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GreatWhite

Active Member
Hi Guys,

Another question has come up before I start building my first rocket. I remember as a kid sanding the leading edge of the fin thinking that I would help improve flight. However, after doing some reading I am finding that it is not necessary and actually not recommended due to possible weakening of the fins. I do plan on brushing on wood filler and sanding that however I just don't know how to finish the edges. Thanks for any input! Happy New Year!

-Patrick

MarkII

Well-Known Member
It is standard practice to round all of the edges except the root on most fins, unless the designer says otherwise. This doesn't require extensive work. All it involves is making the very edge on each side round instead of blunt. Doing just this improves the aerodynamics a great deal and it has no effect on the strength of the fin.

Sometimes you may wish to sand actual airfoils into the fins. An edge that has been thinned down by sanding is structurally weaker, in an absolute sense, than one that has been left at its original thickness, of course. The goal, though, is to create airfoils without making the fin flimsy and prone to fluttering. When they are done right, airfoils do not compromise the strength of the fin. The materials that are used and the manner of construction are chosen to insure that the airfoiled fin will retain its structural integrity and perform as desired during the rocket's flight. In model rockets, which are intended to be reflown multiple times, this means that the airfoiled fin must be durable enough to remain rigid and perform its function in launch after launch. Fin materials that cannot provide this durability are not used to create airfoils.

So yes, model rocket fins can and sometimes are given airfoil profiles, but in those instances, the fins are made of materials and are constructed in ways that are suitable for the task and which do not result in a weak structure. Doing otherwise would be counterproductive and would make no sense.

MarkII

TWRackers

Well-Known Member
My preferred technique is to sand a round profile on the leading edge, which will actually look elliptical to the airflow since the leading edge is usually swept backwards (or forwards, end result is the same). I like to to put a longer taper on my trailing edges, still rounded at the very edge but noticeably elliptical. It's a reasonable compromise between another circular profile and an actual taper to an edge.

The tip of the fin, however, I sand smooth but leave square. My understanding is that a rounded tip is more prone to vortex shedding when the fin is at a non-zero angle of attack than if the tip is left square or brought to a sharp edge. Square is simpler and stronger.

Some contouring of the leading and trailing edges is preferable to reduce the turbulent flow past the fin, as well as the drag. Turbulent flow doesn't provide as much corrective force on the fin, so the fins' effective surface area is reduced, thus reducing stability. So there's a stability advantage to contouring as well as drag.

As far as fins being too thin to put a true airfoil on them without weakening them, there's hope there too. As an extreme example, from what I've read a cylindrical rod (or wire) of a given diameter has about the same drag as a teardrop airfoil 10x thicker. I don't know how the drag scales between a flat plate edge-on of a given thickness versus an airfoil shape of the same thickness, but I'm reasonably sure it's at least a 2:1 advantage. So if you have a kit with 1/8" fin stock and you want to try making your fins with an airfoil profile, you could probably use 1/4" stock and still come out ahead.

If anyone has any real numbers to support or refute this, please pass them along here.

ONAWHIM

Well-Known Member
Interesting discussion, I wonder if anyone has done airflow testing in a wind tunnel using smoke to show the airstream.

I recall that there is another facet to aerodynamics when the control surface (fin) attaches to the airframe at a 90 degree angle. The laminar flow is disrupted and results in turbulence.

I know that this holds true in hydrodynamics as well. Submarines now have a gusset area in front of the sail (conning tower to some) where the apparent attachment to the hull is. It allows the water to flow by more smoothly and results in lest turbulence and noise, increased stealth.
This is similar to the leading edge of the fins on the TRF logo rocket at the top of this page.

What I am suggesting and asking, could other factors be more important to performance than having or not having a beveled airfoil edge?

Stymye

Well-Known Member
I recall the book " Advanced Topics in model rocketry" covered this in depth including wind tunnel testing.

The book "is a comprehensive and rigorous treatment of the trajectory analysis, aerodynamics, and flight dynamics of model rockets" and makes my brain hurt to read.

GregGleason

Well-Known Member
I can recommend a book that I very much enjoyed when I was coming back into rocketry, that addresses your question on fin shape. The book is by G. Harry Stine with Bill Stine and is called Handbook of Model Rocketry, 7th Edition. There is a lot of good information on rockets, from nose cones to fins and a lot of stuff in between.

Greg

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MarkII

Well-Known Member
I mentioned airfoiling as an option. Kit instructions back in the '60's advised the builder to do this far more often than they do now. Standard practice these days is to leave the fin flat but to round off the leading and trailing edges. Most kit instructions also recommend rounding the tip edge as well. Some boosted glider instructions actually advise leaving some or all of the wing edges square. My point, though, is that there are ways to create airfoiled fins that are not weak or fragile. I have sanded airfoils into fins that consisted of 1/64" aircraft plywood, and they were still more than stiff enough.

