Here you go-
MOKI 51 STUNT
Derek Pickard tests the latest European 51 stunt engine to challenge the proven 60s, finds a top performer and explains, the misunderstandings of schneurle porting for stunt engines.
The end of the all-conquering ST60 from production was soon replaced by Supertigre's answer in the form of the ST51. That well-made and good looking engine heralded the start of a new breed in stunt power engines which are lighter, use less fuel and nearly matched the power of the older bigger brother. This is the result of the modern porting.
Unfortunately, the change to schneurle porting incorporated too few alterations from normal radio design which resulted in the engine's power being a bit too enthusiastic for stunt work that lead to the revs winding out in some maneuvers and the engine earning a bad reputation. Only when the porting is subsequently altered by private engine customers to tame the output does the engine begin to behave. But those modifications also take away some of the power, down-rating the reworked 'Tigre to that well below a 60.
Shortly after the stock ST51 fell from grace, along came the MVVS 51 -- a single purpose-designed schneurle ported engine with both the power of nearly a 60 and the behavior of a pro-stunt unit. Although it was expensive it did the job in the air. Only the problem with the impractical rear inlet and its demands for side-mounting was to keep this otherwise excellent unit from better sales success.
And now, on top of the problems of those other two comes the latest 51 schneurle ported stunt motor. There's nothing new about the name Moki even though it may sound strange to some readers. This Hungarian maker has been producing top engines for years, mostly in the ultra-competitive 2.5cc team race and speed business, where the quality of fits is vital.
While the name Moki may have been a secret for most stunters one or two fliers have benefitted from this make. Keen readers of the results pages of Stunt News magazine will recall such a Hungarian motor took a competition victory early in 1994. For the handful of Moki 51 engines in the US, that's a relatively big percentage winning. The failure of most schneurle ported motor to work well at stunt has lead to the incorrect the belief by some enthusiasts that this modem porting system cannot work at producing dependable mid-range power in the traditional manner. That is wrong, as schneurle porting is more efficient than the use of a deflector piston, and when suitably arranged, works better. The fact that so few have been so arranged only reflects the multitude of partially converted radio motors which do not suit us stunt fliers.
Schneurle porting can better fill a cylinder with fresh charge, cause a fuller exit of the old charge, and allow less new gas out the exhaust port. This is potentially the reason why a schneurle 50 can match the output of a conventional 60, and do so while using less fuel.
Too often, partially converted radio motors sold to stunt fliers have incorporated too few effective alterations. This is because most modern radio engines have been developed to influence easily impressed radio flierswith lavish claims of BHPat high revs which has dictated the use of large capacity transfer ports appropriately timed down from the exhaust. But at lower engine speeds (read: stunt flying revs) that porting arrangement is not doing what it was designed to and so the engine's behavior is compromised. The result is a partially-converted engine set to work below it's potential and, worse still, may have an unstable run behavior in the form of it
wanting to get up on its powerband, rev out and stay there.
An example of this is the stock ST51 stunt unit as sold by Supertigre, which has the problem of a high exhaust port (140 degrees) followed by a large 25 degree exhaust lead (transfers at 115 degrees) which, with a 10.8:1 compression ratio, makes the problems stunt fliers have experienced: This engine is too close to radio specs to work well for our use.
For stunt fliers struggling with ordinary schneurle motors, I'd recommend the following order of modifica-tions: if possible, lower the liner to achieve 135 or less degrees exhaust but without uncovering the underneath of the piston; raise the transfer ports heights for an exhaust lead of about 15 degrees; and then lower the compres-sion to no more than 9:1. (I'm informed that filling the third [centre] transfer port and sleeving the internal diameter of the crankshaft port also helps.) Carefully done, these can achieve a satisfactory result. Those modifications will cause a drop in power, but the remaining performance will be more stable and stunt suitable. (Do notice I didn't include the fitting of an ultra small venturi, nor a partially blocked exhaust.)
Schneurle porting can work excellently for a stunt engine if arranged correctly, and this has been known for vears. A good example is the Aldrich-designed Fox 40 of 1970. I have one of those in a classic competition Fierce Arrow, and it's among my favorite engines. Just for the record, it has two schneurle transfer ports timed at 120 degrees, with the exhaust at 132 and the compression is 8.3:1.
The MVVS 51 some 23 years later also works well. Both those engines produce good power, use little fuel, and boast stable running. They have another thing in common -- both were designed for stunt applications, which means moderate size inlet and transfer ports as much as appropriate timing.
