|
RE: SIMLA BUILD THREAD
[quote]ORIGINAL: kingaltair That is going to be one LARGE plane being powered electronically.[:)] Duane [/quote] Yes, and it is an especially well suited one (remember it's an aerobatic motorglider). [:)]
Posted on: 8/20/2012 12:35 PM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=11199870
RE: Equations for Pitch Speed, Tip Speed, CL, CD, & Some Crude Flight Testing Techniques
panther, that is a good idea that you share your knowledge collected with hard work and over a long time. I think it's like "what would I have liked to read when I was a beginner". But it was the thread title what sparked my interest. As to measuring of model airplane speed, there's now even a fourth way: GPS as part of the telemetry system. Admitted, it would make you go beyond the 250$ limit but you already mentioned telemetry so this is for completeness. By the way, radar guns are based on the Doppler effect as well as the sound method you mentioned. However, I don't trust them both, not for some unavoidable inaccuracy but because you don't really know if the airplane flies level. Even a shallow dive may have a very noticeable effect on speed. That's why speed records once had to be flown at very low altitude, just to force level flight. In the 1930s, Heinkel complained that he had to fly speed records at sea level while Messerschmitt was favored having his racetrack at 1700 ft above MSL. Anyway, another time I'd like to gainsay BMatthews. [;)] I think most of us [i]over[/i]estimate the amount of RPM pickup on models with high pitch props once airborne. That may be true only for electrics since I/C engines lose torque when RPM decreases while an electric drive's torque is reverse proportional to RPM. But in both cases torque doesn't increase when airspeed is so low that the prop blades are (at least partially) stalled, instead it may even decrease. That's because prop torque comes not only from drag but even more from lift, which is perpendicular to the blade surface and not in flight direction. This effect is bigger on high-pitch props. You may notice this on low-pitch props as well though, for instance in a full-scale with fixed-pitch prop, say a C172. There are only a few hundred rpm difference between runup, climb, and level off, not that much compared to the runup rpm. Of course leaving full throttle when levelling off would overrev the engine, but still you'd never come close to pitch speed. That's because (1) that would require no prop thrust and hence (2) no airframe drag, which increases roughly with second power of speed, though (an oxymoron). So I would say in [i]any[/i] case someone ended up meeting and beating the pitch speed limit it was due to some dive, not necessarily a death dive. It's sufficient to get a bit beyond that limit to have the prop windmilling and overrevving the engine. I'll add two graphs showing the behavior of an electric F3A drive. You'll notice that thrust decreases rapidly when flight speed is near pitch speed and that windmilling is required to reach it. On the other side, below 20 m/s some blade stall begins so thrust is no longer reverse proportional to speed and torque no longer an inverted parabola. I have to admit that I only skimmed your document, but may I add another unit conversion calculator: just Google for Convert.exe! If you'd like to add more info about electric drives, [link=http://www.rcgroups.com/forums/showthread.php?t=185271]here[/link] and [link=http://www.rcgroups.com/forums/showthread.php?t=182903]here[/link] are nice explanations of the basic equations. If you want to know how I came up with the diagrams, [link=http://time.fh-augsburg.de/cgi-bin/dl.pl?id=ElectricDrive.pdf]here[/link] is the derivation of the basic equation. As to history of R/C model flying, also look [link=http://www.modelaircraft.org/mag/Jan2011/onthefly2.htm]here[/link] and google for Walt and Bill Good and their Big Guff. There are videos.
Posted on: 7/29/2012 4:19 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11173069
RE: SIMLA BUILD THREAD
thom, you may have that fun but not for free. Maybe better Duane should comment on it, so only this: It was part of the Simla resurrection project that I rendered several variants (wing, weight, drive) in a simulator to see how they fly (three years ago). Duane has them all. It's just not RF but Reflex XTR. There might be a FSone rendering as well and if so, Andy Kunz should be able to comment on that.
