Which way around has better aerodynamics?

[dongfang]

Hi,

Time for some wild speculation

Which of the 2 bottles pictured below will reach the ground first, when thrown from altitude at the same time and place?

They are two identical bottles, with tail fins attached. One (let's call it B-1) will fall head down, the other (B-2) bottom down. They have been set up and adjusted to have the same weight (29.0 +/- 0.1 grams). The cap of the bottom-down bottle was cut in halves and glued inside the bottle, to aid stability.

In fact, I already attempted the experiment - but because I forgot to trim the plane with those wings (they belong to another plane), I lost control of it and crashed - So I do not know the answer myself yet, and I will not know until I get the plane repaired - and trimmed, this time.

So what's more important for aerodynamics - a perfect wind-slicing nose cone, or an anti-turbulence tail?

Regards
Soren

[rockets-in-brighton]

Hi,

Time for some wild speculation

Which of the 2 bottles pictured below will reach the ground first, when thrown from altitude at the same time and place?

They are two identical bottles, with tail fins attached. One (let's call it B-1) will fall head down, the other (B-2) bottom down. They have been set up and adjusted to have the same weight (29.0 +/- 0.1 grams). The cap of the bottom-down bottle was cut in halves and glued inside the bottle, to aid stability.

...

So what's more important for aerodynamics - a perfect wind-slicing nose cone, or an anti-turbulence tail?

Good game! I think a blunt nose has a bigger effect than a blunt tail. I'm going to guess that the spout-down attitude (B-1) will have a lower drag coefficient and hence a higher terminal velocity, so assuming they both have time to reach their terminal velocity B-1 will hit the ground first.

[Rusty]

On the simple logic that 'that one looks like it should go faster' I choose B-1 :P
Base down seems to have both a larger surface area and ridges where air could be caught and make it unstable. B-1, although with the pretty aerodynamic nose, looks symmetrical and therefore will probably fly down fairly stably. Also, I have no idea how tail fins work, but I'd take a wild guess that they might not do as much on B-2 as they do on B-1 due to them being slightly depressed and will have less air running over them. On the other hand, I am just as likely wrong seeing as I know nothing about the subject :P
Just my wild guesses, good luck with getting it done! I'm interested to see how it will turn out, and if my random guesses are actually right

[rockets-in-brighton]

Yes, cross section is important in drag, but in this case it's the same either way up. Soren is right to ask if the effect of a streamlined nose is outweighs a blunt tail or vice versa; probably can only be decided by measurement in a wind tuunnel or using very sophisticated flow analysis software.

[Tim Chen]

I voted for B2 because what I know about aerodynamics says that the way the air comes off the back of a moving object is where drag originates. The taper allows the air to slip off more easily.

[Cloud Dancers]

How high are you planning on dreopping them from? I think that both will arrive at nearly the same time as the speed that they fall will probably be too slow for the drag to really make a difference. Maybe a better way to run the test would be to launch both of them and record flight time. Maybe use them as unpowered second stages using the same booster rocket. Use the "nose falls off at apogee" st seperate them from the booster. That way the aero drag will be more of a factor.

[dongfang]

Hi,

I plan to drop the bottles from about 100 meters. That is about as high as I feel comfortable flying the EasyStar. If higher altitudes are needed, I will need to get or borrow an EasyGlider. That has about twice the wing area, and is easier to see at a distance.

Anyway, my planes fly much higher than my rockets.

Regards
Soren

[U.S. Water Rockets1]

Tim has it right. The leading edge doesn't matter as much as the trailing edge. The back of any moving body should be designed to allow the air to slide off without becoming turbulent. The narrow part of the bottle should be facing backwards like a traditional water rocket. Even better would be to put a pointed cap on the bottle to really streamline it.

[dongfang]

Hi,

Thanks for the enthusiastic speculation!

Well the clear winner in 3 drop tests was B-1 - because B-2 was unstable in flight. It tumbled.

I think the tests with this bomb design did not answer the original question (is the tapered end better put at the front or at the back?) so I'm working on B1-B / B2-B, with bigger tail fins and more weight in the "down" end. And on a B-3/B-4, made of 2 bottles each.

I will try repeat the tests when I have a chance, and see if I can get somebody to take pictures. It is quite hard to fly the plane with the bomb load on it - too much workload to take pictures at the same time.

Regards
Soren

[U.S. Water Rockets1]

Hi,

Thanks for the enthusiastic speculation!

Well the clear winner in 3 drop tests was B-1 - because B-2 was unstable in flight. It tumbled.

I think the tests with this bomb design did not answer the original question (is the tapered end better put at the front or at the back?) so I'm working on B1-B / B2-B, with bigger tail fins and more weight in the "down" end. And on a B-3/B-4, made of 2 bottles each.

I will try repeat the tests when I have a chance, and see if I can get somebody to take pictures. It is quite hard to fly the plane with the bomb load on it - too much workload to take pictures at the same time.

Regards
Soren

We can conclude that it must matter that experiments are made with stable rocket designs.

The results of the test when you repeat it with stable rockets should be interesting.

[dongfang]

Hi,

Yeah stability matters - and that was all we got from that experiment But that can happen - I did not know the result in advance, and, well, the answer was that one of the bombs was unstable. One colleague of mine was actually able to predict that, so he voted for B-1.

I have now built:

• Larger fins (half 5 * 8 cm rectangle) and more weight (1 cap in the bottle and one screwed on)

• Larger fins (half 5 * 8 cm rectangle) and more weight (2 caps in the bottle)

• Longer (1.5 bottles), larger fins (half 5 * 8 cm rectangle) and more weight (2 caps in the bottle)

• Longer (1.5 bottles), larger fins (half 5 * 8 cm rectangle) and more weight (2 caps in the bottle)

B1-b and B-2b are both stable.

I have drop tested B1-b and B-2b 5 times now - the pictures speak for the (consistent) result. I did not try the "c" bombs yet.

Kind regards
Soren

[Tim Chen]

Wow! Great photos!

That looks like a real plane dropping bombs!

Do they have competition to drop model bombs on targets? That would be fun!

[dongfang]

Hi,

Hehe, no I don't know of any bombing competitions. It could have been fun, yes. It is actually not so easy to accurately control the direction of a dive maneuver.

But we could make water missile competitions instead! With crash-down rockets. Of course, it would need a safe target area.

Regards
Soren

[U.S. Water Rockets1]

Hi,

Hehe, no I don't know of any bombing competitions. It could have been fun, yes. It is actually not so easy to accurately control the direction of a dive maneuver.

But we could make water missile competitions instead! With crash-down rockets. Of course, it would need a safe target area.

Regards
Soren

You could put a camera with a transmitter on it along the center line of the plane and you could drop bombs remotely like a predator UAV.

[RaZias]

Could someone test the rockets with vertical aerodynamics ridges in the wings and in the bottom of the rocket ? I know that some fight jets use them in the end of the wings as also used saturn V in the bottom.

That would be great to see the same test with a rocket like that !