Cranfield
College of Aeronautics Report No. 8505February, 1985
'LFT
An Assessment of the Suitability of the BHGA Structural Test Rig for Aerodynamic
Testing of Hang Gliders
by E.A. Kilkenny
College of Aeronautics Cranfield Institute of Technology Cranfield, Bedford MK43 OAL, England
Cranfield
College of Aeronautics Report No. 8505February, 1985
An Assessment of the Suitability of the BHGA Structural Test Rig for Aerodynamic
Testing of Hang Gliders
by E.A. Kilkenny
College of Aeronautics Cranfleld Institute of Technology Cranfield, Bedford MK43 OAL, England
ISBN 0947767 18 5
£7.50
i
The views expressed herein are those of the authors alone and do not necessarily represent those of the Institute. "
Summary
In response to a proposal by the BHGA to use their structural test rig to carry out aerodynamic testing of hang gliders, the existing structural test facility and the modifications already made for aerodynamic testing are described. Improvements to the instrumentation are discussed together with an assessment of the flow quality and recommendations made, based on
Summary Contents
List of Figures
1. Background 1 2. The Structural Test Rig 2
3. The Requirement for an Aerodynamic Test Vehicle 3 4. Stability Requirements and the Resulting Tests Proposed
by the BHGA 4 5. Modifications to the Structural Test Rig Needed to Carry
Out the DIP Test 6 6. Improvements to be made to the Measuring System and Possible
Extension of Stability Tests 7 7. Aerodynamic Considerations 8 8. Other Factors Needing Consideration 10
9. Conclusions H References 12 Appendix A - Hang Glider Terminology
Appendix B - BHGA Structural Load Test Requirements Appendix C - BHGA Proposed Pitch Test Requirements Figures
List of Figures
la,b The BHGA Structural Test Rig 2. The Force Balance
3. The British Aerodynamic Test Facility
4. Satisfactory Variation of Pitching Moment Coefficient with Incidence
5. Modifications to the Structural Test Rig for Aerodynamic Testing
1. Background
The British Hang Gliding Association's (BHGA) structural test rig was built in 1983 in order that structural load testing of hang gliders could be carried out by dynamic loading of the sail, instead of the previous technique of static loading with sandbags. The aeroelastic nature of these wings makes this a much more realistic and accurate method and it has been used on the continent since 1975.
The present structural load test requirements for hang gliders are given in Appendix B. For a typical glider, this would involve measuring lift forces in the range -300 kg to +600 kg. Because of the large forces involved, and the nature of these tests, accurate aerodynamic loading took second place to simple operation and cost of the facility. Thus the present system, shown in Figure 1, was conceived.
2
-2. The Structural Test Rig
The test rig is mounted on a trailer which can be towed by most cars. At the top of the support structure is a force balance, shown in more detail in Figure 2, which comprises three vertical load cells, one at each corner of the triangular framework. Since the lift is the only force of interest during the load tests, horizontal forces are not measured and are transmitted directly through the system (together with possibly a very small fraction of the vertical load).
The wing is attached to the balance at the top of the control frame or hang point, about which it is free to pivot. A single strut from the front of the balance frame attaches to each corner of the glider control bar via twin extendable rods. The effective incidence of the glider is determined by the length of these rods. In the past, the keel of the glider has been set horizontal and then the wing, balance and support structure pitched around the base of the structure to the required incidence. This resulted in the force acting perpendicular to the keel, rather than the lift, being measured. This is not the correct information and it is anticipated that lift will be measured in future tests.
The output from the load cells is displayed on the meters shown in Figure lb. These can be carried in the towing car during the trials so that the ultimate loads applied to the wing can easily be seen.
The initial success achieved testing hang gliders resulted in the facility being strengthened for the load testing of microlight wings. However, a
powerful towing vehicle and long runway are needed if the required loading is to be achieved.
3. The Requirement for an Aerodynamic Test Vehicle
In addition to the structural testing of hang gliders, most countries now require aerodynamic testing to ensure satisfactory flight stability characteristics. The aeroelastic properties of hang glider wings prevent the use of scale models and full scale windtunnel tests are prohibitively expensive. Instead, many countries are now using purpose built test vehicles and these have proved to be a satisfactory alternative. The British facility is shown in Figure 3.
The main objective of these facilities is accurate measurement and interpretation of the hang glider's aerodynamic characteristics. However, compared with the BHGA strucutural test rig they are expensive, complex and time consuming to operate.
Experience from previous aerodynamic testing of hang gliders has highlighted the major problem area with these wings - poor longitudinal stability at low
incidences and high flight speeds. Full testing over the whole flight regime is a lengthy process and as of this date, many gliders are still to be tested. As an interim measure, the BHGA examined the possibility of using the structural test rig to carry out a short and simple check on the-stability in this
. 4
-4. Stability Requirements and the Resulting Tests Proposed by the BHGA
Satisfactory static longitduinal stability requires the pitching moment about the centre of gravity (of the wing and pilot) to decrease as the
incidence of the wing increases. As with lift and drag, the pitching moment increases with airspeed. It is usual therefore to refer to pitching moment coefficient which, for a rigid wing, depends only on the incidence and not on the flight speed.