Posted on: 7/27/2012 7:07 AM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=11171114
RE: Gyspy Moth?
Look [link=http://richard.ferriere.free.fr/3vues/D-3vues.html]here[/link] for 3-views of various Moth types (DeHavilland DH-...), including the original (DH-60) and two Tigers (DH-71 and DH-82).
Posted on: 6/14/2012 9:24 AM by Author "UStik"
in the forum "Golden Age, Vintage & Antique RC"
http://www.rcuniverse.com/forum/fb.asp?m=11118034
RE: Complex shape flying wing/delta CG calculator
...
Posted on: 5/7/2012 2:22 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11071133
RE: servo glue ?
What wyowindworks says, except I use special plumber's silicone. It's elastic (the wing can bend), seems chemically neutral, adheres well for several years, adheres to all materials (even the shrink-wrap plastic), yet most times you can even peel it off the wing structure after you cut the servo out of the shrink wrap.
Posted on: 4/30/2012 12:09 AM by Author "UStik"
in the forum "RC Gliders, Sailplanes and Slope Soaring"
http://www.rcuniverse.com/forum/fb.asp?m=11062120
RE: TOM BRETT'S DESIGNS-UPDATE
Hi Duane, that is all [i]very[/i] interesting, as always. Now I'm even provoked to throw in another two Eurocents, or maybe even three. [;)] First, it's good to know that the TBX's main gear swivels. That really makes sense. When you first showed pictures of it I thought it is intentionally rigid. Now I stand corrected. Second, the shipping label on the kit box is not addressed to [i]some[/i] hobby shop, but to the world-famous R/C powerhouse, Ace R/C Inc. These are not my words, my wisdom comes from the well-known R/C Hall of Fame. Look at [link=http://www.rchalloffame.org/Exhibits/Exhibit36/index.html]this page[/link] about the first Kraft proportional, scroll down one quarter or search for "Meet The Runges" to find a short characterization of Ace R/C. Anyway, that might explain why the kit is intended as a contest prize and why it's addressed to Ace R/C (donating such prizes). Third, for me it's really nice to see the lathe. We (my father, my brothers and I) had exactly the same (in fact, my brother still has it). Cees once showed us his one in the Taurus thread but I just can't find it right now. I think he has even the second model (version) while Tom's seems to be model 1, that is the simplest one from the early sixties. It was not a "jeweler's lathe" but intended for use in DIY and modeling (not only flying models) in the first place. It's main asset was to be cheap and it was in widespread use here in Europe. I'm just surprised that it was used also in the US, I didn't know that so far. That's why I only now searched for it (Emco Unimat) in the Web and came up with an entry in the [link=http://www.craftsmanshipmuseum.com/oldmachines.htm]Craftsmanship Museum[/link] in California. Scroll down nearly to the page bottom or search for "UNIMAT DB200/SL1000" to find a short description and a picture. The label seems to be more modern but all else is like Tom's copy. Emco is an Austrian trading company and I don't think that they manufacture anything. Seems the machines are made in Germany, but they are quite simple compared to "real" machine tools. That could have been the way to make the lathe affordable for modelers. Today, they sell [link=http://www.emco.or.at/index.php/maschinen/unimat-4]model 4[/link] of the Unimat line but they claim to be still able to provide all spare parts for the old models. Thanks again for finding and posting this "stuff" and to Helen Brett so generously sharing it.
Posted on: 4/22/2012 8:55 AM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=11051851
RE: Graupner Kwik Fly MK3 ARF
[quote]ORIGINAL: Sport_Pilot The Graupner company bought out Phil Kraft's kit making business in the 60's. [/quote] Wow, another one! I didn't even know that Phil Kraft had an own kit making business, I always thought Jim Jensen kitted the Ugly Stik and the Kwik Fli (even though some people say he kitted only the Mk II). Anyway, Graupner brought out their kit in 1968 and at least the Top Flite kit must have been produced at the same time. So was there really a buy-out?
