Tomorrow’s Plane Today

He envisaged advanced aerodynamics, with twin turbines driving a single pusher airscrew.

It was a bold, typically Learesque concept. The only trouble was the technology of structures wasn’t up to the idea at that time.

Lear pressed on with other projects; he designed the brilliant Learjet, went out of aviation for a spell then bounced right back in again with the long-range LearStar 600 – now modified, it’s under the brand name of the Canadair Challenger.

Bill Lear never was a man to let things die in that agile brain of his; in the mid-1970s the original idea came storming back.

This time there was a basic material from which to work – CFRP, carbon fibre-reinforced plastic.

A basic design was frozen in 1978, after a whole heap of teething pangs and poring over stress charts.

What came out was probably one of the most exciting aircraft ever built in terms of technology and sheer performance. Bill Lear died that year, a year after a project go-ahead was given for Tomorrow’s Twin. His passing put the company a year astern in developmental time, as it regrouped to search for financial backing for its wonder plane. But his work went on . . .

Although the builders had a marvellously pliable material from which to build the Lear Fan, they followed a basic structural philosophy, evolving a semi-monocoque with conventional spars, ribs, stringers and longerons.

LearAvia, the company building the Lear Fan, are already looking for something approaching the true mono concept, trading stiffeners for slightly more thickness; and this is eminently possible thanks to the amazingly light weight of CFRP.

Carbon/plastics form the major part of the Lear Fan’s structure, with aluminium alloy installed in high-strength areas such as undercarriage pickups and wing-fuselage joins.

Kevlar/Nomex honeycomb is used for the two pressure bulkheads and engine divide, baggage and engine compartment floors and control surfaces.

The big thing about this entire design was wind tunnel testing. By March 1978 no fewer than five different tail configurations had been tried on for size in Cal Tech’s tunnel.

LearAvia tried a V, an inverted V, cruciform and a conventional low tail with a fin – and a Y-tail.

The latter proved to be the most efficient in low drag, better stability and control, and propeller protection by the Y’s vertical member.

A crushable honeycomb bumper was fitted to take the impact should the Lear Fan be rotated to far beyond normal limits – the crunch figure was 11 degrees, double the landing flare at which point the big Space-Age Du Pont Kevlar quadruple-blade prop is still 3 ft from terra firma.

In the fuselage manufacture, frames are close together to take the high cabin differential (8. 3 lbs/in) at 41,000 ft.

Lear Fan has been designed for a 15,000 hr/15,000 cycle safe life.

The wings have been computer optimised from an aerofoil similar to the NACA 65 Series. It’s now what LearAvia technicians describe as relatively aft-loaded, with laminar flow being maintained over the front 40 per cent of the wing, a bonus in the lift/drag equation.

The airframe has been designed to limits of + 6 and – 3G, and to a safety factor of 2 instead of the normal 1. 5.

The entire concept is brilliant, and the brilliance really shines at the back end of this magnificent project. Two reliable Pratt & Whitney PT6-35Fs were installed in the basic design, powering through drive shafts to a rugged Western Gear 1500TBO transmission and thence to a scimitar-bladed 12-in-chord Du Pont Kevlar Hartzell-made four-blader. This replaced the initial Hartzell 90-in-diameter prop; the diameter remains identical.

LearAvia tells AUSTRALIAN AVIATION the prop has a pitch range from 28 to 65 degrees. There is no provision for feathering, as according to the manufacturers the chances of a complete loss of power to the prop are negligible. There’s no reverse pitch, either.

With prop speed limited to between 1850 and 1900 rpm, the new Du Pont airscrew does two things – it’s quieter than the original design and also increases the flow of cooling air over the gearbox.

When Hartzell built the Kevlar prop it took into account the high loading coming from two-engine power. In detail, it has produced a supercritical propeller, an airscrew capable of high Mach numbers; more than Mach 0. 8 over the prop’s outer part.

The LearAvia-designed prop’s trailing edge is almost straight, but the leading edge has a pronounced sweep over the outer part.

