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Sep 23, 2007, 04:02 PM
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Bubble Dancer


Hi folks
I have just completed the building of a Bubble dancer type model,I used a laser art kit for the ribs.
Set up C/G to 100ml as recommended ,no warps in wings ,lightened the tip panels.
But it tip stalls like mad, if i just introduce a small turning moment.
Only use small rudder deflection
Any one had a Tip stall problem with a Bubble dancer?
If so did you cure it and how please
I have a lot of years of experience but this one has me foxed
Help please
Good lift
Dandiy
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Sep 23, 2007, 04:22 PM
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jtlsf5's Avatar
This is typical of an aileron plane with no differential or no rudder coupling. You have no ailerons, so obviously not the problem.

Recheck each wing panel carefully. Check for wash-in or apparent neutral especially in the tip panels. Even a little wash-in in the center, mid or tip panels will cause this problem. Once you have the main and mids dead straight, add some washout to the tip. This should cure the tip stall. You don't need much, if you can get the tip TE 1/4" up with the tip panel root level, you should be fine.

JT
Sep 23, 2007, 04:24 PM
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rduder's Avatar
Try increasing the wash-out in the wingtips. (Twist-em down a bit while heating the covering) A degree or so @ each tip will show some improvements in the tip-stall characteristic. Keep tweaking them bit by bit 'til satisfied. You may want to get an inverter & try to operate a monocote heat-gun off of your car battery so this can be done at the field (some have been rumored to use car exhaust heat).
Sep 23, 2007, 05:18 PM
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Quote:
Originally Posted by Dandiy
I have just completed the building of a Bubble dancer type model,I used a laser art kit for the ribs. Set up C/G to 100ml as recommended ,no warps in wings ,lightened the tip panels. But it tip stalls like mad, if i just introduce a small turning moment.
Did you put in the required amount of washout into the mid panels?
This is essential on the BD.

Do not add any washout twist to the tip panels. This just adds high-speed drag, and doesn't do anything for tip stall. The tip panels stay flat, but are rotated overall by the mid-panel washout twist.
Sep 24, 2007, 03:18 AM
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Thread OP
Hi All
Thank you all for your advice special thanks to Mark(the Master)
I must admit I have removed the washout from the mid panel and put about a quarter inch in the outer panel,I will revers this now .
How critical is the C/G position on this section,Would a rear ward C/G cause this?
Thanks Dandiy
Sep 26, 2007, 11:45 AM
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Thread OP
Hi all
I have followed Marks advice.Washout in mid panel only
I have moved the C/G to 93ml from L/e the model is now under sensable control .
I can now set about the real trimming job
Thanks for all the advice
Good lift
Dandiy
Oct 02, 2007, 06:40 AM
Proud member of LISF and ESL
Quote:
Originally Posted by markdrela
Did you put in the required amount of washout into the mid panels?
This is essential on the BD.

Do not add any washout twist to the tip panels. This just adds high-speed drag, and doesn't do anything for tip stall. The tip panels stay flat, but are rotated overall by the mid-panel washout twist.
Mark,

Could you explain why washout from the mid pannel would produce less drag than washout at the tips? I don't quite grasp this concept as it would seem that washout from the center panel would produce a greater degree of twist to the total wing.

I have no doubt you are right, just wish to understand the concept.
Oct 02, 2007, 10:38 AM
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Quote:
Originally Posted by aeajr
Could you explain why washout from the mid pannel would produce less drag than washout at the tips?
On a typical poly wing, tip stall occurs just outboard of the outermost poly break, so that's where you need to reduce the local incidence angle. Adding washout twist to the outermost tip panel reduces the incidence mainly at the extreme tip, and not near the poly break where it's actually needed.

On a 5-panel poly wing glider like the BD or Majestic, twisting the mid panel does just about the right thing. On the more traditional 4-panel wing like an Olympic, you want to twist the inner panel, preferably mostly over the outer parts.

BTW, the main reason why washout has been shunned in RC gliders is that people have been putting it in the tip panels. Because it's ineffective there, you need a lot of it, sometimes 1/4" or more at the TE, which causes high drag at higher speeds. You need a lot less twist if it's put into the mid panels, which gives little high-speed penalty.
Oct 02, 2007, 11:15 AM
Quote:
Originally Posted by markdrela
On a typical poly wing, tip stall occurs just outboard of the outermost poly break, so that's where you need to reduce the local incidence angle. Adding washout twist to the outermost tip panel reduces the incidence mainly at the extreme tip, and not near the poly break where it's actually needed.

On a 5-panel poly wing glider like the BD or Majestic, twisting the mid panel does just about the right thing. On the more traditional 4-panel wing like an Olympic, you want to twist the inner panel, preferably mostly over the outer parts.

