savage@rainbo.DEC (Dennis DTN 282-2614) (08/06/85)
================================================================================ First of all many thanks to those of you who sent me information concerning the flying characteristics of the Grumman AA1A. As always, learning something new raises a few new questions... One of the replies came from A. Gomez. He explained that the Yankee (and the Arrow) don't float like a C-172, that in comparison they drop like a rock, this being due to the fact that Cessna uses a high lift airfoil and the others use laminar airfoils. My question here is perhaps silly but I thought all airfoils had to rely on a laminar flow of air across the upper surface to fly. What is the difference here... and why would anyone want to have a wing that would let them drop like a rock? For those ho are interested the best quote re:the AA1A - "The Yankee is like a mid '50's Italian sports car. Very fast and nice, but you have to be an above average driver to handle it." /Dennis Savage Digital Equipment Corp. ================================================================================
scw@ucla-cs.UUCP (08/08/85)
In article <3460@decwrl.UUCP> savage@rainbo.DEC (Dennis DTN 282-2614) writes: > >One of the replies came [...] uses a high lift airfoil and the others use >laminar airfoils. >My question here is perhaps silly but I thought all airfoils had to rely on >a laminar flow of air across the upper surface to fly. This is true,but not silly (see below). >What is the difference here... and why would anyone want to have a wing that >would let them drop like a rock? The difference between a 'laminar' flow wing and a 'high lift' wing is simple a 'laminar flow' (LF) wing has the thickest point well back (usually in the 40-50% zone), a normal airfoil typically has the thickest point near the 30% zone (25-35 %). The net effect of having the thickest point 'far' back on the wing is that the wing has laminar flow over more of the wing surface. This produces a dip in the l/d curve (called a bucket). (pardon the poor graphics): Regular 'Laminar' . . . . . . . . . . . . . . . . . .. . .. ...... .. .. .. ^^ Bucket One of the first production aircraft to take advantage of this was the P-51 (Mustang) with it's phenomenal range. A major DISadvantage of this type of airfoil is its poor to nasty stall charisterics (some can really get quite harry as the thick part moves aft), another weak point is a requirement that the surface of the arifoil must be *VERY* smooth (no bugs) or you loose the 'bucket' (while retaining the nasty stall), also some of these airfoils have poor pitch moments (pitch moment is the tendancy of the airfoil to pitch (up or down) as the angle of attack changes, large abs(pitch_moment) are a NO-NO). Moderate (40-45%) LF airfoils have all of these features in moderate amounts. For a full (detailed) discussion of this there is a nifty little (well not so little) book called ,as nearly as I can remember, 'The Theory of Wing Sections' (or words to that effect) which should be available in any Eng. Library, and most Public Libraries, it's also available from the SSA and probably from the EAA.