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Questions!


Towner

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Couple of questions;

First, I’m getting a pretty bad shake right at rotation speed that continues for about 2 seconds after leaving the ground. It does not happen in flight, even if I try to copy speed, power, and AoA that happens at rotation. I’m guessing it’s an out of balance tire? I figure it continues a second or two after rotation until the out of balance tire slows or quits spinning? It just seems like a pretty bad shake for a little tire being out of balance. Any thoughts? I guess I should mention it’s the small stock tires and rims.

Next, I get a very slight shimmy under braking but only at a very, very slow speed. Usually just before coming to a full stop. Seems to be something with the brakes and not related to the problem mentioned above. If it was a car or motorcycle, I would think it was a slightly warped rotor. Is this common on our planes? It’s pretty light but I figure if anyone knows what it might be, I might need to fix it before it gets worse.
 

Last (at least for now), I still can’t land this thing! I’ve read every landing technique posted here and have tried them all, so my question may be a little different. I know my airspeed indicator is optimistic at cruise speed. I’m now wondering if it is also high at approach and landing speeds, causing me to carry to much speed when I think I’m not. Anything above 50 knots and 15 degrees and I’ll float forever. I’ve been as slow as 45 knots on short final with 30 degrees of flaps and it still seemed like I had plenty of speed. I did some stalls today with 15 degrees of flaps just to check the airspeed at stall. I was holding the airplane in the stall (mush) and it was showing 35-36 knots. It was just me and about 25 gallons if that makes a difference. Is this a lower than normal speed indication? I know some planes indicate a slower speed at stall just because of the angle of the pitot tube during high AoA. Being new to the ct series, I don’t know if this low stall speed is just due to the angle of the pitot tube and is common to all of our planes or if it may be a leak in the pitot system itself causing my airspeed to be off. Or, it could just be a rusty old pilot trying to learn a new plane!

Thanks in advance for any suggestions and advice!

Jeff

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I have NEVER found an aircraft tire in balance. I balance all tires. Several ask me what I did to their plane. I ask what do you mean. They said it didn't vibrate anymore. I just balanced the tires. Especially the front one. I just balanced a set of 4.00 x 6  8 plys and one took 9 weights and the other 6.

The other thing it may be is the front suspension may need new polyurethane dampeners and or the suspension guide pin. Lift the front enf about 2" and let it down firmly. It should be firm and no play up & down. Lift the front end and try to move the front wheel left / right. No more than about 1/4" free play.

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I'd say for sure an out of balance tire, that is exactly the symptoms I get with a main tire out of balance.  I have small tires and they can shake pretty hard, if you have tundra tires I'm guessing the vibration would be even worse.  You can try tapping the brake lever right after rotation to stop the wheel rotation and kill the shake, until you get a chance to re-balance.

Of course if you check the tire and find no problems you could have something else going on...maybe a loose axle or gear leg?  But I bet you'd feel either of those on landing too.  Do you get the vibration as you slow down after landing?  If I have a badly out of balance tire I'll sometimes feel in there too.

The landing issue is common, it took me about a hundred landings before I really figured this thing out, but I'm a slow learner.  🤪

Your speeds do seem off.  In my CTSW in calm winds I usually run about 55kt on final with 15° flaps, and 48-50kt at 30° flaps, down to 46kt for my shortest possible landings.  At around 45kt with 30° flaps the bottom starts to fall out and the sink rate goes way up.  It's sounding like your indicated speeds are about 5kt lower than those speeds.  You should definitely not see much float at 50kt with 15° flaps; that's about the lower limit for a safe approach and should be quite steep.  I would not put much stock in your low speed stall numbers, since the airspeed indication generally gets inaccurate below 40kt. 

I'd go through and make sure your static port is clear and check all the pitot/static connections to eliminate a pitot or static leak.  IIRC if your airpseed is indicating low than pitot leak is most likely.  But these airplanes are not the most precise, and all of them read a little bit different on speeds.  As long as you can figure out where *your* airplane flies and lands you can compensate for whatever is going on.  Three questions:

1) Is this a CTSW, CTLS, or CTSLi?

2) What is your indicated airspeed and RPM in calm air?

3) What's your wide-open throttle RPM in level flight in calm air?

