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.

[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.

[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.

[av8rps]

Based on what I see with my Model 4-1200, I'd put 4 between each bay even if only 3 are recommended. At higher speeds the fabric pushes up causing drag. So closer false rib spacing would alleviate most of that.

Also, make sure to put false ribs on underside of fuel tanks too. Mine doesn't have those and at cruise you can look right out the window and see the fabric pushed up tight against the bottom of tank, not following the airfoil shape of the rest of the wing.

The minimal weight gain would be more than offset by increased cruise, and fuel efficiency in my opinion.