A Summary Report on an Experimental Investigation into Methods for Quantifying Hang Glider Airworthiness Parameters

Cranfield

College of Aeronautics Report No. 8705

Mav 1987

A Summary Report on an Experimental Investigation

into Methods for Quantifying Hang Glider Airworthiness Parameters

by

M V Cook

College of Aeronautics

Cranfield Institute of Technology

Cranfield, Bedford MK43 OAL, England

~o1

gratefully acknowledged; the Civil Aviation Authority, the Accident

Investigation Branch of the Board of Trade and the British Hang Gliding

Association.

Of the many individuals who have freely given their time and energy

in support of the work the following have provided invaluable help at

various times; Mr H D Ruben of the Civil Aviation Authority, Mr B Blore

formerly of the British Hang Gliding Association, Mr M Southall of Solar Wings Ltd.,

Mr R Carter of Airwave Gliders Ltd and Mr K Shail of Packaging Control Systems Ltd.

Worthy of special mention is Mr T Prendergast of the British Hang Gliding

Association without whose active participation in the experimental work and

in the maintenance of the test rig much would not have been achieved.

Finally, the investigators are very grateful to the Science and

Engineering Research Council without whose support the programme would

not even have commenced.

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The Research Programme Collaboration

Parallel Developments Results

Publications Conclusions

Recommendations for Further Work References

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1. INTRODUCTION

A three year programme of research has recently been completed in which the principal activity was an experimental investigation of those aerodynamic, stability and control parameters which influence the airworthiness of the hang glider. An experimental test rig was used which enabled aerodynemic measurements to be made on full size hang glider wings. Early experiments were aimed at obtaining quantitative data for a number of wings, whereas later

tests were concerned with flow visualization on the flexible wing surfaces. Broadly the principal objectives of the work were to provide a base of aerodynamic data for a representative selection of hang glider wings to

improve the theoretical understanding in the U.K. of hang glider aerodynamics, stability and control and to assist in developing the test rig as a routine airworthiness test facility.

2. OVERVIEW

The hang glider wing has evolved over the last 25 years or so from the relatively crude Rogallo wing having a lift to drag ratio around three to the current sixth generation wing which has a lift to drag ratio of 10 or more. Not surprisingly this improvement in performance is a result of the

aerodynamic development of the wing from the familiar delta shaped fabric parawing to a more rigid swept wing of high aspect ratio. However, this evolutionary process has not been without its pitfalls and casualties.

As a sport hang gliding provides the opportunity to fly at very low cost which implies that the manufacture and development of hang glider wings does not enjoy the level of financial support more commonly associated with aviation. A direct consequence of this is that much of the development to date has been empirical with very little support in the area of theoretical development. A further consequence of this is that there have been many accidents of which a large percentage have been fatal. It would appear that the hazardous characteristics of the hang glider have been identified from experience and the problems are being overcome by evolutionary development, often based on a very slender understanding of the aerodynamics involved. Also, it would seem that many hang glider pilots are blissfully unaware of the magnitudes of the forces and moments acting on their glider at the limits of

The two most common hazardous characteristics of the hang glider wing are the low incidence pitching behaviour and the variation in control power require-ment with flight condition. At low incidence the deflation of the fabric

envelope can sometimes be accompanied by a large and uncontrollable change in pitching moment resulting in a tumbling motion from which recovery would be unlikely. The aeroelastic behaviour of the fabric wing under load results in significantly non-linear aerodynamic characteristics. Consequently the

aerodynamic pitching moment and hence, static stability margin are variable with flight condition. At high speed low incidence flight conditions the wing can become statically unstable and the aerodynamic pitching moment large. As the pilot only has a limited control power available to him by weight shifting it is possible to enter a high speed dive from which recovery is impossible, this

condition can be further aggravated by fabric profile reversal in some areas of the wing. Fortunately evolutionary developments have reduced the severity

of these characteristics significantly, although they are not completely eliminated the hang glider wing is now much safer to fly. Although the physical manifestation of these non-linear aerodynamic effects is recognised and reasonably well understood, no adequate mathematical model exists from which better wing design and safer operating techniques can be deduced.

