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Thread: Scaled-Up Airfoils???

  1. #16
    Registered User Norman's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by SpainCub View Post
    Wait, I thought that the Scale Effect was a possitive outcome from 1-9 10^6 Re, as the skin friction with a turbulent boundary layer decreases as Re increases? Is this not the same a high Re numbers... only subsonic flights and in the rage explained (need to check if that is the limits, but that's my mental note) I recall reading something in Chapter 5 of Theory of Wings to support this...

    The biggest problem is, using an airfoil designed Re 1<10^6 and scaling it below that Re... going the other way should prove positive... hence numerical theory predicts that higher Re yields higher Cl as the turbulent boundary layer is moved backwards on the airfoil... flattening out the parabola (slightly) as you increase Re...
    TOWS was written a lllllong time ago. At that time the NACA 6xxxxx airfoils were the state of the art in airfoil design. They were designed to have a smoothly curved inviscid pressure distribution up to the minimum pressure point and then a straight line to the trailing edge. The idea of a transition ramp was far in the future. This is fine for high Reynolds numbers because the flow transitions to turbulent pretty fast after the change from favorable to unfavorable but at low Re that corner where the pressure distribution changes from favorable to unfavorable will cause the laminar boundary layer to separate and transition off-surface then re-attach thus forming a laminar separation bubble with lots of pressure drag in addition to the skin friction drag. There's also that funny little joggle at the leading edge that bothered Harry Riblett so much but that's another issue. They had stopped doing low Re work by then so nobody noticed these problems.

    So the non lift induced drag is a combination of friction and pressure. A laminar BL produces a fraction as much friction as a turbulent BL so you want laminar flow as far aft as possible without making a large separation bubble. At high Re that's fairly simple because the BL transitions over a short distance and bubbles tend to be small. At low Re (say <500,000) transition takes longer and bubbles can get pretty big. The transition ramp of low Re airfoils is not there to prevent a bubble from forming, just make sure it forms as far aft as possible and not get too big. If you operate one of these airfoils at high Re transition will always happen too far forward and it will have a lot of friction drag. Other than that I don't know of a problem using model airplane airfoils on full sized airplanes.

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    Registered User wsimpso1's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by BBerson View Post
    Sometimes they break the sound barrier, I think.
    Warning - Geek response follows:But they can only get to supersonic flow if they reach sonic before they get to the expansion portion of the duct.

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    Registered User Aerowerx's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by mcrae0104 View Post
    Do you have a drag polar at the lower and higher Re?
    From XFLR5, some curves for the PRANDTL-D root airfoil:

    CL vs CD
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    CL vs Alpha
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    Pressure distribution
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    CL/CD vs Alpha
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    For the MH46,

    CL vs CD
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    CL vs Alpha
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    Pressure distirbution
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    CL/CD vs Alpha
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    All these curves are done at Re=400,000 (little squares) and Re=4,000,000 (little circles).

    I see that XFLR5 does not like the stall region for the MH46 at Re=4,000,00

    I also see the separation point moving forward with higher Re, as Norm mentioned.

    Some squirrely things going on at the leading edge for both airfoils.
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by wsimpso1 View Post
    Warning - Geek response follows:But they can only get to supersonic flow if they reach sonic before they get to the expansion portion of the duct.
    A properly timed spark plug should do it. *


    *full disclosure, I have no actual experience in this area.

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  7. #20
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    Re: Scaled-Up Airfoils???

    Norman, aren't we saying the same thing? I think your explanation is very similar to the wording in TOWS chapter 5, and a direct correlation of benefits of scaling upwards with Re and not the same behaviour when scaling downwards. As with a wing, even at the same scale, the order of magnitude of Induced (Pressure Drag) is greater at lower speeds and decrease as speed increase, and it's inversely in viscus (friction drag)...

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    I know that I had notes somewhere from Dr. Drela which he gave a very precise explanation of scaling on airfoils... but I relied on a free cloud solution that failed and I lost lots of interesting tidbits I noted down over the las couple of years...

  8. #21
    Registered User Norman's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by SpainCub View Post
    Norman, aren't we saying the same thing? I think your explanation is very similar to the wording in TOWS chapter 5, and a direct correlation of benefits of scaling upwards with Re and not the same behaviour when scaling downwards. As with a wing, even at the same scale, the order of magnitude of Induced (Pressure Drag) is greater at lower speeds and decrease as speed increase, and it's inversely in viscus (friction drag)...


    I know that I had notes somewhere from Dr. Drela which he gave a very precise explanation of scaling on airfoils... but I relied on a free cloud solution that failed and I lost lots of interesting tidbits I noted down over the las couple of years...
    We're still not on the same page and it's probably my fault. I'm not an expert on this stuff and occasionally may not use the jargon properly. The transition bubble AKA "laminar separation bubble" is part of the boundary layer and that is a viscous effect. I was using "pressure drag" in the sense of the result of a bad shape not induced drag. There is a pressure disturbance around a large bubble which adds to the airfoil's form drag but is not friction drag so the so called "bubble loss" is lumped in with form drag. This pressure disturbance is visible in the pressure coefficient plot that the OpPoint view shows. At alpha=4.5 and Re=400,000 the upper surface transition point on the MH-46 is at 39% chord but at 4,000,000 it is at 14%. Since the drag of a turbulent BL can be several times that of a laminar BL that can be a lot of drag. Scaling up may not be as bad as scaling down but it's still bad.
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    Registered User Norman's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by Aerowerx View Post
    From XFLR5, some curves for the PRANDTL-D root airfoil:

    Pressure distirbution
    Click image for larger version. 