If you have a pitot leak your cruise speeds should read low as well.  If you are showing something like 90kt at 5000rpm then that confirms a pitot leak assuming your prop pitch is in the ballpark of correct.

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Which wheels do you have? With the Marc wheels the disk can get sticky on the pins. The pins sometimes need cleaned and a slight bit of brake lube. 

If you have Marc wheels they can be balanced on the airplane, just remove the brake disk pins. For Matco wheels you will need some kind of balancer. Like Roger said they all need balanced, and often it takes 2 to 3 ounces of weight. Also another thought is if they had been in balance and now they are not take a close look at the tire. Some of the 4.00x6 6 ply tires had some manufacturing defects that allowed a belt separation and when this happens they get out of balance.

 

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I bought the Marc Parnes motorcycle wheel balancer and it works great when you put it up on a pair of jack stands:

http://www.marcparnes.com/Universal_Motorcycle_Wheel_Balancer.htm

It should work equally well with Marc or Matco wheels (I use it on Matcos).  It's the least expensive wheel balancer I have found that actually works.  Don't get one from Harbor Freight -- the axle rods are not true on them and they will drive you crazy because the wheel will show as balanced and it will still be WAY out.

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2 hours ago, FlyingMonkey said:

I bought the Marc Parnes motorcycle wheel balancer and it works great when you put it up on a pair of jack stands:

http://www.marcparnes.com/Universal_Motorcycle_Wheel_Balancer.htm

It should work equally well with Marc or Matco wheels (I use it on Matcos).  It's the least expensive wheel balancer I have found that actually works.  Don't get one from Harbor Freight -- the axle rods are not true on them and they will drive you crazy because the wheel will show as balanced and it will still be WAY out.

There is no need for a balancer for the Marc wheels. Just remove the disk and balance on the axle. 

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12 minutes ago, Jim Meade said:

After takeoff in my CTSW I always apply the brake momentarily.  WARNING! The brake lever is right beside the throttle.  Make sure you're pulling the correct lever.  If you are not comfortable doing this, then don't do it.

Agreed, that is a concern.  If you tap the brakes and the engine loses power, you've hit the wrong lever!  :D

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2 hours ago, FlyingMonkey said:

How can you ensure the axle is perfectly level?  If not the balance won't be true.

Or is this "close enough" to work in practice?

The first time you land on pavement you will lose a little rubber from one side of the tire, and the balance will no longer be true either. It works good enough.

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Well, it seems pretty unanimous that it's probably an out of balance tire. It's not going to be the polyurethane because I just put them on a month ago. Same shimmy before when I still had the spring. I'll get one of the tire balancers and move forward with that.

As far as my airspeed, checking at at cruise against the GPS, using multiple directions to compensate for wind, I'm indicating 8-10 knots high. 

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2 minutes ago, Towner said:

As far as my airspeed, checking at at cruise against the GPS, using multiple directions to compensate for wind, I'm indicating 8-10 knots high. 

So you are high at cruise and (seemingly) low at landing speeds...hmm.

I'm not a pitot-static expert, maybe somebody here with more experience on that subject has an explanation for that condition.

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It is a ctsw. The front end is solid as far a dampners, pin, axle and overall. It has the Marc wheels. I haven't done a great inspection of the main gear, but from what I've seen, everything looks good. 

Thanks again for everyone's input!

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3 hours ago, FlyingMonkey said:

So you are high at cruise and (seemingly) low at landing speeds...hmm.

I'm not a pitot-static expert, maybe somebody here with more experience on that subject has an explanation for that condition.

I was curious about this too. I didn't know how a leak may affect indicated airspeed overall. I thought it might be possible for a leak to cause a high indication at high speed, but gradually reduce the error as speed decreased, possibly even flopping to a low airspeed. I was just taking a shot in the dark...

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14 hours ago, Towner said:

I was curious about this too. I didn't know how a leak may affect indicated airspeed overall. I thought it might be possible for a leak to cause a high indication at high speed, but gradually reduce the error as speed decreased, possibly even flopping to a low airspeed. I was just taking a shot in the dark...

I would think a pitot leak would show greater error with greater speed as higher pressure air leaks faster, but it's also possible it's the other way around as the higher air volume can't escape fast enough from a very small leak.  As stated, not an expert!  :)

What doesn't seem right is a high indication at one end and a low indication at the other end of the speed range.

