Buckaroo Posted January 13, 2017 Author Report Share Posted January 13, 2017 I think we get a double whammy in the design. We get the constant argument between the prop and the engine and add lower rpm imbalances in the carbs independently running each side of the engine. No wonder we are asked to taxi at 2000 rpm!???? Link to comment Share on other sites More sharing options...
Buckaroo Posted January 13, 2017 Author Report Share Posted January 13, 2017 I think we get a double whammy in the design. We get the constant argument between the prop and the engine and add lower rpm imbalances in the carbs independently running each side of the engine. No wonder we are asked to taxi at 2000 rpm!???? Link to comment Share on other sites More sharing options...
Buckaroo Posted January 13, 2017 Author Report Share Posted January 13, 2017 I think we get a double whammy in the design. We get the constant argument between the prop and the engine and add lower rpm imbalances in the carbs independently running each side of the engine. No wonder we are asked to taxi at 2000 rpm!???? Link to comment Share on other sites More sharing options...
Tom Baker Posted January 13, 2017 Report Share Posted January 13, 2017 Tom means at flight RPM. Generally, the higher the RPM, the smoother the impulses caused by carb imbalances. There might be some shake at low RPM, but it will be virtually non existent at high RPM. No, I mean at idle or just above. 2000 rpm would be a good place to check for the vibration. Link to comment Share on other sites More sharing options...
Roger Lee Posted January 13, 2017 Report Share Posted January 13, 2017 The engine has been working since 1990, 5 million hours and 50K engines. It must be working. Rotax supplies 80-85% of the world's small aircraft engines. The worst thing that happens to a Rotax engine is its owner. Be kind to your engine and just do what's in the manuals and enjoy your flying experience together. Don't do harm and make her angry or you will always lose. Link to comment Share on other sites More sharing options...
gbigs Posted January 13, 2017 Report Share Posted January 13, 2017 I think we get a double whammy in the design. We get the constant argument between the prop and the engine and add lower rpm imbalances in the carbs independently running each side of the engine. No wonder we are asked to taxi at 2000 rpm! The Rotax runs at double or higher RPM than a Continental or Lycoming. The 2000 rpm idle is equivalent to 1000 rpm for a Continental. The 5000 to 5500 rpm range is like the 2500 rpm range in a Continental. Link to comment Share on other sites More sharing options...
Doug G. Posted January 13, 2017 Report Share Posted January 13, 2017 Rotax gearbox 2.43:1 Continental, Lycoming direct drive 1:1. You can figure it out from there if you really have a desire to do so. Link to comment Share on other sites More sharing options...
Roger Lee Posted January 13, 2017 Report Share Posted January 13, 2017 It is 2.43:1 reduction for the 912ULS. 5500 rpm = 2161 straight rpm compared to a Cont. or Lycoming. 5300=1851 2000=823 1700=700 Link to comment Share on other sites More sharing options...
ct9000 Posted January 13, 2017 Report Share Posted January 13, 2017 Because of the short stroke the actual piston speed is slower than conventional lyc. or cont. at max rpm , and therefore is not highly stressed at all . It is the acceleration / deceleration of mass that causes stress . Think of it this way , for one cycle in a long stroke engine the piston must accelerate to a very high speed then decelerate to a stop in order to travel the distance in a given time . A short stroke only needs small comparative speed / load to cover the shorter distance in a given time . The increase in piston speed is the square of the increase in stroke . Link to comment Share on other sites More sharing options...
ct9000 Posted January 13, 2017 Report Share Posted January 13, 2017 Re reading my last post , I forgot to add that rotational speed does not equate to load of stress , think gyroscope . It is reciprocating mass that counts . Link to comment Share on other sites More sharing options...
gbigs Posted January 14, 2017 Report Share Posted January 14, 2017 Because of the short stroke the actual piston speed is slower than conventional lyc. or cont. at max rpm , and therefore is not highly stressed at all . It is the acceleration / deceleration of mass that causes stress . Think of it this way , for one cycle in a long stroke engine the piston must accelerate to a very high speed then decelerate to a stop in order to travel the distance in a given time . A short stroke only needs small comparative speed / load to cover the shorter distance in a given time . The increase in piston speed is the square of the increase in stroke . Neat. Like the physics explanation. Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 So which piston travels farther in one hour a Rotax number 1 piston on a 912 uls at 5500 rpm or a lyc or cont typical 4 cylinder number 1 piston at 2600 or 2700 (say max cruise) rpm? Link to comment Share on other sites More sharing options...
Tom Baker Posted January 14, 2017 Report Share Posted January 14, 2017 So which piston travels farther in one hour a Rotax number 1 piston on a 912 uls at 5500 rpm or a lyc or cont typical 4 cylinder number 1 piston at 2600 or 2700 (say max cruise) rpm? I guess it would depend on how fast the airplane is flying. Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 ????????????????That's a good one right there by golly! This was supposed to be a testicle question! Nerd alert!! Link to comment Share on other sites More sharing options...
