Under Cambered Wings - RC Groups
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Sep 18, 2006, 09:04 PM
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Discussion

Under Cambered Wings


What is the purpose of an under cambered wing design? Is it to enhance a trainer type plane in a consistant slow and stable flight?
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Sep 18, 2006, 11:21 PM
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It starts at a fatter airfoil. The airfoil gets thinner till it becomes undercambered. When its thickness goes to zero, then the mean camber of the airfoil is the same as the undercambered. The real meaning of the the family of airfoils is the meaning of the constant mean camber.

Focus on the thickness and the mean cambered airfoil not to undercambered aspect of the airfoil of the family!

If the function of,"-- a trainer type plane in a consistant slow and stable flight," then consider the airfoil mean camber and thickness.
Last edited by Ollie; Sep 18, 2006 at 11:33 PM.
Sep 18, 2006, 11:25 PM
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M Ashmore's Avatar
An undercambered wing provides more lift.
Sep 19, 2006, 12:54 AM
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Bilbobaker's Avatar
Quote:
Originally Posted by phat23
An undercambered wing provides more lift.
Usually at a cost of more drag. So they are best at slow speed aps.
Commercial airlines probably have the most effiecient air foils right?
High lift high speed?
Bill
Sep 19, 2006, 05:44 AM
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vintage1's Avatar
Quote:
Originally Posted by Bilbobaker
Usually at a cost of more drag. So they are best at slow speed aps.
Commercial airlines probably have the most effiecient air foils right?
High lift high speed?
Bill
Actually gliders are the most efficient, and they camn be pretty fast in a dive.

Mind you a 747 is a sort of glider with some power pods strapped on...
Sep 19, 2006, 08:29 AM
Registered User
In a glide with jets off and clean (up flaps), the 747 has maximum L/D around 18.
The flaps down makes the wing airfoil with more mean camber compared to clean wing. The purpose of lots of mean camber is to slow the plane down and steepen the path (lower the L/D and slower the sink rate).

For cross country racing sailplanes need both high speed and slow speed for working thermals. So there is a compromise between to much or to little mean camber and too much or too little thickness for the best airfoil.

"The diameter of a wire having the same drag is only 0.006 the width of of the airfoil. Such an airfoil may easily develop a lift of 100 pounds for a drag of one pound --." Example: airfoil NACA64-421 (21% thickness) RE 6,000,000.
Last edited by Ollie; Sep 19, 2006 at 09:07 AM.
Sep 19, 2006, 11:57 AM
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Bilbobaker's Avatar
Quote:
Originally Posted by vintage1
Actually gliders are the most efficient, and they camn be pretty fast in a dive.

Mind you a 747 is a sort of glider with some power pods strapped on...

I agree.
I'm allways amazed when I watch large 747s etc. take off and land.
The wings look so small for thier overal size.
Bill
Sep 19, 2006, 01:18 PM
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See DS model sailplanes:
http://members.tripod.com/douglasturner/id27.htm
http://www.sloperacing.com/results/ds-speeds.htm
The airfoil thickness is for a huge strength spar than resist a bending load of more than 30 G's.

On the other hand, indoor FF microfilm models are so slow that they fly along with a slow walking pace. The airfoils are high camber (3% to 5%) and almost zero thickness. Don't sneeze near these models.
Sep 19, 2006, 01:38 PM
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HELModels's Avatar
Maybe it would be more accurate to refer to "undercamber" wings as "no thickness" or "all camber". And if you want to find a camber that is best for your undercambered application, then compare % cambers and locations of high points. Compare polars of various airfoils with similar thickness and then use Xfoil to extract the camber distribution.
Sep 19, 2006, 01:50 PM
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Batmanwpg's Avatar
<An undercambered wing provides more lift.>

Nope. Camber provides the lift.


Quote:
Originally Posted by Bilbobaker
Usually at a cost of more drag. So they are best at slow speed aps.
Commercial airlines probably have the most effiecient air foils right?
High lift high speed?
Bill
Undercamber is used to reduce drag on a high camber airfoil.
Sep 19, 2006, 02:14 PM
Registered User
<Undercamber is used to reduce drag on a high camber airfoil.>

...at slow speed, high Cl/ high AOA flight, that is. At high speeds in level flight, the required Cl for a given amount of lift drops off--->less AOA needed---> a highly undercambered airfoil will likely experience separation around the bottom portion of the LE--->lots of extra drag.

I made a 6 ft span ff glider with a 1/4" solid balsa wing (young and stupid, what can I say?). The first glide tests I did put the L/D around 15:1. After using tape to hold more camber (and subsequent undercamber), I tried it again, and the L/D went up to 20:1 or thereabouts.
Sep 19, 2006, 02:47 PM
Registered User
A powered model, in level flight, has the same constant lift force and opposite and equal constant weight force and flies slowly or high speed or between. Perhaps you are confusing lift force rather coefficient of lift. In level flight, the change of air speed changes with change of angle of attack and change with coefficient of lift.
Sep 19, 2006, 03:54 PM
Registered User
http://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html

The link shows the lift equation. Lift= Cl x density x 1/2 (velocity^2) x wing area.

if the speed goes up in level flight due to a higher power setting, the Cl must go down (to maintain the same lift) for an aircraft to remain in level flight. Cl normally does change proportionally with AOA, but it is NOT necessarily a 1:1 relationship, i.e. doubling the AOA from say, 5 to 10 degrees may instead increase the lift by a factor of 2.3, or 1.6 (depending on wing/ airfoil design), or it may even DROP, we all know that as a stall
Sep 20, 2006, 07:21 PM
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Batmanwpg's Avatar
Quote:
Originally Posted by Flyingwingbat1
<Undercamber is used to reduce drag on a high camber airfoil.>

...at slow speed, high Cl/ high AOA flight, that is. At high speeds in level flight, the required Cl for a given amount of lift drops off--->less AOA needed---> a highly undercambered airfoil will likely experience separation around the bottom portion of the LE--->lots of extra drag.

I made a 6 ft span ff glider with a 1/4" solid balsa wing (young and stupid, what can I say?). The first glide tests I did put the L/D around 15:1. After using tape to hold more camber (and subsequent undercamber), I tried it again, and the L/D went up to 20:1 or thereabouts.
You are stating something that undercambered airfoils where not designed for! Be realistic about the task the airfoil was designed for please.
Sep 20, 2006, 10:32 PM
Registered User
<You are stating something that undercambered airfoils where not designed for!>

I addressed that in the first sentence you quoted, just trying to clarify things a bit, that's all.

<Be realistic about the task the airfoil was designed for please.>

What? High undercamber airfoils work GREAT on my supersonic, propeller-driven amphibious biplane dirigible.


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