CG position

[Matt Piatkowski]

Hello,
Do I hang the RTF model using both or only rear lead-out to find the CG?
Thank you,
M

[Don Jenkins]

I hang the plane by both lead outs to get an approximate lead out position (about 3 degrees nose down) before the maiden flight, based on my limited understanding on how a plane skids around a circle, this has nothing to do with CG. 

Don

[Perry Rose]

Hang by both leadouts and place a line level on the nose. Adjust the slider to get a 1/4 bubble nose down.

[Dennis Toth]

Find the CG location on the fuse, set a straight edge (or a good eyeballing) to get the location at the wing tip. The center of the leadouts should be set about 3/8" back from hear to begin with. Then hang the ship from the leadouts and adjust for 3 - 5 deg nose down.

Best,   DennisT

[Dave_Trible]

Neither.  You don't find the CG by hanging from the lead outs.  That is to establish a beginning lead out position-(1 to 1 1/2 degrees nose down to begin).  The CG is found with the model sitting as is would normally on the ground.  A balance point is found fore and aft on or under the fuselage or placing fingertips under each wing tip until you find a balance point.  I have made a wooden fixture that sets on a table and supports points under the wing just either side of the fuselage.  I put some masking tape on the wing where the fixture touches.  When I find the balance I can reach underneath and mark on the tape with a pencil on both sides.  I then can measure the distance from some reference point (maybe flap hinge line).  In truth I seldom even worry about it.  I just go fly the thing and I'll have a pretty good idea of how it needs adjustment.  I like to use the balance fixture when trying to make like-designs fly about the same from bench trim.

Dave

[Jim Hoffman]

Dave Trible is exactly right.

The hang by the lead out technique is useful for the vertical CG. I believe the question is about the fore/aft CG

[frank williams]

Suppose you hang a weight on a string from the same point that you are hanging the plane.  ...... won't the intersection of this string with the centerline of the fuselage locate the fore/aft cg?

[Steve Helmick]

There's a string/plumb bob thing with a name like "Minerva" or something that you suspend the model in to give the exact CG. Somebody will remember the correct name of it, and you can search for it.  Steve

[Jim Hoffman]

There’s a device called a Vanessa Sling that is used to measure CG.  Most useful on models larger than CL stunters
I found it cumbersome for our stuff.
http://www.rcuniverse.com/forum/tips-techniques-180/708060-vanessa-c-g-rig-how.html

[Matt Piatkowski]

Hello,
Please see the attached. The yellow string is vertical.
I will move the lead-outs 5 mm. forward and fly the overhead maneuvers.
The model shows a tendency to deviate from the intended contour of, say, overhead eight.
The root cause is not known at this moment but I feel that today's tests will show a positive change in the model flight dynamics overhead. If moving the lead-outs 5 mm. forward will not help, I will disconnect the Rabe Rudder and immobilize the rudder 3-5 degrees outside. The rudder's area in this model is small but when the tension of the lines decreases, the rudder effect increases. I will of course watch closely the ninth corner of the horizontal eight, and the second corner of the hourglass. Rabe Rudder helps in these corners by counteracting the gyroscopic moment and I will see most likely a slight increase in the parasitic, momentary, yaw movement. 

Please expect the flight test results in the afternoon.

Happy Summer Flying,
Regards,
M

[M Spencer]

You use both ( leadouts ) to do the hanging . But  try doing it from the F & R while your at it . A massive change of ' sit ' may be detrimental .

A cardboard 3 degree wedge & a spirit level mightnt be a bad idea . to check the fuselage siting levelish

(we drag the other leadout up n down to check control freedom , under ' side load ' / line tension . )
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WHEN its hanging from BOTH , put a Finger Tip on each flap & elevator rear edge , and lever away . With another finger on the fuse to steady. or More .

A marvelous top notch number one plane , will have 5 microns or . 003 degree deflectiom .  Most have about 5 feet , 10 degrees ! 

