A performance test survey of the aerodynamic development of the Slingsby T51 Dart Sailplane

' 6 JUNI 19?3

CRANFIELD REPORT

Aero No. 16

VLlEGTÜ'.GDOUVvKUNDE Klu'/vGrweg 1 - D L L F T

Cranfield Institute

of Technology

A Performance Test Survey of

The Aerodynamic Development of

The Slingsby T51 Dart Sailplane

by

Cranfield Report Aero. No. 16 February, 1973

CRANFIELD INSTITUTE OF TECHNOLOGY

A PERFORMANCE TEST SURVEY OF THE AERODYNAMIC DEVELOPMENT OF THE SLINGSBY T51 DART SAILPLANE

by

H.A. TORODE, BSc.(Eng)., A.C.G.I.

SUMMARY

Comparison is drawn between performance test data gathered from several variants of the Slingsby T51 Dart Sailplane as tested at Cranfield during 1964/5 and also from other contemporary sources. In each case an identical statistical approach has been used in the data analysis. The data shows excellent correllation both between the aerodynamic variants

tested and also between the Cranfield tests and those from elsewhere, and has enabled useful conclusions on the

aerodynamic development of the Dart to be drawn. The success of these comparisons, attributed to the statistical techniques used, would indicate that performance testing may be reaching a state of development where it could be considered as an economic means whereby a manufacturer may monitor the development of his product.

CONTENTS

Summary

1. Introduction 2. The Dart Sailplane

3. Flight Tests

-3.1 Test Conducted at Cranfield 1964/5 3.2 Analysis of Results

4. Interpretation of Results 4.1 Aerodynamic Performance 4.2 Performance Test Comments 5. Conclusions Acknowledgements

References

U

Raw data points

Curve fitting results Data comparison

Wheel down performance Aerodynamic Coefficients Other data

1. Introduction.

During 1964/5 various versions of the Slingsby Dart Sailplane were given performance test evaluations at Cranfield. The work was carried out on a voluntary basis by members of the then College of Aeronautics Gliding Club for the Dart's Manufacturers, Slingsby Sailplanes Limited of Kirby Moorside, Yorks, in order to speed the development of the Sailplane before the World Gliding Championships of 1965, held at South Cerney, Gloucs.

Although the work was competently carried out, it was necessarily done with some haste and it has not been until recently that a full analysis of the data points has been carried out. This memo deals with the results of those tests and with their comparison with contemporary results obtained from other workers who have had the opportunity to flight test Dart Sailplanes.

2. The Dart Sailplane.

The Dart Sailplane was initially designed by Messrs. Slingsby, Reussner and Slater as a Standard Class high performance single seat sailplane of world class competition standard. The prototype (now designated Dart 15) first flew on 26th Noven^er, 1963 (Ref. 1) It was of wooden construction with a single tapered wing using an NACA 64„-620

section with an aspect ratio of 18.1 giving, at an all up weight ^

of 730 lb, a moderately high wing loading (for that time) of 5.6 lb/ft . Also the sailplane proved to have a high speed performance at least

comparable with the larger machines of the day and met with considerable success in its early competitions. However some doubts were raised as to its ability to climb in weak conditions^and with the World Championship about to be held in this country, where the weather is notoriously fickle, a development programme was embarked upon.

In order to produce a competitive sailplane for the open (unlimited) class the design was 'stretched', i.e. a total of two metres was added to its span bringing bonuses both in reducing wing loading and improving

induced drag characteristics. This modification was necessarily introduced at the design stage (first flight November, 1964) and versions were produced both as fixed 17 metre sailplanes or a 15/17 version featuring detachable tip sections permitting the owner to retain the option of using the aircraft as a 15 metre standard class sailplane. This version, although used at the World Championships, was not commercially sucessful since future rulings

caused the terms of reference of the Open and Standard Class to diverge further.

The next development was the introduction of a Redux bonded metal spar in order to: a, save structural weight, and b, to increase wing bending

stiffness with airbrakes open, which was proving to be the critical structural strength case. At the same time a further modification, involving the

addition of some wing area close to the fuselage and reduction of wing

incidence, and a subsequent alteration to the junction filleting was carried out in an attempt to improve the junction flow at high lift coefficients.

