fin airfoils vs other shapes

[lr64]

I see so many rockets with flat beveled and even square edged fins. They could fly higher and faster with an airfoil, unless they are VERY fast. Subsonic rockets could be fine with relatively conventional airfoils. Maybe up to as thick as NACA 009 while maintaining lower drag. It is my understanding* that, with sufficient sweep and a thin airfoil with the thickest point a bit further back, Mach 1.5 or above may be fine. For instance, the F-15 is said to have a NACA 64A006.6 . (Symmetrical, BTW) If we can believe Wikipedia, it's capable of Mach 1.2 on the deck and twice that at high altitudes. It's a "laminar" airfoil, so if built and maintained by a perfectionist, it could have a lot less drag than the NACA 009 and similar foils.
You can look up airfoil use at the Incomplete Guide to Airfoil Usage:
https://m-selig.ae.illinois.edu/ads/aircraft.html

In any case, besides being lower drag, fins with airfoils thicker than the usual flat plates can be made lighter, stronger, and more flutter resistant. At least if you use something less dense for at least part of the construction.
---------------------
Here's a relevant method for airfoils if you're using balsa, plywood, or other sandable material for fins.
https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf
I've done a wing like this, and it seems to work fine. I'm sure it would have been much faster if I'd used a straight taper.

Foam core construction, possibly with a spar, would be legit also. Even built up, I suppose, though maybe full sheeting is a good idea above, I dunno, Mach 0.5? I say this, not from any technical knowledge but because I know some fighter planes in WW2 had fabric covered control surfaces. At a smaller scale, maybe higher speeds would be fine.
------------
*After looking at some technical papers whose names I don't recall.

 

[ReynoldsSlumber]

To put a fine point on it, see this Estes TR-11 comparison of drag on 1/8" thick fins with a 2" chord and 6" span:

 

[Azamiryou]

Is there any data on fins with round leading and square trailing edges?

 

[mjennings]

I see so many rockets with flat beveled and even square edged fins. They could fly higher and faster with an airfoil, unless they are VERY fast. Subsonic rockets could be fine with relatively conventional airfoils. Maybe up to as thick as NACA 009 while maintaining lower drag. It is my understanding* that, with sufficient sweep and a thin airfoil with the thickest point a bit further back, Mach 1.5 or above may be fine. For instance, the F-15 is said to have a NACA 64A006.6 . (Symmetrical, BTW) If we can believe Wikipedia, it's capable of Mach 1.2 on the deck and twice that at high altitudes. It's a "laminar" airfoil, so if built and maintained by a perfectionist, it could have a lot less drag than the NACA 009 and similar foils.
You can look up airfoil use at the Incomplete Guide to Airfoil Usage:
https://m-selig.ae.illinois.edu/ads/aircraft.html

In any case, besides being lower drag, fins with airfoils thicker than the usual flat plates can be made lighter, stronger, and more flutter resistant. At least if you use something less dense for at least part of the construction.
---------------------
Here's a relevant method for airfoils if you're using balsa, plywood, or other sandable material for fins.
https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf
I've done a wing like this, and it seems to work fine. I'm sure it would have been much faster if I'd used a straight taper.

Foam core construction, possibly with a spar, would be legit also. Even built up, I suppose, though maybe full sheeting is a good idea above, I dunno, Mach 0.5? I say this, not from any technical knowledge but because I know some fighter planes in WW2 had fabric covered control surfaces. At a smaller scale, maybe higher speeds would be fine.
------------
*After looking at some technical papers whose names I don't recall.

Your post assumes everyone is chasing max speed and altitude. Non-airfoild fins can be more durable. I have plenty of LPR rockets that I beveled/air foiled that have beat up training edges. But yes as you cross Mach things happen and shapes like bi-convex airfoils are needed. I can't speak specifically to the F15 but most modern fighters have symmetrical airfoils helps with the interesting flight profiles they fly.

 

[SolarYellow]

To put a fine point on it, see this Estes TR-11 comparison of drag on 1/8" thick fins with a 2" chord and 6" span:
View attachment 669305

I wonder what the results would be if the rounded LE was paired with a reasonably (i.e., maybe 4-5 degrees/side) tapered TE so you didn't get the swirling vortex golf ball effect at the TE.

Also, what if just part of the LE was curved like an ogive cross section, say, maybe the first 3/8 inch following a tangent arc to a 1/32 flat that was then rounded?

Doing those two things and keeping most of the sides flat is actually not difficult to do.

Also, what about the simple straight taper LE and straight taper TE. I think most of them end up being done at a steeper than optimal angle, but what if they were done at something like 4-5 degrees?

