Drag Coefficent; what does it mean?

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Well-Known Member
Oh and please don't ask me to look in the "Bible of model rocketry" it's in the mail.

What are concidered high drag coefficents? .3? And how much of an effect does it have on a fight?

MarkII

Well-Known Member
Oh and please don't ask me to look in the "Bible of model rocketry" it's in the mail.

What are concidered high drag coefficents? .3? And how much of an effect does it have on a fight?
If it's in the mail, then you can wait for it to get to you. Then you will be able to look it up yourself. In the meantime, try Google.

MarkII

jsdemar

Well-Known Member
The drag coefficient is a unitless value that represents the affects of shape and surface roughness on drag. It is typically found through wind tunnel testing or backtracking flight data form a logging altimeter.

The drag coefficient will depend on velocity, with a significant change through the transonic region. For typical model rockets, it doesn't change much during most of the flight.

Some people forget that the stability of a rocket will greatly affect the drag coefficient. If the rocket varies its angle-of-attack, it'll have a much higher drag coefficient.

Larger rockets of the same shape will have a lower drag coefficient than smaller ones. This is due to the fact that longer rockets (higher Reynolds number) provide more distance for the airflow to stabilize over the surface (this is a highly simplified explanation!).

Almost forgot to answer your question. The typical value of drag coefficient of 0.6 for small rockets and 0.4 for larger ones, assuming a simple shape without transitions.

Here's something I wrote 15 yrs ago:
http://www.interactiveinstruments.com/pdfs/28.pdf

-John

powderburner

Well-Known Member
What are concidered high drag coefficents? .3? And how much of an effect does it have on a fight?
A cd value of 0.3 would be outstandingly LOW. A much more realistic cd value for most "normal" rockets would probably be more like 0.5 to 0.8, and could easily be higher. (Most people think they build and finish MUCH better than they actually do.)

And I am not trying to sound mean, but if you don't understand what effect it has, you need to do some homework. I strongly recommend that you actually do the numbers for one example to see how it all works.

Do you use EXCEL or any other spreadsheets? This sort of thing is pretty easy to do with a computer helping you; after you program one line of calculations you can copy-and-paste right down the page. If you want to run through an example, PM me and I'll be happy to help you.

cjl

Well-Known Member
The drag coefficient is simply a way of representing the drag of a shape in a method that is unrelated to its size or velocity (hence dimensionless). For our rockets, the drag coefficient ranges from about 0.25-0.3 (on the extreme low end) all the way up to 0.7-1. You could even go higher than that for some odd designs. In general, the drag force on the rocket will be directly proportional to the drag coefficient, directly proportional to the frontal area, and proportional to the square of the velocity.

Also note that you will commonly see incredibly low drag coefficients for aircraft (like 0.05). This is because they use a different reference area, so the coefficients are not really comparable.

Well-Known Member
Wow thats intresting most of my rocksim designed rockets have a drag coefficent of .3 ; not to shabby... even my really fat ones have .3 drag coefficents.....

MarkII

Well-Known Member
Remember, though, that in many places in RockSim, you can arbitrarily set any drag coefficient you want. You could, if you wanted to, create virtual rockets in RockSim that had impossibly low levels of aerodynamic drag that would never exist in reality. Achieving a very low Cd in an actual built rocket is quite a challenge. To determine how much drag the real deal has, you need to use a wind tunnel.

MarkII

Well-Known Member
where does one obtain a wind tunnel?

RoyAtl

Well-Known Member
where does one obtain a wind tunnel?
Oh, any middlin' to large aircraft manufacturer will have one laying around.

A few aerospace colleges and university programs have smaller ones.

You can even build a fairly simple low speed one (Vern Estes published plans for one back in the 60's, and they were going to get updated by Bob Cannon and Dave Babulski, until illness got in the way).

Someone even made a makeshift one out of a large pvc pipe strapped to a car (with the caveat that the wind tunnel speed could only match the car speed + a little more due to the convergence)

But even if you had a small wind tunnel, you'd need the instrumentation to measure the forces (the tunnels that Vern, et al. designed were for stability tests that were slightly more sophisticated than the swing test, not for drag measurement)

JDcluster

Well-Known Member
Years ago in 7or8th grade we had a simple wind tunnel to measure the drag on our CO2 powered cars. It was a simple blower motor that ran through a set of boxes with smaller boxes or tubes to straighten out the air flow.
The car was held against a rod that connected to a triple beam scale to measure the force.

JD

RoyAtl

Well-Known Member
Years ago in 7or8th grade we had a simple wind tunnel to measure the drag on our CO2 powered cars. It was a simple blower motor that ran through a set of boxes with smaller boxes or tubes to straighten out the air flow.
The car was held against a rod that connected to a triple beam scale to measure the force.

JD
That is essentially the Estes wind tunnel. Nice that you measured against the scale.

