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Old Jan 20, 2012, 05:39 PM
G_T
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I believe if you read my posts in this thread I have been quite clear that the drag of the turbulators is not accounted for in the graphs I have presented. I have also been quite clear that real world testing is in order. I am not lucky enough to have access to a suitable wind tunnel...

Gerald
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Old Jan 20, 2012, 06:10 PM
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Gerald,
Thanks for the reminder. I just thought that maybe the transition had been plotted on one of the pics and I'd missed it or something.

Is it possible to do any of this testing on existing wings? Or is it something that would only have benefit on a designed airfoil? Getting some kind of difference on an existing wing would indicate that making a new wing would be really useful.

We did some testing several years ago, but the turbulators were placed near the spar locations and were simply strips of narrow tape. Still, I have vague memories that when only one wing was done, it influenced the the turn pattern of the plane. Probably AG series airfoils.

Gary
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Old Jan 20, 2012, 06:21 PM
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Philip,

Thank you for your EXCELLENT post on this subject, it is much appreciated!

Gary,

Gerald posted earlier how to test for trip strip effectiveness. Essentially, if you put a strip on one wing, you should see a yawing moment in the direction of the opposite wing as it would have more drag if the trip strip was effective. Make sense?

Now, Philip described a method to determine the location of the trip strip for a single chordwise station at a single point. Of course on an existing wing this can be further complicated by the use of different foils along the span of the wing, and locale Cl variation the wing sees. Therefore optimizing the location of the trip strip will take some effort and mostly be an approximation-it would be exactly correct at 2 points, or if molded into the wing could be made more or less 100% accurate. The other method would be to design a series of foils intended for trip strip use that allow for an accurate linear approximation of the boundary layer trip point, which is what Gerald has done. Of course one must still optimize the trip location for a desired Cl as Philip described.

Tom is testing the trip strip on the molded Zone V2, when the sun finally decides to shine! If he could stop sending nasty weather my way I could perform similar testing. I would be most curious to see the AG455ct results from previous experimentation; if these were fruitful, I wonder why more folks aren't flying with trip strips?
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Old Jan 20, 2012, 07:25 PM
G_T
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There are predicted differences in lift as well as drag.

If the turbulator works, and in a fashion that thins the upper surface boundary at the trailing edge of the wing, then the overall air mass is displaced downwards to a greater extent than if the turbulator does not work or is not present. Thus the foil behaves as if alpha is slightly increased.

Or I really should say that a bubble that thickens the boundary layer at the back of the foil makes the foil behave as if alpha is reduced, with respect to Cl.

Of course this will be alpha and Re dependent, as it is relative to bubble location and boundary layer thickness.

Gerald

PS - Anecdotally, Tom told me of when he did the 1-wing test on a strong throw I'll let him tell you his experiences...

PPS - As I guess I have to keep saying, the drag of the turbulator itself is not represented in these graphs.
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Old Jan 20, 2012, 07:40 PM
G_T
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If someone wants to do a quick and dirty turbulator test, try this.

Find the point where your wing's chord is half of the root chord. Make that the innermost location of the turbulator. Run the turbulator out to the tip.

At the innermost location of the turbulator, make it start just in front of the hingeline.

At the tip, have the turbulator hit about a half inch in front of the hingeline.

Some turbulators are more effective than others. Zig-zag is one of the better simple ones. One can make a usable zig-zag turbulator using pinking shears. The angles are not optimal but it should be better (I think) than a straight turbulator. Play with the thickness. The thinnest that works is best.

There is nothing really magical about these numbers...

I chose half chord as the starting point, as this is where the foil synchronization is starting to get rather bad. The turbulator should make an improvement in this. Also, the tips have great leverage. This part of the wing will give the most visual indication of yaw (drag) and lift differences.

The turbulator is relatively far back because I am looking to improve moderately high speed behavior, rather than low speed behavior.

If things work, when flying too slowly (say, not much faster than stall), both wings should behave similarly. There should not be a strong roll or yaw tendency. But, when flying a little bit faster than optimal (for the camber setting) then both yaw a roll may be different.

Be careful NOT do do full power launches until you get things sorted out. Otherwise you might not have a plane left. At high speeds, small differences in lift coefficients can give a noticable roll rate.

It is possible the turbulator location I am specifying may not be quite enough forward for best results. But I think it will be sufficient to show some differences. The best location is going to be dependent on the wing design.

The testing needs to be done under calm non-turbulent conditions, to show the greatest effect. It is then that the pilot can fly hands-off the most effectively anyway.

The plane needs to be well trimmed first.

Gerald

PS - If the half-chord location is closer to the tip than about 8", then start at about 8" (20cm). The last few inches of the tips have airflow that is not going to be very close to the 2D modeling in most cases anyway.
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Old Jan 21, 2012, 12:41 AM
G_T
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Quote:
Originally Posted by oakman7004 View Post
Over the years we(RCG) have discussed this topic and I always follow with
big interest since I have used trips towards the tips for many years now.

