Randy Ryan · Nov 05, 2001 08:24 PM
#0 sourceOK, here's my question. How would you define "stagnation point"?
Randy
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Airfoil Question
Stuka Stunt Main Forum · 13 of 13 known posts recovered
Preston Briggs · Nov 05, 2001 08:40 PM
#1 sourceThe way Fancher (and perhaps others (I'm confessing ignorance here, not try to twit Ted)) use it, the "stagnation point" is like the continental divide. Air molecules that arrive at the airfoil above the stagnation point flow over the airfoil. Those that arrive below the point, flow under. And we think of the point as infinitely small, so molecules actually bump into it.
Thi is my interpretation of the discussion in the "Doctor" article.
Preston
The problem, as I understand it, is that (especially) with blunt airfoils, this point can be inconsistent--the "decision" is actually made by pressure changes out ahead of the foil where flow changes occur due to compression effects.
turbulence can occur along a foil since at one point in span flow is differennt than at the adjacent points--some over, some under.
turbulence can occur along a foil since at one point in span flow is differennt than at the adjacent points--some over, some under.
Addition of the LE "trip wire" as described in the recent stab/ele configuration article in Stunt News and other threads, creates a "defined" stagnation point and therefore potentially reduces the turbulence.
I believe this is a different effect completely from the application of turbulators on a smooth wing, which are there to trip laminar flow away from the surface in order to change L/D situation.
Don't take this as absolute truth--I may have interpreted this stuff all wrong but its the best of my recollection. Hopefully someone else will validate this.
FWIW
Curt
LAST EDITED ON Nov-06-01 AT 09:15 AM (CST)
The wire along the leading edge of the stab is exactly what you discribed. It creates a definite seperation point along the surface. The theory is the the rounded leading edge on the stab will create some instability because the seperation point changes due to the rounded edge. I am not sure I follow this. I dont think the point of seperation moves. The most forward point is always going to be point where the air touches the stab. As long as you build your stab straight and the leading edge is true it should work fine. I beleive there is a direct relationship between the leading edge shape of the wing to the stab. I dont really know this for sure. You never see blunt wing blunt stab. Sharp wing Sharp stab. You always see The opposite.
As for Vortex generators and laminar airlfow go to this website it explains it all.
http://www.avweb.com/articles/vortexge.html
The laminar airflow across the wing becomes turbulant and seperates at a certain point aft the highpoint.(at low speeds) When the surface is smooth as galss the airflow actually seperates from the wing at low(landing) speeds and causes wing stalls.(It would actually fly better if it had a satin finsh over a gloss shine finish) With small turbulators attached to the wing at a certain point along the highpoint of the wing this will cause controlled turbulance in a spiraling manner. This way the air is not relying totally on the shape of the foil for it direction. As it passes the highpoint it hits the turbulator and is set into a spiraling motion as it travels past the aft section of the airfoil. This spiraling airflow will stay attached to the wing at much lower speeds creating better tracking a landing abilities for the wing in question. In a since it is actually cleaning the airflow and keeping it in a controlled motion as it passes along the aft section of the airfoil. You can actually fly the plane below its tested stall speed and still have plenty of stability and control. I have heard that it has been done on a stunter using small strips of tape and it was very successful. It would turn sharp and no stall. Check out the website. It will answer alot of questions about turbulators.
DMoon
Doug Moon
Turbulators and turbulators.
DMoon, in your referred article they are speaking about vortex generators and Cessna type wings. It's not the same as turbulators and laminar flow/profiles.
As a glider pilot (1:1 scale) i know that in laminar flow wings the turbulators are used to reduce drag, not to help stall speed or airflow at stall. On laminar wings the laminar flow changes to turbulent at some point and in that point it creates "laminar separation bubble". That causes plenty of drag and you get less drag if you change the flow to turbulent with turbulators just before that separation point. The flow changes, but there is no big laminar bubble. Almost all modern gliders use turbulators on the lower surfaces of the wings. They are positioned at about 65 - 85% from leading edge, depending on glider and profile.
