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Improving speed in my CT.


Roger Lee

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This post was copied from another site and is posted here as an educational tool. For most of us rpm is the major determining item for speed and other flight performance that we find favorable. Being over propped and only turning 5200 rpm WOT at you normal cruise altitude is detrimental to your overall performance in all facets. I hope this will generate some thought and post.

 

 

 

 

Improving CTSW Speed?

 

Ron

 

Any advice on improving CT speed performance? Had the VP 3-bladed prop installed by Flight Design together with a 2-axis autopilot. On PPL there are no speed restrictions. Currently average between 105kts and 115kts TAS, at -12 deg flaps, no wind and engine RPM (Rotax 912S) of 4700 to 5000rpm. VP prop is not constant speed and for ideal performance, I find that setting one notch above fully coarse is best. On autopilot, if I trim the nose to a slight downward attitude, it steps up speed about 3kts.

 

Am doing a lot of X-country destination flying and an extra few knots would be great...well psychologically at least.

 

This may have been dealt with already, if so kindly direct me to the forum page,

 

Kind regards...Ron (ZU-ELZ)

 

Sandpiper

 

 

Over here most of the CT's have their props set to give 5500 RPM, full throttle in level flight at the altitude you most often fly. I cruise at 5000 to 5200 RPM which gives me a TAS of about 120 knots at 3-5,000 feet MSL. We only are allowed -6 flaps. Minus 12 flaps, like you have should increase speeds 3-5 knots. I don't understand why you are so slow. Must be your prop pitch and the lower RPM's you run. Have you measured speed over a known course?.

 

CharlieTango

 

 

Your best speed is at 7,500' density altitude, flaps reflexed, and your prop pitched so that you get 5,500RPM at Wide Open Throttle.

 

"one notch above fully coarse" sounds like the biggest issue, for speed you want to be flat not coarse.

 

I"m pretty sure you could find 125-130kts true air speed while being kinder to your engine but it will be a lot louder.

 

cruising @ 4,700 to 5,000RPM doesn't tell the whole story, you have to consider mainfold pressure too. 5,000RPM @ WOT will be faster then 5,000RPM @ 50% throttle.

 

 

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The common misconception is that pitching your prop is a compromise between climb and cruise performance. The compromise is between climb/speed vs economy but economy is probably false economy since the Rotax is designed for higher rpm. The economy at worst case is still very good.

 

If you pitch so that you can realize 5,800rpm you will be optimized for climb at the DA that you can see 5,800 max. This makes little sense, mostly because the CT is such a good climber and you are limited to a few minutes at 5,800RPM.

 

Because the CT is normally aspirated speed will improve with altitude but only to a point and that point is 7,500' DA. The fastest configuration is with flaps reflexed, 7,500' DA and 5,500rpm with a wide open throttle.

 

The WOT rpm will vary with conditions and seasons and I have learned that it isn't worth adjusting the pitch time and again.

 

The CT is loud @ 5,500 WOT so good headsets with ANR that works at the Rotax's high pitch are important.

 

An aft CG can add speed by reducing the negative lift from the tail but this is destabilizing and not recommended by me. :unsure:

 

If you are limited to negative 6 degrees then make sure you are realizing neg 6 and not neg 5 or more (like neg 3-4). Same goes with neg 12.

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In the UK a colleague and I have found CTSW's with the static port fitted different ways.

 

Our group CT is fitted with the flush to the skin part forward and the raised step facing rear.

 

Another CTSW at our field has it fitted with the flush to the skin part facing to the rear, and the raised step at the front.

 

We believe the second way creates a venturi effect and reduces the static pressure in the static line. The altimeter and ASI will be affected. The aircraft will show a significant speed increase. We think there could be a 10kt increase in indicated speed depending on the airflow and a 100ft change in indicated altitude.

 

Does anyone have any thought on this?

 

John.

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This was a discussion a long time ago on another forum and it was determined that the "Dam" should be up front. I have seen them in all different positions.

But tell you what I'll do, tomorrow I'll call FD and get a definitive answer.

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  • 2 weeks later...

I have also wondered about the static port and asi. Mine was delivered with the dam facing backwards.

I had Roger turn it around 180 degrees to see what would happen and I ended up with erratic air speed. I was getting 40 knots idling on the ground. I had the port turned facing back again.

I tested my air speed with 2 of my friends and found my asi was reporting 5 knots slow at 80, 95 and 105 knots(my friends could not fly any faster, I like the ct). My asi was tested when I had my transponder tested, the asi was accurate to 1/4 knot at 130 knots isolated from the system. When I disconnected the static port from the back of my asi I got back the 5 knots I was missing.

My static port is right below my feet in the storage compartment, yours may be different. I would be interested if anyone else who has verified their air speed was off has tried this test.

 

Freedom

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  • 2 weeks later...

My experience was that with the dam in the back, as delivered, the IAS was about 17 knots too high at cruise (this confirmed by multiple flights up and then downwind using GPS groundspeed to calibrate). It seems to me the dam in the front would create a low pressure area behind it, which should increase the IAS. We rotated the dam to the rear and the IAS was much more accurate. I am sure each plane is different, though. - WF

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  • 1 month later...

I note that the round-the-world CT guys, flying way over gross weight, set their props to 5300 WOT. I asked why on the other forum and they said that they tried several setting snd this gave them the best gas mileage! I don't see how this squares with the way our thinking has evolved toward 5500 WOT. Any ideas? WF

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I note that the round-the-world CT guys, flying way over gross weight, set their props to 5300 WOT. I asked why on the other forum and they said that they tried several setting snd this gave them the best gas mileage! I don't see how this squares with the way our thinking has evolved toward 5500 WOT. Any ideas? WF

 

that isn't precise, 5,300 WOT @ what DA?

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