Tom Baker Posted January 14, 2017 Report Share Posted January 14, 2017 Well the Lycoming piston will travel farther, because it has a longer stroke. I think the question you meant to ask is which piston will travel a greater distance within the cylinder. If someone else doesn't do it first I'll look in my manuals tomorrow and figure it out. Link to comment Share on other sites More sharing options...
FredG Posted January 14, 2017 Report Share Posted January 14, 2017 From the internet, the Lyc O-360 has a 4.375" stroke. The 912 ULS has a 2.40" stroke. Assume that the Lyc will run at 2200 RPM at cruise and the Rotax will run at 5400 RPM at cruise. So, For each minute of flight with the Lyc: 2200 RPM x (4.375" stroke x 2 strokes per crankshaft revolution) = 19,250 inches per minute For each minute of flight with the Rotax: 5400 RPM x (2.40" stroke x 2 strokes per crankshaft revolution) = 25,920 inches per minute The Lyc piston travels further per crankshaft revolution, but the proportionally greater engine speed for the Rotax results in greater piston travel distance per unit of time. The Rotax piston travels further regardless of variation in cruise RPM for either engine over realistic ranges. For example, the Lyc would have to turn at almost 3000 RPM in order to have the same piston travel per unit time as the Rotax at 5400 RPM. If the Lyc is turning 2700 RPM and the Rotax is turning 2400 RPM, the Lyc and the Rotax are close, but the Rotax piston still travels a bit further per unit time. I'm sure if I made an error in my calculation someone will correct me. Fred Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 I'm just wondering wear wise which one travels farther within the cylinder. I understand the longer stroke puts more stress on the components. My lyc and cont friends think their 2600 rpm rigs are running with ease compared to ours at 5500 rpm. I guess in reality they maybe working harder! This will be a interesting discussion at the airport coffee! Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 Great post Fred! Now with all things taken into consideration I wonder which engine would go longer? I'll bet Rotax! Water cooled heads, tighter tolerances, less pressure impulses and the biggest reason it's what I currently own!???? Link to comment Share on other sites More sharing options...
WmInce Posted January 14, 2017 Report Share Posted January 14, 2017 The cylinder walls also have a special coating. I recall it is of a ceramic type material. Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 Yes they'll run 43 seconds without oil!???? Link to comment Share on other sites More sharing options...
Tom Baker Posted January 14, 2017 Report Share Posted January 14, 2017 Fred, the only issue I see with your calculations is the cruise RPM used for the Lycoming is to low. In any case the Rotax piston travels a greater distance per unit of time. The other thing to consider is the accelerate and stop of the mass. The Lycoming piston and rods are going to be heavier, so you have more mass to deal with per unit of time than you do with the Rotax. Also because of the larger combustion chamber in the Lycoming I would expect stronger power pulses leading to more vibration. Link to comment Share on other sites More sharing options...
Doug G. Posted January 14, 2017 Report Share Posted January 14, 2017 I think the 43 second run (I have not seen this anywhere, BTW.) is largely due to the extremely small tolerances between the piston and cylinder walls. Which is also why the 912 typically does not burn oil. If you want to run your engine without oil you are much better off with a Lycoming or Continental.???????? Link to comment Share on other sites More sharing options...
Buckaroo Posted January 14, 2017 Author Report Share Posted January 14, 2017 I've owned 3 Rotax powered vehicles. Two were Aprilia motorcycles (Falco and Millie Italian croch rockets) and current Can Am 800 side by side. I sold the Falco (1000 cc 60 degree inline twin) to a navy seal in Temecula Ca. He commuted to Coronado Island almost every day and put over 100,000 miles on the bike without problems. These motors are beautifully built and bulletproof and go forever. Link to comment Share on other sites More sharing options...
FredG Posted January 14, 2017 Report Share Posted January 14, 2017 Tom, regarding piston travel distance, the Rotax and the Lyc have the same piston travel per minute when the Rotax is turning at 5400 RPM and the Lyc is turning at 2960 RPM. I do not know the most common cruise RPM of a O-360 and looked a bit on the web for a ballpark value. Regarding other loads on the engine, sure, reciprocating mass is a critical contributor to wear and potentially to vibration (and to lost engine efficiency - getting those pistons moving from a dead stop twice per crank revolution uses energy from fuel that is no longer available to turn the propeller). That too is a bit complex as the loads increase as the square of the piston/rod speed but as a linear function of the mass (I think...) fg Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 14, 2017 Report Share Posted January 14, 2017 Rotax gearbox 2.43:1 Continental, Lycoming direct drive 1:1. You can figure it out from there if you really have a desire to do so. 2263 prop rpm at 5500 engine rpm. Link to comment Share on other sites More sharing options...
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