Trying to get the most speed out of my CTLSi

[Andy A]

I adjusted the prop on our CTLSi last week and picked up a few knots.  Before the adjustment I was seeing 105 to 107 as TAS during cruise.  After the adjustment, at 3,500' (my most commonly used altitude), WOT will produce 5,600 RPM.   I am now getting about 112-115 knots TAS as my max speed.  If I pull it back to 95% or about 5350, I am getting 109-110 knots cruise speed.  Is that the best I can expect from this airplane?  By adjusting the pitch, I now can access the full range of the green RPM arc.  It will just barely hit 5,600 rpms...I have seen 5,610 rpm one time.  Since I have no intention of operating it above 5,500 rpm (in fact, I plan on using about 5,300-5,350 rpm for cruise) is there any reason to adjust the pitch of the prop any more?  Maybe the tundra gear on our plane is causing it not to go quite a fast as some of the others, or maybe its the additional weight of the fuel injected engine.  Is there anything else I can do to get closer to the 120 knot max speed without over reving the engine?

[Ed Cesnalis]

You are pulling back to what feels to you like 95% throttle but you are prolly down around 70% power judging by your speed.

Two things will pick up a bunch of speed, 1) flatter pitch so you are 5,500 at your cruise altitude; 2)Cruise setting becomes max speed setting.

Consider if you added two thousand feet at least to your cruise altitude, you speed would go up and your fuel burn down and noise will go down.  Personally I don't think it makes much sense to optimize for max cruise speed at low altitude because its too noisy and not comfortable.

[Roger Lee]

"WOT will produce 5,600 RPM"

Leave it here. You're in a good spot. Prop pitch should be set up at your average altitude which can be different for other pilots. Some may live at sea level and never really get over 2K ft. and others fly at 9K+ ft. The rpm should remain the same, but the prop pitch will be different.

When I did my big prop research project years ago 5600-5650 is the seat spot for a BALANCED climb, cruise, fuel economy and engine temp set up. We will all have days we may fly lower and some higher. Playing with anything more than you already have will just be chasing your tail. Each flight will always give different results. How you take off (flaps vs no flaps) angle you climb at, outside air temps, winds and the big factor is the human at the controls. No one fly's like a machine that does it EXACTLY the same each flight even if they think so. 

[Andy A]

Thanks for the advice.  Just to clarify two things.  First, the 95% power setting is what I am reading off the Skyview display.  This is the fuel injected version, so I am not sure if the carbureted models have this same reading on the display.  Secondly, I picked 3,500' altitude because I live next to the coast and pretty much at sea level.  All of my "fun" flying is below 3,500', most is around 1,500' and 2,000', where I am not too concerned about speed.  If I am going somewhere, 4,000 to 6,000 is the desired elevation, dependent on the winds.  Our field elevation is 90'.

[sandpiper]

Maybe your TAS readout is not accurate.

[Ed Cesnalis]

17 minutes ago, Andy A said:

Thanks for the advice.  Just to clarify two things.  First, the 95% power setting is what I am reading off the Skyview display.  This is the fuel injected version, so I am not sure if the carbureted models have this same reading on the display.  Secondly, I picked 3,500' altitude because I live next to the coast and pretty much at sea level.  All of my "fun" flying is below 3,500', most is around 1,500' and 2,000', where I am not too concerned about speed.  If I am going somewhere, 4,000 to 6,000 is the desired elevation, dependent on the winds.  Our field elevation is 90'.

 

That 92% sounds mighty suspicious, do you have a power / tourque curve for the Si?  

 

You can leave your pitch where it is but if your tach isn't  5,500 and your throttle isn't wide open you'r not getting best speed.

I have tundra gear and see TAS well above 120kts most of the time.  Pulling back to the 5,3xx range would cost me 10kts at least.

 

 

 

[Andy A]

My throttle is probably only 70% open when I am at 95% power.  I have to push it the other 30% further to get the final 5% of power out of the engine.  I have always thought that was strange.

[Ed Cesnalis]

Just now, Andy A said:

My throttle is probably only 70% open when I am at 95% power.  I have to push it the other 30% further to get the final 5% of power out of the engine.  I have always thought that was strange.

My guess is at wide open, because your prop is too flat you are less than 92%, probably less than 90%.  AT 70%throttle / 5,350  might be even less than 70%.

 

Do you have a power / torque curve / graph?

[Roger Lee]

"Do you have a power / torque curve / graph?"

The power and torque curve graph in the Rota manual doesn't really apply to us. It is predicated on a constant speed prop and other variables. It's only good as a general reference. Plus you have to factor in gauge inaccuracies, elevation differences and temp differences and aircraft differences.

5600-5650 is and will always be the best BALANCED rpm at WOT at your average altitude setting. Anything less than 5500 rpm WOT is over pitched and has zero redeeming qualities. A CT set at 5600-5650 will always be a tad faster than 5500. This was proven during the prop research testing.

