Please Help Open Rocket Screen Shot Two Different Fins?

[Scott Chase]

I don't know enough about fin design. The small fin will hit higher altitudes than the larger fin and is more stable. I just don't know what the smaller fin is going to act like off the pad. Can someone please help me understand the difference between the two?

 

[Homer_S]

Did you mean the larger fins are more stable? The CP is moving towards the rear (~31" for the small ones v. ~35" for the larger ones, measured from the nose or 3 Cal v. 5 Cal)... more stable. The small fins result in less drag, so more altitude. Key will be looking at the velocity off the rod/rail in the sim window. Also, make sure to sim the motor to see where the CG goes...

One other comment, are the fins TTW with essentially a minimum diameter? If so, they look like they peek into the motor. You can switch to end view to make sure the fin tabs aren't too long.

Homer

 

[Rex R]

while I have seen a case where smaller fins Were more stable...this is not the case here. an upscale hy-flyer with its' long fins, making the fins smaller and shorter moved the cp aft.
Rex

 

[Buckeye]

while I have seen a case where smaller fins Were more stable...this is not the case here. an upscale hy-flyer with its' long fins, making the fins smaller and shorter moved the cp aft.
Rex

Can you post a sim file that exhibits this behavior?

 

[Scott Chase]

Did you mean the larger fins are more stable? The CP is moving towards the rear (~31" for the small ones v. ~35" for the larger ones, measured from the nose or 3 Cal v. 5 Cal)... more stable. The small fins result in less drag, so more altitude. Key will be looking at the velocity off the rod/rail in the sim window. Also, make sure to sim the motor to see where the CG goes...

One other comment, are the fins TTW with essentially a minimum diameter? If so, they look like they peek into the motor. You can switch to end view to make sure the fin tabs aren't too long.

Homer

I did some more sims between the two rockets the small fin did produce a flight that was 400 hundred feet higher with a 29mm G55. The fins only look like TTW because I put a Tab on them, not thinking about what I was doing. I'm going to go with the larger fins 400 feet does not seem worth it. I'm going to learn how to check the velocity off the rod. What velocity should I be trying to achieve?

 

[Scott Chase]

while I have seen a case where smaller fins Were more stable...this is not the case here. an upscale hy-flyer with its' long fins, making the fins smaller and shorter moved the cp aft.
Rex

Thank you for your reply are you suggesting that the larger fins would make a better high flyer ?

 

[Scott Chase]

Can you post a sim file that exhibits this behavior?

I can try? Not so good with the sim program yet.

 

[Rex R]

the Estes hi-flyer is one of those rockets w/ long fins and the cp on the original version is aft of the beginning of the fins(the cg was mighty close as well...so close that it was unstable on a C6-5). when When Estes upscaled it to the hi-flyer xl they noticed that the upsized fins looked like elephant ears so they down sized the fins by 10%. this also moved the cg and cp. however the cg moved a bit further forward than the cp moved.so it worked well. I noticed it as I was doing sims of an hi-flyer XXL(flies great on an E28-6) sorry to say all my open rocket files are at home.
Rex
*edit*
you should be looking for a speed off the rod/pad between 30 fps(no wind) 40-45 fps(mild wind)

 

[Scott Chase]

the Estes hi-flyer is one of those rockets w/ long fins and the cp on the original version is aft of the beginning of the fins(the cg was mighty close as well...so close that it was unstable on a C6-5). when When Estes upscaled it to the hi-flyer xl they noticed that the upsized fins looked like elephant ears so they down sized the fins by 10%. this also moved the cg and cp. however the cg moved a bit further forward than the cp moved.so it worked well. I noticed it as I was doing sims of an hi-flyer XXL(flies great on an E28-6) sorry to say all my open rocket files are at home.
Rex
*edit*
you should be looking for a speed off the rod/pad between 30 fps(no wind) 40-45 fps(mild wind)

Rex,
when you say off the rod are you talking about vertical velocity, vertical acceleration, total velocity or total acceleration?

 

[Nytrunner]

Rex,
when you say off the rod are you talking about vertical velocity, vertical acceleration, total velocity or total acceleration?

Vertical velocity. In no-to-low winds, you need at least ~44 ft/s upon rod exit