Testing of rescue systems parachutes for aircraft categories UL and LSA.
Sequence taken from the test of rescue system for the American aircraft Cirrus with the parachute BRS.
Many ultralight aircraft are equipped with integrated parachute systems activated by rocket engines. As other products under the LAA ČR administration, these systems fall into relevant certification as well. For this certification, thus the Type certification, must be proved that prescribed regulation requirements are met for both, the whole set parachute-rocket and the parachute itself. By the parachute must be proved its strength, opening time and descending. Producer defines basic parameters for the use of given parachute, its loading capacity and maximum operational speed, more exactly highest speed at the time of parachute opening, and as well, the minimum time necessary for parachute unfolding and inflating. From this a usable height for the rescue of aircraft and crew is derived.
For an aircraft designer the tough assignment proves to be to tune parachute opening so that a requirement for the opening time would be met. The longer time of parachute opening, the lower dynamic impact and thus a softer demand on the parachute strength including fastening items, thereof a softer demand on a strength load on airframe fixing points.
Unfortunately on the other hand the time is prolonged and, with it, the necessary height for parachute inflation. The designer has to design a parachute so that it would comply with a given tolerance not only with a regulation (e.g. according to the German DULV- the time of opening 4.5 sec.), even with the compromise of requirements mentioned above. It can be tackled by series of technical steps – canopy shape, central opening size, possibly systems with various flaps and slots, and above all size, shape, kind of material and slider area which slows down the canopy opening time. The slider is for instance of fabric or a mesh ringlet, which has grommets on the outside perimeter through which parachute lines run. The slider is loosely put on the canopy bottom side. While the parachute opens the slider does not allow canopy to open fully – it opens only to the slider diameter size. In this phase the parachute is in a pear shape and reduces the first dynamic impact. Only consequently, by overpressure, the canopy starts to widen and the slider follows to slide along the parachute lines down- as a result the parachute opens slowly till the full opening. To tune the speed of opening it is necessary to work with the slider diameter, its area and other above mentioned canopy parts. Against the effort to slide down reacts the dynamic pressure on the slider surface in relation to the aircraft speed at the time of canopy unfurl.
Simply, this all is a matter of rich experience and long-time testing and each company keeps this knowledge. In designing parachute rocket rescue systems not only knowledge of their designers and aircraft manufacturers requirements are of importance, but, as well, valid regulations. Currently we know and use three regulations for rescue systems (further Z.S.) We present their most important items in following paragraphs.
Picture from the drop test of parachute for the Galaxy rescue system
Regulation ZS 2 – LAA ČR
Rescue system is not obligatory. Maximum allowed descending is 6.8m/sec to AMSL. Parachute strength is verified at maximum weight and maximum speed, which is increased by coefficient 1.05. At least drop tests must be carried out for maximum counted speed. Canopy opening time for weights 450 – 560 kg is not determined, the producer counts or measures minimum opening height at the speed of 65 km/h and the figure states in the technical parameters of rescue system. Along with it must be in these tests always verified the opening shock. Further, drop tests verify stability (oscillation, swinging).
Regulation ZS 2 demands as well at least three firing tests from a trailer behind vehicle. Of them two tests are intended for canopy opening check at the speed of 65 km/h and at least one test must be carried out at parachute firing upright over a towed hurdle behind vehicle at the speed of 100 km/h. The hurdle simulates aircraft aft with T shaped empennage.
It is 2 m high in 4 m distance from the firing point. The test must in this case prove sufficient canopy stretching, which guarantees that catching by horizontal empennage cannot occur.
(Deutche Ultralight Verband)
German Federal Republic
All UL aircraft in Germany must be equipped with rescue systems.
It is resulting from the German rule.
The rescue system parachute strength is verified at maximum weight and maximum speed. It is not multiplied by any coefficient. The regulation demands at least three drop tests at a speed, that must not be lower than the aircraft VNE. Along with it the opening shock must be stated. Further drop tests then verify stability, similar as it is by the Czech regulation.
