Question: feet position vs. mast centerline


#1

I’ve been looking at a few builds and the last vid I watched I noticed that the person had both feed ahead of the mast. Since I’m just about to cut up my board and I’d only like to do it once what is the best position of feet vs. mast.

something like:

back foot —6"— mast CL —12"— front foot

or

mast CL —2"— back foot —18"— front foot

etc. What do you recommend?

does this question make sense?


#2

HEllo,

The position of the mast doesn’t really matter, what matters is the relative position of your center of gravity(CG) vs the two horizontal lifting surfaces.
The closer the mast is to the main front wing the less your fuselage has to resist bending.

What I am saying is that you need to look where the weight of the rider seems to be relative to the horizontal wings. Ideally your (you + board+ gear) CG should be slightly forward of the main wing. That does mean that you require a downforce on the back tail to balance the moments but that is a stable solution. The further forward your CG relative to the main wing the more down force the aft wing will need to generate and the more stable you will be but the less fast as the main wing is working harder.

Does this make sense?

JB


#3

@ jbungener

Yes, thank you - that explains why the person in the video was where he was. It also changes my plan. For whatever reason I was assuming you had your back foot basically on top of the mast and the front foot about 18 inches forward, but the CG explanation makes more sense.

My main goal (right now) is to make sure my ESC box isn’t where one of my feet will need to be, since it sticks up a little bit.

Thank you!


#4

You also need to take the trust into account…
Using the same mast and wings (liquid force impulse) for surfing, kiting, and e-foil my feet end up in different position.
For surf and kite back foot is between on top of the mast to a couple inches forward, that wing creates way more lift than the original foilfish wing for which my back foot was a couple inch behind the mast.
For for efoil, to take off, my back foot is at least a foot forward, and i put my weight quite a lot toward the front, but when it’s flying, i slide my back foot back a at least half a foot and bring my weight back, it’s much more comfortable…


#5

I’m beginning to think about putting the ESC box behind the mast position - sounds like my feet will always be somewhere in front of it. I’ll draw a diagram of some sort with the proposed positioning and post here. Thank you!


#6

Something else you need to consider is the distance from your ESC to your batteries. If it is more than 15cm you may need to add an extra capacitor bank to avoid hammer (voltage spikes) issues that would damage the ESC


#7

That’s a good thought. Unfortunately I don’t have any space in my ESC box for additional caps. I do have gigantic wires from the battery box to the ESC box, so should be ok.


#8

Doug, from what I’ve read having extra thick battery wires won’t stop the voltage spikes, the wires need to be short regardless of the thickness. Certainly worth some more research.


#9

Here is a very good article for that topic. Looks like a serious problem.

https://www.rcgroups.com/forums/showthread.php?952523-too-long-battery-wires-will-kill-ESC-over-time-precautions-solutions-workarounds


#10

If you have troble with voltage spikes, low-esr caps are cheap. Get some smaller ones and go for more. I got 470µF 63V ones for mine. Then just attach some protection diodes on your ESC. I used LITTELFUSE SMBJ48A over the input voltage ceramic caps.


#11

As for your CG, you can use this tool here (https://www.ecalc.ch/cgcalc.php?) to approximate your CG. The foil is essentially an Aircraft under water, same physics.
This tool, however, doesn’t take the motor/propulsion into account. For a stable, stationary “flight”, the moment around the CG has to be equal to zero, otherwise you’re pitching forwards or backwards. The motor will generate a backwards-pitching moment, that’s why the CG has to be a lot further forward on an E-Foil than on the same Wing/Stabilizer Combo without a motor.
Hope that helps and doesn’t confuse too much.