Over the years some effort has been made to remedy this situation. Research at various levels into those aerodynamic, stability and control characteristics which have a profound influence on airworthiness has been undertaken, usually by hang glider enthusiasts who have the necessary academic skills. In general the level of activity has not been very great and has frequently been initiated by the manufacturers. Most of this activity has been centred in the USA and Europe with virtually no such activity in the UK. It is therefore perhaps a little surprising to learn that competitive UK hang glider pilots equipped with wings developed in the UK are currently amongst the best in the world.

In an attempt to improve the safety record of the sport the controlling bodies in most countries have established minimum standards of manufacture and airworthiness for hang glider wings. Compliance with these standards usually requires a hang glider wing to be submitted to a series of simple tests using various test rigs developed especially for the purpose. The airworthiness tests usually require the measurement of a number of fundamental aerodynamic parameters which determine stability and control characteristics, interpretation of the results then depends on a number of limiting criteria some of which have been determined empirically and some of which have been determined from theoretical considerations. Largely due to lack of resources the BHGA has been particularly tardy in establishing an airworthiness test

procedure and the procedure that has been recently introduced is based

largely on an empirical foundation. The need for a UK based programme

of research into hang glider airworthiness is therefore well founded.

In 1979 the CAA and AIB jointly commissioned the construction of

a test rig for the purposes of measuring the aerodynamic characteristics

of the hang glider wing. This activity was initiated primarily to investigate

a particular wing which had been involved in a fatal accident, the results

of that investigation are reported in reference 1. It was intended that

the test rig should subsequently be used for research and for routine

airworthiness testing. At this time an interest in hang glider stability

emerged in the College of Aeronautics and after initial contacts had been

made it was agreed that the CoA would conduct a research activity based

on the use of the test rig. Initial work was compounded by numerous

shortcomings in the test rig so progress was very slow; the results of

this initial activity are reported in reference 2. Meanwhile a longer

term research programme was agreed with the interested parties and funding

for a three year programme was obtained from the SERC. This report summarises

the activities and results of that programme.

3. OBJECTIVES

To ensure that the research programme provided the maximum benefit

to those interested the original objectives were refined after discussion

with the organisations involved. The original proposal for the programme

included the principal objectives which were agreed with the CAA, BHGA

and AIB and are re-iterated as follows;

(i) to develop the test rig and data reduction system as a valid source

of aerodynamic and stability data.

(ii) to write computer programs to process the data obtained from the

test-rig.

(ili) to derive equations that describe the hang glider attached to the

rig and to relate these to the glider in free flight.

(iv) to study in detail the measurement of static stability parameters,

in particular longitudinal static stability.

(vi) to carry out experiments to identify simple models of non-linear effects, such as wing camber and twist, and to incorporate these effects into the equation describing static stability.

(vii) to suggest a technique for airworthiness testing of hang gliders using the test rig. In particular, to define stability criteria similar to "stick free" and "stick fixed" criteria as applied to conventional aircraft.

4. THE RESEARCH PROGRAMME 4.1 Progress

Measuring achievements and progress made against the original objectives described in section 3 indicates a reasonable degree of success. The essential basic activities were completed satisfactorily but the difficulties encountered in so doing extended the time required substantially thereby leaving less

time for the later work. Also, the nature of some of the aerodynamic modelling is far more complex than might have been assumed at the outset

implying that an adequate theoretical analysis would occupy considerably

more time than was available in this programme. In retrospect it is considered that the original objectives were optimistic assuming that all could have

been pursued to completion. However, some attention has been given to all of the original objectives and varying degrees of progress have been made in each. Additionally, a small number of additional topics have been addressed which were not included in the original objectives. 4.1.1 Test Rig Development

Initial activities were concerned with the development and calibration of the test rig and digital data reduction equipment, the objective being to establish a reliable accurate facility capable of producing repeatable

results. This activity took what was subsequently to become a disproportionate amount of the total time available to the programme. Many of the difficulties arose as a result of shortcomings in the mechanical design of the rig

and certain limitations in the data acquisition system. Many of the

difficulties could have been avoided had the rig been subjected to a period of testing prior to commissioning. However, this was not possible as

the rig was pressed into service as soon as it was completed in support of the accident investigation referred to in reference 1.

