RE: Having engine, wing and horiz. stab on a common centerline as opposed to having it otherwise.
I have run into trouble with two of my own design models that had their symmetrical-airfoiled, non-dihedraled wings and horizontal stabs in line. The symptom was a sudden uncommanded pitch up or down. I think that the cause was that the stab was in airflow that was split by the wing, with a thin, slightly lower velocity, turbulent flow sheet hitting the stab right on its airfoil centerline. I think that if this lower velocity flow regime shifts just slightly up or down due to a slight change of wing lift coefficient, the stab suddenly sees higher velocity air above or below it, which tends to abruptly raise or lower the stab. I fixed the problem by increasing wing incidence by about half a degree, which avoided having to relocate the stab. I now make sure that the stab is at least a half inch above the wing airfoil centerline. Stab dihedral or anhedral would also probably fix this problem. I suspect that for non-symmetrical wings, the stab should be slightly above or below the airfoil zero-lift line. Many of my control line models had their symmetrical wings and stabs in line. without any sudden pitch problems, possibly because slight building errors moved the stab slightly out of line with the wing.
Posted on: 9/7/2012 6:17 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11221257

RE: Enlarging the tailplane..
If you considerably increase the area of the horizontal stab and elevator, you will have to move the CG back quite a bit, since the horizontal tail will carry more of the airplane's weight. Otherwise, the airplane will be nose heavy, and require a lot of additional elevator travel to perform maneuvers and to flair for landings. Doubling the area of a typical horizontal tail would probably require moving the CG back from something like 33 percent of the wing chord to something approaching 50 percent, which will lower the effective wing loading and reduce stall speed significantly, and quite possibly, considerably improve tracking in maneuvers, although it might look a bit odd. Many competition aerobatic models have horizontal tails with about 30 percent of the area of the wing, along with rather long tail moments, while the average sport model normally uses a horizontal tail of under 20 percent of the wing area. Elevator servo torque requirement will tend to increase somewhat, particularly if the elevators are given a rather wide chord. Increasing both the span and chord would result in a more effective tail than would simply doubling its chord. Simply doubling the chord would result in a tail with a rather low aspect ratio with a poorer lift/drag ratio than that of a normal tail.
Posted on: 4/17/2012 6:45 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=11046492

RE: What are these little fences for?
Control surface fences are intended to reduce leakage of air at the ends of the surface, increasing control effectiveness, and reducing drag and yaw. Somewhat similar to gap sealing. Evidently, they have been considered to be worthwhile on some competition sailplanes, improving low-speed aileron effectivness, and more than offsetting their slight additional drag in straight flight. Nice conversation piece at the field, too. It would be relatively easy to tape temporary fences to a wing for a quick low speed handling test, taking care to ensure that they won't come adrift and jam an aileron.
Posted on: 2/27/2012 4:44 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10978933

RE: Tri Plane decalage ; best settings?
The upper wing on an equal-winged bipe usually develops more lift then the lower wing, and, according to theory, induced drag should be minimized by having the wings develop equal lift, by giving the upper wing lesser incidence. However, full-scale wind tunnel tests conducted on biplanes many years ago revealed that induced drag was very slightly reduced by giving the upper wing something like 1-1/2 degree more incidence than the lower one, contradicting theory I find that my own design aerobatic bipes seem to exhibit better line-holding on vertical uplines and downlines with about a half to one degree less incidence on the upper wing - in other words, negative decalage. I have never flown an RC tripe, but would be inclined to give the top wing about the same incidence as the other two, or maybe a degree or so less, and would be prepared to experiment a bit with still lower incidence for the top wing to see how it affects handling.
Posted on: 1/29/2012 5:59 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10934255

RE: aerofoil for Extra 300
As I mentioned previously, I used a similar section, (15 percent thickness, max. thickness at about 14 percent chord), and it worked just fine, on a very low wing loading competition fun fly bird, although I had to use quite a bit of wing washout to avoid sudden tip stalls at low airspeed, probably because the wing had 60 percent taper. It also has large chord coupled flaperons, which may have made the stall more abrupt.
Posted on: 12/16/2011 5:21 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10862960

