Wind speed vs. parachute descent rate

How does wind speed affect a parachute's descent rate?

I've observed the same rocket do this on two different days:

Flight One-
Descent time of 46 seconds
Light wind, 5 MPH, sunny

Flight Two-
Descent time of 37 seconds
Moderate wind, 13 mph, partly sunny

The difference could be lack of thermals, both flights were about 800', does wind have the effect of 'knocking down" a parachute?

In the stiffer wind the parachute drifted much further, but landed quicker!

Sounds like a TARC question to me.

You're likely to experience more thermal updrafts in calm air rather than in moving air that would blend the temperature variations better so that may be part of the difference.
Was the load swinging more in the moderate wind as opposed to the light wind? If so, the chute may have been "spilling" some of its contents and that could result in a more rapid decent.

Ken, you're very perceptive about this being a TARC related question...

I didn't consider that a stiff wind would counteract a thermal, and yes, the payload section was swingin' wildly!

We were going to try a bigger 'chute but were worried about the trees, and of course the heavy rain ended everything.

You have already noted that the wind could have partially collapsed the deployed chute and changed the descent configuration. There are probably as many more variables as you care to keep looking for:

Was the chute folded/packed the exact same way each time?
Was the ejection wadding packed the exact same way each time?
Did the recovery system come to the "fully open" condition in the same amount of time? (or did the chute stay wadded up for a fraction of a second longer?)
Did any of the shroud lines snag on any other part of the rocket or recovery system?
Was the nose cone inserted in the exact same position each time? (or does the nose cone fit more tightly/loosely if rotated to a new orientation?)
Did each motor have the same amount of ejection charge material, and burn that charge the same way?
If you are using the Estes "tri-fold" internal mount for the shock cord, did that big fat wad of obstructing cardstock remain consistently glued down to the inside of the BT?

Last but not least, it really isn't fair to make a comparison like this based on only two flights. A much more rigorous set of tests needs to be done in order to draw valid results. The statistical variations are going to kill your analysis with only one or two launches from each condition.

Most likely the largest difference is altitude, due to weather cocking of the flight in wind.

Gerald

granafratz said:

Ken, you're very perceptive about this being a TARC related question...

I didn't consider that a stiff wind would counteract a thermal, and yes, the payload section was swingin' wildly!

We were going to try a bigger 'chute but were worried about the trees, and of course the heavy rain ended everything.

Click to expand...

Thanks; I've been involved with TARC for a while now and you were talking about all the right numbers.

Have you been coming up to the CMASS launches to practice? If not, our next scheduled launch is 08 November in Amesbury. Lots of people on the filed who can answer your questions and discuss various theories.

Actually, the second windier flight was 20 higher in altitude (flight was relatively straight), both flights appeared to deploy fully just after apogee.

I think Ken's assertion that the stiff breeze counteracted the thermals, and that the swinging parachute/payload spilled air out of the canopy, is more likely.

Interesting that the separate booster and it's 'chute was not swinging, I suppose if a payload gets swinging due to deployment it just gets worse in a stiff wind?

More experiments to come!

Ken, Granafratz = Tom Kahmann, we know each other well!

I know for certain that wind can have an effect on parachute decent rates.

On a day that really wasn't suitable for launching, I went out with a D12-7 powered clone of the Estes Exocet 38mm Missile (1925). I launched it into the wind, and it weathercocked. The parachute deployed with the rocket only about 100' above the ground. It then blew back towards me, well over my head, and managed to catch an updraft created by the hill behind the field where I was launching. That rocket, with it's 18" parachute (and a 6" hole in the center) stayed aloft for what seemed to be about half an hour as it was stuck in the updraft on the back side of this hill. I had to run around the hill, stopping every couple of minutes to see where it was, and I was stunned how long it stayed in that little bubble of air, until a gust, or a drop in the wind let it go.

The yard (an old burned out lumber yard) has since been developed, so I won't be able to launch there again .

granafratz said:

Actually, the second windier flight was 20 higher in altitude (flight was relatively straight), both flights appeared to deploy fully just after apogee.

I think Ken's assertion that the stiff breeze counteracted the thermals, and that the swinging parachute/payload spilled air out of the canopy, is more likely.

Interesting that the separate booster and it's 'chute was not swinging, I suppose if a payload gets swinging due to deployment it just gets worse in a stiff wind?

More experiments to come!

Click to expand...

Put a spill hole in the apex of your payload canopy to eliminate the airflow spill oscillation. it WILL help the decent rate. Depending on the diameter of the chute no more the 3/4" hole should be all that's needed. Will also help lower the swing impact speed of the egg capsule.

As several folks mentioned a spill hole in the center of a chute will go a long way to stabilize a chute and reduce the oscillations on descent which spills air and increases the descent rate. A spill hole that is 10% of the diameter (1.5" on 15" chute for example) only reduces the chute area by 1%! (0.1x0.1=0.01=1%) so the descent rate is not dramatically changed while the swing is dampened).

As Kenn mentioned, your rocket will always weathercock into the wind and will reach a lower apogee the stronger the wind blows. Angling the rod downwind several degrees will minimize the altitude loss as the rocket will turn more vertical as it accelerates lowering the apparent wind angle. Run some sims to find the relationship.

To regain the lost altitude, you must reduce the weight of the rocket. Run some sims to determine this sensitivity.

The biggest effect of wind is to increase the sideways drift of the rocket. In your case, the sideway drift in the 5 mph wind was about ~337' whereas in a 13 mph wind it increases to ~705'which is more than twice the walk......

Bob, also a CMASS TARC mentor

Also remember, relative to the rocket during decent, there is no wind. The rocket is moving with the air mass so there is no air moving sideways past the chute. That is why it drifts, it's moving at the same speed as the air.

What ever is causing the faster decent on the windy day, it is not the direct affect of wind on the rocket during decent. Lack of thermals or an unstable/swinging parachute would be my first guess. If the rocket tilts into or with the wind it could have a much larger sideways component to its speed at deployment and that could contribute to the excessive swinging of the rocket under the chute. Each time it swings to a side, the chute spills air and drops faster.

As Bob explained, a spill hole can help alot.

Were there any barometric readings for either flight?
What about air temperature?
Air density would also play a roll in decent.


JD