ORIGINAL: jaka
Hi!
Britbrat...The Magnum XL is like any other sport engine, it develops its max power at around 16500-17500rpm. OS FX .40 ,Super Tigre .40, MVVS GFS 6,5cc, TT. 40 SEetc ... all of these engines develope their max power at about 17000rpm. There is no difference!
But saying that just because an engine develops its max rpm at 17000prm doesn't mean you should prop your engines in your high winged trainer or low winged sport airplane at these rpm.
To the contrary, the right way to prop a high winged trainer is for good trust at low to moderate flying speeds and not for high rpm and max speed (there really isn't much difference in speed though, whatever prop you choose for a plane like that).
For all .40 to .46 sport engines this mean using a 11x5- 11x6 prop. This prop size will give nice and reliable flying characteristics (good trust, low noice, steady and calm engine behavoir with less aggressive behavior when giving full throttle in an emergency situation and good reliable idle) suitable for a newcomer at all ages.
Regards!
Jan K
Sweden
Hi Jaka. Actually they don't all develop peak power up there -- the OS 40 & 46 LA being good examples (~13,500). I'm using dyno data -- not the manufacturer's phony advertising (OS being particularly bad in that regard). However, when a question is asked "how do I get the most out of my ---blah, blah blah engine?", I look at the airframe application & the engine characteristics, then offer a solution. There is no problem at all in running an engine in the 12,000 -13,000 RPM range to get decent power from it --which you pretty much have to do with those feeble LA 40/46's. If he has a sport flier, then there is nothing wrong with winding an engine up to 16,000 if that's where it likes to go & he wants real performance.
However, you have never seen me suggest that a trainer be operated at 16,000 - 17,000 rpm engine speeds
The Magnum is an interesting case, down in the 12,000RPM zone it is only slightly more powerfull than a 46 LA, but let it hit 17,000 & it is a particular monster in that class. It is not nearly as well suited to a trainer as a TT 40 Pro for example, or an OS 40 FX -- both of which peak at high revs, but which also have substantial grunt at lower revs.
There are horses-for-courses & no one-prop-fits-all answers.