The mechanical design of the rig has shown a number of defects which have delayed the research programme whilst modifications were effected It would appear that the limiting magnitude of some forces and moments applied to the rig super-structure by the wing under test were not fully appreciated as some joints and components failed. Also, many joints in the rig structure have failed, or partially failed under the relatively high levels of vibration experienced in normal running over a typical runway surface. This problem continues to bedevil the operation of the rig from time to time.

The onboard digital data acquisition system was generally found to be quite reliable in operation, however, as a purpose built equipment it lacked the flexibility that would have been a distinct advantage during the development period. The equipment was designed to input, scale and digitise analogue signals from the sensors. The digital data was then subject to calibration laws, preprocessed and stored on cassette tape.

Subsequent to a test the stored data could then be read back into the computer for conversion to standard aerodynamic parameter form, some very limited analysis and printout. This latter facility proved to be so absurdly slow that

an alternative off-line method for data analysis was developed at the commencement of this programme. The onboard equipment was therefore used for data acquisition and monitoring only. In this role the main problem was that the calibration constants were hardwired into the computer on

EPROM's, so every change to the system during development requiring re-calibration involved returning the EPROM's to the equipment manufacturer for re-programming, this procedure introduced significant delays into the earlier part of the programme.

Problems encountered with the sensors and their associated wiring were somewhat more difficult to resolve. The load cells incorporated

into the structure were not optimised for scale range nor were they temperature compensated. The former problem was further proof that the operating

loads on the rig were not fully appreciated at the design stage, it also led to difficulties in cases where the load scale range was far less than that of the load ceTT measuring it. This gave rise to relatively high levels of noise on some resolved aerodynamic parameters.

The airspeed and direction measuring equipment also gave rise to a number of problems. The equipment comprised a combined wind vane and anemometer together with dial indicators and signal processing as supplied for use in sailing boats. The problem arose when it was discovered that indicated wind speed was resolved into the direction of the longitudinal axis thereby giving rise to speed errors when the wind direction was off

the runway centre line and when low levels of turbulence were present. The resolution of this problem was difficult because of the design of the equipment and was costly in terms of the time required for liaison with the manufacturer. Other less serious difficulties arose from time to time due to the ingress of moisture into sensitive electrical components on the rig.

Most of the above rig defficiencies were overcome with varying degrees of success and the test rig was very carefully calibrated using a wind

tunnel test wing. Aerodynamic data for the wing was obtained from the rig andfroma series of wind tunnel tests at similar conditions. By this means a high level of confidence in the rig as a data acquisition facility was achieved. Details of this work are described in reference 3.

4.1.2 Data Reduction

It was recognised some time before the commencement of the present programme that data reduction and analysis on the scale required would have been totally out of the question if it was based entirely on the on-board computer. This was simply due to the slowness of the process and the very large amounts of data obtained during testing.

Consequently an interface unit was designed and built to enable the data to be read from tape and transmitted via an IEEE data bus to

a CBM computer. At the commencement of the research programme this facility was already working but had not been fully developed. Most effort was

concerned with the writing and development of software programs to read, process, analyse and output the aerodynamic data in a suitable format.

Briefly, the work was largely concerned with the revision and further development of the programs described in reference 2. The main processes handled by the software are; to read data from tape, to convert the data from hexadecimal to decimal, to sample the data in such a way as to reduce the noise content and the number of data points for a given test, to apply some further calibration adjustments, to calculate the standard aerodynamic parameters in coefficient form and to output the processed data either in tabular form or as graph plots. The processed data is also stored on floppy disk.

Even with this facility the time taken to process the data from one test was measured in hours, largely due to the speed limitations of the cassette tape equipment. Consequently rig operating procedures were developed to minimise the data record for each test point which had only a small impact on data reduction time.

4.1.3 Test Rig Operation

As a result of the mechanical shortcomings of the test rig the ideal operating technique and conditions were necessarily limited. It was

firstly necesary to ensure stable temperature conditions for the load

cells which was achieved by wrapping them in insulating material and allowing the rig to stand outside for some time prior to carrying out the test.

