Construction rules and standards for the construction of sailplanes
It will probably be of interest for pilots to know something about the safety features and checks during the production of sailplanes. These checks ensure future pilots a safe flight and landing without construction faults or defects.
These checks are done according to the rules of the JAA (“Joint Aviation Authority”), an organization composed out of the European “FAA’s”. The rules are updated irregularly but they are always based on new information and research development.
An aircraft has to be built according to the regulations that exist at the time of it’s initial development or planning.
But this is exactly one of the main problems.
Some aircraft, that are in today’s production got their type certification a long time ago. If for example a new wing is being manufactured, with a change to the airfoil, the old fuselage doesn’t need to be recertified. The fuselage can be built according to the regulations long ago. Only the wings need to get a new certification.
This reaches a point at which “old-timers” can be built and sold as brand new aircraft although they would never pass a new certification process.
A good example is the “new” Cessna 172. That aircraft got it’s type certificate in the 1950’s. It is being built and sold with the old type certificate today as a new aircraft, but it can by no means be compared to a newly designed and certified Diamond Katana or DA 40 with it’s superior construction and safety features of the fuselage.
Naturally that doesn’t mean, that such a Cessna will break apart, but the obstacles that have to be passed in the process of a new certification are a lot “higher” than the ones 30 or 40 years ago.
A DG-100, the first aircraft of Glaser-Dirks would not get a type certificate today either, due to the horizontal stabilizer and elevator design. The construction demands are simply different (higher) than in the 1975`s when the DG-100 received it’s type certificate. Anyway, those planes can certainly be used and sold without any problems or doubts. Things only need to be changed when an air worthiness directive is being issued. Those directives are issued whenever weaknesses in the construction or in the building process that are relevant to the safety or that might have caused accidents are discovered.
A real new construction, with the need of rectifying the whole aircraft happens very rarely. Usually components of other aircraft are being used in “new constructions”. The engine powered DG-800 was a brand new construction. The DG-1000 is “nearly” brand new.
Old sailplanes usually don’t show the best performances – alongside with technical and research disadvantages. On the second view we notice that modern designs offer a greater degree of passenger safety and improved safety in the handling of those aircraft’s.
Building regulations are defining those standards!
One of the most interesting aspects in the certification process is always the stress test of the wing. This is used to certify it’s internal strength. For this test a wing is being built without the steering devices. After the test often the wing is stored by the manufacturer and can be used if needed in order to check if a wing of a crashed aircraft had any differences to the wing that was used in the initial tests. Therefore it’s got to be built in the same way as any future wing of the specific aircraft. No manufacturer will even try or think of improving the strength of that wing in order to get better results in test. In case an FAA or EASA official would notice only the slightest tolerance the result would be devastating for every manufacturer!
In a stress test a wing will be fixed to a frame and weights are put on it slowly exceeding the calculated limit load factor.
For simple stress testing the fuselage and wing can be mounted upside down in a frame. Then slowly sacks of cement are put on the wing. The wing has to sustain the stress without deformation or structural damage for at least 3 seconds. No force in the air can last longer than 3 seconds. Once this test has been accomplished successfully the limit load factor is officially considered proven.
But how high is the calculated limit load factor?
Usually the unit that measures those forces is the “G”-scale. For example the DG-800 wing’s specifications are +6.4g/-4g. Aerobatic figures cause stress within the frame or structure of aircraft, but that’s not the reason for a test.
Manufacturers can easily certify aircraft’s for “Acro” or aerobatic use if they really want to and happen to have the money left for the certification.
To accomplish that certification a maximum of +7.0 and -5.0g has to be proven.
A while ago we certified the DG-303 for ACRO use, after strengthening the construction in a few places. We did not certify the DG-800 for ACRO use though, because of the enormous costs of the whole process – we simply felt that it would be unnecessary.
But the DG-1000 already is certified for Acro purpose.
Well, aerobatics are not the reason for a +6.4/-4g certification.
Many of the readers will ask: “So what’s it all about then?”
Our answer is simple but understandable: “The weather”!
The planes have to be strong enough to withstand a lot of stress for example in a wave- rotor. The stress on the structure of any aircraft under these conditions are often higher than in any kind of aerobatics.
This is also the reason for the different speed limits. One for turbulent air and one for steady and smooth air. The absolute max. (VNE) – red indication on the airspeed indicator, is only for smooth air. (I wonder if all competitive pilots know that). The second limit is the design maneuvering speed (VA). Any airspeed in excess of VA can overstress the airframe during abrupt maneuvers or turbulence. Then there is the speed limit from which on no full aileron, rudder or elevator can be applied. In case of our DG-800 both speeds are the same (190 km/h).