Straight tapers are super easy to set that up to do very quickly with a variety of different tools. Doesn't have to involve sanding all night for a week.

How close would these get to the performance of the full airfoil? Leaving them out of the chart makes the argument for sanding forever a bit of a straw man, IMO.

 

[lr64]

Your post assumes everyone is chasing max speed and altitude. Non-airfoild fins can be more durable. I have plenty of LPR rockets that I beveled/air foiled that have beat up training edges. But yes as you cross Mach things happen and shapes like bi-convex airfoils are needed. I can't speak specifically to the F15 but most modern fighters have symmetrical airfoils helps with the interesting flight profiles they fly.

I don't assume that. However, I'll admit that, every time I look at a square edged fin, I see a little drogue.
Non-airfoiled fins can be more durable if you make them as fat as an airfoil. You can also increase the trailing edge thickness of a real airfoil, to some extent, without huge losses, as long as you cut it off square.

Bi-convex foils or "hexagonal" are two ways to deal with supersonic speed, but not the only ones. Symmetrical is not the same as biconvex or bevelled aka hexagonal.

I got a bit carried away. Going back to the Incomplete Guide, we have the following with real root airfoils:
5 percent or thicker:
F-14 (9 percent, lots of sweep available)
F-18
F-100
F-101
F-105
F-111 (10 percent or so! Benefit of extreme sweep.)
Thinner than that, but still real:
CF-105 Avro Arrow
F-5, T-38, F-20
F-102
F-106

 

[lr64]

Have left off the questions I have no answers for.

Also, what about the simple straight taper LE and straight taper TE. I think most of them end up being done at a steeper than optimal angle, but what if they were done at something like 4-5 degrees?

Some supersonic aircraft have that sort of airfoil, though I don't know the proportions. For instance, the XB-70. Then there's the Douglas X-3, which looked very fast but couldn't break Mach 1! Then there's the Orbital Sciences Pegasus, but I think that's a speed range far beyond anything seen in amateur rocketry so far..

Straight tapers are super easy to set that up to do very quickly with a variety of different tools. Doesn't have to involve sanding all night for a week.

Are you making your fins out of lignum vitae? Using the method linked to in the first post of this thread, I made a 36 inch wing with a curvy planform in 10 hours. I have no doubt it would be much faster with straight taper or constant chord. For a subsonic rocket, an airfoil that's an ellipse with the thickest point at 20 or 25 percent of chord, followed by tangent lines to the trailing edge, would be fine. Very similar to this:

Some in this series come to a point at the trailing edge. Some are thicker. I suspect this one could be thicker at the Reynolds numbers seen on many model rockets.

How close would these get to the performance of the full airfoil? Leaving them out of the chart makes the argument for sanding forever a bit of a straw man, IMO.

Not so much a straw man as just not having the data, probably. Maybe someone else does. I would guess that if you made them as thick as is feasible with a real airfoil, they'd be quite draggy.

-----------
I just estimated the drag coefficients corresponding to the Estes graph, for the 400 fps velocity.
0.55, 0.3, and 0.084 respectively. I've assumed that tip phenomena and intersection drag are minor. The latter is a relatively large assumption, I guess. I've also assumed no lift. The Reynolds number works out to about 426,000.
For comparison, a fin with a circular cross section might not be above a Cd of 1! With precision, far lower drag coefficients are possible with airfoils. Hypothetically, as low as .003, but I think .010 or .020 would be more realistic. A hand made, imperfect 12 inch chord model of a NACA 009 was measured at a Cd below .007 in a wind tunnel at a lower Reynolds number than in the Estes graph. See Soartech 8: https://m-selig.ae.illinois.edu/uiuc_lsat.html Which even includes data on how far off the model was from the nominal shape. The same publication includes data for a built up, partly open bay model of a NACA 6409. This airfoil has a lot of camber, but still got down to a Cd of about .011 at a Reynolds number of only 200,000.

 

[Art Upton]

Most Rocket Modelers that are not in NAR competitions really don't want to mess with it.
And Square and Rounded are more durable.

Someplace in a drawer I have a Nike model decades old with dings all over the sharp edges, and a broken fin... and dust, how does dust get in a drawer?

Didn't this post happen before some months ago?

*Model must have gotten dust on it before being put in drawer

 

[SolarYellow]

Are you making your fins out of lignum vitae? Using the method linked to in the first post of this thread, I made a 36 inch wing with a curvy planform in 10 hours. I have no doubt it would be much faster with straight taper or constant chord.