Posted on: 4/16/2012 4:51 AM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=11043877
RE: Graupner Kwik Fly MK3 ARF
Wow, that sounds disgusted! Could you specify a bit more what you mean? All I know is that Graupner brought out a licensed version as early as Feb. 1968. There's a [link=http://www.koralpe.biz/kwickfly.html]nostalgic web page[/link] about it (be sure to scroll down to the red links to an explosion view and a 3-view). There were marginal modifications like better nose landing gear, smaller canopy, and structural strengthening. Of course the ARF may have different internal structure due to modern design and production methods. And of course they modified the nose to accomodate a modern engine and cowled it (even though it still could be mounted upright). And they added a retractable tall landing gear to allow for the bigger props in use today and because people like retracts and would ask for them, anyway. But I thought planform and general shape are still the same. Do you know more about that? Here's a maiden-flight video: [link=http://www.youtube.com/watch?v=rAWqabrz_3g]at YT[/link] (engine mounted the old way).
Posted on: 4/15/2012 4:13 AM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=11042364
RE: OS .15 Steel Fin?
Strange, my OS-702 muffler (about 1968) has no pressure tap, just the bushing for priming. The tap in the back cover plate requires a check valve, the tap in the case below the carb doesn't, but both are for real pressure tanks and a regulator (like [link=http://www.rcuniverse.com/forum/fb.asp?m=8550127]this one[/link]) would be needed. At least I thought so. [link=http://www.osengines.com/history/ostimeline03.html]http://www.osengines.com/history/ostimeline03.html[/link]
Posted on: 4/10/2012 1:59 AM by Author "UStik"
in the forum "Golden Age, Vintage & Antique RC"
http://www.rcuniverse.com/forum/fb.asp?m=11035562
RE: OS .15 Steel Fin?
Hello Lomcevak, I still have my .19 which has steel fins on the cylinder. Maybe you have just the .15 version. We ran these engines on 75% methanol and 25% castor, no nitro. Throttling was just fine, with the baffle or with a muffler, but a glow plug with an idle bar helped. The muffler was made by OS and had no pressure tap, just a bushing for priming the engine with a long needle on a syringe. The carburetor had enough suction to do without tank pressure if only the tank was at about the same level as the carb.
Posted on: 4/9/2012 2:23 PM by Author "UStik"
in the forum "Golden Age, Vintage & Antique RC"
http://www.rcuniverse.com/forum/fb.asp?m=11034911
RE: Maximum altitude??
Well, that flat-bottom airfoil is just what I had readily available because I calculated my Telemaster. With some amount of work I replaced it by SD7037 what gave an airframe better suited to the task. Actually I think the OP really needs an airplane with something like a RG15 airfoil and a F3A drive as he already considered. It would not resemble a F3J machine but rather a F3C or even F5B one, but the drive would have the efficiency assumed for this project. It was just not what I had at hand data-wise. I do have data for another F3A drive (AXI) which show that those 20x13 F3A props are really optimized. The Graupner CFK 20x12 is just not suited to such an application. Notice efficiencies well over 60%. The UAV would just climb with more than 20 m/s airspeed and more than 10 m/s climb rate, but why not with a RG15? The airframe could have less span and aspect ratio but should be a composite build for rigidity and accurate shape.
Posted on: 4/9/2012 7:24 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11034269
RE: Maximum altitude??
Sorry, made a mistake. I was so fixated on getting the max. drive efficiency at the airframe's best climb speed that I overlooked that the motors were underloaded. One cell more (6s1p 12 Ah for each motor) brings them into their range of best efficiency. Overall drive efficiency is now 50%, initial climb rate is 3.9 m/s (760 ft/min) at 12 m/s (27 mph) airspeed, and maximum height is 9800 m (32k ft) at 13.5 m/s (30 mph) airspeed. Two 6s1p 10 Ah batteries would still give 8100 m (26.5k ft) maximum height and two 6s1p 8 Ah would give 6500 m (21.3k ft). Sounds fine especially regarding the structural benefits of having the heavy drives outboard on the wings. Counter-rotating props would be fine as well but I think there are no left-turning props. Could check now the same airframe with a bit higher-kv motors and/or a different airframe with faster (less cambered) airfoil and less aspect ratio. Would give small differences, though. What would be really needed is different prop pitch. The props linked to in post #113 look interesting and the aeronaut props as well but I have no data about them.
