Sailplane & Gliding 1990

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SAILPLANE

Magazine of the

British Gliding Association

& GLIDING

Kimberley House, Vaughan Way

Leicester, LE1 4SG

Tel Leicester 0533 531 051

December 1990-January 1991

Volume XLI No. 6

YOUR LETTERS 305
H. c. A. Dale (reply by B.
FINAL RESULTS
Morris), P. J. Stratten, M.

S&G CLASSIC
Boydon, F. G. Irving, C. 0.
J. Torode, A. Cronin, J. Buntin,
EDITOR
Vernon (reply by J. C. Gibson), 306
Gillian Bryce-Smith
Ann and R. Procter
C. White, C. A. P. Ellis, S.
281 Queen Edith's Way, Cambridge, CSI 4NH

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Ottner, A. M. Segal, H. G. 307 LOOKING BACK
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285 Peters, V. Davies, M. Abrahams W. E. Malpas
(reply by W. G. Scull)
CONSULTANT EDITOR
OPEN CLASS NATIONALS
310
Rika Harwood
290 TAIL FEATHERS M. Bird
66 Maisemore Gardens, Emsworth. Hants, POlO 7JX

Platypus
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314
FINAL RESULTS
FLYING TECHNIQUE AND

SUBSCRIPTIONS

Bev Russell
291 WIND-GRADIENT
THE 1991 COACHING
BGA Office
I. W. Strachan
PROGRAMME
315
C. C. Rollings
COMMITTEE
WINDS ALOFT AND

A. W. F. Edwards (Chairman ), M. Bird. 292 WEATHER BELOW
316
M. F. Cuming, B. F. R. Smyth T. A. M. Bradbury BGA & GENERAL NEWS
ADVERTISING MANAGER
BGA ACCIDENT SUMMARY
ChHeeirloenn PRrretcshsi eL
td
295 GOOD BEGINNINGS 317 K Shipley, D. A. Wright
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322
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0494442423 or 0860 510407
296 SPIRIT OF THE PLACE
GLIDING CERTIFICATES
I. Wright

1991 NATIONALS
PUBLISHER

British Gliding Association
298 A DREAM COMES TRUE 323 QUALIFYING LISTS
(Barry Rolfe, BGA administrator)
J. West C, G. Corbett
326
GERIATRIC RIGGING DEVICE
300 REGIONALS' RESULTS
D. H. G. Inee
328
WINNING WAYS OF A CLUB NEWS
TUTOR
301
N. James 339 TO NURTURE A DREAM
R. Witter
INTER-CLUB LEAGUE FINAL

302
M. B. Jefferyes
THE BGA POLITICAL
COMMITTEE
340
TWO VIEWS OF THE JUNIOR J. Holland
NATIONALS

304 Karina Hodgson and Kerry 341 WAY OFF TRACK

Lomas Penguin

Member of the ~\!! b"
Royal Aero Club and the
~
Cover. Dave Smrth of Derby & Lancs GC, flying an Fed~ration A~ronautique
Astir. took this beautiful photograph of the Peak Internationale
District with Mam Tor in the middle distance and
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December 1990/January 1991 283
LIS
SAILP S LTD.
NOT A GOOD IDEA individually and possibly even lookout and A "MYTH" REVIVED
Dear Editor, effect of elevator? There is no order from Dear Editor,
The other day while helping at the launch above that the pupil must not touch the rud­ In spite of (or maybe because of) John Gib­
pOint I had a word with a visitor to our club der, only that the AEI will deal with the co­ son's kind reference to my slender monograph
who was getting ready for his very first flight in ordination of the rudder, since the rudder (ref 1) in his piece on Static Stability (see the
a glider. To my surprise I saw that his legs exercises have not been introduced. Any pro­ August issue, p187), I cannot resist making a
were stretched! out fla\. "Can you operate the posal for an improvement in our methods few comments.
rudder comfortably like that?" I asked. Before should also address the wider implications of He poses six questions and proceeds to

he could answer the AEI in the back seat that change and offer a practicable com­ argue that the answer to all of them is "no". I

explained that he had set the rudders fully for­ promise. When can I arrange for my head to would agree that this is so for all except the

ward so that the pupil could not reach them. be examined? first.