I can put straight tapers on fins with really good precision and essentially perfect repeatability with a few minutes of setup and a minute or two per taper. Most of that time is getting the fin set in the fixture; the actual cutting only takes a few seconds. So, say, four fins with four tapers each, as perfect and all the same as reasonably possible, in under a half hour. And it doesn't matter whether they are balsa, bass, plywood, or G10. Also doesn't matter how big they are. 1/16-in to 1/4-in thick or more only makes a little difference.

Working on a project to be able to do the same with a tangent ogive section.

Also have plans to hot wire foam cores with a NACA 64A-10 airfoil (will probably squish it to 6% thick, but I was only able to find decent resolution in profile points for the -10) and fiberglass them for a 4-inch rocket. That will take a lot longer, but it's just something I want to do.

 

[rfjustin]

Show me how to get a proper airfoil on G-10 FR4 or carbon plate and I'll join in the conversation.

 

[lr64]

Most Rocket Modelers that are not in NAR competitions really don't want to mess with it.
And Square and Rounded are more durable.

Someplace in a drawer I have a Nike model decades old with dings all over the sharp edges, and a broken fin... and dust, how does dust get in a drawer?

Didn't this post happen before some months ago?

*Model must have gotten dust on it before being put in drawer

I think I mentioned it someplace. Kind of my pet peeve. If you're worried about thin trailing edges, look at the picture of the HT-21 above. Several other HT airfoils can be found at https://charlesriverrc.org/articles/on-line-plans/mark-drela-designs/drela-airfoil-shop/ Two of them aren't symmetrical, though.

Models that have less drag can get the same performance with cheaper motors. Also, models that don't flutter can get far better performance with cheaper motors, and you don't have to buy a new kit for every flight.

----------
Justin:
Most of the material in G-10 or carbon plate is just ballast. Make a curved core and put relatively thin sheets of that stuff on it. The stuff near the outside surface does most of the work. OTOH, I recall that carbon composites wet sand pretty well. A plywood fin with an airfoil shape will be much stiffer and probably somewhat stronger than a thin carbon plate fin. If you're worried about damage, make a glass or carbon trailing edge. If your rocket goes Mach 2, that's another story.

 

[neil_w]

I’m a little confused by the purpose of this thread… I think most rocketeers are well aware of the benefits of airfoils, they just don’t want to deal with it. I sure don’t.

How do beveled leading and trailing edges compare to airfoils?

 

[lr64]

I don't know about beveled airfoils at subsonic speeds, but if you're going to go Mach 3, they're probably just the ticket. As long as they don't flutter.

The odd thing is that I see flat plate or beveled fins in heavy, floppy, solid materials even on subsonic rockets that people have put a whole lot of other work in. Like this one:
https://sedstufts.notion.site/CARM-Technical-Documentation-2e8ebba6ad954b1c946b6060394e9795

 

[Art Upton]

Because it is not needed to put in anything other on that rocket for what they are doing.

 

[dhurstell]

I see so many rockets with flat beveled and even square edged fins. They could fly higher and faster with an airfoil, unless they are VERY fast. Subsonic rockets could be fine with relatively conventional airfoils. Maybe up to as thick as NACA 009 while maintaining lower drag. It is my understanding* that, with sufficient sweep and a thin airfoil with the thickest point a bit further back, Mach 1.5 or above may be fine. For instance, the F-15 is said to have a NACA 64A006.6 . (Symmetrical, BTW) If we can believe Wikipedia, it's capable of Mach 1.2 on the deck and twice that at high altitudes. It's a "laminar" airfoil, so if built and maintained by a perfectionist, it could have a lot less drag than the NACA 009 and similar foils.
You can look up airfoil use at the Incomplete Guide to Airfoil Usage:
https://m-selig.ae.illinois.edu/ads/aircraft.html

In any case, besides being lower drag, fins with airfoils thicker than the usual flat plates can be made lighter, stronger, and more flutter resistant. At least if you use something less dense for at least part of the construction.
---------------------
Here's a relevant method for airfoils if you're using balsa, plywood, or other sandable material for fins.
https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf
I've done a wing like this, and it seems to work fine. I'm sure it would have been much faster if I'd used a straight taper.

Foam core construction, possibly with a spar, would be legit also. Even built up, I suppose, though maybe full sheeting is a good idea above, I dunno, Mach 0.5? I say this, not from any technical knowledge but because I know some fighter planes in WW2 had fabric covered control surfaces. At a smaller scale, maybe higher speeds would be fine.
------------
*After looking at some technical papers whose names I don't recall.

