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All Forums >> RC Airplanes >> Aerodynamics >> airfoil thickness
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airfoil thickness - 7/17/2004 7:44:23 PM   
hexonxonx


 

Posts: 188
Joined: 5/29/2004
From: carlyle, IL, USA
Status: offline 		i see a variety of wing thickness and also pizza box flyers or flat wing type.what is the benifit of a thick airfoil .im working on a delta and have ended up with a very thin wing,not much curve on the top of the wing. any info?
       Post #: 1

RE: airfoil thickness - 7/17/2004 7:54:09 PM   
dick Hanson



Posts: 10191
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		little light plane- ? foamy?
thick is NO advantage
actually the opposite
lightest thinnest which is still STIFF-is best

The whole reason for "thick is make the friggen thin less sensitive to pitch -also stronger
All of the carefully worked out airfoils are simply for ONE FINITE application - change speed and AOA and it all turns sour.
But what the heck - do what you like -It's modeling
no one gets killed -------------

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RE: airfoil thickness - 7/17/2004 9:33:07 PM   
Gringo Flyer



Posts: 2248
Joined: 1/16/2004
From: Formosa, ARGENTINA
Status: offline 		I'm no aero pro but a thick airfoil creates a lot of drag. It also creates a lot of lift. Thats why lots profiles and 3D planes have them. Slow flying bc of the high drag and lift. Another effect of the thick airfoil is that the plane dives a lot slower. Which I guess is an advantage in a 3D type plane.

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RE: airfoil thickness - 7/17/2004 9:53:20 PM   
dick Hanson



Posts: 10191
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		lots of drag and less lift --but a broader range of AOA
dives about th same -if wing area is about the same -

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RE: airfoil thickness - 7/17/2004 11:20:27 PM   
flywilly


 

Posts: 899
Joined: 2/3/2002
From: glen allen, VA,
Status: offline 		Glad to see you're still keeping an eye on us, Dick. Or is it your dog who's watching...
Sorry you left the kit business (not to mention the R & D).
Regards,
Will Bartlett

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RE: airfoil thickness - 7/18/2004 12:25:32 AM   
Gringo Flyer



Posts: 2248
Joined: 1/16/2004
From: Formosa, ARGENTINA
Status: offline 		Dick, it would seem to me that a thicker airfoil would mean that there is a bigger curve in the foil and hence more room behind it that would have low pressure and that would in turn create more lift. (not sure I explained that well) Like I said I am no pro in airfoil design but it would seem to be that would be the case. Also, I cant remember who told me the slower dive thing but I am almost positive I read it on a post. But it would have to dive somewhat slower if for nothing more than the thicker airfoil creates more drag.

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RE: airfoil thickness - 7/18/2004 1:47:16 AM   
dick Hanson



Posts: 10191
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		Will, that's Libby watching --
I will be reading and hear a slight noise -look down and there is that fixed gaze --
Gringo - do not confuse our model foils with the text book efficiency generalizations for airfoils developed for man carrying machines - .
The foil in the sizes we use really means little -unles tit isdeveloped for a given load and speed.
for rip **** tear around flying - square edges and flat foils work fine -
What makes the big differences?
wing loadings and power -

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RE: airfoil thickness - 7/18/2004 2:09:35 PM   
hexonxonx


 

Posts: 188
Joined: 5/29/2004
From: carlyle, IL, USA
Status: offline 		thanks folks, i geuss ill continue with the build .it sounds as if this ship may still fly. when i get it done i will post pics in s.p.a.d.

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RE: airfoil thickness - 7/19/2004 5:40:05 AM   
Jimmbbo



Posts: 1179
Joined: 10/1/2003
From: Fresno, CA, USA
Status: offline 		Can't resist throwing my $.02 in...

Thick airfoils do create more lift and drag (both induced and parasite), and are structurally stronger than thinner sections. In FS airplanes, the section chose becomes important depending on the mission the airplane is designed to accomplish (as can be seen when comparing an A-10 wing to a 757..)

Since most of our models have power loadings (lb/hp) that are astronomical when compared to FS airplanes, drag is easily overcome by our overpowered engines and is not a practical issue, and I agree with Dick that wing loading and thrust/power are the relevant numbers.... Which is why you can see flying model doghouses, pizza boxes and lawnmowers... Put enough power on just about anything, and it'll fly

Cheers!

Jim

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RE: airfoil thickness - 8/3/2004 11:24:47 PM   
adam_one


 

Posts: 410
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		Hi,
It has been found that at Re <20,000 (like many indoor R/C models and foamies), a thin flat plate actually generated more max lift than a Clark Y airfoil.
However, a properly designed airfoil optimised for that operating condition would probably outperform both of those.