Posted on: 4/9/2012 2:12 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11034079
RE: Maximum altitude??
Please don't stop, your posts are inspiring! It's good to have other ideas to compare. Two different propellers are a great idea. I just tried to keep it simple and use only commercially available components. You had the idea how that could be done with a twin configuration, but - without really thinking about it - wouldn't that require two full-blown motors making for some weight? And wouldn't one motor/prop suffice because the mission / drive selection is dominated by climb? I just checked the twin with two AXI 2860/12, AXI PG4/33 4:1 gear, the Graupner 20x12 prop, and a 5s 12 Ah LiPo battery for each motor (effectively 5s2p). That is more battery than before (8s1p 12 Ah) and more weight. Now the drive's peak efficiency [i]is[/i] at 12 m/s airspeed as is the airframe's lowest power demand. But the drive efficiency is 10% lower, prop efficiency is the same, so there's virtually no benefit. A bit more maximum height comes from the bit more battery. Climb rate is really low. It's no practical solution. Seems it's better to use the single-drive variant (push or pull doesn't matter), put a bit more battery in it (15 Ah instead of 12 Ah), and - above all - make the airplane faster by using a less cambered airfoil. The aspect ratio could be reduced.
Posted on: 4/8/2012 10:45 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11033190
RE: Maximum altitude??
It's not a Graupner prop, it's one of the aeronaut CAMcarbon linked to above (I think they are better, design by Rudolf Freudenthaler). The Skylark UAV is smaller and seems faster than what is discussed here, of course that prop fits. Hand launch is as well possible with a 16.4 ft span, 20 lbs weight UAV, but I'd prefer a bungee launch because it's safer / more predictable.
Posted on: 4/8/2012 9:50 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11033037
RE: Maximum altitude??
Thanks! As to mathematics - count me in! I can't imagine these things in abstract form (equations) and need something visual. That's exactly why I made these spreadsheets. And once I had them I found them very enlightening. The 18k ft case has been adjusted (Reynolds number, airfoil coefficients) and isn't quite as "quick" as before. Looks even better now, but still more drive efficiency is needed, that is less power per prop disk area. The airframe's power demand is a given so more prop disk area is needed. Since I don't know of any bigger folding props I'd try the twin solution with the same prop and two smaller geared outrunner motors. Could be a true twin boom configuration with the inverted V-Tail split into halves (paddles). The two booms could be located quite far outboard for more aileron effect and a better mass distribution (wing bending load would be much smaller). The booms could be just tacked to the wing's flat bottom. [:D]
Posted on: 4/8/2012 4:43 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11032750
RE: Maximum altitude??
