1/1/2011CPU Power and Thermals 1 CPU Power Performance and Thermals 101 Efi Rotem, Jan 2011.
On Self-CenteringIn Thermals - Oakland Cloud...
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8 On Self-Centering In Thermals Hans Gremmer - an advance in its understanding As in generally known, a full-size glizer pilot must rely on his instruments in thermal flight, not only for finding lift but also for adjusting the turn radius for the best rate of climb. The rate of climb is highest in the center of a thermal. Without complex equipment the pilot would never succeed in centralizing the glider, which has a tendency to drop out of thermals. An RC-glider pilot also has to \'Iork hard to center the model in the thermal, usually by observation, trial, and error. On the other hand, it is always puzzling to observe the thermal i ng of a free fl i ght gl i der. If properly trimmed, it seems to remain on a thermal for hours on end, alway s centeri ng in the thermal core by itself. A dethermalizer is indispensable: the World Records for distance and duration by free-flight gliders prove that they are capable of flying almost indefinitely in suitable thernal conditions. Where do these self-centeri ng properti es of a free-fl i ght-gl i der come from? It is true that constructional outline, rigging and trimming have a considerablle influence on theor rhermaliong properties. In this contex I assume that the major distingui sing feature of d free-fl ight model - the dihedral wing - has the greatest impact on the centering properties of a glider in a thermal, and we shall deal wi th thi s phenomenon 1 ater on. Fi rst we have to explain that this only comes into play when the model meets with the thermal shell itself. So the questions asked are: 1. What is the nature and structure of a thermal shell ? 2. Why do only free flight models (with dihedral wing) use thermals to best advantage, especially in respect of working their way towards the center? As is well known, a thermal is not just a column of rlslng air. Thinking of a thermal shell resembl- ing a huge balloon filled with warm air comes some- what closer to the mark, but is not the whole story. There must be something unique regarding the struc- ture of a thermal. If it were only a motionless mass of buoyant air, a model or a bird would glide down from the top to the bottom and leave the thermal. This view is quite common, and as a proof we may show a misrepresentation published in a recent issue of an RC magazine. On the contrary, it can be observed that bi rds or models often enter a thermal at very low altitudes, e.g. buzzards 20 to 30 m above ground level, and leave it at great height. From this it may be concluded that they rise in the thermal bubble itself. Therefore a thermal must have an entire circulation mechanism, in which the air rises in the center and drops outside. When a buoyant mass of warm ai r separates from the ground, the friction of the rising warm air at the outside of a thermal causes the cooler air around the bubble to circulate. The warm air itself takes up the shape of a gi ant smoke ri ng and looks like a doughnut around which cooler air revolves descending at the periphery, returning at the bottom to the center and there rising again. In this way a continuous flow of cooler air is pumped upwards in the center, and its upward speed is hi ghest in the plane of the vortex ring. Hence any flying objects such as dust, pollen, spores, seeds or model gliders can be taken aloft and will rise if their sinking speed is less than the rate of updraft w,ith to the vortex ri ng. Thi s rotary system 1 s shown 1 n Fig. 2. It may be considered as the universal key to the understanding of thermaling: Thi s rotary system can attract flyi ng objects towards the center of the lift and keep them within the shell. How this system works in real flight can best be shown by the strange behaviour of F1E-models entering a thermal. Such models have to fly straight ahead through a thermal shell: when approaching the thermal, then always lift their tails, simUltaneously speeding up tremendously as if attracted by a giant magnet. They then rise like an elevator, after a while slowing down, sometimes even being pushed back Seemingly, they always enter a thermal at the bottom, for fi rst they have to fly through a downdraft on the outside of a thermal, where they sink. The acceleration and the subsequent slowing down are caused by the so-called "entrainment" (the in- flowing cool air into the center of a thermal). When approachi ng a thermal. they encounter a ta il wi nd, caused by the synchronous rotation, as it were a co- rotation, but when having passed the center they have to overcome headwi nd caused by the