And just the other week I serached for a statement by Dr Drela were his says that the drag from the tips will in reality not be noticable. Not even in launch...(but I failed to find this statement in my seach efforts)

However, for me this is very much inline with my findings over the years. For me the trips (used on several different airfoils) gives noticable improvements (low speed) and no noticable drawbacks, not even in launch. I am using trips, today, that in theory are to high by 0.1mm the last 10 cm. Last, the improvemets are more noticable with higher wingloadings (like 15-18g/sqrm).

So my conclusion is that the drag increase is so small and other factors are way more important, e.g. fuse area, airfoil. and therefore one cannot notice the different in the increased drag.

/Jonas Ekman
Thanks for the feedback!

Jonas, could you post a picture of one of your turbulated wings? Also anyone else who has results they consider an improvement? It may help to see examples of what works. Also examples of what doesn't work could be useful...

To all those who have contributed to this thread, THANKS!

Gerald
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Old Jan 21, 2012, 01:30 AM
Graham Kirkland
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This is a very interesting thread - thank you Gerald. However, I get the feeling that I am missing something somewhere. If a strip of tape that is 0.1-0.2mm thick is sufficient to cause transition to turbulent flow, then surely our hinge gap on the top surface will cause transition. The hinge gap is a much larger disturbance to the airfoil shape than the tape. If this is true, then all our wings are turbulated at 70% of chord.

Graham
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Old Jan 21, 2012, 02:37 AM
G_T
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Moldies typically have taped hingelines...

Bagged planes often do not. But bagged wings are at times not as smooth. Surface smoothness is going to have an affect IMO.

If a gap is narrow such as might often be the case for a bagged wing, then the gap may have sufficient resistance to air flow that it might be fairly invisible until the flaps are dropped. Just in front and just behind the gap, at the surface of the wing, the airspeed is 0 (viscous flow sticks to the surface). Airspeed is low just above this, increasing as one moves away from the wing (assuming not under a bubble). What does it really mean for air of speed 0 to hit an edge or a narrow gap?

In a moldie though, there is usually a large channel under that gap. There may be a pressure differential from root to tip. This channel would then provide a pipe for air to move along. The flow is at roughly right angles to the flow over the wing. There is no reason for this flow to be at airspeed 0 at the surface of the wing.

So, this potential cross-flow may be what causes the notable difference with moldie wings compared to bagged wings. In a bagged wing the channel is much narrower therefore the resistance to flow is much greater.

Additionally, this channel may allow the formation of a vortex. The air may swirl rather than traveling linearly.

Anyway I think this is part of what is going on with gaps at flaperons in DLG wings.

Gerald
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Old Jan 21, 2012, 02:40 AM
G_T
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Here is the forgiving series at +4 degrees float setting. The usual caveats...

Gerald
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Old Jan 21, 2012, 05:43 AM
agnotology
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From Mark Drela, 2003:

"On my SuperGee and my XP3, the flow is laminar all the way to the TE in the fast reflex condition, on both surfaces. I verified this with an acoustic transition probe in a wind tunnel. This is with roughly a 3/32" hinge gap on top, and no gap seals.

In the high-camber high-CL case, upper surface transition is naturally at or just ahead of the hinge. This is close to the ideal transition location for this operating condition, so tripping due to the more-open hinge gap is not an issue."

http://www.rcgroups.com/forums/showp...7&postcount=15

I have been using tip trips on DLGs and various other gliders for years. I think the circling behaviour differences are quite large.

I have tried trips on only one tip many times over the years, but I cannot detect any yaw, even on gliders with known problem airfoils and big circling behaviour changes.

Selig et al did quite a bit of turbulator testing which is in Soartech 8. I'll have to look again, but it seems to me the drag increase from the turbulator at high speeds was below their measurement error.

I have found the effective position is quite far ahead of the XFOIL transition location prediction. I only became aware of the technique of looking for the negative friction area in XFOIL a few months ago, and haven't looked to see how that lines up with my experimental position.

Kevin
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Old Jan 21, 2012, 07:40 AM
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Philip thanks for your contribution on this subject!


Quote:
Originally Posted by G_T View Post
Jonas, could you post a picture of one of your turbulated wings? Also anyone else who has results they consider an improvement? It may help to see examples of what works. Also examples of what doesn't work could be useful...
I have done some experiments with tip turbulators (about 0.3 mm thick placed at the upper surface just in front of the hinge line of the flap). On some planes (e.g. Twister 1) I did not noted any difference and removed them. On my Turbo 2007 and Twister 2 I noted that the ailerons feel more direct when flying against stall speed. No other difference was noted during launch or during flight (both handling and performance). Due to this I decided to let them on.


A first “test” for XFLR5 could be to duplicate the tests performed at Re 100.000 shown in the paper of Dr. Gopalarathnam.

Regards,
Bas Breijer
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Old Jan 21, 2012, 01:54 PM
agnotology
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That is a great paper on turbulators, Benjamin/Bas! And thanks for the detailed info Phillip.