Thanks, now it's clear. [:)] Just to show what "quick check" means a few diagrams. I know it's not really an explanation, at least for non-engineers, but anyway (because it's fun for me). The NEU-F3A-1 (1300 kv, 6.7:1 gear) was assumed with the data published on the website. Impedance of ESC, battery (8s 12 Ah LiPo), and so on is only estimated. The data for the Graupner CFK 20x12 prop are calculated with Martin Hepperle's program and quite reliable, except near static. A rather simple airframe was assumed with 20 lbs weight, 2200 sqin wing area and 5m (197") span, giving 17.6 aspect ratio. (Quite ambitious, and seems no case for a composite construction, at least for wing and tail.) Horizontal stab is 20% of wing area and vertical stab 10%. Tail moment arm 51". Airfoil is Anderson SPICA flat bottom (sorry BMatthews) because it's quite cambered (4.62%) and I have data. Now all is put together in spreadsheets. I made these 10 years ago and unfortunately didn't automate them so far because I dislike Excel programming. So they are virtually useable only for me. I will share them anyway if requested. First a comparison of drive rpm and efficiencies at ground level (standard atmosphere), full power and cruise power. Yellow is rpm, dark blue motor-gear efficiency, medium blue prop efficiency, light blue total drive efficiency, that is the power effective on the airplane. All dependent on airspeed. The airplane's minimum power demand is at 12 m/s (27 mph) airspeed. At full power the drive's maximum efficiency would be at 23 m/s (52 mph) so in this case the prop's pitch is too big. Second diagram the same except at partial power (0.465 of full power). That's just enough to cruise at 12 m/s which is not the absolute minimum speed/power but still gives a small amount of speed stability. Now the drive's maximum efficieny is at 10 m/s (22 mph) so the prop's pitch is a bit too small. That's what I meant in my previous post, but it's merely a "theoretical" comparison. What really matters you'll see now. Yellow is drag of the airframe, pink is drive thrust at full power, blue at cruise power. It's not easy to imagine at which speed the height will be maximized. Next diagram: yellow is airframe power consumption, pink is power delivered to the airframe at full power, dark blue at cruise power setting. Light blue is difference between full drive power and airframe power demand. This difference is climb power. Dotted pink is reachable altitude/height with maximum 6700 m (22k ft) at 16 m/s (36 mph). Climb rate is 5.6 m/s (1100 ft/min). Of course that's ground air density. But next two diagrams show the case of about half ground density, that is 18k ft. Because the air is thinner the airplane flies faster (airspeed 17.5 m/s, 39 mph) for maximum reachable height. This reachable height is slightly reduced (6300 m, 20.6k ft) because the power delivered is lesser and a bigger part of it is needed to overcome drag. Here you see that the airframe's aerodynamic quality is still of minor importance (maybe except aspect ratio). But cruise flight requires a higher power setting (0.605). Climb rate is 3.1 m/s (610 ft/min). So even though some parameters change with altitude and climb rate gets lower and lower, target altitude could be reached with even a bit charge in the battery for some loitering and landing approach. There are no reserves which means the accuracy (not to mention correctness) of this calculation would be crucial. So no responsibility is taken for the correctness of this information. [;)]
Posted on: 4/8/2012 4:37 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11032166
RE: Maximum altitude??
That's all accounted for. There are just no big enough folding props. And I think fine-pitch is a makeshift if your drive is actually turning too fast. At least the prop can be big enough for good efficiency, but instead of gearing the drive rpm down the prop pitch is reduced. Peak efficiency is generally achieved at advance ratio of about 1, however well the prop is designed and made. I don't know any 3D model, heli or even parkflyer that has an efficient prop. Would be just too big and slow. Even full-size props on smaller airplanes are not efficient because they would be too big, low-rpm engines would be too heavy, and a gear would be too delicate and expensive. By the way, I have a restraint writing your RCU name because this name does mean something to me. Are you the inventor of the jet engine or is it your real name? [;)]
Posted on: 4/7/2012 5:10 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11031664
RE: Maximum altitude??
Did only a quick check and it showed that the prop pitch could be too big (the Graupner 20x12 would be better) and the NEU drive could be too powerful with a 10s battery (8s would be better). The drive's efficiency would be below 50% but 18k ft could still be made with a 12 Ah battery. Yet the whole mess has to be re-thought again. (Maybe even less pitch, maybe even bigger prop or, since there is no such prop anyway, BMatthews' idea of a twin.) After all this was only a quick check, not a thorough one. By the way, if you define two different mission profiles it could well be that for the 18k ft mission, which is determined by climb, a low-pitch prop is best and for the 5k ft mission, [i]if[/i] it's determined by cruise, a high-pitch prop.
Posted on: 4/7/2012 12:45 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11031559
RE: Maximum altitude??
