89-0903-CP TETHERED PARAFOIL TEST TECHNIQUE

Glen J. Brown* Vertigo Inc. P.O. Box 117

Lake Elsinore CA 92331-0117

Abstract

A test technique has been developed and is now in use that is effective in measuring the performance of full scale Parafoils. The method involves tethering the Parafoil under test to an instrumented test rig on a truck. Measurements of airspeed, tether tension and tether angle are used to determine the Parafoil's L/D and lift coefficient. For development testing a device is added that allows trim and camber changes to be made while running. The test rig has been used with Parafoils from 18 square ft. to 800 square ft. Data reduction techniques and examples of results are presented.

Backaround

The accuracy of wind tunnel testing of Parafoils is limited by scale effects such as Reynolds number, porosity and the difficulty of scaling lines. The best utility of wind tunnel tests is in parametric studies. Glide performance measured in wind tunnels usina subscale models is often of questionable accuracy.

Flight testing of Parafoils is useful for determining control characteristics for manned applications. Performance measurements in flight are difficult and inaccurate unless ideal atmospheric conditions exist and costly tracking instruments are available.

Basis of Test

The angle from the vertical of a kite string, or in this case a Parafoil's tether, is related to the forces on the kite and its lines as shown in Figure 1.

The relationship between the tether angle and the L/D of the tethered system is:

L/D=l/tan (tether angle)

This relationship is the basis for a convenient method of measuring Parafoil L/D, using a protractor calibrated in units of L/D to measure tether angle.

*Director of Engineering, Member AIAA

Copyright O American Institute of Aeronautics and Astronautics, Inc., 1989. All rights reserved.

TETHER POINT

Fig 1. Vector diagram for tethered Parafoil.

Descrivtion of Eauivment

The tethered test rig is illustrated in Figure 2. The Parafoil tether cable is attached to a load cell mounted in the base of the rig. The tether cable runs past a large protractor board that is calibrated in units of L/D. A bubble level, airspeed indicator and load cell readout are mounted to the protractor board. A video camera is mounted on an arm that places the protractor board and the various readouts within its field of view. The pyramid structure protects the test instruments by limiting the tether cable angle.

Fig. 2 Protractor board instrumentation.

Recordina Data. Data is recorded with a video camera mounted to the side of the test rig with a field of view that includes the L/B protractor scale, the airspeed indicator and the load cell readout. A title card is recorded before each run for record keeping purposes and audible comments recorded from time to time. It is important to mark bad data that occurs when the truck is changing speed or turning. This is accomplished by holding a flag or a hand if front of the camera when the data is to be ignored.

Control of Parafoil. The Parafoil must be actively controlled. to keep it above the truck. A technician stands in front of the test fixture and controls the Para foil. with steering lines. Care must be taken to minimize the influence his control inputs have on the measured data.

s a g e Z ~ A larye Parafoil is capab1.e of lifting or overturning the truck in a gust of wind. When testing anything over 400 square feet a release nechanism is included in the tether line.

Performance Testinq

Launch. Initial deplcynent and inflation of the Parafoil is accomplished using a technique borrowed from the sport of Parapente. The Farafoil is spread on the ground in back of the truck with the bottom surface up and the trailing edge forward. The truck then accelerates quickly to apprax imatel y 15 mph while the Paraf oil inflates and flies overhead. The Parafoil i.s brought under control before the tmck accelerates slowly to the test speed. A test in progress is shown in figure 3.

Fig. 3 Parafail under test.

Tests can be run on any reasonably snooth surface of sufficient length. However, to obtain high enough accuracy for meaningful quantitative results, it i.s necessary to test on a very smooth surface in no wind conditions. We find those conditions at times on El Mirage dry lake in the Mojave desert, generally for several hours after sunrise. Very smooth wind is theoretically acceptable, but even small gust components add significant noise to the data.

Even in perfectly calm conditions on a perfectly smooth surf ace, the Paraf oil. under test will. exhibit a sursincr motion. The - - following method of averaging a large number of data samples is used to improve data precision. A videa camera is mounted i:s a position with a field of view of the tether protractor and all data readouts. A data frame is selected by viewing the tape and selecting a 2 second segment of valid data. The data from each of the 60 video iinages is then tabulated and averaged. A t least .5 data frames are obtained in this way for each test condition, which are then compared, in dimensionless f o m , to determine the standard deviation of the results.

