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  #1  
Old 05-22-2007, 10:13 AM
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Default Transitional vs. Laminar

From what I have read, the airflow over the strakes is transitional, not laminar.
How do you get to a laminar airflow without disrupting the design?

I have found that the strakes are designed to be a non-lifting airfoil but stop short of producing laminar airflow.
I am guessing that the rational for this was due more to maintaining the 'EZ' label to the construction of the plane and avoid overly complex construction techniques where the rate of return was lower.

That said, I could use a symetrical airfoil which would satisfy the non-lifting airfoil criteria as well as the laminar airflow objective but as Marc pointed out, the original design is not a symetrical airfoil.

Has anyone ventured into this area?
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Old 05-22-2007, 11:33 AM
Marc Zeitlin Marc Zeitlin is offline
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From what I have read, the airflow over the strakes is transitional, not laminar.
Where would you have read that?

Over ANY airfoil (depending upon reynolds #) you'll have an area of laminar flow near the LE, then a small transitional region (where the transition is from laminar to turbulent), then the rest of the surface will have turbulent flow. For the canard (GU or Roncz), the laminar flow region continues to about the 50-60% chord point. On the main wings, it continues to about the 25-30% chord point. On the strakes, it's probably closer to the 15-25% chord point (all assuming clean surfaces - no bugs, no rain).

The transition area is relatively small, and the remaining chord length of the airfoil will have turbulent flow over it.

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How do you get to a laminar airflow ....
You create favorable pressure gradients so that the boundary layer stays thin.

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....without disrupting the design?
You don't. Especially on the strakes.

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I have found that the strakes are designed to be a non-lifting airfoil ....
There's no such thing as a "non-lifting airfoil". A barn door will produce lift if it's at a positive AOA. The strakes do not produce much lift at aircraft cruise AOA's (they're definitely lower on the lift curve slope than the canard or wings), but they are most certainly producing lift at all speeds, especially at slower speeds, such as during a climb.

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....but stop short of producing laminar airflow.
See above for the laminar flow region description. There's laminar flow over the front of all airfoils for some distance.

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That said, I could use a symetrical airfoil which would satisfy the non-lifting airfoil criteria as well as the laminar airflow objective but as Marc pointed out, the original design is not a symetrical airfoil.
First of all, the symmetry of an airfoil has absolutely nothing to do with the extent of the laminar flow region on it's surface. You can have symmetric (or non-symmetric) airfoils that have laminar flow over the first 20%, or over 60-65% of the chord length.

Since ALL airfoils will produce lift at appropriate AOA's, the symmetry of the strake airfoil may or may not keep the lift distribution the same, depending upon what incidence angle it's mounted at with respect to the fuselage.

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Has anyone ventured into this area?
What, exactly, are you trying to achieve?

The "King Kozy" was a re-airfoiled COZY that had tremendously cambered strakes to provide loads of extra lift at lower speeds, for attempted use out of much shorter airstrips. It never flew. A few folks (on the canard-aviators mailing list, IIRC) have changed the airfoil shape of their strakes on VE's or LE's. I've not heard of before/after comparisons of performance, or any indication that those aircraft perform substantially different than stock aircraft.
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Old 05-22-2007, 11:59 AM
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What, exactly, are you trying to achieve?
I am trying to avoid this:
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The transition area is relatively small, and the remaining chord length of the airfoil will have turbulent flow over it.
In the Long-EZ design the top & bottom of the strake is relatively flat. Most airfoils I have studied have a gentle curve to them which (in my non-rocket scientist understanding) preserves the laminar airflow over a longer span of the chord before transitioning to turbulent flow. Somehow I have an association between turbulent airflow and drag.

This is from a previous thread......which leads me to believe that this is an area where an improvement could be made.
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I don't believe your supposition that the strakes are symmetric airfoils is true (and the fact that they've got flats in them doesn't keep them from being an airfoil - they're just not very efficient ones).
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Old 05-22-2007, 12:24 PM
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Originally Posted by Marc Zeitlin View Post
Over ANY airfoil (depending upon reynolds #) you'll have an area of laminar flow near the LE, then a small transitional region (where the transition is from laminar to turbulent), then the rest of the surface will have turbulent flow. For the canard (GU or Roncz), the laminar flow region continues to about the 50-60% chord point. On the main wings, it continues to about the 25-30% chord point. On the strakes, it's probably closer to the 15-25% chord point (all assuming clean surfaces - no bugs, no rain).
Looking at eracers tufted wing/vert stab and Lynns tufted under cowl, the laminar flow seems to go beyond this, unless there is flow over Micro turbulence.
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Old 05-22-2007, 12:32 PM
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Looking at eracers tufted wing/vert stab and Lynns tufted under cowl, the laminar flow seems to go beyond this, unless there is flow over Micro turbulence.
You are confusing "laminar" flow with "attached" flow. Both laminar and turbulent flow are attached. You cannot tell whether flow is laminar or turbulent by looking at tufts - all you can tell is whether it's attached.

What Jack's tufting of the winglet intersection shows is that he's got far less detached flow than previously, but I guarantee you that the flow over the aft 2/3 of the wing/winglet has a turbulent boundary layer - not laminar.
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Old 05-22-2007, 12:39 PM
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Originally Posted by TMann View Post
In the Long-EZ design the top & bottom of the strake is relatively flat. Most airfoils I have studied have a gentle curve to them which (in my non-rocket scientist understanding) preserves the laminar airflow over a longer span of the chord before transitioning to turbulent flow. Somehow I have an association between turbulent airflow and drag.
While laminar flow is usually less draggy than turbulent flow, just adding "curvature" doesn't guarantee extended laminar flow. Very specific shapes are required to create the favorable pressure gradients. As stated, the Eppler 1230 airfoil used for the main wings is NOT an laminar flow airfoil - only about the forward 30% or so has laminar flow over it.

