Aviastar 150 reviewed by Clarence Lee (RCM January 2000)
We have another new engine of Chinese manufacture to take a look at this month - the Aviastar 150.
Crankcase : As we have mentioned several times in previous reviews, there are three basic methods of casting a crankcase. For the benefit of our newer readers, these are sand-casting, investment of lost wax-casting, and pressure die-casting.Sand-casting is generally used for prototype development and short production runs due to being the least expensive method of producing an aluminum casting. Investment casting is the next least expensive method, although the cost per part is the highest of the three methods. Pressure die-casting is most generally used by the model engine manufacturers for large production runs and, although the initial cost of casting is high, running $30,000.00 and up, the cost per part is low. Aviatronics has chosen the investment-casting method to produce the Aviastar crankcase. Investment casting has one big advantage over pressure die-casting in that a disposable internal core is used for each casting. This allows the transfer and boost port channels to be angled and positioned exactly as wanted - something that cannot be done with pressure die-casting where removal of the permanent core has to be taken into consideration.
The crankcase has been cast in one piece with removable head and backcover. The majority of the machine work has been performed with latest CNC machining equipment resulting in extremely smooth surface finishes and very tight tolerance control. When we disassemble these engines, we always check for the sleeve fit in the crankcase which, in this case, was a smooth slip fit, the fit of the bearings in the case and on the crank, etc. This always gives us an indication of the quality and tolerance control. Many times over the years we have had engines go through our hands that either had the bearings fit too loose or too tight. If a bearing is too loose in the bore, it will spin and gradually become looser. If too tight without sufficient internal clearance, it will run hot and be very short-lived. We have had engines that by merely shaking the case, the front bearing would fall out but, on the other hand, the rear bearing would seem to have been installed with a 10 ton press. The bearing fits in the Aviastar 150 crankcase were ideal, being a light press-fit. Obviously, Shu June Ye, the engine's designer, has had quite a bit of experience properly fitting ball bearings. Operations such as honing of the sleeve is done by hand, not automated equipment, with each piston individually fit to the sleeve which assures the best possible piston/sleeve fit.
Crankshaft And Bearings : The massive crankshaft has been machined in one piece from chrome alloy steel, hardened and finish-ground on all bearing surfaces as well as the counterbalance face and outside diameter. This makes for a beautifully made part. Although finish-grinding of the counterbalance face and o.d. rather than leaving a lathe-turned finish is a cosmetic feature, it is still an indication of quality and attention to detail. The counterbalance has been milled away on either side of the crank pin for counterbalance action but retains a .072" thick solid disc section at the rear for added strength. The counterbalance would balance out the full weight of the con rod and 3.5 grams of the wrist pin's 6 gram weight. Although this would appear to be a shade on the light side, the engine's vibration level was no higher than any of the other engines in this size range that we have tested, actually being relatively smooth for an engine of this displacement size.
The crankshaft is supported by a giant 42mm (1.654") o.d. x 25mm (.984") i.d. steel-caged, single shielded bearing at the front. The portion of the crankshaft main journal directly forward of the rear bearing has been reduced slightly in diameter to 23mm (.905"). Normally, it is customary to reduce this area about .001" to allow the rear bearing to slip over the crankshaft up to the bearing seat. The reason for the 2mm reduction is hard to figure other than to possibly save a little weight. The crankshaft's intake port was timed to open 42 degrees after BDC and close 55 degrees after TDC for a total intake duration of 193 degrees. The 55 degrees closing time could be considered bordering on the late side which would favor higher rpm operation rather than low-end lugging power.
Piston, Sleeve, And Rod : The piston has been machined from a high silicon content aluminum alloy and utilizes a single .048" wide expansion ring pinned to prevent rotation. Alvin So tells me that the piston material is the same as that used for Honda Civic pistons and the piston ring material is the same as that used in Honda motorcycles. The piston, in turn, runs in a hardened, chrome alloy steel sleeve with a heavy .120" wall thickness to prevent any distortion at the elevated temperatures that occur on those lean runs that modelers occasionally subject their engines to.
One of the piston's wrist pin holes has been step-bored to prevent the wrist pin from passing through the piston so that only a single snap ring is used to retain the 7mm dia., hardened steel, tubular wrist pin. Unusual is the use of two small wire springs slipped over the wrist pin on either side of the con-rod that center the rod in the piston. This, in turn, keeps the rod from moving rearward and rubbing on the backplate - again, attention have been given to a minor detail.
The beefy con rod has been machined from bar stock aluminum and bronze-bushed at both the crank pin and wrist pin ends. The bronze material being the same as that used by Mercedes-Benz for the wrist pin end of their automotive con rods and cam bearings. Shu June Ye obviously sourced a lot of materials when designing the Aviastar engines. Three oil holes provide lubrication to the crank pin end of the rod-one on top and two off-set so as to not be in line with each other at the bottom. Wrist pin lubrication is provided by single holes on both the top and bottom sides of the wrist pin. Obviously, assuring an ample supply of oil to the rod was the goal here.
