New plank airfoils - RC Groups
 Sep 22, 2011, 04:56 PM closed due to popular demand ... Last edited by ibuild; Oct 19, 2013 at 08:09 PM.
 Sep 22, 2011, 09:30 PM Just fly it! Looks pretty good. That TE will by very difficult to build accurately and strong. So very thin back there. Adam
 Sep 22, 2011, 10:50 PM closed due to popular demand ... Last edited by ibuild; Oct 19, 2013 at 08:09 PM.
 Sep 23, 2011, 02:25 PM closed due to popular demand ... Last edited by ibuild; Sep 27, 2011 at 10:18 PM.
 Sep 24, 2011, 12:26 AM Registered User 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?
Sep 24, 2011, 12:55 AM
Just fly it!
Quote:
 Originally Posted by c.walker 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.
 Sep 24, 2011, 10:47 AM closed due to popular demand ... Last edited by ibuild; Oct 19, 2013 at 08:08 PM.
 Sep 24, 2011, 11:10 AM Just fly it! 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.
 Sep 24, 2011, 03:41 PM closed due to popular demand ... Last edited by ibuild; Oct 19, 2013 at 08:08 PM.
 Sep 24, 2011, 04:09 PM Just fly it! 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?
Sep 24, 2011, 05:10 PM
Embrace the suck
Quote:
 Originally Posted by wyowindworks Won't this require a trim change?
Depends on where the CG is.

Nauga,
who likes neutral stability
 Sep 24, 2011, 05:13 PM closed due to popular demand ... Last edited by ibuild; Oct 19, 2013 at 08:08 PM.
Sep 24, 2011, 05:30 PM
Just fly it!
Quote:
 Originally Posted by nauga 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?

Last edited by wyowindworks; Sep 24, 2011 at 08:22 PM.
 Sep 24, 2011, 05:51 PM closed due to popular demand ... Last edited by ibuild; Sep 26, 2011 at 10:40 PM.
Sep 24, 2011, 07:40 PM
Embrace the suck
Quote:
 Originally Posted by wyowindworks 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:
 Originally Posted by wyowindworks ...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