Gipsy Engines and the Art of Propeller Swinging

Brian Lecomber leads you through the everyday life of a Gipsy Major engine from before starting to shutting down.

PRE-FLIGHT

Starting the inspection on the left rear of the engine, you will find that the throttle and mixture rods emerge from the firewall and link on to a cross-shaft arrangement with additional bellcranks which move the magneto cam-rings and thus advance the spark as the throttle is opened.

Check the little ball joints and so on for wear since it can become embarrassing when they fall off. Also check the various housings on the back of the engine for oil leaks; this may sound fatuous since a lot of Gipsies look like one big oil leak at the back anyway, but take a look nonetheless; you can always get lucky (unlucky?) and spot some crevice which is oozing more than usual.

Moving forward along the engine, check the crankcase top cover joint for leakage. If this starts to weep it must be looked at, since the next stage might be a cracked crankcase or cover. Look also for little wisps of oil-like hairs on the case and cover castings; these can be the first indications of a crack starting. Another common fountain-head for external lubrication is a weeping front crankshaft seal, the first indication of which is usually an oil stain on the back of the propeller blades near the bub.

Moving down the cylinders, check that the cooling duct cowlings are intact and then squint down inside the duct, where you will often find a rich film of oil and dead beetles. This does not matter (the oil being general seepage from the pushrod tubes), but do look down the line of the head/barrel joints to see if there is a shiny black area of shellacked oil around any of them: this is caused by a blowing head joint and should be dealt with forthwith.

Reaching the right hand side of the engine, make the same checks for blowing head gaskets, etc, and also check the inlet and exhaust manifolds for hairline cracks, loose securing nuts, and obvious signs of corrosion in the vicinity of the hot-box around the vertical bit of the inlet manifold.

Moving up to the carburettor, have a good goop at the bitty little top steady bracket joining manually same to the crankcase. If this slackens off - which they do tend to do – you are headed for a kit-form inlet manifold. Check the movement of the warm air flap and look for slop where the flap spindle pivots in the carburettor casting. This latter is a common wear point, and a switched-on Gipsy engine man will usually bore the holes out and make up a couple of bushes for the spindle. Look also for slop in the throttle linkage bellcrank behind the carburettor. This is another common malady, and as the bellcrank develops more and more sideways rock, it will begin to stop you getting full throttle.

Moving to the right rear of the engine, you come to the oil filter. This has (or should have) a T-handle sticking out of it, which should be turned through 360 degrees before the first flight of the day. This can easily be done by hand or even better by using a special tool designed for the purpose.

Then, finally, it is just a question of topping up the oil. With its inverted cylinders the Gipsy Major's lubrication system has to be partially total loss, so that you can expect a normal oil consumption of between a pint and a quart an hour. Since the engine tends to breath out the first pint or so fairly rapidly, I generally keep the oil level about a pint below the full mark once it has warmed up. If the pistons have received the ‘ring mod’ expect the oil consumption to be a great deal less.

Right then. Having pre-flit the device, the next thing to think about is starting it. Although some Tiger Moths have been fitted with electric starters, we shall gloss over that convenience and concentrate on the subject of hand-swinging the propeller.

PROPELLER SWINGING

The first points are basic ones of safety. If you are elected for prop-swinging you should

(1) make sure you are not wearing any loose clothing such as a tie, a neckless, pendant or an open shirt or jacket which could catch in the fan

(2) park the aircraft in such a position that you will not be standing knee-deep in a cow product, puddle or on a slippery surface when you come to do your stuff

(3) take hold of your pilot and point out firmly that since it is your ass in a sling if anything goes wrong, you are the one who is going to command the operation during the starting phase.

You also make it ever so clear to the pilot that there is no such command as "On". Magneto switches are either “Off” or “Contact”. You can take it from one who has nearly acquired a three-piece head on a couple of occasions that the "on" is much too easily confused with the word "off” when there are other noises going on around you.