Surprisingly the accelerations in a light aircraft are a lot stronger than in a heavy aircraft. Example:
Whenever there is a lot of water in the wings and therefore the wings are heavier than normal, it takes a lot of time for the air to pull the wing up or push it down. The limit load factor is calculated by using the weight of parts that don’t help the aircraft to stay aloft in proportion with the weight of the “necessary” parts such as the wings and so on.
A big problem that we face are engine-powered sailplanes. A lot of weight is concentrated in the fuselage (the engine) and therefore the wings need to sustain more stress and are built stronger than the one’s in “regular” sailplanes.
An engineer can calculate the limit load factor by measuring the wing area (in sq. ft), weight and max. airspeed. (In case of our DG-800 it’s 6.4/-4g.) But that is not the limit load factor that has to be proven in the test! We have to show that the wing can withstand stress on a continuing basis that is 1.725 times greater than the calculated limit load factor!
This safety factor has been increased from 1.5 to 1.725 lately by the way.
(Therefore new wings have to be at least 15% stronger than old wings!)
Our DG-800 wing sustained a stress exceeding 12g before it broke apart. This was luck for us. Due to some changes the aircraft got to be heavier and the wings had to be recertified after the development of the winglets as well. Due to the result of our initial test it was easy for us to get the certificate for this heavier wing.
The limit load factor is always measured until the wing breaks apart. The interesting question is only: Will the engineers self-confidence break apart prior to the wing, with the wing or will the engineer “survive” the test?
It is not always the case that a “limit-load-test” is a success in the first trial. Many manufacturers suffered the problem of wings that would not sustain the limit load. It’s not only DG who suffered though. If a wing doesn’t pass the test it needs to be redesigned and all sales will be heavily delayed until all tests are completed successfully.
The second obstacle is the so called fluttering test.
It is at least as difficult to pass as the limit load factor test. An engineer has to show that the wing will not start to flutter.
The underlying basis for this test is once again pure mathematics. Tests have to be made analyzing the flexibility and elasticity of the wing. In order to do that the wings are mounted in a frame and manually oscillated. Once this is completed without problems the designer has to fly the aircraft at 10% above VNE (max. airspeed) trying to get the wings to flutter using every anything that is possible.
This test is done at a height of more than 3 km. Therefore true airspeed is far exceeding 300 km/h that are shown by the airspeed indicator in the cockpit.
Would you like to do such a test??
(I wouldn’t but I feel comfortable flying an aircraft that has been flown under those circumstances by it’s designer. He will not do those tests if his calculations would be false….)
It’s good to carry a parachute with you if you do those tests. In case a wing starts to flutter the whole aircraft will be ripped apart within seconds. We had a case of a DG-600 wing starting to flutter a while ago at a speed exceeding 300 km/h….. it was caused due to week aileron struts.
The fluttering tests have to be repeated for each modification of an aircraft. Even larger water tanks in the wings or redesigned winglets make new tests necessary.
Finally there are a number of rules
considering the flying abilities of an aircraft.
A plane has to react harmlessly in a stall situation. We had to install a special stall warning system in our DG-600 whereas the DG-800 is really nice at low speeds. The DG-600 wing was good if you would look at the pure numbers, but it was critical at low speeds in the version without winglets.
Further on the stall speed is not allowed to exceed a speed of more than 80 km/h.
If a plane has a higher stall speed due to water in the wings valves need to be installed that enable the pilot to get rid of the water very fast.
Due to it’s superior weight compared to simple sailplanes engine-powered aircraft are commonly equipped with flaps to achieve a stall speed below 80 km/h.
The required effectiveness of the air brakes is also a difficult task to achieve. At a speed of 30% above the minimum speed (around 54 to 57 kts – 100 to 105 kph) the sink rate must equal a glide angle of 7:1. Both the DG-800 and the DG-1000 only just manage to achieve this and are still known for their effective air brakes.
These aspects were just a view standards/rules and laws that one has to keep in mind when building an aircraft. Other rules are focusing on the maximum weight of an aircraft, the crash durability and lately even a cockpit crash test is required. The first fiber-glass sailplanes had a cockpit-crash durability of today’s VLA’s. It would be impossible to attain a new type certificate for those sailplanes today.
One factor is not covered under the JAR construction rules though – the noise pollution. Anything that has to do with noise is covered by national laws. As you might know those laws are very strictly enforced in Germany. Therefore engines have to be quiet over here.
The JAR/EASA construction rules apply for the whole of Europe.
The construction rules always make sure that a new aircraft is always state of the art when it’s being sold.
New safety and construction features cost a lot of time and money, but they are worth your safety and ultimately your life!
– friedel weber + w-dirks –
translated by Thiemo Gorath