### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### pictsidhe

##### Well-Known Member
In the AC103 appendices, there are graphs to work out acceptable designs without having to do bothersome and accurate testing of stall and top speeds.

Skip to airfoil lift coefficients, namely Cl=1.6; Camber over 7%, that's easy enough to determine, but relatively flat bottomed? I'm looking at a flatish bottomed airfoil: the F5FRAS13 looks about right for my tandem; Page Title

Does anyone know how 'relatively flat-bottomed' is defined by the FAA?

Last edited:

#### Autodidact

##### Well-Known Member
In the AC103 apendices, there are graphs to work out acceptable designswithout having t do bothersome and accurate testing of stall and top speeds.

Skp to airfoil lift coefficents, namely Cl=1.6; Camber over 7%, that's easy enough to determine, but relatively flat bottomed? I'm looking at a flatish bottomed airfoil: the F5FRAS13 looks about right for my tandem; Page Title

Does anyone know how 'relatively flat-bottomed' is defined by the FAA?
The 1.6 is not a coefficient of lift, it is a "lift factor" - it's just a number that the appendix gives you to plug into its graph, and it is based on its own definition of camber and flaps, and for the purpose of ac.107 the camber is not the same definition as usual either, it is the distance between the chord line and the top surface of the wing, big difference. In the aerodynamics texts, camber is the distance between the chord line and the "mean" line; ac.107 is talking about something different.

Does anyone know how 'relatively flat-bottomed' is defined by the FAA?
The precise, scientific definition is; "A Clark Y-ish kind of a thing..." :cheeky:

Last edited:

#### pictsidhe

##### Well-Known Member
I'd noticed their odd definition of camber, I'd discounted my airfoil as counting as counting as '1.6' until I read it. When I work the numbers, the lift factor does look like Cl

I suppose I'm going to have to guess if the airfoil I want to use is going to count as 'flat'

Maybe I should take a poll

#### Autodidact

##### Well-Known Member
I'd noticed their odd definition of camber, I'd discounted my airfoil as counting as counting as '1.6' until I read it. When I work the numbers, the lift factor does look like Cl

I suppose I'm going to have to guess if the airfoil I want to use is going to count as 'flat'

Maybe I should take a poll
I figured I was being overly pedantic; and the number does look like a Cl, but you're given the same "Cl" regardless of what your planform is, and there's a lot more to section Cl than just camber (I think...) so it's kind of arbitrary, but the guys who wrote it I think did a good job of making it simple and keeping the resulting aircraft within a certain performance range.

#### pictsidhe

##### Well-Known Member
Actual Cl does vary with aspect ratio and other things, but lump the area together and average the Cl and the lift factor is the Cl needed at stall. I'd need much more wing to really get the stall offered by the appendices, I'm just wondering if I can get away with the 1.6. I've reflexed a few low drag airfoils as possible substitutions, but they wouldn't be so docile, or produce as much lift. Hmmm, legal or safe...

#### Autodidact

##### Well-Known Member
Actual Cl does vary with aspect ratio and other things, but lump the area together and average the Cl and the lift factor is the Cl needed at stall. I'd need much more wing to really get the stall offered by the appendices, I'm just wondering if I can get away with the 1.6. I've reflexed a few low drag airfoils as possible substitutions, but they wouldn't be so docile, or produce as much lift. Hmmm, legal or safe...
As long as you conform to the appendix, you're legal it even says in the language of the appendix (particularly in the "drag points" computaion section) that "we've given you some leeway here..."

PS, you know, I never even thought of their "lift factor" as a lift coefficient; I'm used to determining wing area from a relatively large number of factors - the appendix just says "You got this kind of airfoil and flaps, so you get this much area.", and so my mind said "This is not a lift coefficient" onder:. You are using the 170lb FAA pilot for the gross weight, aren't you?

Last edited:

#### pictsidhe

##### Well-Known Member
Conforming to the appendix means knowing how 'relatively flat-bottomed' is defined. I'm trying to determine what my legal options are, but the law is being relatively fuzzy...
Yep, for FAA legality calculations, I'm 170lb, + 30lb of fuel onboard . For actual calculations, I'm being more realistic and allowing 300lb of pilot, fuel and luggage/future beergut extension.

#### Dana

##### Super Moderator
Staff member
If more than half of the bottom surface looks flat, or if you can lay it on the floor and it doesn't rock, it's "relatively flat bottomed".

Dana

#### pictsidhe

##### Well-Known Member
That sounds like a reasonable definition, is it sort of official?

#### Autodidact

##### Well-Known Member
It's sort of a strange definition (AC 103-7's, not Dana's). I've seen symmetrical airfoils that have a higher section Clmax than some cambered airfoils. You can have two airfoils with the same camber and a thin version will have a very flat bottom while a thicker version will be pretty rounded on the bottom. The FAA wants to avoid inspecting these things, so I would think that anything with discernable camber can be considered "relatively" flat bottomed. And, relative to an airfoil with no camber, it is.

#### Dana

##### Super Moderator
Staff member
No, not official, even "sort of". But it's a "reasonable" definition of "reasonably" flat bottomed.

Or go one better, and make your ailerons flaperons; then you can use the 1.8 lift factor regardless of airfoil. You don't actually have to use the flaps...

