Evan Pimm

Right answer Simon, this is how the experts find the 'most rearward' balance for their gliders. S&L at a nice cruise speed, then gently ease the stick forward to about 30 deg nose down dive. Without touching the elevator or trim (stick free), watch the model. It will tend do one of three things, either nose up quite positively, continue in a straight line, or continue to increase speed and dive angle. The first indicates that the balance can come back, the second shows the max rearward, and the third shows a balance too far back. Re-balance to obtain a gentle pull-out from the dive. You will have to re-trim the 'cruise setting' a bit as the balance changes, but in essence you are finding the best balance for your particular model set-up.Evan.

David, if you are converting a F/F to radio you will have to change both the wing/tail angular relationship, balance and possibly the tail section, if only for strength reasons. Fitting an elevator to a model means that you are now capable of varying the airspeed, which means greater loads, and the need for a more forward balance, so stronger and longer wing dihedral braces and spars and less (around +2deg?) wing/tail angle difference. As the tail needs now to 'lift' both ways a zero camber section is desirable.John, a tailplane cannot affect the cg (balance), only weight added at the front or back can do that . The reason the old models were stable is that the balance, even if at the trailing edge of the wing, is still sufficiently forward of the aerodynamic centre, (the lift cg, if you like) that there is still enough longitudinal stability for the model to be speed stable, although the closer the two get, the narrower the speed range is. The glider guys will tell you about 'tailplane takeover' a condition where the model bunts, no matter how much up elevator the pilot applies. A result of the balance being just a bit far back, and the model being flown just a bit out side the stable speed range. But you guys know all this stuff anyway. Evan.

To answer the original question, no, a tailplane cannot 'support the weight of a tail heavy model and rebalance the cg.' In simple terms the cg (balance) determines the longitudinal stability of the model and is fixed by you, the builder. The tailplane is the 'speed control' of your model, and you vary the speed with elevator trim. The tailplane will then maintain the required main wing AOA by either 'lifting' up or down, hence the preferred use of a zero camber (symmetric) section for this surface. The 'lifting' (cambered) sections referred to are an old specialised free flight method of controlling the flight, with other trim methods, of a 'fixed surface' model and is used to maintain a fixed speed (single AOA), a pretty artificial form of flight. As alluded to in previous postings, if anything upsets this trim, the results can be spectacular, as anyone who has witnessed an FAI power model 'blow up' will agree. Evan. 

Sounds like the model is stalling at the top of the loop. If it only does this on tight loops, but not on a bigger radius loop, then I would say that would confirm the suspicion. There is no real cure, you can limit the elevator travel so as you can't loop that tight, enter the loop with a higher airspeed, if possible, or just get into the habit of easing off the elevator as the model goes over on its back. Evan.

If you really did flick roll the model, then the wing was stalled. Flick rolls are autorotations, just a horizontal spin. If, again, you tried to recover a bit too quick, then the wing might not have unstalled. Adding elevator will only cause another flick, and as these are also high drag gyrations, the model will lose airspeed all the time. Sounds like you was a little slow, due to the apparent effects of the high local windspeed, when you entered the first flick, and it all went pear shaped from there. Pilot error? Probably, more like jut not enough compensation for local conditions, difficult to get it right when you ain't sitting in it. Evan.

Being 'international' so to speak (NZ) ;1. Mode 1 (Learned to fly Mode 2 first) 2. always flown in N.Z.3. Self taught, now instruct. With modern radios and buddy leads I don't care what the pupils' mode is.  Evan.

In a word...no. Yes, I know it looks a lot, but it just doesn't seem to notice it. What the dihedral will do, once the model is trimmed properly, is allow to use the rudder as a flight control, as the full size one can steer around the sky reasonably tidily 'rudder only'. I have flown other modellers 'flat wing' versions, and most of them were pretty nasty flying machines, mind you they used foam wings too, which does not help. Stick to the original ideas, the guys that designed the original, way back, knew what they were doing, probably much more than the average 'club expert ' does nowadays. You will not be disappointed.Evan.

Terry Whiting, Yes, I know the term 'semi-symmetric' is locked in modelling yore, but it is a nonsense term, something is either symmetric, or it ain't, and if accurate exchange of information is the goal, then we shouldn't use nonsense terms, specially with single interest subjects. As for the camber line, you will note that I said it separates equal areas above and below the line itself, to do that then, it must be equal distance between the top and bottom of the section at any point on the section. Depending on the shape of the section then, the camber line will either be straight or curved, ie the section will be symmetric (non-cambered) or cambered. Even that old term 'undercambered' shouldn't really be used, it is simply a cambered section where the bottom curve of the section has gone past the 'flat bottom' bit and, because of the amount of camber and the thickness of the original streamline, is now curved to follow the top. But I am only an old grouch and no matter what I may say, it won't make any diff at all, but it is fun to natter away about it all... Evan.

