section of air gets thicker. This region is known as the boundary layer, which fortunately is usually quite thin for most of our airplane applications. Here is an illustration of a boundary layer building up from the leading edge of a flat plate. The ladder-like cross-sections show the velocity changing with distance from the plate until the free stream value is reached. At the extreme left-hand side, I ve represented the velocity as being constant from top to bottom. We usually find it easier to imagine the body as standing still and the air moving past it rather than In this particular case, the velocity profiles are typical of a laminar flow boundary layer. Now I haven t mentioned laminar flow up to now, so I d better define it. When flow is laminar, all the streamlines are generally parallel to each other as if they were in laminated strips. The alternative is turbulent flow, which is just what you d expect from this definition. The flow forms lots of vortices, like roller bearings. Intuitively, this is fairly easy to imagine. If you think of waves coming into the shore, when the velocity is small, the flow over the beach is quite smooth and one layer flows easily over the others. However, if you increase the speed, a point is reached where waves form and the water tucks in, breaking up the smooth pattern. The analogy isn t exact, because water waves are affected strongly by gravity, while airflow essentially is not. However, we don t need to worry about that for our demonstration purposes. Turbulent flow has by nature a higher drag than laminar flow, which is why, particularly during the latter part of World War II, there was a lot of effort spent on trying to keep the flow over wings laminar for as far downstream as possible. I m going to jump ahead just for a moment because I know some of you are going to ask why we put turbulators on free flight models to induce turbulence and so improve performance. I ll get to that answer soon and it has to do with flow separation. But first I want to introduce Reynolds Number. Lord Osborne Reynolds was a rich 19th century English aristocrat, who lived at the time when it was fashionable for such people to have their own scientific laboratory behind the gazebo. He was also a competent scientist who noted that there should in principle be some way to predict

7 when transition from laminar to turbulent flow took place. He probably observed that the smoke from his after-dinner cigar rose straight up in a laminar fashion until some point where it abruptly became turbulent. What was the criterion for when this change came about? He developed the mathematics into a non-dimensional criterion which relates the inertial flow (Newton s law that says things keep going until you do something to change them) to the viscous flow, which is a measure of the tangential frictional forces. This is the Reynolds Number. It is defined as the density of the fluid (in our case air) times the flow speed (the inertial force) divided by the viscosity per unit length (the frictional force). Writing it out, this is density x velocity x length / viscosity. For our purposes, density and viscosity are just functions of altitude and temperature, and can be considered to be constants for any particular airfield. At sea level on a standard day (15 degrees C or 59 degrees F) Reynolds Number equals 9333 x velocity in m.p.h. x length in feet. As we don t know Reynolds Number phenomena all that well, it s accurate enough to remember as 10,000 x mph x length in feet. Experimental observations show that transition from laminar to turbulent flow occurs naturally on a flat plate at a Reynolds Number between 100,000 and 1,000,000. This doesn t seem to be very exact, but it s about as good as we can do bearing in mind that different wind tunnels have different intrinsic turbulence, and pressure gradient as we go downstream has quite a big effect. So what does this mean for our model airplanes? Let s assume that the lower limit applies. Then at 50 m.p.h., transition will occur naturally 2.4 inches downstream of the leading edge of the wing. Our wings may be about 10 inches chord, so transition occurs at about the 1/4 chord line. For a small free-flight airplane flying at 15 m.p.h., transition would occur about 8 inches downstream, which is generally greater than the wing chord. Now let s look into why we would want to make transition happen artificially at a much shorter distance downstream. The answer is to do with flow separation. If the pressure increases as we move downstream over the wing, it will tend to feed forward in the much slower moving boundary layer and push the boundary layer away from the surface. This is called flow separation. As we have seen earlier, lift is generated on a wing by flow over the top surface accelerating rapidly round the leading edge causing a very low pressure which literally sucks the wing upwards. As the flow goes along the wing surface, it slows down as it has more room to expand, and the pressure rises towards the atmospheric value. The velocity profile looks something like this for a flow which is starting to separate.

8 laminar on both sides of the ball, and there s no knuckleball effect. And finally, in tennis matches, the players always like to have new balls because they go faster with turbulence-producing fuzz on them than when they have got smoother and are therefore more likely to have easily separable laminar flow. The top chain-dotted line shows the top edge of the boundary layer and the lower chain-dotted line shows the line of zero velocity. Below this line the air is flowing backwards. The line PQ represents the streamline which separates the normally flowing air from the separating air. Laminar flow will separate much more readily than turbulent flow because it doesn t have the higher energy from flow which is spun toward the surface. Thus although attached laminar flow has lower drag than turbulent flow, it is much more liable to separate and thus cause drag associated with this phenomenon. Three examples from the world of sports come to mind. A golf ball has dimples on it to induce turbulence. Without those dimples the flow would be laminar and would separate right at about the maximum diameter of the ball. With dimples creating artificial turbulence, the flow has higher energy and stays attached much further round the ball. Thus the wake is much smaller and the drag is reduced. A Boeing engineer recently came up with a golf ball that had intersecting hexagons on the surface instead of the more common circular dimples. Theoretically, these would be better at promoting repeatable turbulence because of the sharper edges, but I don t know whether the idea ever caught on. A baseball player s knuckle ball must be thrown with no rotation and at a speed very close to that of natural transition. Because of nonuniformities in the ball shape caused by the seams, the flow will change to completely laminar on one side as the ball slows down. The flow will separate off that side while remaining substantially attached on the other side, with a large change in net side force. Knuckleballers always complain about pitching in mile-high Denver. The reduction in density with altitude means that they should increase their pitching speed so that the product of the speed and the density stays the same. Generally they can t make this adjustment and so the flow stays O.K. back to model airplanes. What we ve seen now is that for small slow-flying airplanes, flow will generally be laminar past the point where the pressure on the top of the wing starts building up, and so will easily separate from the surface. This not only creates substantial drag, but also makes any trailing edge-mounted controls ineffective. So we put so-called turbulating strips along the span of the wing to induce turbulence, and prevent premature flow separation. Larger, faster models are generally operating right in the zone of natural transition, but some could benefit from turbulators to make the ailerons more effective. This is probably why a lot of modelers write about larger airplanes flying much better than smaller ones. Remember that a model flying to scale will increase its natural cruising speed proportional to its increase in size, so the Reynolds Number goes up as the square of the scale. A large model (say Giant Scale - 80 inches plus in wing span) will be comfortably in the naturally turbulent flow region, while a medium sized model could be right at the critical Reynolds Number, where some laminar flow separation could occur. This seems to be more than enough for one article, so I ll look forward to your comments. A couple more items On page 115 of the current issue of Model Aviation is an article about slope soaring at our own Sunset Beach in Watsonville. Of even greater interest is that the airplane belongs to Jack Trotter, the distinguished looking gentleman on the right of the picture. Jack was a member of our club

9 some years ago (I've been trying to persuade him to rejoin for some time), and will, I am sure, be remembered by those of us who are long-term members. If you read this, Jack, please rejoin! Unfortunately, his suggestions underestimate the strengths required by at least a factor of four. I know that few of you make the kinds of airplanes to which he refers, typically WW I or earlier, but if you do, beware. John is an outstanding aeromodeler and flyer, for whom I have a great deal of respect, so this may seem like a nit-picking comment, but it might save someone's airplane. I have written to MAN about it, and await a reply. The second item refers to an article in the current Model Airplane News. John Glezellis has an article about rigging for scale models, and shows how to determine the strength needed for rigging cables. And that does it for another month. See you all Down by the River!