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New Rotax E-Learning video on the causes of detonation and pre-ignition.

Roger Lee

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It occurs to me after all the posts about needle jet clip position, carb heat and detonation that we all sort of do have mixture control since our bing carbs do not really have chokes (which only work with a closed throttle body) but rather have an enricher circuit which should work with any throttle setting. So if you did have overly hot CHT or EGT due to possibly bad fuel, cold/dry air, over-pitched prop or whatever, then you could tweek the 'choke' to richen up the mixture and see if that corrects things. Usually we just can't take advantage of mixture control simply because the Rotax is already setup to run significantly ROP from MSL to outer space

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So what Rotax are now saying is that one should be achieving 5500/5800 wot on take off?.........Bit of a change!


The prop on my Sky Arrow (Warp Drive) is pitched exactly as it came from the factory and I've verified its what the Maintenance Manual specifies.


I see just over 5,000 rpm on takeoff. About 5,400 rpm in full throttle cruise (which I virtually never use).


Assuming my tach is correct, I barely see 5,000 rpm, if that, in a full power climb. This is climbing through 8,000':






I've given thought to trying just a little less pitch. After all, an engine that never sees redline also never sees full rated power.


Maybe one of these days.

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Hi Oz and All,


Remember Rotax writes these things for everyone around the world. Don't forget that may have in flight adjustable props so a little interpretation needs to be done even though it isn't spelled out which is where the Rotax documentation falls a little short sometimes. These are things you learn in Rotax school.


If you only have a ground adjustable prop then we try to balance the climb/cruise setup up and make the engine happy at the same time. The best WOT flat & level rpm is 5500 WOT. This gives you good climb, good cruise speed and the best fuel economy for an individual rpm setting. Balance is the key. This is a good place to be too if you have to fly your plane at WOT because of an emergency like a broken carb cable on one side then you can do that without any problem at 5500 WOT. If you have it set over 5550 then you would be limited to a 5 min. run time. 5500 is Rotax's continuous run rpm. A good cruise rpm (with the max WOT rpm at 5500) is 5000-5300 depending on the plane and application. I prefer 5100-5300 for cruise. If you run on only 100LL then cruise at 5300 to help keep the lead blown out. At a 5500+ WOT rpm your take off rpms will be up to 5000 or more. This take off rpm will depend now on what flap setting and what climb angle you chose that will either load or help unload the prop. Your plane"s over all configuration will play a part in this, too. Never set your engine up to only run (i.e. 5000-5200) WOT rpm. It loads the engine too much and over time it will cause problems that you won't see until it's too late. These low rpm settings really overload an engine at take off with rpms well below 5000. Having an engine setup to run 5300 or less rpm WOT throttle is counter productive. You get worse climb, worse cruise speed and worse fuel economy. I have proven this to hundreds of people so there really isn't a valid argument to set your WOT to low cruise rpms. If you have a special need for a better climb prop, like living and flying at very high airports, altitudes or maybe a float flier, ect... then you may need a higher rpm setting to accommodated the special circumstances, plus you will get better performance while flying at the high altitudes with a little flatter pitch. Everyones pitch on the prop setting will be different because we all live and fly at different altitudes, have different props and different fuselage aircraft. I will say that if you truly want to see 5200 on take off as Rotax wants then your WOT flat and level is closer most likely 5700 or more. For me personally this isn't a good balance.

Having the prop to get 5500+ rpm WOT has another big plus over low rpm settings. If you screw up a landing and need that full throttle save then the prop/engine rpm will spin up quick and be more likely to save your tail. If the rpm was set too low then rpm spin up is slow and it will result in poor save my tail time.


All bets are off for the lucky few that have an in flight adjustable prop. They can get the best of all worlds. But even with these guys if they have set the prop to only turn 5100-5200 WOT during cruise then it's counter productive and over loads the engine. Just because they have an in flight adjustable prop doesn't always mean they use it properly.

Cruising in the 4000 rpm range for extended periods is not good for your engine. It is ok for a short look at something and landing setups, but not for a constant use. Your engine was specifically designed to run all it's life in the 5000's. It was never intended to run in the 4000's and is really hard on the engine over time and has much more vibration. Just because you can't feel the stress and vibration doesn't mean it isn't there.


Be nice to your engine it cost $20K and it is one of the only things keeping you in the air and safe.

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