RE: aerofoil for Extra 300
As Cludwig mentioned previously in this thread, I think that the subject airfoil may have originated with the Eppler 242 series. Coordinates are in the URL below, for 12.1 percent thickness, which could easily be translated to any desired thickness. http://rg65.iq-web.net/foils_index.php?class=Eppler&profil=e472.txt I suspect that the airfoil used on the Extras, Edges, and Sukoi could be very closely approximated simply by using a elliptical nose, with maximum thickness located somewhere between around the 14 to 17 percent chord, spliced onto tangent chord lines that run perfectly straight to nearly the trailing edge. A small amount of reflex (inverse) camber, maybe 1/2 percent, might have been used. Recent NASA low reynolds number airfoils frequently use reflex camber, evidently for better L/D ratio at high lift coefficients.
Posted on: 12/14/2011 2:12 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10859537

RE: What are boost tabs???
Here is a link to a MAN article with some pics of my boost tab-assisted 84" span bipe that uses only one standard servo for each flight control to get full-aerobatic control response at its top speed of about 85 MPH. http://media.radiocontrolzone.com/mair/online_articles/BiplaneSecrets.pdf
Posted on: 11/1/2011 8:10 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10792989

RE: What are boost tabs???
Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18" high, with average chord of about 7", on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2", spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement. That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos. After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane. I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.
Posted on: 11/1/2011 7:59 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10792975

RE: What are boost tabs???
Wobblewobble mentioned having crashed because of a boost tab. I had my rudder servo linkage disconnect once, and the rudder went hard over - more than 45 degrees, with a very large 3-D type rudder. I had to fly the airplane in knife edge to a gentle upwind crash landing that slightly damaged a wingtip. I had been experimenting with the travel of the passive boost tab, where the servo controlled the rudder directly, with the tab assisting. It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.
Posted on: 10/31/2011 6:00 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10791305

RE: Fokker Dr1 Triplane problems
The basic handling problem with the DR-1 is excess aileron yaw caused by insufficient vertical tail area, along with its use of only two ailerons. The same problem afflicts many WW1 designs; the Tripe more than most. An enlarged rudder of the same shape would not be all that noticeable, and should give a big boost to handling. I have fixed several evil-handling model bipes by simply enlarging their fins and rudders, but haven't as yet tried this trick on a triplane.
Posted on: 10/27/2011 8:07 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10784938

RE: Elevon question
The taper used in the elevons reduces their mass near the tips, where their mass is most likely to induce flutter because simple wing flexing vibratory excursion is highest near the tips. Control surfaces should be as light as practical while retaining sufficient strength to do the job. What they need in order to minimize the likelyhood of flutter is high stiffness to weight ratio, not maximum possible stiffness at the expense of weight. I would be strongly inclined to employ mass balancers at the tips of the elevons. I employ them on all of my airplanes that are expected to fly fast enough to make flutter a possibility. The mass balancers, if mounted at the tips, should statically balance about a third of the static unbalance of the elevons. Excessively heavy mass balancers can be more dangerous than none, since they may induce flutter a flutter mode where the wing or stab flexes in a more complex manner than simple flexing of the tips up and down. Such higher harmonics usually only appear at rather high airspeed, where flutter can be extremely brutal, and break up the airplane in a fraction of a second.
Posted on: 10/2/2011 10:38 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10744995

