B Yen writes: >I scratch-built a polyhedral HLG out of a $10 foamy free-flight toy >glider... It seems to fly good as a HLG, with no >adjustments. However, the balance point is towards the rear of the >wing.. .. 1 3/4" back of airfoil-apex ... Is this a problem? (the balance >point is no where >near the apex-point of the airfoil). ?? Everywhere I've read, the >balance point is near the apex of the airfoil.
C/G location is not that simple. The design of the entire airplane is involved, particularly the size of the tail and the length of the tail moment arm. If the tail is big enough and/or far enough aft of the wing, you can have the C/G very far aft of the wing's high point (perhaps even well aft of the trailing edge!) and still have positive static pitch stability. If the model flies the way you want it too, with adequate stability and control response, then this C/G is obviously not a problem. Don't worry if some book, or some expert, says (grossly oversimplifying in the process) that the C/G "must" be at some particular spot. If the model behaves the way you want it to, then just don't tell your model what that book or that expert said, and it will continue to fly the same as before! >Towards the inboard section of the wing, there is a gap between it & the >TE stock. (see picture). Is this enough, to destroy lift? Yes, but the question is how much? Ideally, it's best to have your tape hinges on top, with the gap on the bottom. That can still give you drag problems at high speed, but I suspect you won't be spending a lot of time in that particular corner of this model's flight envelope.Getting rid of the step and gap on top will improve lift and drag at lower speeds. If possible, both the upper and lower surfaces of the trailing edge stock should be flush with the wing surface ahead of them. Better yet, on the surface that has the gap, have the trailing edge stock slightly thicker than the wing, and generously round the leading edge corner of the T.E. stock on that side. This will encourage the flow to re-attach after it crosses the gap. >What about incidence-angle for the tail-section? There are ways to calculate it, but the bottom line is that you should use whatever angle makes the airplane fly at the airspeed you want it to fly when the elevator is at neutral. The angle required to do this depends on a lot of thins, particularly the wing's characteristics and the C/G location. Also, remember that the incidence you pick results in one particular airspeed in flight (unless your model is exactly neutrally stable in pitch), and you will have to add some elevator deflection to make the model fly at any other airspeed. You need to decide what that trimmed airspeed should be. >I kinda stumbled my way thru the "design". The first step in model design is getting clear in your mind exactly what it is that you want the model to do, and in what operating envelope you want it to do that in. Start with a clear "mission profile" for the model, then figure out the characteristics required to achieve that mission. Without these two crucial steps, you have no "yardstick" to measure the effects of your design changes. > Is there an "orthogonal" >method of tweaking the design. I.e., 1st set main-wing incidence angle, >then main-wing area, then fuselage length, tail-section area, >tail-section incidence-angle. I just guessed at the fuselage length, >base on the tail-section area (constrained to what I had to work with). There are analytical methods that work reasonably well. In addition, understanding the theory behind those methods will help you understand what you see when you fly the model, and therefore what parameter to change to make the model do what you want it to. However, once again, the bottom line is how the model flies, in comparison to how you want it to fly. Properly used, the analytical methods will help get you to a level of performance and handling with fewer steps of prototyping and test flying (of course, improperly used they can slow the process down!). Ultimately, though, all analytical methods involve some degree of simplifying assumptions, and therefore are never an absolutely perfect prediction of how the model will behave, no matter how sophisticated the analysis. Even Boeing still has to build one and fly it before they can be sure of the merits of a new design. That said, as far as some simple analytical methods to help get your design in the ballpark, try browsing the "Ask Joe and Don" section of our website. In particular, read up on the use of "tail volume coefficients". These are a simple way to study and understand the relationship between wing size & shape, tail size, and tail moment arm. There are a number of other references that help, but for most of them you need some initial background in model design theory to understand how to use them. There's a series of articles by John Roncz in Sport Aviation from 2/90 to 2/91 that covers basic aircraft design in surprisingly plain language, and includes some spreadsheets that can be modified to work with R/C models. If you know some aircraft design theory, Blaine Beron-Rawdon's "Plane Geometry" spreadsheet is very good, but fairly advanced. Although I personally have some differences with some of its details, a lot of folks swear by Martin Simons' book "Model Aircraft Aerodynamics". >The CF rod fuselage broke at where I drilled it out (to attach it to the >blue-foam nose). So, I have to figure out a a different way of >attachment. Any suggestions. Yes, don't drill holes, even small ones, in carbon fiber if you can possibly avoid it! Carbon/epoxy can be very sensitive to notches, holes, and other stress concentrations. If you do have to drill a hole, put the hole at a spot where there is little or no stress in the carbon. Alternatively, epoxy a plate to the side of the carbon rod or a bulkhead with a hole in the middle onto the carbon rod, then fasten to that plate or bulkhead. > Someone recommend a u-shaped wood brace >to "clamp" the rod to the foam. He also suggested drilling small holes, >to let the epoxy attach to the CF rod. Do NOT drill holes or file notches in the carbon rod. You can get a good bond to the carbon by roughing up the rod's surface with medium sandpaper. > I keep thinking the blue-foam, >will just "tear out". (that's what happened yesterday. I used some >wall anchors, those plastic things just tore out of the foam..the >landings put a lot of leverage on the CF rod) 1. You need to spread the load out over the foam. It has low strength, so trying to apply a concentrated load to it (such as with your wall anchors) will cause it to fail at those points, as you saw. Gluing it to a surface, such as the face of a thin plywood plate, will spread the load out and allow the joint a chance to survive. The foam will then tear away right next to the joint. Structural engineering is tricky. It's very easy to beef up one weak area and only succeed in moving the problem to the next weakest area. Even worse, your beef-up may create a stress concentration that weakens another area that was formerly strong enough before you compromised it. There was a British-government-built zeppelin in the 20's that buckled some stringers in the middle of the belly. They added some splices and bragged that the belly was now considerably stronger than it was originally. Sure enough, the next time they flew it, it broke completely in half (right next to the ends of the splices!) and crashed, with much loss of life. One method that works well is to have two bulkheads in the foam pod, a couple of inces apart. You could epoxy one to the aft end of the pod, then cut the pod vertically about 2" ahead of the aft end. Cut out a thin (1/8" lite ply or 1/16" birch ply) bulkhead to match the cross-section of the pod at the cut, then epoxy the front and back pieces of the pod back together with the bulkhead sandwiched between them. Drill a hole the same diameter as the carbon rod through the aft end of the pod and both bulkheads, then epoxy the carbon rod into the hole. 2. Styrofoam isn't very tough. If "bounceability" is a requirement for you, then try to find some EPP foam (it looks like beaded white syrofoam, only rubbery). 3. Learn to not hit the ground so hard. ;-) Don Don Stackhouse @ DJ Aerotech [EMAIL PROTECTED] http://www.djaerotech.com RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