Also, it was found that good quality results could only be obtained when the prevailing wind had a velocity of five knots or less and was directed along or close to the centre line of the run way in use. At first it was thought that an improved understanding of the way in which the rig operates and the development of a more exacting data reduction process

would permit tests to be conducted in less than ideal conditions. However, in the event this was not to be and, as a result, the number of tests

achieved in the programme although sufficient were not as numerous as had been hoped.

4.1.4 Aerodynamic Tests

During the course of the research programme a comprehensive series of tests were carried out to obtain aerodynamic data for a number of representative modern hang glider wings. In some instances tests series were repeated when discrepancies were apparent as a result of some of

the rig shortcomings already mentioned. In most cases the information obtained comprised lift, dreg and pitching moment characteristics for 'variations in air speed and angle of incidence. No significant tests were carried out to investigate sideslip effects largely due to time constraints. As a result of these tests a data base has been established

and great care was taken to ensure the validity and accuracy of the aerodynamic

Much later in the programme a further series of tests were undertaken to investigate upper and lower surface distortion under locc anc' local

flow characteristics. This was achieved photographically with the wing surfaces fully tufted in order to visualize local flow conditions. The result of this work was a very large number of photographic records of various areas of the upper and lower wing surfaces, again for various combinations of air speed and angle of incidence. The interpretation of this qualitative information proved to be a daunting and time consuming task and the finaings are discussed in detail in reference 7.

4.1.5 Theoretical Studies

Progress in the theoretical studies has not been as great as was originally intended as a result of increased time given to the essential rig development work and the experimental programme. However, some progress has been made in establishing relationships between a theoretical model of the longitudinal static stability of the hang glider ana its measured characteristics. Interpretation of the simple linear stability model used for conventional aircraft enables the hang glider characteristics to be interpreted correctly. However, such a model can not be regarded as an adequate representation ot the hang glider because of its pronounced

aerodynamic non-linearities, particularly in its pitching moment characteristics, Certainly, some progress has oeen made towards the identity of a suitable

non-linear model but the time required to interpret tne experimental data ana lo construct an adequately general empirical model would have been well beyond the scope of the present programme.

During the course of the programme published material has been

acquired on a continuing basis and contact has been made with some researchers in the USA, Europe and in Australia. The interpretation of relevant material in the light of the present programme has indicated that the empirically based criteria currently used in an attempt to ensure that hang gliders have acceptable flying and handling qualities are reasonably well founded. However, the 'foundation laying' of the present programme would appear to offer a sufficiently complete basis from which a reasonably adequate approximate mathematical stability and control model could be built.

4.1.6 Pilot Tests

At an early stage in the work it became obvious that the aerodynamic forces acting on the pilot and control frame of the hang glider constitute a significant proportion of the total and must therefore be included in the stability model. As an additional item of work, a brief series of full scale wind tunnel tests were carried out on various pilots, harnesses and flight equipment. The results of this work were quite thorough in their content and were found to compare very favourably with similar work undertaken elsewhere. This work is reported in reference 4.

4.2 Equipment 4.2.1 The Test Rig

The mobile test rig, shown in figure 1 complete with glider wing, comprises a Citroen CX2400 chassis with a demountable superstructure to which the glider is attached. The superstructure incorporates load cells, which provide the equivalent measuring capacity of a six component wind tunnel balance, and a means for continuously adjusting the glider pitch and yaw attitudes. The rig also includes a digital data acquisition system, shown on figure 2, which reads and preprocesses all sensor output signals prior to their storage on a magnetic cassette tape for future analysis. A complete description of the test rig is included in reference 3. 4.2.2 Data Reduction Facility

A micro-computer, printer, plotter and purpose built tape reader were procured specifically to facilitate further off-line data reduction and analysis. Although slow in operation this equipment proved both reliable and invaluable and is still in use in support of the test rig.

4.2.3 Glider Wings

Although a number of wings were made available and tested the main results were confined to observation of the following three wings since they were most representative of current hang glider types;

(i) a Skyhook Sail wings Ltd "Silhouette". (ii) a Hiway Gliders Ltd "Demon 175".. (iii) an Airwave Gliders Ltd "i^iagic III".