RE: Correcting adverse yaw
The Wright brothers used wing warping, not ailerons, and repeatedly crashed their 1901 glider due to warp yaw, and wing warping is by far the aerodynamically cleanest way to get lateral control. The basic problem is that if you want more lift in order to raise one side of the airplane, you have to pay, with drag. There is no free lunch. The Wrights fixed the problem by adding vertical tails to their 1902 glider. The rest is history. Differential travel is a well-proven method of controlling aileron yaw. Frise ailerons work even better, by causing the up-going aileron to produce parasite drag, and thus offset the drag of the down-going aileron. I tried a fair bit of aileron differential one one of my fun-fly aerobatic models, and ran into a fair bit of roll/pitch coupling, and was forced to remove nearly all of the differential. The problem with aileron differential in aerobatics is that the differential is reversed in inverted flight, causing bad aileron yaw in inverted slow flight. I use nearly none on my aerobatic designs, and fine-tune differential by extensive flight testing in order get rolls that are as axial as possible. I have often run into models with bad aileron yaw, and have found that a very effective fix is to simply add vertical tail area to reduce the yaw. A simple taped-on cardboard extension on the rudder trailing edge can serve as a quick and dirty test of the additional area needed, along with a proportional reduction of rudder travel. Some bipe designs that I have test flown exhibited particularly bad aileron yaw. The Ultimate is a good example of a bipe with adequate vertica tail area.
Posted on: 4/7/2011 12:26 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10449293

RE: How much throw
Aileron and rudder throw are entirely up to the individual flyer. Elevator throw, however, is a bit more involved. In my view, for normal (non-3D) flying, elevator throw should be just adequate to stall the wing, and no more. This ensures that the elevator is able to persuade the wing to develop its maximum lift, without excess danger of stalling. Excess elevator throw can produce an inadvertent snap roll at very inconvenient times, such as during a panic pullout from a low-level steep dive, resulting in loss of control and a crash. To adjust elevator travel, I give the airplane full power and full elevator, while flying at a safe height. If the airplane performs several consecutive loops with no indication of stalling, it needs more elevator throw. If it immediately stalls and wanders around drunkenly, it has excess throw. I find the ideal to be 1/2 to 3/4 of a loop before the airplane rolls and wanders with full up or full down elevator. If it goes wonky before finishing 1/4 loop, it has too much elevator travel. Application of full elevator in a dive pullout may result in disaster. I do the same exercise with both up and down elevator. Airplanes often require different up and down throws.
Posted on: 3/24/2011 12:00 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10420066

RE: Determining CG
ChiefK, sounds like your CG is a bit too far forward. You might try a bit of tail ballast, which will also remove the need for a lot of the negative stab incidence, and also dictate the use of lower elevator travel, to avoid the increase in pitch sensitivity that accompanies CG that is further aft. I like to set the CG so that the airplane requires virtually no "down" elevator while flying inverted, but this can sometimes make the airplane a bit hard to land smoothly in a gusty wind.
Posted on: 2/13/2011 7:38 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10334268

RE: Determining CG
A useful trick for determining the accuracy of CG-locating softward is to plug in the dimensions of a successful canard (tail first) airplane, just to see the whether the algorithm gets totally confused, or takes it in stride. Da Rock's suggested program, copied below, failed miserably, unless I screwed up in some way, so I must conclude that it contains significant errors, not only for canards, but for conventional configurations. Maybe someone else could throw a canard at it, and see how it handles it. A good program should be able to work out the distribution of lift of a canard, as well as a normal layout, sufficiently accurately to locate the CG. If not, it is probably in error in calculating the effect of downwash on tail lift. http://www.geistware.com/rcmodeling/cg_super_calc.htm
Posted on: 2/13/2011 2:05 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=10333476

RE: Wing flutter
I learned the hard way that if you aren't using control surface mass dampers (balancers), you are skirting disaster with every flight. The violent kind of flutter described requires an initial deflection to set it off, usually a series of air bumps spaced about right to get the wing or stab into oscillation at the critical amplitude, after which it just takes off, with amplitude increasing rapidly with each cycle, until something breaks, detuning the system. If you are lucky, an aileron or elevator may simply break off, leaving the rest of the structure in flyable shape. In severe cases, you hear a bang, accompanied by a cloud of hundreds of airplane pieces. Properly designed mass dampers will remove the danger 99 percent of the time. Removing slop from linkages will seldom make any difference with this type of flutter, and may just raise the airspeed of onset, making the flutter even more destructive.
Posted on: 9/4/2010 9:38 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9980883