In response to the original poster's question, sanding in airfoils could indeed make some fins weak and fragile, and it obviously should not be done in those situations. Any potential performance advantage would be negated by fins that flutter or shred. But putting airfoils into fins does not always cause these problems; it depends on the fin design and material. Whether or not creating symmetrically airfoiled fins will result in improved performance is a matter of debate. I suspect that in order for them to be able to contribute this advantage, several other features of the rocket must also be optimized, too. Simply airfoiling the fins alone in the absence of other improvements will probably not add very much.

MarkII

GreatWhite

Active Member
Thanks for all of the replies! I decided to just round the leading and trailing edges of my first project. I think I will get more advanced as I continue to build rockets.

-Patrick

luke strawwalker

Well-Known Member
I usually airfoil the fins (round leading tapered trailing, flat root and tip) edges and then paper the fins-- the paper greatly strengthens the fins and more than makes up for any so called 'lost strength' from airfoiling while making the fins nice and smooth for finishing in one easy step.

Good luck! OL JR

powderburner

Well-Known Member
Interesting discussion, I wonder if anyone has done airflow testing in a wind tunnel using smoke to show the airstream.
Been done with smoke for flow visualization, with pressure taps to measure chordwise pressure distribution, and by other testing forms. And depending on how old you are, this research was possibly begun before you were ever even born.

What you are talking about is classic subsonic airfoil theory, and lift coefficients, and drag coefficients, and other aerodynamic characteristics. I can highly recommend a book like Theory of Wing Sections by Abbott and von Doenhoff for some explanation and a ton of airfoil data. (Amazon.com for under $10) Or see if your local community college offers any instruction in basic aero, if you are really curious. Long story short: a rounded leading edge (LE for short) makes a significant difference in airfoil performance versus a square LE. Long story still long: Actually, a square LE has the general aerodynamic characteristics of a "flat plate" airfoil section---which stinks. The round LE needs to be smooth (as does the rest of the airfoil surface) and the fin thickness also needs to taper from root to tip in proportion to the local chord. The LE shape should ideally be more of a half-ellipse with the LE shaping extending back to 20-30 percent of the local chord. On the rear half of the chord, the thickness should again taper down to a thin trailing edge (TE) on the ideal airfoil. You should end up with a cross-section that looks like a stretched-out teardrop. On a more practical note, LE shapes for subsonic model rockets still work nearly as well with a simple rounded cross section that extends only 5 or 10 percent of the chord, a wide zone of flat fin shape back to 80-90 percent chord, and a taper down to a sharp TE across that last 10-20 percent. This is only really important if you are building a competition altitude or duration rocket. The thin fin TE shape will damage more easily and for "sport" rockets many builders trade off the improved durability of full-thickness TE shapes against the drag reduction and extra performance of tapered TE shapes. Then you have the even more practical aspects of airfoil details like the Gurney flap. Race car driver Dan Gurney found that if he added a short flap at the TE (perpendicular to the chord) that he got more lift (or down-force, for a race car driver) with no measureable increase in drag, which goes completely backwards from classic airfoil theory. Other experimenters have found that they can still get most of an airfoil's aero performance after increasing TE thickness from a knife-edge to a measurable thickness (which is easier to build), which also goes completely backwards from classic airfoil theory. Finally, you have wings on things like insects that operate at extremely low Reynold's numbers, generate lift just fine, and look exactly like flat-plate airfoils with rough surfaces (hairs, blood vessels, etc). Go figure. Our model rockets don't (in general) quite fit into the extremely low Reynold's number class, unless you count modroc BG and helicopter wings. Our rockets are big enough (and have long enough fin chords) and fast enough that classic airfoil shaping rules still apply. Even the highly swept designs (which get into three dimensional flow effects) still benefit from a rounded LE shape. Yes, it's worth taking the time to do a little LE shaping. RocketsNorth Well-Known Member I can recommend a book that I very much enjoyed when I was coming back into rocketry, that addresses your question on fin shape. The book is by G. Harry Stine with Bill Stine and is called Handbook of Model Rocketry, 7th Edition. There is a lot of good information on rockets, from nose cones to fins and a lot of stuff in between. Greg I have to agree with Greg. This is an awesome book full of very useful information and well worth the$15 or \$20.

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Zack Lau

Well-Known Member
Interesting discussion, I wonder if anyone has done airflow testing in a wind tunnel using smoke to show the airstream.

What I am suggesting and asking, could other factors be more important to performance than having or not having a beveled airfoil edge?
Asymmetry may induce a spin that will reduce altitude. But, perhaps the biggest enemy is excess weight, particularly for low impulse model rockets. At higher impulses, one can often shoot for an optimum weight, but at lower ones, this may not be possible. The rocket will fly higher and higher, as weight is reduced, until the rocket fails due to instability or structural failure.

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