4.2.4 Pilots and Harnesses

The wind tunnel tests of pilots and harnesses were carried out on volunteers and various types of harnesses provided through cooperation with the BHGA. This equipment is described in reference 4.

4.3 Visits

During the course of the development of the test rig a large number of visits were made to equipment manufacturers for the purposes of resolving equipment problems. Clearly these visits were essential but were of little direct benefit to the research. However, with the help of the BHGA a small number of visits were made to hang gliding sites in the UK to observe gliders operating in both a sporting and competitive role. These visits were considered particularly valuable as a means for gaining insight into

the nature of hang glider behaviour in an operational context which was not possible with the test rig. Additionally, a visit was made to West Germany to learn from the experiences of the German researchers. This exercise reinforced an already established and valuable link with the Germans which has been maintained since.

5. COLLABORATION

Varying degrees of collaboration with a number of UK organisations was enjoyed throughout the research programme. Those organisation with whom the collaboration was significant were as follows.

5.1 The British Hang Gliding Association

The support of the BHGA has been invaluable since they have provided active participation in the experimental work and have acted as the interface with the UK hang gliding sport and industry. They also provided the test

rig on loan and were responsible for its routine maintenance. 5.2 The Civil Aviation Authority

A continuing liaison has been maintained with the CAA since they were jointly involved with the AIB in the procurement of the test rig and have a vested interest in all new hang glider developments in the UK.

5.3 The Accident Investigation Branch of the Board of Trade

Since the AIB procured the test rig for the purposes of investigating

the type of gl ider involved in a fatal accident, considerable colloaboration

was enjoyed in the earliest days of the research programme. AIB personnel

were extremely cooperative and were willing to share their experiences

during the process of learning to operate the test rig. Once the data

reduction equipment was working considerable help was given to the AIB

in reducing and interpreting the data which was the subject of the accident

report, reference 1.

5.4 Airwave Gliders Ltd

Since Airwave Gliders Ltd designed and built the test rig they have

maintained a continuous interest in its use and have been helpful in

supporting its development. They also provided an experimental "Magic III"

glider wing for test purposes.

5.5 Packaging Control Systems Ltd

Packaging Control Systems Ltd designed and built the test rig computer

and the off-line interface unit for reading data tapes. During the development

and calibration of the test rig they were consulted frequently and freely

gave advice and assistance in modifying the computing equipment.

5.6 Mainair Sports

Mainair Sports provided on loan a Skyhook Sailwings Ltd "Silhouette"

glider wing which was the subject of an extensive series of tests.

6. PARALLEL DEVELOPMEi\!TS

During the course of the research programme a number of significant

developments have occurred in the hang gliding and light aviation world.

Activities and developments in which some interest was maintained were

as follows.

6.1 Microlight Aircraft

Regulations for microlight aircraft, as distinct from the powered

hang glider, were derived from the CAA regulations for power assisted

6.2 BHGA Airworthiness Committee

Active participation in the BHGA Airworthiness Committee has been, and continues to be maintained. A major activity of this committee in the period of interest has been the production of a workable airworthiness scheme for hang glider operations in the UK, reference 6.

6.3 Structural Test Rig

Recently the BHGA have developed a second test rig for structural testing of hang glider wings by means of aerodynamic loading. Since this rig is also capable of providing some limited aerodynamic information its suitably for this purpose was evaluated and reported in reference 5. 6.4 Hang Glider Tow Launching

Hang glider tow launching techniques have evolved rapidly and have been accompanied by a number of accients. As a result a liaison was formed with Captain J Taggart of the Army Hang Gliding Centre with a view to

developing a sound tow launching technique. Unfortunately the working relationship was of short duration since Captain Taggart was himself killed in a hang gliding accident. Subsequent involvement in this activity was minimal since an "acceptable" technique was developed elsewhere.

6.5 Other Developments

(i) A number of hang glider projects have been undertaken by students in various UK universities and, when requested, information, advice and guidance has been given.

(ii) A tenuous link has been established, via the CAA, with ONERA in France as they have carried out a series of full scale wind tunnel tests on an earlier generation of hang glider wings. Their published work provided a useful basis from which to build.

(iii) A similar link was established with a research worker at Stanford University in the USA who was carrying out a similar programme of research in the same time period.