RE: What are boost tabs???
Boost tabs have been employed on full-scale airplanes of much later vintage than the China Clippers. The DC-9, which later became the MD-80, uses a combination of boost tabs and servo tabs to control its ailerons and elevators. The elevators and ailerons are statically counterbalanced. The pilot has direct cable manual control solely over the servo tab, which is assisted by a separate boost tab. The result is relatively light control forces, weight reduction, and excellent feel. Should hydraulic oil be lost, the airplane remains fully controllable - a boost to safety as well.
Posted on: 8/15/2010 10:07 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9936145

RE: Need Help With CG Determination
Flying wings have their centre of pressure at about 25 percent of the chord, similar to the wings of airplanes having horizontal tails. Tailless airplanes must have their center of gravity no farther aft than 25 percent of mean chord before becoming dynamically unstable and nearly impossible to fly. I have found, during wrestling with several tailless designs, that CG about 22 percent of mean chord may the the practical maximum aft location for friendly handling. Tailless airplanes will usually be more sensitive in pitch, although this broad-chord example should exhibit reasonably friendly handling with the right CG.
Posted on: 7/13/2010 9:41 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9861954

RE: Zero-Zero glider design
We used to trim outdoor hand-launched gliders quite close to zero-zero; otherwise they would enter a loop immediately upon the very vigorous launch, instead of attaining the target height of 100 feet or so. With the zero-zero setup, the CG has to be well aft, or the airplane would just dive into the ground from full altitude. I think that a lot of the secret in handlaunched gliders was in selecting just the right balsa density for the wing. The wings normally had flat bottom airfoils, of about 6% thickness. At high airspeed, diving moment developed by the campered wing would twist the wingtips to a slight negative angle of attack, preventing looping until the airspeed fell off near the top of the climb, whereupon the airplane would enter a stable glide.
Posted on: 5/3/2010 7:23 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9707992

RE: pitch-yaw coupling
Short of major re-arrangement of area, another way to adjust yaw-induced pitch is to use sweepback (like a Cessna) or sweepforward (like a Mooney) of the rudder hinge line. A slightly swept-back hinge line causes the rudder to supply some pitch-up, countering a previously existing tendency to pitch toward the belly in knife-edge. A bit of sweep-forward does the opposite, countering a tendency to pitch toward the canopy. One way to do this is to temporarily tape on a wedge-shaped extension to the fin's hinge line, and hinge the rudder to it. It may take a couple of tries to get the hinge line sweep just right. With a computer radio, of course, you can simply mix a bit of up or down elevator with rudder, depending on the direction of the unwanted pitch, and determine the correct amount by experiment.
Posted on: 5/1/2010 1:58 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9702516

RE: Taper change on a simple flatbottom foiled wing
I routinely use about 40 percent taper on my aerobatic models, with good results, in combination with full-span ailerons, which work with the unstalled inner portion of the wing to give solid roll control, even when the tips are stalled. I tried 50 percent taper on one aerobatic pattern model that had barn door (partial span) ailerons, and coupled flaperons. It tended to tip stall and drop a wing on landings, unless I touched down at a couple of knots above full stall. Wind tunnel tests conducted many years ago revealed that 40 percent taper reduced induced drag by about 12 percent, and profile drag by about 3 percent. Taper also helps lighten the wing structure, since it gives a deeper mainspar near the root, where bending loads are highest. A deeper spar can use less material for the same strength. Taper also moves the center of pressure inboard, further reducing bending load on the inner portion of the wing, where loads are highest.
Posted on: 4/24/2010 9:35 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9684969

RE: Biplane Incidence ?
Full-scale wind tunnel tests that were conducted back in the early 1930s revealed that bipe wings developed very slightly less induced drag when the upper wing of the pair had a degree or so more incidence. The reason for this appears to be unknown, since theory says that minimum drag should be achieved when both wings work at the same lift coefficient, which would require slightly lower incidence in the upper wing. My own experiments with model bipes seems to point in the direction of slightly lower incidence in the upper wing, which, I think, helps aerobatic line-holding. The commonly held opinion that increasing incidence in the upper wing makes for a gentler stall was demolished by the old wind tunnel tests. The tests revealed that the slightly earlier stall of the upper wing did tend to cause a stabilizing nose-down moment, but this moment was vastly overpowered by the large nose-up pitching moment caused by the huge increase in drag that accompanies a stalled wing.
Posted on: 4/6/2010 10:53 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9641266