counter-rotati ng bubble. Some well known West-German F1A fl iers (Nordic A-2) who also compete in F1E-events profit from these observations when tow launching, especially in stronger winds when tow circling is impossible. A straight flying model on tow shows the same thermal behaviour as a straight flying magnet model. Now let us come to the question of why a circl- ing model tightens its turn according to of lift. The rotary system also causes the entraln- ment" at the base of a thermal shell, and this en- trainment drifts a model to the center of lift. The stronger the lift the stronger is the inflow of cooler air - the "entrainment" - and the greater is the central drift resulting in a tightening of the turn. But why does a full-size glider obviously drop out of a thermal and not drift into the center by itself? It always encounters more lift at the inner panel of the wing which is nearer to the center of a thermal. As a result the glider is banked away from the core, unless the pilot counteracts. Thi s counteracti on is equally made by the outer Wingtip of a dihedralled model: the airstream flow- i ng to the center causes a greater angl e of attact at the outer pannel, thus counter-balancing a pos- sible excess of lift at the inner pannel which is nearer to the center. In addition, the inflow of air to the center affects the upturned wingtips the 1atera 1 area of whi ch (not vi si bl e, only projected) render the drift possible. Experiments were made with different dihedral angles in order to test the thermal-seeking qualities. "It became very clear that models with little dihetral (only sufficient for a stable flight in calm air) usually dropped out of a thermal, whereas those wi th greater di hedra1 stayed in. We also know from observation of thermal-soaring birds, that from time to time they fold up their wings to a positive dihedral. In conjuction with upturned pinions this may give a better central drift. Buzzards are real "model s" for studying thermal flight.
Transcript of On Self-CenteringIn Thermals - Oakland Cloud...
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On Self-Centering In ThermalsHans Gremmer
- an advance in its understandingAs in generally known, a full-size glizer pilot
must rely on his instruments in thermal flight, notonly for finding lift but also for adjusting theturn radius for the best rate of climb. The rate ofclimb is highest in the center of a thermal.Without complex equipment the pilot would neversucceed in centralizing the glider, which has atendency to drop out of thermals. An RC-gliderpilot also has to \'Iork hard to center the model inthe thermal, usually by observation, trial, anderror. On the other hand, it is always puzzling toobserve the thermal i ng of a free fl i ght gl i der. Ifproperly trimmed, it seems to remain on a thermalfor hours on end, alway s centeri ng in the thermalcore by itself. A dethermalizer is indispensable:the World Records for distance and duration byfree-flight gliders prove that they are capable offlying almost indefinitely in suitable thernalconditions.
Where do these self-centeri ng properti es of afree-fl i ght-gl i der come from? It is true thatconstructional outline, rigging and trimming have aconsiderablle influence on theor rhermaliongproperties. In this contex I assume that the majordistingui sing feature of d free-fl ight model - thedihedral wing - has the greatest impact on thecentering properties of a glider in a thermal, andwe shall deal wi th thi s phenomenon 1ater on. Fi rstwe have to explain that this only comes into playwhen the model meets with the thermal shell itself.So the questions asked are:
1. What is the nature and structure of a thermalshell ?
2. Why do only free flight models (with dihedralwing) use thermals to best advantage, especiallyin respect of working their way towards thecenter?
As is well known, a thermal is not just a columnof rlslng air. Thinking of a thermal shell resembling a huge balloon filled with warm air comes somewhat closer to the mark, but is not the whole story.There must be something unique regarding the structure of a thermal. If it were only a motionless massof buoyant air, a model or a bird would glide downfrom the top to the bottom and leave the thermal.This view is quite common, and as a proof we mayshow a misrepresentation published in a recent issueof an RC magazine.
On the contrary, it can be observed that bi rdsor models often enter a thermal at very lowaltitudes, e.g. buzzards 20 to 30 m above groundlevel, and leave it at great height. From this itmay be concluded that they rise in the thermalbubble itself. Therefore a thermal must have anentire circulation mechanism, in which the air risesin the center and drops outside.
