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Old Mar 21, 2012, 08:04 AM
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Miami Lakes, Florida, USA
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How should a yaw string behave in a proper thermal turn?

I set up my 4-meter Xplorer with a few inches of yarn suspended by a wire above the nose and a keychain video camera mounted in front of the leading edge to record how it behaves during flight. The videos come out good but I'm not sure how to interpret the results. What should I expect the yarn to do if I make an ideal thermal turn?
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Old Mar 21, 2012, 08:50 AM
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It should point straight at the camera. Pointing left or right suggests the nose is slipping in a turn or the fuse is yawing in level flight.
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Old Mar 21, 2012, 09:10 AM
SoarNut
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Some info on real sailplanes use yaw strings:

http://en.wikipedia.org/wiki/Yaw_string
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Old Mar 21, 2012, 10:28 AM
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I used to fly full-scale gliders. The yaw string is much shorter on those (relatively speaking) and the convention was to keep it straight back.

Yours, Greg
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Old Mar 21, 2012, 11:00 AM
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Mike,

You might try some specific manuevers in order to gage their effect on the yaw string. Each manuever should be preceeded with a period of hands off level flight. First try just yawing the plane with the rudder. Slowly to one side or the other just to see how the string reacts. You may need some aileron to keep the wings level.

Then try some aileron-only slow rolls - maybe to 10 to 20 degrees. This time using elevator to maintain level flight. If your aileron differential is right for the speed the yaw string should not move. If it goes to one side or the other you can see if you have too much or not enough differential. You can also experiment with rudder mixing to see if it is better for canceling a yaw vector.

Look at the recordings and see how the yaw string has reacted. After you are comfortable interepreting the sting's (probably erratic) reaction you can try for coordinated turns.

Rick

PS - Where (or when) did you get the Explorer 4.0? Should Jamie be frightened????? (I am.....)
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Old Mar 21, 2012, 02:45 PM
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Mike,
Becasue the string is up near the nose, in a steady, coordinated turn, the free end of the string will point slightly away from the centerline, opposite side of the turn.

So, if you are in a steady left turn, the free end should be pointed slightly to the right of the fuselage center line.

If it is in line, then the turn is a slight skid. The tighter the turn, the more it should deflect to be coordinated.

For this reason, I have the string right in front of the wing and camera is then about 18" back.

Now when there is a rolling input, the string's action can be confusing. In a perfectly coordinated roll, moving ail should not deflect the string. You will have to pay close attention to which way the string (free end) moves as you deflect ail. If it moves the same way as the roll, you have a skid command and if it moves opposite the deflection you have a slip. The strings action here indicates if the ROLLING MOTION is coordianted or not.

Once the turn becomes steady and there is not a rolling motion anymore, the string now will show how coordinated the turn is. This is the coordinated turn part, which is a whole lot easier to get right than the rolling part.
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Old Mar 21, 2012, 04:57 PM
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Quote:
Originally Posted by fnnwizard View Post
Mike,
Because the string is up near the nose, in a steady, coordinated turn, the free end of the string will point slightly away from the centerline, opposite side of the turn.

So, if you are in a steady left turn, the free end should be pointed slightly to the right of the fuselage center line.

If it is in line, then the turn is a slight skid. The tighter the turn, the more it should deflect to be coordinated.

For this reason, I have the string right in front of the wing and camera is then about 18" back.
I had to think about that for a while, but I believe I understand what you mean. I can see that, because the fuselage is straight, it can never really be aligned throughout its length with the curving path of the plane. Would it be correct to say that aligning the wing perpendicular to the direction of travel is the real goal, as opposed to worrying about how the fuselage is aligned?

I'll work on relocating my camera and yaw string.

Rick: I flew that plane at FSS #1. You were there.
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Old Mar 21, 2012, 05:08 PM
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Quote:
Originally Posted by Miami Mike View Post
I had to think about that for a while, but I believe I understand what you mean. I can see that, because the fuselage is straight, it can never really be aligned throughout its length with the curving path of the plane. Would it be correct to say that aligning the wing perpendicular to the direction of travel is the real goal, as opposed to worrying about how the fuselage is aligned?

I'll work on relocating my camera and yaw string.

Rick: I flew that plane at FSS #1. You were there.
I should write that my mount for the string is right in front of the LE of the wing. The string is then over the wing when the plane is in flight.

