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MK II Weight and Balance

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Mark K

Joined: 17 Aug 2014
Posts: 32
Location: Great Bend, PA

Posted: Thu Jan 24, 2019 6:52 am Post subject: MK II Weight and Balance

I did some searching on the list here and have not found what I am looking for in regards to W&B for Kolbs in general, not to mention MKII.  I may have missed it so pardon my re-asking.  

While going through my Light Sport ground school, we have been going over W&B extensively.  So now that my rebuild is progressing I'm starting to think about my MKII W&B.

As far as the weights, arms and moments I'm good with that.  My big question is where is the center of lift/pressure?  I've looked through the plans and build manual and am not seeing any reference.  

Per my understanding it is easy to determine the CG, it is determining if the CG falls within the acceptable envelope of % of wing cord that I'm not finding.

Thanks in advance.

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Mark
Twinstar MKII
Great Bend, PA

byoungplumbing(at)gmail.c
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Posted: Thu Jan 24, 2019 7:29 am Post subject: MK II Weight and Balance

it is determining if the CG falls within the acceptable envelope of % of wing cord that I'm not finding.Thanks in advance.--------Mark



 	  Quote: 	
		  Mark

	


You said you can find the center of gravity. Take the center of gravity measurement call it cg.     Cg times  100  then devide by the measurement of the wing cord (should be listed in the plans)     the wing chord for example in the Mark 3 is listed in the plans at 66 in.

Example for mkiii with 66 inch wing cord.

Cg    20 inches

(20 X 100) ÷ 66   =   30.3 % of wing cord

My mkiii plans say the results should be between 25 and 35 %

Hope this helps

Boyd Young 

Mkiii Utah

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victorbravo(at)sbcglobal.
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Posted: Thu Jan 24, 2019 9:27 am Post subject: MK II Weight and Balance

The center of pressure is not anything that is important during the weight and balance. Center of Pressure is an aerodynamic factor of the wing's airfoil, and is very important during the design and engineering phase. The aero engineer(s) determine the Cp. Then they determinea safe CG range or envelope  (which is based on where the Cp is, among other things). 



The reason that Cp is not shown on the plans is that (for most purposes) it's of no concern to the mechanic or the pilot, only the designers and engineers.



All the mechanic / technician / inspector needs to be concerned about during "weight and balance" is... weight (in pounds or kilograms) and balance (as in the center of gravity) .



Bill Berle

www.ezflaphandle.com  - safety & performance upgrade for light aircraft

www.grantstar.net           - winning proposals for non-profit and for-profit entities



--------------------------------------------

On Thu, 1/24/19, Mark K <seanote(at)echoes.net> wrote:

 

 My big question is where is the center of lift/pressure?  I've looked through the plans

 and build manual and am not seeing any reference.

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Mark K

Joined: 17 Aug 2014
Posts: 32
Location: Great Bend, PA

Posted: Thu Jan 24, 2019 3:16 pm Post subject: Re: MK II Weight and Balance

Guys, thanks for the info.  Very helpful.  Where can it is indicated on the plans for build manual the allowable envelope for the CG?  % wing chord?  Thanks in advance.

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Mark
Twinstar MKII
Great Bend, PA

Richard Pike

Joined: 09 Jan 2006
Posts: 1671
Location: Blountville, Tennessee

Posted: Thu Jan 24, 2019 5:08 pm Post subject: Re: MK II Weight and Balance

Can't say anything about the MKII, but for the MKIII scroll to sheet 41:

http://oh2fly.net/oldpoops/Lexan%20rear%20enclosure.html



I would think that they are pretty similar.

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Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

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Mark K

Joined: 17 Aug 2014
Posts: 32
Location: Great Bend, PA

Posted: Fri Jan 25, 2019 3:14 am Post subject: Re: MK II Weight and Balance

Richard,



Thanks for the great info.  These diagrams are part of my build manual for plans. 



Mark

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Mark
Twinstar MKII
Great Bend, PA

Mark K

Joined: 17 Aug 2014
Posts: 32
Location: Great Bend, PA

Posted: Sun Jan 27, 2019 7:03 am Post subject: Re: MK II Weight and Balance

Richard, Thanks, I meant to type this information was "not" in my build manual or on the plans. Much appreciated!

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Mark
Twinstar MKII
Great Bend, PA

rickofudall

Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA

Posted: Sun Jan 27, 2019 3:44 pm Post subject: MK II Weight and Balance

Can't speak to the Mk II specifically but the 25 to 35% is probably a nice conservative range that will not catch you out. I've flown the Mk III at 1200 lb loading and cg calculated at 18%. I had 1/2" between the stick and the seat frame tube. Not a lot of margin but she flew with fingertip pressure and flared nicely. At that weight with horizontal stab gap seals (stabilizer to boom tube) I put her into a stall and held her in it for 1000' using only rudder input to keep the wings level. When I released back pressure she flew away. I've flown her at 40% at design weight (1000 lb) with no problems, too. Stick pressures were light but not necessarily too light. In both cases I was flying on a high pressure day with no discernible convective activity. Turns were limited to 30 degrees bank. I don't advocate flying at these CG extremes but for phase 1 testing only. My personal limits are 20 to 35% for everyday flying. If you decide to test at those or similar limits do so ONLY after you have some time in the airplane and approach further limits gradually. I highly recommend horizontal stabilizer seals to the boom tube. They really increase rudder authority.If you're working toward a Sport Pilot ticket as well as rebuilding a Mk II, please have a more experienced pilot do the first flights of your airplane then give you some dual once it is out of phase 1.

Rick Girard

PS When doing measurements to calculate moment arms be sure the aircraft is positioned properly. In general this is with horizontal stabilizer level. If the Mk II has specific instructions in the plans use them. As you're just learning weight and balance you might want to consider using a zero point that is in front of the nose so that all measurements are positive. Having to screw around with negative and positive arms can be tricky. Good luck and have fun.

On Thu, Jan 24, 2019 at 8:54 AM Mark K <seanote(at)echoes.net (seanote(at)echoes.net)> wrote:



 	  Quote: 	
		  --> Kolb-List message posted by: "Mark K" <seanote(at)echoes.net (seanote(at)echoes.net)>

 

 I did some searching on the list here and have not found what I am looking for in regards to W&B for Kolbs in general, not to mention MKII.  I may have missed it so pardon my re-asking.  

 While going through my Light Sport ground school, we have been going over W&B extensively.  So now that my rebuild is progressing I'm starting to think about my MKII W&B.

 As far as the weights, arms and moments I'm good with that.  My big question is where is the center of lift/pressure?  I've looked through the plans and build manual and am not seeing any reference.  

 Per my understanding it is easy to determine the CG, it is determining if the CG falls within the acceptable envelope of % of wing cord that I'm not finding.

 Thanks in advance.

 

 --------

 Mark

 Twinstar MKII

 Great Bend, PA

 

 

 

 

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