Finally, the option of a retractable undercarriage was added before the aircraft entered series production. This modification was available both on the open class sailplane (Dart 17R) and also on the

15 metre version (Dart 15R),in anticipation of a ruling allowing

retractable wheels in the standard class. No Dart 15/17's were produced with retractable undercarriages.

Although the Dart continued to be modified and in particular for the 1968 World Championships, no further flight performance tests are available. For completeness these modifications included a modified (Wortman) leading

edge to the wing section; the introduction of a fully faired nose section and transfer of the pitot source to the fin. These aircraft (two built) were designated Dart 15W and were subsequently converted to 17 metres as Dart 17WR. Finally some late production Darts had tailplanes fabricated from Aluminium Alloy.

The Dart is still considered to-day as an advanced high performance sailplane but is understandably unable to hold its own with the current generation of large flapped glass fibre sailplanes.

3. Flight Tests.

3.1 Tests Conducted at Cranfield (1964/5).

In all,three versions of the Dart were tested at Cranfield.

(a) A prototype, probably the second example in the basic condition. (b) A Dart 15/17 constructed for the 1965 World Championships.

This aircraft was tested in the 17 metre configuration. It featured no other modification.

(c) A production Dart 17R loaned by D.D. Carrow. This aircraft feature all production modifications including metal spar, wing root mods, (with reduced incidence) and retractable undercarriage.

For basic performance test techniques the reader is referred to refs. 2 and 3;suffice here to say that all tests were carried out by the classical partial glide technique, flights being made in early morning in order to establish as smooth conditions as possible. Data acquisition

was carried out photographically from calibrated A.S.I, and altimeter. Glides were typically of the order of two to four minutes long. This data considered

with the relevant calibrations, (including a tenq>erature ascent) was fed in a computer data reduction routine where instrument calibrations

(including P.E.) were applied and the data reduced to equivalent speeds and sink rates at sea level in a standard atmosphere. Data gathering was concentrated in the high lift, low speed regime since necessarily this was the area of principle interest in the development of the sailplane. Sufficient points were gained in this region to establish clearly the departure of the sailplane from normal first order behaviour as it enters the stalled condition. This work has been supplemented by in flight photography of the junction flows.

It was this state that the t m l s were left in 1965 (ref. 5 ) . Reporting on them herein is necessarily brief since the present author was not

personally involved. However he would not wish to diminish this amount of effort given on a voluntary basis by the individuals concerned, and is well aware of the effort required to establish a test and data reduction routine and to test three separate aircraft.

3

-3.2 Analysis of Results

Following the establishment of the MOD (PE) sailplane research contract at Cranfield various statistical analysis tools have become available and have been applied to the Dart tests. Also Dr. G.R. Whitfield of Reading University was contacted and agreed to allowing his highly exhaustive Dart 15R performance test (ref.4) to be used for comparison with the existing Cranfield data (fig. 1 - 4 ) .

The results from all four sailplane tests were analysed in an identical manner using the classical least squares

routines:-— r x ^ illustrated Q

and also the same curve fit derived for the raw sink rate speed equation

V - AV^ + B ^^^^ A ^S ^ illustrated A where; A » § . C^^^ (HI) ITW 2pS/R . K

Where C is the zero lift profile drag coefficient and K is the overall lift dependant drag factor.

These constants are analogous to, but distinct from sectional

profile drag coefficients and the induced drag factor K. In particular it should be remembered that K, the lift dependant drag factor contains not only a contribution due to induced drag but also contribution from such lift dependant drags wing body junction, interference drag and lift dependant wing sectional drags.

The curve fit equation II is prefered for the reasons set out by Whitfield in ref. 4. Additionally these curves are supported by local 5 knot mean sink rate point as arfe shown in fig. 5 - 8 .