Forgot to illustrate BMatthews' advice to use a multi-panel wing with an odd number of panels. The picture shows a 5-panel wing were the center segment is a piece of its own. The biggest bending stress of the whole wing is in the middle of this segment where no wing joiner is. The main spar is C/F reinforced, otherwise the wing is a traditional wood construction. It's a three-piece wing with two joiners quite far outboard which can be simple and lightweight. And the pieces are shorter than the fuselage while two pieces would be longer. You see the upright V-tail and may imagine that it produces an adverse roll when used as rudder. The inverted V-tail produces proverse roll. The effect is small, though. Just for the record: Such high-performance electric gliders have usually geared drives for reasons mentioned above. Efficiency of a good gear is about 95% because nearly all friction is rolling friction, like railway wheels on rails have. The bearings are ball or needle bearings and the toothed wheels have involute toothing. That's also why there is nearly no wear. Just an annoying whine due to straight toothing...
Posted on: 4/6/2012 12:32 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11030370
RE: Maximum altitude??
The AXI and the Neu motors are not comparable power-wise, the AXI is more powerful and weighs 840 g more. But even if a direct drive and a gear drive are comparable and the weight difference is smaller then this weight put into more battery usually outweighs the 5% gear losses. It's often like in this case: The low-kv motor having the required kv is just one power class higher than the higher-kv gear combination and thus has considerably more weight. AXI offers geared outrunners for a reason: "The possibility of spinning extremely big propeller compared to the size of the unit".
Posted on: 4/5/2012 11:41 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11029540
RE: Maximum altitude??
Yeah, those low-kv motors are heavy, that's just what I wanted to show. The Neu drive with gear seems to be well suited, maybe even a bit too powerful. You see, 1300 motor kv divided by 6.7 gear ratio gives 194 drive kv. That's a bit more than the AXI but the recommended props (about 20x14) would even fit. And I think you would find a matching centerpiece for the 10 mm prop shaft. An efficient propeller needs a low power loading, that is Watts power per square meter prop disk area. The slower the plane flies the lower the power loading has to be. You may read about the theory [link=http://www.mh-aerotools.de/airfoils/propuls4.htm]here[/link], especially the second picture (diagram) about optimum efficiency. Compare a prop drive to a jet engine: the jet has to accelerate a small amount of air to high speed to get the same thrust as a propeller which accelerates much more air to much less speed. The latter is more efficient, same airspeed provided. According to the theory, you have to choose a big enough prop disc for the power the airplane needs for climb and for the intended airspeed. If you choose a too small prop disk you need a stronger motor because more power is wasted, and that is done by turning faster. So you can't compare to different kv motors on the same prop. The high-kv motor would have to be much more powerful two turn the same prop with more rpm. The bigger the prop disk is for a given thrust needed the slower the prop has to turn. So for the same plane: big prop - slow rpm [i]or[/i] small prop - much rpm (and losses). Sorry, this explanation isn't really thought-out. Anyway, for cases as this one a motor-gear combination is virtually always much lighter than a low-kv motor. Don't even think about doing without a gear. By the way, airbrake effect is well possible with a folding prop, no problem. It's just that the effect lessens with speed so it's more for glide angle control. And you have to test the effect before relying on it, maybe on another model. And you have to check that the ESC doesn't prevent it.
Posted on: 4/5/2012 9:36 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11029375
RE: Maximum altitude??
Double post deleted.
Posted on: 4/5/2012 9:32 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11029368
RE: Maximum altitude??