In tests of rescue systems according to the German regulation the coefficient k is important. It is set by a rate of maximum allowed speed to aircraft weight, which is relation VNE/m. Verifying of canopy opening minimum height and thus the rescue, is set by this coefficient. The coefficient determines as well the speed at which the system is tested.
The Mi-8 helicopter used for described drop tests
Right picture: Container with the weight of required weight before the drop test
According to the German regulation at least three times must be carried out firing tests to verify the system function and the canopy opening at the coefficient k lower than 0,4 (testing speed is ranging according to the drop speed from 45 to 65 km/h).
At a higher coefficient than 0.4, the test to verify the minimum time of canopy opening is carried out by a drop from flying aircraft or helicopter at the speed of 120 km/h.
The rescue system canopy opening time at the aircraft weight of 472,5 kg must not be longer than 4,5 sec. after the rocket engine activation. The German regulation strictly demands this figure.
By rescue system canopies determined and tested for higher speeds, the verified level of minimum rescue aircraft height with crew is shifted from 60 – 80 m over the ground (proved cases of rescue with the GRS system in the past) to 120 – 160 m over the ground – thus the safe possible rescue height by these rescue systems almost doubles. Therefore a pilot determination to fly slowly and low under the level of 150m is not, when using rescue systems aimed for higher speeds, very safe. Simply, currently it is not possible to achieve a low ultralight aircraft rescue system weight by which it would be possible to rescue a crew with aircraft from 60 m over the ground at critical speed around 300 km/h. The rescue height is in this case shifted at least to 120 m over the ground.
The parachute rescue system descending for UL aircraft must not be higher than 7,5m/sec.( measured 30m from the ground by means of a drop cord).Re-count on 1000m AMSL each aircraft anchor point along with connecting slings and cables must hold out the measured canopy opening shock multiplied by the safety coefficient 1,5. In connection with the stated opening time 4,5 sec applies according to the German regulation that the lighter and smaller parachute, the bigger opening shock. It means that a desired save on the canopy weight causes ( regarding required rescue system anchor points strength) to disproportionate increase of the aircraft weight. Thus, the more aircraft anchor points, the heavier is the aircraft airframe comparing aircraft on the German market with those on US or other countries markets who respect ours or American regulations counting with different strength of front and rear anchor points of bearing slings or cables leading to the canopy. According to our, or American regulation the airframe and the whole rescue system are then much lighter.
Partly open canopy of the rescue system after the drop.Well seen is the slider ring.
The Czech republic accepts both regulations and the German tests can be carried out in our country in presence of the main Czech inspector. It is further important step in cooperation and mutual trust of the DULV to the LAA ČR.
Regulation for category LSA – USA – ASTM F 23-16
American category LSA comprises light sport aircraft up to the weight of 600 kg. Rescue system equipment for these aircraft is not obligatory.
Maximum allowed speed of LSA category aircraft in horizontal flight is 222 km/h. Coefficient of speed increase for safety comes out from the speed in 75% of engine power, it corresponds to 109kts (i.e. ca. 222km/h). Safety multiple is 1,21 (the test speed is then ca. 245 km/h).
The regulation allows a slight tolerance for testing. Let us present an example:
By weight the coefficient could be 1,22 and for speed 1,23. Cumulative safety multiple must be then at least 1,5 (1,22 x 1,23 = 1,5006; the rate of these figures must be balanced. It is set by the American regulation of FAA for this category. Along with it must be stated The opening shock (in pounds or in kN.)
The Cirrus aircraft under the BRS rescue system tested in USA.
To verify the descending rate and determination of the speed opening the parachute must be tested at operational weight (for instance at 472,5 kg and 90km/h of speed).
To verify the strength, the canopy must be dropped three times. (For instance in maximum operational weight – rescue system loading capacity- it will be 473 kg, which after safety multiple of 1,25 gives you the weight of 591 kg, drop speed will be then 245 km/h).
The regulation in USA does not solve a canopy descending rate and a necessary time for its opening. But the descending is recounted to the height of 5000 ft / 1500m AMSL.
In USA it is all left on companies (and an invisible hand of the market). Producers do not have to publish the figures.
