The Tale of Tails

[Beasho]

One of our local FOIL Amigos, Kyle, has been pushing the limits of equipment with a focus on downwinding.  There have been plenty of discussions about front foils but not quite as much about tail wings.  So I decided to dust off the formulas and “run the numbers” on tails and their influence on the LIFT and DRAG of our Foil Systems.

My Conclusion:  Tails Count Twice

In Simple Terms:  The Lift Generated by the Main Wing (Front Foil) has to carry:

Weight of the Person  +  Weight of the Foil board + Downforce of the Tail (?)

In my case ~200 lbs of Person with Wetsuit + 25 lbs of Board and Foil = 225 lbs.
 
Because Tails create DOWNFORCE for stability this Center of MASS sits slightly in front of the Center of Pressure of the Front wing.   The Center of Pressure is typically at the ¼ Chord meaning 25% back from the leading edge of the wing. 

[Beasho]

Using some assumptions about SPEED ~ 14 mph and the Area of an Axis 1099 with 375P tail, the distance from Center of Pressure to the Front and Back foils and the Coefficient of Lift, constant angle of attack, I came up with the following estimates:
 
225 lbs (Person + Board) + 35 lbs (Tail Down Force) = 260 lbs of LIFT from Front Wing

[Beasho]

Center of Mass therefore sits 4” in front of Center of Pressure to Offset 35 lbs of Downforce on the Tail.

The Front Wing has to Provide 35 lbs MORE lift than the weight of the RIDER and the BOARD to accommodate the 35 lbs of TAIL downforce. 

This means that there was an additional 70 lbs of “LIFT” (+35 lbs offset by -35 lbs) created by the tail.   260 lbs + 35 lbs = 295 lbs @ 16:1 Lift to Drag Ratio = 18.4 lbs of DRAG.

DRAG is always acting perpendicular to LIFT.  In this case BACKWARDS. 

[Beasho]

Full View.

Conclusion:  A No Tail Solution would have 31% less Drag than my current Axis 1099 and 375P combination.

BUT How Far can we go towards NO TAIL?

[Beasho]

Because AXIS publishes all their metrics I built this quick MATRIX of Front Foil vs. Back Foil with an Area Ratio.

I am suspecting that an efficient relationship is in the 13% to 14% range.

With the highest efficiency we can maximize glide, connection between bumps . . .  and therefore HAPPINESS!

Thoughts??

[Beasho]

OK!  Here is some more motivation. 

Last week Kyle and I were out at our Local spot.  Conditions were 10 feet @ 10 seconds.  There was No Wind but a considerable amount of hangover in the water.

The takeoffs on the outside were 7 - 9 foot faces, I was riding my 7' 4" x 31" SUP foil with an Axis 999 and 375P tail.  Kyle was riding a custom 6' 1" homemade Kalama style board with the Axis 1099 and 350P tail.

Kyle can typically pump loops around me and he was whining about how terrible the conditions were "I can't do pump outs, it's terrible out here . . . blah blah blah." 

So I said "Then don't pump out.  Take it to the Pinnacles and show us how its done." 

I was taking off 75 yards outside of Kyle because his board was too small for the bigger waves.  The Pinnacles are ~ 350+ yards from the takeoff and require transiting a section of deep water.  My longest ride ever at this spot was 1,300 yards taking off 400 yards further out and riding 300 yards past the pinnacles. 

Here were my tracks from the morning.  I was pretty happy with a 560 yarder to and through the Pinnacles. 

[Beasho]

So Kyle takes off on an inside wave and starts to glide through the flats.  He had completed a Downwinder the day before with Jeff Clark from Genentech in South San Francisco to 3rd Avenue ~ 14 miles.  He was frothing over what he had learned,

How to connect bumps seeing the energy like Nero in the Matrix.

Our local objective was always to ride a wave from the outside and "Take it to the Skate Park" a local halfpipe 2 miles away next to Surfers Beach.   Kyle took off and just went over the Horizon.  No Wind, riding short period constructive energy for 2.3 miles. 

OMFG!!!!

[Beasho]

His tracks overlaid on a graph of the

"North Pacific Ocean"

with a smaller callout to "Half Moon Bay".

No Wind, Not a Downwinder, Not a Shorerunner, no Heroic Flat Water style takeoff, Just Efficiently Weaving the Swell and Chop.

So much to look forward to.  So much to learn. 

[Hdip]

Conditions were 10 feet @ 10 seconds.  There was No Wind but a considerable amount of hangover in the water.

Just because there is no wind doesn't mean it's not a downwinder. It's wind swell from somewhere.

That was a downwinder. That's why James Casey is trying to teach everyone how to do it.

[PonoBill]

Good stuff. I'm determined to do downwinders without a wing, even if I have to cheat to do it. So yeah, a motor. But more importantly, increasing the efficiency of high-lift wing/fuselage/tail combinations. This project with Admin to screw around with designing something that works better with our limited capabilities might yield some results. I've been playing with tails this year on Maui and mine are getting smaller and thinner. I going to pick up a few from KD, but I've also ordered some G10 sheets to experiment with my own ideas.

After spending time with a slightly re-profiled Axis 400/60 tail (I made it thinner) I rode my 999 with a 340 tail that previously felt really good. I came in after one pass thinking I had something stuck on the tail.

I'm also going to try 3D printing some tails. I got some polycarbonate filament which should hold up long enough to test some ideas.

