False bottom rib

[Flyboy66]

What is the benefit to installing false bottom ribs on an S7?

[jiott]

It helps hold the proper airfoil shape; otherwise the high pressure on the bottom of the wing (which is necessary for lift) tends to push up on the fabric into a concave shape. The concave, undercambered, shape is more like the old earlier models that were draggier and therefore slower.

[Flyboy66]

The rib option doesn't cost much, but is does add a little weight. Does anyone have an idea about the speed difference with and without the rib option? Does it affect stall speed?

[av8rps]

I don't know if anyone has ever proved the speed difference, but based on my personal experiences I believe they make a large difference. Jiott did the shortest and best explanation why to use them. I know I wouldn't build a Kitfox without them. And don't skimp, put the max number suggested between each set of ribs (4 as I recall).

[Esser]

It probably only adds about the weight of an apple.

[Flyboy66]

I don't know if anyone has ever proved the speed difference, but based on my personal experiences I believe they make a large difference. Jiott did the shortest and best explanation why to use them. I know I wouldn't build a Kitfox without them. And don't skimp, put the max number suggested between each set of ribs (4 as I recall).

Do I need to specify the number I want, or are you saying some people do not install all the ribs they are given?

[jiott]

I believe the SS7 kit option for bottom false ribs comes with 3 ribs per bay. That is what I used, but I see from this thread that some folks are installing 4 per bay. I think that is overkill, especially if you have a good tight 350 degree shrink on the fabric.

[Flyboy66]

I believe the SS7 kit option for bottom false ribs comes with 3 ribs per bay. That is what I used, but I see from this thread that some folks are installing 4 per bay. I think that is overkill, especially if you have a good tight 350 degree shrink on the fabric.

Are they fabricating the extra ribs or ordering them from the factory? I cant see that it would be worth the time trying to fabricate what I presume is a very inexpensive part to buy.

[David47]

.... Does it affect stall speed?

Simple answer - yes. If you don't maintain some semblance of the original aerofoil shape your stall speeds will change to .... ?. i.e. I don't know that anyone would be able to tell you with any certainty. Adding these is a no brainer IMHO.