Important are markings of all dangerous functional parts of the system. For instance, with a sticker is designated the opening through which the rocket is going after activation (similar it is by the LAA ČR). Relevant markings are in the cabin as well. On producer websites must be available so called Emergency contact saying what to do in case of an accident to prevent any injuries due to a wrong handling with the rescue system. The instructions must count with firemen and rescue workers, which is important as well. It is done in USA very well indeed. As for aircraft in the category experimental in USA, there can be installed any rescue system and nobody cares an installation and nor its parachute. With aircraft of this category though, it is not possible to do any commercial activity.
American requirements on rescue systems for the Cirrus aircraft.
Ing. Milan Bábovka: Aircraft of the company Cirrus are sole types of the category General Aviation in the World standard equipped with ballistic rescue systems.
Maximum take-off weight of the newly designed Cirrus aircraft due for the year 2007 is set to 1724 kg. For use is counted with one, two or three parachutes in one unit.
By the company was announced a classic tender to deliver a rescue system with parameters on the edge overcoming current technical possibilities. In this tender participate Czech companies, the company Galaxy as well.
Current aircraft Cirrus SR22-G2 and SR20-G2, the weight is up to 1500kg, are using one canopy with the time of opening between 6,5 to 6,8 sec., which complies with the desired requirement to rescue a crew on an airport circle where roughly 80% of all accidents happen. Mostly it is due to pilot mistakes caused by a stress from flight traffic. The new Cirrus is to have a rescue system at least the same or even more efficient as the current one. The time of opening is to be up to 6,5 sec. (respectively 5,0 sec. by the canopy) at the speed of 90km/h. The test requirements are similar as by the LSA. The Cirrus aircraft is a fast five-seat machine. To construct for it a parachute device complying with prescribed requirements is very difficult. As a root to increase safety coefficient was set the border of manoeuvre speed of 296 km/h. When multiplied by the coefficient 1,2 it corresponds to the tested speed of 356 km/h by the weight of 1724 kg x 1,2 = summary 2068 kg. It complies with the exemption for reduced total coefficient from 1,5 to 1,44.This exemption was awarded to the Co. Cirrus by the FAA after a lengthy negotiation because the original requirement of 1,5 was not possible to accomplish for the manoeuvre speed.
To crack into this tough technical proposition will be very hard for the newly designed system as well, moreover when this parachute technology has limitations for very short opening time from the system activation and limited weight of the rescue system. By the currently produced types of Cirrus no system has achieved the desired coefficient 1,5. The new assignment allows the total rescue system weight up to 27 kg.
Showing the progress in increasing the safety for crews of smaller aircraft the company Cirrus has set the hard act to follow. As a prove that rescue systems in aircraft work is the fact that up to this date already ten of aircraft with occupants were rescued (in one case even with a technical malfunction of control).
The company Cirrus manufactures from two up to three complete composite aircraft a day in a new factory. Canopies for these aircraft are completely thoroughly checked including descending rate which must not be higher than 7 m/sec.(recounted to 1524 m ASML) In seats muffling materials are used to reduce the impact on occupants in case of hard landing. The rocket is consistently separated from occupants, because due to burning of chemicals used as a fuel for rockets in USA a poisonous hydrogen chloride evolves, which by smokeless powders used here in Czech rep. is not possible (one worry less for us). We have a better fuel from the Synthesia, now the Explosie a.s., thanks to their clever staff ( simply, tradition and our handy golden hands).
The company Galaxy carried out on 27.April 2006 parachute tests at the Hořín airport near Mělník under the supervision of the LAA ČR. Tests were done by drop from the MI-8 helicopter. The crew was made up of captain Ing.J.Černý, second pilot Ing.J.Rajda and onboard operator S.Fuxa, who looked after the parachute drop with the weight.
The required weight was made up of steel container with additional steel plates. Between the weight and the parachute sling was placed a connecting piece – strength measuring device based on the principle of a copper cone deformator placed in a dynamometer. In testing while parachute opens there is a dynamic opening shock. The strength and force react to the cone, changing the shape and thickness from which we can read the strength at opening.
The parachute opening initiation is set by the connecting sling length (4m), which after electric activation opens a release mechanism of the parachute.