(iv) By far the most significant parallel activity to the present research was the work being undertaken in West Germany by various research groups. Since they have a similar test rig and their overall objectives were similar so a very useful exchange of information was, and continues

(v) The Hang Gliding Federation of Australia has set up a group based at Newcastle University, New South Wales to establish an Australian Hang Glider Certification Standard. A link has been made with this group and most of our experiences and findings have been conveyed to them since they are also building a test rig for certification and research purposes.

(vi) During the course of the research programme information and advice has been given to the investigators of a number of hang gliding accidents. 7. RESULTS

7.1 Test Rig Development

The test rig was subject to a lengthy and very thorough calibration and development process prior to its use in experiments. The load cells were calibrated by the application of known static loads and the wind

speed and direction measuring equipment was calibrated in a wind tunnel. Final calibration of the rig was undertaken with a rigid test wing having a known and accurate aerodynamic performance. As a result of this exercise the test rig was developed to what is considered to be the highest possible standard subject to the limits of the equipment. Subsequently the rig

proved to be reliable in operation and capable of producing accurate repeatable results to a high level of confidence. Part of this confidence is attributed to the test technique adopted and to the data reduction process which

was developed simultaneously. A full description of the results of this exercise is included in reference 2.

7.2 Aerodynamic Wing Tests

The test rig was used to run several series of aerodynamic tests on three hang glider wings in which the objective was to measure lift

coefficient, drag coefficient and pitching moment coefficient as a function of keel incidence and speed. At each incidence-speed point a measurement data sample was taken over a short time interval whilst the conditions were maintained steady. During the data reduction process excessively large and small values of a single record were discarded and a running average taken of the remaining data in the record. By this means and in conjunction with a careful test technique, good quality results were obtained. A typical example of the longitudinal aerodynamic coefficients

as determined for the "Demon" glider wing are shown on fig. 3, fig. 4 and fig. 5. Even after repeating some of the tests a moderate amount of scatter in the results is evident. However, since this is a function of the equipment used for the tests little further improvement would be expected without substantial rig modification. The results are presented in full in reference 7.

7.3 Pilot Tests

A brief series of wind tunnel tests were carried out on various pilots of different stature with various types of harness. It was found that the only significant aerodynamic force arising from the pilot is his drag. It was found that the drag was largely independent of pilot size, type of harness and wind speed. The largest factor to influence

the drag was the attitude adopted by the pilot. A typical result, expressed as drag area, the product of drag coefficient and pilot reference area, plotted as a function of speed for various harnesses is shown on fig. 6. The drag area shown refers to pilot and control frame, the drag area of the pilot alone being about two thirds of the total value. Typically, a pilot drag area is nearly constant at about 0.2 m^. These results were compared with the results of similar tests carried out in West Germany, France and Switzerland and a good degree of agreement was found. The results of this work are reported in reference 4.

7.4 Flow Visualization Tests

Flow visualization tests were carried out in which an extensive photographic record was obtained of the flow over the tufted upper and lower surfaces of the glider wing for a comprehensive incidence-speed envelope. The thorough, but tedious, analysis of the photographs produced some interesting observations which vindicated the excessive time required for this work. The upper and lower surface flow characteristics were interpretted in the form of a series of diagrams as typified by fig. 7 for the "Demon" glider wing. The shaded regions indicate detached flow and the spanwise line on the under surface indicates the seam where the

upper and lower surface fabric skins are joined, this seam plays a part

in determining the under surface flow patterns as may be observed. Considerable insight was gained from this work since it was possible to relate luffing,

camber changes, wing twist and stall effects to observed flow characteristics. A further significant benefit was that these observed effects could be

related directly to the peculiarities in the measured aerodynamic coefficients. Confidence in this work was enhanced since the observations and their interpretation

correlated very well with observations made in flight by hang glider pilots. The results of this work are reported in detail in reference 7.

7.5 Theoretical Studies

The theoretical studies were largely concerned with stability and control since airworthiness is very dependent on these characteristics. The studies were divided into two parts,

(i) A review of basic stability theory and its application to the hang glider.

(ii) Stability and control calculations based on the theory defined.

