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Modifying Stabilizer Incidence - 7/7/2012 11:17 PM   
georgebonn


 

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Status: offline 		I purchased a very well kit-built Great Planes Piper J3 Cub (40). Today I measured the incidences of the wing and stabilizer. Each has an incidence of 6 degrees positive. The plans specify 2.5 degrees positive.

If the incidences are left as is, how will the flight characteristics differ from the prescribed 2.5 degrees ?

If the easily changed wing incidence is changed to 2.5 degrees with the stab incidence at 6 degrees, how will this affect the flight characteristics ?

The stabilizer is glued in place and covered. Is there a way to change the incidence to 2.5 degrees ?

Thanks to all who reply.............George

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RE: Modifying Stabilizer Incidence - 7/7/2012 11:30 PM   
jester_s1


 

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Status: offline 		Incidence changes can have a drastic effect on flight characteristics. If the incidences are that far out of spec, you do not have a well-built model.

I have to ask though, how did you measure your incidence? Did you level the plane on its datum line and use a quality incidence meter, or did you do it some other way? It sounds a little bit like you measured them with the plane sitting on its tailwheel, which will put the datum line at an upward angle.

Now for your questions, I need to clarify something. Do the plans specify 2.5 degrees of positive incidence for the wing relative to the stabilizer (which normally would be set at 0) or do they specify that both surfaces be set at 2.5 degrees positive to the datum line (which would be 0 degrees to each other). Those are two very different things. Given that it's a Cub, I would expect the former but I'm not familiar with that particular kit.

If you change the wing to 2.5 and leave the tail at 6, you'll nose over as soon as you advance the throttle for takeoff most likely. I'd be surprised if it would even be possible to get enough up trim to make the plane fly.

As for changing the stabilizer, all you can do is pull the covering and use an x-acto knife to break the glue joint. It's not a huge operation, but it will require a covering repair.

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RE: Modifying Stabilizer Incidence - 7/8/2012 12:12 AM   
big max 1935


 

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Status: offline 		Having built several of the Great Planes Cubs of all sizes I have always relied on the cutting of the kit parts for wing & stab placement .Real hard to build one of their kits wrong. Never had any flying problems & never bothered to check incidences . Cubs even the real ones want to fly with a nose down attitude if I remember right.I think your plane is OK as is. Max H. AMA 14958

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RE: Modifying Stabilizer Incidence - 7/8/2012 12:16 AM   
georgebonn


 

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Status: offline 		Thanks for your response.

The method by which I measured the incidences of the stabilizer and wing...........

1. placed the plane on a table on its main gear wheels and tail wheel

2. marked on the side of the fuselage a line from the centre of the propeller to the front edge of the stabilizer

3. raised and supported the rear of the fuselage such that the line from the propeller to the stabilizer was level

4. used a Robarts incidence meter on the stabilizer and wing to measure the incidences at 6 degrees positive

The plans and the Great Planes website specify .........."Degrees Incidence: stabilizer: 2.5 degrees; wing: 2.5 degrees (flat bottom is 0 degrees to engine)"

I interpret this as meaning that the wing and the stabilizer have 0 degrees incidence relative to each other; and 2.5 degrees relative to the datum line.

What will be the effect on flight characteristics is the wing and stabilizer incidences are left at 6 degrees ?

Again, thanks...............George

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RE: Modifying Stabilizer Incidence - 7/8/2012 12:33 AM   
Chad Veich



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Status: offline 		First of all, a line drawn from the center of the propeller to the leading edge of the stab may be handy as a reference but it is not necessarily the datum as defined by the designer. As long as both the wing and tail are zero to one another per the instructions there should not be any issue. The only other measurement that matters is the engine thrust line relative to the wing/stab. If the thrust line is parallel to your reference line then basically what you have is 6 degrees of down thrust which sounds a bit excessive. The first thing I would do is find out what the thrust line is in relation to the wing and stab and then go from there. I would agree with the assesment by big max above that, most likely, the airplane is built per the instructions and will be just fine. Unless the builder purposely messed around with the design or it was very poorly assembled then chances are good it is close to what the designer intended.

