Corrected Boyce Maxi Pershing 1A CG/CP locations

[burkefj]

Hi all, I just completed my Boyce Pershing 1A Maxi kit that uses bt-101 tubing, and balanced it as per the instructions at 3" back from the forward end of the body tube, which would be about 22.3" back from the tip of the nose. I did one test flight using a G-74-6 which took a hard right turn about 50' up and knew that it was marginal at this setting. I set about to build a quick rocksim/openrocket simulation to confirm what a good CP is and found that they had not used the same size fins as the maxi kit, they are slightly smaller which makes things worse. I've done many pershing 1A rockets and found that barrowman is conservative by quite a bit compared to rocksim equations so I'm comfortable using Rocksim stability for a pershing 1A and using about 1.3x caliber for CG location.

One Mistake I've found people make is to rely on a rocksim for the maxi pershing 1A that is on rocketreviews, this is a big mistake as that has 4 fins per section not 3 so gives a very optimistic CP location and it should really be removed and burned.....

I've moved the CG now to 1" back from the end of the body tube or 20.3" back from the tip and will refly on monday to confirm.

I'll include both simulations here, both rocksim and openrocket, these are dimensionally correct and the CG and total weight are forced at the top level of the design to be what I have using a 36" top flite chute and a G-74-6 motor. I don't have any of the internals because they aren't needed for this. You can see how conservative barrowman is for openrocket as compared to rocksim stability.

I did let Alex know and he will update his online instructions once I've confirmed that this setting is ok. I've used about 5-6 ounces of lead shot in the nose to get this CG with this motor. Using a G-80 or G77/78/79/64 are heavier and will need more weight of course.

 

[burkefj]

I re flew today in 8mph winds and it flew great, nice and straight.

My kids were working so I had to try to video and you can see they are much better than I am

 

[kuririn]

Thank you for posting that correction.
Glad I didn't fly mine yet.

 

[ronklogan]

Thank you for the Rocksim file!!!! I've finished assembling mine and wanted to work out how much weight I need and how high it will go with various motors. I wish manufacturers would always make these available when we buy their kits.