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Andy A

-12 Degree Flap Microswitch

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I was told recently that the reason the CT’s in the USA are limited to -6 degree flap settings is to prevent their speed from exceeding 120 knots.  I was also told that changing out a flap microswitch would upgrade a CTLSi to the -12 degree setting.  I recently purchased a CTLSi with tundra gear and was trying to find a way to pick up a few extra knots to offset the loss of speed because of the larger wheels.  Does anyone know if the information I received was correct?  If so, where can I get one of those microswitches?

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i Andy,

I tested this on 4 CT's early on. We all flew side by side and take off side by side. The -12 did not get any better speed. If we could use a constant speed or in flight adjustable like the rest of the world we may gain a few knots, but as it sits in the US with a ground adjustable doesn't happen. I did  6 years worth of test with these CT's against other. It just wasn't worth the -12 or the -6. Their argument for -12 may sound good, but it didn't happen. FD's early idea to have the prop over pitched to 5200 rpm at WOT to slow it down also was a false. I did slow it down, but to its detriment and letting the rpm be up around 5600 doesn't make it cruise too fast. matter of fact 5200 WOT could cause case cracks.

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My CTLS has tundra and no fairings on. -6 flaps.

It hits about 123 knots.

I do not know why, but it's a fast one. Always has been.

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2 hours ago, Anticept said:

My CTLS has tundra and no fairings on. -6 flaps.

It hits about 123 knots.

I do not know why, but it's a fast one. Always has been.

This time of year I hit 127kts.  123kts is easy.

The single biggest item to tweak for speed on a CT is prop pitch.  If you are not cruising WOT @ 5,500 then you are not realizing all make able speed.

The now favorite pitch, recommended on this site,  that allows more RPM obviously sacrifices speed, you have to fly with a partial throttle.

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18 minutes ago, Roger Lee said:

Just remember your instruments are as accurate as you may like to think.

I don't rely on a simple read out in one direction.

One of the easiest ways to know if your flight instruments are accurate is to judge if the resulting computed winds are accurate.  If your IAS is overstated for instance you will calculate excess headwind component.  The resulting wind calculations are sensitive to error from your flight instruments.

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2 hours ago, Ed Cesnalis said:

This time of year I hit 127kts.  123kts is easy.

The single biggest item to tweak for speed on a CT is prop pitch.  If you are not cruising WOT @ 5,500 then you are not realizing all make able speed.

The now favorite pitch, recommended on this site,  that allows more RPM obviously sacrifices speed, you have to fly with a partial throttle.

Same here.  I have the smaller wheels, which helps a lot with drag it seems, as I think Ed has as well.  I usually see 127kt TAS when I'm trying to get somewhere, and I can get 130kt TAS at max continuous RPM of 5500 at optimal altitudes of 6500-8500ft.

I have flown quite a bit with Bill Ince, whose prop we set up identically to mine and his CTSW is a similar weight, but he's got tundra wheels.  My CT can walk away from his.  Strangely though, my airplane seems to be able to get the same speeds even with my main wheelpants off, *maybe* a knot or two difference.  So it seems that wheel size makes a big difference, but wheel pants not so much.  

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49 minutes ago, Roger Lee said:

Just remember your instruments are as accurate as you may like to think.

I have two different airspeed indicators in my airplane, they read within a knot or two of each other.  Also I flew beside another CTSW and we called out various speeds as we went to different settings and our numbers matched.  If my instruments are off, they seem to be off in other CTs in the same way.  In zero wind my GPS ground speed usually matches my IAS at low level, which also confirms my instruments are fairly accurate.

My speeds are high enough that I questioned my instruments, but I have spent a good deal of time verifying they are pretty accurate.  CTs in general are definitely among the fastest LSAs out there.

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The reason for the limit to -6° flaps is not because of top speed, but rather stall speed. The stall speed can not be more than 45 kts CAS in the clean configuration. The FAA considers the clean configuration with the negative flaps. With -12° flaps the CAS is more than 45 kts CAS. This info came from the Flight Design engineers at one of the shows when I was working in the booth.

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1 hour ago, Tom Baker said:

The reason for the limit to -6° flaps is not because of top speed, but rather stall speed. The stall speed can not be more than 45 kts CAS in the clean configuration. The FAA considers the clean configuration with the negative flaps. With -12° flaps the CAS is more than 45 kts CAS. This info came from the Flight Design engineers at one of the shows when I was working in the booth.

