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Rudder Trim tabs for MIIIC w/ 912UL
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John Bickham

Joined: 10 Jan 2006
Posts: 170
Location: St. Francisville, LA

Posted: Fri Mar 30, 2007 1:12 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

Hello list,



Getting close to finishing up the repairs and modifications on my Mark IIIC.  Finished cutting and fitting new Lexan yesterday.  Got some leftovers and started thinking about rudder and aileron trim tabs.  Not sure if there is a preference between aluminum or Lexan. 



Looked in the archives for sizes and angles.

From John H, May 2000:

 	  Quote: 	
		  The 21.75" X 3" tab bent 30 deg is too much trim.  I will

cut it down to 14.75" X 3" and test again.  Most likely I

will stay  this dimension since I do not want to drill out

anymore rivets, even though it is easy to drill the alum

fabric rivets.  I use a 2" lip to attach the trim tab to the

ribs on left side of rudder.

	




Any pictures and descriptions would be much appreciated.

_________________
Thanks too much,

John Bickham
Mark III-C w/ 912UL
St. Francisville, LA

I know many pilots and a few true aviators. There is a distinct difference that I have the greatest respect for.

JetPilot

Joined: 10 Jan 2006
Posts: 1246

Posted: Fri Mar 30, 2007 3:49 am Post subject: Re: Rudder Trim tabs for MIIIC w/ 912UL

THe lexan looks a lot better, but it is much heavier than aluminum.   Just be careful not to get to much weight out on the end if the control surfaces, or you can induce flutter.  I used aluminum on my ailerons and elevators for this reason, its also easier to bend if you need to adjust it.



Mike

_________________
"NO FEAR" - If you have no fear you did not go as fast as you could have !!!

Kolb MK-III Xtra, 912-S

Paul Petty

Joined: 11 Jan 2006
Posts: 226
Location: Mississippi

Posted: Fri Mar 30, 2007 5:43 am Post subject: Re: Rudder Trim tabs for MIIIC w/ 912UL

Hey John,



Wanna race to the finish? Im shooting for easter hehehe



do not archive

_________________
Paul Petty
Kolbra #12
Ms Dixie
912 UL 70" warp

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Fri Mar 30, 2007 5:55 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

| Any pictures and descriptions would be much appreciated.

|

 | John Bickham

Morning John:



The description of my trim tab was a little confusing.



Overall dimentions are:  14.75" X 5"



.025 aluminum works well.  I prefer it over lexan.



Bend a 2" lip on it 30 degrees.



Attach to rudder ribs with aluminum fabric rivets.  I like the large 

flange heads.  I think I used three rivets per rib, and it covers 

three ribs.



Attach trim tab to left side of rudder.



Works great.



john h

mkIII



PS:  Got my center section parts primed yesterday.  First time the 

spray gun has been out since I rebuilt the left wing in 2000 and 2001. 

Took me half a day to get set up.  Turned out ok despite loads of 

pollen, wind and dust.  Had to paint outside under a tent fly.  Also 

primed the gear legs which are holding up well considering some of my 

more spetacular landings.  If I am smart, and I have my doubts, I will 

wait until we have rain, which is finally forecast for this weekend, 

before I try to shoot Pontiac Red Aerothane.  Aerothane shows every 

little bit of dust, pollen, lint, and other contamination.

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Fri Mar 30, 2007 5:57 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

| Any pictures and descriptions would be much appreciated.

 |

| John Bickham

John B:



If you still need a photo of trim tab I will see if I can get you one. 

I have the fuselage in the basement, but the horizontal stabilizers 

are taped up to the rudder and vertical stabilizer.



Let me know.



john h

mkIII

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

Paul Petty

Joined: 11 Jan 2006
Posts: 226
Location: Mississippi

Posted: Fri Mar 30, 2007 11:06 am Post subject: Re: Rudder Trim tabs for MIIIC w/ 912UL

Gang,



What is the cause or source of air that requires our planes to need a trim tab on the rudder? Should I go ahead and make one for my Kolbra before it is flown for the first time?

do not archive

_________________
Paul Petty
Kolbra #12
Ms Dixie
912 UL 70" warp

Paul Petty

Joined: 11 Jan 2006
Posts: 226
Location: Mississippi

Posted: Fri Mar 30, 2007 11:21 am Post subject: Re: Rudder Trim tabs for MIIIC w/ 912UL

Mike,



I just re-read your post on trim tabs. You have them on your ailerons and elevators? If so can you take some photos and post to the list or send me directly? 

do not archive

_________________
Paul Petty
Kolbra #12
Ms Dixie
912 UL 70" warp

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Fri Mar 30, 2007 12:12 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

| What is the cause or source of air that requires our planes to need 

a trim tab on the rudder? Should I go ahead and make one for my Kolbra 

before it is flown for the first time?

 |

| --------

| Paul Petty

Paul:



Don't worry about trim tabs until after you thoroughly test your 

airplane.  Then, before you decide on what you what to correct and 

how, make sure you fly a couple long cross country flights, 

maintaining a heading and altitude.  Your arm will tell you exactly 

what you need and how much.