A general review of the longitudinal static stability of aircraft reveals that the stability margins determine the magnitudes of control displacement and force to trim. Unfortunately, a similar analysis applied to the hang glider rapidly becomes intractable since the aerodynamic

pitching moment characteristic is very non-linear. An alternative approach, pioneered by the German researchers, assumes quite correctly that control displacement and force characteristics are determined by stability. Learning from the German experience a detailed analysis of the control characteristics was undertaken and the broad requirement for a stable controllable glider was established satisfactorily. The mathematical model developed in this

analysis was limited in scope by justifiable assumptions about likely operating conditions and by the availability of numerical values for the variables

in the various equations. The results of this work produced equations relating the essential aerodynamic and control parameters for trim and notional stability diagrams of the form typified by fig. 8.

Using the data obtained from the wing tests a comprehensive numerical analysis of the stability and control characteristics of the gliders tested

was undertaken. A typical example of control angle to trim characteristic

of the "Demon" glider is shown on fig. 9 and is the practical realisation of the theoretically derived plot shown on fig. 8. In interpretting fig. 9 it must be remembered that not all of the flight attitudes shown could be achieved in normal flight. Results such as this are discussed in some detail in reference 7 in the light of the understanding provided by the

flow visualization work. Since pilot control power is primarily a function of his weight, hang point position and hang strap length, further extensive evaluations were undertaken to evaluate the effects of varying these

parameters on the stability and control characteristics. In general all three do not cause the stability to vary dramatically provided the variation are typical and the flight envelope is "normal". However, all three have a significant effect on stability at negative incidence angles and all three, not surprisingly, have a significant impact on control to trim over the entire flight envelope. In general it was found that practical observation correlated reasonably well with theoretical deduction for normal flight conditions. Outside these conditions practical and theoretical

observations tended to diverge somewhat. However, most observations were satisfactorily explained.

8. PUBLICATIOPJS

(i) Cook, M.V. "Hang glider airworthiness". Article in "Aerogram" Vol. 2, No. 5, College of Aeronautics, Cranfield Institute of Technology, May 1982.

(ii) Cook, M.V. and Kilkenny, E.A. "Research into the aerodynamics, stability and airworthiness of hang gliders". Article in

"Wings!", British Hang Gliding Association, July 1982. (iii) Kilkenny, E.A. "An evaluation of a mobile aerodynamic test

facility for hang glider wings." College of Aeronautics

report 8330, Cranfield Institute of Technology, November 1983. (iv) Kilkenny, E.A. "Full scale wind tunnel tests on hang glider

pilots." College of Aeronautics report 8416, Cranfield Institute of Technology, April 1984.

(v) Kilkenny, E.A. "The aerodynamic characteristics of hang glider

pilots". Article in "Aerogram" Vol. 3, No. 4, College of Aeronautics, Cranfield Institute of Technology, May 1984.

(vi) Kilkenny, E.A. "An assessment of the suitability of the BHGA structural test rig for aerodynamic testing of hang gliders." College of Aeronautics report 8505, Cranfield Institute of Technology, February 1985.

(vii) Kilkenny, E.A. "An experimental study of the longitudinal

aerodynamic and static stability characteristics of hang gliders." College of Aeronautics Ph.D. thesis, Cranfield Institute of

Technology, September 1986.

(viii) Cook, M.V. and Kilkenny, E.A. "An experimental investigation of the aerodynamics of the hang glider." Proc. Int. Conf. on "Aerodynamics at low Reynolds numbers". Royal Aeronautical Society, October 1986.

9. CONCLUSIONS

Considered in the light of the original objectives the principal findings may be itemised.

(i) A full scale aerodynamic test facility for hang glider wings has been established which is capable of producing accurate and repeatable results.

(ii) A simplified theory for the longitudinal static stability and control of the hang glider has been reviewed and evaluated using data obtained

from the experimental programme. The method reviewed was found to be

suitable for general use when the flight conditions applying were "normal". It was also found that a proper assessment must consider the wing with pilot and that both control force and displacement must be evaluated.

