to aero engineers: impact of stab size / lower limit ?

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jmf1 said..
Hello,

This is a question to aero/hydrodynamics engineers (if any on the forum) about the lower limit for the stab size and the impact of a significant size reduction. The reason is that I have bought a second hand Moses foil that has a unusually small stab associated to the wing.

I use the W683(S) wing (about 1000 cm2). The normal stab in the S450 which is 326 cm2. I have the S421, which is only 220 cm2. I wonder If I should buy a S450 or not. For making the decision, I would be happy to have a numbers crunching/theory part. I also have a W873 - 2100 cm2 wing, to use with the same stab.

326cm2 to 220 cm2 is a 30% surface reduction. As I use the same fuselage, it is also a 30% reduction of the "Stab volume" (basically stab surface x distance to wing) parameter. It also moves forward the neutral point. So it could impact the static margin.

Would someone be able to provide some help to crunch numbers and see up to when we can decrease size and still have stable / healthy configuration. And is there would be a low threshold not to go below ?

Some competitors and experienced users said to me that stab size and shape had only a minor impact on the behavior of the foil. There may be some truth in it, but maybe there is a limit :-)

I also "dream" of a tool/excel sheet to be able to run small simulations, assess orders of magnitude about our foils, and estimate meaningful key parameters.

JMF

A bit of background first, as there sometimes seems to be a bit of confusion on these forums about stabiliser sizing.

The role of the stabiliser is to provide downforce behind the main foil. This is no different from an aeroplane, which if you look carefully has an upside down aerofoil and a negative angle of incidence on the stabiliser. So why is this necessary if we could just move our weight slightly behind the centre of lift of the foil to encourage it to adopt a positive angle of attack?

The answer is that we could do that, but it would be totally unstable in pitch. The smallest amount of additional lift, from a bit of chop for example, would lift the foil upward, but the weight being behind the centre of lift of the foil would result in the nose of the board pitching up slightly, resulting in more lift, resulting in more pitch up etc. etc. Similarly a reduction in lift on the foil would result in the nose pitching down, resulting in more loss of lift, resulting in more pitch down etc. etc.

To gain some measure of longitudinal stability, we want the centre of gravity in front of the centre of lift of the foil, as in this case an upward motion of the foil will result in a slight pitching nose down which results in less lift. So the foil will try to reach a stable equilibrium.The distance between your centre of lift and the C.G. is your static margin.

To achieve this in practice however we need to apply a downward moment to the rear of the foil to balance our weight in front of the foil. This is where the stabiliser comes in. The stabiliser applies a moment, which is force x a lever arm. This means that a larger stabiliser on a shorter fuselage can apply a similar moment to a smaller stabiliser on a longer fuselage. The force developed by the stabiliser is dependent on a couple of factors: its area and its angle of attack. A smaller stabiliser at a higher angle of attack can produce the same downforce as a larger stabiliser at a smaller angle of attack.

So people who cut down their stabilisers in order to make their board go faster could just have easily achieved the same effect by reducing the angle of attack of their stabiliser, particularly as overall drag at reasonable angles of attack is more related to span than it is to area. Very often they have just bastardised several hundred dollars worth of carbon fibre stabiliser to achieve something that could have been done with a shim worth next to nothing.

Generally the tradeoff is that while you will reduce overall drag by reducing stabiliser angle of attack, you will also reduce longitudinal stability, so the board will be a bit more twitchy in pitch. If you have good skill levels this is probably fine, after all foils have to be designed to suit the average user, so there is some margin for destabilising them for highly skilled users.

So for your specific case, you can achieve sufficient downforce by shimming the stab to a larger angle of attack to generate more downforce if required. The limitation will be whether the stabiliser ventilates more easily when you are close to the surface, as it will be working at higher lift coefficients. If it doesn't ventilate when you are close to breaching the main foil, it should be fine the rest of the time.

However, if you have adequate longitudinal stability, I would just leave it as it is.

Thanks Pacey for all those explanations. From my experience in RC gliders, and the basic knowledge about aerodynamics, I understand this relation between surface, Angle of Attack and static margin. Well explained in your email.

I was wondering if there was a "second order" effect over the speed range of the foil. When trimmings with similar static margin between "big stab + lower AoA" and "small stab + bigger AoA", will there be difference on the handling of the foil over the speed range, from minimum foiling speed to Vmax ?

Exemple, will you have to move more forward with speed in one case than in the other ?

Or some other foreseen second order side effect ?

At first order, I understand that you confirm the idea that many stab can do the job (surface, shape...) as long as you shim them correctly and don't reach ventilation.

Kind regards,

JMF

Pacey said this, too. Notice that the longer the fuselage, the smaller the stab can be. You see this on racing foils. It is about downforce moment. The Starboard GT with 75 cm fuselage has a very large stab. The Starboard Race with the 115 cm fuselage has a tiny stab.

So, not only are the racing foils fast due to a small stab, but the long fuselage also yields better pitch stability.

If you want another datapoint, the Fanatic Flow 1000 has a relatively tiny 215cm2 rear stab, and good pitch stability. I believe its the same rear stabiliser across their range of front foils from 900 to 1500cm2.