Failing is fine, as long as we are learning.

[Simon_betts10]

Inspired by George Mallory and his sometimes controversial story of conquering Everest I decided to build a rocket that I was sure may fail.

The reason I was sure it might fail, I used long fin, this was to see if I could experience flutter in a potentially destructive way. To ensure it, I used an F motor (read streak I think). The idea was to observe the effect and analysis the debris to see what had happened to the rocket during flight.

The results were spectacular, a fantastic launch and lots to learn. Lots of papers were sent to me by the experts at the club and I learnt a lot of Information about flutter and what affects its chances of being destructive.

 

[smstachwick]

Very nice!

What conclusions did you draw from that? Any surprises?

 

[Hobie1dog]

thanks for all the pics

 

[cls]

Inspired by George Mallory and his sometimes controversial story of conquering Everest I decided to build a rocket that I was sure may fail.

Some very cool logic there, hope your experiment design yields results...

 

[OverTheTop]

So how will you choose to try to cure the flutter? How will you stop the fins from flexing and resonating?

 

[Simon_betts10]

Very nice!

What conclusions did you draw from that? Any surprises?

I was surprised to see the airframe had unzipped itself, must have been during the period of unstable spin proceeding the separation of the fins from the airframe.

Otherwise most of this was confirmation. Fins were severed close to the mounting point which would have been the area of maximum moment.

Was incredibly to watch.

 

[Simon_betts10]

So how will you choose to try to cure the flutter? How will you stop the fins from flexing and resonating?

There are a couple methods. One is to reduce the distance from the root to the tip. Another is to reduce the distance of the tip cord, thus giving a more triangular fin design. Thankfully I have had some great advice from @PhilC who really knows his way around this issue.

 

[lakeroadster]

Any chance you could post the geometry of the fins, along with what they were made of, so we could run fin flutter calc's?

Or maybe if you have already done those calc's. and a simulation of the flight you could post them up for us to enjoy?

I've thought that using calculations to build a rocket with lets say (6) fins, of varying shape, thickness and material would be an interesting experiment... to see which would shred, and which wouldn't. Strategically spacing the stronger fins to ensure stability, while still having a test bed for analysis of the weaker fins.

 

[Simon_betts10]

Any chance you could post the geometry of the fins, along with what they were made of, so we could run fin flutter calc's?

Or maybe if you have already done those calc's. and a simulation of the flight you could post them up for us to enjoy?

I've thought that using calculations to build a rocket with lets say (6) fins, of varying shape, thickness and material would be an interesting experiment... to see which would shred, and which wouldn't. Strategically spacing the stronger fins to ensure stability, while still having a test bed for analysis of the weaker fins.

Will see what I can pull together.

 

[Steve Shannon]

I was surprised to see the airframe had unzipped itself, must have been during the period of unstable spin proceeding the separation of the fins from the airframe.

Otherwise most of this was confirmation. Fins were severed close to the mounting point which would have been the area of maximum moment.

Was incredibly to watch.

Zippers are very common with destructive fin flutter. Almost every shred I’ve seen, in anything less than fiberglass, results in a zipper. Once the fins go away the rocket becomes unstable, tumbles, and the nosecone comes out, dragging the chute with it. When the chute pops open it causes a zipper.

 

[PhilC]

I had a flutter problem in the days when I was starting out, and before I found the NACA report. I reproduced it in a rocket and used a cheap keycam & mirror to film down the rocket towards the fin. It had two or three good ply fins and one in balsa which was intended to fail. The video wasn't brilliant (needed a higher frame rate) but it showed some twisting before failure.

 

[Simon_betts10]

Zippers are very common with destructive fin flutter. Almost every shred I’ve seen, in anything less than fiberglass, results in a zipper. Once the fins go away the rocket becomes unstable, tumbles, and the nosecone comes out, dragging the chute with it. When the chute pops open it causes a zipper.

Interesting comment I will bear in mind.

 

[Simon_betts10]

I had a flutter problem in the days when I was starting out, and before I found the NACA report. I reproduced it in a rocket and used a cheap keycam & mirror to film down the rocket towards the fin. It had two or three good ply fins and one in balsa which was intended to fail. The video wasn't brilliant (needed a higher frame rate) but it showed some twisting before failure.

Very cool way of seeing the effects. What was the purpose of the mirror? My assumption is less drag, camera inside of the airframe and mirror external?

 

[OverTheTop]

For some further learnings, have a look here:
https://en.wikipedia.org/wiki/Second_moment_of_area
https://en.wikipedia.org/wiki/List_of_second_moments_of_area
The second moment of area is also a measure of stiffness. Look at the diagram in the second link with the solid rectangles. Ix is proportional to h^3. That shows that stiffness (Ix) goes up proportionally to h^3. What conclusions can you draw from that?

 

[PhilC]

Very cool way of seeing the effects. What was the purpose of the mirror? My assumption is less drag, camera inside of the airframe and mirror external?

Correct. The camera was big & awkward to stick in the airflow so I mounted it inside. The mirror was outside in a small fairing. The mirror restricted the field of view but allowed the fin to be seen without adding too much drag.

 

[Simon_betts10]

Any chance you could post the geometry of the fins, along with what they were made of, so we could run fin flutter calc's?

Or maybe if you have already done those calc's. and a simulation of the flight you could post them up for us to enjoy?

I've thought that using calculations to build a rocket with lets say (6) fins, of varying shape, thickness and material would be an interesting experiment... to see which would shred, and which wouldn't. Strategically spacing the stronger fins to ensure stability, while still having a test bed for analysis of the weaker fins.

I have attached the geometry of the fins. Currently I have not done any mathematical proofs that the flutter was the cause. Mostly been focusing on building intuition via practical experimentation. If you have calculations I would be very much grateful if you would share them. I used 4 fins for this design and a 2 or 3mm ply.

 

[lakeroadster]

I have attached the geometry of the fins. Currently I have not done any mathematical proofs that the flutter was the cause. Mostly been focusing on building intuition via practical experimentation. If you have calculations I would be very much grateful if you would share them. I used 4 fins for this design and a 2 or 3mm ply.

Theoretical Flutter Velocity (NACA TN 4197, Equation 18)

Aircraft Plywood (Birch):​

2mm thick: 599 ft/sec (409 mph)​

3mm thick: 1102 ft/sec (752 mph)​