Thread: what percent power to run

I had a conversation with one of the Rotax reps at Sun and Fun Friday morning and found out we may be running our engines to hard at times. I started the conversation with the question "is it ok to run 5500 rpms all day on the 912uls?" His response was "how many rpms are you set at in WOT at cruise?" He said its not ok to run 5500 all day if thats what your pitched for WOT after 2 mins in level flight. You would need to run at 5800 WOT in level flight to run 5500 all day and not be running your engine to hard. So if you can only get 5500 WOT at your current pitch setting than you need to back off to 5300 or less for extended cruising times. Likewise if you are pitched to 5300 WOT than you cant run there all day either. You must pull back the power slightly to not run the engine at 100 % power. Maybe everbody knew this allready, but I was never schooled about not running full throttle for extended cruising periods. Any other info on this subject would be appreciated as I have never attended a Rotax class.

The issue the Rotax rep is speaking of, is not running the engine at 100% power, but rather loading the engine due to a less than optimal prop pitch. The engine is redlined at 5,800. If all you can make at wide open throttle is 5,500, you are not at 100% power on the engine. Rather, the prop is taking a bigger bite of air, and is causing a load back into the engine case, slowing down what the engine is trying to run at. This will cause engine damage over time. Back in 2006 and 2007, most of us were delivered airplanes with this condition. Wide open throttle, I was seeing a max of 5,200 rpm's at 4,000 feet. Go higher, and performance fell off dramatically. At 11,000 feet, I was lucky to see 4,700 rpm's wide open. I've since had the prop repitched. Yesterday, I saw a max of 5,710 rpm at 2,000 feet, wide open, straight and level. This resulted in 124 knots indicated, and 129 knots true airspeed. I see 120 knots true when I cruise at 5,300 rpm.

I agree. Glad some one else said it, too.
This is a subject that has been around since the beginning of the forum. It is good to set your plane up to achieve 5500-5600 rpm WOT and I agree that cruise should be between 5000-5300. If you run 100LL all the time then the 5300 would be better choice to help with the lead issue. FD used to set them to 5200 WOT and that was too much load. Rotax told FD factory the same thing. Just took them a while to listen. CT's now come set to 5500+/- WOT from the factory. I prefer to be closer to 5600 rpm WOT. The reason is, just a little more kick on take off, a littler better rpm at higher elevation fields, a little better performance on both those on high density days and a little better rpm/performance at higher altitudes where many of us cruise. Since you don't run at WOT at cruise then 5600 is no big deal. You will never see that rpm on climb out and you never cruise at 5600 rpm any way. The guys I set to 5600 are faster than the ones at 5300-5500 at WOT. There are only a few reasons to set the WOT setting higher than 5600 and the large majority will never need it and few circumstances require it under normal operations. (Forget the extremes, normal operation)

Roger,

I personally find it hard to believe that someone at FD (and other LSA makers as well) thought that limiting top speed by too coarse of prop pitch was a good idea. It's like setting your variable speed ceiling fan on high, but then hanging weight on the blades to slow it down. It would have been better to set it to medium speed to begin with, and they could have done that by adjusting the throttle cables instead. Same result in limiting power, but at least it wouldn't cause additional load on the engine. I know that Rotax was preaching that message to FD back in mid-2007, but it took a long time for FD to get it.

Hi Roger F.,

It's been a while. Nice to hear from you.
I couldn't agree more that it was the wrong idea. I guess better late than never. I would urge any one else with a CT set at 5200 WOT to make the adjustment. You'll like it and never look back, not to mention your engine will be happier.

I know this is going to drive some to say; DAAAAAH! But here goes, What is WOT and is it a matter for a ROTAX certfied A&P to set it and log it??

Originally Posted by Roger Lee

It's been a while. Nice to hear from you.

Roger,

Looking forward to seeing you in Columbia. Bummer I was forced to cancel our get together for a Bay Tour flight last month. The weather couldn't of been better around San Francisco.

Originally Posted by Teamplayer

I know this is going to drive some to say; DAAAAAH! But here goes, What is WOT and is it a matter for a ROTAX certfied A&P to set it and log it??

WOT = wide open throttle. The adjustment is a prop repitch. I believe it needs to be done and logged by an LSR-M (Light Sport Repairman - Maintenance) at the minimum.

Hi Teamplayer,

"Wide open throttle" WOT should be set between 5500-5600 rpm for an over all best performance. With our ground adjustable prop we need to balance as many of the performance factors as possible. If you have extenuating circumstances, like a high elevation airport to operate out of then another rpm setting may be a little more appropriate. It should be easy for any A&P to do this and just log it. If your A&P isn't sure how you guys are welcome to call me. It is easy.

This is something we're going to change at the annual next week. How many degrees does the prop generally have to change to move it from the old FD setup to the new?

If when you go up to your normal cruise altitude and give it WOT for at least 30 sec. - 1 min and the WOT rpm levels off and it is 5200 rpm then I would flatten the pitch 1.5 degrees and once in a while it takes up to 2 degrees. 1.5 is a good starting place If you are closer to 5300 then 1.5 is good. If you are closer to 5350 then .75 - 1.0 degree. It is not an exact science and doesn't need to be. Just shoot for at least 5500 and if it gets closer to 5600 then all the better. So if it's some where in between 5500-5600 you are good and will see a difference on the performance with your plane. If you go from 5200 to 5550 then every aspect of the planes performance will be enhanced. 5200 was a terrible rpm for WOT. Everything suffered.