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    All these curves are done at Re=400,000 (little squares) and Re=4,000,000 (little circles).

    I see that XFLR5 does not like the stall region for the MH46 at Re=4,000,00

    I also see the separation point moving forward with higher Re, as Norm mentioned.

    Some squirrely things going on at the leading edge for both airfoils.
    This is a polar graph showing the upper surface transition point at different AoA. The pressure distribution and coefficient of pressure are shown in the OpPoint view. Here's a screen shot of it with some annotations. The large arrow inside the airfoil shows the center of pressure. It stays stationary within the range of AoA wher the pitching moment curve is linear. That will show you where the actual aerodynamic center is.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    -------.~.
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    -(^^)---(^^) -Norm
    We didn't throw the tail away, we just hid it

  10. #23
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    Re: Scaled-Up Airfoils???

    Hi Norman, I'm on the same boat as you are... not an expert on this either... so it's probably my fault. Im also a little confounded that I lost all my notes... and probably was quoting Drela before and it could have been Selig, but who knows...

    Here are some snippets from some documents which I lost the relationship of the original document:

    Reynolds Number
    Reynolds number is the ratio of the fluid's inertia forces to the viscous forces in the boundary layer of the fluid. It is an important parameter in deter- mining the dynamic similarity of flow around models and full-scale aircraft. When the model data are obtained at much lower Reynolds numbers than those encoun- tered at full-scale conditions,the inertia forces of the fluid on the model are much lower in proportion to the viscous forces than those on the full-scale airplane.
    As a consequence, the flow conditions are no longer dynamically similar.
    The point of transition from laminar to turbulent flow, the thickness of and velocity in the boundary layer at any streamwise station on a surface, and the angle of attack at which the flow field separates from the surface are all functions of Reynolds number. The boundary-layer (viscous flow) conditions on any configuration affect the drag coefficient throughout the angle of attack range and the maximum lift and stall characteristics of the aircraft. The precise effect depends on the particular airfoil and planform used, and on the interference effects of the fuselage and nacelies or pods.
    As Reynolds number increases, the point on the surface along the flow line at which the boundary layer changes from laminar to turbulent moves forward. The precise point or locus of transition is affected by the geometry of the surface or body and by the resulting pressure distribution, surface roughness or wavi- ness , and the magnitude of the velocity fluctuations in the airstream. A s a result , it is difficult to extrapolate model test results of natural transition effects obtained in present test facilities to full-scale Reynolds numbers. Efforts are frequently made to simulate flow conditions typical of higher-than-test Reynolds numbers by artificially fixing the transition using strips of roughness particles (grit) or
    other flow-tripping devices. The test results at several Mach numbers are then extrapolated to full-scale Reynolds numbers .
    Same page as you I believe... If I'm understanding you correctly.

    an increase in Reynolds number in the region around 4 X 10^6 causes a thinning of the boundary layer with a resultant rearward movement of the separation point, a narrowed wake, and, thus, a decrease in drag. Extending the benefit of the Scaling Factor, it should yield better performance data at higher scale in 3D aerodynamics.
    What made me make the comment of Scale... and the effects of Inertial forces.

    I should have also mentioned that Airfoils design for low Re numbers will not scale upwards properly, same as Higher Re Airfoils do not work well in Low Re...
    From the XFLR5 data for this Airfoil... looks like not the best airfoil to scale upwards.


    BTW; These references are in my notes, but I do not find such documents so I might not have read into them:

    Chambers, Joseph R .: Status of Model Testing Techniques. Stall/Post- Stall/Spin Symposium , paper J , A i r Force Flight Dynamics Lab. (FGC) Wright-Patterson AFB , Ohio, 15-17 Dee. 1971, pp. J-1 to 5-25.

    Libbey, Charles E . ; and Burk , Sanger M ., Jr .: A Technique Utilizing Free-Flying Radio-Controlled Models To Study the Incipient- and Developed- Spin Characteristics of Airplanes. NASA Memo 2-6-591; , 1959.

    Selig, M. S. and McGranahan, B. D., “Wind Tunnel Aerodynamic Tests of Six Airfoils for Use on Small Wind Turbines,” ASME Journal of Solar Energy Engineering, Vol. 126, November 2004, pp. 986–1001.
    Last edited by SpainCub; January 7th, 2017 at 06:41 AM.

  11. #24
    Registered User Aerowerx's Avatar
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    Re: Scaled-Up Airfoils???

    Quote Originally Posted by SpainCub View Post
    .....I should have also mentioned that Airfoils design for low Re numbers will not scale upwards properly, same as Higher Re Airfoils do not work well in Low Re...
    From the XFLR5 data for this Airfoil... looks like not the best airfoil to scale upwards.
    What airfoil would be a better choice?

    Or would the scale and full size have to have different airfoils?
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    Re: Scaled-Up Airfoils???

    Speaking of bubbles, try flying a radio controlled model just before dark, after dew starts to form. If you fly the entire flight at the same speed, you'll see a pattern in the condensation showing where the separation bubble is. You can also see how small imperfections trip the flow before the bubble, and at what angle the disturbance spreads out. I haven't done it in a while, so I don't remember for sure, but I think the condensation is within the bubble?

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    Re: Scaled-Up Airfoils???

    A fairly concise and "easy reading" (comparatively) explanation of Reynolds Number is found in John Thorp's June 1960 article in Sport Aviation "Which Airfoil Section.

    It provides a good "baseline" for this thread.

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