Conventional Schnuerle scavenging is utilized with a single transfer port on either side of the divided exhaust and a single boost port directly opposite the exhaust. The exhaust was timed for an open duration of 150degrees, the transfer ports 124 degrees, and the boost port 120 degrees - ideal timing for the engine's intended operating range that would allow the use of a conventional muffler, but still show a good gain if a tuned pipe were used.
Cylinder Head : The deeply finned head has been machined from bar stock aluminum and utilizes a .215" deep x .860" wide, bowl-shaped combustion chamber surrounded by a .200" wide squish band area set at a 3 degree angle. The measured combustion chamber volume was 2.5cc which computes to a compression ratio of 8.4:1 figured with the closing of the .315" high exhaust. A compression ratio of 8.4:1 is right in the ballpark for a glow ignition engine utilizing low nitro fuel being neither too high nor too low.
Carburetor : Like many of the model engine manufacturers in recent years, the Aviastar 150 uses a 2 needle, rotating barrel design carburetor with an 11mm (.433") dia. intake . Besides the normal "O" ring used to seal the carburetor against the crankcase intake, two other "O" rings are utilized - one at the bottom of the carburetor neck and the other at the outer side of the carburetor barrel. The high-speed needle valve as well as the idle mixture needle valve are also sealed with small "O" rings - no chance for any air leakage with the Aviastar carburetor. In operation, the carburetor performed beautifully ( in conjunction with the engine's overall design ) with a nice broad high-speed mixture adjustment range and non-critical idle mixture adjustment.
Performance : Very extensive break-in /operating instructions accompany the engine personally written by Dave Gierke. For break-in , Dave recommends a fuel mixture containing 5% nitromethane, and 24% oil content composed of 1/3 castor oil and 2/3 synthetic. Dave, like myself, is a firm believer in plenty of lubrication, especially during the break-in period. Following break-in , the 5% nitro recommendation is retained but the oil content can be lowered to 18-20%, still retaining the 1/3 - 2/3 castor / synthetic ratio. As most fellows do not have the chemicals for mixing their own fuel, Dave recommends that five ounces of castor oil be added to a gallon of commercial fuel which will bring a fuel with oil content of 20% up to 24%. However , many fuels on the market nowadays are only using oil percentages in the 17% range which would mean adding nine ounces of castor oil to reach the 24% recommendation. For the purpose of testing, we mixed up fuel consisting of 5% nitromethane, and 24% oil consisting of 1/3 Klotz Racing Castor and 2/3 Klotz KL-200 synthetic, with the balance methanol. No glow plug accompanied our review engine but a K&B KB-1L glow plug is recommended which was initially installed in the engine. However, with the rich mixture setting and high oil content fuel, we experience a problem getting the engine to transition from high idle to full throttle without cutting out even with battery heat applied. Installing a K&B R/C (idle bar) glow plug solved the problem. After the break-in period, when we were able to lean the engine in more fully, the transition problem clear up completely and the KB-1L glow plug now worked fine.
Following a 30 minute break-in period consisting of six 5-minute runs with cool time between runs, the following power figures were recorded. The Temperature of the day was a warm 76 degrees F, the Relative Humidity 48%, and the Barometric Pressure standard at 29.92 inches of mercury. All propellers were Zingers.
16 x 10 ----- 8,500
16 x 8 ----- 9,200
18 x 6 ----- 8,200
18 x 8 ----- 7,800
18 x 10 ----- 6,750
20 x 6 ----- 7,200
20 x 8 ----- 6,750
These are very respectable figures, making the Aviastar 150 one of the more powerful engine in this displacement size we have tested. Although an electric starter is recommended, we had no problem hand-starting the engine and never resorted to the electric start. The engine exhibited very docile and "user friendly" handling characteristics. Most surprising was the low idle speed of 1,750 rpm with 16" props, 1450 rpm with the 18", and 1,200 rpm with the 20" - pretty impressive. Acceleration from idle was almost instantaneous after the engine had acquired 30 minutes of break-in time and deceleration was only a matter of a few seconds. We did note that the engine should be allowed to idle for a few minutes so as to warm up thoroughly before it would accelerate cleanly.
The engine carries a retail price of $259.95 with the muffler selling for $39.95 additional. A very reasonable price for an engine in this size range with the quality and attention to detail that has gone into the Aviastar 150. As a promotional inducement, a $24.95 aluminum engine mount is also included in the price of the engine. In the Mandarin Chinese language, if something is very good or done very well , it is termed " Ding Hao ." We found the Aviastar 150 to be one Ding Hao engine.
Reproduction of this article with the courtesy of RCM and Mr. Clarence Lee