This single-purpose design advantage is equally true of the new Moki 51 stunt. The Hungarian motor is powerful, economical, and run-stable. And, to put that into perspective, the power is every bit that of an ST60, the economy matches a mean 40, and the stability ensures both those output characteristics are delivered at all times. Property designed stunt motors are a delight -- it's the compromised ones which give the trouble.
Moki 51 Stunt incorporates a combination of case
construction that has the exhaust fitted to the
outside of the circle and a down-tilting muffler
which ensures exhaust muck doesn't cover the
model. This engine has all the grunt of an ST60
but with less fuel load and absolute tops in
quality.
While this Moki was being tested, I was redeveloping a recently-released 46 radio motor for stunt and experimenting with various conversions as part of a wide ranging comparison of the 46 stunt engines. When eventually finished and proven to give the optimum stunt run, it was noticed that this excellent schneurle ported engine produced less power per capacity than the Moki at stunt revs. Although the 46 had a slightly better power than a Tigre 46, the Moki 51 (vhich is really only a 49 capacity) is up there with Tigre 60 output. That 15 to 20% lift can be partially explained by the 6% bigger capacity but the main reason for the increase is the Moki incorporates the optimum transfer ports for stunt revs of between 8,000 to 10,000 rpm. It also has to be said that if I'd re-timed the sleeves of both engines to work at 12,000 rpm, the 46 would certainly have shown an advantage ... unless I could have enlarged the 51 's three transfer ports.
To view transfer port sizes as a secondary throttle is an over sitnplification even though part of the issue is gas speed. The best transfers for a specific job are those which incorporate the appropriate shape, capacity, timing and direction.
As regards the remainder of the mechanicals, this is a twin ballrace, ringed engine, of extremely good quality. The 22mm bore and 21 mm stroke give 8cc. Compression ratio is just 8.6:1. The single ring is pegged, the conrod is bushed, the needle is supplied straight so the owner can bend it to suit individual preference, and the case construction is such that the engine is assembled so the exhaust emits to the outside of the circle in the inverted position. Extra head shims of two different thicknesses are supplied so the user has the option of further reducing the compression. Mention must be made of the fact that this manufacturer is one of the very few which can design and make a decent piston ring for this size motor.
And just in case any doubt still lingers, there's nothing wrong with using a piston ring instead of an ABC piston/liner, providing the design, metallurgy and machining is correct. Too often model engine rings have been too big in cross-sectional area in relation to the bore and unavoidably distorted during initial fitting, or badly made in inappropriate fitting ringlands. The result has been long run-in periods and/or bad cylinder sealing for the working life of the motor. Correctly done, a simple ring is smart and effective in it's operation, and capable of doing a good job with eventual refurbishing being inexpensive.
The single ring and its fit in this Moki 51 is every bit an example of what other makers should strive to achieve. Unfortunately, the majority of engines are built to a price instead of a specification and the vital ring fit is inadequate.
The steel cylinder liner has three transfer ports in the schneurle fashion. These ports are medium-sized which are fully efficient at maximizing mid-range torque. The timing is 130 degrees for the exhaust and 118 for the transfers. As previously explained, that spells our kind of performance.
The weight is 11 ounces with the efficient muffler adding just under another ounce. The well-made 2 inch Moki spinner weighs an ounce.
Test ship was a 64 inch span Epic that weighed in at 63 ounces.
TheMoki flew it wth all the power of an ST60 on less fuel, but
without the problems normally associated with schneurle ported
engines.
Praise must be heaped on the muffler. The unit does everything right in that it is small, light, of simple construction and emits little noise while being angled downwards for the oil muck to clear the underside of the wing. It is of three piece hollow, all-alloy construction with the front and rear parts being annodized black. If such simplicity can result in so good a performance then other makers should copy it.
Everything about the fits, materials and design oozes quality. This is a superbly built motor that immediately impresses the moment it is taken out of the box, to the way it runs-in and performs. The maker claims 0.9 bhp; which may not seem much by contetnporary standards but it indicates the maker is both honest and understands engine applications. Clearly, this Moki has been built for the mature enthusiast who is both prepared to pay for quality, demands efficiency at the one application and isn't fooled by meaningless power output claims.
The engine has the same mounting width as the OS 40/46 and similar weight. It has longer bearer mounting lugs which means that thin alloy pads to prevent the engine digging into wood beam mounts are not vital. Fitting the Hungarian 51 in place of the VF is as easy as drilling two extra holes towards the front of the stock beam bearers.. Because of this, the engine was slotted into my 64-inch Epic -- a plane initially designed to be powered by an ST60, but in this case built for a VF46 plus pipe, and then used to test the Irvine 40 piped engine. With the Moki in place the weight came in at 63 ounces, which is acceptable for the 770 square inches.