Really chaps, we must get away from this 'flat bottom/semi symmetric etc' nonsense and start to use 'cambered sections' and 'non cambered sections'. Then it becomes much easier to understand this 'zooming' thing. The camber line of a section (that line that separates the streamline section into equal areas above and below) gives a clue to how much drag and/or lift the section can generate (in broad, laymans terms) and how 'speed sensitive' it may be. Generally the greater the camber, the more 'lift'  (and drag) the section can produce at an airspeed. That being so, the greater the difference in lift and drag with changes in speed. So, if your turn in any direction in relation to the wind direction (the aeroplane does not know the wind is blowing) results in a speed change, then the model will climb or lose altitude, depending whether you allow the nose to drop or rise during the turn. In the full size there is both an ASI and Altitude indication, and it is relatively easy to turn without changing you airspeed, but with models this is not easy, the best indication is the engine note, if it speeds up then your airspeed is increasing, and the model will 'zoom' when you straighten up, and if it reduces, then the model will lose speed and altitude when straightening. The whole thing is pilot induced, and practise is required to get to know just how much 'back stick' is needed to produce a smooth, level turn. Obviously a non cambered section is much less sensitive to this malady, and another good reason to use such a section on almost any sport R/C type. Evan.

Or you could point the thing into the prevailing wind direction, and widen one end for the most usual crosswind, like a 'blunt'  tee, if you get my drift...just as useful as a big square, but about half as much grass to cut and maintain. Modellers forced to use a 'tight' strip learn how to land accurately very quickly, and you will find that other clubs and flyers tend to be very poor circuit flyers with some really bad landing habits... Evan.

I have heard of the Airsail Volksplan, Beaver and Auster being electrified, but not a Chippie, yet. Come to that I haven't heard of a Tomahawk being sparked either, but I don't get to hear everything. And yes, the Chippie is a small scale model, the wing area is not great, and it was designed for a .40. Given that most scale models tend to be quite a bit bigger, 2 metre span and more, then you can see that, despite the quoted span, the actual thing is really only a wee thing and will respond magnificently to the addition of lightness. Evan.

Or so they claim. We just did some 'poly C and .75 oz glass cloth' on a wing for a scale aerobat, and there is as much weight gain as h/weight tissue and lots of dope coats. In its defense the finish (before paint) is quite good, and much less work than epoxy and glass. On a big model it makes sense, on something as small as the Chippie one needs to be very careful. Lightweight tissue and  dope is the way to go. You must realise too, that the kit manufacturer has little control over the wood supplied in the kit, whilst every effort is made to match the wood with the intended task, he is always limited to what is available at the time of kitting. If the modeller is not satisfied the the wood should be replaced with what you consider suitable. These little scale models were never intended as a 'first scale build', being small and quite close to scale they require quite a bit of experience before you attempt a build, hence the instructions are aimed at an experienced scale modeller and are quite sufficient for anyone who has built from, and designed his own, plans and models. Evan.

Seems that you should first get a good drawing of the real thing, it had plenty of dihedral anyway. It has also got 4 ailerons, the easiest way to drive them, depending in the size of the model, but I'm guessing it's pretty small, will be a central servo in the lower wing driving the two lower ailerons in the usual way. (pushrods and bellcranks). These can the drive the upper ailerons via a removeable trailing edge pushrod, again its common practise and you can piccies in any of the mags, or ask at your local modelshop/club. I would advise aginst torque rods as the additional weight of the upper ailerons could unduce a bit of unwanted twist or flutter in the sysrem. Oh, and your hinges and links should have no stiffness in them at all, the whole lot should 'flop' under its own weight. Evan.

A .70FS? The poor thing was designed around a .40 two stroke. You better hope that between the glassing of everything and the final covering (that fabric Solarfilm stuff?) and the extra servos you don't end up with an overweight, overpowered bomb. The originals were Solarfilm on the open areas and dope and tissue on the sheeted bits. They also had admirable flying qualities, I think there was a writeup in one of the English mags where the builder mentioned that the engine he used was way too much power and he had some trouble getting a low enough idle to land the thing. I think he was using a .54 FS. He was, however, most appreciative of the flying. I would advise caution about anything that adds to the weight, and isn't actually needed. Evan.

If you use a plastic tube bearing for the aileron rods, you will find that they will bind as the bellcrank moves. Best just to let them run through biggish holes in the ribs so that they can move fore and aft as needed. Hence the reason for just holes in the ribs, the support offered by the servo at one end and the bellcrank at the other is plenty as the actual rod length is not great. Evan.

On a model as small as the Airsail Chipmunk, a standard servo is plenty for flaps. Evan.

The balance point on any conventional model is always in the same place, around 25 to 30% MAC. For your Mustang with its simple tapered wing the simplest method is to measure half way out along the wing from the fuselage side to the tip, then dividing the chord at this point into four. At the 1/4 point on this line from the leading edge, project this point to the fuselage side. This will be, as near as dammit, 25% MAC, and a safe place to start. Flight testing determine you preferred positon after that. Evan.

