Ye Olde Rocket Forum - View Single Post - Interesting offering from BMS...

Quote:

Originally Posted by marslndr

Hmm... any nose down angle should make things worse I would think. The motor is already above the CG, that should give it a nose down moment. Angling the motor tube nose down should increase boost problems.

But you did not see this on yours? Strange.

The Space plane solved the problem of residual lift during boost with spin tabs, that is not an option on this rocket without an extensive redesign.

One thing I find curious is that people have only had stability problems in the pitch axis, not the yaw axis. It looks like it does not have enough fin to correct yaw instability, the dihedral will help but it does not look like enough to me.

Anyway, I ready to attach my motor tube assembly so I need to decide soon.

Thanks

Mark

Mark,

Been there, done that. At least three times. It's not written in any of the plans that I'm aware of, but this is the trick that makes the Invader fly right. I found this out the hard way. The Invader has plenty of yaw stability with those twin fins, but it's during the boost phase where you see the pitch instability. The downthrust in the motor tube is equal to the downthrust you'd build into the motor mount of a high-wing, flat-bottom RC trainer airplane. That big pie pan of a wing produces a lot of lift relative to its weight. The result is, if you mount the motor parallel to the wing root, you get the back-flipping power prang of death. Mount the motor with the 2-degree downthrust, and it counteracts that pitchover. The downthrust is only for the power phase of the flight, not the glide.