Design theory: incidence and CG

[Jyoder]

Most foils are designed like airplanes with CG forward of center of lift and positive incidence angle such that increase in speed leads to a positive pitching moment of the nose at a static stab trim setting. That’s why big lift foils are hard to hold down at high speed without a lot of forward foot placement and forward CG.
Benefits of this in aviation is safety and maintenance of control authority in horizontal stabilizer, pitch down at low speed for stall recovery.

But we control hydrofoils with weight shift and change of CG, not control surfaces.

What if we made an “unstable” foil with negative incidence angle and CG behind center of lift. It would pitch up at low speed- good for start, and would maintain trim across whole speed range without needing as much CG movement, right? Faster and stab could be symmetrical airfoil for less drag? I think I’ve inadvertently done something close to this with my DIY surf wing, as it has these characteristics of pitching up at low speed and maintaining level flight easily at higher speed without increasing front foot pressure the faster I go. I need to try to measure the angles and my CG position at stable flight.

Thoughts?

[tkaraszewski]

A center of lift behind the CG is inherently unstable, regardless of the angle of incidence of the horizontal stabilizer. This is because as the attitude of the airplane changes, the mass of the airplane accentuates the misalignment and causes the change in attitude to get bigger.

Imagine a paper airplane that you’re pushing through the air. As soon as the nose of the plane moves out of a perfectly straight line, the push you’re applying from the tail will cause the nose to quickly move completely off to one side of the plane or the other.

On the other hand, with the CG forward of the center of lift, the mass of the plane tends to dampen these movements. Imagine putting an airplane through the air by it’s nose. Even if the tail is a bit out of alignment behind the plane, the pull from the nose tends to drag the test of the plane back into alignment behind the center of mass.

So the negative incidence of the stabilizer is there to make up for the forward CG. With the nose weighted rather heavily, the plane will tend to pitch down, so you set the stabilizer’s neutral position to counter that.

As you move the CG back closer to the center of lift, you need less stabilizer incidence to counter the weight, as it’s not hung so far out in front of the center of lift, and this flatter stabilizer position is more efficient, because it makes less drag.

You could continue this trend and move the CG aft of the center of lift and counter this with a reversed stabilizer incidence, but as soon as you’ve moved the CG behind the CL, you’ve lost your inherent positive stability and every movement of the wrong tends to accelerate itself rather than damp itself back to neutral, which would make for an exceptionally difficult (impossible?) foil to ride.

You’re still going to need more AOA at low speeds than high speeds, and so you’re still going to need weight forward as speed increases to keep the nose down. But if you’re not balancing you’re own weight forward of the main wing with some downforce from the stabilizer, you’re probably going to end up with very small CG movements making a huge difference when both wins are acting with positive lift.

I’d think nearly symmetrical airfoils and a very low angle of incidence would get you the closest to what you want, but not so much that you moved the CG behind the CL or got into negative angles of incidence. This would probably be draggy low speeds as you’re relying entirely on AOA through the water for lift, and would become efficient as it flattened out at high speed.

There are probably typos and weird autocorrect mistakes above, I wrote this on my phone.

[tkaraszewski]

This PDF discusses this issue on page 5-13 and 5-14 under the headings of "static stability" and "dynamic stability":
https://www.faa.gov/regulations_policie ... ak_ch5.pdf

And this video illustrates the same stability problem in a car trailer with the weight both in front of and behind the "center of lift" (i.e., the trailer axle in this case):

[Peter_Frank]

You are correct Jyoder.

In fact racefoils somewhat seek this trim, and many freeride foils are close to this also, so you dont need more front foot pressure, at least insignificant, can not feel it on my foils really.

But there still have to be some stability for most riders, so a compromise is usually best.

As you can see one can ride without a stabilizer at all, and here you have "negative" stability too because of the wing pitching moment, but because of the lever from the longer mast and our muscle memory we can still ride.

It is all a tradeoff and you can choose any "spot" between or at too much stability and too much instability, personal preference only, but most want just a tad stability so it is close to neutral and still easy to ride

Fast competition model gliders are always designed with this fully neutral trim you talk about though, so CG quite rear quite opposite to "aviation" planes, thus being able to ride at insane speeds or normal speeds in a straight line without needing pilot input or trim changes, and you can obtain the fastest speeds still having control this way.