When it comes time to make a rudder trim tab, talk to John W and Mark 

G, who have been flying their Kolbras for some time.



Roll correction can be made with Steven G's excellent tool that 

changes the angle of attack of on wing.



Pitch correction can be made with elevator forced trim, and also flaps 

which you can adjust to give you some reflex if you have a tendancy to 

drop the nose.



Ov e4r the last 2,500+ hours, the tang that keeps the flap handle in 

place at neutral flaps has worn approximately 1/8".  The other day I 

popped a 4130 tab behind the worn one, cut a new knotch init with a 

5/32" chain saw file, and now I have flaps that are going to be a tad 

reflexed, I hope.  That'll help bring the nose up and maybe give me a 

little more cruise speed.



john h

mkIII

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

dhkey(at)msn.com
Guest

Posted: Fri Mar 30, 2007 5:26 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

Why does everyone have essentially the same sized trim tab the same 

direction on the rudder. It seems like something on the plane could have 

been designed differently.

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Fri Mar 30, 2007 5:33 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

| Why does everyone have essentially the same sized trim tab the same

| direction on the rudder. It seems like something on the plane could 

have

| been designed differently.

David:



It is not the airplane, it is the prop blast and the way it hits the 

tail section.



My mkIII is trimmed perfectly with the engine shut down and no trim 

tab on the rudder.  But when the power is brought up, the nose wants 

to go left and the trim tab takes care of that.



john h

mkIII

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

knowvne(at)aol.com
Guest

Posted: Fri Mar 30, 2007 6:42 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

Blame Rotax



 They build the 912 so it spun the prop in the wrong Direction....



 Mark



 --

Ed in JXN

Joined: 24 Mar 2006
Posts: 122

Posted: Fri Mar 30, 2007 6:42 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

Hi David,



        The culprit is a combination of forces present in all single-engine 

prop-driven aircraft (and most twins), too.  They are:

        Torque:  The twisting force the engine exerts on the prop.

        Spiraling slipstream:  The 'corkscrew' effect of the airflow over 

the aircraft structure, especially the tail group.

        Gyroscopic precession:  Think of the prop as a huge gyroscope, which 

it is.

        'P' factor:  The uneven (asymmetric) loading of the prop, more 

prevalent at slow airspeeds and high angle-of-attack.



        The need for trim tabs isn't the fault of the designer.  Maybe Sir 

Isaac Newton.



        The forces most prevalent in cruise are torque and spiral 

slipstream.  A good explanation for things like this is "The Pilot's 

Handbook of Aeronautical Knowledge", AC 61-23(X?), I don't know what the 

latest version is.  You can order it from the GPO or Sporty's (sportys.com).



        For those desiring even more punishment, "Aerodynamics for Naval 

Aviators" should keep you occupied.  Recommended only for those lacking any 

type of social life.



Ed in JXN

MkII/503



---

pj.ladd(at)btinternet.com
Guest

Posted: Sat Mar 31, 2007 1:53 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

Torque:  The twisting force the engine exerts on the prop.

 	  Quote: 	
		  >


	


Hi Ed,

in this instance is it not the twisting motion which the engine imparts to 

the AIRFRAME not the PROP which is the force which needs correcting?



Just a thought.



Pat

pj.ladd(at)btinternet.com
Guest

Posted: Sat Mar 31, 2007 1:56 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

It seems like something on the plane could have been designed differently>>



Hi



you are dead right. The fin could have been built slightly offset, and some 

are,  but think of the complications in the building process and even then 

it would only be absolutely correct for one speed and/or throttle setting.



Cheers

Pat



do not archive

Steve Boetto

Joined: 11 Jan 2006
Posts: 365

Posted: Sat Mar 31, 2007 3:31 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

In a message dated 3/30/2007 10:43:01 P.M. Eastern Daylight Time,  knowvne(at)aol.com writes:

  	  Quote: 	
		  

Blame Rotax



They build the 912 so it spun the prop in the    wrong Direction....



Mark

	


 

 Hi Mark,

 The 447 spins in the opposite direction, and guess what, the trim tab goes  on the other side.  Bryan Melborn of Custom Air mounts the same tab on  every plane and gets consistent results.