A Parapente glider was obtained far test purposes and run on the test rig (figure 4). This Parafoil has 13 cells, an aspect ratio of 3.66 and extensive line cascading. We were unable to obtain usable data on this Parafoil because of surging. An L/D of better than 6 was observed. This nay indicate a limitatian of the tethered test technique for testing very high performance Parafoils.

Fig. 4 Parapente glider under test.

Pavload-Eff ects

Trin 7--f;hiEt. The moment center that determines the trim of the Parafoil for tethered tests is the confluence point of the suspension lines. Lift and drag forces are reacted at this point, but external moaents are not. If the Parafoil-payload system has an apparent center of gravity that does no coincide with the confluence point, then the drag (or thrust) of the payload times the vertical distance between the system cg and the confluence point is an applied moment that will shift the trim point o f the Parafoil.

Estimation of trim changes due to payload drag and thrust are made using standard linearized aircraft equations. The details are beyond the scope cf this paper. The largest correction the author has calculated for an application is CG0.05.

Payload Draq. Measured L/D applies to the canopy and lines only and does not include payload drag. This is the most meaningful measure of Parafoil performance because payload drag is application specific , LID is adjusted for the effect of payload drag using:

where :

LjDp,= 41) corrected for payload drag L/Dv,= L/D measured without payload CDApl= "drag areaw of payload C b Parafoil lift coefficient S= Parafoil wing area.

As an example, consider a personnel Parafoil with a neasured L/D of 4.0 at a CL of 0.6 and an area of 200 square feet. A jumper has a drag area of approximately 6 square feet. This reduces the L/D to 3.3, which is closer to the observed glide ratio for typical personnel Parafoils.

In comparing two canopies with different line sizes or different cascade arrangements, it may be interesting to know the relative performance of the canopies alone. For example, consider a parafoil with a full line set of 4 0 lines, each 15 feet long with a mean diameter of . 0 4 inches. Use a drag coefficient of 1.2 to arrive at a drag area of:

Applying this value as a "negative dragwto the Parafoil considered above shows that the canopy alone has an L/D of 4.35 .

Table I is a summary of results obtained in a series of tests of three popular personnel Parafoils and a special purpose prototype Parafoil.

PF400a 3.55 .55 5.6 3.90 PF400b 3.90 - 4 0 5.6 4.51 PD 260 4.09 .51 3.4 4.60 MANTA 266 3.77 .52 4.3 4.23 ALPHAMI11 3.54 - 5 4 5.9 4 . 0 4

Table I. Test Results Summary

In order to compare the canopies, which had different size lines, line drag area was estimated, tabulated under "AlineS1, and used to calculate canopy-only L/D, tabulated under i8L/Dcanfl.

Trim Studis

The trim angle rigged into a Parafoil deternines its trimmed lift coefficient and has an effect on glide performance. The line lengths that determine trim angle are usually determined based on experience and are verified in flight. Tethered testing provides an opportunity to explore trim effects in a controlled environment and to more nearly optimize line lengths.

A trim bar was constructed to facilitate trim testing (figure 5.) It consists of a bar with attached turnbuckles that allows convenient adjustment of either trim angle or the adjustment of any line group separately. Trim testing requires the use of a full line set. If the Parafoil is to have cascades then a full line set is attached with "slip cascade^'^ at the cascade locations.

Each line group is attached to a turnbuckle, i.e. all A lines are attached to one turnbuckle, all B lines to the next, etc.. This gives the Parafoil a different "archvt than it has with separate risers and is a

source of some error. However, it i s felt that the error is a small absolute value that does not interfere with obtaining syt-elativesy values to determine optimum trim.

Fig. 5 Adjustable trim bar

Results obtained for the PD421, a 360 square foot tandem Parafoil with exceptional glide performance, showing the effects of trim and brake settings are summarized in figures 6 , L/D is improved with small amounts of nose- up trim compared to the starting configuration.

Fig. 6 PD42P Trim Effects.

Acknowledqements

The experimental work on which this paper is based was supported by Develapmental Sciences Corporation, division of GEC. The author thanks the company and Dr. Gordon Harris, Vice President of Engi:ieer.i ng for their support. The author also thanks Mr. Bill Coe of Performance Designs 3nc for his assistance.

References

1. Lingard, J. S . , *The Perf ornance and Design of Ram-Air Parachutestt, Technical Report 81103, Royal Aircraft Establishment, Farnborough, 1981

Conclusian

Tethered Parafoil testing has been used for both developmental purposes and to obtain specific quantitative measurements. It has been found to be a useful and economical method. Limitations are the need for favorable weather conditions and the surging of some Parafoils.