same rocket, motor, & chute - very different decent rates (~45 seconds different)

[Dave S.]

I launched the same rocket two days in a row - same motor and parachute used each time. I noticed that the videos of the flights are different time wise from launch to touchdown.

The time from launch off the pad till chute opening is about the same for both (~14 seconds).

The difference is clearly in the descent time under chute. The shorter flight pretty much matches what was predicted by OpenRocket. The second flight is about 45 seconds longer. (I am comparing the video of both flights - launch to touchdown).

It seems weird that in one case it would take almost one minute longer to come down. This second flight only went 35 feet higher, but landed a loooong ways away.

It was almost perfect calm for the shorter first flight and a bit of a wind at ground level for the longer 2nd flight. I'm thinking probably a big difference in the wind high up, even if not too different at ground level?

This rocket was plain old motor eject w/o a JLCR. I am thinking I should use the JLCR next time and maybe a chute with a spill hole to stop that swinging and the hard smack it gets at touchdown.

Any other suggestions?

 

[mh9162013]

Dual deployment?

Also, maybe thermals contributed to the lengthened descent rate. You could work in identifying thermals and trying to avoid them?

 

[David Schwantz]

Big puffy clouds with flat bases have thermals under them. Also look for birds circling in them.

 

[BEC]

Thermal activity would be my first guess as well. I flew the same model 9 times last Friday and even on similar altitude flights, the durations sometimes varied greatly. On two cases there was clearly thermal action...not enough to take it back up but certainly enough to slow its descent significantly under 'chute.

 

[Steve Shannon]

Wind can make a difference also. With a wind your parachute tilts with the wind and develops a little lift.

 

[robopup]

Also may be worth paying attention to air density - it's proportional to drag.

 

[J Blatz]

How about the variation in delay times? Depending on the motor you can see significant swings in delay for identical configurations. This could mean one of the fights had a couple seconds longer delay thus a shorter time under canopy.

 

[Dave S.]

The delay times and altitudes were about the same. It sounds like thermals or winds at elevation are the most likely explanation for why there was such a significant difference in the time it took for the rocket to get from apogee down to the ground.

Thanks for all your suggestions I really appreciate it

 

[BABAR]

Probably not the answer, but worth considering.

Did you use a plastic parachute? Incomplete opening is common with these. However I would expect if this WAS the culprit, your longer duration would more likely match the sim, and the short one would be due to incomplete opening. EXCEEDING sim predictions, IF all the data inputs are correct, would suggest thermals.

 

[Jeremy Sim]

Thermal activity I'd guess

 

[Handeman]

The chute can definitely develop lift when air moves across it horizontally. This usually doesn't happen because the chute drops vertically while moving horizontally with the air mass. The air has little or no horizontal velocity relative to the chute. In other words, since the chute is moving with the wind, there is no "wind" being felt by the chute.
If the air mass is very turbulent or the chute is swinging side to side, you can develop more horizontal air movement across the chute which will create lift. A spill hole is a good way to stop swinging and greatly reduce any lift that might be created.

 

[Dave S.]

It was definitely swinging. Enough to make me consider a new chute with a spill hole