Card board cut out?

[bradycros]

I've seen referances on the forum about a card board cut out method. Can someone please explain what it is, how do you use it and how do you make it?

 

[stantonjtroy]

It's an elementary method for locating the CP (Center of Preasure) of your model. The relationship of the CP to the CG (Center of Gravity) will determine the stability of your model. In short the "cardboard cutout" is an exact, proportionate sillouette of your model. You balance it via the "see-saw" method and mark the balance point. this is the CP.....Sort of. this is accurate for a realitive crosswind of 90 degrees. Not totally accurate. The Barrowman equations are a more accurate method. It's a non-calculas series of formulas that, though daunting at first glance, are fairly easy to figure. For stability the CG needs to be at least one gauge (body tube diamiter) forward of the CP. Most kits have the stability question worked out but a scratch build will need the stability checked. There are a number of online sources for the Barrowman equations ( though not one comes to my mind at the moment )
Hope this helps.

 

[stantonjtroy]

Try here: https://my.execpc.com/~culp/rockets/Barrowman.html

 

[Peartree]

The cardboard cut out method is still useful for those really odd designs that are not suitable for use with the Barrowman equations, things with complex fin arrangements, side pods, tube fins, etc. The newest version of Rocksim (computer simulation software) version 9.0 is supposed to handle more of these types of things, but for now, for me, it remains unaffordable so I still find occasions where this method is a useful test. The cutout method is not without its problems as Troy noted so while it's useful, you cannot rely upon it 100 percent.

 

[shreadvector]

I've seen referances on the forum about a card board cut out method. Can someone please explain what it is, how do you use it and how do you make it?

It is shown in the reference material I linked to in the top sticky thread on the top of this forum. The material covers so much and helps get new folks the info fast and eliminates the need to try to re-type the material over and over again.


Here are a couple of those links again:

https://www.esteseducator.com/Pdf_files/1976clas.pdf

https://www.spacemodeling.org/JimZ/manuals/cmrdm.pdf

 

[Micromeister]

The Cardboard cutout method gives what's referred to as an LCP Lateral Center of Pressure based on the overall area of the object involved.
The method is exceedingly helpful with odd shaped or otherwise very difficult to draw models. Particularly ODD-Rocs and PMC's.

By balancing the cardboard cutout on the edge of a ruler, that point will serve as the LCP of the model, Giving it the modeler a starting point for locating a forward placed workable CG that can be transfered to the actual object during the building process. Below is an example.
Hope this helps.

 

[luke strawwalker]

I've seen referances on the forum about a card board cut out method. Can someone please explain what it is, how do you use it and how do you make it?

I would add to the excellent information provided here by others, that the cardboard cutout method provides the CENTER OF LATERAL AREA of the rocket. When you cut out the rocket's silouette, the balance point is the center of lateral area. As someone pointed out above, this is where the CP would be if the rocket were pointed 90 degrees to the airstream, in other words, pointed completely sideways to the direction of flight. That's about as much of a 'worst case scenario' as you can get! Therefore, the CP derived from the cardboard cutout method is about as conservative and 'worst case' as you can get.

Once you have the balance point of your rocket from the cardboard cutout method (center of lateral area) the actual rocket can be loaded with the desired rocket motor, wadding, chute, etc. so it's flight ready (less ignitor) and balanced using a loop of string slid along the body until the rocket balances horizontally, which determines the Center of Gravity, or CG. This is the ACTUAL CG of the rocket. Measure from the end of the body tube to this balance point, and compare to a similar measurement made of the cardboard cutout-- if the CG is at least one body diameter ahead of the center of lateral area, you should have a good stable rocket.

Center of lateral area DOES give you an over-conservative "CP" location usually, which is why rockets designed by cardboard cutout methods tend to be overstable. Still, that's better than understable, (neutrally stable, or completely unstable!) That's where Rocksim or some of the other programs using their methods or the Barrowman Equations come in handy... they get you actually closer the real CP point.

CP varies in flight, it moves around based on the outside conditions of the rocket in it's flight, and the air it's moving through. One important thing to remember is that, generally speaking, the CP tends to move FORWARD the slower the rocket is travelling, or the greater the angle of attack of the airstream it's moving through. That's why rockets tend to become more stable as they pick up speed during flight, and also why high angles of attack like rod-tipoff (rod whip) and gusts of wind and other things can make rockets go unstable or do other unwanted things at slower airspeeds (like just after liftoff). That's why you need at least one caliber (body diameter) stability, so account for the CP moving forward.

Fortunately with solid-propellant rockets, the CG also moves forward as the propellant burns, so this helps offset the CP movement effect. (Incidentally, liquid fuelled rockets are the opposite-- the CG moves REARWARD as the propellant is expended and is getting to lower levels in the tanks-- helpful to know if you're flying Cold Power rockets, as some folks on here are!)

Good luck and hope this helps! OL JR