I have yet another paper that indicates there is quite a difference between a Laminar Separation Bubble (LSB), and the transition bubble used on newer RC sailplane airfoils. A LSB appears to be pretty rare. It is indicated by hysteresis in the lift curve slope and a drag increase in the middle of the drag bucket over a small AoA range. Their analysis and testing indicates that a LSB is just laminar separation with no reattachment at all.

It says the transition bubble on most airfoils at most RE and AoA, is very thin, and does not increase the drag. Since XFOIL only calculates the airflow right next to the wing profile, it does not predict the thickness of the bubble or any unsteady effects. Interesting reading anyway.

http://www-scf.usc.edu/~jmcarthu/Fin...aper_final.pdf

I have gone through Soartech 8 again, and they did measure a drag increase for turbulators. The increase varies with trip height, as you would expect.

Kevin

Edit: I just remembered this interesting site with some wind tunnel testing of turbulators on an RC glider. He had noticeable differences between a straight and a zig zag turbulator strip, which is different than the Selig testing:

http://scherrer.pagesperso-orange.fr....html#lastcure
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Old Jan 21, 2012, 02:16 PM
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Quote:
Originally Posted by G_T View Post
Thanks for the feedback!

Jonas, could you post a picture of one of your turbulated wings? Also anyone else who has results they consider an improvement? It may help to see examples of what works. Also examples of what doesn't work could be useful...

To all those who have contributed to this thread, THANKS!

Gerald
I did start to place the turbulators like Bas but ended up more forward since the turbulators behind 60-70% just affected the aileron efficiency.

I today say that the sweetspot is 55-60% of the cord. I have had them at 50%-73% and were I have then now they do the job well for me. I must thank Kevin (who is hell of a lot better in the aerodynamic theories than I am) who at least confirmed my findings with his some years back.

/Jonas
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Old Jan 21, 2012, 02:40 PM
G_T
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Here is essentially what we have to work with. This plot shows XFLR5's prediction of drag for a comparison between the Zone-V2 moldie and the forgiving series, in cruise camber. XFLR5 of course predicts lower drag for the turbulated wing. Quite a bit lower in the sweet spot, and only very slightly lower when one flies way too fast or way too slow.

The reality of course is the turbulator will add drag.

At the higher lift coefficient range, the turbulator should be buried within the bubble. Things could be strange then... The flow in the bubble may be reversed. The turbulator might remove a little energy from the bubble and return it as reduced drag. Pure speculation though, and I expect that effect if present would be extremely small.

At lift coefficients around 0.2 and smaller, I'd expect the drag added by the turbulator to increase the drag above that of a conventional wing. But at least we are not usually trying to fly at a lift coefficient around 0.2 when in cruise camber!

Clearly around Cl=0.4 there is room for the turbulator to add quite a bit of drag and still come out ahead.

Cl=0.6 is the middle of the Cl range for cruise camber for these wings.

Gerald
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Old Jan 21, 2012, 02:46 PM
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Hi Gerald,

let me ask two questions: Do you read other peoples posts at all - in particular what Philip Kolb wrote on page 2 (#29) in this threat?

Quote:
Originally Posted by G_T View Post
Find the point where your wing's chord is half of the root chord. Make that the innermost location of the turbulator. Run the turbulator out to the tip.

At the innermost location of the turbulator, make it start just in front of the hingeline.

At the tip, have the turbulator hit about a half inch in front of the hingeline.
Where did you get those numbers from?

Well, let’s do the analysis then. Starting with the cl-cd polar in your post #34 we find that the performace of your airfoil should be improved by a turbulator between cl~0.4 to ~0.55. Starting the analysis with cl=0.4 we use the skin friction coefficient (cf) to determine the location of laminar boundary (BL) separation. Plotting cf in Fig. 1 we find that the boundary layer seperates at x/c~0.55.
Since the boundary layer is quite thick at these Reynoldsnumbers and the laminar separation bubble is eliminated via forced transion, cl over alpha is changed sigificantly. The turbulator (if properly sized) will be ineffective when it is inside a laminar seperation bubble. Hence we should check with forced transition when laminar separation starts to occour upstream of the transition location. At cl=0.53 the boundary layer is just before separation at ~40% chord as shown in Fig. 2.

For a transition location at x/c=0.55, I would guestimate to place the turbulator at least 5-10% upstream of the desired transition location. If you place the turbulator further downstream (inside a laminar separation bubble), it would have to be quite thick to add disturbances into the boundary to destabilise it and force transition.
Fixing the transition location at x/c=0.55 will improve the airfoil performance between cl=0.4 to ~0.53. The part of the polar where airfoil performance is critical (cl=0.1…0.4) suffers quite significantly due to increased viscous drag as shown in Fig. 3.

This is why I think it is not possible to design a competitive DLG wing with turbulators. Guess I have said that before…

Gerald, it is really nice that you are making your designs available for everyone, but please check the results more carefully. Most people will not have the chance to do a check of the data on their own.

Cheers,
Benjamin
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