Chocks should be placed under the wheels with the ropes laid out so that when you pull them away they are "twisted" out: if you arrange them so that the rope gives a straight pull you may end up having to grovel around on your hands and knees if they get stuck. If you have both chocks on the same bit of rope they ought to be removed from the left side of the aircraft and, as is my own practice whenever pulling chocks away, always to keep one hand or an elbow or something in contact with the wing as I walk in to get them. If you make a point of doing that there is no way on most aircraft that you can absent-mindedly stick your swede into the propeller. You might bean yourself on the pitot head, of course, but nobody is perfect...!

Hand Swinging

Now to the actual swinging.

It is best to stand with your feet apart, your right foot parallel to the centreline of the fuselage and your left foot roughly parallel with the propeller. Park yourself quite close to the centreline and quite close in to the propeller arc. The commonest mistake swingers make is to stand too far out, which has the undesirable effect of tipping you forwards as you swing. Then, when you take hold of the propeller, reach out with your right hand only and cop hold of the blade as far out as you can reach. It does not matter if you have to stretch a bit, because as soon as you start the swing you will be coming back into balance.

Grip the trailing edge of the blade with as small a fingertip purchase as you can get away with. In text-book fashion, in fact, you should not wrap your fingers round the blade at all, the thinking being that if you have got a big fistful of propeller and the engine kicks back, you are going to end up with a short-fingered hand. “All the power of the swing”, says The Book, “should be transmitted with your hand flat on the front of the camber”. However, not being a gorilla I have never managed to swing a propeller with the flat of my hand, and I do not know anyone else who has; so what it boils down to is simply to grip the blade using as little of your fingers as possible. Anyway, from that starting position you are well set up for a hefty, safe swing.

The movement is like a golf swing, with the weight of your body doing the work rather than your arm muscles. As your arm comes down you are shifting your weight from the right leg to the left and at the same time twisting your body away from the propeller as you 'follow through'. Try to avoid dancing away each time at the end of the swing but do not worry if you end up taking a step or two back now and then. The main thing is that your own inertia is at all times taking you away from the filleting machine and not into it.

Engine Starting

Now to the start itself. If your Gipsy engine is cold, then it will first need priming.

You (the propeller swinger), bawl out:

"Fuel on, throttle closed, switches off, priming."

The pilot responds, and at the same time sticks his paw out of the cockpit with his thumb down as a confirmation that the magneto switches are down and “off”.

Tiger Moths are primed merely by pressing the tickler button on the carburettor. Continue priming until fuel pees out of the manifold drain pipe, and then wait for it to stop running before doing anything else.

When the peeing stops, lock the cowlings and move round to the front of the propeller and sing out:

"Throttle closed, switches off, sucking in."

The pilot responds, still with his thumb down, and you pull the fan through four compressions to suck the mixture into the pots. In spite of the switches being “off”, treat the propeller exactly the same as if it were live - because one day, when a switch lead falls off, it is going to be. Quite a lot of people over the years have contracted flat heads by winding a propeller round too casually when they thought the magneto switches were “off”.

During the sucking-in bit you will, of course, be able to check the compressions. Some variation in a Gipsy Major compression is acceptable, but if one compression is missing altogether - a not unusual circumstance - then you will have to decide what to do about it.

The cause of a total compression loss might just be a sticking valve, and the pundits will tell you that the easiest solution to this on a Gipsy engine is to shrug philosophically and start it up on the remaining three, whereupon ninety nine times out of a hundred the offending valve will immediately unstick all by itself and report for duty. This theory quite often works, but it does rather overlook the fact that having stuck open once, the valve may quite possibly elect to repeat the performance again, at some other time, perhaps in the air. It is not very likely once the engine is running, but it has happened and so the possibility must be considered. I would certainly refuse to fly behind a Gipsy engine which had a recent history of repeated sticking valves on start-up.

Having sucked in, you are then ready to go. You call:

"Throttle set, contact".

The pilot responds, switches on the impulse magneto only, the forward switch, and sticks his grubby thumb up. You then give the propeller a swing-with-intent and the engine should start, at which point the pilot switches on the other magneto.