Im sorry, did you say F-15???

I am now very interested in this thread

 

[lr64]

Because it is not needed to put in anything other on that rocket for what they are doing.

That depends on how you look at it. If the mission is to use up all of the cash and do at least a certain amount of design work, you'd be entirely right. If there was any desire to save time and money, then doing the fins right, which is simple, would allow some other compromises elsewhere on things that would otherwise be complicated. Or allow the use of less expensive materials. Or just make the rocket a bit lighter, with less drag, so it could fly higher on less fuel.

Im sorry, did you say F-15???

I am now very interested in this thread

I did write F-15. Plus a whole bunch of other jets in the 6th post. Keep in mind that there's an upper limit for using a more or less normal airfoil, but that limit is well above Mach 1 on swept fins. If you're planning to burn the paint off your rocket with speed, that might be above the limit. If I recall correctly, the fins need enough sweep to stay within the shock cone, if that's the right term for it. I think that might mean 63 degrees at Mach 2, though someone with more knowledge should correct me if I'm wrong.

 

[Capt. Eric]

You're assuming that all rocketeers are looking to reach the maximum limits of speed or altitude possible. That's true for some people, but I find that most folks just want a nice looking rocket that flies in a predictable manner, without getting into the weeds of design and technology. Most of us not in competitions don't really care if it reaches 600' or 800', as long as it looks good going up and coming down. It didn't reach 800'? Contrary to your assumption, I am not going to spend more money on a larger engine to reach that. I'll just say "cool, where's my next rocket?"

To each his or her own. If your interest is in reaching certain performance goals, more power to you. But if that's not everyone's goals, you have to accept and respect that. No head scratching necessary.

 

[bad_idea]

If I recall correctly, the fins need enough sweep to stay within the shock cone, if that's the right term for it. I think that might mean 63 degrees at Mach 2, though someone with more knowledge should correct me if I'm wrong.

Mach angle is arcsin(1/M), where M is the Mach number. For Mach 2, that is 30 degrees, which we generally interpret as a 60-degree sweep back. So you'd want a sweep back > 60 degrees for Mach 2.

 

[ghostfather]

A few years ago I 3D-printed the Naca 009 profile for some small rockets, and did some comparative altitude tests using Estes D motors, and the airfoil certainly makes a big difference. I was consistently getting about 22% higher flights on the same (tiny) rocket using the airfoil, compared to rounded flat 3D-printed fins. I was astounded.
I later did some 3D-printed symmetrical Von Karman profile fins, which flew to about Mach 1.2 on my H altitude record in Australia. The mid thickness was 8% of the length, like the Naca 009. I was using high temp CPE filament, with the thinnest layer of glass and epoxy to smooth it out, which was probably overkill. Furthermore, I also used a hollow core in the printed fin to attach it to a small rectangular piece of G10, which was mounted in a slot on the airframe, and epoxy fillets.
I think I still hold the H altitude record in Australia, probably because no one Down Under takes an H record seriously. It was only 8811 ft.

 

[Dave P]

Aircraft fighters and missiles designed to evade and attack each other are an apples to oranges comparison for high performance rockets.
Agility is the requirement for fighters and missiles, that's why some missiles have canards in front. They're unstable on purpose.

 

[lr64]

Aircraft fighters and missiles designed to evade and attack each other are an apples to oranges comparison for high performance rockets.
Agility is the requirement for fighters and missiles, that's why some missiles have canards in front. They're unstable on purpose.

True enough, but low drag at low angles of attack is still desirable for high speed flight.

 

[lr64]

You're assuming that all rocketeers are looking to reach the maximum limits of speed or altitude possible. Most of us not in competitions don't really care if it reaches 600' or 800', as long as it looks good going up and coming down. It didn't reach 800'? Contrary to your assumption, I am not going to spend more money on a larger engine to reach that. I'll just say "cool, where's my next rocket?"

You're assuming what my assumptions are. My assumption is that people don't like throwing money away.

Would you spend more money for a bigger motor it your rocket was only getting to 300 feet?

 

[Capt. Eric]

You're assuming what my assumptions are. My assumption is that people don't like throwing money away.

Would you spend more money for a bigger motor it your rocket was only getting to 300 feet?

Depends on the rocket, but generally speaking no, I wouldn't. As long as the flight was stable and looks good doing what I want it to, I don't care how high it flies. In fact, lower is better on the small field that I usually fly on.

 

[mjennings]

If you're looking for airfoils data find yourself a copy of Abbott and Von Doenhoff's Theory of Wing Sections. The book is mostly appendices with airfoil data in percent chord. 64A010 is nothing special and is likely in there.