"As an AEI I must not let the pupil touch the Most of what he says under the heading

rudders. He moves the stick while I control the THAT DIAMOND BADGE "Centre Of Lift" is only precisely true when

rudder." , viewed as a consequence of thin aerotoil
I had previously heard of instructors in other Dear Editor, theory applied to two-dimensional aeroloils
clubs who did this sort of thing through Some details of Mike Cuming's suggestions (see ref 2). Under these idealised circum­
ignorance, thinking that this way of breaking in the August issue, p173, on how to stances, an element of a 2-D wing will pro­
down the task might facilitate learning. But rebalance the Diamond badge might not go duce a lift force but no drag and one can
here was a perfectly sensible chap from my down too well with those who don't have the deduce the results displayed in Mr Gibson's
own club dOing this because of orders from luxury of more than 1 :35 and a choice of 365 Fig 1. It is, perhaps, slightly surprising that real
above. gliding days per year. aerofoils of finite thickness in a real (Iow­
As a psychologist with a long-standing pro­ Perhaps a handicapping system would help speed) flow behave very nearly in the same
fessional interest in the acquisition of skill I am put the goal and distance legs into perspec­ fashion but witll, of course, a small drag. The
horrified to learn of this practice. When hand­ tive, especially now that some people are aerodynamic centre is typically located be­
ling three-axis machines, the co-ordination of actually claiming the Diamond distance having tween 0.24 and 0.27 times the chord aft of the
stick and rudder movement is the very core of flown 500km in an Open Class glider! leading edge.
flying skill. Pupils need to learn tIlat whenever PETER STRATIEN, Bicester, Oxon If one has a three-dimensional wing with
they move the slick sideways they must apply taper and perhaps sweep (eg that of the K-13)
rudder pressure simultaneously. Such essen­ THE ANSWER IS TO COMPLAIN it is possible to define an equivalent rect­
tials need to be emphasised from the very angular wing with the same properties, so far
start. To do otherwise is a grave disservice to Dear Editor, as moments are concerned, as the real wing
the pupil. The practice must also create a I was intrigued to read the two letters (refs , and 3). This wing will produce curves
serious bad habit among our future instructor (August issue, p177) by young pilots referring much like those of Mr Gibson's Fig 1 provided
population. to the stagnant old fogey element in the glid­ that the lift is assumed to act at the mean
Whoever thought up this aspect of the AEI ing movement. aerodynamic centre, together with a moment
syllabus needs their head examiningl If the lads were upset enough to go into about that point. Tile moment coeffiCient must
HAROLD DALE, Beverley, North Humberside print why didn't they name the clubs involved be referred to an appropriate mean
to spare other youngsters the same misfor­ aerodynamic chord.
tunes? I certainly would like to know where I Let it now be said that there is nothing par­
Bemie Morris, BGA Instructors' Committee am going to be cheated out of my money by ticularly sinful about the concept of the centre
chairman, replies: Mr Dale has identified one geriatric old fools with the "we didn't do it so it of pressure. The reality is Ihat we have a wing
of the AEI compromises which caused con­ can't be done" attitude. with some distribution of pressure acting on its
siderable debate at the Instructors' Committee I learned to fly at 17 and was positively surfaces. The effect of these pressures.
and which is linked to our methods of formal encouraged to the extent that an instructor resolved perpendicular to the mainstream
flying training. The sequence of formal instruc­ loaned me the money for my 50km. Maybe all direction, can be expressed as a lift force act­
tion is - lookout, effect of elevator and speed clubs aren't the same but as usual only those ing through a point on the chord line whose
control, effect of ailerons, aileron drag and with grievances write letters. I think I should location varies with angle of attack or lift coef­
need for rudder. Rudder is not for turning the have got my 16 year two week-old pilot with ficient. This point is the centre of pressure and
glider and then turning. These exercises are 30 solos and two Bronze legs or my 20 year­ it is the paint at which the lift acts. There is no
abbreviated for the AEI who does not cover old assistant Cat with a Gold badge and two object in arguing that "Common sense says
aileron drag, rudder and formal turning at all Diamonds to have written in praise of the way that lift which is generated by pressures on the
but introduces the ailerons which roll the glider the young are treated. surface of a wing cannot be centered so far
and result in it turning. If you don't like the food or service in a res­ away from it ..." Common sense doesn't deal
The trial lesson conducted by the AEI is not taurant surely you don't go back. Reading be­ particularly wet I with concepts like infinity or
intended to be a formal flying lesson, rather an tween the lines of our "Dismayed Student" he infinitesimally small quantities. It is rather like
overall flying experience which may resul! in a has been going back for four years. At least arguing thilt common sense dictates that the
formal training course. The control handling is Amir-Reza Amiri had the right idea. He went ceotre of mass of an object must lie within the
only a small part of the complete experience and flew solo somewhere else. material body of the object. That 01 a horse­
and is intended to persuade the "pupil" that he Clubs with bad attitudes to young pilots will shoe lies In the fresh air. The centre of pres­
could easily learn to fly. The two day course soon realise that the average age of ils mem­ sure may lie between the leading and trailing
for AEls is insufficient to deal with more flying bers is increasing when they have pu1 a stair edges of an aerofoil, but it may also lie beyond
exercises and the trial lesson mostly too short lift up to the winch and they have bingo them and there is nothing outrageous about
for the pupil to have more than a "quick go" evenings in the bar. Eventually nature will take such a state of affairs.
on the controls. The AEI's patter and its course and these clubs will die oul. Instead of having the lift act at some variable
demonstrations have been standardised to the In short, do what you do with everything point on the chord line, it can equally be rep­
formal training exercises as much as possible else in life - complain! If you don't get results, resented as acting al some fixed poinl,
to reduce difficulties on upgrading to assistant write a letter to the club committee naming Ihe together with an appropriate moment about
instructor. individuals who made your life unpleasant. Ask that point. As Mr Gibson explains, and as is
There is no argument that aileron/rudder co­ the club to remember your "stolen flying fees" elaborated above, If that fix.ed point is the
ordination needs to be taught very early on, and go and fly somewhere else. Maybe they aerodynamic centre, then the coefficient of the
and It is, but should it be from the very start? don't realise what they are doing wrong! moment about that point is constant. The con­
Should II precede the effects of the controls MICK BOYDON, CF1, Wrekin GC venience of using this representation is that we ...
December 1990/January 1991 285
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286 SAILPLANE & GLIDING
YOUR LEITERS

