RPM/Density Altitude/% Power

[S3flyer]

Always kind of wondered this. What is the approximate percent power as a function of DA and RPM? This data is in the manuals for Part 23 aircraft but I can't seem to find this for non-constant speed Rotax. Basically, looking for something like:

 

0' 4800rpm 65%

0' 5000rpm 75%

....

3000' 4900rpm 65%

....

9000' 5300rpm 70%

.....

[FastEddieB]

Do you have the ROTAX Owner's Manual? I'm pretty sure it's in there.

 

Its available online.

 

If any of that is a problem, I could do a screen shot of the .pdf and attach it here.

[Ed Cesnalis]

Always kind of wondered this. What is the approximate percent power as a function of DA and RPM? This data is in the manuals for Part 23 aircraft but I can't seem to find this for non-constant speed Rotax. Basically, looking for something like:

 

0' 4800rpm 65%

0' 5000rpm 75%

....

3000' 4900rpm 65%

....

9000' 5300rpm 70%

.....

 

you need a variable to represent prop pitch or manifold pressure. something close to this represents best performance: 7,500' DA, WOT, 5,500RPM = 75%

 

your samples are simpler because those aircraft have fixed pitches otherwise they use mp.

[207WF]

It is confusing because we know that rpm is not enough to determine power at a given density altitude. Someone who is pitched coarser will be drawing a higher % of power and using more throttle to get the same rpm. The rotax operators manual, fig. 10.1.1.2 in my version, gives power output assuming full throttle at various rpms between 4000 and 5800, graphed against density altitude in meters. (Interestingly, it says full throttle is OK all the way down to 4000 rpm.) The y axis is output in KW. Max or 100% is 60 KW, so you can compute the percentages of power here. For less than full throttle I am not sure how to get the data from this.

WF

[S3flyer]

I saw the graph and it really didn't make sense to me. I know that to be correct you'd have to go through various prop settings, along with RPM and DA to get an accurate chart but I thought there had to be a reference setting somewhere.

 

I guess I'll go with a rule-of-them for non-turbo engines: full throttle at 7500' is 75% power. Unless of course, you're CharlieTango high up in the mountains

[FastEddieB]

I don't know if this is any help or not:

 

 

It's from my Sky Arrow.with as ROTAX 912ULS2.

 

The speeds will be higher in the CT, but fuel flows should be similar.

 

BTW, I'm not sure I trust the figures for the Sky Arrow as far as speed and range - my fuel burn does not seem to go up with altitude as the chart would suggest.

[S3flyer]

Thanks, Eddie. I saw the charts for your SkyArrow in another forum once and they don't make sense to me either. I can't see how 75% power is the same at 2K and above. I also see fuel flow increasing with altitude given the same RPM. This doesn't match my experience.

 

For instance, I flew 2.2 hrs, 5100 RPM and 4500' burning right at 10.0 gallons. This is basically 4.5 gph.

 

Not a big deal -- was just curious. I'm guessing for my prop setting: 5000 rpm/75% power/3000', 5150 rpm/75%/5000' and 5300/75%/7000' (plus or minus 100 rpm).

[207WF]

Again I have to think that Fast Eddie's chart is for WOT, as with a constant speed prop, like the tables in the manual that I referred to. If we can find a density altitude in those charts for which our WOT rpm matches what is shown (even by interpolation), then we would have a pretty good sense of the % power and fuel flow at that particular setting. The question then seems to be, if I back off WOT for lower rpms, how can I figure how much less power I am pulling? I would guess that with fuel flow data at the lower rpm setting we could get a good approximation, as the specific fuel consumption of the engine should be close to a constant over that range of rpms. In other words, the ratio of the fuel flow at the lower rpm setting to the fuel flow at the higher rpm (WOT) setting should be the ratio of the %power at the lower rpm setting to the %power at the WOT setting. WF