Volume 32 No 1 Feb-Mar 1981.pdf - Lakes Gliding Club

• J.~:;.~":~:~~~;,'fg!l~!tt~~:f'.·Fig J. lllcvrpvrated in the heading is a diagrammatic iIIustrativlI vfa ,hermic rotor.I was having a bad wave-day: that is, I wasn't getting any. I wasa fairly early sol'o pil0t. tlying an Oly 28. and a combination oflack of ain:mft penetration. inexperience and sheer bad luckhad caused me 10 miss the wave, three times nlllOing. Admittedlythe day wasn't the easiest. The bottom of the wave variedfrom about :\000 to 4000ft. and the lowesllayer of air, below themain temperature inversion. was thermic: the illstability brokeup the bottom of the wave. making the wave-lift itself difficultto recognise. and transforming the wave-bars into large, illdefinedareas of cumulus.On the way back from my third aerotow, having been droppedtoo low and too far back from the leading edge of the cloud.frustrated and depressed. I found a thermal, or rather it foundme. Four knots, nearly over the airfield, and still enough heightto play with. We 1'1 this isn't wave, I tfuought. only a poor substitute.but I might as well. tay off the deck.This thermal was strong. surging, with narrow cores and Ifound some difficulty in staying wj,thil. The ground wind wasstrong ,md I assunled that within a few minutes I would have toleave the thermal and tly Llpwind to regain the airfield. To mygreat surprise as 1 kept re-centring I remained approximately inmy original position over the airfield. After about 20min 1 lostthe thermal. couldn't find it again and assumed it had collapsed.1 came in and landed but in view ofthe observations recorded inthe rest of this article, 1 was prob.ably wwng to give up soreauily.May have wider application1 ought to say at Ihis point that al') of my ohservations onthermic rolor have been made in the area near RAF Dishforth,although they cover both the normal westerly wave off the Penninesand the mu..:h rarer north·easterly wave o~f the Clevelandhills. It may be. however, that these observations have a widerapplication.Rotor may he conveniently divided into th~ee types. based onthe soarahle qua'lities of the rotor, or lack of them. Taking th.elatter variety first. we have:(I) Negative rotor. This is lhe ordinary kind, very turbulentand gusty. wiln the downs bigger th,lIlthe ups. In a glider,you come down slowly or quickly, depending on the localconditions.(2) Positive tolor. This occurs when rotor merges with ,thelowest pari of the wave (in the zone of lift). The situationis still very turbul.ent and gusty, but the ups more thancancel the uowns. If you tly steadily into wind, easillgbaek 011 the surges and pressing firmly forward (withincreased speed) when the surges lIie, you will normallygain height and with any luck will arrive in the bottom ofthe wave itself within a few minutes.Cl) Thermic rotor. This occurs when rotor becomes involvedwith unstable air, somewhere between the ground and thebottom of the wave.The symptoms may be described as follows. Strong, surginglift - 6kt ,is typical - occms in narrow cores, often interspersedwith patches of sink; normally the sink is fairly heavybut less strong than the lift. The period of the surges is in theregion of I-Smin - they are vertical squalls, ra'ther than gustsbutshort-period turbulence ,is generally also present, and maybe s.evere. The cores vary rapidly, both in strength and position:however, repeated centring, and observati'Ofls on othergliders; show that the cores tend 10 remain in line same generalarea. say within a zone about one mil'e in diameter.Too long for ordinary turbulenceThe position of this zone is evidently determined by the airtlowpatterlil of the rotors below tne wave, and therefore by thewave itself, and ultimately by the obstruction causing the wave.The period of the sLlrges is too shont to represent the repeatedgeneration of thermals by a specific heat-source on the ground,and for that maller t00 long for ordinary turbulence. The processevidently consists of unstable air being dragged up on thewindward side of a rotor (see Fig I,), in the manner describedand illustrated in Alcide Sant,illi's excellent al'llde "Into Wavesfrom Thermals", in the book Advanced Soaring edited by John:loss.How can we be sure that thermic rotor isn't s,imply strong,well-organised posi(,ive rotor? First of al'l, thermic rotor occursmainly uuring spfing, summer, or autumn, when thermal effectswouId be expected: in fact, I cannot remember the phenomenonoc.curring during the winter (although I stand to be correctedabout this). Secondly, ordinary thcrmals mfty show a transitioninto thermic rotor. For example, on one occasion, I was thermailingat aboul 1500ft near Dishforth when a light westerlystarted up. As 1 gained height the thermal became surgy butincreased in strength. Fortunately I was well centred and continuedto gain heightl"apidly. At about 4000ft the lift suddenlybecame smoother. 1 lurneu into winl!, begun 10 S·turn andtloated up to 5700ft in gentle wave (one of the other pilots made10 OOOft!). From these and other similar observations, it seelllsclear that thermic rolor is n very distinct phenomenon nnd notsimply a Iype of positive rotor.The highest parts of the surges in thermic rotOr often reachthe same level as the lowest part of the wave. This obviouslymakes ilt possible to thermal into wave by using thermic rotor,as indicaled by Santilli. However. the actual' proCess of thermailinginto waVe raises a number of quest,ions, in particular forthe early solo pilot."How do I know it's real thermic rotor'? And what's more torhe point, holY on earth do 1 soar it? I'm tense and ratherscared. and I'm overcontrolling. ,I can',t centre p"~)perly, and6 SAILPLANE & GLIDING