The crew flies in the set height of 200 – 300 meters according to requirements of respective drops and according to prearranged speed conditions, for instance at 90km/h, 120km/h or 250km/h.
Pilot maintains a set height and speed, for higher accuracy the second pilot announces figures and revises the flight to the place of drop and the onboard operator releases the parachute with weight. To keep all the parameters the crew must coordinate its work exactly. The drop is recorded on camera from the ground. From the recording is then possible to phase the course and measure times of canopy opening.
Altogether six drops were carried out at these tests.
How to set up a comparative test diagram of the opening shock?
The diagram is set up that from one copper rod of given diameter are taken out from each side samples from which the test cones are made. They are then tested in a test-room (from-to certain figures of kN), tested batches are mutually averaged and the measuring diagram is set.
Description of the Galaxy rescue system
The parachute canopy is placed in a container, which is drawn when activated. Then unrolled after stretching of parachute lines, connecting sling (5,5 m long) and fixing slings. In this phase the system is drawn to the height ca. 18m above an aircraft as a package and then starts inflation. The advantage of this system is that when used the canopy is unrolled sufficiently far from an aircraft. It is favourable mainly in high speeds when a possibility of entangling of the canopy to the aircraft construction or its flying debris is considerably reduced. By the way, the company has this principle patented since 1994 under the number 1859-94.
Description of one test for the DULV
This DULV test verified minimum parachute opening time using a test weight of 472.5 kg and a deployment speed of 120km/h. The drop height was 250m above ground level and the opening time recorded was 4.45 seconds from time of release from the hook of an MI-8 helicopter. The maximum opening time of 4.5 sec. was set by the regulation. In reality this time is considered to be the total time of opening, including the time required to position the canopy above the aircraft after inflation.
The recorded time from the drop time to the release of the container has been shown to be equal to the realistic deployment scenario time from rocket firing to parachute deployment and inflation above the aircraft.
In designing a parachute, it is important to utilise as much of the canopy opening time allowed by regulation as possible, as this allows the system to be deployed at higher speeds. The tested canopy was designed for speeds up to 320 km/h.
The Mi-8 flight for the drop test and the onboard operator Mr.Fuks before the moment of drop
After completion of this test all results were immediately analysed, including measuring of the dynamic opening shock. This is done by measuring the deformation of a copper cone dynamometer used for measuring dynamic shock. It showed a width of 3.6 mm. According to the test table, this corresponds to a dynamic shock of 20.8 kN.
The test proved that the canopy complies with the required opening time and maximum dynamic shock.
Another test of the same canopy
This time the parachute was tested for an operational weight of 472.5 kg for the Ultralight Category. Our first step was to verify whether the canopy intended for the DULV test, and with the designated opening time of 4.5 sec., would pass one of the tests required by the US-LSA standards.
The test weight was increased to 580 kg, an increase of 22.7% over the previous test weight. The tested speed was 250km/h, an increase of 23%. Therefore, the total safety coefficient met the required 1.5.
Looks from the MI-8 helicopter during the drop test.
Right pictures: Sequence from the drop test of the Galaxy rescue system
The opening shock was measured at 34 kN, with an opening time of 3.75 sec., including the initial drop time. Without the 1.25 sec drop time, the net time of full canopy inflation was only 2.5 sec.
The canopy made it through the test without any damage. The cone deformation was 2.59mm.
Commentary about these tests
Ing.Milan Bábovka adds: It is not recommended to pursue these experiments further, because all canopies have their absolute strength limits, and with ever-growing kinetic energy loads there are very high “g” loads on opening. These would result in additional reinforcement of airframes to sustain these loads, and along with this large weight increases.
From our long-time testing and measurements of parachute drops we have learned that it is not possible to test a canopy only at a speed of 300 – 320 km/h just by dropping it at required operating weight and then using these figures for LSA aircraft. We have found that it is necessary to include a speed correction factor for the drop itself. Therefore, for a required opening speed of 321 km/h, the drop aircraft must in reality have a calibrated airspeed of 350 km/h at time of drop.
