Fuselage length vs tail angle

In the tuning podcast (at about the one hour mark), Erik was saying that a longer fuselage will require more downward angle on the tail to maintain the same front foot pressure compared to a shorter fuselage. Mike hesitated a little and mentioned leverage being a consideration. I tend to agree…at least for airplanes, a longer fuselage should require less tail down angle to maintain the same state of trim/CG. Can we discuss this so I understand what Erik is saying?

Side note…I think we REALLY need to give concrete labels to tail angle tuning. My suggestion is to label it as “tail down force angle” for the LE of the tail wing pointed downwards. Positive, negative, etc. just makes a mess of all the info out there!:joy:


I thought the same , The longer fuse has more leverage therefore you need either a smaller tail or less downforce/angle on the existing tail to give similar front foot pressure.

Negative shim = less lift
Positive shim = more lift
Doesn’t matter if your stab is top or bottom of fuse!!!
The only difference is where the shim is placed……
Top mount a negative goes under front bolt of stab
A bottom mount a negative would go under rear bolt of stab.

I like to take things to an extreme to find the trend. If you think about a tail with a massive AOA, say 45 degrees and two fuse lengths. 5cms and 3meters. Then think of the radius the foil would draw without rider input. The 5cm radius would be much tighter than the 3m radius. So to get the same radius, which equates to front foot pressure, you need more AOA on a longer fuse. That makes sense to me mathematically, and it’s also what I feel. That said, I’m running my tails pretty flat and pushing the foil super far forward to account for that. My current tuning is lining up my tail with the tail of the board. I find that feels the best and has the lightest swing weight in our surf.

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Gave some thoughts here fwiw in the episode thread

Yes agree, it is move the tail leading edge UP OR DOWN
The brands have all done different things with +ve/-ve so it’s a mess to reverse that

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It’s confusing i think the error may be assuming it flies straight forward at its angle of attack.

Okay…so here adds to the confusion.:joy: If I just quote this portion of your entire post, there’s no way for anyone to understand this without further explanation.

When you say less/more lift, is this more or less lift from the tail? Or is it more/less lift from the main wing? Because they are not the same thing…more lift on the tail (I’m going to assume you’re talking about tail lift), depending which way that lift goes, up or down, will create the exact opposite on the front wing.

If you said negative shim equals less (or more) tail down force, then I’d understand what you mean and I’d also know what the end result would be. From the rest of your post, this is what I understand you to mean. But I had to get the full explanation in order to understand this. See what I mean? :call_me_hand:t4:

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The tail wing can be overpowered…that’s essentially your front foot pressure. I’m no genius, but I want to say your example with the ultra short fuselage might hold true if it was just the foil flying at a relatively slow speed with no one standing on the board. It might be able to maintain a tight arc. The long fuse would also do as you’ve described.

Given some speed (just moderate speed), I don’t think you would be able to ride the long fuselage without blowing out of the water. Well, actually, you’d have to reduce the AOA to something more reasonable than 45 deg to prevent stalling, but I think the leverage would be so great, it would force the front wing up and be impossible to overpower.

What I’ve been trying to figure out is how what you’re feeling during your testing can hold true. I don’t doubt what you’re saying, it doesn’t make sense to me. There must be something I don’t understand…

This works great! Tells me, without question, which way to go.

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I don’t quiet understand either ,I don’t doubt what you feel, personally I haven’t noticed any difference between different fuse lengths with the same tail/shim combo in regard to front foot pressure , but maybe that’s cause I’ve only tried 50-60 cm fuses.I understand the longer arc a long fuse needs to travel which may take more time (less pitch sensitivity). which could give the sensation of less front foot pressure? but you would imagine the extra leverage of the long fuse would create even more front foot pressure eventually.

I think these are correct. Worth looking into planes from which we are taking the physics.

Fuse Length

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Tail Shim
(need to have a a diagram for top and bottom mount, the bottom mount is easier to interpret)

I like to think about tail shim like bicycle gear for pumping

  1. LE down is easier gear, more leverage, reduced top speed
  2. LE up is harder gear, less leverage, increased top speed

And then for turning (this I’m less certain on because I can’t really turn that well…) Anyone agree with this?

  1. LE down is tighter turn, more push, more drag (like rockered shortboard)
  2. LE up is larger turn, less push, less drag (like a longboard)

I agree with everything you’ve said

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Nice. Also in the drawing I exaggerated the airfoil shape of the tail, as only on the older tails do you noticed such pronounced downward camber, most of the modern ones are much more symmetrical, and the lift is entirely from the designed angle (Axis reckons you don’t need to shim their setups much)

Image photoshopped to show downward camber for a tail :metal:

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You’re correct in that there’s a longer lever so more force, but with the longer lever you get less distance on the front foil. With a longer fuse you can get away with a smaller tail, but you need more AOA. That’s part of why I’m gravitating to longer fuses. Smaller, more efficient setups and it stretches out the pump. And if I get the mast far enough forward the turning angles are still tight. Now our surf is generally small, but I still prefer long fuses in bigger surf and faster speeds. Lets me relax into it more. More stability so I can drive more. Id rather have a setup that I trust and can push at 100% than a theoretically more nimble setup where I’m managing instability and can only surf at 80%. You can see it in style and upper body balancing. Wider arms. I like lateral instability, not pitch instability.

Ahh ha ! Now I understand what your saying Erik and I totally agree !

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