The problem with static testing is that it will not give you any idea how the model will actually fly. The prop that will you the best static thrust might not move the model fast enough to actually take off. It will find the prop that gives the most static thrust, of course. But the best prop to match your engine/airframe/flying? Sadly, no. You will have to take a wide selection of props to the field and fly the things. Evan. 

Then again, if you look back at the photo that started the thread, and realised that for the tip vortex to still be able to seen some time after the prop as moved on shows that the air behind the moving prop must be fairly stable/still, otherwise you would have thought that the vortex would have been blown away/shredded by that blast of air from behind the prop. A pretty good photo to show that here ain't that much commotion behind a moving propellor... Evan. 

And I would be a bit cautious about anything written in 'Wiki whatever'. Air in a twisting helix around the fuselage? Never seen it mate, as what this discussion is all about, specially at low speed/high power situations, hovering IMAC models with smoke on, for instance. As for torque, the pilot most definately does not counter the effect with aileron, and it does not matter which way the prop is rotating, he uses rudder, left or right depending which way the aircraft want to yaw. Aileron trim does not produce adverse yaw, use of aileron can, but only in specific aircraft types. Generally speaking aerobatic aircraft do not suffer from the malady, nor do most of our models. I have a 1/4 scale 'Monocoupe' which certainly does, but its about the only model I have which does. So those particular references do not actually stand the light of close scrutiny. I'll bet 'Spifire ' pilots would be surprised to learn that their aircraft suffered from adverse yaw. No, not yet convinced. I will have to stick telltails all over my models and see which way they blow. Meanwhile, think on this. Which way does your hovering IMAC model want to rotate if left alone? Mine always roll left. Spiral airflow theory would have it rolling right. Evan. 

I have flown one of those light electric models, after my Sicroly it was very light, and the old up elevator trick very nearly stopped it dead. With little weight/momentum, aerodynamic drag has a huge effect on the foamie things. Be gentle the first couple of times. Evan. 

Well spotted Simon, that is exactly what is happening. So why, as Andy points out, do we use thrust offset to counteract the effect? That comes from the old F/F thing, where there was no other way to control the effect other than rudder trim, which had a range of other effects as well, not all of them wanted. With modern Wireless we use rudder to control the effect, which is our Yaw control, instead of Aileron, as the torque is definitley trying to roll the model. Oh dear, another conundrum. Two reasons, one, you need to use rudder to control the torque effects on the ground, cause ailerons don't work until the wheels leave the ground, and two, the damned thing is spinning too, so there are precessional effects. Torque doesn't disappear when the model takes off either, as any full size pilot will tell you, you still need to hold a bootful of rudder as you climb too, otherwise the ball whips off to one end of the old 'Turn and Slip' indicator. Matter of fact it must always be present, otherwise the prop will stop turning. What to do? Why don't the aiplane start rolling? Left to its own devices it probably would, but we pilots trim the damn things to fly 'S&L'. What happens is that one of the wings actually develops a tiny bit more lift than the other, just enough to counter the torque effect. It does this because thats what we trim the airplane to do. As we all know, lift and drag go hand in hand, and this little bit of extra lift gives the wing making it a bit more drag, which will yaw the airplane, so using rudder is just the ticket. Some of the IMAC boys re now coupling rudder and throttle in their computer radios, as they are finding out that thrust offset only works (is balanced) at one specific prop/rpm, and they need it to be balanced at any power setting.  Evan. 

Me too Mike, it's a bigger prop, probably no heavier though, but I have no idea how fast you are spinning it. I do believe that it is a torque related effect though, as you have probably guessed. There may be some slipstream related effects, but these are minimal compared to that caused by the big fan on the front. Evan. 

I did say 'Self respecting'...Seriously though, I fly competition aerobatics, and yes, for reasons specific to the nature of the competition, some thrust offset is used. Strangely, it's different amounts for different models, and often different amounts for two of the same model. And it's all to do with what we are talking about. But for most of us, with the normal sort of sport flying that the other 90 odd % of us do, this side thrust offset simply isn't needed. And i'm being specific here too, side thrust offset, not down thrust, which is used for an entirely different reason. Evan. 

I remain unconvinced, I have watched too many aerobats hovering with smoke on and no sign of spiral airflow to be convinced. Looking at it another way, the energy required from the engine to rotate the mass of air behind the prop, given that this air will pull more air in from the surrounding, would leave precious little for thrust. Consider too, that as soon as the model starts moving the thrust starts to decrease and the velocity vector would start to 'straighten' the flow anyway, and I think that the 'spiral airflow' has little or no effect on our models. We have to look somewhere else for the 'right thrust' requirement for models. Apart from it being a hangover from free flight models, no self respecting radio model actually needs right thrust, and in general (full size) flying, where you would expect the effect to be much more obvious with much more power and slower, higher pitched fans, they actually go to some trouble to make sure that the motor points in the direction of travel... But, I hear you say, what about the offset fins, cambered fins etc seen on many old and some newer full size stuff? That is a story that will have to wait for another time, but it is related to the answers to this conundrum... Evan.