3/32 horns will have a ton ,1/8 wire horns less . If your putting ' proportional ' loads to flight loads on em .
Some moving one ( say ) flap , will have all the others moving in sympathy .

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a Olde Tip (e) from the 60s was , pitching , in level flight , low . sets the rear C G limit / Groove .
my de H 88 , if one muffler fell off , it would have slight pitchng , at 5 foot Alt. Level .
If both fell off , it would steadlly porpoise .
Gaining altitude has a steadying influance ,
So the Idea is to see if its dead steady , low .
a ETA 29 Early Bird on 60 foot lines could fly at 6 inches , leaving a speed boat plume of early morning dew , or rain , in its wake ! .
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SO , the ' first ' set up , control do dah , is hang the wing , and pull n push the pushrod ( only ) to see theres no give at the primary interface , in noo speak .

about 10 ounce pressure at the trailing edges should get only a few millimetre ' give ' . Hopefully . Better like a rifle bolt than stirring jelly .

[Matt Piatkowski]

Report from the field.

Four full pattern flights. Ideal weather: 6 miles steady wind, broken clouds, ambient temperature 70 deg. F.
If the lead-outs position change was working, I was unable to see or/and feel it.

Next: I am disconnecting the Rabe Rudder. I will set the rudder's angle initially at 3 degrees and incrementally increase it.

By the way: the thrust direction is 3 degrees outside and in the X-Y plane of the model Cartesian coordinate system.

Thank you,
M

[PJ Rowland]

There are countless things that effect overhead 8 tracking.

Id start with moving the leadouts forward 1/8th  and maybe less tipweight by 2g and retest.. keep repeating till it goes away.. if its possible..

My biggest issue with using the hanging line technique is it doesnt take into consideration ANYTHING to do with leadout position,  relative to a motor pulling the plane, relative to line drag, relative to tip weight, relative to wing..

The Static hang test has been proven to be a wives tale. Take a plane that is "currently in perfect" trim then static hang and measure.  I guarantee its not level.

Or use Lineii and get it in the ballpark,  have 6 flights to closely adjust it. Then measure it ..

[Dave_Trible]

Just to throw a few things out to try if overhead line tension and therefore tracking is what you are working on:
If the airplane is a little too nose heavy it may seem to fly very well everywhere EXCEPT in overheads, especially if flying on the slow side.  Simply that excess weight begins to turn in at you on knife edge flight.  Retrim the airplane a little more tail heavy and reset the leadout adjustment rearward to follow the CG adjustment-these two work hand in hand.  If the airplane seems to fly away at the top of the vertical eight when the airspeed is at it's slowest, that's a good bet.  You can narrow your line spacing at the handle if the feel gets too sensitive by adding the tail weight.
You haven't mentioned lap time.  You can simply hit a wall where nothing is going to help much if you are flying too slow.  Also you could try lines that are a bit shorter.  Line drag could be excessive for the power you are using-more of a problem if your inboard wing is of longer span than outboard inducing more inboard drag.  You mentioned your engine offset-that should be plenty.  A fuselage that is not quite straight or a wing not installed truly perpendicular to the fuselage can cause these issues.  It can usually be overcome to some satisfaction with more of something else in the trim tool kit.
The choice of prop can make a huge difference.  More pitch and especially towards the tips of the prop can greatly improve overhead performance.  You may also try a slightly smaller diameter prop with a bit more rpm. 
Be very sure the wings are level upright and inverted and the feel of line tension is the same for inside and outside loops.  If it feels a little lighter one direction then you will find that manifests itself in bad ways overhead as well.

Dave

[Tom McClain]

First maiden flight checklist:

1.   CG at 18% of mean aerodynamic chord. Tank empty.
2.  Adjustable lead outs set 2 to 3 degrees back. This is done by hanging the plane by its leadotuts and seeing that the nose is hanging 2 to 3 degrees nose low.
3. Rudder, if adjustable, and even not, 2 degrees deflected to the outside. This is a good starting point for Rabe rudders.
4. Flaps level with bell rank at neutral and elevators slightly deflected down.  Bottom of tapered elevator parallel to bottom of horizontal stabilizer.
5. Enough tip weight that outboard wing definitely falls down when plane balanced on nose to tail centerline.
6. From rear of airplane make sure the flaps and elevators are level with their partner surface.
7. Slight toe in on main landing gear.
8. Tail wheel neutral or slightly deflected to steer outboard.
9. Test run engine to make sure of reliable operation upright, inverted, and outboard with  wing low with definite 2 4 break when raising the nose vertical.

