Rotax Power

[solarguy54]

So we have twoCTLS aircraft, a 2009 with a Neuform prop and a 2012 with a Sensenich.  The 2009 has always out-climbed and had a higher airspeed at WOT than the 2012.  I have both props pitched to run at close to 5500 RPM at WOT at cruise altitude.  I assumed the difference was the prop.  So yesterday, I took the 2012 up and recorded flight specs.  Climb rate, RPM, OAT, cruise TAS, etc. Landed, quickly switched props, and repeated the tests.  Turns out that the 2012 has a lower climb rate and about 10 knots lower cruise turning about 5200 RPM at WOT with the Neuform prop than the Sensenich.  So, it’s putting out lower HP than the 2009 CTLS.  Compression is fine.

I remember that the owners who flew it to AZ from Dallas for delivery mentioned that they thought performance had dropped from when they had last flown it 9 months before.  

So, I’m suspecting varnish in the carbs.  Both carbs are balanced at idle and at 4000 RPM.  Think it’s time for a carb overhaul?

[Tom Baker]

My airplane has experienced a few times when the power seemed down. In all cases I found the carb heat stuck in the on position with the control in the off position. One time the problem was on the back of the air box,  the other was an issue on the back of the control.

[coppercity]

Had the same issue with my 2009 one time after Cheryl complained of poor climb performance and lower cruise RPM.  Carb heat rtn spring had broke on back of airbox so carb heat was on with control fully in.  

[WmInce]

45 minutes ago, Tom Baker said:

My airplane has experienced a few times when the power seemed down. In all cases I found the carb heat stuck in the on position with the control in the off position. One time the problem was on the back of the air box,  the other was an issue on the back of the control.

 

34 minutes ago, coppercity said:

Had the same issue with my 2009 one time after Cheryl complained of poor climb performance and lower cruise RPM.  Carb heat rtn spring had broke on back of airbox so carb heat was on with control fully in.  

Can we assume both of these were CTLS's?

[Tom Baker]

9 hours ago, WmInce said:

 

Can we assume both of these were CTLS's?

Yes

[solarguy54]

Thanks, guys.  I'll double check that.

[Roger Lee]

Minor things can account for this discrepancy. Both plane must weigh almost the same. Both times the pilot must be exact with the same take off procedure. The flaps may be slightly different settings. Prop pitch is the usually culprit then aircraft weight second. Atmospheric conditions from one flight to other other makes a difference. This can change by the minute. Instrument inaccuracies also make a difference especially between two different planes.

When I did my big prop research project all these variables were removed. What I find with most pilots is wishful thinking. Like someone who says these wheels pants give me 2-3 more knots speed.

The only real way to test this that isn't fraught with inconsistencies is to fly each plane side by side on take off. Split the runway and slightly stagger the planes.

Unless the difference is huge I don't think I'd lose any sleep over it.

 

[solarguy54]

Wow, you guys were RIGHT ON!!!  I had to pull the engine mount/engine to remove the airbox and give it a thorough check.  Turns out that when the carb heat was OFF, the fresh air intake was partially closed.  And it was significant.  So I've lubed it and adjusted the cable.  Now it opens FULLY!!!  Can't wait to try it out and experience the increased HP.

Thanks guys!!!

Sid