Slick Aircraft Structural Integrity
During display or competition flying, or practise for that matter, the aerobatic pilot has enough on his mind not to have to be concerned about the integrity of his aircraft. For this reason we designed the Slick 360 to have superior performance, excellent handling qualities and unquestionable structural integrity.
Any aircraft structure should be designed for an optimum balance between required strength and minimal mass. Too strong implies too heavy, and in all things aeronautical weight is the enemy! Save some weight and the loads decrease, resulting in more possible weight saving. This is an example of the “weight spiral” going downwards! The secret is to build every part just strong enough.
How is “strong enough” defined? The designer adopts an airworthiness standard (in our case the Federal Aviation Regulations Part 23, or FAR-23 of the United States of America), which defines a minimum limit load factor for the relevant category of aircraft (normal, utility or acrobatic category). Load factor is simply the number of times the load experienced by the structure during a manoeuvre is more than that experienced during unaccelerated level flight. Load factor is often expressed in terms of “g’s”. Straight and level flight is flight at 1 ”g”, and a co-ordinated turn with a 60 degree bank angle requires 2 “g’s”. FAR 23 requires a minimum positive load factor of +6 “g” and a negative load factor of –3 “g” for aerobatic category aircraft. To accommodate competition aerobatics we chose limit load factors for our Slick 360 aircraft of + 10 ”g” and – 10 “g”. It is unlikely that the pilot could exceed these values, but we still apply a factor of safety of 2.0 rather than 1.5 as required by FAR-23. This means that no part of the Slick aircraft should fail before it reaches +20 or –20 “g” loads.
Structural integrity is often regarded as the strength of the wings only. Whilst wing strength is certainly very important, every other structural component needs to be strong enough as well. To demonstrate structural integrity the major components of the first all-composite Slick airframe were subjected to 15 “g” loads in static test rigs.
Initial tests on the fuselage indicated an area on each side, just aft of the firewall where the carbon fibre skin started buckling at about 8 “g” and the bond to the firewall flange failed at about 11 “g”. To rectify this the skin area where buckling occurred was fitted with a “sandwich panel” by incorporating an 8 mm PVC foam core and two more plies of carbon fibre. A subsequent test was performed to 15 “g” without any signs of buckling or impending failure. In fact the test rig failed at this load!
Investigation of the wreckage of a new all composite aerobatic aircraft which experienced a wing failure at less than half of its design limit load indicated a manufacturing flaw in the wing main spar. This went undetected during manufacture and subsequent inspection. To eliminate risks of this nature, Global Composte Solutions, the company manufacturing the Slick 360, will test every single wing in a static loading test rig to +11 “g” before integrating it with the fuselage. In this test rig, called a whiffletree, the aerodynamic loads on the wing are simulated through a system of beams and links connected to a single hydraulic actuator for each wing. The load applied to the wing is measured by means of a calibrated load cell. The wing needs to sustain the test load (11 “g” in our case) for at least 3 seconds. At the same time the aileron movements must be shown to be free and easy. After the test no signs of residual deflection or structural failure (cracks, delaminations etc) should be evident. This test establishes the structural integrity of each specific Slick 360 wing before it is flown for the first time.
In order to prove the design up to limit load one Slick 360 wing was tested to +15 “g”. Again the test rig failed and had to be strengthened twice before this load could be demonstrated. It is planned for some time in the future to test a wing to failure, which should occur at 20 “g”. This will verify our design process. In the meantime we are confident the wing can sustain loads 50% in excess of the design limit load.
Another factor playing a role in structural integrity is flutter. Flutter occurs in an aircraft component when the frequency of an aerodynamically excited vibration coincides with an undamped natural frequency of that component. This phenomenon generally occurs at high speed and is capable of destroying an airframe in a fraction of a second, usually with catastrophic results.
To determine the flutter characteristics of our Slick 360 the completed aircraft was subjected to Ground Vibration Testing by a group of specialists at the Council for Scientific and Industrial Research (CSIR) under leadership of Dr Louw van Zyl, an internationally recognised expert in this field. In this test procedure a number of accelerometers are fitted to the airframe to measure the response to vibration input over a range of frequencies. Once the airframe vibration modes, frequencies and damping are determined in this way a computer simulation of the aerodynamically excited vibrations and the airframe response can predict flutter which may occur in flight.
The Slick all composite airframe is constructed using mostly carbon fibre material. This provides superior rigidity of the structure, which is beneficial from a flutter standpoint. The results of ground vibration testing indicated that no flutter modes in the Slick aircraft reached zero damping (flutter indication) up to flying speeds of 500 knots! Since our maximum dive speed is limited to less than half that speed, it is probably safe to conclude that Slick is flutter free throughout its operating range.
The most common flutter occurrence in an airframe is probably aileron induced wing flutter. For this reason ailerons need to be balanced with their centre of gravity coinciding with, or close to, the hinge line. By using a horn design to induce a leverage benefit an on the basis of ground vibration tests it was determined that the aileron (and elevator) mass balance requirements could be somewhat relaxed, resulting in a huge weight saving!
To verify the flutter characteristics of the aircraft, a series of in flight flutter measurements were made. Two aerodynamic “shakers” were fitted to the wing tips, which could induce vibrations through a frequency range during flight. Again accelerometers were fitted at certain points on the airframe and the response to vibration input recorded for a range of flying speeds up to the maximum dive speed of the aircraft. The results showed the Slick to be flutter free to way beyond its VNE of 260mph.
Extensive wind tunnel tests were conducted at the CSIR on various aileron shapes and spade configurations. We have now optimised the ailerons so that they are "snatch free at high speeds and provide crisp harmonised controls without the "bobble" effect during rolls.
We are confident that the Slick is one of the strongest, lightest and most precise aerobatic aircraft available on the market today.