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RE: Modifying Stabilizer Incidence - 7/8/2012 1:33 AM   
Gray Beard


 

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Status: offline 		I don't recall anything about the last GP Cub I build but I'm sure they showed the datum line that they made the measurements from. It sounds like it was set at zero zero by the builder and your just shown a 6 degree from where you have it setting at what you think is level. Most any Cub will be able to be trimmed to fly well at zero zero. The datum line is usually shown on the plans. 6 degrees is a big bunch!!
If you can't find the datum line or don't have the plans give tower/GP a call and find out where these measurements were taken from. I think they are open 24/7. You do have a pretty good meter!

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RE: Modifying Stabilizer Incidence - 7/8/2012 1:43 AM   
georgebonn


 

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Status: offline 		Surprisingly, the datum line is not shown on the plans and I could not find it on the GP's website. My understanding is that all aircraft's datum lines are drawn from the centre of the propeller to the bottom of the stabilizer. Is this not correct ?

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RE: Modifying Stabilizer Incidence - 7/8/2012 1:51 AM   
Gray Beard


 

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Status: offline 		Kinda sorta? On some plans there is a line, sometimes two of them, one straight through the crank and beyond a bit, that is usually the datum line and you can use a straight edge to finish drawing it through to the aft end of the fuse. The other one is shown/drawn at an angle showing the engine thrust angle. It works very well as a datum.

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RE: Modifying Stabilizer Incidence - 7/8/2012 2:03 AM   
SeamusG



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Status: offline 		Get a 48" straight edge and align it with the bottom surface of the stab - draw a line and call it your datum line. Assuming, of course, that the stab is at 0 degrees incidence. Some kit plans extend the thrust line that's aligned with the crankshaft center line. If there is no down thrust built into the firewall then it will be parallel with the datum line.


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RE: Modifying Stabilizer Incidence - 7/8/2012 5:59 AM   
Chad Veich



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quote:

ORIGINAL: georgebonn
My understanding is that all aircraft's datum lines are drawn from the centre of the propeller to the bottom of the stabilizer. Is this not correct ?


Actually what we are referring to as a "datum" is, I believe, more correctly called a reference line. The datum is something entirely different and is actually perpindicular to the reference line we are discussing. See the link below for the definition of a datum line. I have never heard that the reference line must be drawn from the prop center to the bottom of the stab and, quite honestly, this would result in some very odd reference lines on some designs. Rather the reference line is simply a fixed line defined by the designer to be used as, wait for it, a "reference" to which dimensions and other data can be measured and compared. From my experience it is often the thrust line or the center line of the basic lofted shape of the fuselage but I don't think their is a fixed rule for where it "has" to be. I'd love to hear from one of our resident aircraft designers on the subject as I could be all wet in my understandings!

Definition of datum line as it pertains to aircraft design: http://aviationglossary.com/datum/

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RE: Modifying Stabilizer Incidence - 7/8/2012 7:19 AM   
Lnewqban



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quote:

ORIGINAL: georgebonn

Surprisingly, the datum line is not shown on the plans and I could not find it on the GP's website. My understanding is that all aircraft's datum lines are drawn from the centre of the propeller to the bottom of the stabilizer. Is this not correct ?

http://en.wikipedia.org/wiki/Angle_of_incidence

If both angles are equal, then, the airplane has the proper incidence angles and zero decalage (difference between incidence angle of the wing and tail).

In engineering and drafting, a datum (the singular form of data) is an arbitrary reference point, surface, or axis on an object against which measurements are made.

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RE: Modifying Stabilizer Incidence - 7/8/2012 2:55 PM   
WhiteRook



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Status: offline 		doesnt +6 degrees mean the leading edge of the stab is higher than the trailing edge ? which would make the nose want to
head up, no?

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RE: Modifying Stabilizer Incidence - 7/8/2012 3:07 PM   
LesUyeda



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Status: offline 		"which would make the nose want head up, no? "

No. The horizontal stab WILL fly level, unless you force to do otherwise, and the plane WILL take on the attitude that accomplishes that.

