-The Cherokee has a slightly better glide ratio than a 172.
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-Why do you say soft field performance is bad? The specs on both the 140 and the 172 are pretty darn close.
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-The Cherokee 140 has the same power plant as the equivalent 172's.
Published specs may be close, but don't necessarily reflect reality. Real-world experience confirms that the 172's higher-aspect-ratio wing and different airfoil make it a more efficient performer at higher angles of attack. The PA-28 "hershey-bar" wing's "laminar" airfoil has higher induced drag at high angle of attack (such as in soft-field operations).
I have 530+ hours in C-172 and 620+ in Cherokee 140 and have owned both. No way the Cherokee 140 glides anything close to a C-172. If you rely on published specs, try this one -- with the same engine, the published service ceiling (max density altitude for 100 fpm climb) of my '77 Cherokee 140 was 11,000'; while for a C-172M it's 13,100' -- with a MGW 150 pounds higher than the Cherokee's. In practice, my Cherokee 140 struggled to get much above 10,000', even lightly-loaded; while I've taken my C-172 (which then still had its original, tired old O-320-H2AD) up to 14,500' density altitude.
That's why Piper adopted the tapered wing for the original PA-28-151 Warrior -- it too has the same engine as the PA-28-140 and the C-172I-M. But the Warrior's wing looks more like the 172's, in area, aspect ratio and airfoil (in the outer panels, at least), and therefore the airplane performs more like a 172.
I'm not bashing the Cherokee 140. I like it. It's a good, safe, roomy (in front, at least), easy-to-fly, economical airplane. But in some respects its performance suffers in comparison to a C-172. Those stubby little wings just won't do as well at lower speeds with the marginal power from an O-320.
Almost 500 hours of my Cherokee 140 time was as a CFI giving dual. I think the Cherokee 140 was too easy to fly to be an effective trainer. It could cover a multitude of student sins. Thus Piper dropped it as a trainer in favor of the PA-38 Tomahawk, which was designed to have "edgier" handling.