When a buoyant mass of warm ai r separates fromthe ground, the friction of the rising warm air atthe outside of a thermal causes the cooler airaround the bubble to circulate. The warm air itselftakes up the shape of a gi ant smoke ri ng and lookslike a doughnut around which cooler air revolvesdescending at the periphery, returning at the bottomto the center and there rising again. In this way acontinuous flow of cooler air is pumped upwards in
the center, and its upward speed is hi ghest in theplane of the vortex ring. Hence any flying objectssuch as dust, pollen, spores, seeds or model gliderscan be taken aloft and will rise if their sinkingspeed is less than the rate of updraft w,ith respe~tto the vortex ri ng. Thi s rotary system 1 s shown 1 nFig. 2. It may be considered as the universal keyto the understanding of thermaling:
Thi s rotary system can attract flyi ng objectstowards the center of the lift and keep them withinthe shell. How this system works in real flight canbest be shown by the strange behaviour of F1E-modelsentering a thermal. Such models have to fly straightahead through a thermal shell: when approaching thethermal, then always lift their tails, simUltaneouslyspeeding up tremendously as if attracted by a giantmagnet. They then rise like an elevator, after awhile slowing down, sometimes even being pushed backSeemingly, they always enter a thermal at the bottom,for fi rst they have to fly through a downdraft onthe outside of a thermal, where they sink.
The acceleration and the subsequent slowing downare caused by the so-called "entrainment" (the inflowing cool air into the center of a thermal). Whenapproachi ng a thermal. they encounter a ta il wi nd,caused by the synchronous rotation, as it were a corotation, but when having passed the center they haveto overcome headwi nd caused by the counter-rotati ngbubble.
Some well known West-German F1A fl iers (NordicA-2) who also compete in F1E-events profit fromthese observations when tow launching, especially instronger winds when tow circling is impossible. Astraight flying model on tow shows the same thermalbehaviour as a straight flying magnet model.
Now let us come to the question of why a circling model tightens its turn according to the"stren~th
of lift. The rotary system also causes the entralnment" at the base of a thermal shell, and this entrainment drifts a model to the center of lift. Thestronger the lift the stronger is the inflow ofcooler air - the "entrainment" - and the greater isthe central drift resulting in a tightening of theturn.
But why does a full-size glider obviously dropout of a thermal and not drift into the center byitself? It always encounters more lift at the innerpanel of the wing which is nearer to the center of athermal. As a result the glider is banked away fromthe core, unless the pilot counteracts.
Thi s counteracti on is equally made by the outerWingtip of a dihedralled model: the airstream flowi ng to the center causes a greater angl e of attactat the outer pannel, thus counter-balancing a possible excess of lift at the inner pannel which isnearer to the center. In addition, the inflow ofair to the center affects the upturned wingtips the1atera1 area of whi ch (not vi si bl e, only projected)render the drift possible.
Experiments were made with different dihedralangles in order to test the thermal-seekingqualities. "It became very clear that models withlittle dihetral (only sufficient for a stable flightin calm air) usually dropped out of a thermal,whereas those wi th greater di hedra1 stayed in. Wealso know from observation of thermal-soaring birds,that from time to time they fold up their wings to apositive dihedral. In conjuction with upturnedpinions this may give a better central drift.Buzzards are real "model s" for studying thermalflight.
Figure 1 - A false representation of the flight pathof a soarer in a thennal shell drawn as a cylinder(implying a volumn of rising warm air) in which thegliders sink from the top to the botton - accordingto a French RC-magazine 1980.
Fi gure 2 - A correct representati on of the rotarysystem wlthin a thennal shell, as pUblished in "TheScientific American" April 1962. The originalexplanation of the drawing says: "THERMAL SHELL iscomposed of a vortex ring (torus, or doughnut) aroundwhich a current of cooler air circulates in closedstreamlines (broken lines). The soaring bird fliesin a circle the radius of which gives the bird anaerodynamic sinking velocity equal to the velocityof the updraft. In this equilibrium position withinthe shell it is carried aloft as the shell rises .
