Data on fin design?

[dr wogz]

I saw a Breast Cancer rocket here a few years ago. It was all pink, and wrapped in a pink feather boa. so the whole rocket was feathery & fluffy. The launch pic shows how "flat" feathers can get under force & under air pressure!! that was draggy for sure!

I've seen other make their airframe from a pool noodle. again, kinda draggy.. (it had a proper tube up the middle)

 

[Ronz Rocketz]

I would say no when comparing a round ball to a wiffle ball (extreme example). Doesn’t the stitches on a baseball help the pitcher curve the ball (not in a straight trajectory)?

 

[steveh.jae]

I would say no when comparing a round ball to a wiffle ball (extreme example). Doesn’t the stitches on a baseball help the pitcher curve the ball (not in a straight trajectory)?

Boundary layer is THE KEY element. Therefore wiffle ball example irrelevant.

pitches (type/degree[velocity, spin rate & moment, etc.]/effect) react to forces the balls’ stitches impart to the boundary layer after it leaves the pitchers hand as it moves to the plate.

I guess my question was just another example of “chasing after the wind”. Thanks everybody for your inputs!

 

[Ronz Rocketz]

A less extreme example off the top of my head is a sputnik rocket with a styrofoam ball.



Sometimes it's fun to chase the wind or someplace where there is no wind.

 

[steveh.jae]

All of my copies are the original edition. I have not seen the newer edition. I purchased my first copy in 1973 or 74. It has a little rain damage from rain coming in an open window, and many of my handwritten notes. Around 1980 I lent it out to a model rocketeer and it did not come back. When one or more of the authors sold off a few cases from their stash, I purchased two more copies. I wanted to be able to loan one copy out while not risking my only copy. With my second purchase, I received an errata sheet. I'm guessing that the 2003 edition simply has the changes noted in the errata sheet. My first copy was eventualy returned, so that I now have thee copies.

How much did you actually pay? I see a lot insanely high prices asked, but that does not often equate to what people are willing to pay for them.

Boat drag is very different from model rocket drag, because most of the drag is surface wave drag. However, water is an incompressible fluid. Air is compressible, but for the low speeds of most of our model rockets, it is essentially incomressible. Many papers useful for model rocketeers can be found in the Hydrodynamic literature.

Alan, Thanks for your response. Previously my most expensive college textbook purchase was Michael Bartons’ Bonds Biology of Fishes ($310/new) several years ago. Reference books are dear indeed, particularly when they are so very rare. I have no idea how large the printing run was for the 1973, and 2003 editions. I recently paid $350 for the 1973, 1st edition hardcover copy in vg condition. Would you be so kind as to scan a copy of the errata page for me? Thank you!

Steve

 

[steveh.jae]

A less extreme example off the top of my head is a sputnik rocket with a styrofoam ball.

View attachment 420838

Sometimes it's fun to chase the wind.

Thanks rklapp! Ecclesiastes 1:14

 

[steveh.jae]

I am a mechanical engineer, and most of my engineering experience has dealt with structural mechanics, not fluid dynamics. We are talking about boundary layers, Reynolds numbers, etc which is pretty complicated. As mentioned above, boat drag is different from projectile drag. But for years, rocketeers have discussed the advantages of using a rough surface to improve the drag characteristics of a model (think dimpled golf ball). I've never seen anybody post/publish work done with "rough" model surfaces, but I am pretty sure people have done R&D projects on the subject. I recall some articles in MRm (see the Ninfinger link above) about using tape to trip the boundary layer, but don't remember what they concluded.

Thanks!

 

[steveh.jae]

View attachment 420027


That $3.99 shipping really kills the deal for me.

It was the $0.82 for me. (?)

 

[blackjack2564]

This shape is currently the "go to" for high performance, and has proven itself many times . Of course there are minor variations. If body tube is 2 in diam. then fin span should be slightly more than 1 caliber or 2.25in. This holds true any diam. fin span slightly over tube diam. The leading edge should mimic the shock wave angle the fin will encounter.

What happens to paint when approaching Mach-3. I prefer 4 fins. Good luck with your project.

 

[steveh.jae]

This shape is currently the "go to" for high performance, and has proven itself many times . Of course there are minor variations. If body tube is 2 in diam. then fin span should be slightly more than 1 caliber or 2.25in. This holds true any diam. fin span slightly over tube diam. The leading edge should mimic the shock wave angle the fin will encounter.
View attachment 420991
What happens to paint when approaching Mach-3. I prefer 4 fins. Good luck with your project.

Would high-heat aluminum paint fare better? Cool pic btw. How fast, how high, what motors?

 

[steveh.jae]

https://www.flightliteracy.com/effect-of-wing-planform/
Pedro, I think you're getting some things confused, or are jumbling a few concepts together which are leading you in the wrong direction. And, i have't seen your other post yet.

Boeing make planes, and in your example, you say the 777. The wing of an airplane is design to offer as much lift with as little drag as possible. And to be efficient at a given air speed, and air density (altitude).

Different wing shapes are designed to accommodate different flight characteristics; wanted results. Look at different airplanes just to see the different varieties to achieve the different, wanted results.

The elliptical does offer great performance for subsonic, and a cross a wide variety of air speeds & densities.

Remember, even the first few jets to reach (and pass) the sound barrier, wings were straight. they then started to sweep them back, as airspeeds increased, settling in at about 37° swept back.

There are other factors which come into play, and our little rockets don't really have the mass or large areas to require precise calculations & specific geometries to achieve their intense function. Chances are: wind, air pressure, humidity, surface finish, etc.. will have more of an effect on an LPR rocket than just the fin shape.

Dr Wogz, are you aware of drag literature specific to “surface finish”? Please advise. Thanks!

 

[Ronz Rocketz]

I just bought these for sanding airfoils like the picture. Seems to work great...

https://northcoastrocketry.com/products/airfoil-assistants

 

[TonyL]

going back to the OP, the 'blended winglets' and 'feathered winglets' you see on commercial aircraft are nothing particularly new. They would be categorized as 'wing end fences', similar to what has been used on race cars for coming up on 50 years. Their purpose is the make the airfoil more efficient at producing a normal force, generally in one direction, and they work well at this task by controlling spill off the end of the wing.

For a rocket, the angle of attack is normally pretty low and can be either direction. Spill off the fin tips should play a role to some degree, but I suspect that the gains are small for the complexity required. It would make an excellent science fair project to see if fins with end fences could be meaningfully smaller [and/or lower drag] than fins without. A small scale wind tunnel would be a desirable way to evaluate designs since one would like to quantify both drag and restoring forces.

I notice that I have never seen an end fence on a aircraft rudder [so far, and T-tails don't really count as it is a full wing, not a fence] nor on a missile. If they were very useful for the bidirectional case, one would expect to find some examples somewhere.

br/

Tony

 

[dr wogz]

Dr Wogz, are you aware of drag literature specific to “surface finish”? Please advise. Thanks!

Hi Steve,

Sorry, no. just eh basics. I know that there are different shapes, and that the different shapes had different stall characteristics. That was what I was trying to demonstrate with the link I attached for Seinior.

 

[steveh.jae]

Hi Steve,

Sorry, no. just eh basics. I know that there are different shapes, and that the different shapes had different stall characteristics. That was what I was trying to demonstrate with the link I attached for Seinior.

Thank you!