Thermal Flying for RC Gliders

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As I write this, it does seem as if this awful winter is finally releasing its grip on the flying fields —and not before time. After all these weeks of frost and snow, the prospect of being able to go flying without being kitted out in arctic fashion is an attractive one. I don’t know about you, but my flying since the end of November has been more than somewhat restricted, due to the travelling problems, and sheer cowardice on my part in the face of the freezing conditions. However, it is a fact that even the most unpromising conditions can produce some exhilarating flying. For instance, one cold but sunny day a few weeks ago, I bungeed my rc plane up whilst standing on a downwind west-facing slope, up to the waist in a snowdrift (the crust collapsed at the moment of launch!). Much to my amazement, I picked up and centred a very nice wind-shadow thermal, and rode it out over the valley for over 16 minutes. Definitely very satisfying, despite third degree frostbite (I was concentrating too hard to move out of the snowdrift!). Despite such diversions, however, and the winter pleasures to be had in the workshop (and I am talking about building model aeroplanes!), the prospect of summer, and of drifting a model around under a hazy, blue, warm sky, becomes more attractive by the week—let’s hope it comes true this year!

Area Rules 0.K?

Students of the latest design trends will have noticed that many of the more recent models (and in particular multi-task designs) feature fuselages which are quite bulbous at the front and rapidly reduce to much smaller cross-sections at the wing location. This “fashion”, which seemed to originate with the Austrian Sitar brothers’ multi-task and world speed record holding models, appears to be an application of the “Area Rule” principle, developed in the full-size world on jet fighters of the 1950s. Before asking ourselves whether the concept is really applicable to model sailplanes, it would be worthwhile to look into the background of area ruling and explain, in broad outline, what it means. Back in the days when jet engines were still (relatively) low in power, certain fighter designs (notably the delta wing Convair YF-111) were having trouble in delivering up their design performances. The problem seemed to be a sharp rise in drag in the high subsonic speed region and, after research, the YF-111 appeared with a reshaped fuselage featuring a “waisted” shape. This conquered the problem to such a degree that the original design performance was exceeded. The fuselage profile had been reshaped in accordance with the Area Rule, the basis of this being as follows: Take a well streamlined body, with good drag characteristics at high speed. If we now add a wing to this, the resulting drag shape appears as in Fig. 3, where the frontal area of the “belt” around the body is equal to the frontal area of the wing. It does not need a wind tunnel to devine that this is not ideal from the point of producing a low drag vehicle. What the clever chappies in the aircraft industry did was to perform the (in retrospect) glaringly obvious trick of subtracting the wing frontal area from the cross sectional area of the original streamlined body. The fuselage is then “waisted” in so that the cross-sectional area at the point of maximum wing thickness, plus the frontal area of the wing, equals the original cross-section. In theory the equivalent drag shape of the fuselage plus wings is then the same as the original streamlined body, without wings. Obviously, this is a simplification; other elements must be considered, and some pretty painful maths are involved. However, for our purposes the ex-planation is near enough. In the full-size world, the resulting drag reduction certainly revolutionised the performance of the YF 111 on the available power. It is obvious when considering a thin-wing delta fighter, with its small wing frontal area and relatively vast fuselage, that this solution can be applied to the full, since the amount to be subtracted is still large enough to leave an adequate cross-section at the “waist”. When we look at a typical thermal soarer, however (you see, I haven’t forgotten what this column is supposed to be about!), the situation is rather different. The frontal area of even a thin wing (say 100 in. by 0.8 in. thick-80 sq. in.) will be equivalent to a pretty rotund fuselage (in this example about 10 in. diameter—n-r2 and all that), where 3 or 4 inches would be the normal size. it is clear, then, that we will not be able to apply the rule fully—but bear in mind that the theoretical cross-section from which we would subtract the wing frontal area is larger than that remaining at the nose, since we are trying to achieve a shape equivalent to that shown in Fig. 1. In practice, then, we normally finish up with something which looks like Fig. 5. Does it work?? Well, I must admit to being a little sceptical, since, at first sight, something developed for supersonic delta fighters would seem to have little relevance at our speed range.

However, the proof of the pudding, as they say—and the amazing speeds reached by the Austrians on their world record runs, plus the very quick speed times returned by this type of model in F3B speed tasks, indicates that they are certainly low drag machines. There it is, then—fashion trend or actual aerodynamic improvement, I am not sure which, but perhaps someone can shed some more scientific light on the matter? Until then, there is only one thing to do—try it. At least the end result is an attractive fuselage, so there is obviously some truth in the old saying “if it looks right, it will fly right.” More on the (same) lines … My comments about knotting nylon monofilament lines produced an interesting response from Harry Campbell of Glasgow (where else with that surname?), which makes some points which are new to me.

I quote . . Regarding your comments about nylon monofilament line, as a long time sea angler, with considerable experience of the stuff, I thought I might be able to add some points of interest:

(I) Knots must never be pulled up dry, or friction damage will almost certainly result. Most people lick is before pulling up, but if possible, soaking in water for a minute or two makes the knot pull up very smoothly without damage. (Mono fines actually absorb water and in fact work better when wet rather than dry).

(2) When pulling the knot up, this should ideally be done in one go; that is, one pull to full tightness, since multiple pulls will tend to cause friction damage.