Ah, here's something... http://www.aerorocket.com/

cjl

Well-Known Member
Wow thats intresting most of my rocksim designed rockets have a drag coefficent of .3 ; not to shabby... even my really fat ones have .3 drag coefficents.....
Rocksim often assumes unrealistically smooth surfaces and transitions, which would cause a laminar boundary layer and less drag. In reality, unless you're in the habit of polishing your rockets before each flight, your surface will be rougher than Rocksim's default, causing a higher drag coefficient. Second, the drag coefficient is independent of diameter. A really fat rocket might have a high drag, but a low drag coefficient. This is because the drag itself is proportional to the drag coefficient (which describes the drag inherent to the shape) multiplied by the frontal area (big things are draggier) multiplied by the square of the velocity (fast things have more drag). Because of this, a 4" rocket will have 4 times more drag than an otherwise identical 2" rocket (with the same Cd and velocity).

I'd be pretty surprised if any actual rockets you build get under about 0.45, and in most cases, they'll be higher than that.

MarkII

Well-Known Member
Rocksim often assumes unrealistically smooth surfaces and transitions, which would cause a laminar boundary layer and less drag. In reality, unless you're in the habit of polishing your rockets before each flight, your surface will be rougher than Rocksim's default, causing a higher drag coefficient. Second, the drag coefficient is independent of diameter. A really fat rocket might have a high drag, but a low drag coefficient. This is because the drag itself is proportional to the drag coefficient (which describes the drag inherent to the shape) multiplied by the frontal area (big things are draggier) multiplied by the square of the velocity (fast things have more drag). Because of this, a 4" rocket will have 4 times more drag than an otherwise identical 2" rocket (with the same Cd and velocity).

I'd be pretty surprised if any actual rockets you build get under about 0.45, and in most cases, they'll be higher than that.
From the "what is the Best way to get a small rocket down quickly" thread:

http://www.rocketryforum.com/showpost.php?p=78590&postcount=10
http://www.rocketryforum.com/showpost.php?p=78592&postcount=11
http://www.rocketryforum.com/showpost.php?p=78632&postcount=13
http://www.rocketryforum.com/showpost.php?p=78636&postcount=14

My feeling is that the "Polished" setting represents a standard of perfection that is rarely if ever achieved in low power rockets. I think it is more appropriate for Level 2 and 3 rockets whose builders have utilized professional-grade auto body finishing techniques. It's not a grade of finish that you can get by simply rubbing a rattle-can paint job with some 1500 grit.

MarkII

Well-Known Member
i need to remember that

where does one obtain a wind tunnel?
The sky above you makes for a pretty good wind tunnel. After the rocket motor burns out, the acceleration you can measure on board is directly proportional to the rocket's aerodynamic drag. As long as the speed is high enough to get significant deceleration, the drag calculations you make that way can be quite accurate. Here are some Cd vs. velocity plots I calculated from some flights in the last 2 years, using a Parrot altimeter:

A 29mm rocket flying a conical nosecone and an Apogee 29mm nosecone, both with G80s

A 38mm rocket that went almost 16,000 feet on an I600, using a Von Karman 7:1 nosecone

The increase in drag near Mach 1.0 really stands out in these plots. And notice how much lower the increase is for the Von Karman nosecone, which was invented specifically to produce the theoretical minimum wave drag for low Mach numbers.

The drag coefficient was calculated using the measured rocket diameter, the as-weighed landed mass of the rocket, and altitude-specific atmospheric density. A drag coefficient of 0.3 is definitely achievable without polishing; these rockets were smooth and a little shiny, but not mirror-finished. What some may not realize is that it's a lot easier to get a low Cd with a fat rocket than it is for a long, slender rocket, since the Cd for rockets is normalized against the frontal area. A long skinny rocket has a lot more skin area compared to its frontal area, and that shows up in the Cd. A short, fat rocket doesn't have much drag for its frontal area, which is what the Cd is all about. I have flown a very short, matte-finished rocket designed for the F10 motor that had a drag under 0.3; it was 0.27 to 0.28 if I recall correctly.

But like cjl said, the drag is the 1/2 x area x the Cd x the velocity squared, and each of those factors is just as important as the other. Some people think the aero drag losses go down if the rocket breaks Mach, just because the Cd goes down. In reality, the drag force at 1200 mph was over twice what it is at 800 mph for the 38mm flight, above, even though the Cd was lower.

cjl

Well-Known Member
But like cjl said, the drag is the 1/2 x area x the Cd x the velocity squared, and each of those factors is just as important as the other. Some people think the aero drag losses go down if the rocket breaks Mach, just because the Cd goes down. In reality, the drag force at 1200 mph was over twice what it is at 800 mph for the 38mm flight, above, even though the Cd was lower.
More accurately, the drag is 1/2 x density of fluid x area x the Cd x the velocity squared

(For reference: the 1/2*density*velocity squared term in the equation above is often just referred to as the dynamic pressure)