. . : : : ..... : . .3 Mixed zone
Glider speeding up in Glider slowing downthe ~ynchronous rotation in the counter-rotation
Figure 3 - No caption.
Figure 4 - Left: Side drift of a straight flyingmodel in a crosswind.Right: Central drift of a circlingglider in a thennal.
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\ ~--------~
~1D1~ rom.,.- ,
Fi gure 5 - Full si ze gl i der banked away from thecore of the thennal caused by excess of lift at theinside wing.
Fig. 6 Free-flight glider with its outer panelcounterbalancing possible excessive lift at the innerpanel, simultaneously offering a side area forcentral drift.
10 Centering in Thermals, Gremmer: Cont.
Now we know that a sufficient dihedral helps amodel to stay in a thermal and that a thermal soarertightens its circles due to the influence of thecentral drift. But why does a model speed up at thesame. time? . (Remember, this does not apply tostralght f1Ylng magnet-models, but only for circlingmodels). The speeding up of a circling model in athermal is caused by the increased circular airfloweffect of tighter turns. Since the decal age reducesitself according to the radius of the turn, themodel acce1 erates and the c i rc1 e becomes stilltighter as already caused by the central driftwhich often ends in a lethal spria1 dive. Hence th~modern wing-adjustment of wash-in at the innerpanne1, which is said "to hold the inside wing up."Some interesting articles hve been published in thesubject of tril1111ing for therma1ing, e.g. by Will aimBaker in the NFFS Sympo 1980 "How to trim free flightmodels for therma1ing."
We also explored the thermal properties for rearsteered magnet-gliders, when the straight flight~ourse is interrupted for a subsequent circlinglnterva1. For the uninitiated it must be said thatno assymetrica1 warps can be employed with thesemodel s, because warps have a prejudica1 effect onthe model's straight-fl ight trim. However, wesucceeded in renderi ng thi s type suitab1 e for selfcentering thermal flights. First, very slow flyingspeeds and very light wings allow very tightcircles, hence the model picks up the slightest bitof a thermal activity. Moreover, we appliedtrim-change systems 1ike V. T. I., whi ch can preventspiralling. But the real breakthrough came with theapplication of new stabilizers with an aeroe1asticcovering, which inverts the curvature at negative
angles of attack (as occurs in dive conditions) andproduces strong negative lift in this way. With aC.G. of 90 to 95'.1', chord these stabilizers are aseffective in terms of spiral stability as normalones at C.G. of 60'.1', chord, assuming the same tailvolume. (Also see the article "Magnet-gliders forlong duration f1ughts").
Comp1 ete steeri ng units can be ordered with acertified bank check of 30 Oeutsche Mark (30 DM)from:Anton FRIESERSch1esische Str. 2, D-8832Weissenberg, West-Germany
More information can be obtained by writing to:Hans Grel1111erOberbretenauer Str. 11, D 8300Landshut, West Germany
ReferencesNFFS-Sympo 1977 and 1980The Scientific American, April 1962: "The SoaringFlight of Birds," by Clarence D. Cone Jr."Vom Ba1 sa-Gl eiter zum Hoch1 ei stungs-Seg1 ern by HansGremmer, pub1i shed at "F1 ug- und Modeltechni k", D7570 Baden-Baden (= "From the ba1 sa-g1 i der to thehigh perfoJllllance soarer" the only book onfree-flight gliders in the Western World)
Acknowledgments:I want to express my hearty thanks to TrevorFaulkner, lecturer in Sheffie1d/GB, for correctingthe text, and also to Professor ARGHIR, Rumania, forhis drawings 3, 4, 5 and 6. T. Faulkner andG. Arghir are also active F1E-f1iers.
DOD