This, according to the figures, stops the entire leading edge from hitting the wake at the same time, reducing noise and vibration.

And LearAvia is already experimenting with structural and aerodynamic refining of the initial design. . .

With the basic design frozen, there was still one more change to make; fuselage skin panels were increased to seven from the basic four and this resulted in a one-foot stretch- if one could call it a bona fide stretch. And the entry door was shifted to the front of the cabin.

Testing went on apace throughout the entire program, LearAvia knowing it would have to break new ground in proving components and the overall structure before certification could become a reality.

Each of the Lear Fan’s 2000 components has been tested and re-tested for more than four years. A significant comparison of CFRP and aluminium as far as strength goes has been given in a specific instance; an aluminium stringer failed in compression at 11,700 lbs. A similar stringer made of CFRP failed at 21,300 lbs.

Problems are apparently a non-event when it comes to fatigue in the Lear Fan. LearAvia points out the almost unanswerable logic that fatigue begins in metal when repeated loads of more than 15 per cent of the ultimate strength are applied.

The figure for composite material is claimed as about 85 per cent, but the aircraft has been designed so the figure never approaches more than 75 per cent of ultimate strength.

There’s an admitted ‘but’ to carbon fibre’s performance in brutal impact. Where aluminium deforms plastically before complete failure, absorbing energy in a relatively gentle fashion, carbon fibre deforms elastically right up to failure.

Instead of releasing impact energy gradually by yielding. carbon fibre tends to store this up right to the failure point- then let the lot go in a violent manner which could present an additional crash hazard by having bits of carbon fibre flying around the immediate landscape at high speed.

This comparison will form part of the ultimate certification. And, like every other new type, LearAvia is prepared to put its product through an administrative wringer to prove its concept’s viability and reliability.

And it appears that Britain’s CAA will be hot on the trail of this crashworthiness when the time comes. . .

Delamination and bond failure are two other maybes, but LearAvia says that any damage should be easily detectable. At this stage, the entire aircraft is tested ultrasonically before leaving the Reno, Nevada, plant, showing up delaminations, air and resin pockets and air bubbles down to 0. 001 in.

Repairs in the field are a breeze, according to the Lear Fan’s makers. A minor repair is made in a similar fashion to that on a fibreglass boat, and a hair dryer can be used for curing skin repairs!

Other parts damaged enough to need replacement can be easily fitted, such is the acute tolerance demanded during construction. Optically-aligned tooling has achieved tolerances of 0. 002 in for the building of major sub-assemblies.

Lightning strikes have been catered for by the introduction of flame-sprayed aluminium in key points. Avtur and its effects on CFRP and bonding agents have been thoroughly tested and found to be undiscernible.

The whole program is an adventure. LearAvia’s Marketing VP ‘Torch’ Lewis has admitted that his company will “take a bath” on the first 20 Lear Fans sold, as they will have to be shifted at the original price now way down below current costings brought on by inevitable inflation.

Late last year a Lear Fan cost a cool $1,745,000 US, plus two per cent outside North America. Against that initial figure, LearAvia pitted some down-to-earth facts. Conventional twin turboprops have a specific range between 50 and 60 per cent of the Lear Fan. Lightweight structure accounts for about 50 per cent of the Lear Fan’s performance improvement. Composites usage has chopped gross weight to an estimated 65-75 per cent. The rest of the performance is achieved by lower drag – specifically, a very smooth skin with a scant number of fasteners or rivets, plus smooth jointing.

Laminar flow hanging on to at least the front 40 per cent of the wing; a high aspect ratio of 9. 5, resulting in low induced drag; low frontal area engine installation, and a higher operating altitude.

Asymmetries is another point where the Lear Fan shines. On most twins, one engine ‘dead’ means an 80 per cent performance figure gone with the wind. In the Lear Fan, the percentage loss is claimed to be only 62. 26 per cent. Nearest competitors to this figure, according to US Department of Transportation statistics, are the Commander 690A with 68. 66 per cent and the Mitsubishi MU2-J (68. 59 per cent).

