This Leopard claims a new spot

DN Staff

February 20, 1995

8 Min Read
This Leopard claims a new spot

Herts, England--Using a suite of software packages, engineers at Chichester-Miles Consultants Ltd. (CMC) are designing a four-seat jet that will travel at 500 miles per hour. The company says the plane, which it calls the Leopard, will be the first production jet of that size.

Critical to the plane's design is the extensive use of engineering software. Among the programs playing a key role: Algor, AutoCAD, and NISA II.

A marketable idea. During his 30 years in the British aerospace industry, Ian Chichester-Miles, chairman and chief executive of CMC, witnessed the development of jet airplanes ranging in size from large aircraft to executive jets. In 1980, he decided to take jet propulsion technology one step further by applying it to a basic four-seat plane.

Armed with several pencil drawings, he set out to create an airplane nimble enough to get in and out of small airfields easily, yet powerful enough to achieve full-scale jet performance. His goal was to fly 500 miles/hour at 40,000 ft or above.

But meeting those requirements wasn't enough. Chichester-Miles also wanted to ensure a high comfort level for passengers. That meant reducing noise and vibration, and providing sufficient space-features that are difficult to incorporate in small planes. "You can't just scale down a big airplane unless you are designing to carry babies," he says. "People don't just change size as they go from big aircraft to small ones."

To reduce internal noise, Chichester-Miles designed the Leopard with the engines placed at the rear of the plane, well behind the passenger cabin. But he still needed a compact airplane configuration that could accomodate four people and the necessary operating equipment at a low cost. The solution: model the plane's interior in a style similar to a high-performance sports car.

The influence of automobile designs does not stop there. CMC looked at environmental control systems on several large aircraft to determine how they could be simplified and made less costly. Typical air cycle systems found in large aircraft are both light and efficient, but are also very expensive. The final decision was to use a fluid cycling system similar to those used in cars.

Back to basics. The Aircraft Research Association Ltd. (ARA) in Bedford, England, was responsible for the 3-D shaping of the wings on the original prototype aircraft (001) for CMC. ARA used their in-house 3-D Full Potential Flow method to create an aerodynamic design; while the structural design was done largely by hand. Mainly a crude, proof-of-concept prototype, the 001 was first flown in December 1988 at the Royal Aircraft Establishment in Bedford, England. The plane completed some 100 test flights with only one real problem emerging: a pitching instability that was easily fixed by changing the pivot axis position of the horizontal stabilizers.

Since then, CMC has been hard at work updating and redesigning the Leopard. The new version has an increased payload and much higher performance than the 001, and it is pressurized. This means engineers can no longer rely on hand calculations to achieve the highly efficient and reliable structural design they need. As a result, engineering software has played a crucial role in the design process.

Engineers used Algor's Linear FEA software with Composites add-on to do a stress analysis of the Leopard's stabilizers. Chris Burleigh, chief designer at CMC, needed to know the distribution of the internal loads among the ribs and spars of the stabilizer structure. He was also interested in the overall bending and torsional stiffness of the stabilizer, as well as the magnitude of the local bending effects in the skin panels due to the distributed pressure loading.

Since the stabilizer's structure is mostly stitched carbon-fiber fabrics in an epoxy resin matrix, Burleigh chose to analyze the area as orthotropic plates, knowing the laminate properties could easily be calculated by hand. He calculated the thickness and modulus of the plates to give the same membrane and bending stiffness as the equivalent composite. He obtained the panel bending by separating panel membrane loads and bending moments, and correcting for each layer of material.

The analysis of the roof of the pressurized passenger cabin also required the use of Algor software. "By this time I had gained enough confidence in Algor and my ability to use it to go straight into the Composites modeler," says Burleigh. He put the roof's honeycomb sandwich panels with glassfiber/epoxy skins into the Algor model as thick sandwich elements, using the actual properties of each layer. A simple laminate analysis provided stress output that indicated areas of weakness. Burleigh eliminated these areas one by one by adjusting the skin and core construction until he was satisfied with both the deflections under load and the stresses.

