RC BEES of Santa Cruz County, Inc.

Transcription

1 RC BEES of Santa Cruz County, Inc. Newsletter May 2011 Next Meeting Editor: Alan Brown, 388 Aptos Ridge Circle, Watsonville, CA Phone: (831) Web site: Old Business Thursday, May 19th, 2011, at the EAA building, Aviation Way, Watsonville Airport, 7:30 PM. Treasurer s Report Beginning Balance $ Income Donations $67.00 Dues $ Subtotal $ Expenses FunFly Expenses $ Field Mowing $ Field toilet service $65.40 Subtotal $ Ending Balance $ April meeting Thirteen members attended the meeting on April 21st, President Steve Boracca called the meeting to order at 7:30 p.m. and the minutes of the March meeting and treasurer s report as published in the April newsletter were approved. Allen Ginzburg reported that there may be water in the camera box. It had not been working for two days. The power seemed to be O.K. and possibly a simple gasket would be all that was required. ( I just clicked on as I am writing this newsletter, and saw myself landing my Li l Extra this morning, so I guess our repair crew did a great job!). New Business The float-plane fun-fly which is to be held after the May meeting was discussed. Because the river is behind the pits and the normal seating area, we will have to make somewhat different arrangements for spectators. One suggestion was to have people sit on the runway, which will not be in use, but this was voted against because folks would not be able to see the airplanes take off, which is one of the primary interests. The second idea was to confine spectators to the area east of the first two or three pits, which would allow them to see the action without impeding the flyers, who would be launching perhaps close to the third and fourth pits. Remember that this will be our first attempt at this type of program, so we should expect it to be a learning process to some extent. It was noted that we should trim the bushes near the take-off area both for safety and visibility, and Bob McReynolds commented that we should have a boat ready for retrieval, and not be blowing it up at a critical time! Mark your calendars!

2 Another question raised was whether we should have competitions as for previous fun-flies. It was generally agreed that this may not be very practical, but we could certainly have audience participation in judging take-offs and landings, and scale appearance. That s Sunday, May 22 nd. Again, it was suggested that we combine a swap meet with the fun-fly, and so this will be done. Bring your swappables! May 22 nd! Bill Boone reported that Don French, who is in management at the Watsonville Airport, had suggested that we could do a park flyer model airplane demonstration in the field across from the terminal to coincide with the EAA Young Eagles Day on the first Saturday of each month, perhaps for about an hour. Show and Tell Don Good showed his Stearman PT-17, which he had ordered from NitroPlanes based on the fine similar model that John Williams has. Don was disappointed in that he received a different color scheme (yellow) from the one he ordered, and found that some of the assembly was not as straightforward as he might have expected. However, you can all see that it looks pretty good now. The meeting was closed at 8:15 p.m. Down by the River Don Good sent in some nice photos again, so here s a few of them. The first is, I think, Marcelo s L-39, which somehow eluded the photographers at the previous fun-fly. A very nice panned picture of it coming in to land. Next, your editor s ex-john Nohrden Li l Extra, a nice every day flyer. Then Ricky Wright s Stearman, Joe Parisie, flushed with success at selling his P-51 at the previous fun-fly, brought along an old Escape pattern plane, ex-joe Sluga, complete with retracts and an O.S. Max 61, which is up for sale. Call Joe if you d like to have it. Here it is. with Don Good s airplane. Your editor has some cryptic notes about a World Models Spot On 50, by Dan Southwood, but no photos available. A very nice-looking pattern plane. Bob McReynolds brought along his F-22 trainer, which didn t seem to be as docile as one might have preferred, although Bob had removed the trainer wing additions.

3 Don Wilden sent me a photo of my late-lamented Santos Dumont Demoiselle. A great flying little airplane, but I took my eye off it just long enough for it to become a pile of sticks! Here it is taking off, and here being brought back home by fellow-pilot, Steve Jones, after the engine died. Allen Ginzburg epitomized the modern RTF trend the other day, to your editor s mortification! Alan (not Allen) had just explained how he was doing with his scratch-built 1920 s Heinkel floatplane (he had cleared off his work bench, got his plans all ready, and had started on cutting wing ribs), while Allen (not Alan) had ordered his RTF Coota Amphibian from Nitroplanes on Monday, received it on Wednesday, and had it at the field on Thursday! And it s a very nice-looking airplane, as you can see, coming complete with motor, receiver, transmitter, ready to fly out of the box. The Coota is a model of a home-built airplane, the original put together by one of the original flying car designers. You can see why we scratch-builders are a dying breed! Other events of the month included Paul Weir unusually putting his F-86 in the trees at the end of the runway. New member Kevin Crews is having fun with his foam Cessna 182. Scott Steiner has a new almost ¼ scale Extra from Extreme RC, which he is flying with his usual aplomb. And Curt Gabriel was at the field for his spring visit to Santa Cruz County and brought a Katana from Precision Aerobatics, and an Align helicopter, which Dennis Kanemura is going to help him to master. Aero 101 Aspect Ratio and Taper Ratio We haven t had one for a while, so here goes. This all started when I was asked recently why a tailless aircraft would tuck in, almost to the extent of being irrecoverable, when it was put into a descent. Well, I have a very nice tailless aircraft, a Westland-Hill Pterodactyl, and it certainly doesn t tuck in when it descends, so that had me puzzled at first. Further discussion showed that the airplane in question was a model of the low aspect ratio deltawing Avro Vulcan, very different from my high aspect ratio, moderately swept Pterodactyl. What was happening, I think, was that at low to moderate speed in horizontal flight, the delta wing s wingtips were perhaps stalled while the rest of the wing, much further forward, was lifting in the normal way. When the stick was pushed forward, the speed increased, the wing to some extent unloaded, and the angle of attack decreased. The wingtips were no longer stalled and started working normally, thus moving the center of pressure backwards, and inducing a nose-down pitching moment. Hence the observed tuck-in. As will be shown later, the model did not reproduce the exact wing section of the original airplane.