Pitching moment coefficient is defined as
C^ = M/JpV^Sc where
m
M = P = V = S = c =pitching moment coefficient
pitching moment (Nm) air density (kg/m^) air speed (m/s) wing area (m^) mean wing chord (m)
A satisfactory variation of pitching moment coefficient (taken about the centre of gravity) with incidence is shown in Figure 4. In the case of a rigid wing, this would be independent of airspeed. However, the flexible nature of
hang glider wings results in their aerodynamic coefficients varying with flight speed. In particular, a number of hang gliders have a tendency at low incidences for the pitching moment coefficient to decrease with airspeed and become negative. This results in a \jery unstable glider. There have been a number of occasions where pilots have been unable to recover from steep dives because of this dangerous feature.
Hang gliders are controlled by weight shift and it is often impossible to measure directly the pitching moment about the various centre of gravity positions of the wing and pilot. It is more usual, and easier, to measure the lift, drag and pitching moment of the wing about the top of the control frame, and combine these with relevant pilot force and moment characteristics to calculate the pitching moment about the centre of gravity. However, an indication of the trend of the variation of pitching moment about the centre of gravity can be obtained by analysing the pitching moment of the wing about the top of the control frame.
In view of this, the BHGA proposed to measure the pitching moment of the wing about the top of the control frame, in the low incidence region, and check its variation as the airspeed increases. If, with the keel at zero incidence, at any speed below a specified maximum dive speed, the
pitching moment should become negative, the gliderwould fail the test. This would be known as the Dive Incidence Pitch (DIP) test,
Although this test is not sufficient to guarantee dive recovery, it will highlight gliders with serious problems in this flight regime. The intention is for this to be a quick, simple check rather than a rigorous and complicated test. Hopefully gliders with poor characteristics could be modified on site and retested the same day rather than wait weeks for the results as in the case with the aerodynamic test facility.
6
-5. Modifications to the Structural Test Rig Needed to Carry Out the DIP Test
The modifications made to the structural test rig in order to measure the pitching moment of the glider about the top of the control frame are shown in Figure 5.
The glider is attached to the original balance at the top of the control frame (or hang point) and is free to pivot about this point. However, the bottom bar fixings, which used to hold the glider at a specific incidence, have been replaced. The bottom bar fixes under point A and the pulley system allows alterations of the incidence of the wing. A spring balance is fitted adjacent to this which measures the horizontal force holding the bottom bar.
In order to measure the pitching moment about the top of the control frame, the force being measured should be perpendicular to the control frame. Thus the system is only valid when the control frame is within a few degrees of the vertical. However, this is usually the case when the wing is at zero incidence.
The system in operation is shown in Figure 6. It is necessary for at least two people to ride on the test rig platform; one to read the spring balance and a second to hold the anemometer, for which there is at present no fitting, and read the airspeed.
6. Improvements to be Made to the Measuring System and Possible Extension of Stability Tests
The BHGA acknowledge that the present system is inadequate and have proposed several modifications. The first is to replace the spring balance with a load cell whichwill measure the force acting perpendicular to the
control frame. To do this, it may be necessary to redesign the measuring system. In this event, additional load cells could be incorporated into the balance to enable drag and sideforce to be measured. Provision is also being made for the fitting of an anemometer and possibly a yaw vane.
Having improved the instrumentation of the test rig, the BHGA are keen to make full use of it. Additional stability tests have been proposed over amuchlarger flight envelope and these are shown in Appendix C.
However, although this test vehicle was adequate for measuring the
large loads applied during structural tests, stability tests involve measuring changes in relatively small aerodynamic forces and moments. Consideration must be given to accurate reproduction of the wing's aerodynamic environment.
8
-7. Aerodynamic Considerations
Correct airflow over the wing is important if its aerodynamic
characteristics are to be correctly reproduced. This requires the hang glider to be situated in a position such that the towing car, the support structure and the ground do not significantly affect the airflow around it.
The influence of the ground on the flow around a wing is determined by the height of the wing above the ground and by its geometry. It is generally accepted that a wing will be in ground effect if its ground clearance is less than its span (see Reference 1 ) . Typically, hang glider wing spans exceed 30 ft whilst the height of the wing above the ground on the structural test vehicle is approximately 12 ft. Ground effect could therefore be significant. This will result in the wing being at an effecitvely higher incidence, with
a resulting increase in lift coefficient and change in pitching moment coefficient, combined with a reduction in the induced drag coefficient.