Leading the ‘nearests’ is the Gulfstream II with 64. 94 per cent. But these are all just figures; what about the aircraft?

On January 1 last year, the first Lear Fan 2100 flew amid a blare of ballyhoo. And who wouldn’t make a noise about this type? After all, it was the first turboprop aircraft with an all-composite primary structure.

After the fanfare came the hard work; testing, compiling performance charts, and getting the act together to satisfy FAR Part 23 requirements in certification testing.

In a few short months the first-off had notched 380 mph in level flight, 460 mph in a dive, and climbed to 31,000 ft.

In test flights, it was proved that the centre of gravity movement limits of 15 to 35 per cent of MAC (mean aerodynamic chord) gave out good stability and control in steady and manoeuvring flight throughout the entire speed envelope.

Stall characteristics were classified as normal, with plenty of pre-stall buffets and conventional recovery. Stall entry averaged out at 72 knots and so far test pilots have reported a marked lack of wing drop.

The normal control layout featuring rod and cable linkage has proved to be just that – normal. Flaps have been installed on a simple one-piece hydraulically-actuated segment. The empennage may look ‘different,’ but the two V-layout surfaces operate conventional elevators, the downward-poking part of the Y proving to be a conventional-feel rudder surface.

Even on the first few flights, it was possible to produce significant figures. Range at one stage was carded at 2300 statute miles at 350 mph, carrying seven passengers in the upto-nine-seat cabin at Flight Level 410.

The twin-engine climb rate was initially given at 3500 fpm, with the single-engine climb rate given at 1900 tpm. Truly an impressive engine-out performance. The single-engine ceiling is listed as 30,000 ft.

The publicity blurb issued by LearAvia in Reno pulls some intriguing figures and statements out of the typewriter: –

It’s a single-pilot aircraft with a development cost of more than 100 million dollars US, empty weight is about 4000 lbs, and at economy cruise burns off 40 galls/US/hr (about 270 lbs).

That’s about one-third of most popular bizjets, one-fifth that of a GE-powered Learjet, and less than half of most turboprops – in fact about ten miles/US gallon, up to 90 passenger miles per gall/US. In fact, about the same as a family automobile.

Union Carbide’s ‘Thornel’ carbon fibre composite is used for just about everything including the seats, giving twice the strength of aluminium at half the weight.

Amazing how facts can be made to sound so exciting; but the Lear Fan is undeniably the biggest thing since sliced bread, a giant leap for aviation.

With its unique appearance and the fact that Northern Ireland is deeply involved as part of Britain’s deal with LearAvia, in the project, it wouldn’t surprise AUSTRALIAN AVIATION in the slightest if someone nicknames the Lear Fan the Irish Aeroplane!

Back on the test-flying scene, the decision to instal a glider type reflex upward flap setting of minus five degrees is a good one in practice, as it reduces drag and attitude in the cruise configuration.

Controls have performed normally, with the ‘butterfly’ elevators biting satisfactorily as the Lear Fan approached takeoff knots. There is apparently only a trivial torque swing when power is applied for the rollout, taking into account that the scimitar-bladed prop has to absorb about 1300 shp.

We haven’t yet sighted Manual’s handling notes, but it’s pretty certain that there will be a high Vie, the undercard acting as a speed brake on what could otherwise be a rapid descent.

For the pro, avionics are conventional with Rockwell-Collins navcoms, a Sperry SPZ-650L autopilot/flight director, Collins WXR-250 weather radar and the bigger WXR-300 available as an option, as is the Global VLF/Omega.

For the salesmen among our readers, LearAvia has more than 260 Lear Fans already sold, including no fewer than 34 last November.

The second Lear Fan of the line will be the actual certification aircraft, and partial certification is due in January next year.

The firmest information on production is that by the mid-1980s up to 30 Lear Fans will come off the line each month.