Dana

#### pictsidhe

##### Well-Known Member
It's sort of a strange definition (AC 103-7's, not Dana's). I've seen symmetrical airfoils that have a higher section Clmax than some cambered airfoils. You can have two airfoils with the same camber and a thin version will have a very flat bottom while a thicker version will be pretty rounded on the bottom. The FAA wants to avoid inspecting these things, so I would think that anything with discernable camber can be considered "relatively" flat bottomed. And, relative to an airfoil with no camber, it is.
I'm torn between a flat bottomed airfoil with a CLmax at my span loading of about 1, well away from the assumed 1.6 and the draggier sorta flattish bottomed one with a Clmax of 1.35. The flat bottomed one is going to be out of it's drag bucket if I enlarge it to really hit the stall speed. The flattish one is going to struggle if I make the wing bigger than 1.6 as it's L/D also goes down the pan.
At this point, I'm leaning toward a completely paper legal speedster using the flat bottomed airfoil and nearly as much wing as a 1.4 needs, it'd still stall a lot faster than a 1.6 sized preferred sorta flat airfoil.

##### Well-Known Member
For the record, my understanding is that the NACA 2414 and NACA 4414 are considered "relatively flat bottomed" as used on a bunch of designs. Also both reasonable airfoils for a wide range of ultralights.

#### pictsidhe

##### Well-Known Member
The 2414 doesn't look that flat to me, maybe I should ask what other airfoils are considered 'flat'.
I've been messing around in Javafoil with my low drag, low lift airfoil to get more lift, looking promising to have a lowish drag, moderate lift, flat bottomed airfoil, being able to use a higher aspect ratio helps too. My span is fixed at 7.2m
I probably should have said that this is for a tandem wing, so most airfoils need a chunk of reflex added to get the Cm down. That often trashes lift

##### Well-Known Member
In my mind, flat-bottomed = not single surface (concave bottom), not symmetric (overly convex bottom). Something where the weird AC103 camber definition actually roughly corresponds to 2x the airfoil camber, basically.

#### Himat

##### Well-Known Member
I'm torn between a flat bottomed airfoil with a CLmax at my span loading of about 1, well away from the assumed 1.6 and the draggier sorta flattish bottomed one with a Clmax of 1.35. The flat bottomed one is going to be out of it's drag bucket if I enlarge it to really hit the stall speed. The flattish one is going to struggle if I make the wing bigger than 1.6 as it's L/D also goes down the pan.
At this point, I'm leaning toward a completely paper legal speedster using the flat bottomed airfoil and nearly as much wing as a 1.4 needs, it'd still stall a lot faster than a 1.6 sized preferred sorta flat airfoil.
I am not sure I do understand your design problem here.
As I understand it, a "flat bottomed" airfoil is by the wording in the regulation an airfoil where the side intended to be the underside of the wing have to be flat. Now, if that is flat all the way from the leading edge to the trailing edge is not stated. The Clark Y is mentioned and I do like Dana's definition in post #8 and find that reasonable. If as much as two thirds of the underside is a straight line there should be even less question.

But then, if you look at a profile like the symmetrical Eppler 472 it would fit the bill of "flat bottom" with a little modification. A modification often seen on model airplanes. A straight line is substituted for the gently curved line aft of max thickness. If this modification is due to laziness or ignorance I don't know, but it's not that detrimental to the performance. If you play around with both the thickness, thickness distribution and the chamber line you can have a "flat bottomed" profile with the chamber you want. It will not be optimal like it would be without the "flat bottom" constraint, but for practical purposes close enough. Referring to the model aircraft world again, in model sailplane competition the less optimal profiles chosen for reasons of easier build and so on perform close to or as good as the "optimal" profiles.

#### Dana

##### Super Moderator
Staff member
If it's a tandem wing you may not be able to use the appendices... somewhere it says it's only for conventional configurations.

Edit: I was mistaken; I thought there was something about conventional configurations, but I couldn't find it. Perhaps I was just recalling the bit about not counting canard area that BJC mentioned.

The approach I was going to take for my biplane project (on hold but it might get resurrected someday) was to use flaperons, allowing me to use the 1.8 lift factor even with a semi-symmetrical airfoil (23012 for its good aerobatic potential). Nothing in the rules says you actually have to use them or that they even have to be effective. Nor is there a minimum stated size for flaps, though I suppose they should be reasonably believable.

Dana

Last edited:

#### BJC

##### Well-Known Member
HBA Supporter
At this point, I'm leaning toward a completely paper legal speedster using the flat bottomed airfoil and nearly as much wing as a 1.4 needs, it'd still stall a lot faster than a 1.6 sized preferred sorta flat airfoil.
(Perhaps this has already been suggested, I'm just catching up on this thread.)

Why not design a wing with simple flaps that extend to > 50% of span, get the 2.0 lift factor, and use the smallest wing area that meets the chart's stall speed?

BJC

#### Himat

##### Well-Known Member
If it's a tandem wing you may not be able to use the appendices... somewhere it says it's only for conventional configurations.

Dana
To the devils engineer there is some latitude between a tandem wing and a Junker flap to be exploited...

#### BJC

##### Well-Known Member
HBA Supporter
If it's a tandem wing you may not be able to use the appendices... somewhere it says it's only for conventional configurations.

Dana
If a canard configuration is used, the canard wing area is not to be included in the wing area when using the stall speed chart.

BJC