If the engine fires but does not run, and the propeller stops in its normal position, you simply try again. But, if the propeller stops in the wrong position, straight up and down, for example, then call for the switches to go “off” while you re-position it for another go.

Never 'bounce' the blade against the compression before you take your swing. Apart from the obvious risk of a kickback, you will lose some of the compression past the piston rings so that the engine will not start on that swing anyway.

If, after several swings the engine still remains mute, call "switches off” and pull the propeller slowly past a few compressions.

At each maximum pressure point there should be an audible click from the right-hand magneto as the impulse unit works and, if that click is not there, you are suffering from the very familiar malady of a sticking impulse unit. Your hairy old Gipsy men can hear the absence of that tell-tale click from a hundred yards away, upwind.

One solution to the problem is to whip off your starboard shoe and clout the magneto smartly with same in order to jar the impulse until free. This usually works but socking the magneto with a beetle-crusher can hardly be very good for it, and anyway it creates a bad impression among the bystanders. The more civilised answer is to locate the actual impulse casing itself, which is the round alloy-coloured thing between the magneto distributor cap and the crankcase, and then tap it gently with a stick as if you were hitting a billiard ball. Two or three taps and you will usually hear it click free. If it does not free itself, or if it sticks repeatedly, then do the gentlemanly thing and howl for an engineer; the unit either needs lubricating (a special process) or the magneto needs to be slightly re-positioned on its base to achieve the correct amount of end-float on the drive dog.

With the impulse magneto working properly and the cylinders fully primed, a cold Gipsy Major engine will usually start quite easily. An engine man will confirm that with fuel and a spark meeting in the right proportions at the right time, there should be no other result. The same will apply to a stinking hot Gipsy Major, one which has been shut down just a few minutes before. In that case you do not prime and you do not suck in; merely set the throttle a tiny crack and hit it with a live swing straight away.

The time the bloody thing will not start is when it is neither hot nor cold; as when it has been standing for half an hour or so. Then you never quite know where you are: the mixture might be too rich, it might be too weak, or it might be simply staring up into the sky with its mind on other things. When this embarrassment occurs then every Gipsy-powered pilot in the world has his own theories of what to do about it. Some prime it but do not suck in; some suck in without priming; some assume it is flooded and blow it out, and still others give up and smooch off for a cup of tea.

Most of these methods work some of the time with engines, but I have only ever found one method which can be more or less relied on to work all of the time on all Gipsies. And that is to prime, suck in four swings, and then to immediately clear the resultant flooding by 'blowing out'. Call for the magnetos to be “off” and the throttle wide open, then turn the propeller backwards (so that the impulse magneto does not work) over eight compressions. Then crack the throttle and give it contact and the engine should start first go. Well, second go, perhaps ...

After Starting

Once the engine has fired and is running, the oil pressure should come up within 30 seconds. On a winter’s day a stone-cold Gipsy Major may take four or five minutes of ground running at about 1,000rpm before the oil is warm enough for takeoff. You can tell when you are doing the magneto check if the oil is too cold, by the fact that the oil pressure will reduce (probably due to cold oil cavitating in the pump) as you increase the rpm. The magneto check is done on most Gipsies at 1,600rpm, by which speed the oil pressure should be a minimum of 30psi; the maximum ‘mag drop’ is usually quoted as 100rpm, and the maximum differential 70rpm.

THE GIPSY ENGINE WARM AIR SYSTEM

The warm air system could use a few words of explanation. The difference between that on a Gipsy Major and a flat engine's hot air arrangement is that most flat engines take their carburettor heat from a hot-box around the exhaust. This means that when you select ‘heat’ you really do get hot air which is why you get such a big rev-drop, of course. You need this on a flat engine because the carburettor is a long way from any big-time source of heat and is, therefore, very prone to icing. In a Gipsy engine, however, the carburettor sits bang alongside the cylinders, so icing is fairly rare in the first place. Perhaps for this reason, de Havilland elected not to incorporate a hot air system at all.