One reason is that the air molecules don't "see" a thin plate airfoil the same way we see it.
Some air approaching the wing goes over the top and some goes underneath.
The point where the air splits near the wing's LE is called the stagnation point.
As the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below what we consider the wing's LE. As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
The result is that the air molecules "see" a thin plate airfoil with a positive AOA as it was a cambered airfoil.

< Message edited by adam_one -- 8/4/2004 11:21:02 AM >

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RE: airfoil thickness - 8/4/2004 2:17:47 PM   
dick Hanson



Posts: 10191
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		Thats the beauty of the flate plate!
Simple , easy to make and very effective.
The Leading Edge Sneak --is simply caused by slight pressure "bubble" being formed -as you noted .
The easy way out is --over the top
Nature abhores unbalance in pressure -so when in doubt -just look for the easiest path- that's where the air will go .

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RE: airfoil thickness - 8/4/2004 5:12:58 PM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline
quote:

It has been found that at Re <20,000 (like many indoor R/C models and foamies), a thin flat plate actually generated more max lift than a Clark Y airfoil.
However, a properly designed airfoil optimised for that operating condition would probably outperform both of those.


adam_one,

This relationship between Reynold's number and the 'best' thickness for maximum lift is a pretty fundamental one, and worth understanding. As air flows from the leading edge over a typical airfoil, the pressure goes down, roughly until the thickest point of the airfoil, and then starts going back up, or 'recovering', as the airfoil guys would say. During the pressure recovery part of the flow, the air is flowing from a lower pressure to a higher pressure, which makes it much more likely to separate. The higher the Reynold's number, the more likely the air is to acheive pressure recovery without separating. Thicker airfoils have a bigger variation in pressure over the chord, and so the pressure recovery is more extreme. For this reason, wings operating at lower Reynold's numbers cannot be as thick as wings operating at higher Reynold's numbers without separating. This is why 'good' low Reynold's number airfoils are thinner, and tend to resemble flat plates, and also why flat plates compare more and more favorably to traditional airfoils as the Reynold's number gets small. Having said that, it is the thickness that relates to Reynold's number, not the flatness. For a given thickness, the optimum airfoil shape will not be a plate with constant thickness (i.e. a flat plate), at least in terms of lift and drag. If aerobatic requirements dictate that a wing be stallable on command, then a flat plate may have some favorable characteristics, although those could be acheived by reducing the leading edge radius as well. If, in addition to the ability to stall on command, you want high drag, and don't need high lift, then the flat plate starts to look really good, especially since it is so simple to make. We shouldn't overstate this by saying that flat plates are just as good as traditional airfoils for low Reynold's numbers, even though they are obviously perfectly good choices for lots of planes.

banktoturn

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RE: airfoil thickness - 8/4/2004 5:56:49 PM   
adam_one


 

Posts: 410
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From: Stockholm, SWEDEN
Status: offline
quote:

For a given thickness, the optimum airfoil shape will not be a plate with constant thickness (i.e. a flat plate), at least in terms of lift and drag.


I agree, but in case of an extremely low Re along with a rather thin flat plate wing, the air molecules "reshape" the plate into a cambered airfoil (at a positive AOA).
As the Re gets smaller, the required thickness gets smaller, but there are also drawbacks for being too thin e.g., the max thickness point has to move forward and its distribution along the airfoil becomes critical as well.
If you want an airfoil for an extremely low Re that also has a wide speed range, the job gets rather tricky.
I think designing really good airfoils for extremely low Re is one of the most difficult challenges in the model aerodynamics.

< Message edited by adam_one -- 8/4/2004 7:20:19 PM >

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RE: airfoil thickness - 8/4/2004 6:12:14 PM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline
quote:

I agree, but in case of an extremely low Re along with a rather thin flat plate wing, the air molecules "reshapes" the plate into a cambered airfoil (at a positive AOA).
As the Re gets smaller, the required thickness gets smaller, but there are also drawbacks for being too thin e.g., the max thickness point has to move forward and its distribution along the airfoil becomes critical as well.
If you want an airfoil for an extremely low Re that also has a wide speed range, the job gets rather tricky.
I think designing really good airfoils for extremely low Re is one of the most difficult challenges in the model aerodynamics.


adam_one,

I understand how the 'reshaping' can happen, but it is not unique to airfoils that are thin or flat, and it is not the entire explanation of the performance characteristics of thin airfoils.

Low Re airfoils are tricky, mostly because the body of knowledge we would like to take advantage of is all aimed at higher Re airfoils, and hence often not applicable. Beyond that, it is tricky to optimize an airfoil for any set of operating conditions, whether it is low Re, high Mach number, etc.

banktoturn

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