4. Interpretation of Results, 4.1 Aerodynamic Performance.

The four basic performance test results are plotted together as figure 9 and a resume of the aerodynamic coefficient for each sailplane are

given in table 1. Notable features are the clear superiority of the retractable wheel versions over the fixed wheel Darts at high speed, and secondly the superiority of the 17 metre Darts over their 15 metre

counterparts at high lift. This is predictable from basic theory but before detailed comparison can be made the effects of the various wing loadings must be considered. Figure 10 shows the speed polars corrected to a standard wing loading of 5.22 lb/ft 2, the test loading of the 17 metre sailplane. The minimum speed of all variant has levelled due to this

process at 39 knots ( C L max • 1.013). From ref. 7 this would appear to be a'drag rise' limit rather than a stall limit. These curves show the

comparative 'aerodynamic' performance of this group of like sailplanes, and now relate directly to the later graphs showing aerodynamic drag co-efficients. It is worthwhile to note that, in general terms the performance of the Dart has been improved by a total of some 12Z

throughout the speed range and in particular the best lift drag ratio has been improved by 31 to 35.5.

Dealing firstly with profile drags, the zero lift profile drag co-efficients for the four sailplanes are plotted as fig. 11 and may be relied on for reasonable accuracy from performance tests. The main contribution to improving high speed (profile drag) performance would appear to be the addition of the retractable wheel giving a reduction of some 0.001 (or 102 of total) in profile drag co-efficient. However since the sailplanes with retractable wheels also both had the root modification no conclusions can be drawn directly as to how much of this

improvement is due to that modification, but since this modification

was embodied to improve drag at high lift the contribution to profile drag should be small. It would be enlightening to test a late series production Dart 15 which has the root modification but a fixed wheel in order to check this hypothesis.

Whitfield also tested his Dart 15R with the large retractable wheel extended and these tests (fig. 11 & 13) that this contributes some 0.003

to the profile drag co-efficient; three times the value for the semi buried fully faired wheels of the fixed wheel Darts.

A comparison of lift dependant drag characteristics is shown in

fig. 12, however, these results must be considered with some caution due to the poor accuracy of determination of the lift dependant drag factor K from tests of this kind. This is unfortunate since the overall parameter should contain contributions from such increments as trim drag, function interference drag and lift and speed (i.e. Reynolds Number) dependant forms of both fuselage and wing profile drags, many of which are drag increments which may be easily altered during a development programme. Particularly the Dart 15 result seems unusually optimistic. Further comparison with the profile drag co-efficient shows this to be somewhat high, indicating some interaction within the curve fit, which is quite possible with the limited range and number of data points and particularly the lack of high speed information in this test. Zachers results (fig. 14) also serve to support this suspicion. This accpeted the comparison serves to show that the root modification has improved the lift dependant drag characteristics by reducing K by approximately 0.1 for the 15 metre version or 0.2 for the 17 metre version. The discrepancy in these results remains unexplained but is probably within the limits of accuracy of measurement. Tha situation is, however, supported by popular opinion which holds that the Dart 17R is significantly superior to all other variants.

Theoretical estimates render a planform induced drag factor of t-;056 c^t. Ft'j-i:?. to which must be added a contribution of 0.175 for the lift dependant

nature of the wing sectional profile drag (ref. 7 ) . These figures total 1.23 which in comparison with fig. 13 shows that the Dart series would appear to be free from any serious deficiencies in lift dependant drag due to such phenomena as junction drag. Reference 7 also indicates a

5

-zero lift profile drag of some 0.006 for the Dart section, i.e. some 50Z of total, which would seem an unusually low contribution to total for a high performance sailplane. Generally sectional profile drag contributes some 70 - 80Z of total. This indicates the existance of some further increment of drag present throughout the speed range. It is probable that either (a) the aerofoil is not achieving stated results through waviness or roughness, or (b) the fuselage is an aerodyanamically poor design. The second reason is considered most

likely and indeed an attempt was made to improve the fuselage design in 1968,but in that case qualitative opinion was not encouraging. However the potential would appear to be there should it be wished to be exploited» but at this stage in the design life this is only

likely to be undertaken by a private enthusiast.

4.2 Performance test comments

The exercise has raised some useful aspect of the basic philosophy of performance testing and its application to development programmes. Principly it is worthy of note that in this cave, five performance

tests have been taken from three different sources, and the results have been mutually in agreement to a sufficient extent to be able to draw useful aerodynamic conclusions.

Also, in several cases herein, tests using a minimal number of experimental data have rendered meaningful results. Both these points are considered to be the result of the use of statistical analysis of performance test results and future development in this area could further reduce the practical drudgery of performance testing.

This situation, which certainly would not have been predicted some few years ago, seems most encouragipg and it may be hoped that in the not to distant future such progranmes as this one may become

standard practice in the development of new designs.

