CG and CP question on MADCOW Batray

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AfterBurners

AfterBurners

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Recently I completed the build on my MADCOW Bat Ray.

023.jpg

The question I have is based on the ROCKSIM file of my Bat Ray the CG ranges from 1.00 to 1.09 (Margin) depending on the size of the motor that is loaded in the rocket.

Is this a safe number to fly the rocket. It's already built so there's not much I can do about it right now. When I measure from the tip of the nose cone back 17.75" (MADCOW suggests this is where the CG should be) it lines up perfectly in the center with my launch lug placement. I'm using a 1/4 x 3" lug. The weight of the rocket according to the specification sheet suggests 8.0oz. My Bat Ray comes in at 8.3 oz and probably because I have on an Aero-Pack motor retainer.

The original balsa nose cone, which was defective had a hole in it and the instructions suggested stuffing the end of the shock cord in the hole and filling with epoxy and adding weight if necessary? My replacement nose cone from SEMROC had no hole obviously and I used an eye screw for an attachment point.

So do you guys think it should be fine to fly? Here's the ROCKSIM file - View attachment mcr_batray.rkt

Scratch builds I have done in the past I always try to stay between 1.15-1.50, but I would image every rocket is a little different.

Just looking for some feed back.

Right now I'm working on the MADCOW Scooter kit and I was consider changing the CG slightly if the number comes in too low?
 
Last edited by a moderator:
just checking my facts...where is your measured cg? the rocsim file says 15" from nose tip. do the nose cone demensions match the file? right now the file would worry me.
rex
 
I loaded a G79 and it put the CG right 17.85" from the tip of the nose cone. The CG is measured and according to the MADCOW instructions when the rocket is loaded with the largest motor that one is going to fly in it. I don't see myself using anything larger than a "G" motor.
 
grumble, they should say the heaviest motor(which may not be the largest). I you plan on using the G79 then you will need to add noseweight to move the cg forward sry, I really doubt that there is enough margin to play with. the good news that it shouldn't take much. a 3/8" bit, some lead shot, epoxy and some 3/8" wood dowel would be my choice, just be carefull drilling the nose(from the base of the shoulder) and you shouldn't have problems.
rex
 
those odd fins goof up ORs cp calculations, so one kinda has to use Madcow's number of 17.75"(loaded/15"no motor) for cg purposes. that said, I would take the easier route...use a lighter weight motor :). an 8.5 oz rocket will scream on an F50 / F52 and will likely go out of sight(OR guess 1700') unless you like going for a 1/2 mile (oneway) walk...a G79 should put it up to approx 2600'...:).
rex
oh right
1.25 cal would be my lower limit.
 
Yeah you're right Rex...probably just fly it on a different motor. What I found out is that it flies well on a bunch of different motors so I have a lot of options. Thank you for your suggestions...it's appreciated! :)
 
So I just finished a build on the standard Batray, and Open Rocket is telling me I need about 4.5 ounces of nose weight (including the nose eybolt) to get to 1.3 caliber stability on a G76. Which seems like a lot on an 8 ounce "dry" rocket, but then again it's a relatively stout design. I probably won't fly on those too often (it sims to 2700 ft), but limiting to Fs doesn't help a whole lot. Does that amount of mass seem reasonable?
 
if you can get the cg(pad weight) at or forward of Madcow's cg point, you should be good to go. Madcow has done the math for us to determine a safe cg point of one cal.
Rex
 
Does anybody here have experience with flying the batray with the published cg? The instructions give 17.75" from the nose, but the cp from the rocksim is only about 18.5" from the nose - less than half a caliber. But then again I converted the file into open rocket, and the wacky fin shape probably doesn't sim exactly right.
 
I could certainly swing it, but I'm still curious about other flyers' real world experience with it, just to get an idea of if I'm in the ballpark.

Basically I want it to be stable but light enough to fly on an F22, since really a bat deserves a smoky motor.
 
So I think I figured it out. Open Rocket is giving me a CP of about 18.7", making the recommended CG of 17.75" only marginally stable. But I downloaded the Rocksim trial and opened the same file, and got a CP of 20.1" - making the recommended CG right at 1 caliber.

Has anybody else noticed a similar discrepancy with this design or ones like it? I'm not familiar enough with the exact details of the calculations in each program to know which is more trustworthy here, though I'd suspect Rocksim just because it generally has better support for "exotic" shapes.
 
AfterBurners said:
Recently I completed the build on my MADCOW Bat Ray.

View attachment 111981

The question I have is based on the ROCKSIM file of my Bat Ray the CG ranges from 1.00 to 1.09 (Margin) depending on the size of the motor that is loaded in the rocket.

Is this a safe number to fly the rocket. It's already built so there's not much I can do about it right now. When I measure from the tip of the nose cone back 17.75" (MADCOW suggests this is where the CG should be) it lines up perfectly in the center with my launch lug placement. I'm using a 1/4 x 3" lug. The weight of the rocket according to the specification sheet suggests 8.0oz. My Bat Ray comes in at 8.3 oz and probably because I have on an Aero-Pack motor retainer.

The original balsa nose cone, which was defective had a hole in it and the instructions suggested stuffing the end of the shock cord in the hole and filling with epoxy and adding weight if necessary? My replacement nose cone from SEMROC had no hole obviously and I used an eye screw for an attachment point.

So do you guys think it should be fine to fly? Here's the ROCKSIM file - View attachment 111991

Scratch builds I have done in the past I always try to stay between 1.15-1.50, but I would image every rocket is a little different.

Just looking for some feed back.

Right now I'm working on the MADCOW Scooter kit and I was consider changing the CG slightly if the number comes in too low?
Click to expand...

I built on a few years ago and used epoxy to attach the shock cord.Why don't you contact Madcow,I'm sure they will send a replacement nosecone.Built as instructed and with the motors recommended you should have no problems flying it.Mine flies great.Madcow has very nice kits.I have the striker in my build pile.Momba is another I built.Stickershock made me the cow patches.
 
FWIW my version of the kit came with a long eyebolt and a drilled nose cone, with the intent to epoxy the eyebolt along with any nose weight into the drill hole. Since it sounds like the manufacturer recommended CG works, I'll weight it to that, swing test it, and fly it in a couple smallish motors first to get a feel for if its marginal.

Still curious about the difference between rocksim and OR CP though.
 

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