If you're trying to reduce drag on your aircraft, there are far more opportunities for doing so via cooling mods, gear leg fairings, intersection cleanup, etc. than with changing strake airfoil shapes. This is way down on the histogram of important drag components.
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Old 05-22-2007, 01:15 PM
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Okay, but I would assume a gain exits in this area, yes? I would venture that improved airflow in any area would lead to improved performance.
I've already covered such bases as NACA scoops. I bypassed the landing gear drag by going with the Infinity R/G.
I'm researching fillets for the best results when I get to that point.
No ouside antennas.

The strakes have always stuck in my mind as an area that could be improved.
I have a symetrical airfoil that I have been working with that provides a smooth transition to the center section spar area.
Everything looks good in AutoCAD but in noting your comment as quoted in an earlier thread regarding the fact that the plans do not have a symetrical airfoil, is this a problem?

I have the technical aspects as far as the actual construction of these strakes worked out.
I assume that while this may be a long way to go for a very short stick, it would be an improvement just the same.
I may be wrong but I don't recall seeing a 'flat top' airfoil on any aircraft.
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Old 05-22-2007, 03:30 PM
Marc Zeitlin Marc Zeitlin is offline
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Okay, but I would assume a gain exits in this area, yes?
Sure. But it may not be a measurable one. Gaining 0.01 kts really isn't worth any effort at all, is it?

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I would venture that improved airflow in any area would lead to improved performance.
Again, it has to be detectable.

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Everything looks good in AutoCAD but in noting your comment as quoted in an earlier thread regarding the fact that the plans do not have a symetrical airfoil, is this a problem?
Could be, but probably not. Given the infinitesimal drag decrease by doing it, though, why bother?

You need to look at the drag polar of whatever airfoil you think you're going to use (if one is available) and see what the drag coefficient is at the lift coefficient you plan to use. For cruise flight in these aircraft, the lift coefficient is very low, and the drag is consequently pretty low, too. The AOA's are low, and the chance of decreasing the drag enough by an airfoil shape change to be able to notice it is pretty minimal.

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I assume that while this may be a long way to go for a very short stick, it would be an improvement just the same.
Penny wise, pound foolish, in my estimation, but it's your $$ and time.

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I may be wrong but I don't recall seeing a 'flat top' airfoil on any aircraft.
You are wrong. Many of the new supercritical airfoils are substantially flat on top. Of course, they're made for minimizing drag in the transonic region, which doesn't apply to us at all, but they do exist.

See:

http://www.aerospaceweb.org/question...ls/q0003.shtml

for an example. By Whitcomb, no less.
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Old 05-22-2007, 03:59 PM
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Whitcomb supercritical airfoil
The basic design approach behind these shapes is to flatten the upper surface of the airfoil to reduce flow acceleration and to use a highly cambered aft section to generate the majority of the lift. The disadvantage of this approach is that aft-loaded wings shift the center of lift back which necessitates moving the wings forward. This design tradeoff results in larger pitching moments, the need for larger and heavier control surfaces, and the need for stronger and heavier wing structures.
Yes, as you said it seems that would not apply to our aircraft.
And yes, as you said, the plans strake airfoil is not very efficient.

So, back to the original question which is has anyone tried to improve upon this?
And.....would a symetrical airfoil suffice?

The objective here is to lower the reynolds number.
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Old 05-22-2007, 04:01 PM
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Quote:
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Yes, as you said it seems that would not apply to our aircraft.
And yes, as you said, the plans strake airfoil is not very efficient.

So, back to the original question which is has anyone tried to improve upon this?
And.....would a symetrical airfoil suffice?

The objective here is to lower the reynolds number.

mmmm - might you also reduce fuel - on dis one i agrees - there is MUCH lower hangin fruit to pick
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Old 05-22-2007, 04:13 PM
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mmmm - might you also reduce fuel -
In reality, by changing this to a symetrical airfoil, you increase the volume of the strake thus increase the fuel capacity.
As far as what this does to my project scope.......it is part of the original scope (no scope creep here.)
This is not a retrofit. It's not like I'm cutting off my winglets or something!

Bottom line is ...... this is part of my project.
I brought this to the forum to inquire as to what experience may exist in this area.....not to justify my design.
This is no different than what I see the REAL plane manufacturers building.
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Old 05-22-2007, 06:45 PM
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So, back to the original question which is has anyone tried to improve upon this?
IIRC, Bill James has a more airfoily shape on his V.E. strakes.

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And.....would a symetrical airfoil suffice?
Suffice for what?

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The objective here is to lower the reynolds number.
There are two ways to reduce the reynolds number (well, three). Shorten the chord, or lower the airspeed (or fly higher - reduce the density). The shape of the airfoil has absolutely nothing to do with the reynolds number. See:

http://en.wikipedia.org/wiki/Reynolds_number

The next question would be, why do you think you want to reduce the RN? Generally, drag coefficients are higher at lower reynolds numbers. If anything, you should want to increase the RN. Not that you can, but that's what you'd WANT to do.

I get the impression that you've done some cursory reading of aerodynamic theory, not understood it very well, and then attempted to redesign the aircraft without understanding the ramifications of the modifications you're proposing. I will say, you're in some good company here.
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Old 05-22-2007, 07:13 PM
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Thanks Marc.......as usual, very helpful.
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Old 05-22-2007, 07:33 PM
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Attached is what I am talking about regarding the strake airfoil.
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Old 05-22-2007, 07:47 PM
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Attached is what I am talking about regarding the strake airfoil.
I could swear by that drawing you are developing a higher pressure zone UNder the strake due to that larger curvature differential than on the strake top.
Am I seeing your drawing right?
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