A follow up on The best engine for the job?

By Alec Bender

The engine is, as Bob Stephens says, the most important part of any aeroplane, after all it governs the performance of any given aircraft and at the end of the day your life depends on it. So you want performance, economy, availability of spares and above and beyond all else total reliability. This being an island nation, at some time you are going to fly over water, now if you are apprehensive about this you want the very best engine you can get and not some cheap old engine that seemed like a good idea at the time.

I think firstly from Zodiac News’ point of view, is what model are you building, because the basic 600 series design revolves around the VW engine which was not really satisfactory, but it does give a yardstick for engine weight and C of G. Now if one is building a 601 HD you have a wider choice of engines than the HDS, but the VW is a non-runner because of lack of power, and if you stretch the VW or otherwise tune it the reliability deteriorates dramatically with pulled studs, blowing joints and overheating.

However the Limbach 2000, does give the power and reliability and is half the cost of the Rotax 912. The Limbach achieves reliability by having new castings, new design, new metallurgy and liquid cooling which also prevents shock cooling to the cylinder heads when descending with the engine idling. This engine will probably be acceptable to the PFA if fitted to the HD, but not to the HDS because of its higher stall speed, and because of the single ignition fitted to the Limbach.

The scene gets more complicated still when the Lycoming or Continental is considered, because again we are back to very old technology and although nobody could question the reliability, these engines are large capacity, relatively heavy and thirsty. The weight is probably high for the Zodiac nose leg and a considerable mass on the front of a very light aircraft. The added work and expense of magnetos over electronic ignition has to be considered and the fact that these engines are certified, limits the amount of work you can do yourself - legitimately. The attraction of course is that they are available second hand at a cheaper price, however, caution is the watchword here as knowing the history of any second-hand engine is vital.

The Subaru 100 hp engine I think is very good - most Japanese things are well engineered, the only problem is that as far as I know, the dual ignition systems both work through a single plug per cylinder. For this reason it may be acceptable to the PFA for the HD but again not for the HDS.

I am a two stroke fan, but I have changed my view on their use in aircraft, mainly because they have a bad reliability record. I think the fact that there is no ‘wear’ indicator i.e. the oil pressure gauge, you get no warning of impending disaster and things usually go bang in a big way. On the subject of wear indicators, its other use is on extending the life of a ‘permit’ engine after TBO time has been reached, so long as oil pressure is good and compressions are good your inspector can extend the engine life.

I was impressed with the Mid West Rotary (Ex Norton say no more) when I saw it at Wroughton. However, on investigation the rotor cooling and oil recovery system seemed an added complication. The engine though is modern with reduction gear and turbine smooth it would be nice to see one in an HDS, but I do wonder about the future spares situation. People with ARV Super 2 fitted with the Hewland engine are now in a bit of a fix and I wonder if more thought about engine choice would have been wise.

The Rotax 912 is a modern engine that I suspect owes a lot to the original 1200 cc VW in that it is very compact, very smooth and very economical. Technically though its is fairly advanced with hydraulic tappets, electronic dual ignition and a reduction gear with shock load protection. Full glycol cooling allows higher than normal temperatures and full thermal efficiency, plus, in the Zodiac range no need for carb heat (so long as you don’t overcool it 90 - 110 oC is the range quoted). For doubters of reduction gears I would just quote the most famous aircraft engine of all time THE MERLIN and say no more!

The efficiency of an engine and propeller combination is the ultimate test for an aircraft installation and the slower and larger the prop is the better. Now the engines with a reduction gear score in this respect because they produce maximum power but keep the prop revs down and are thus more efficient. Tricycle aircraft allow a larger prop arc because the tail isn’t coming up and again helps in the direction of efficiency. The propeller turning at crankshaft speed must not allow the prop tips to exceed (or get close to) the speed of sound, so this restricts the diameter of the prop and the prop arc on engines that rev.

Thus the ultimate test of efficiency - that is taking off fully loaded from a short strip is a large and slow revving propeller.

Two stroke fuel economy is always worse than a four stroke because it is firing twice as often, and with less precise ‘valving’ and thus some fuel wastage. Also included in fuel consumption figures is a quantity of oil lubricant.

Thus I hope I have provided some food for thought on engine selection. Armchair, brochure perusal in the depths of winter seems a million miles away when you line up for your first take off in a new aeroplane with your new engine and prayer mat induced knee shake. Imagine how you will feel about flying over large amounts of water, and then reflect, there is usually no cheap panacea for good economy, good efficiency, good reliability and PEACE OF MIND! The final choice of engine may not be your first choice but more a compromise against:

Which aircraft you are building. What the PFA will approve. What you wish to pay. Length of strip. Taildragger or tricycle. Reduction gear or not. Propeller choice. Fuel economy - range affectedType of flying you wish to do.Weight of engine.New or second-hand.What’s available.2 stroke or 4 stroke.Dual or single ignition.PEACE OF MIND!

I use a Rotax 912 with a warp drive carbon fibre prop (66 inch) in my HDS and I am well pleased with it on all counts. Safe Flying - Alec Bender.