(iii) The results of the quantitative aerodynamic tests and the qualitative flow visualization tests were most successful. It was possible to relate observed flow patterns and wing fabric deformations to measured aerodynamic characteristics over a wide incidence-speed envelope. The understanding derived from this work correlated well with reported hang glider behaviour in normal operations. There is no doubt that this aspect of the work

provides a very sound basis on which to build any future research activity. (iv) Drag was found to be the only aerodynamic characteristic of the

hang glider pilot of any consequence. It was also found to be relatively insensitive to pilot size or type of harness. However, pilot weight, hang strap length and hang point position were found to be quite critical in determining stability and control characteristics. It is therefore considered prudent to evaluate every hang glider for a range of these critical "pilot parameters".

(v) Lateral stability and control was not considered due largely to time constraints.

10. RECO^i^!Ep!DATIOMS FOR FURTHER WORK

(i) The test rig load cells should be replaced with temperature compensated load cells with a scale range more appropriate to the task.

(ii) In the interests of greater flexibility and more rapid data reduction, the test rig computer should be replaced with a standard commercial micro-computer and interface unit with, ideally a disk drive for data recovery.

(iii) In any future work a high priority must be to define an adequate mathematical model for the purposes of stability and control investigation. As this depends on quantifying the aerodynamic non-linearities with a

suitable approximation the task will not be easy. However, it is felt

that the present work constitutes a real basis from which such a development could emerge. Not until this is achieved will it be possible to find

a new "break-through" in determining more comprehensive stability criteria. REFERENCES

1. "Report on the accident to Wasp Falcon IV powered hang glider at Wittenham Clumps, near Didcot on 21 May 1978". Aircraft Accident Report 1/83, Department of Trade. Published by Her Majesty's Stationery Office.

2. Sweeting, J. "An Experimental investigation of hang glider stability." College of Aeronautics M.Sc. thesis, Cranfield Institute of Technology, 1981.

3. Kilkenny, E.A. "An evaluation of a mobile aerodynamic test facility for hang glider wings," College of Aeronautics Report 8330,

Cranfield Institute of Technology, November 1983.

4. Kilkenny, E.A. "Full scale wind tunnel tests on hang glider pilots." College of Aeronautics Report 8416, Cranfield Institute of Technology, April 1984.

5. Kilkenny, E.A. "An assessment of the suitability of the BHGA structural test rig for aerodynamic testing of hang gliders." College of

Aeronautics Report 8505, Cranfield Institute of Technology, February 1985. 6. "BHGA Airworthiness scheme for hang gliders". British Hang Gliding

Association, December 1985.

7. Kilkenny E.A. "An experimental study of the longitudinal aerodynamic and static stability characteristics of hang gliders". College of Aeronautics Ph.D. thesis, Cranfield Institute of Technology,

FIG. 1 THE TEST RIG

-20 -10 0 IB 2B KEEL INCIDEMX (DEGREES)

40 50 A 8. e M/S D10. e M/s 012.5 M/S T M . 2 M/S e 15.9 M/S

FIG. 3 VARIATION OF LIFT COEFFICIENT WITH INCIDENCE - DEMON

a

(J

0 10 KER INCIDENCE (DEGREES)

r

0.40 A B. e M/S D 10.8 M/S O 12.5 M/S • 14.2 M/5 O 15.9 M/S -10 0 10 20 KEa INCIDENCE (DEGREES)

40 50 60

FIG. 5

VARIATION OF PITCHING MOMENT COEFFICIENT WITH INCIDENCE - DEMON

8.6 8.5 -8.4 B.3 z 8.2

8

«3 STIRRUP HARNESS I CNO PARACHUTE) O STIRRUP HARNESS 2 CWITH PARACHUTE) » FULLY ENCLOSED HARNESS CUITH PARACHUTE)

% « I o ê m O - I 1 I 1 I 1 I • • • • • • 8 2 4 6 6 18 12 14 16 16 28 22 24 26 26 38 32 34 VELOCITY CM/S)

in region-.A MQ nose up B MQ nose down

6(a) for equilibrium

Stable if

.-da

control angle (6) to trim,

FIG. 8 TYPICAL REQUIREMENT FOR CONTROL ANGLE TO TRIM VS INCIDENCE

-IB

-60 -40 -20 JP 20 40 PILOT'S CONTROL ANGLE.TO TRIM (DEGREES)

60 60 100