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30 minutes ago, Tom Baker said:

The static if open to the cabin will cause a higher than indicated airspeed at cruise.

When I did my panel upgrades last year I noticed the static line (which often has extra length allowing removal of panel with instruments still attached) had the line rubbing on things with poor routing.  I noticed one location that had nearly broke through the wall thickness, which I replaced and then added spiral cable wrap around it for protection going forward.

I'd suggest looking at the lines from instruments all the way back to firewall, and zip tie things up / cable wrap, even if no issues noticed.  Those Teflon tubes are not bullet proof.

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57 minutes ago, Tom Baker said:

The static if open to the cabin will cause a higher than indicated airspeed at cruise. If at slower speed with a higher angle of attack the pressure in the cabin increases it could cause it to be off in the opposite direction.

Well there you go, check your static system!

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13 hours ago, Towner said:

Well, I really appreciate all of the information, but at the same time it is frustrating! How do I get work done when I just want to go flying! Thanks again and I guess I'll do some tinkering instead of flying...

If everything is working well otherwise and you can fly safely, you can defer the tinkering to a "bad weather" day.  ;)

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Did you try the brake application trick after take-off to see if vibration goes away?  If so, it's balance.  If not, it's a defective tire(s) or a axle/wheel bearing issue as mentioned above.

A suggestion for landing.  This takes a long runway.  Do some take-offs at the flap setting used for landing (at full throttle, of course).  With slight back pressure on the stick, bring the front wheel off the runway.  Determine airspeed.  Now, while holding the pitch "as is" allow the airplane to rise off the runway on it's own. Determine airspeed and note where the horizon is out the windshield . The speed it comes off the runway and the angle of the plane relative to the horizon is what you will be looking for just before touchdown when you land.  I am a "40 flaps" lander so I find my CTSW has about a 40kt. final landing speed for touch down.  I can land without throttle but this is an "acquired ability" on our CT's and must be learned with many landings.  At all landings, depending on wind conditions, throttle application is sometimes required to prevent dropping to the runway.  One can get behind the power curve easily and the CT will drop fast needing immediate throttle to recover and arrest sink rate.  Instead, for the beginner, I would recommend some throttle, around 3,000 rpm on final landing.  This will cause some float and a slightly longer landing but it provides some air flow past the elevator and helps lessen the quick and unexpected drop to the runway getting behind the curve can cause.

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3000 RPM is pretty high in my opinion, but if you have 2500 RPM while gliding at 60 knots it will work nice. It just adds a little cushion, making the airplane feel like it has the kinetic energy of a heavier airplane. Most people have their idle set lower than that. That would equate to around a 2000 RPM idle on the ground.

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To each his/her own, and there are many ways to skin the "landing a CT" cat.  But I don't generally like planning to land with throttle.  I usually try to land power off, and use throttle as necessary if I'm not going to make the runway completely power off or the sink rate is too high at some point.  I will concede that a little power in will often make for smoother, flatter landings for beginners, but then you are training that method in from the beginning and it could be hard to "untrain" that later. 

One notable exception to my thoughts above is minimum speed landings.  If you are doing an approach down around 46kt, such as landing at a short grass strip, a little power in can tame the sink rate somewhat without increasing approach/landing speed.

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Tom, agree with the 2,500 rpm under certain flaps settings and runway lengths/conditions.   3,000 kts too high for 15 or less flaps on short runways. When landing with power on local 6500 ft. runway at 40 flaps doing easy touch & go's, I use 40 flaps at 3,000 rpm and this seems to work when I want to analyze my landings.  As Andy says, it slows the sink and doesn't add to speed.  Gives a slowed down landing for beginners and requires the new pilot to have some patience for letting the plane settle down and not be in a hurry to land.

Andy, I started out learning to land in the CT with power.  To transition into power-off landings in our light planes with very little inertia does require a pilot to teach himself to get the timing right for controlling sink, distance above the runway and speed during those final few feet above runway.  Our rotax engine's power and our CT's ability to easily climb with full flaps saved me a lot when I needed to abort a landing during my early power-off landings learning period.

 

 

 

 

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