Kinda like this:
Untitled (1 min 2 sec)
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Old Mar 21, 2012, 06:23 PM
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Originally Posted by fnnwizard View Post
I should write that my mount for the string is right in front of the LE of the wing. The string is then over the wing when the plane is in flight.
How's this?
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Old Mar 21, 2012, 07:25 PM
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Originally Posted by Miami Mike View Post
How's this?
Perfect... or should I say X-Xellent !
If you post your videos, I can help analyze them.
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Old Mar 21, 2012, 07:52 PM
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Yaw string

Quote:
Originally Posted by fnnwizard View Post
Mike,
Becasue the string is up near the nose, in a steady, coordinated turn, the free end of the string will point slightly away from the centerline, opposite side of the turn.

So, if you are in a steady left turn, the free end should be pointed slightly to the right of the fuselage center line.

If it is in line, then the turn is a slight skid. The tighter the turn, the more it should deflect to be coordinated.

For this reason, I have the string right in front of the wing and camera is then about 18" back.

Now when there is a rolling input, the string's action can be confusing. In a perfectly coordinated roll, moving ail should not deflect the string. You will have to pay close attention to which way the string (free end) moves as you deflect ail. If it moves the same way as the roll, you have a skid command and if it moves opposite the deflection you have a slip. The strings action here indicates if the ROLLING MOTION is coordianted or not.

Once the turn becomes steady and there is not a rolling motion anymore, the string now will show how coordinated the turn is. This is the coordinated turn part, which is a whole lot easier to get right than the rolling part.
I agree w/ fnnwizard for the most part.

In this videos
Yaw string on 2-meter Spider--take one (2 min 9 sec)
, the yaw string is on a post behind the trailing edge of the wing, and I was just letting the rudder mix and differential aileron settings take care of it. I seemed to have the mix about right for keeping the yaw string centered during rolling motions. Very little aileron in either direction seemed to be needed in constant-banked turns, so the rudder was basically staying centered, and the yaw string blew just a tad to the outside. I'm guessing that this was probably more efficient than giving some inside rudder to fully center the yaw string, and (since centering the yaw string would eliminate the rolling-out torque from the interaction between sideslip and dihedral) then adding some outside aileron to compensate and keep the bank angle constant. Seems like the latter would be less efficient, especially in this glider with a streamlined slender fuselage.

Tip: To film the yaw string deflection as the bank angle is changed, do reversing turns from 45 degree bank left to 45 degree bank right-- any adverse yaw will be more visible here, than in a simple roll-in from wings-level. The yaw string should stay fairly centered. Most people might want to tweak their rudder mix and/or differential aileron settings to accomplish this.

Tip: to film the yaw string deflection in a constant-banked turn, do lots of 360's.

You never would want the yaw string to stream to the INSIDE of a constant-banked turn.

Related thread: http://www.rcgroups.com/forums/showthread.php?t=1597991
See esp posts 4 and 8 (yaw string videos) and 3 and 11 (theory)

Related link arguing for leaving the yaw string slightly off-center (streaming slightly toward the outside) to reduce the need for outside aileron in a constant-banked turn (especially if you would otherwise tend to need outside aileron in a constant-banked turn) (pertains even to a yaw string over the wing, but naturally more so to a yaw string up at the nose where the deflection will be larger) : http://www.wisoar.org/Documents/Holi...Efficiency.pdf

Steve
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Old Mar 22, 2012, 12:55 AM
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In the movie the Final Countdown you can see a piece of yarn taped to the wind screen of the Tomcat. Yarn is the most accurate and fastest instrument you can put on an A/C. It's also used for airflow testing or tuft testing as it's called. I put some on the B-2 back in the early 90's.
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Old Mar 22, 2012, 08:00 AM
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Quote:
Originally Posted by Miami Mike View Post
I had to think about that for a while, but I believe I understand what you mean. I can see that, because the fuselage is straight, it can never really be aligned throughout its length with the curving path of the plane. Would it be correct to say that aligning the wing perpendicular to the direction of travel is the real goal, as opposed to worrying about how the fuselage is aligned?

I'll work on relocating my camera and yaw string.

Rick: I flew that plane at FSS #1. You were there.
Duh!
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