[jondrums]

Thanks for the first several posts.

If you're questing for minimum drag, then you've hit on exactly the right topic - the tail!  You're already on a premium front wing that has lowest drag you can buy (with probably a few exceptions such as Mike's lab).  Everything from there has to be done with the tail.

So, you're now in a battle between drag and pitch stability.  In the extreme, you ditch the tail altogether and really improve drag.  But pitch stability is a nightmare. 

So how to find the best balance of drag and pitch stability? 

You have three key variables:
1) tail size
2) fuselage length
3) tail angle of attack (shims)

Generally, you're going to want a longer fuselage for minimum drag.  If you go too long the surface friction of the fuselage itself starts to add up.  But the fuselage surface friction is a smaller factor than the tail's lift induced drag.  So longer is generally going to allow you to get the same pitch stability with less tail downforce and therefore less drag.

Think of it this way - if you put on a fuselage that is 25% longer, you can use 25% less downforce and still have the same pitch stability.

So now you should take your tail and start shimming it little by little to reduce the downforce.  Go ride it and see if you like the pitch stability.  You have to move the foil a little bit forward in the boxes each time you reduce the tail downforce.

When you find the right balance of downforce and pitch stability - start working down in size on the tail.  A smaller tail will need more angle of attack to get the same downforce as a bigger tail.  But has less drag to produce the same downforce (to a limit).

I think after all this you're going to end up realizing that you want a really smal tail and a fairly long fuse.

I'm on KD Maui 12" (very similar to axis 300P) and a standard fuselage.  And I've shimmed the tail to about 3deg negative angle of attack versus the front wing.  Its very low drag and still very maneuverable because of the small tail.  Pumps great.

Have fun with this process!

[PonoBill]

I've found shimming to be a critical element in reducing drag quickly, and I consider 3 degrees to be almost a magic number. One of the experiments I want to make is reducing the fuselage to the minimum structure that still has good riding characteristics. I think Admin's notion of moving the mounting of the fuselage/wing to the structure of the wing and using the fuselage solely to hold the stabilizer may open the door to a somewhat less draggy structure. The success of the advanced fuselage that Axis has recently released makes the wing-mounted design a more reasonable focus. Given that most of the force the fuselage needs to resist is vertical, something almost blade-shaped might be satisfactory. Of course, I'd want something that survives my clumsy handling, but I think that's possible.

One of the more obvious improvements will come from simply going faster. Lift increases as the square of velocity. Doing what we can to reduce drag will ultimately mean reducing drag of all the components to enable higher speed before liftoff. That's obvious to me in practical terms since I need to get my 999 really galloping to get my 225 pounds off the water. But once I accelerate to the higher speeds the foil permits I actually have too much lift. At max speed (for me) I'm one tiny weight shift away from a bruising yard sale. 

Tailless designs do work, I've seen one here on Maui that offered some pitch stability. It used a sweptback wing, which is not a great solution for reducing drag since the flow across the wing has to travel further, across sections that don't deliver much lift. But some compromise between straight wings and a delta shape is possible.

Lots to play with, and lots to learn. Not the least of which will be all the tools and techniques necessary to build stuff that offers some incremental advance.

[jondrums]

To measure angle of incidence, I'm using this digital level

I made some plywood parts that cup the leading and trailing edge with a V-groove - you can see how it works here.

[sflinux]

Conclusion:  A No Tail Solution would have 31% less Drag than my current Axis 1099 and 375P combination.
BUT How Far can we go towards NO TAIL?

I would approach it from the perspective of projected area:
Total Area 1099 + 300 = 1289 (for reference)
Total Area 999 + 375 = 1223 <<<
Total Area 999 + 350 = 1187 (Step 1)
Total Area 999 + 325 = 1176 (Step 2)
Total Area 999 + 300 = 1158 (Step 3) {James Casey's preferred downwind tail foil}
Total Area 999 + 275 = ? (Step 4)  {James Casey's preferred prone tail foil}
Total Area 999 + 250 = ? (Step 5)
Total Area 1099 + no stab = 1141 (Step 6)
Total Area 999 + no stab = 1010 (Goal?)

As you go down in surface area you will find a limit for what provides enough lift for the speeds you are going at your weight.
As you step down I would start with the longest fuselage.  Then gradually go shorter in fuselage to get ready for the next step.
Besides surface area, the other thing to look at is efficiency.  It may be worth talking with KD Maui about making a custom tail
KD Maui Marlin (higher aspect ratio):
Total Area 999 + 450/15/188 = 1198 (tail aspect ratio 7.72)
Total Area 999 + 350/14/165 = 1175 (tail aspect ratio 7.66)

I would think no stabilizer would make it harder to pump, just have to explore to find that magic combo.

[sflinux]

Proected area of smaller front foil combos:
Total Area 899 + 375 = 1043
Total Area 899 + 350 = 1007
Total Area 899 + 325 = 996
Total Area 899 + 300 = 978
Total Area 899 + 275 = ?
Total Area 899 + 250 = ?
Total Area 899 + no stab = 830
Total Area 799 + 375 = 920
Total Area 799 + 350 = 884
Total Area 799 + 325 = 873
Total Area 799 + 300 = 855
Total Area 799 + 275 =?
Total Area 799 + 250 = ?
Total Area 799 + no stab = 707