Peter

[jumptheshark]

That vid makes sense to me. I changed foils this year and went from moving my front foot, to now changing my back foot. On a Lift 170 currently, and my front foot is pretty much always in the same spot across the speed range. I shift my hips with sudden hits of speed, but I don't have move the front foot. On my Zeeko I had to move that front foot with big increases in speed, but never really changed the position of my back foot. The Lift has a bigger stabilizer on a shorter fuse than the Zeeko. On the Zeeko, I wore a very defined spot in the pad with my back foot, but shifted my front foot enough to wear a much bigger area. Now its the opposite and there is a much more pronounced effect on the ride. On the Lift, my front foot is always in the same spot. With my back foot over the plate, it is so steady and stable its almost a bore. Nice for resting, traveling distances, upwind, or through nasty chop, but if I want to have any fun and carve hard, I shift the rear foot back behind the plate and the foil comes alive. With the weight forward the stabilizer is big enough to dominate giving stability. Shifting back lets me overpower it and there is more response to any given input.

[Jyoder]

That vid makes intuitive sense to me. I changed foils this year and went from a front foot dominated foil to a back foot dominated foil. On a Lift 170, my front foot is pretty much always in the same spot across the speed range. I may shift my hips with sudden hits of speed, but I don't have move my front foot. On my Zeeko I had to move that front foot with big increases in speed, but on my current foil It's the rear foot that changes everything. This Lift has a bigger stabilizer on a shorter fuse than the Zeeko. On the Zeeko, I don't think I moved that back foot much, but shifted my front with big changes in speed. Now its the opposite and there is a much more drastic change in the ride. With my back foot over the plate, the Lift is so steady and stable its almost a bore. Great for resting, traveling distances, upwind, or through nasty cross chop. If I want to have fun carving, I shift my rear foot back behind the plate and the foil comes alive. With the weight forward the stabilizer is big enough to keep everything steady. Shifting back lets me overpower it and there is more response to any given input. When airing, I have to have that back foot back to get the sudden pitch change needed to ollie up, but when landing, I need that back foot to come down over the plate for all the stability I can get when re establish normal flight. The front foot doesn't change position at all.

For pitch control, it shouldn’t matter where your feet are on the board as long as the net foot pressure/CG is in the right place.

But for turning, carving like you said, it’s probably better to have a certain foot width and maybe slight pressure imbalance- especially if the foil is tuned for stability like PF says.

[jumptheshark]

Maybe for pitch stable flight, but the harder you carve on a foil, the more pitch becomes a dynamic component of the turn. In slow turns with the rider remaining mostly vertical throughout, it is mostly yaw, but once heeled over hard in a carve, it's rear foot pressure that alters the pitch so the main wing's lift enters the and equation and produces the desired G forces. The shorter fuse of my current foil that I can step back to better leverage.... or no stab at all for the mono guys is what makes them so responsive.

Pumping is a great example of pitch change contributing enough to make for propulsion. If my feet are in the right place to pump efficiently, they are in the right spot to carve hard.

[Kamikuza]

That’s why big lift foils are hard to hold down at high speed without a lot of forward foot placement and forward CG.

Some, not all.

What I find with my gear is I need to spread my stance out much wider than usual to give maximum leverage for the necessary fast response to pitch control -- front foot way forward, rear foot way back. But pretty much the same balance point as before.

[Kamikuza]

In fact racefoils somewhat seek this trim, and many freeride foils are close to this also, so you dont need more front foot pressure, at least insignificant, can not feel it on my foils really.

As far as I can tell, all racers have their rear foot behind the mast, compared with the many "free riders" that have their rear foot on or in front of the mast...!

I can't understand why people do that, because for carving the board around you lose leverage -- basically what jump is saying. Giving up the leverage makes the ride more about riding that board than driving the board, if you get my meaning.

IMHO

[joekitetime]

but if I want to have any fun and carve hard, I shift the rear foot back behind the plate and the foil comes alive.

I have no intention of making any comments on this thread (I have to look up too many words and the concepts stagger me)... but...

I have very much found the above comment to be true...