4 This made me think that an article on the effects of aspect ratio and taper ratio might be appropriate, so here goes. First, let s define aspect ratio. It s the ratio between the span and the average chord of the wing or whatever surface we re talking about. Because average chord is often tricky to determine, we take the equation for aspect ratio, A.R. = span/average chord, multiply both top and bottom of the equation by the span and get a more easily defined number, A.R. = span 2 /area. Taper ratio is straightforward as long as the leading and trailing edges are straight and continuous from root to tip, when it is just the ratio of tip chord to root chord. For rounded tips, extend the leading and trailing edges straight out to the tips, and use the projected tip chord. For other plan forms, it gets a bit more nebulous, but we won t worry about that here. A wing looked at from the front has a high pressure region below it and a low pressure region above it. The difference is generally greater in the middle, even for an untapered wing, and tapers off towards the tips. The high pressure air wants to flow from the bottom to the top at the tips while it s also flowing rearward. Consequently there will be a net inflow over the top surface and a net outflow over the lower surface as shown in the figure below. - + Front view Flight direction Plan view Most of this differential will occur near the tips and will cause a downflow at the trailing edge. This effectively reduces the local angle of attack of the wing, but not uniformly across it. The amount depends on the taper ratio. If the wing is untapered, most of this effect is felt out near the tips and so the tips operate at a somewhat lower angle than the center part of the wing, which means that they are less likely to stall than the center section. This is a stable configuration. If the wing is tapered to a point like a delta wing, then there is very little vortex strength near the tips and consequently there will be more downwash further inboard where there is more meat for the differential pressure to grab on to. The inboard section is now at a lower angle and so is less prone to stall than the tip. This will cause the airplane to drop a wing tip at stall, which is less desirable. This is exacerbated by the fact that the smaller chord at the tip is usually less efficient because of Reynolds Number effects, and so it would probably stall at a smaller angle anyway. It is normal to think of wingtip vortices as being the way in which pressure leaks from the bottom to the top of the wing, but in fact vortices are generated all across the wing because of the differential lift associated with decreasing chord on a tapered wing. The vortex distribution looks something like this. Flight direction The most efficient lift distribution is given when the vortex strength is constant across the wing, and each part of the wing is essentially operating at the same angle of attack. This allows the wing to operate at its maximum efficiency, although it is neutrally stable as far as stall characteristics are concerned. This condition results from an elliptical planform. A closely straight-tapered wing to an ellipse in terms of vortex uniformity is that on a P-51, which is why this airplane was so effective. Anyone who has built a model Spitfire knows that you really should build in washout at the wing tips to gain stability at the stall at the cost of some wing efficiency. A good rule of thumb is that you should always build washout into a wing if the taper ratio is 60% or less. Classic full-scale examples are a number of De Havilland aircraft, like the 1930 s racing Comet, the biplane Dragon Rapide and the Mosquito. Now what about delta wings? They clearly have the worst possible arrangement, as the wings taper close to a point at the tips. Tip stalling must be a serious problem with these aircraft. It is interesting to note that the Avro Vulcan went through a wingtip change early in its development program. The prototype airplane had a very straightforward deltawing configuration as shown.