With reference to the statistical analysis of these results,two oth«robservations have been made. Firstly, that, during the analysis of a set of results, particular a numerically small set, one is not able to separate entirely an interaction between lift dependant and zero lift profile drags. Obviously any curve fitting process will have maximum accuracy in the mid C. ranges due to maximum concentraction of points. However maximum measurement accuracy is achieved at low lift co-efficient where profile drag is dominant. It may well, in future be worthwhile

to define profile drag accurately using several very high speed runs and having established the zero lift profile drag co-efficient using a first order correction,use the remaining data to establish the lift dependant drag. Using the present system, refined as it may be by using the correctly weighted curve fit (II), may bring about minor changes in curve fit slope

(K) causing large losses of accuracy in the zero lift co-efficient intercept

(CDO).

Secondly there is some evidence both from this data and other data collected at Cranfield that curve fit (II) (the raw speed data fit) may differ systematically from the Cj) - C L fit, the result being higher

values of lift dependant drag (and lower values of profile drag) resulting from the raw speed fit. This is not yet statistically proven and no

5. Conclusions.

The performances of four variants of the Slingsby T51 Dart have been established by the statistical data reduction of flight test data from various sources. The data has been found to hold a higher degree of

consistancy than may have been expected from this kind of flight performance tests enabling conclusions to be drawn as to the advantages of the separate modifications embodied during the sailplanes development.

ACKNOWLEDGMENTS

This note constitutes the coiq>arison of much practical data collected over a period of years by, in many cases, voluntary effort. The author has, with only small exception, taken no part in this often

laborious work andgratefully acknowledges the considerable assistance of the following parties.

Mr. T. Coldwell. Mr. J. Blackmore. Mr. D. McQue. Slingsby Sailplanes Ltd. Mr. D.D. Carrow. Dr. G.R. Whitfield.

Organisation of Cranfield Flight Tests. Piloting and Assistance at Cranfield Tests Data Logging of Cranfield Tests.

Co-operation and loan of aircraft in the interest of development by performance testing.

Loan of Dart 17R for Flight Tests. Contribution of Dart 15R results.

1. Irving F.G. 2. Machin K.E. 3. Torode H.A. 4. Whitfield G.R. 5. Carrow D.D. 6. Zacher H. 7. Abbott I. & VQn Doenhoff A.

Flying the Slingsby Dart. Flight International 14.7.64.

Performance testing of the Slingsby Sky.

Journal Royal Aero. Society Vol. 58 July, 1954, Perfomance testing of Sailplane.

College of Aeronautics Report

Automatic Recording and Analysis of Glide Performance Testing.

12th OSTIV Congress Alpine U.S.A. 1970. Aero Revue 4/72.

Guinea-pig performance t r i a l s .

Sailplane & Gliding P86 Vol. 17, 1966,

10th OSTIV Congress S. Cerney 1965. Aero Revue 2/68.

lO 2 0 3 0 4 0 SO 6 0 7 0 8 0 90 IOC IIO KT 3 •Us . 4 KT 6 0 0 > 1 \ 0 > ^

CRANFIELD TESTED POINTS FOR DART 15 PROTOTYPE FLYING WEIGHT 730lb

— . - _ . .

EAS 1

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FIGURE 1 CRANFIELD TESTED POINTS FOR DART 15 PROTOTYPE FLYING WEIGHT 730 lb 10 20 3 0 4 0 SO 6 0 7 0 6 0 9 0 100 KT 4 V. 5 EDS 6 KT 7 • < S • • 0 1 o e • 1 • P • t 5 1 • 1 1

CRANFIELD TESTS ON DART 17 PROTOTYPE i FLYING WEIGHT 780lb

EAS

FIGURE 2 CRANFIELD TESTS ON DART 17 PROTOTYPE FLYING WEIGHT 780 lb

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DART I5R FLYING WEIGHT 780lb

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Printed by courtesy of the author, N. Ellison, "British Gliders and

Sailplanes", published by A and C Black.

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ERRATA

Page 2, l a s t para, line 1: for ' t r a i l s ' read ' t r i a l s '

Page 4, para 4, line 5: for 'function' read'junction'

Page 4, para 5, line 1: induced drag factor of 1.046

c.f. Fig.13.

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