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Old Sep 22, 2011, 03:56 PM
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Old Sep 22, 2011, 08:30 PM
Just fly it!
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Cody, WY
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Looks pretty good. That TE will by very difficult to build accurately and strong. So very thin back there.

Adam
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Old Sep 22, 2011, 09:50 PM
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Old Sep 23, 2011, 01:25 PM
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Old Sep 23, 2011, 11:26 PM
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Perhaps a silly question, but, for a plank, would you rather have a Cm that's constant with Re, or a Cm that's as close to zero as possible over the Re range?
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Old Sep 23, 2011, 11:55 PM
Just fly it!
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Cody, WY
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Quote:
Originally Posted by c.walker View Post
Perhaps a silly question, but, for a plank, would you rather have a Cm that's constant with Re, or a Cm that's as close to zero as possible over the Re range?
Both.
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Old Sep 24, 2011, 09:47 AM
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Old Sep 24, 2011, 10:10 AM
Just fly it!
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Ideally you want an airfoil that has a neutral Cm at all speeds.

The challenge is that the airfoil has to fly at different AoA at different speeds and typically still requires trim changes.
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Old Sep 24, 2011, 02:41 PM
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Old Sep 24, 2011, 03:09 PM
Just fly it!
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If the wing is flying slow the wing has to fly at the required angle of attack to generate enough lift to maintain level flight along the slope. As the wing flies faster it develops more lift and then needs to fly at a lower AoA to fly level along the slope. This is why most wings require subtle trim changes as the speed changes.

Example:

Your glider weighs 45 ounces and has a wing area of 350 square inches. Lets also say that it uses your TK Plank 1a. The MAC of this wing is 6.333"

So at 40 mph the wing needs to generate a CL of .282
at 80 mph the wing needs to generate a CL of .071

Using the MAC in profili:
At 40 mph the RE at MAC is 195,000. The wing needs to fly at 2.1* to generate a CL of .282 to fly level.
At 80 mph the RE at MAC is 391,000. The wing needs to fly at .3* to generate a CL of .071 to fly level.

Won't this require a trim change?
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Old Sep 24, 2011, 04:10 PM
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Quote:
Originally Posted by wyowindworks View Post
Won't this require a trim change?
Depends on where the CG is.

Nauga,
who likes neutral stability
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Old Sep 24, 2011, 04:13 PM
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Old Sep 24, 2011, 04:30 PM
Just fly it!
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Cody, WY
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Quote:
Originally Posted by nauga View Post
Depends on where the CG is.

Nauga,
who likes neutral stability
Nauga, I'm glad you chimed in.

Can you help me understand this?

I too like neutral stability. So when a nuetral stable plane increases it's speed the wing generates more lift... right? So this change in lift and speed would change it's elevation but not necessarily it's attitude. If I want the plane to maintain it's elevation at this new speed (from my perspective on the ground) I would need to change it's attitude...right? Or not?

Adam
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Old Sep 24, 2011, 04:51 PM
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Old Sep 24, 2011, 06:40 PM
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Quote:
Originally Posted by wyowindworks View Post
I too like neutral stability. So when a nuetral stable plane increases it's speed the wing generates more lift... right? So this change in lift and speed would change it's elevation but not necessarily it's attitude. If I want the plane to maintain it's elevation at this new speed (from my perspective on the ground) I would need to change it's attitude...right? Or not?
If by the above you mean that as you increase speed you need to decrease AOA to maintain level flight, you are correct. As for the trim to maintain that new AOA, a neutrally stable airplane does not require a different trim surface deflection to maintain that new AOA regardless of speed. Think of it as having the CG at the aerodynamic center of the whole airframe (i.e. at the neutral point).

Quote:
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...you disabled your PMs...
My PMs are open and have always been, and I regularly exchange PMs with other groupers. Not sure why you got indications otherwise.

Nauga,
and his rate command system
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