(3) Only the ring and line should be pulled. The end will take care of itself if a little spare is left, and if the knot is correctly formed, it can be cut off very close if desired.

(4) This knot, which is a basic form of “blood knot,” is reckoned to be good for 90% of the line’s breaking strain.

(5) The biggest single enemy of nylon line is ultra-violet, so lines should not be left lying in direct sunlight if possible.

6) My experience is that permanent stretch is alarming, even when temperatures are not in the seventies (as much as 25?, after only short periods of use). Obviously, then, lines must be checked regularly, as you said. (There does come a time when the line will tend to stop stretching if it lasts that long, but the diameter will be so reduced that breaking strain is lowered considerably and the line is useless for anything. They need to be changed frequently, whatever they are used for.)

 

Interesting stuff indeed, and this certainly clarifies in my mind some of the previously inexplicable line breaks which I have witnessed and experienced. Perhaps the snapping of nylon will be a less frequently heard sound this season, if we all heed this good advice. Another Oglesby special .. . Regular readers of this column (assuming that there are any!) will remember that Denis Oglesby unusual 100S model was featured a year or so ago.

The successor to this model made its appearance part-way through last season and, besides being somewhat  original in appearance, it has made a lasting impression with its fine soaring performance, the minimum sinking speed glide being particularly exceptional. Denis kindly provided drawing and the following comments for the model, which he calls Charmed Quark . . This model is the 1978 development of the 0-Gulf, the feature on which, in last February’s R.M., ended: “Next year’s version should clear up odd minor design faults, such as second rate performance and falling to pieces every other landing!” I am pleased to report that this has been achieved, although the knock-off sub-fin creates the illusion that almost every landing is a disaster-1 like to keep my friends entertained! It is always difficult to assess performance, but I am fairly sure that for duration flying it can just about match the best of them, all the way through from calm to moderate to fresh winds. In its first ten contests it made 2nd place twice; its

average placing however was only mid field, for which I blame my “non-stick” fingers. A final point about the 0-Gull is that, during one relatively fast F3B speed run, it doubled its dihedral. I found that this so improved rudder response that the resultant consistent circling was equivalent to a per-formance increase. In consequence, the Charmed Quark has 10 inches of dihedral. During this speed run the earlier model was carrying wing ballast close to the fuselage, so the new model has wing ballast boxes further out with space for 20oz. in each wing. So far, I6oz. per wing has been carried with no noticeable effect on handling. The wing joiner, by the way, is four times stronger than that in 0-Gulf–1 don’t want any more dihedral! An obvious peculiarity of this model is that large triple-knock-off-mode all-moving 54.1 b-fi n . Mode I is a clean swipe off backwards, mode 2 snaps a disposable balsa tongue side-ways and mode 3 declutches the servo. I wanted the tailplane on top of the fin so that it would only have two zones of interference drag compared to a mid-fin tailplane which has four, and the adopted layout has some advantages, at the expense of complexity, over a normal “T” tail. The wing incorporates a number of features in the hope of obtaining yet more efficiency from a “1005” wing. How many people realise that a parallel-chord wing develops lift equivalent to only 88% of its area? It is like dragging an extra tailplane around.

Here, I have aimed for close to 100% effectiveness by approximating to an elliptical plan form, and maintaining undisturbed airflow over the top surface. Tip stalling is prevented by changing progressively along the wing to a section capable of slightly more lift despite the small tip chord—i.e., a free-flight duration section. Approximately elliptical lift distribution is maintained by aligning the zero lift angles of the main and tip wing sections as illustrated. This wing shape has allowed the main working part of the wing to use a 9in. chord instead of the 8in. normally found on 100S models, whilst actually reducing induced drag. The planform deviates most from the elliptical at the wing root, to trigger the stall first there. It should then spread rapidly outwards, a price I am prepared to pay for extra performance.

The Eppler 387 section was preferred, but the only supposed co-ordinates I could find for this section turned out to be those for E174, and this was adopted. E174 is excellent for duration, but becomes an airbrake at anything greater than twice stalling speed (i.e. it suffers a large drag increase at this speed). For profile accuracy the wing is fully sheeted I/32in. high-density balsa over lin. pitch ribs. The TE is I/32in. thick; finish is smooth, top and bottom. Everything seems to have worked out in practice; it usually stalls straight, but relatively violently. Performance is always excellent, becoming even more so when flown very close to the stall, but this is demanding of the pilot and control system, and it is difficult to regain slowest trim quickly after that stall! In light conditions the big 9in. chord helps to maintain high efficiency when flying very slowly without ballast (even when it is raining!). In windy conditions, plenty of ballast makes it extremely potent. That airbrake effect mentioned is useful for “dethermalising” quickly without picking up excess speed, but the multi-task speed run is limited to twenty point something seconds—still not too bad for a duration design though. Next version? I want to try an E392 section, as I suspect that this can marginally improve duration performance whilst (unbelievably?) approaching the Austrians’ E193 speed potential—that will start some debates! I also want to try polyhedral as l need rapid roll response to experiment with wind turbulence soaring techniques (also to reduce the size of that darned sub-fin). For non 1005 events I want to include flaps, purely to assist towing and landing when ballasted to very high wing loadings. Many thanks to Denis at R-C.UK for these interesting insights into the design procedure. Note, by the way, the “hammerhead” fuselage with all the equipment contained in the nose bulge, and refer to the “area rule” piece above.