This figure could of course improve as more streamlined techniques on handling and forming a new material evolve.

And there’s a lot to learn about making what has now become known as a plastic plane . . .

Your Lear Fan starts life as rayon, run on a hot mandrel to make the carbon fibre, which is eventually woven into ‘cloth’.

The fibre has to be kept cool until ready and is usually transported amid dry ice. Tops at room temperature is 21 days before it starts curing itself and becomes brittle and unusable.

As can be anticipated, autoclaves form a significant part of the curing process once the component is formed by layering with up to four layers of ‘cloth’.

Curing is normally a four-hour job at 90 lb/sq. in. The initial hour and a half of curing is done at 120 C, with 177 C taking up the remainder of the time.

Lear Fan Ltd in Belfast already has more than half its gigantic factory in operation, and by the end of this year hopes to have three factories in full production with its batteries of autoclaves.

For the record, the first 42 Lear Fans are to be assembled in Reno, with the proportion of Ulster-built parts building up to 100 per cent with the 43rd Lear Fan due in September next year.

From that time the Lear Fans will be made and assembled entirely in Northern Ireland and flown to the US for fitting-out.

As one of the possible under-the-microscope parts to be given the treatment during certification trials is the power train let’s look at how it works.

The two PT6s are separated in their aft mounting position by heat-blanked compartments able to withstand a 1093 C fire for 15 minutes before the composite structural material begins to lose its strength.

The two turbine engines are coupled by drive shafts by Bendix, made of aluminium alloy sheathed with a graphite/epoxy combination for stiffness. They have flexible couplings on each end to permit angular misalignment.

If a coupling failure occurs, anti-flail bearings stop the shafts from whipping. Each shaft coupling is tested to a maximum ten million cycles under torque and bending loads before installation.

The actual transmission is designed, as mentioned earlier in this article, by Western Gear for a 1500-hour TBO. It’s a simple, rugged arrangement, with pinions in the two shafts meshing with a single bull gear. Both shafts are connected to their driving pinions by a sprag clutch which can only transmit torque in one direction – rather like a ratchet. So there’s just no way a failed engine can create any drag on the ‘good’ engine.

An engine torque runaway is cancelled out by a shear pin at the clutch end of each shaft, which disengages the shaft if it’s overloaded.

Chip detectors warn of any metal slivers in the gearbox. LearAvia already has its own answer ready for the gearbox reliability knockers; it simply compares its gearbox to that of a twin-engine chopper – which has to be just as fault-free yet operate in a much harsher environment.

The Lear Fan’s gear ratio of 3:1 compares more than favourably with a helicopter’s typical 30:1 ratio. The chopper’s transmission must support the entire weight of the machine and give the thrust as well. Lear Fan’s gearbox operates at less than half its design load most of the time. . .

The nub of the whole thing is that if something goes ‘sproing’ down the back end of the Lear Fan in the engine compartment, it must not be allowed to damage the rear fuselage.

So LearAvia have grasped the nettle and planned a test of tests – the running of a gearbox with some of its teeth deliberately removed, to demonstrate its confidence in Western Gear’s design.

Be all this as it may, from where we sit it looks like a laydown mizzaire that the Lear Fan will get the certification it needs. It’s already well into its hot and high trials, de-icing trials are a natural, and unless something goes radically wrong with the world of professional aviation, the aircraft looks like becoming a part of every nation’s scenery as the Aircraft of Tomorrow which arrived today.

All the line drawings, cutaways and in-flight pictures we’ve so far seen make one thing apparent – and one can be sure that Bill Lear, wherever he is right now, is working on it. And that’s the plain tact that the propeller is the only thing breaking the lovely lines of this flying orange pip.

It took courage, a deal of travelling winning money from here, there and even everywhere to prove one’s faith in a product one believes in. . . but LearAvia have done just that. They have gone in boots and all, muck or nettles, to prove their faith in something both naturally beautiful, beautifully natural and economically, operationally a sure-fire winner.