There is not much choice of motor kv and prop diameter/pitch, anyway, so you don't have to think that hard about optimum drive. There are only a few big folding props, those I know of are the Graupner CFRP 20x12 (and maybe 17x10, [link=http://www.graupner.de/en/products/a19e1365-97ab-4565-bb27-a7b06a1c1fea/productcategory.aspx]here[/link]) and the aeronaut CAMcarbon 18.5x8/10/12/14/15 ([link=http://www.aero-naut.de/en/products/aircraft/airplane-accessories/propellers/camcarbon.html]here[/link]). I found one really low-kv motor ([link=http://www.modelmotors.cz/index.php?page=61&product=5360&serie=20&line=GOLD]AXI[/link]) which is even made for a 10s LiPo. They recommend their special 30x20E propeller but it's not a folding prop. It might still be good for this project though. I didn't check what matches. But may I remind of further problems that already have been mentioned. The long runtime of the drive means there will be some heat to be dissipated from the battery. Maybe the 10 cells should be installed separately and not as a pack to get rid of the heat. Maybe it's not a problem because half an hour runtime means only a 2C load, but it has to be checked. As well has to be checked if the battery has to be warmed or at least insulated during motor-off flight (descent in the cold air at 15k ft). Due to these problems I would use a radio with telemetry (besides the UAV control devices) because an ESC with integrated telemetry would save weight and hassle. Add temperature sensors for motor and battery and you have the drive under full control. Unfortunately, Multiplex announced those ESCs more than one year ago but still doesn't ship them, so there seems to be a problem with them. Maybe it's a problem with the switching BEC, though. And an idea regarding flying the thing: The ESCs have a break that stops windmilling so the prop blades can fold by the airflow and drag is reduced. You may want to have drag, though, for landing approach control. If the throttle stick is slightly advanced from the off/break position, the motor turns slowly but the blades unfold and windmilling makes for an airbrake effect. Saves separate spoilers but requires some charge left in the battery for landing (which is good for stretching the approach as well.) But it has to be checked if the drive would give enough braking effect at all. And yet another flying thing: A R/E plane may be tricky to land in gusty wind because there's no separate yaw and roll control, even less crossed controls. And worse, a high-aspect-ratio wing makes for big yaw and roll inertia. So I would just practise in a simulator, even if the plane had ailerons, or are you used to gliders?
Posted on: 4/4/2012 10:53 PM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11028899
RE: Maximum altitude??
[quote]ORIGINAL: iron eagel I always though that an electric motor was most efficient at it upper limit of rpms... [/quote] That's right, but as you see in the diagram the drive (including prop) doesn't "unload" much with more speed, or the other way around, even static rpm is quite high. So the motor's efficiency is not far from maximum and the propeller's efficiency is paramount.
Posted on: 4/4/2012 1:08 PM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11028275
RE: Maximum altitude??
Gear should be no problem, there are maintenance-free gears which virtually never fail. Maybe you end up with a gear-less solution but you shouldn't exclude a gear in the first place. Maybe you just can't find a low enough kv motor. The mentioned AXI F3A has only 235 kv but is fed by a 10s LiPo battery, which is a good idea. But then it turns just too fast for your project. A gear practically reduces kv, just divide the motor kv by the gear ratio. The problem of lower air density mentioned by BMatthews is another thing to watch out for but I thought we get the basic points clear first. Aerodynamically, all gets faster with altitude, that is airspeed increases and the prop turns faster. The motor is unaffected by air density but turning faster means giving less torque. So there is just a maximum altitude (ceiling) for the airplane and it needs a stronger drive than the average model to get that ceiling above 15 k ft. Temperature and moisture effects are yet other (smaller) influences and have been mentioned as well. I forgot: The "paddles" have some aileron effect but are no full replacement, especially not with a high-aspect-ratio wing. What you need is dihedral for stability and basically a R/E plane. Polyhedral wings handle nicely and [i]effective [/i]dihedral should be 9 degrees, that's a good rule of thumb. To illustrate what I mean: The blue/yellow parkflyer has 3 / additional 12 degrees dihedral on the inner/outer wing panels, giving 8.8 degrees effective. The Motorglider has 3 on the wings and additional 30 degrees on the tiplets, giving 9.0 equivalent. Both are typical shoulder-wing R/E models. Edit: Naah, I was wrong. I forgot that the average model [i]has[/i] a strong drive. Just tried with a calculation for my Sr. Telemaster with a weak drive. It's not exactly overpowered but would still climb at 18k ft (half the ground air density), just with only 58% climb rate and 18% more air speed. It would need about 40 minutes to get at 18k ft, so that's the reason for a stronger drive (which I have).