have exchanged a variable centre of pressure load necessary for stabiJity?" As John says, include Munk's 1924 theory in NACA TR 191.
location for a fixed moment coefficient, thus no, it is not. He explains why in Fig 5. I sum­ He showed that lift due to angle of attack acts
reducing by one the number of variables to be marise it by pointing out that it usually works at 25% chord and lift due to camber acts at
considered. The use of the aerodynamic centre out that for many conditions, in particular at 50% chord, an elegantly simple model which
is therefore a mathematical and conceptual high speed (lOW lift coefficient) and/or C of G combines to give the standard lift and pitching
convenience but nothing more. forvvard, especially with highly cambered sec­ moment at 25% chord universally used to
The de-bunking of "longitudinal dihedral" as tions, a down load is needed for trim, and this define section characteristics. It firmly anchors
a contributor to longitudinal stability is long may be what has given rise to this particular the lift to the aerofoil in a physically sensible
overdue. How sad, then, that a recent pubJica­ myth! way. Its replacement by a single force at the
tion (ref 4) says " . . . it is the longitudinal Thirdly question 5 asks "does the tail setting CP which may be beyond the moon is
dihedral that influences the production of a angle affect stability?" Again as John says, no, mathematically exact but physically absurd.
favourable restoring moment . . ." as Mr Gib­ it does not - it is chosen purely for trim (Fig 5 Can we please never mention centre of
son so neatly explains, it doesn'!. again relevant), ie as a suitable starting point pressure again?
The reader who has persevered this far may from which the elevator does its work of
like to know that there is an excellent mono­ controlling and manoeuvring. InCidentally it
UNIVERSITY GLIDING
graph, published by OSTIV, devoted entirely was years before I found out what was meant
to the static stability of gliders (ref 5). by longitudinal dihedral, and when I did I could Dear Editor,
never fathom why anyone had bothered to We at Bristol UniverSity are very keen to
References:
invent the term. encourage communication and exchange visits
1. Irving. F. G.: An Introduction to the Longitudinal
Static Stability of Low-Speed Aircraft (Pergamon, CEDRIC VERNON, Guildford, Surrey between university and polytechnic gliding
1966); 2. Abbot, I. H. & von Doenhoff, A. E.: Theory clubs all year round, not just at the annual
of Wing Sections (Dover 1959); 3. Yates, A. H.: Inter-University task week. I have compiled a
"Notes on the Mean AerodynamiC Chord and the John Gibson replies: At the level to which fairly comprehensive list of these clubs which I
Mean AerodynamiC Centre of a Wing" (Royal my article was addressed, it was naturally am happy to send to anyone interested. Also,
Aeronautical Society Journal June 1952, pp461 -4 74); based on the simplest model of Jift and pitch­ if you would like help and information on start­
4. Barnard, R. H. & Philpott, D. A.: Aircraft Flight ing moment. This is still entirely adequate for ing a gliding club at your university or
(Longman, 1989) and 5. Morelli, P. "Static Stability the design of multitudes of aircraft and gliders polytechniC, please don't hesitate to contact
and Control of Sailplanes" (OSTIV, 1976).
without further elaboration. me.
FRANK IRVING, Imperial College, London I am perfectly happy to agree with Frank CHRIS WHITE, captain of Bristol University
that the centre of pressure is a respectable GC, University of Bristol Union, Queens Road,
Dear Editor, concept, useful particularly for determining Bristol BSB 1LN
I was pleased to see John Gibson's article structural loads. In early texts, however, it was
in which he debunks some of the curious always expressed as the centre of the resul­
THE BGA'S LIMITED RESOURCES
legends which somehow grew up on the sub­ tant force of all the pressures around the wing,
ject of longitudinal stability. not as the centre of lift. The latter was found to Dear Editor,
However, I have to disagree with him on one be at the aerodynamic centre, where alt In his article in the August issue, p217, Bill
of his six pOints, and whilst writing I take the changes of lift can be expressed as forces at Scull quite rightly points out that the BGA's