BTW, the main reason why washout has been shunned in RC gliders is that people have been putting it in the tip panels. Because it's ineffective there, you need a lot of it, sometimes 1/4" or more at the TE, which causes high drag at higher speeds. You need a lot less twist if it's put into the mid panels, which gives little high-speed penalty.
Thank you for your superior explanation on why I do not use wash out on my designs, center panels included.
Instead I use turbulation that is tunable for my wing tip panels. Speed run testing taught me to stay away from the tip by at least one chord length. Working from the wing joint out, the 12% turbulator is about a foot in length for a 40-mil thickness (that I can trim back to use the minimum to get the job done). To date I have found no situation that would force me to return to what I believe is the ‘archaic caveman twist’.
Oct 02, 2007, 11:16 AM
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Monster Mash's Avatar
I have the wind dancer and when I first got it I had a tip stall problem, since the wing was built by Polecat I just left it alone and adjusted my flying style.

I found that as long as I kept the speed up the plane would not tip stall. I have been flying it for a couple years now and it is definately my favorite electric sailplane.
Oct 02, 2007, 02:14 PM
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Mirage?


Quote:
Originally Posted by markdrela
On a typical poly wing, tip stall occurs just outboard of the outermost poly break, so that's where you need to reduce the local incidence angle. Adding washout twist to the outermost tip panel reduces the incidence mainly at the extreme tip, and not near the poly break where it's actually needed.

On a 5-panel poly wing glider like the BD or Majestic, twisting the mid panel does just about the right thing. On the more traditional 4-panel wing like an Olympic, you want to twist the inner panel, preferably mostly over the outer parts.

BTW, the main reason why washout has been shunned in RC gliders is that people have been putting it in the tip panels. Because it's ineffective there, you need a lot of it, sometimes 1/4" or more at the TE, which causes high drag at higher speeds. You need a lot less twist if it's put into the mid panels, which gives little high-speed penalty.

Prof. Drela,

Does your advice hold on true on a three panel wing such as the Mirage? I have decided to build a Mirage based on your comments in another thread and have ordered the short kit.

Thanks in advance,

JPHeritage
Last edited by jpherit; Oct 02, 2007 at 03:34 PM.
Oct 02, 2007, 02:25 PM
Registered User
Quote:
Originally Posted by jpherit
Does your advice hold on true on a three panel wing such as the Mirage? I have decide to build a Mirage based on your comments in another thread and have ordered the short kit.
It's tough to recommend how to add washout to a 3-panel wing, since it's not obvious where the tip stall will occur. The Mirage design does have some washout in the tip panels, but I'm not sure whether it's to give more tip stall margin, or to reduce the induced drag during slow flight. It would be necessary to build one without the washout to see what happens. Maybe someone has already tried this.
Oct 02, 2007, 02:52 PM
Registered User
Quote:
Originally Posted by markdrela
It's tough to recommend how to add washout to a 3-panel wing, since it's not obvious where the tip stall will occur. The Mirage design does have some washout in the tip panels, but I'm not sure whether it's to give more tip stall margin, or to reduce the induced drag during slow flight. It would be necessary to build one without the washout to see what happens. Maybe someone has already tried this.

Thanks for your comments. Unless I hear otherwise, I'll build in washout according to the plans.
Oct 02, 2007, 05:44 PM
Quote:
Originally Posted by jpherit
Thanks for your comments. Unless I hear otherwise, I'll build in washout according to the plans.
I flew the Mirage for 6 years with wing tip anti stall turbs successfully. The blank strip started 1 chord length from the tip and about 12 inches in length inward (40 mil ht.). I nibbled about 1 inch off, and it stayed that way. This machine did cross countries off a short high start, and placed in a few unlimited events. [There was no evidence of a drag problem when loaded up to 11 oz/ft.]
Oct 02, 2007, 06:56 PM
Registered User
Quote:
Originally Posted by histarter
I flew the Mirage for 6 years with wing tip anti stall turbs successfully. The blank strip started 1 chord length from the tip and about 12 inches in length inward (40 mil ht.). I nibbled about 1 inch off, and it stayed that way. This machine did cross countries off a short high start, and placed in a few unlimited events. [There was no evidence of a drag problem when loaded up to 11 oz/ft.]

Thanks for your comments. Let me repeat what you said in case I did not understand clearly.

You attach a 40 mil high strip 12 inches long on each of outer panels. The strip starts one cord length from the wing tip.

Question: Where did you nib off one inch, tip side or root side? Also, how wide is the strip and how far from the leading edge? And finally, did you build all three panels flat and assemble them without any twist between panels?

Finally, don't the strip spars that run the entire wing length on the front top of the wing act as turbulators? Is your turbulator in addition to these built-in ones?


Thanks

JPH


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