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  #21  
Old 04-14-2015, 11:22 AM
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Quote:
Originally Posted by tmacklin
...Has anyone ever actually flown one of these beauties?


Yes.
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  #22  
Old 04-14-2015, 11:37 AM
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Quote:
Originally Posted by kevinj
Do two cardboard cutouts, one for top view one for side view. Find where they balance and measure the distance from the nose to those spots. Average the measurements and you get a very conservative cp location.

Should get you in the ballpark.

kj


Thanks Kevin! I think I can use the paint patterns that came with the plan pack as templates and then upscale the results to actual size. At least that's what I'm going to try before I fly!
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  #23  
Old 04-14-2015, 11:38 AM
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Quote:
Originally Posted by shrox
Yes.


Do you care to talk about it?
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  #24  
Old 04-14-2015, 12:20 PM
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Quote:
Originally Posted by tmacklin
Do you care to talk about it?


I just looked over the instructions, looks like I left out the CG mark!

It should balance just in front of the leading edge of the main wing.
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  #25  
Old 04-14-2015, 01:46 PM
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Quote:
Originally Posted by shrox
I just looked over the instructions, looks like I left out the CG mark!

It should balance just in front of the leading edge of the main wing.



Thanks Doug.

With an Estes E9 dummy engine installed, the center of gravity (CG) locates just past the mid point of the launch lug, which I attached as per your design. The center of pressure (CP) should then be 2.60 inches aft to achieve one caliber of stability. I'm going to proceed with Kevin's suggestion just to be on the safe side and will post my findings here. If I can devise a way to attach a string without damaging the finished rocket I'll also do a swing test.
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  #26  
Old 04-14-2015, 01:54 PM
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Quote:
Originally Posted by tmacklin
Thanks Doug.

With an Estes E9 dummy engine installed, the center of gravity (CG) locates just past the mid point of the launch lug, which I attached as per your design. The center of pressure (CP) should then be 2.60 inches aft to achieve one caliber of stability. I'm going to proceed with Kevin's suggestion just to be on the safe side and will post my findings here. If I can devise a way to attach a string without damaging the finished rocket I'll also do a swing test.


Post your findings here if you would be so kind.
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Old 04-14-2015, 01:58 PM
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Quote:
Originally Posted by shrox
Post your findings here if you would be so kind.


Absolutely!
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  #28  
Old 04-14-2015, 06:43 PM
Madison Alum Madison Alum is offline
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Put together a rocksim file for the Skonkwulf. CP using Barrowman was 15.0 inches. Using the rocksim method it was 15.7. I built using thickwalled bodytube and corresponding nose cone with a resultant measured weight and CG of 360 gm and 10.75 inches. Have only flown once on an E28, but flew great. Have yet to attempt the paint scheme so it is just a plain primer white.
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  #29  
Old 04-14-2015, 08:38 PM
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Quote:
Originally Posted by Madison Alum
Put together a rocksim file for the Skonkwulf. CP using Barrowman was 15.0 inches. Using the rocksim method it was 15.7. I built using thickwalled bodytube and corresponding nose cone with a resultant measured weight and CG of 360 gm and 10.75 inches. Have only flown once on an E28, but flew great. Have yet to attempt the paint scheme so it is just a plain primer white.


That's amazing! Earlier tonight I made two cardboard cutouts directly from a copy of the side and top views on the paint schemes which came with the Skonk Wulf plan pack. The average center of pressure using this method turned out to be a bit over 15 inches. I'll take photos tomorrow and post them here along with my math, such as it is.

I couldn't do the Barrowman method if my life depended on it. Fortunately, it does not!
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  #30  
Old 04-15-2015, 12:51 PM
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Yesterday evening I made cardboard cutouts and balanced them as closely as possible to get some idea of the centers of pressure in both the side and plan views.

First I made a copy of the Skonk Wulf decal placement and color guide which came with the instruction pack. These appear to be proportionately correct although no scale is given. Next I taped this copy onto a thin piece of corrugated cardboard and carefully cut out the silhouettes with a fresh Exacto blade. These silhouettes were then laid flat over an architect's scale and the balance points marked.

The overall length of the actual rocket I built is 23.75" nose to the end of the motor mount tube. The length of the cardboard silhouettes from the same two points of reference measures 7.0625".

The center of pressure looking at the side view silhouette measures 4.375" from the nose, or 61.9% of the total length. Thus, 0.619 x 23.75" = 14.71" on the actual rocket.

The center of pressure looking at the plan view silhouette measures 4.750" from the nose, or 67.2% of the total length. Thus, 0.672 x 23.75" = 15.81" on the actual rocket.

The average between these two shapes is 14.71" + 15.81" / 2 = 15.26" on the actual rocket.

With an E9 dummy weight of 67 grams installed, the center of gravity locates 16.625" from the nose. To obtain one full caliber of stability I will then have to add enough nose weight to shift the CG 3.965" forward.

I built this rocket with Apogee thinwall BT80 tubing and and an Apogee PNC-66A nose cone. The empty weight of my build stands at 8.5 ounces. The total lift off weight with an Estes E9 stands at 310 grams or 10.9 ounces as is. In order to achieve one caliber stability I'll have to add at least 3 ounces of nose weight, which will bring the total lift off weight to about 14 ounces. It looks like any 24 mm composite would be a better bet.

Anyway, here's some pictures I took this morning.
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