opportunity to comment on a couple of the this fixed pOint and where therefore all lift resources are limited and member clubs fre­
others. effectively acts. It cannot simultaneously act at quently expect too much from them.
Question 1 says "Does wing lift act at the the centre of pressure, which includes the Dare I suggest the solution lies in that part
centre of pressure (CP)?" John says no, but force effectively resulting from pressures caus­ of the article that states: "There are really only
oh yes it does, by definition! He also says "it is ing moments but no lift. two things that matter for gliding, airspace in
the point where a force equal to the lift would Since complete misunderstanding of this which to fly and sites to fly from." If the BGA
have to be applied to generate the observed distinction is the cause of the widespread attempts to do everything that its clubs
pitching momen!." Yes, certainly it is, but that stability mythology, it is essential to picture the demand it must inevitably have fewer resour­
pitching moment can be measured about any Jift and pitching moment separately. The CP ces for these two vital aspects of our sport.
pOint one cares to select, and the term centre causes total confusion. Even though it is Consequently I should like to see the BGA
of pressure simply means the pOint of the mathematically respectable. It is simply too dif­ Executive concentrate resources upon these
chord line (or on some other defined datum ficult to see through it to the reality of static essentials and make it clear to member clubs
line) about which the pitching moment is zero, stability. The answer to the first question must that other requirements can be covered only if
or, as one usually thinks of it, the point remain: "No, lift does not act at the CP". they produce the necessary volunteers to man
through which the lift vector passes. As to where it acts "by definition", I regret them.
"Lift ... must act at a fixed distance from the that in my very compressed article I did not The need to establish priorities for the
measurement reference point." No - the thing BGA's limited resources was foreseen by the
that is fixed is the aerodynamic centre. This is London GC many many years ago when at an
not the centre of lift, it is the centre of the Association AGM it successfully promoted a
change of lift due to change of angle of attack, resolution which Barry Rolfe has recently con­
and it is the pitching moment about this pOint firmed is still extant.
that is constant for a given wing section. This CHARLES ELLIS, liford, Essex
latter is what is called the zero lift pitching
Fluorescent Orange Windcones made
moment; its value is the same for all measure­
in Nylon or Ministry of Defence GLIDING ON THE TELLY
ment reference pOints since for zero Jift it is a
Hypalon"'.
pure torque. Dear Editor,
Flagstaffs suitable for Windcones sup­
Thus, one gets the correct balance by Thanks to Channel 4's showing of "Lifting
plied and fitted.
applying the lift at the aerodynamic centre and The Blues" 1.5m people are now more aware
adding the zero lift pitching moment as a pure Landing Marker Sets in bright orange. of gliding. Admittedly small by TV standards
torque, as correctly shown in Fig 4. All types of Flags and Banners. (in fact about 1 in 12 of the people that were
For stability, of course, it is the above watching TV at that time) but significantly more
PIGGOn BROTHERS & CO. LIMITED
change of lift which counts so that the impor­ Stanford Rivers, Onger. Essex eM5 9PJ than are directly or indirectly involved with the
tant thing is the position of the aerodynamic Tel: [phone] Telex 995457 (Piggot) sport. This can only be to the good of the
centre. Fax [phone] movement, or is it? (See also the last issue,
Secondly, question 4 asks "is a tail down p231.) ..
December 1990/January 1991 287
Make Insurance problems
just plane sailing ...
CONSULT THE AVIATION INSURANCE SPECIALISTS
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AND POWERED AIRCRAFT
Lowndes Lambert House, 53 Eastcheap
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Tel [phone] Telex 8814631
Member of 1l.I.I.IlA
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288 SAILPLANE & GLIDING