RE: Biplane aileron?
My first bipe design had two wide-chord, full-span ailerons on the lower wing only. Its roll rate was lazy. In desperation, I increased aileron travel to nearly 45 degrees each way, but the roll rate, although OK for scale-like gentle sport flying, was far too slow for snappy aerobatics. Full aileron deflection produced excessive, sloppy yawing and very noticeable drag. The model practically screamed for four ailerons. One might think that two double-width ailerons would do the job of four narrower ones. They don't. Lowering an aileron on the lower wing of a bipe lowers the pressure, not only on the on the upper side of the lower wing, but also over much of the underside of the upper wing spanned by the aileron. This lowered pressure tends to suck the upper wing down, opposing the roll. The no-aileron wing also strongly resists being rolled. Rolling induces a change in effective angle of attack that results in a large lift force that opposes the roll. The result is something like trying to steer a car by only one front wheel, with the other wheel locked straight ahead. Bipes with upper and lower wings of unequal size, with generous aileron area on the larger wing have less need for four ailerons. Roll resistance of the no-aileron wing decreases very rapidly as its dimensions shrink.
Posted on: 4/6/2010 8:22 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9640968

RE: Tapered wing or constant cord wing
A 40-percent tapered wing will produce about 12% less induced drag, and over 3% less profile drag than a constand chord wing of the same aspect ratio. It also generates less aileron yaw, and is structurally lighter. Note spelling: "chord", not "cord". If the structural complication of taper is not attractive, a well-rounded wingtip, where planform curve at the trailing edge extends inward about one chord, produces at least 6% less induced drag than a wing with square tips.
Posted on: 3/3/2010 9:04 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9550972

RE: gp 40 ultimate biplane flutter
I have used both boost tabs and spade balancers, with good results in both cases. Spade balancers cause more drag that boost tabs, but they can also function as mass balancers, and help prevent flutter. Spade balancers go back to the 1920s, when the were normally called "paddle balancers". Paddle balancers are basically the same as spade balancers, except that they are longer, normally requiring two or more supports. I tend to prefer boost tabs to spade balancers, mainly because spade balancers cause more drag than boost tabs. The Douglas DC-9 used both boost tabs and servo tabs on its elevators and ailerons. Both were free to weathervane into the slipstream; the pilot having no direct mechanical control. His control column was attached by cables to the servo tabs, which moved the control surfaces, assisted by the boost tabs. The result was a sweet-handling big bird, with excellent control feel and response, without the addional weight and complexity of hydraulic assist.
Posted on: 12/27/2009 10:29 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9364996

RE: AR vs. Winglets for reducing induced drag
I remember reading that early flight tests of the Lockheed F-80 surprised the engineers by revealing that the airplane could attain higher altitude with tip tanks fitted than with clean wings. Evidently the tip tanks reduced induced drag more than enough to offset their additional parasite drag. One problem with winglets that I don't recall having seen mentioned is that they impart a dihedral effect, producing proverse roll/yaw coupling that is undesirable for aerobatics. I had to use almost as much winglet area below the wing to get rid of the dihedral effect. Such winglets are easily damaged by ground contact, requiring breakaway mounts.
Posted on: 12/18/2009 4:51 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9341656

