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  #1  
Old 09-02-2010, 05:57 PM
blackshire's Avatar
blackshire blackshire is offline
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Default Estes Viking CG question

Hello Doctor (and all other readers here on YORF),

The folks at the Northcote Heavy Horse Centre www.northcotehorses.com will be incorporating model rocketry activities into their special event days, and I am sending them an initial batch of 24 Estes Viking kits along with 1/2A6-2 or A8-5 motors (ordered from Sierra Fox Hobbies in Italy) and other launching supplies. Due to the relatively small size of their available flying field (their jousting field), they will use only "1/2A" or "A" motors in the rockets. I have a question about the Estes Viking's CG (Center of Gravity):

The "worst-case scenario" (in terms of forward CP [Center of Pressure]) Viking fin configuration is just three fins, glued on with their longest edges as the root edges and with their wider ends forward, as in Moe Bertrand's photograph (attached below) from the EMRR article on the Estes Black Diamond (see: www.rocketreviews.com/images/flightlog/bd1e.jpg and http://www.rocketreviews.com/review...k_diamond.shtml ), a re-packaged Estes Viking in a different "livery" that came in the "Launchables" (#1452) Starter Set. (This Viking configuration resembles the Estes Cherokee-D.) My question is: With this "worst-case scenario" fin configuration, is the Viking aerodynamically stable with 1/2A6-2 and/or A8-5 motors, or would it need clay ballast in the nose cone? (I know that the Estes Viking [although not the old Centuri Viking and Akela-1] *definitely* needs clay nose ballast if flown using heavier "C" motors with this fin configuration.) If necessary, I will provide them with clay for ballast.

Many thanks in advance to you and/or anyone who can help!
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  #2  
Old 09-03-2010, 08:25 AM
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Shreadvector Shreadvector is offline
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It does not need clay in this configuration for the 1/2A6 and A8 motors IF AND ONLY IF the fins are glued on perfectly. If they are crooked and two of them are deflecting the rocket in the same direction it will go unstable.

In other words, if the fins are all tiled and generate a spin, that is OK, but if the fins are tiled in opposite directions, they will not spin the rocket, but they will deflect the back end sideways and destabilize.

Just tell them which fin configuration they MUST NOT use.

Here is a cut & paste of past advice on this kit that I supplied to someone via 2 e-mails:

*************

Simple explanation, but semi-long.

The Viking started out in the 1970's as a Centuri kit - the Akela-1 designed for the Cub Scouts. It was sold in bulk packs of 12 rockets with bulk packs of 12 A8-5 motors. Other motors were Ok and recommended such as the 1/2A6-2 for smaller fields.

Estes and Centuri were both owned by Damon Corp and ran semi-independently for a long time. Eventually they closed the Centuri offices in Pheonix and moved everthing to Penrose and eventually stopped selling Centuri branded product. Some Centuri items lived on as Estes versions (the Centuri ST-13 tube became Estes BT-56 tube and it is a teeny bit bigger then the Estes BT-55. Many Estes kits with plastic fin units use the BT-56.) .

The Akela-1 became the Viking under the Estes name and the stability was reduced when they switched from the heavier Centuri injection molded nose cone that fit the ST-7 tube to the lighter Estes nose cones to fit the BT-20 tube. And then Estes stopped making A8-5 motors for many years, so any kits that recommended them had their instructions changed to A8-3. An A8-3 will eject the recovery system while the rocket is still moving upward at very high speed, most likely damaging it. you want to eject the recovery system when the rocket has fully coasted to the top of the flight and slowed down (or stopped and turns to head back down but before it picks up too mcuh speed on the way down).

For the Viking, you want to use a 1/2A6-2 or an A8-5 or a Quest A6-4 motor. Other options include the Estes 1/2A3-4T mini-motor inside a motor adapter. Your next question may be: 'what's the difference between the 1/2A6-2 and the 1/2A3-4T?' The answer is simple: the 1/2A6-2 is perfect for models that have been built with all 5 fins and have a tiny bit more drag so they will slow down faster and if they made a 1/2A6-3 motor it would be perfect. With only 3 fins, they model will have a little less drag and will coast a bit more, so the 1/2A3-4T in an adapter is optimum. For general flying, the A8-5 or the Quest A6-4 are nearly perfect for any stable fin combination. Note that an Estes A8-5 actually only has about 4 seconds of coasting time since they usually come in on the low end of the delay time, but the Quest motors usually come in exactly as advertised, so an A6-4 will coast for 4 seconds.

*********************

No need to add clay to the Vikings. They are very stable as long as you have the motor block installed at the right depth (so the motor does not stick out the back too far) and the fins are "Sticking out" from the tube and not glued "close" to the tube. The Estes instructions for this model are online (and their server is slow, so give it time to load):
http://www.estesrockets.com/assets/instructions/001949_VIKING.pdf

On page 2 of the instructions you will see step number 2. It shows 6 examples of the possible fin configurations. the configurations that are bad are the 3rd and 4th from the left on the top left row and the 3rd from the left on the bottom right row. Best is 5 fins followed by 4 fins and with the fins sticking out into the airstream. the best of the best is the last one on the bottom row, followed by the first one on the bottom row.

*************
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  #3  
Old 09-03-2010, 01:46 PM
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blackshire blackshire is offline
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Fred, I thank you very much for posting that detailed information on the Viking! While I know the kit's history with Centuri and Estes, other "cheechacko" (newcomer) YORF members may not, and it's good for them to know about the kit's past. The A6-4 sounds like the perfect motor for the Northcote Centre's flying field (the A8-5 seems a bit too "hot" given its dimensions, and while I wondered about the A6-4 I'd never tried a Viking with one). I'll provide them with a copy of the Viking instructions that has the "to-be-avoided" fin configurations prominently highlighted.
__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see:
http://www.lulu.com/content/paperba...an-form/8075185
http://www.lulu.com/product/cd/what...of-2%29/6122050
http://www.lulu.com/product/cd/what...of-2%29/6126511
All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com.
NAR #54895 SR
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