5 of lift and drag backward as shown in the next figure. Fairly early on in the development program the wingtips were modified by addition as shown in the next picture, to give drooped leading edges. (This particular photograph is of the airplane that was used to test the Concord engine, underslung on centerline). This was clearly a hasty modification done to alleviate tip stalling. Remember that this airplane was designed in the 1940 s, when delta-wing design was in its infancy. This should be sufficient to demonstrate wingtip stall on highly tapered wings. Now let s go to aspect ratio. First, let s agree that we re not discussing extremely low aspect ratio like on a F-117A stealth fighter. When you get to that geometry, the simple assumption of the so-called horseshoe vortex (in plan view) goes out of the window. We ll also confine the arithmetic to an elliptical planform with constant geometric angle of incidence. This is not limiting. It just makes it simpler. Any other common planform will work similarly, but the mathematics are a little more complicated. It is sometimes thought that a swept back wing is more prone to tip stall than a straight wing. This isn t really true, but it often seems so because a swept back wing frequently has a smaller taper ratio. The effect of the vortex pattern coming off the trailing edge of the wing is to reduce the effective angle of attack, which in turn rotates the directions The vertical induced velocity, w, reduces the angle of attack from α to α 0 and tips the lift vector backwards by the difference of the two angles. This vector now has an extra component in the drag direction in addition to the basic profile drag of the airfoil. Without going into any details, the value of the new equivalent angle of attack is given by α o = α - C L / π. AR where AR is the aspect ratio. A little more transposition shows that the ratio of the two angles for a given C L is given by α / α o = / AR Loud cries of So what! from the audience at this point. Well, the point is that the stall angle increases according to this simple formula, but the lift coefficient and therefore the stall speed don t change. For example, a 2-dimensional wing (infinite aspect ratio) might stall at 15 degrees. The same airfoil on a wing of aspect ratio 6 will stall at 20 degrees, while if the aspect ratio were reduced to 2 (something like an extreme Profile Hots 3D airplane), the stall angle would be 30 degrees. The stall for this latter case would be apparently gentler because the whole lift curve is stretched out sideways by a factor of two. Think about the Concorde, which lands at about 35 degrees angle of attack. Now what about the extra drag term? Is it really worth concerning ourselves about it? Well, the profile drag for a wing alone might be about 1/30 of the lift. An L/D of 30 for an infinite wing is quite attainable. The so-called induced drag caused by the lift term bending backwards is approximately equal to the lift times the angle in radians or C Di = C L 2 / π. AR Let s calculate the wing lift/drag ratio near the stall, (which is also near the landing speed), for different

6 aspect ratios. This will not include the drag of the rest of the airplane. I m going to assume a stall lift coefficient of about 1.4, which is reasonable for a lot of our aircraft. We ll divide the lift by the sum of the fixed profile drag and the induced drag to get the overall lift/drag (L/D) ratio. AR: Infinite L/D If we increase the speed by about 40%, then we only need half the lift coefficient, which in terms of L/D is the same as doubling the aspect ratio. So the new values of lift/drag ratio for the higher speeds will be as follows at the same aspect ratios as above: L/D So it becomes pretty clear why low aspect ratio aircraft drop like a stone at approach speeds. Remember, the real lift/drag ratios will be worse than illustrated because we didn t include the drag of the rest of the airplane. This is only for the wing! Remember also that the tangent of the glide angle is one over the L/D ratio. So our aspect ratio 2 airplane is dead-sticking at a 14.4 degree descending glide angle while its angle of attack is 30 degrees to its flight direction or 15.6 degrees to the horizontal. And that s if the rest of the plane has no drag! A sailplane with a 15 : 1 aspect ratio under the same conditions would have a glide angle of 3.6 degrees, but its angle of attack to the horizontal would be 13.4 degrees. So when an airplane is landing deadstick, it s attitude relative to the ground isn t affected much by aspect ratio, but it s sink rate is affected dramatically. That seems like enough for one article, so I ll quit and, as usual, look forward to your comments. This picture shows it taxiing into the water. And here it is coming out. Without going into too much detail, I ll just hit some highlights and another picture. Floatplane Fun-Fly, Sunday, May 22 nd. Don t forget to come to the fun-fly even if you don t have a float-plane. Bill Moore will be serving one of his great lunches, and we should also have some great launches! Don Wilden has a friend, Fred Tuxworth, who has built and regularly flew a model Loening Amphibian OL-6, at 1/5 scale, 9 feet wingspan. He flew the full-size airplane during WW II in the Aleutian Islands among other places. This is an old gas engine, possibly a Quadra 50, where all the cooling is done by simulating the original ducting past the baffled engine, for a substantial reduction in engine temperatures. The 3- bladed propeller is home-built from laminated walnut and maple! The fully-functioning rigging rods were fabricated from.035 stainless steel sheet sheared to 5/64 width and hand sanded to streamline section, with

7 2-56 right and left-handed threaded ends attached by silver brazing. Hints for flying boat design. Probably still very reliable. So if any of you fancy making something like this for the fun-fly, you have just about a week to do it! Nostalgia Corner We Brits used to hang on to the prose from our monthly Aeromodeller, and even more exciting was the Aeromodeller Annual, somewhat on the lines of Frank Zaic s Year Book. Looking through some of my sixty-year old copies, I came across these gems. And this lot should keep you occupied for another month!