The airflow around the wing will also be influenced by the wake of the towing vehicle. The magnitude of this effect will be determined by the type of vehicle used and its forward speed. The wake will create a low pressure region behind the car which will alter the effective incidence of the inboard portion of the wing, with resulting changes in lift, drag and pitching moment coefficients.
It is very difficult to predict the magnitude of this effect but a suitable choice of towing vehicle should minimise it considerably.
Significantly more important are the inteference effects from the support structure. The base platform will alter the effective incidence of the inboard part of the wing. The main support member (and the people riding on the
platform) will affect the flow underneath this region of the wing, espacially near the trailing edge. In particular, the aerodynamic behaviour of stability devices near the inboard trailing edge of the wing; such as the luff lines, could be incorrectly assessed.
Considerable improvements are possible however.
1. The height of the wing above the base structure could be increased. This would reduce the magnitude of all these effects.
2. A streamlined fairing could be fitted to the support structure to minimise the flow disturbances underneath the wing.
3. If the fitting of the load cells and anemometer, with suitable instrumentation, goes ahead then there will no longer be a need for people to ride on the platform.
Aerodynamic effects should also be taken into account when fitting the anemometer. Unless an attachment is made to fit on to the wing being tested, it may be necessary to mount the anemometer on a long boom to keep it out of the disturbed air flow.
In order to assess the flow quality and accuracy of the measurements, it has been suggested that the BHGA carry out tests on a Demon 175 wing already tested on the aerodynamic test facility. As well as a full range of aerodynamic data being available, flow visualisation using wool tufts has been carried out on this glider and photographs are available of both upper and lower surface flow patterns.
Atmosphere conditions during the trials are also important. The wind should be as calm as possible and certainly gusty conditions should be avoided. The fitting of a yaw vane with the anemometer should enable the tests to be carried out in minimal crosswind flows, a factor found to be important on the aerodynamic facility.
10
-8. Other Factors Needing Consideration
Similar vehicles, for aerodynamic and structural testing, have been used, both in Europe and North America, for many years. It is therefore advantageous to consider results from experimental programmes carried out using these facilities when reviewing current test procedures.
One particular problem has become apparent whenever a single facility is used for both aerodynamic and structural tests. During structural tests it is not unknown for hang gliders to break up, with little prior warning, in an almost explosive manner (see Reference 2 ) . This shock loading together with the proximity of the flapping wreckage has resulted in extensive damage to aerodynamic load measuring equipment. It is clear that there is a
conflicting requirement for a very robust structural test rig and an accurate and highly sensitive aerodynamic facility. Before modifying the existing balance, consideration should be given to using a separate balance for aerodynamic tests.
An international standard of testing techniques has yet to be agreed on. The American regulations still maintain the measurement of pitching moment coefficient about the top of the control frame as adequate. They
require it to be above specified minima and this is the. basis of the additional tests proposed by the BHGA. However, gliders meeting these standards have not always passed other European tests (see References 3 and 4 ) .
The leading European countries now measure lift, drag and pitching moment of the wing about a convenient point and relate these to the combined wing and pilot centre of gravity, as is done with results obtained on the British aerodynamic facility. This necessitates lengthy calculations after
the tests are complete and is the main reason for the quick, interim measurements originally proposed by the BHGA. Perhaps the more detailed stability
tests should be left to the established procedure of the aerodynamic facility, and the structural test rig used only to carry out the quick DIP test as
9. Conclusions
1. The BHGA structural test rig is adequate for structural testing of hang glider and microlight wings.
2. Considerable improvements to the facility's instrumentation are required if any realistic aerodynamic tests are to be carried out. Principally, these should include a pitching moment load cell together with an
accurate and correctly located anemometer.
3. The flow around the wing is influenced by the proximity of the ground, the towing vehicle, and the support structure. The extent of the problem is difficult to estimate, but could be determined by tests on a Demon 175 wing already tested on the aerodynamic facility. If the effects are found to be significant, improvements could be made by modifications to the facility.
4. The Dive Incidence Pitch Test is a useful procedure for investigating a potentially unstable region of the flight envelope. Such a test
should highlight gliders with major instabilities, but would not guarantee dive recovery.
5. The additional pitch tests proposed by the BHGA, based on American guidelines, are not an alternative to testing on the aerodynamic facility. Gliders meeting these tests in the past have not always passed the more rigorous European requirements. Tests carried out on the British Aerodynamic facility are based on similar standards as the European requirements.