Flying at 5500 vs 5300 usually doesn't make a 10 knt difference. This is where the human factor (changes in pitch whether you think your level or not and cross controlling, gauge inaccuracies and wishful thinking play a part. The extra fuel consumption and few extra knots usually isn't worth the extra rpm.

Unless less someone has four identical planes, 14+ different props and the ability to have only one set up person and take off side by side and fly side by side at the same time the study facts don't lie. These numbers have proven to be good for almost all that have set their system up to these specs.

The issue then becomes the human factor. Just like some said early on they could set 150 knts with -12 flaps. Same as before--Hogwash.( I tested that)

 

Bottom line chasing a few knots isn't worth the time effort and is usually faulty due to the human flying. It will always be different because no two flights have the exact characteristics. 

 

[Ed Cesnalis]

1 hour ago, Roger Lee said:

The power and torque curve graph in the Rota manual doesn't really apply to us. It is predicated on a constant speed prop and other variables. It's only good as a general reference. Plus you have to factor in gauge inaccuracies, elevation differences and temp differences and aircraft differences.

5600-5650 is and will always be the best BALANCED rpm at WOT at your average altitude setting. Anything less than 5500 rpm WOT is over pitched and has zero redeeming qualities. A CT set at 5600-5650 will always be a tad faster than 5500

The graph comes from a dyno and applies just fine.  Arguing that you are faster above the 5,500 cruise RPM limit is the same as arguing that you would be faster with a super dirty air cleaner restricting the flow.  Its a dumb argument because you don't throttle back to go faster.

[Roger Lee]

You throttle back because the extra rpm waste more fuel than speed increase. It isn't an efficient use of the resources at hand.

[Ed Cesnalis]

1 hour ago, Roger Lee said:

You throttle back because the extra rpm waste more fuel than speed increase. It isn't an efficient use of the resources at hand.

That's a change of subject from best speed (this thread) to an economy cruise setting.  The two are mutually exclusive, best speed and economy cruise.  Your prior post said the RPM range was proven faster but that's nonsense and this explanation fits but doesn't get him to best speed but rather a lower speed.

[coppercity]

10 hours ago, Andy A said:

I adjusted the prop on our CTLSi last week and picked up a few knots.  Before the adjustment I was seeing 105 to 107 as TAS during cruise.  After the adjustment, at 3,500' (my most commonly used altitude), WOT will produce 5,600 RPM.   I am now getting about 112-115 knots TAS as my max speed.  If I pull it back to 95% or about 5350, I am getting 109-110 knots cruise speed.  Is that the best I can expect from this airplane?  By adjusting the pitch, I now can access the full range of the green RPM arc.  It will just barely hit 5,600 rpms...I have seen 5,610 rpm one time.  Since I have no intention of operating it above 5,500 rpm (in fact, I plan on using about 5,300-5,350 rpm for cruise) is there any reason to adjust the pitch of the prop any more?  Maybe the tundra gear on our plane is causing it not to go quite a fast as some of the others, or maybe its the additional weight of the fuel injected engine.  Is there anything else I can do to get closer to the 120 knot max speed without over reving the engine?

What you are seeing with your new prop pitch seems about right for what I have experienced in the CTLSi's I have flown.  The injected motor does have a "power" and "economy" setting depending on throttle position.  In economy mode the power and fuel flow is decreased along with your speed, a throttle position slightly above this setting ramps up the fuel flow, power and RPM.  It can take a few prop adjustments to find the setting that can allow a 5300-5400 rpm comfortable cruise in the "power" band vs having to throttle back below 5500rpm and getting into the "economy" band unintentionally.

[Roger Lee]

The 912iS engine is in economy below 5300 and anything at 5300 or higher burns more fuel than the SW. Prop pitch plays a big part here.

[Grateful]

Try moving your CG aft with some weight in the baggage compartment.

[Ed Cesnalis]

120kts in Econo mode probably not going to happen.  WOT Baby!

[Andy A]

Its my understanding that the CTLSi is in Economy mode below 98% power as measured on the Skyview.  The engine is automatically leaned in Economy mode.  Any setting over 98% and the engine will go full rich, which increases fuel burn from around 5 gph to 6.5 to 7gph.

[Ed Cesnalis]

2 hours ago, Andy A said:

Its my understanding that the CTLSi is in Economy mode below 98% power as measured on the Skyview.  The engine is automatically leaned in Economy mode.  Any setting over 98% and the engine will go full rich, which increases fuel burn from around 5 gph to 6.5 to 7gph.

functionally that's the same or similar to my carb setup on my CTSW.  I can make the top speeds, as high as 127kts true and as low as 118kts.  I sure can't due it without WOT which is full rich and burns over 6 gallons even at high altitude (which makes sense since there is no leaning