Aerofoiling fins

[mpitfield]

I am at the fin stage of my Madcow Tembo, a 4" diameter airframe 35.5" long, a nice stubby rocket, with relatively short aspect ratio 1/4 ply fins. I am building this as a single deploy hopefully for my level 1 certification. I would like to try to aerofoil the fins, nothing radical as this rocket is not likely to see a huge benefit however I like to use each build as a exercise to learn more techniques.

The research I have done on this subject is minimal but as usual eye opening and just reminds me of how little I know on the subject of rocket science. What I have been able to determine is the final design will practically place strength over performance gain however I still want to pursue this. After reading an article off Apogee's site (News letter 305) I have interpreted that the best suited design for subsonic rockets is, as described right from the article "fin’s leading edge is rounded. The trailing edge is sharp. There is a fin maximum thickness that occurs at a fixed percentage of the fin cord (maximum length from the leading to the trailing edge) length back from the leading edge...basically like a symmetrical plane wing fatter up front and more narrow on the trailing edge...potentially maximum fin thickness at 25% of the fin foot length.

So obviously I need to settle on design before technique! Based on this what advice do you have on the subject of design? Then what techniques have you used to consistently get the desired shape for your proposed design?

 

[powderburner]

First, I am going to suggest that you use the TRF button for running a search, both on the current TRF and on the archived posts from TRF "1.0" (not trying to be glib here, but there is a ton of discussion on this already waiting for you)

While you are doing that, I will be drafting a follow-up post

 

[powderburner]

The whole "perfect" fin airfoil thing is based on the assumption of a low drag aerodynamic shape that looks like a teardrop. Round in the front, gently reaching a max diam near the middle, then tapering down to a sharp point at the rear. If you look in all the classic aero textbooks for minimum subsonic drag, they all hold up this shape as the holy grail. Which is true.....sort of.

For purposes of building model rockets (or even mid- and high-power rockets) that spend most or all of their flight time at subsonic velocities, you can greatly simplify these airfoil recommendations. If you round the fin leading edge, and blend this "roundness" into the flat portion of the fin over the first five percent (or so) of the chord, you will accomplish 99% of the aerodynamic benefit of following the front half of the teardrop shape. The bulk of your fin can be left flat between the leading edge and trailing edge. You will of course still want it to be smooth for reduced drag, but flat is just fine here.

Technical reasons for this: our flying hobby objects travel at velocities that put them into a range of Reynolds numbers (Rn) that is far, far below the Rn values for aircraft airfoils that are shown in all those textbooks. We are just about down in the Rn range of bumblebees and dragonflies, which manage to fly quite well with flat-plate wings with considerable surface roughness. Yes, a teardrop airfoil shape might still prove to have lower drag, but you are gonna have a tough time measuring the difference on our little bitty fins.

Secondary (practical) reasons: it is much easier to shape a flat fin (hey, it's ALREADY flat), and with the potential damage our model rockets face due to handling (they call dents on big airplanes "hangar rash") and rough-field-landing, it is often wise to leave the trailing edges at full thickness. And most of the reason for making the fin nice and smooth is merely to make it LOOK better, not for any measurable aerodynamic improvement.

The rounded leading edge is kinda important for subsonic airflow because it does reduce drag slightly, but more importantly it helps preserve smooth airflow over the fin when the rocket tilts to a small angle of attack (wobbles in flight) and helps the fin generate more force to correct the rocket's tilt. A square leading edge in subsonic flow tends to cause flow separation at the leading edge, reducing the corrective aero force that can be generated and leading to turbulent flow and higher drag.

Hope some of that made sense....

 

[hornet driver]

The whole "perfect" fin airfoil thing is based on the assumption of a low drag aerodynamic shape that looks like a teardrop. Round in the front, gently reaching a max diam near the middle, then tapering down to a sharp point at the rear. If you look in all the classic aero textbooks for minimum subsonic drag, they all hold up this shape as the holy grail. Which is true.....sort of.

For purposes of building model rockets (or even mid- and high-power rockets) that spend most or all of their flight time at subsonic velocities, you can greatly simplify these airfoil recommendations. If you round the fin leading edge, and blend this "roundness" into the flat portion of the fin over the first five percent (or so) of the chord, you will accomplish 99% of the aerodynamic benefit of following the front half of the teardrop shape. The bulk of your fin can be left flat between the leading edge and trailing edge. You will of course still want it to be smooth for reduced drag, but flat is just fine here.

Technical reasons for this: our flying hobby objects travel at velocities that put them into a range of Reynolds numbers (Rn) that is far, far below the Rn values for aircraft airfoils that are shown in all those textbooks. We are just about down in the Rn range of bumblebees and dragonflies, which manage to fly quite well with flat-plate wings with considerable surface roughness. Yes, a teardrop airfoil shape might still prove to have lower drag, but you are gonna have a tough time measuring the difference on our little bitty fins.

Secondary (practical) reasons: it is much easier to shape a flat fin (hey, it's ALREADY flat), and with the potential damage our model rockets face due to handling (they call dents on big airplanes "hangar rash") and rough-field-landing, it is often wise to leave the trailing edges at full thickness. And most of the reason for making the fin nice and smooth is merely to make it LOOK better, not for any measurable aerodynamic improvement.

The rounded leading edge is kinda important for subsonic airflow because it does reduce drag slightly, but more importantly it helps preserve smooth airflow over the fin when the rocket tilts to a small angle of attack (wobbles in flight) and helps the fin generate more force to correct the rocket's tilt. A square leading edge in subsonic flow tends to cause flow separation at the leading edge, reducing the corrective aero force that can be generated and leading to turbulent flow and higher drag.

Hope some of that made sense....

I don't think I could have put it better !! I've moved from making a knife edge at the rear ---A LOT OF WORK AND HARD TO GET EVEN--NOT TO MENTION WEAK--to just cleaning it up for paint purposes---Do it for looks, I can live with that!! Performance --not so much.

 

[mpitfield]

First, I am going to suggest that you use the TRF button for running a search, both on the current TRF and on the archived posts from TRF "1.0" (not trying to be glib here, but there is a ton of discussion on this already waiting for you)

While you are doing that, I will be drafting a follow-up post

No glib received, all good feedback. I did do a look around the forum and ultimately came upon the Apogee article during a Google. After looking around a bit more your advice seems very practical and accurate, plus I like simplicity when warranted and I believe this is one of those times. So simple rounded edges it will be, I will save the more complex airfoiling for my supersonic birds, when I get there that is.

 

[Nathan]

Perfect airfoil fins with sharp thin trailing edges look nice but are likely to get damaged when the rocket hits the ground, fins first.

I usually just bevel the leading and trailing edges, like this:

 

[Handeman]

Perfect airfoil fins with sharp thin trailing edges look nice but are likely to get damaged when the rocket hits the ground, fins first.

I usually just bevel the leading and trailing edges, like this:

Same here. A bevel is all I do. If you are going to sand the bevel into the fin, I have found that if you put a piece of masking tape on the fin where top of the bevel will be, it works as a great guide when sanding. If you're sanding the tape you're going too far.