RE: gp 40 ultimate biplane flutter
Full-scale applications of boost tabs usually employ a boost tab with 5 to 10 percent of the area of the control surface. The reason that a boost tab can be relatively small is that the center of pressure on a control surface is normally about 1/3 chord behind its leading edge. It would be very interesting, as rmh suggests, to investigate a boost tab having 50 percent of the area of the control surface, by simply slapping in an additional hinge line along the middle of the control surface. Such a boost tab would require a very small angular deflection. I think that it would perform OK, but the divided control surface would be far less resistant to torsion and thus more prone to flutter, unless the entire control surface was made far stiffer, which would invoke a heavy weight penalty. Following this line of reasoning, I think that a boost tab of 1/3 the total area of the control surface would be a better solution, producing a sort of variable camber control surface and possibly reducing drag slightly, since the boost tab would need a much smaller deflection than usual. I have used boost tabs that extended over the entire length of the trailing edge of rudders with good results. Those tabs had about 1/10th the chord of the rudder, and required a deflection of about half that of the rudder. I screwed up on my first such rudder, and gave the boost tab too much deflection. Result: far too much boost. The servo had to resist the boost tab, and supply torque in the opposite direction, resulting in a rudder that refused to trim. The boost tab would move the rudder slightly from one side to the other within the free play of the rudder actuating linkage. A boost tab of 1/3 the chord of the control surface would probably require a deflection of only about 12 percent of that of the control surface before the boost tab provided excess force, destabilizing the control. There is no question that boost tabs reduce the effectiveness of a control surface if a given area and deflection, simply because part of the control surface is deflected in the wrong direction. The control surface thus has to be made slightly larger in order to obtain the same control authority.
Posted on: 12/17/2009 1:53 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9338673

RE: gp 40 ultimate biplane flutter
Boost tabs tend to add weight at the trailing edge of the control surface, and thus would tend to make a control surface slightly more prone to flutter. I normally mount them well inboard, where their additional mass is far less likely to cause problems, and make them as light as possible, using, for example, aluminum pushrods in place of the usual steel ones. Boost tabs have worked well for me on several original aerobatic models. The tabs appear to be able to reduce servo torque needed by up to about 85%, although the control response can sometimes get a bit non-linear when the tabs supply more than about 75% of the effort, particularly on very large rudders. They have worked well for me for 3-D type flying, working well up to 45 degrees surface travel, if the tabs are made larger than usual, with proportionally reduced throw.
Posted on: 12/17/2009 9:34 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9338027

RE: AR vs. Winglets for reducing induced drag
Very interesting that winglets were found to reduce lift - have never previously heard of that. I read where some engineers were unsure of the optimum angle for winglets - straight up and down, or some other angle. Tests revealed that the best angle was straight out. In other words, a slightly longer wing produces less induced drag than a shorter wing with winglets, which seems to vindicate the sailplane designers. I heard that Boeing engineers were attracted by the idea of increasing the wingspan of their then-new 747-400 series to reduce induced drag, but decided to go with winglets simply because the longer wing would take up too much ramp space. A wing with winglets can probably be built slighter lighter than a longer wing, all else being equal, since the winglet probably imposes additional bending moment on the wing than a wing extension would.
Posted on: 12/14/2009 10:12 AM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9329638

RE: High vs Low Aspect Ratio for speed
Just to set the record straight, a higher aspect ratio wing of the same area will not have more frontal area, although frontal area of a wing is not normally considered in drag calculations. Consider an extreme case, where you take a low aspect ratio wing design, and keeping the same airfoil and area, double its span and cut its chord in half. The new wing will be half as thick as the original. With double the span and half the thickness, its frontal area will be the same as the original's. It will probably have a slightly higher minimum drag in straight and level flight since minimum drag normally increases as Reynolds number decreases, and with half the chord, it will fly at about half the Reynolds number, a product of chord and airspeed. However, with a much smaller root chord, interference drag between the wing and fuselage will be probably be decreased enough that the total drag would probably be no higher than with the original wing. Maximum lift of the higher aspect ratio wing will probably be slightly lower than that of the original, again due to the lower Reynolds number. For a given wing design, induced drag is inversely proportional to aspect ratio. With four times higher aspect ratio, the new wing will have one-quarter of the induced drag of the original, for vastly lower speed loss in turns. On the downside, a longer wing tends to produce higher aileron yaw, so the fuselage may have to be increased in length, or the vertical tail area increased, or both, probably resulting in increased overall drag of the airplane.
Posted on: 12/6/2009 3:43 PM by Author "Rotaryphile" in the forum "Aerodynamics"
http://www.rcuniverse.com/forum/fb.asp?m=9308418