Get a copy of Paul walkers trim chart.

Where did this checklist come from.  Paul Walker, Ted Fancher, Dave Fitzgerald, Brett Buck,  Windy Urtknowski, Keith Trostle, Bob Whiteley, Bob Hunt, Scott Richlen, Dick Houser and the NVCL gang, to name a few.

[Matt Piatkowski]

Report from the field: part 2.

The Rabe Rudder elimination and setting the rudder at 5 degrees helped a little bit in wingover and OH eight but the second turn of the hourglass and the ninth turn of the horizontal eight are a little worse. I feel that whatever I may gain in the judges' scores for some maneuvers, will be offset by the lower scores for other maneuvers.

By the way: Max Bee II has a small rudder and its effect is somewhat weak but deflection, when the Rabe Rudder was used, was close to 15 degrees. I still do not know if this deflection was correct. Some Max Bee II fliers use a very large momentary deflection, 25 deg. or so, some use only 5-6 degrees. How to find a proper range? This is one of those things you can only check if you are recording your flights and see them in slow motion.

Hi Dave,
The plane is not nose-heavy. It is very sensitive in the pitch channel and the CG is 1/2" aft w/r to the location shown on Igor's plans. The lap time is 5.3 seconds and I like it. I was flying the same plane with a lap time of 5.15 seconds and it always felt a little bit too fast in maneuvers.

To sum it up: I still have more questions than answers and will probably start testing another plane, called Big Yellow, that has the rudder motionless and set at 0.0. Perhaps by watching the flight dynamics of Big Yellow, I will learn something about Big Red? Please see them both ready for flights yesterday afternoon. Happy Flying, M

[Tom McClain]

Report from the field: part 2.

The Rabe Rudder elimination and setting the rudder at 5 degrees helped a little bit in wingover and OH eight but the second turn of the hourglass and the ninth turn of the horizontal eight are a little worse. I feel that whatever I may gain in the judges' scores for some maneuvers, will be offset by the lower scores for other maneuvers.

By the way: Max Bee II has a small rudder and its effect is somewhat weak but deflection, when the Rabe Rudder was used, was close to 15 degrees. I still do not know if this deflection was correct. Some Max Bee II fliers use a very large momentary deflection, 25 deg. or so, some use only 5-6 degrees. How to find a proper range? This is one of those things you can only check if you are recording your flights and see them in slow motion.

Hi Dave,
The plane is not nose-heavy. It is very sensitive in the pitch channel and the CG is 1/2" aft w/r to the location shown on Igor's plans. The lap time is 5.3 seconds and I like it. I was flying the same plane with a lap time of 5.15 seconds and it always felt a little bit too fast in maneuvers.

To sum it up: I still have more questions than answers and will probably start testing another plane, called Big Yellow, that has the rudder motionless and set at 0.0. Perhaps by watching the flight dynamics of Big Yellow, I will learn something about Big Red? Please see them both ready for flights yesterday afternoon. Happy Flying, M

I recommend start over with trimming the plane and work from there. Or you will be chasing your tail.

[pmackenzie]

My 2 cents - your low overhead tension is because the lap time is on the slow side.
Watch Igor's videos, usually he is 5.1 to 5.2 seconds/lap.

[Matt Piatkowski]

5.1 sec. per lap is too fast.
I feel comfortable with the lines' tension everywhere while flying 5.3 sec. per lap.
I feel comfortable with the model turning, grooving, and tracking abilities.
I feel comfortable flying all overhead maneuvers.

