What causes the increase in lift with thickness ratio

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wwalton

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I'm trying to help my daughter prep for CFI check ride. So she asked why does it increase lift at thicker ratios? My first thought was just due to increased distance on the top, assuming not symmetrical, thus creating more differential of pressure? But then I went down a rabbit hole of Reynolds numbers and delayed separation lost myself and confused her.

1642044620428.png

I did a search and found this graph and several like it in my Dan Raymer textbook, but even the graphs with specific airfoils did not explain the why they just showed the result.
Thanks for any insights.
 

Voidhawk9

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Thinner airfoils also end up with smaller LE radius, which tends to reduce stall AoA. Flow cannot as easily stay attached when turning the tighter corner of the thinner leading edge.

The effect of Reynolds number is a deep but interesting rabbit hole. Seems to me this is largely ignored at the GA level - most aircraft use a 'one-size-fits-all' NACA airfoil and solve the problems of reduced Re (and induced upwash) on tapered wing tips with washout.
 

ragflyer

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There are very thin (highly cambered) airfoils as well that generate very high lift and very thick airfoils that have low lift. Max lift is most closely correlated to camber.

In general the lift slope (increase in lift per deg of AOA) of all airfoils (infinite span) are very close to identical (0.11 per deg). This means the only way to get a higher max lift is to postpone stall for as long as possible. As pointed out earlier in many thin airfoils the leading edge radius results in premature stall. In many cases a slightly thick airfoil does this best but not in all cases.
 

wsimpso1

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Info above is decent guidance on the actual physics.

FAA exam questions and "correct" answers though... they have a long history on many topics of being incorrect. Best advice there is to take the practice exams that use the actual questions, look hard at the "right" answers, make mental note of them, and use those when presented with the questions. Classics have to do with aviator's breathing oxygen, the fuel flow gauges, and explaining how wings lift.
 

TFF

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Yes. Right answer is what the FAA wants you to answer. For testing, its regurgitation. They want you to know bureaucracy makes airplanes fly. I’m sure, since she is working on CFI, she has learned that.

On note of aero answer, you do have to attach to the aerodynamic answer that there is points of diminishing returns for each airfoil profile. It doesn’t go infinitely. The NACA 0012,15,18 are the meat. The others loose too much, thick or thin. I believe that is symmetrical too so there is going to be dramatic differences at thin.
 

wwalton

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Excellent, thanks to everyone for your input. I totally agree with the "right" answer for the FAA. I appreciate this advise most of all. It's easy to say something that's not the "rote" answer the examiner is looking for and end up in trouble.

I'm going to say the answer is camber and leave it at that...more thickness equals more camber. (yeah I know that's a blanket but I think it's safe)

Like I told her they can dig all day and if they want to fail you they will. It's one person with full veto power...what could go wrong?

Thanks again for the help. Love this board of experts.
 

wwalton

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This is preparation for the actual check ride. I should have been more clear. The oral part of questioning is always much more challenging than the multiple guess portion. And especially on the CFI, when I was walking out to the plane to start the flight I was very relieved just to get to that point.

In reality you learn more from mistake and failures, that don't kill you, than any other way. But I'm not gonna say how many pink slips I got, I'll just say I learned more from some check rides than others.
 
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