After the brief period of running-in, the result of accurate machining, the engine was fitted with a 12x6 Bolly prop and hooked on 70 foot lines on a windy day. The preferred weather for Stunt News engine tests is wind; not gale force, but definitely fairly strong and demanding. As during the running-in, starting was easy, adjustment a delight, and the ship was flown running my normal 5% nitro/25% castor fuel. The ground four stroke setting of 9,100 rpm gave 5.5 seconds per lap. Pulling the nose up and making the engine go to work in the maneuvers resulted in a smooth switch to about 9,400 rpm with which the engine totally controlled the plane
throughout the pattern.
Towards the end of the flight it was noticed that the lap times had increased from 5.5 seconds to 5.2 seconds yet the engine still avoided any form of wind-up in loops with wind assistance. The test was repeated with the same results.
The cure turned out to be lowering the compression. This was done by removing the head, peeling off the one thin shim fitted by the factory, and replacing it with one of the thicker shims supplied in the engine box. Although this work wasn't been measured, I guess the result was a drop to below 8:1 compression.
From that point on, the Moki 51 was stunt perfect and did everything right. The conclusion is obviously that from new this stock engine is set tip for straight FAI fuel (80% methanol, 20% castor), and the use of nitro demands a decompression mod as this motor's performance is compression sensitive.
This subject of compression in our form of two stroke is the cause.for much confusion. The situation is straight-forward: all the compression ratio numbers I quote are those measured by myself the simple and obvious way - filling the combustion chamber at the top of the stroke with a carefully measured amount of liquid, and arithmetically relating that small amount to the known total swept volume. This is in reality no more than a static compression ratio as in operation the working compression can vary enormously depending on the porting and exbaust. While the latter in its most advanced form can have a supercharging effect and force a compression ratio from 8:1 to well past 16:1, with our model plane engines running conventional mufflers (read: technically
unsophisticated exhausts), the range is much smaller. The difference is that although a motor like the ST60 has a statically measured compression ratio of 12.7:1, as some of its incoming gas is lost out the exhaust during the scavenging mode of the engine's operation, the actual working compression is lower. This is in contrast to a modern schneurle ported engine that has better control of gas direction, and so loses less or none of the inlet charge out the exhaust before it can contribute to the engine's combustion. In reality, the fact that the ST60 and the Moki 51 both have the same power output is indicative of the way the newer motor uses all its charge and probably operates on a true compression of 8:1 whereas the old Tigre's breathing inefficiency causes it
to run a lot less than nearly 13:1 - probably around 10:1 or even lower. This point is backed up by the way the older Italian motor uses 50% more fuel to obtain the same power as the Moki. It is part of the problem of bad economy inherent with the conventional deflector piston layout which forced the design's replacement with schneurle porting for many two stroke applications some 45 years ago. And, before some smart enthusiast arranges a test rig where these two engines are spun to operating revs by an electric motor, and the compression is measured through a hollow plug to a pressure guage, be advised that such measurements are not representative of much as the combustion process has a significant effect on exhaust emptying and fresh charge filling.
Now back to the Moki and what it does in the air: At all times in subsequent flying, this compression-corrected Hungarian powerhouse started instantly, provided fistfulls of sweet torque, and did a top job. Such lovable behavior is as guaranteed as the dawn with this excellent engine, and it handled the big Epic with complete authority. Only if the wind gets up very strong would it be necessary to maybe shorten the lines. But with higher wind, the engine speed can also be confidently increased by the reliable reaction to weakening the needle setting.
This means more power in gently predictable stages and with it the realization that this new Moki has got as much real oomph as a big plane can demand. This Moki has a delightfully progressive behavior to the needle setting -- as good as any engine I've tested. And in the air it stays right.
Where previous European 51 schneurle ported motors have failed due to one thing or another, this Moki delivers. It is powerful yet progressive, and always smooth and dependable. The total weight with its quiet muffler is just 12 ounces, and only 120cc of fuel is required for the pattern on 70 feet lines. It all adds up to a great stunt engine.
Conclusion: This superbly made motor is the first schneurle ported 51 to both deliver the power of a top 60 while being stunt-dependable and fully practical. Very highly recommended.
Declaration of interest Derek Pickard did not receive a free engine for this test but paid $172 from the US Distributor
Topic: Moki .51 engine review in Pampa stunt news 1981-1991 era (Read 2407 times)