Les

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RE: Modifying Stabilizer Incidence - 7/8/2012 3:30 PM   
pkoury


 

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Status: offline 		Just for the fun of it how about this.

Raise the tail of the Cub until the horizontal stab is at 2.5 degrees. Measure the wing incedence. If they match then the builder probably went with the kit parts without modifications in which case I would test fly it before making any changes.


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RE: Modifying Stabilizer Incidence - 7/8/2012 3:33 PM   
SeamusG



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Status: offline 		Datum line: In SIG plans they have a line that runs thru the fire to the tail. The line intersects the firewall where the engine's thrust line intersects the firewall and it's at 0 degrees incidence. The line extends from a point "in front of the engine" to a point beyond the tail. SIG calls this line a "datum line" on their plans.

Yup, it's a reference tool that looks like a line (actually a plane - but in a 2D drawing it's a line) and is at 0 degrees incidence.


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RE: Modifying Stabilizer Incidence - 7/8/2012 3:39 PM   
gaRCfield



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quote:

ORIGINAL: georgebonn

My understanding is that all aircraft's datum lines are drawn from the centre of the propeller to the bottom of the stabilizer. Is this not correct ?


No, it's not correct. The aircraft designer will put the wings and stab at a strategic location, based on the desired performance. Some aerobatic designers may want the stabilizer right in the air flow, to achieve stab control or elevator control at low air speed by having prop wash flow over the tail. Some designers may put the stab higher or lower, to have 'clean' air flow over the stab during cruise conditions, etc.

An easy example is a "T-tail", typically seen on a jet where the stab is raised up to stay out of the jet exhaust gases.

Also don't forget that the engine will likely have right and down thrust.

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RE: Modifying Stabilizer Incidence - 7/8/2012 3:42 PM   
gaRCfield



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Status: offline 		..and agreed, that if the airfoils are to have the same incidence and they do, the error is probably in the reference line. Call the wings and stab 2.5deg, or whatever they are, and calculate the reference line from that. Then measure your thrust line to see if it's where it should be. It would be a lot easier to just change the thrust line than both flying surfaces.

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RE: Modifying Stabilizer Incidence - 7/8/2012 7:25 PM   
Gray Beard


 

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Status: offline 		I have never seen any model plane plans or instructions refer to it as a reference line so I will stick with what they call it, Datum. That way we will all be on the same page when talking about it.
I no longer have a complete wall to hang my plans from but I used to tack them up on the wall of my shop.{bigger shop} and line them out like Seamus said. At some point there is a straight line you can use. On the plans for the planes I'm building right now incidence isn't mentioned or showed. I so have a straight line through the crank and it has a angled line for engine thrust but the best line I found was doing as Seamus said. I used a rule along under the stab and discovered the perfect datum line is along the top of the fuse sides before the rest of the formers and sheeting is installed. In my case the incidence is set at zero zero with a flat bottom gull wing. It was easy, thank you very much Mr. Ziroli!!

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RE: Modifying Stabilizer Incidence - 7/8/2012 8:03 PM   
SeamusG



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Status: offline 		Just for the record - LT-40 plans

And, as Gene stated, datum stands on its own.


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RE: Modifying Stabilizer Incidence - 7/8/2012 10:12 PM   
tony0707


 

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Status: offline 		Hi
Put the plane in a foam plane cradel
Put a level on the sab and MAKE the stab read ZERO on the level - (dead center on the bubble)-shim it so it stays there
Now put your incidence meter on the wing (radio on if you have the meter placed on the ailerons )
The reading you get on the meter will be your wing incidence ( always with regard to the stab that is ZERO )
In this case the meter sb reading 2.5 degrees positive if correct
That means the wing sb 2.5 degrees higher at the leading edge than the trailing edge
You need to adjust the wing seat as needed to have the wing at 2.5 degrees positive location if it is not there already
Be sure to use your means of fastening you wing downthe incidence must be the same after all is tightened
This is a very good time to check your wing to be sure it is centered on the fuse
I use a piece of nylon fishing line with knots in it
Example lace one end of the nylon line at the end of the aileron (closest to the wing tip ) and extend it to the joint between the stab an rubberdo the same thing to the other side of the wing
When both lenghts of line are equal the wing is centered put a mark on the fuse so you can always have the wing square to the plane every time you install the wing on the plane
Building and flying more than most for 26 years
This sb the info you need to know and how to get there
REGARDS TONY