 

[SolarYellow]

https://www3.nd.edu/~ame40431/AME20...ingSectionsIncludingASummaryOfAirfoilData.pdf

A hard copy can also be had very inexpensively at https://www.addall.com/
ISBN 9780486605869

 

[Chuck Rogers]

The following fin airfoils are included in the RASAero II software. The CNalpha, Center of Pressure (CP), and lift and drag of the fins with the selected airfoil will be calculated for subsonic and supersonic Mach numbers (and hypersonic Mach numbers).


From the RASAero II Users Manual:






For the lift and drag of the fins at selected Reynolds numbers, you can use the Aero Plots and Run Test features in the software. (See the Table of Contents in the RASAero II Users Manual for the Aero Plots and Run Test features.)

Are airfoiled fins worth it for low total impulse model rockets, or low speed (low Mach number) rockets? Run the rocket on the RASAero II software with the different airfoils and you can see the performance impact of airfoiling, or at least rounding, the airfoil on the fins.

Note that the rounded airfoil was added to the RASAero II software not just for model rockets, but as others have mentioned, with very thin carbon fiber fins it can be very difficult to airfoil or even bevel the leading edges of the fins. It may be the lowest drag solution to have very thin (but strong) carbon fiber fins with just rounded leading edges.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[SolarYellow]

...with very thin carbon fiber fins it can be very difficult to airfoil or even bevel the leading edges of the fins. It may be the lowest drag solution to have very thin (but strong) carbon fiber fins with just rounded leading edges.


Charles E. (Chuck) Rogers
Rogers Aeroscience

That's something I wonder about. One of the arguments for airfoiled fins is that they generate lift (restoring torque) more effectively at smaller angles of attack than flat fins, and thus will maintain the rocket overall closer to zero angle of attack throughout the flight. Thus, (the hands wave in the air) the airfoiled fin may cause the rocket to have less drag overall throughout the flight, even though somewhat thicker airfoiled fins themselves might have slightly more drag at zero AOA. Do you have any sense of where those lines might cross?

 

[Chuck Rogers]

Chuck Rogers wrote:

<< For the lift and drag of the fins at selected Reynolds numbers, you can use the Aero Plots and Run Test features in the software. >>

Note that the Reynolds number displayed in the Aero Plots and Run Test is the Reynolds number of the rocket body based on the total length of the rocket.

To get the fin Reynolds number you have to use the ratio of the fin average chord to the total length of the rocket. The fin Reynolds number is calculated internal to the software, but not displayed in any of the outputs.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[lr64]

That's something I wonder about. One of the arguments for airfoiled fins is that they generate lift (restoring torque) more effectively at smaller angles of attack than flat fins, and thus will maintain the rocket overall closer to zero angle of attack throughout the flight. Thus, (the hands wave in the air) the airfoiled fin may cause the rocket to have less drag overall throughout the flight, even though somewhat thicker airfoiled fins themselves might have slightly more drag at zero AOA. Do you have any sense of where those lines might cross?

At subsonic speeds, a good airfoil of moderate thickness has lower drag than a flat plate. In the Princeton wind tunnel tests included in Soartech 8 (see link below), a flat plate of 12 inch chord and 1/4 inch thick was tested. It had a rounded leading edge and the aft 3 inches tapered down to 1/32". The Cd at 0 lift was about .013 at a Reynolds number of 300,000. A NACA 0009 had a Cd of about .007 at the same Reynolds number. All the symmetrical airfoils in those tests had drag coefficients well below the flat plate at that Reynolds number.

Except in unusual cases. the lift curve slopes of airfoils are about the same, so the flat plate would generate about as much lift at a given angle of attack as an airfoil. At low Reynolds numbers, some symmetrical airfoils actually have a small deviation of the lift curve around 0 lift, so the flat plate might actually provide slightly more stability. The results for the 0009 in Theory of Wing Sections, linked to in post 25 above, show less, if any of that deviation than is seen in the Princeton tests. That data is for Reynolds numbers of 3, 6, and 9 million. 300,000 is what a small and or slow rocket might see. For instance, a fin with a 2 inch chord going at about 280 fps. For 3 million, you'd need a 10 inch chord going at at around 560 fps or so. As you get up near Mach 1, the results would probably be different.


Soartech 8 aka Princeton tests can be found here:
https://m-selig.ae.illinois.edu/uiuc_lsat.html

 

[SolarYellow]

Crazy that page has a thumbnail of Helios on it. I worked on that program as an intern. Brings back a lot of memories.