The reason for this required correction is due to the weight resistance before the canopy is inflated, and due to airspeed indicator error. Of course there are also computer programs and tables that can be used to make these corrections. It is essential to state these figures correctly so that the final design of the rescue system has the same performance parameters as stated when installed on an aircraft with corresponding weight/speed.
This 22 – 25% increase in weight verifies canopy strength at time of opening. It is not necessary to use the maximum test speed.
Such a test is essential for all the weights in the scope of the LSA Category in the USA. Thus, testing canopy opening time at minimum speed to 4.5 sec., as required in Germany, is not practical or real. Canopy opening time is inevitably prolonged and with this comes a growing height necessary for full opening and canopy inflation. However, in General Aviation the safety factor of 1.5 is set by regulation and must be kept. There are some minor exemptions like the one granted to Cirrus. For this reason most rescue system manufacturers are not able to find a satisfactory solution to this problem, that would allow an aircraft to be saved when making the turn from base to final, at 150m above ground level. This would require a total opening time in the range of 6 – 6,3 sec. So, there is a mere 5.0 sec. remaining for canopy opening.
At the end about some more tests.
At the airport near Mělník the new GRS 6/600 SD LSA canopy (aimed for the American category LSA up to 600 kg) was tested at operational MTOW with a 25% increase.
- The tested opening time at the minimum speed of 90 km/h and the weight of 600 kg was recorded at 5.8 sec. including the drop time of 1.25 sec. The measured opening shock was 22.5 kN (this figure is not necessary for an aircraft designer or aircraft user). Calculated rescue height with fully opened canopy was 130 – 140m above ground level.
- The tested opening time at the maximum speed of 250km/h and at the weight of 750 kg was 5.0 sec. including the drop time. Measured opening shock was 32.5 kN (this figure is used as proof of canopy strength).
- The tested opening time at the maximum speed of 250km/h and the weight of 600kg was 5.34 sec., but most notably, at the same dynamic shock of 22.5kN (this figure is the one used by an aircraft designer and for verifying of the highest opening shock level.)
The canopy inflates better when braking from a higher speed with the regulating slip speed of the slider. And, if the canopy is designed correctly, the opening shock remains the same. Ing. Milan Bábovka adds the following: With these parachute drop tests in May, 2006, the Galaxy GRS s.r.o. company has completed a two-year development cycle of rescue systems for General Aviation aircraft and aircraft that fall into the LSA category (the USA standard). A total of 130 rockets were fired from a vehicle mount to compare canopy opening times. A total of 65 parachutes were dropped from the L-410 Turbolet aircraft and from the MI-8 helicopter. Along with this, the company filed an application to patent the new series 6 canopy design for the weights of 360 kg, 473 kg, 600 kg, and 650 kg, all with a verified safety coefficient of 1.5. The designation of the new ZS are: GRS 6/360, 6/473, 6/600, 6/650 SD LSA, year 2006.
In conclusion, it is necessary to state that the GRS 6/600 SD LSA weight, including a 6m long parachute sling and the rocket engine is only 12.3 kg. The descent rate is only 6.9 m/sec. At present this is probably the best rescue system in its category on the world market.
Described tests were conducted at the time the regulation ASTM 2316-08. For this reason, it was necessary to test the weight increase by a factor of 1.25 and 1.21 speed koficientem that the resulting safety parachute had reached safety factor of 1.5 for the intended weight and speed.
With the new regulation issued by the ASTM 2316-12 test was modified for testing parachutes to test parachutes carried out on the aircraft or with concentrated loads (Dead load).
Since our company has performed and conducted all tests with concentrated loads is the strength factor of 1.5 has been directly included in the test values (weight-speed) .It is therefore necessary to test these values continue to rise.
Conversely, thus tested parachutes are under an amendment to 2316-12 strength and higher safety factor to 2.25 (1.5 x 1.5).
Testing involves as well the opening shock check – the magnitude of shock is measured by the size of deformation of the copper cone .( On the palm you can see the cone before and after the test)
Tests supervised and the text compiled the LAA ČR chief engineer Ing Václav Chvála,
Commentary written by Ing.Milan Bábovka of the Galaxy.