  

  

 Steve B

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  



See what's free at AOL.com. 

  [quote][b]

Steve Boetto

Joined: 11 Jan 2006
Posts: 365

Posted: Sat Mar 31, 2007 3:41 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

It is interesting to look at an example of a Tail-less Pusher like a  Long-EZ and see the results. On mine with a 160 hp engine and a 64x79 pitch wide  blade prop there was almost no rudder change across idle to full power. P factor  and torque are real issues but I agree with the Kolb Masters that the main  culprit is wash on the vertical tail.

  

 steve b



See what's free at AOL.com. 

  [quote][b]

d-m-hague(at)comcast.net
Guest

Posted: Sat Mar 31, 2007 3:45 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

At 05:52 AM 3/31/2007, pat ladd wrote:

 	  Quote: 	
		  



Torque:  The twisting force the engine exerts on the prop.



Hi Ed,

in this instance is it not the twisting motion which the engine imparts to 

the AIRFRAME not the PROP which is the force which needs correcting?


	


Newton:  Equal and opposite reaction.



                                 -Dana

--

--

The citizens of the United States are getting the government they deserve. 

The problem is that I'm also getting the government they deserve.

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Sat Mar 31, 2007 5:39 am Post subject: Rudder Trim tabs for MIIIC w/ 912UL

| It seems like something on the plane could have been designed 

differently>>

|

| Hi

|

| you are dead right. The fin could have been built slightly offset, 

| Pat

Patrick:



My thoughts exactly.



I tried to get the old Kolb Company to experiement with this solution, 

off setting the leading edge of the vertical stabilizer.  They never 

got around to it and I got tired of flying 1/2 ball out of trim.



I off set mine several times, eventually ending at 1 1/8" with very 

little improvement.  After all that drill, about three or four 

different locations, I repositioned the leading edge of the vertical 

stabilizer back to the center.



Then I went back to the rudder trim tab which was one rib bay long. 

Tripled the length to three rib bays.  That was too much.  Cut one rib 

bay off the length and came up with the dimensions and photos I shared 

a couple days ago.  Works perfect now.



I might add I also experimented with many different engine to airframe 

changes without success, eventually putting the engine back where 

Homer put it in the first place.



john h

mkIII

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

JetPilot

Joined: 10 Jan 2006
Posts: 1246

Posted: Sat Mar 31, 2007 6:57 am Post subject: Re: Rudder Trim tabs for MIIIC w/ 912UL

 	  Steve Boetto wrote: 	
		  

 

 Hi Mark,

 The 447 spins in the opposite direction, and guess what, the trim tab goes  on the other side.  Bryan Melborn of Custom Air mounts the same tab on  every plane and gets consistent results.

  

  

 Steve B

  

	




That say a lot for Bryans building skills.  If all the trim tabs on all his planes all turn out to be the same, thats as close to perfect as you will ever get.



Mike

  

  

  

  

  

  

  

  

  

  

  

  

  

  



See what's free at AOL.com. 

   	  Quote: 	
		  [b]

_________________
"NO FEAR" - If you have no fear you did not go as fast as you could have !!!

Kolb MK-III Xtra, 912-S

jbhart(at)onlyinternet.ne
Guest

Posted: Sat Mar 31, 2007 6:09 pm Post subject: Rudder Trim tabs for MIIIC w/ 912UL

From: N27SB(at)aol.com

Date: Sat, 31 Mar 2007 07:41:12 EDT

 	  Quote: 	
		  It is interesting to look at an example of a Tail-less Pusher like a 

Long-EZ and see the results. On mine with a 160 hp engine and a 64x79 pitch 

wide blade prop there was almost no rudder change across idle to full power. 

P factor and torque are real issues but I agree with the Kolb Masters that 

the main culprit is wash on the vertical tail.

Steve and Kolbers,

Out of curiosity, I looked at the magnitude of the propeller torque that has 

to be resisted by the FireFly in the roll axis for level flight.  Assuming 

that under cruise conditions the engine is developing a torque of 40 foot 

pounds.  Multiplying by the belt ratio (2.7) gives 108 foot pounds torque 

that the air frame must resist.  To compensate most add trim some where in 

the roll control system.  Another possibility is that one could add weight 

to the propeller down swing wing tip to dynamically balance out the torque.  

I would have to add 108/11 or 9.82 pounds to the right wing tip to prevent 

torque roll off at cruise.  For those who are moving up the hp scale, the 

amount could easily double and triple.  I would not want to try to takeoff 

or land a FireFly with a 10 pound weight on one wing tip.

Not practical but interesting.

Jack B. Hart FF004

Winchester, IN

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