The Lycoming O-540 Engine and Why We Use It

One of the questions we’re often asked about our Robinson R44 helicopters is what type of engine they use. Many helicopters used for pleasure flying, aerial work and other types of flying are often turbine powered. The Robinson R44 has a piston engine, as does our famous Hiller film star helicopter. On hearing this, car enthusiasts in particular become very interested in how the Lycoming O-540 engine we use compares to their own car engine. They’re often quite surprised when the hear some of the figures.

So here is a short rundown especially for car enthusiasts, with some interesting facts on why the Lycoming O-540 engine is built the way it is, why it’s used in aviation, in Robinson R44 helicopters particularly and why its difficult to make a direct comparison with a car engine.

At first glance, the performance statistics of the O-540 are rather underwhelming, compared to a car engine. It’s a 540 cubic inch, or 8.9 litre, air cooled flat six engine, but it only revs to 2,700 rpm and makes just 220 bhp. This is probably the number one thing that surprises people more used to the statistics from a conventional car engine. To have an engine which is almost nine litres in capacity, yet only develop less that 250 bhp seems incredibly inefficient. After all, a similar flat six engine in a modern Porsche 911 develops more than 300 bhp from just 3.4 litres. Modern 4.0 litre car engines can make 500 bhp. Even a family turbo diesel 2.0 can make 200 bhp.

Lycoming O-540 - industrial strength engineering
Lycoming O-540 – industrial strength engineering

It also has a carburettor. We stopped buying new cars with those on a long time ago. OK, there is an injected version in the R44 Raven II available too, but most of the Lycoming O-540s in the world have carbs.

It burns what seems like a lot of fuel, with lead in it. The Lycoming engine, like most aviation piston engines, burns 100LL fuel. While the lead content is far lower than automotive fuel used to be, it does still contain some, due to the age of the engine design. It also burns around 14 gallons of fuel per hour.

So why do we operate an engine like the Lycoming 540 when in automotive terms it may appear at a glance to be an antique?

Well firstly, it’s actually difficult to make a direct comparison to car engines. While Lycoming aero engines and car engines may be both piston in design, they’re developed to run in totally different ways. A car engine will have a gearbox with different ratios, either manual or automatic, so the engine is rarely at the same RPM for long, ranging up and down the rpm quite a lot.

An aircraft engine is designed to run at a single power setting almost constantly, very much like a generator. Robinson R44 helicopters need lots of torque to turn those 33 foot rotor blades and drive the tail rotor and the Lycoming has lots of that, almost 800 ft lbs. We need that torque low down the range so that we can lift from small clearings and climb at more than 1,000 ft per minute if needed. An engine like a car’s which may well rev to beyond 7,000 rpm is of limited use to helicopter pilots.

High revving car engine, or big torquey aviation engine?
High revving car engine, or big torquey aviation engine?

So as we said at the beginning, it’s difficult to make a direct comparison. While the engine may burn 14 gallons per hour, consider that in a Robinson R44 we cruise at 110 knots, which is just under 130 miles per hour. This works out very roughly at around 9 miles per gallon.

That may sound pretty poor, but if you consider that taking a car of equivalent value, say a Bentley Turbo and drive that at 130 mph, it will probably be returning similar figures. Then add in the fact that we are flying in pretty much a straight line, with no roadworks, speed cameras, congestion or detours and we can cover ground very quickly. Journeys that in a car may take a frustrating 120 road miles and two hours can be flown directly ‘as the crow flies’ in as little as 60 miles and 35 minutes in a helicopter. So while we may be burning a lot of fuel, we’re not doing so for anything like as long.

We are also very efficient with weight. A Robinson R44 Raven with full fuel, before passengers, weighs just over 700 kg. A typical Bentley or similar will weigh over 1,800 kg. So while we may not have the most advanced engine design, what we do have, we use very well.

But with modern technology, why are we still flying so many helicopters and fixed wing aircraft with these engines? After all, if you could fit some of the modern automotive technology to our Lycoming, just think what could be done? To a very large degree, it comes down to economy of scale and the sheer cost of certification of aviation engines.

Developing a new engine in aviation takes, literally, decades. It’s is a hugely expensive process, with multiple authorities across the globe that all have to be satisfied that your engine is safe and fit to be flown overhead the general public. Only then can you even start to market it to the aviation world. Unlike airliners, the market for general aviation engines is very, very small. Our engines last for decades if well maintained and can be rebuilt to fly again and again. The chance to sell a new aviation engine doesn’t come along every day.

Engine reliability is paramount in aviation, with no breakdown service at 1500 feet
Engine reliability is paramount in aviation, with no breakdown service at 1300 feet

And finally, you have aviation safety. If your car should break down, you’ll be stuck on the hard shoulder, frustrated and waiting for assistance. Aviation engines must have multiple levels of redundancy built into them by law. Things that would stop a car engine may present a problem for our Lycoming, but it will still keep flying. Here are some examples.

We have two completely separate sets of spark plugs and ignition leads. Each system can supply a spark to continue flight if the other should fail. We use magnetos to generate that spark which means that we can continue to fly, if needed, without an alternator, should our generator fail. These are both elements that would result in a car engine breaking down.

Our engine is air cooled. Very similar to a classic Porsche 911 engine, air is drawn in and around the engine by a rear mounted fan and then the warm air is expelled to cool the engine. We have no liquid cooling radiators.

It’s a pushrod engine with only two valves per cylinder. No overhead camshafts or multi valve cylinder heads. Cam chains and cam belts are forbidden in the world of aero engine certification. Belt and chain driven engines can snap and lead to failure, so any overhead valve systems must be gear driven. The chances of a push rod engine failing totally through valve train failure is remote. Even if a pushrod should fail, the rest of the engine will probably still keep running, even if it sounded pretty rough, enabling us to land safely.

So while the Lycoming O-540 engine may seem a little antiquated, there are both economic and regulatory reasons for this. Yes, it would be wonderful to take our Lycoming engine design and incorporate some more modern technology, but that’s unlikely to happen any time soon.

However, one thing that car enthusiasts do agree on. The sound of a big 8.9 litre O-540 barking into life is a pretty sweet sound.