12
-References
1. Dole, Charles E.
Flight Theory and Aerodynamics: A Practical Guide for Operational Safety John Wiley and Sons, Inc. 1981
2. Schonherr, Prof. Michael
The Way to the Safe Hang Glider - The Development of Measuring Vehicles 'Drachenflieger' Magazine 1983-1984 (translated from German)
3. Schonherr, Prof. Michael
The Need for a 3-Component Measuring Test Vehicle. 'Drachenflieger' Magazine 1984 (translated from German)
4. Bundi, Arno and Oprecht, Ulrich.
Fundamental Considerations for Experiments with Hang Gliders. Verein Delta Test
Technisch-wissenschaftliche Mitteilungen, January 1984 (translated from German)
keet crossboom rigging wires hang strap tip rod control frame • /
control bar/bottom bar parachute
Appendix .A
Appendix B BHGA Structural Load Test Requirements
STRUCTURAL LOAB T!!STS
.1 The manoeuvering factor, n, is the number by which the load experienced in steady trimmed rii/rht in still air (n = 1) must
be multiplied to represent the maximum load to be considered for the flight condition specified.
Por hang gliders in the utility Cateffory, appropriate for hill and thermal soaring, the relevant values
are:-Maximum positive ri = iA Maximum negative n = '2 .2 Factor of Safety Fs
The factor of safety must not be less than Ps = 1.5
An increased factor may be necessary to cover any uncertainty in the applied load or the strength of the material or to cover possible loss of strength through service and wear.
. ;• Test Rig
Thf? structural load test mus' be carried out on the '^HGA Structural Test Rig. STR.
.A Test hang glider selection and trim See section 3 paras 1 it 2
.5 Crossboom Tension
If a means is provided i.e. by "musclebox" to change the cross boom tension in flight, the tests must be carried out at both the minimum and maximum limits of operation of the control.
K Ultimaic- test load *see note.
The ultimate test load to be set on the 3TR is iriven by:-Ultimate test load, Kg = Fs x n x pilot weight' Ultimate positive load = 6 x pilot weictït-' Ultimate negative load = 3 x pilot weight*
"•• NOTE the load measurement system of the STR is such that the weight of,the hang glider under test is added to the test load.
. • Positive load test
. ST;i load frame base set to +50^'
. Hang Glider keel set parallel to load frame base
. Required load setting - 6 x pilot weight (see 4 above) . Maximum load achieved is held on meter 1
. Load is applied by increasing the rig speed until: : failure
: or the required test load is achieved : or the rig speed reaches 65 mph airspeed ,8 Negative load test
. STR load frame base set to -10
. 'r\r\,ng Glider keel set parallel to load frame base
. Load is applied by increasing the rig speed until failure or the required test load is achieved.
.1 Tnilslide test
. STR load frame sot horizontal
. Han/p; Glider set up tail first with keel parallel to load frame base.
. Load is applied by slowly increasin^r rig speed to 25mph airspeed. .10 Pass criteria
The hang Glider must complete all three above tests without permanent deformation.
APPENDIX C BHGA Proposed Pitch Test Requirements
5 PITCH T3STS
.1 Test Rig
The pitch tests will normally be carried out on the adapted BHGA
Structural Test Rig (A) STR, but the BHGA Aerodynamic Test Rig ATR
may also be used at BHGA discretion.
.2 Test hang glider selection and trim
See section 5 paras 1 4 2
.3 Cross Boom Tension
See section 4 para 5
.4 Dive Incidence Pitch Test
. Mount the hang glider by its hang point with the keel horizontal
NOTE: Due to the keel pocket, the sail will be at a negative
incidence of -5 to 10 .
. Gradually increase the speed of the test rig to 50 mph airspeed
. The control bar force must not at any point become negative
i.e. tend to pitch nose down
.5 Pitching Moment Curves
. Mount the hang glider by its hang point
. Run the rig at a constant 20 mph airspeed
. Measure the control bar force at various angles of incidence
(relative to the keel) from zero up to stall incidence.
. Run the rig at a second constant speed of 50 mph or more, measuring
the control bar force at various angles of incidence frota zero to 30
. For each speed plot a curve of coefficient of pitching moment vs
Incidence,
. The curves must be of acceptable shape
. The coefficient of pitching moment at zero incidence must exceed
.05 ^t 20 mph
,025 at JOmph
zero at 40mph
P X h X g
Cm = ^
r i x d x v x 5 x c
Cni = Coeff of Pitching Moment
F = Bar force Kg
h = Perpendicular distance to hangpoint m
g = gravity = 9.81 metre/sec 2 S = Projected sail area metre
c = mean chord = area/span metre
V = Airspeed in metres/sec
= Airspeed mph x .4513
d = Air density Kg/metre
= .3481 P/(T + 273)
P = Atmospheric pressure millibar
T = Air temperature c
.7 Further Tests;
Results of the above tests may indicate that further tests are
necessary.
.8 Test Development
Test methods and criteria will be refined as the test equipment
and data base are improved *
Figure .lb
Figure.la
Fig ure. 3
Figure. A
Figure. 5
! .
rm
dJp^
Figure. 6a
The D.I.R tests in progress
Figure. 6c
Figure. 6d