He also used Algor to model solid components and combine them with composite element models. Case in point: A large fitting at the inboard end of the wing flaps that is used to transmit the flap actuator load into the flap structure. The flap itself was represented by thin composite plate elements, and the fitting by solid brick elements. The models were constructed by creating the geometry in Algor's Superdraw and then pulling out each component to create separate, smaller models. Burleigh decoded and recombined each of these models. "This made the models and their input data easier to handle. It also allowed modification and deletion of pieces of the model without interfering with the whole thing," he explains.

Surface modeling steps in. When designing the engine pods at the rear of the aircraft, the goal was to minimize the frontal area by fitting the cowlings as closely as possible around the engines. Burleigh was equally concerned with providing a smooth streamlined shape and incorporating an "S"-shaped intake duct.

The engine and pod were first drawn by hand to scale. Burleigh constructed the basic cross sections and lines in Algor's Superdraw as arcs and circles, and converted them into smooth closed NURB splines. After dividing the closed splines and deleting one half, he transferred the remaining parts to Algor's Supersurf and created a NURBS surface over them. He then used a mirror image to replace the missing half.

Burleigh checked the clearance above the engine by observing cross sections at strategic points along the pod surface. He modified splines and constructed new surfaces to attain the 0.25-inch clearance required at the critical points. Several cross sections were plotted full-size so a "negative" mock-up of the available space could be created in wood. When the mock-up showed modifications were still needed, Burleigh reconstructed the surface and adjusted the model. He made prototype tooling directly from the full-size plots by sticking the plots to fiberboard and cutting them out with a bandsaw. "Overall," he says, "Supersurf has proven to be extremely useful."

Carlton Matthew, an outside consultant to CMC, conducted an analysis of the wing structure using NISA II FEA software from Engineering Mechanics Research Corp. (EMRC). He created a complex model of the wing using thin composite plate and orthotropic composite brick elements. Matthew ran several analyses to develop the design and to establish correlation with prototype test results. Eventually, he created an entirely new model for the wing design.

Back at the drawing board. Electrical and avionics designers at CMC use AutoCAD LT to produce wiring diagrams and drawings for sheet metal brackets. Burleigh notes the package has been very cost-effective and is adequate for even fairly complicated components.

Meanwhile, other engineers have been using AutoCAD Release 12 to produce most of the 2-D drawings for the Leopard. The company claims its techniques are not very advanced because it doesn't use solid models or complex assemblies, but that hasn't been a problem. According to Burleigh, "With AutoCAD we can produce good working drawings very quickly, and modify them if required without delay." The best part, he says: "We're doing it without a huge investment in equipment or manpower."

Moving forward. The 001 prototype featured two low-power NPT301 turbojets from Noel Penney Turbines (NPT) Ltd. in Coventry, England. The converted missile engines were intended solely for the 001, and NPT was slated to develop new engines for the final version of the aircraft. But when NPT went bankrupt, the entire Leopard project was delayed for one year until CMC teamed up with Williams International Corp., Walled Lake, Michigan.

Williams developed the FJX-1 engine for CMC's first production-configured aircraft, the 002, now under construction at the Designability R&D Center in Dilton Marsh, England. Rated at 680 lb-thrust, the FJX-1 has more than twice the power of the NPT301. That's okay for the 002, but it's not quite enough for the final aircraft. Williams is currently researching the proposed 1,000 lb-thrust engine for the final jet.

Material sacrifices. The 002 is made of all composite parts, mostly carbon or graphite epoxy. To keep tooling costs down, CMC is using low-temperature curing resins instead of high-temperature ones. But not without penalty: the efficiency of the structure in terms of strength-to-weight is not as good as with high temperature resins, so the plane weighs more. Still, the company says the ratio is better than with the equivalent metal structure.

The Leopard seems to offer everything a business traveler could want: speed, comfort, and the convenience of a personal plane. But what about safety? Chichester-Miles insists the Leopard will be just as safe as any other properly certified aircraft. In fact, he says the plane's design parallels that of larger executive jets, right down to its standard anti-icing system for the wings and tailplanes.

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