Turbulators are also used in free flight models and then again for an other reason. So there are plenty of types and uses for turbulators.
DMoon, in your referred article they are speaking about vortex generators and Cessna type wings. It's not the same as turbulators and laminar flow/profiles.
As a glider pilot (1:1 scale) i know that in laminar flow wings the turbulators are used to reduce drag, not to help stall speed or airflow at stall. On laminar wings the laminar flow changes to turbulent at some point and in that point it creates "laminar separation bubble". That causes plenty of drag and you get less drag if you change the flow to turbulent with turbulators just before that separation point. The flow changes, but there is no big laminar bubble. Almost all modern gliders use turbulators on the lower surfaces of the wings. They are positioned at about 65 - 85% from leading edge, depending on glider and profile.
Turbulators are also used in free flight models and then again for an other reason. So there are plenty of types and uses for turbulators.
regs, Jyrki
Although I have not tried it, I remember reading years ago about guys adding wire to the wing LE to create a turbulent separation on relatively blunt airfoils. Some tried stand-offs to move the wire various distances in front of the LE. I THINK the final result was that for model speeds, right on the LE was best.
Consider also that laminar flow turbulators are probably not needed if you leave the finish on the model "less than perfect".
But then, perhaps those of us who apply the crappy finishes could not tell the flight difference anyway
George
Consider also that laminar flow turbulators are probably not needed if you leave the finish on the model "less than perfect".
But then, perhaps those of us who apply the crappy finishes could not tell the flight difference anyway
George
Mark Mitchell · Nov 07, 2001 10:19 AM
#7 source> But then, perhaps those of
>us who apply the crappy
>finishes could not tell the
>flight difference anyway
> George
I'm heartened to know that I'm not a crappy finisher but rather an aerodynamic sophisticate. Do dope brush hairs count as turbulators?
>Do dope brush hairs count
>as turbulators?
>as turbulators?
They may make just a "hair" of difference
George
Brush hairs are only useful if they lay on the wing perpendicular to the airflow. Cat hair works finer. Don't ask how I know.
Scott Riese · Nov 08, 2001 12:07 PM
#10 sourceWARNING....WARNING.......OVERLOAD........ERROR,ERROR
TMI..... SHUT DOWN!
TMI..... SHUT DOWN!
LAST EDITED ON Nov-08-01 AT 05:34 PM (CST)
Frog hairs don't work!
George
P.S. Geez...speaking of stagnation point...sorry!
Steve Helmick · Nov 08, 2001 09:10 PM
#12 sourceLAST EDITED ON Nov-08-01 AT 09:12 PM (CST)
Interesting subject, turbulation of stunt wings. As an old Free Flight flier, I've seen a lot of theories on turbulation. The guys from Scandenavia generally use a sharp LE (on FF's), with the idea that it creates turbulation. Dick Mathis used this idea on his FF designs and carried it over to the Excalibur and Coyote stunters. Others think a well radiused LE will create high pressure and help attach the airflow to the surface. This is a laminar flow attempt, mostly used on Hank Cole's CH-407 and other sections of his design. This particular airfoil was popular until the advent of zoom launching (trying to speed the glider into a climb above towline height of 50 meters/164'). It just would not go fast. If this makes you think of Ted Fancher's blunt LE's, you got it right.
Airflow tends to become detached (non-laminar) from the highpoint aft, which worries me when we are using flaps. I would not think flaps would be very effective if the airflow is not pretty well attached (laminar) to the hingeline. The FF guys (some) use spanwise strips aft of the highpoint called "Invigorators" to fix this situation, and I think it would be worth a good look. The NFFS has a lot of publications with articles on such things, and you can link to NFFS from AMA's website. This would make a great test project/article for Stunt News, and would be a natural for an engineering student project. If Invigorators do what I think they will, be ye prepared to revise flap areas and/or control ratios. Steve