Makes sense.  So...what is the purpose of 12° reflex is if gives a higher stall speed but no more speed? 

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1 hour ago, FlyingMonkey said:

Makes sense.  So...what is the purpose of 12° reflex is if gives a higher stall speed but no more speed? 

I didn't say that it doesn't give higher speed, only the reason was because of stall speed not Vh. It does give higher speed, but you really need the controlable prop to take advantage of it.

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I have tundra gear and custom tiny wheel pants.  

The neg-12 raises the stall speed above LSA minimums and it is a faster configuration on the top end.  Roger did testing that doesn't agree of course.

I find in smooth air I can get a lot of speed in descents.  When I do this I might bust the 5,5000 continuous number for up to 5 minutes.

I try to keep my TAS from exceeding 145kt.

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The 45 knot stall speed is the main reason that FD didn't use -12. Also when flying at extreme altitudes -12 far less efficient climb. Zero flaps work better.

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2 hours ago, Roger Lee said:

 . . . when flying at extreme altitudes -12 is more harmful to climb.Zero flaps work better.

:tut_tut-3315: . . . now look what you've started!

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When I purchased my ct I would see 130kts IAS, when I got home I put the pitot/static test equipement on and found a static leak on the airspeed case, then I got 115kts IAS. These airspeed guages are cheaply made and with time will leak around the glass sealing gasket. These airspeeds don't even have an adjustable snubber to control needle shake but that can be done in other ways.

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9 hours ago, Roger Lee said:

The 45 knot stall speed is the main reason that FD didn't use -12. Also when flying at extreme altitudes -12 far less efficient climb. Zero flaps work better.

I have been climbing with negative flaps for a while now.  I'm not seeing that down side.

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With the fact the carbon cub got around the 120 knot top speed by putting a restriction in the AFM not to run the engine over a certain RPM for 5 minutes, I wonder if flight design could have done the same about not allowing landing with negative flaps.

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40 minutes ago, Anticept said:

With the fact the carbon cub got around the 120 knot top speed by putting a restriction in the AFM not to run the engine over a certain RPM for 5 minutes, I wonder if flight design could have done the same about not allowing landing with negative flaps.

I don't think so, because the stall limitation is in all conditions, basically it can't stall above 45KCAS *ever* in straight flight, not just on landing.

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Doublechecked the definition, you are correct. It can't stall higher than 45 knots in any VS1 configuration.

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3 hours ago, Anticept said:

With the fact the carbon cub got around the 120 knot top speed by putting a restriction in the AFM not to run the engine over a certain RPM for 5 minutes, I wonder if flight design could have done the same about not allowing landing with negative flaps.

Carbon Cub did not have a power limitation to avoid going over 120 kts at maximum continuous power. The reason for their limitation has to do with maximum empty weight, and the original ASTM formula for determining it. IIRC it was 2x 190 for pilot and passenger + 1/2 pound of fuel for each max continuous HP. There max continuous limit was 80 HP. This meant their empty weight couldn't be more than around 900 pounds.

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On 3/14/2018 at 5:42 AM, Ed Cesnalis said:

The now favorite pitch, recommended on this site,  that allows more RPM obviously sacrifices speed, you have to fly with a partial throttle.

What is the so-called "favorite pitch"?

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28 minutes ago, MileHighCTLS said:

What is the so-called "favorite pitch"?

able to achieve ~5,650 RPM at cruise altitude at cruise.

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17 minutes ago, MileHighCTLS said:

What is the so-called "favorite pitch"?

I'll bite and take a guess.

A prop pitch setting, which results in a wide open throttle engine RPM of 5600-5650, at your routine cruise altitude.

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7 hours ago, Tom Baker said:

Carbon Cub did not have a power limitation to avoid going over 120 kts at maximum continuous power. The reason for their limitation has to do with maximum empty weight, and the original ASTM formula for determining it. IIRC it was 2x 190 for pilot and passenger + 1/2 pound of fuel for each max continuous HP. There max continuous limit was 80 HP. This meant their empty weight couldn't be more than around 900 pounds.

That's really weird.

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6 hours ago, Anticept said:

That's really weird.

People often think the Carbon Cub is restricted because of speed, but I don't think you could make it go 120 kts if you wanted to. Take a look at their specs, and you will see its performance is no where close to the 120 Kt limit. In fact the VNE is only slightly more than 120 kts. Weight is also why American Champion's Champ is in the standard category instead of being offered as a SLSA. It is to heavy to comply with ASTM standards.

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