Volume 8 No 3 Jun 1957.pdf - Lakes Gliding Club

Detection and Location of Thermalsby Air Commodore L. P. Moore, C.B.E.I None's limited experience of thermalflying, the first symptom almost invariablynoticed on ~tering or lea:ving athermal has been a qUIck: change ofaIrspeed.But only recently has otle found that thenature of this change is related to directionof fl.ight in relation to wind direction, asfollows:-(4) Entering thermal into wind or"falling out" downwind-suddendecrease in airspeed.(b) Entering downwind or falling outupwind-sudden increase in airspeed.(c) Falling out crosswind on downwindside of thermal-rapid airspeedchanges up and down and burble.The significant thing about these gusts isthat they are usually the first indication ofentering or leaving a th~rmal; the sequencebeiIli a sudden gust or lull followed by asense of acceleration up or down followedby a variorneter reading; the lag betweenthem being the result _of the sailplane'sinertia. It seems therefore that gusts andlulls may be a good-if not the bestindicationof the presence and location ofthetmals.It occurs to me that the above experiencemay well be explained by the accompanyingdiagram if we agree the theory now widelyaccepted, that thermals drift downwind at alesser speed than that of the Wind; treatingthe thermal as an obstruction in the path ofthe airstream.By reducing t he ground speed of thethermal to zero andconsidering a hori~ontalcross-section ofit and ofits environment onthe plane in which the sailplane is circling,we have the air around the thermal streamingdownwind past it, producing eddies inthe wake and a local increase of windspeedon the flanks. The diagram depicts andel\plains, I hope, adequately how I believethe phenomenon is caused.If this explanation is right, a device thatwould measure airspeed differential betweenwingtips ought to go much further in providinga means for detecting and locatingthermals, as follows ( opposite sides of a diaphragm could beDIAGRAM r.AI!t$1JLl..P />IUC1..E.NTTA(. :lltNJluN. A .TH"ItAfIt~01