Posted on: 4/4/2012 12:21 PM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11028175
RE: Maximum altitude??
[quote]ORIGINAL: BMatthews So for THIS case it is the climb phase rather than the cruise phase that will be the mode that shapes the design of the craft. [/quote] Yes, I agree, that's what I wrote in my first post. Even if I still don't know whether a marked cruise is intended at all.
Posted on: 4/4/2012 10:36 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11028113
RE: Maximum altitude??
I agree, but I didn't want to go into that detail. I forgot another aspect: Flying such a thing will be much different to flying F3A, and I don't even mean the instrument flying using the control system in the FPV. It has been said as well, a stable airplane is required since even if it can be flown "IFR" it is much workload. And why use an expensive and (a bit) heavy autopilot when a stable plane flies on its own, anyway. Like in the old days, it should be trimmed for a good power-off glide and then the drive set up (downthrust) for best climb. The plane could even have a polyhedral wing without ailerons. There are such 5 meter rudder-elevator gliders! And there's an idea to place the two halves of the inverted V-tail seperately quite far outbords on booms attached to the wings. This way they have a rudder and aileron effect at the same time. Here are pictures. Not exactly a pusher but you get the idea.
Posted on: 4/4/2012 9:30 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11028029
RE: Maximum altitude??
Let's be helpful - very good idea. Seems to be not that easy though... To throw in my 2 Eurocents, I think the general concept is a given and requested is only information helpful to put it into practice. Shoe showed on the back side of an envelope that it can be done, so we think about how it can be done. Climbing to 15k ft and cruising there is possible with an airframe as already suggested. For me the discussion showed that a narrow (high aspect ratio) wing is most important, but not planform or streamlining. Airfoil may play a role. The airframe's overall design determines how good it climbs and at what speed. Now the electric drive has to match it to get the high efficiency required (65%) [i]at that speed[/i]. It has been said here several times that an as big as possible propeller should be used. That's far more important than pull/push or other design features. There are some well-designed (basically efficient) carbon folding props, just the suitable diameter and pitch has to be found out. High efficiency at low speed requires a slow-turning prop and drive. There are some low-kv outrunners, but maybe a more conservative outrunner or even inrunner with a gear would be better. (Gear has been mentioned before as well.) There are ready-made outrunner/gear combinations (e.g. AXI). Attached pictures show characteristics of an F3A drive by AXI. They go to show that the prop determines the drive's overall efficiency (prop, gear, motor, ESC, battery, cables, connectors). And that efficiency has a marked maximum at some speed (the light blue line in the second diagram). In this case, though, the required 65% are achieved between 27 and 46 m/s. That should be too fast for this project but that just means one has to choose/layout the drive differently. (That can't be done on the back side of an envelope, but it's not that hard, either.) Another small but maybe important aspect comes to mind. Climb and cruise are quite different modes for the drive. To get better efficiency in both modes full-size airplanes have variable-pitch propellers. There is even an automatic variable-pitch hub for model folding propellers, as presented in the glider forum a while ago ([link=http://www.rcuniverse.com/forum/m_10815942/tm.htm]here[/link]). Might help a bit. (Dang. Seems to be no longer available.) Last picture a typical pusher configuration, for aerial pictures in this case.
Posted on: 4/4/2012 8:51 AM by Author "UStik"
in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11027974
RE: Ed Kazmirski's Taurus
Look at Ray's post #828 on page 34. The whole article was not known so far...
Posted on: 1/14/2012 1:54 AM by Author "UStik"
in the forum "Classic RC Pattern Flying"
http://www.rcuniverse.com/forum/fb.asp?m=10907981
|
