Adjusting Prop pitch, engine RPM's, vs EGT overheat at less than full RPM's

[All_In]

Has Vance describes my current state of mind with the fewest words?
Confused at a higher level.
The platform:
Rotax 915iS on an Airgyro AG915 Centurian with a new four-blade prop.
The max engine RPMs are supposed to be 5800, right?

I'm learning by starting with 18 degrees and working my way to 25 degrees.
I'm at 21 degrees today and will finish tomorrow with 25 degrees.

I'm speculating now that is no way I'm going to get 5800 out of the new prop.
At 18 - 20 degrees if I hold anything over 5100 RPM for less than a minute the EGT overheats. It will allow 1621 and turn yellow after that.
At 21 degrees I can hold 5200 RPM before it hits 1622. Better...

Tomorrow I fly 22 - 25.

This seems backward... Why more pitch keeps the EGT lower at a higher engine RPM?

 

[JEFF TIPTON]

Perhaps it is the combustion is more complete in the cylinder before going to the exhaust pipe.

Good morning, John.

 

[Aerofoam]

Static test, or flying?

 

[All_In]

Perhaps it is the combustion is more complete in the cylinder before going to the exhaust pipe.

Good morning, John.

Good one. I did not consider that.

 

[All_In]

Static test, or flying?

These results were test flying her each time.

I tried static first. But at 18 and 20 degrees the max RPMs were only 4,500 at 19 degrees.
1300 less than Max of 5800 and equated to be 800 RPM lower than test flying her.

Going to fly 20 again this morning when it is cooler than 75 degrees and not in the 90s as it was yesterday during the test.
High EGT may have been caused because the larger degree tests were performed when it was 10 degrees cooler in the late afternoon?

 

[DonBishop]

You might want to try asking the Rotax "experts" on the Rotax forum.

Rotax-Owner.com - Home

Rotax-Owner.com is a web based support system which provides a common access point for locating key Information - Education - Support for their Rotax Aircraft Engines such as the Rotax 912, Rotax 914, Rotax 912 iS, Rotax 915 iS, Rotax 503 and Rotax 582.

www.rotax-owner.com

 

[Aerofoam]

Just thinking out loud, possibly the increased pitch is causing better airflow across the cooling system.
Or. you have found a pitch that is more efficient for the given speed.
If you were static, I would be wondering about cavitation (Blade stall) which would generally increase RPMs because less work is being done and would reduce airflow....
Flight testing each change is the best, sounds like you are dialing it in.
Is max RPM a goal? Seems like less than max and good flight performance would be better...
Also, is 4 bladed the best for this?
usually more blades equals less efficiency, but more can be better for low speed, lower RPM use...

 

[Vance]

Has Vance describes my current state of mind with the fewest words?
Confused at a higher level.
The platform:
Rotax 915iS on an Airgyro AG915 Centurian with a new four-blade prop.
The max engine RPMs are supposed to be 5800, right?

I'm learning by starting with 18 degrees and working my way to 25 degrees.
I'm at 21 degrees today and will finish tomorrow with 25 degrees.

I'm speculating now that is no way I'm going to get 5800 out of the new prop.
At 18 - 20 degrees if I hold anything over 5100 RPM for less than a minute the EGT overheats. It will allow 1621 and turn yellow after that.
At 21 degrees I can hold 5200 RPM before it hits 1622. Better...

Tomorrow I fly 22 - 25.

This seems backward... Why more pitch keeps the EGT lower at a higher engine RPM?

Glad to read you are up flying John.

I recently flew a 915 powered Cavalon at near maximum takeoff weight on 100LL and the high exhaust temperature was what limited our cruise speed to around 80kts.

Sea level to around 6,000 feet didn’t seem to make much difference in the indicated air speed that put the temps in the yellow.

A mixture adjustment appears to me to not be an option with the Rotax 915.

It may be there is an adjustment that I am unfamiliar with.

My experience is lower engine RPM for the same power makes for higher exhaust gas temperatures.

We did not experiment with the Cavalon prop settings.

 

[All_In]

Glad to read you are up flying John.

I recently flew a 915 powered Cavalon at near maximum takeoff weight on 100LL and the high exhaust temperature was what limited our cruise speed to around 80kts.

Sea level to around 6,000 feet didn’t seem to make much difference in the indicated air speed that put the temps in the yellow.

A mixture adjustment appears to me to not be an option with the Rotax 915.

It may be there is an adjustment that I am unfamiliar with.

My experience is lower engine RPM for the same power makes for higher exhaust gas temperatures.

We did not experiment with the Cavalon prop settings.

This data point is very helpful. However, it turned out to be not testing with the same airport temperature.
This morning at 75F I was able to re-fly 19 and 20 degrees and no EGT overhead at all. Even to 5620 RPM.
It was over 90 the last time.
I'm back home waiting for the temperature to drop before doing more learning/testing which should be about 3 or 3:30 PM.
I was disappointed with myself I did not record the temperature for the first tests. I know better!


With this new 4-bladed prop I get a 10-knot cruise speed increase compared to the original fatter 3-bladed prop. This is when pitched to low-degree settings (= now 90 knots with new prop) and a 20-knot increase (now 100 knots) with 22 degrees, higher top speed settings. She used to fly at 93 MPH (80 Knots) with two people and 103 MPH with only me in her.
These test are all with two people and full fuel.

 

[All_In]

PS:
VNE is 130MPH. At 22 degrees I'm almost there.
It was 126MPH/ 109 knots and a 29-knot increase. So think I'll be done testing at 23 degrees and have to back off.