Moving the lead-outs forward was a bad idea for this plane. The lead-outs were most likely in the optimum position.
I do not know how to evaluate Rabe Rudder and its effect on the modern pattern without a camera and slow-motion analysis. Perhaps I will get one and start recording my flights?

Happy flying,
M

[M Spencer]

And LASTLY : https://stunthanger.com/smf/stunt-design/rabe-rudder/ Kieths set up . added Last . just now. After the rest . Read L to R however . 

How to find a proper range? This is one of those things you can only check if you are recording your flights and see them in slow motion.

Geees Wayne ! By FEEL , looseness , YAW primarilly . Banging ( instant full up & down ) the squares , you get YAW . ya wanna getherudderto gettito Fly straight . then n There .
The Top of the coicle , the top of the hourglass , and into the clover - in wind , count to a great degree too .

http://www.tulsacl.com/RabeRudder.html

theres some info , re setting a rudder .



Essentially set it around 1/4 in at T E out , neutral elevator , 1/8 out 2 full Up , 5/8 out @ full down . Rod from elevator L E Bottom . i.e. 45 from hinge down & back . 6 mm pivot from hinge .

looks like the photo just missed the front pin . But you can guess it from that . T E offsetrather than degrees .

essentially it seems you find a full up critical setting & a full down ditto . then you can adjust movement , and neutral , to match .

there was recently in the  a thread on kieth cam track operated set up , used in his Bearcat etc . a more refined version .

[M Spencer]

Al Rabe disc , 5.000.000 words . think his rudder set up , is in the mustunt III artical . https://www.youtube.com/playlist?list=PLkHUtj971Jgk_k-N5vVQX_as0dcVexiz4

A low winged diheadraled scale based Comp. stunt ship , due to its configureation , will trim out differant to a conventional set up .

heres some P 51 stuff . https://stunthanger.com/smf/open-forum/is-this-the-father-of-modern-stunt-ships/ to get a picture of progress .

You wanna read it through a few times ( appropriate section ) form a few questions , then digest the relevant Info . Eventually 12 somethings later , youve pretty much got it .

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oh dear ; https://stunthanger.com/smf/at-the-handle/let's-talk-flaps-cg-tails-and-asssociated-stunt-stuff-here's-a-start/ .
Your red ship , the elevators were below normal proportions . Normal Split is 60 / 40 ish T P / Elev . 50 / 50 for fingertip jumpey more hair trigger - fly every inch type types  .

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I cant find the picturesOf Kieth trostles rudder set up , but theres a tad of info here : https://stunthanger.com/smf/at-the-handle/rabe-rudder-on-'pusher'-props/msg572621/#msg572621
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NOW , Trim settings will CHANGE with the PROPELLOR , so you want to go for ballpark range initially . a guess at neutral - max inbd. & max out . remembering getting it to fly straight /
eliminate yaw ( & speed loss ) is the start of it . Its not gunna suddenly get 500 points better cores . But should eliminate some ragedness , get more even pull at the handle , and a SMOOTHER presentation ,
particularly flown hard & in wind .

If you do a 12 kilomerte jog , have a cold shower , then stick a finger in the prop to get a adrenaline hit , you should be on top form .        

[Steve Helmick]

There’s a device called a Vanessa Sling that is used to measure CG.  Most useful on models larger than CL stunters
I found it cumbersome for our stuff.
http://www.rcuniverse.com/forum/tips-techniques-180/708060-vanessa-c-g-rig-how.html

Jim got it right. This is the most accurate way to determine the CG. Be sure you don't drop the plumb bob through your model. Put a piece of tape on the model centerline (top or bottom) and mark where the plumb bob points. If you put it on the bottom, you can probably leave it on there and it won't be noticed during appearance judging if you are concerned about that. I'm pretty sure that Bruce Perry and Mike Haverly use this device.

As for the local users of the Rabe Rudder, I think all have had better results with less, rather than more.  Steve

[Mike Haverly]

Here

[John Carrodus]

Matt- Did I see yellow trimtabs on the elevator of your red stunter? Cheers
John Carrodus