The Tiger Moth's automatic ‘warm air’ system works by staying in warm air for approximately the first two-thirds of throttle movement, and then moving into cold air as you open up for full grunt. If you think you are getting icing in a Tiger Moth, what you do not do is gradually open up the tap to compensate for the rev-drop: the method is to throttle back a bit, say to about 1,800-1,900rpm, to make sure you are firmly in the warm air range of the throttle movement.

Maximum drive rpm varies slightly according to the Mark of the engine, but if you are stuck for a guideline use 2,350rpm for all of them except the Gipsy Major 10-2, which can go up to 2,650rpm. Comfortable cruising revs are likewise 2,100rpm for most Marks and 2,200rpm for the 10-2. Tick-over should be 500-700rpm, and you want to keep the normal ground running speed up to 1,000-1,100rpm to stop the plugs oiling up. If the plugs do oil up regularly, you probably need a new set of piston rings.

AEROBATICS AND OIL USAGE

During aerobatics the Gipsy Major will survive upside down with no oil pressure for quite incredible lengths of time. I limit myself to 15 seconds at a stretch, but opinions do vary. One thing you do need to watch when doing aerobatics, though, is your oil consumption or, to be more accurate, the quantity of oil the engine chucks out.

Most Gipsies have a breather on the rear of the crankcase top cover; during most manoeuvres this has no particular significance, but when you start doing long-held inverted 45deg climbs for Reverse Cubans, for example, this breather suddenly becomes the lowest point in the 'sump'. And then bingo! From using a litre or two of oil an hour during aerobatics the engine suddenly starts slinging out eight or nine litres an hour! Most of this ends up along the fuselage and over the tailplane, but the point is that if you do not keep an eye on the situation you can end up with too little oil in the tank to maintain full oil pressure.

The trick is to watch the oil temperature gauge, if you have one: if you are doing aerobatics solidly the temperature will rise a little initially and then stabilise, so if it starts to rise again at some point after that it means you have chucked out so much oil that the rest is getting warmish. Time to go home.

Talking of aerobatics brings us to the rocker lubrication. Gipsy Major valve gear is lubricated by the rocker box covers which form four separate oil baths, independent of the engine's main oil system. The covers are removed at check-time and filled with fresh engine oil up to the wire level mark halfway up the small stand-pipe which caters for rocker box breathing. If you do a lot of aerobatics, however, the oil baths may need replenishing at shorter intervals than the normal check-times. The need for replenishment seems to vary from engine to engine; some Gipsies never seem to lose their rocker oil, while others scatter it all over the engine bay and need to have the rocker boxes off and checked every 20 hours.

Some aerobatic pilots try to keep the spillage at bay by blocking-off the breather pipes, but this is not a very good idea because any compression leakage past the valve guides then tends to pressurise the rocker box and blow the oil out somewhere else. Probably the best thing to do is to make up a four branch drain tube like a sort of mini exhaust manifold. It will not actually stop the leakage, of course, but it might discourage the oil slick from reaching the tailwheel more than once a day.

The last point on the operating side is to avoid closing the throttle suddenly after a long climb, particularly if you are doing something like glider towing. This is a no-no for any engine, of course - the too-rapid cooling causing distortions all over the place, but it particularly applies to the Gipsy engine. Anyone who makes a habit of it is headed for a cracked crankcase, a cracked head or a valve seat breaking up.

SHUTTING DOWN

The normal stopping method is to make a magneto dead-cut check by switching off and on one magneto, followed by the second, then bringing the engine back to tick-over for a few seconds, switching off both magnetos together, and just before the propeller stops rotating, opening the throttle smoothly all the way. This causes the engine to take in a great big gulp of air and prevents running-on as the engine comes to a stop. Some Gipsies have a 'stopping ring' which you pull to cut off the fuel supply inside the carburettor. This is the best way of stopping the engine since you do not leave unburned fuel vapour in the cylinders to wash away the oil film, but if you are going to want a re-start in the next half hour or so, then stop the engine on the magnetos. That way you will leave some juice in the pots and, you never know, when you want to start, she may even go first swing!