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RE: Modifying Stabilizer Incidence - 7/8/2012 10:47 PM   
Gray Beard


 

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Status: offline 		Let see if these wonderful servers will allow me to post two photos at one time?? I won't shoot for three, I know better. To show how Jim was locating his datum line and setting his incidence I took a couple of photos for you. One, I discovered my new build wanted a zero zero so I saw that the stab {flat, no airfoil} was sitting on the fuse and if I followed it along the fuse it ended evenly with the line going through the spinners center. SHAZAM, Datum LINE! You can also see the extra line from the spinners center showing a bit of down angle, what do you know, the engines down thrust angle. How simple is that?

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RE: Modifying Stabilizer Incidence - 7/8/2012 10:55 PM   
jester_s1


 

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Status: offline 		As alluded to by others, it's not super important that the stab, wing, and thrust line are right with the datum line nearly as much as it's important that they are right with each other. As you said, the stab would be hard to change so using it as the zero point (the datum line for practical purposes) will work to get everything flyable. Use Tony0707's method to get the wing and engine set to the right angles per the plans and go fly your Cub.

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RE: Modifying Stabilizer Incidence - 7/8/2012 11:00 PM   
Gray Beard


 

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Status: offline 		To go along with 707. I knew the stab was at zero and the wing so the first thing I did was set and mounted the stab at zero and made sure it was staright and level, it's perfect. To set the wing incidence I used my meter on the stab and made sure the fuse was perfectly set at zero in my building stand, used rubber bands to hold the fuse so it didn't move then used the meter to set the wing at zero. I also use a levels to get everything as close as I can, I have several sizes and shapes of levels to get things straight. However you do it try to be perfect when setting up a plane. A perfect plane will trim easy and fly straight, it's worth the time it takes. I think the OP just didn't have his plane on a correct datum when checking incidence and I would bet the plan was set at zero zero.

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RE: Modifying Stabilizer Incidence - 7/9/2012 12:18 AM   
Mastertech



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Status: offline 		Wings and stabs are what fly through the air, the fuse is just along for the ride and to hold the wing and stab in the correct relationship to each other. The wing provides lift and the stab keeps the wing at the proper angle to climb , descend or fly level otherwise known as AOA (angle of attack)

Your 6 and 6 is really zero-zero.

I've had airplanes I've moved the wing and stab 3-4 degrees and the only change is how the fuse looks flying through the air, otherwise it flew exactly the same.

BTW there is no airplane that flies at zero zero without some elev trim to keep the wing at a positive incidence.

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RE: Modifying Stabilizer Incidence - 7/9/2012 1:47 AM   
pkoury


 

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quote:

ORIGINAL: tony0707

Hi
Put the plane in a foam plane cradel
Put a level on the sab and MAKE the stab read ZERO on the level - (dead center on the bubble)-shim it so it stays there
Now put your incidence meter on the wing (radio on if you have the meter placed on the ailerons )
The reading you get on the meter will be your wing incidence ( always with regard to the stab that is ZERO )
In this case the meter sb reading 2.5 degrees positive if correct
That means the wing sb 2.5 degrees higher at the leading edge than the trailing edge
You need to adjust the wing seat as needed to have the wing at 2.5 degrees positive location if it is not there already
Be sure to use your means of fastening you wing downthe incidence must be the same after all is tightened
This sb the info you need to know and how to get there
REGARDS TONY


If the plans call for both the horizontal stab and wing to be 2.5 degrees and he starts with the stab at zero and sets the wing at 2.5 then his decalage will be off. If both wing and horizontal stab are 2.5 degrees then the decalage is zero. If the builder did not modify the stab and wing mount then it should be close to the 2.5 degrees called for in the plans. If he sets the stab at zero and adjusts the wing for 2.5 then his decalage would be 5 degrees.

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