Short field/Flaps/172A differences

[BrianW]

I was reading the 170B POH, and the 1956 172 POH this morning and noticed that the 172 POH recommends no flaps for takeoff with obstacle clearance, and the 170 POH says to use 20.

Strange since these planes should be very similar in performance, same engine, prop, wing. Any idea what caused Cessna to change their recommendations for flaps on takeoff when they came out with the 172?

[voorheesh]

I’m not familiar with either POH from mid 1950s 172 or 170 airplanes. However, as a CFI teaching short field takeoffs with obstacles, I can tell you that later model 172 Owners Manuals specify that using 10 degrees of flaps on takeoff shortens the ground run while taking off with no flaps yields better performance in clearing an obstacle. (See page 2-14 Cessna 172M Owners Manual titled Takeoff Flap Settings). That might have something to do with it.

[GAHorn]

There’s a couple of things to keep in mind when using the data in those Owners Manuals…not the Least of which is Neither is “Approved”.
They were probably most-influenced and published by Cessnas’ Marketing Dept….so the paragraphs of “Operating Recommendations” are only that…

The “official” performance data (without the advice of how they think you should fly the airpalne) is identical to the Owners Manual charts, and is published in the AFM (required to be carried in the Airplane….not something often told to student pilots when “AROW’ is taught.)

Anyway, doubtless Cessna intended to provide accurate operating information and if you study the two manuals you will see they are NOT Apples-to-Apples comparisons.

The Zero-flap climb to obstacle for the 172 is performed using that models’ Va of 69 mph IAS, while the 170B data used that models’ Vy of 76 mph IAS. (and despite that the 170B beat the 172’s angle of climb slightly.)

Also, the paragraph in which the 170B recommended 20-deg flap climb to obstacle points out that the recommendation is made …not for climb-rate performance…but because of the slower climb-speed used allows the best clearance…. and keep in mind that the data also Warns against using flaps at density altitudes of 4K and higher, where flaps will penalize both ground Run AND climb Angle.

Here’s my CFI-brain writing: Airplanes generally have better rates of climb with zero flaps because of the penalty of drag due to deployed flaps. It’s rare that the shorter ground-run makes up for the reduced climb from flaps. (But with similar or same models comparisons, that data clearly must be collected using the same speeds. The two manuals did not use the same speeds, despite the general agreement they are almost identical airplanes. (However, not identical…. that 172 has the added drag of a nosegear sticking down out there…and a very slightly difference model propeller was used which may be why Cessna used different speeds…a discussion worthy of it’s own thread but we will disregard that minor detail in this one, just noting that these are Not Really the same airplane…. just so-very-closely similar as to make it an interesting comparison.)

But take a look at the performance charts for the two models… The 170B-model clears the 50 ft obstacle slightly with slightly less distance than the 172A when using the identical flap settings….. under standard conditions, gross wts, the 170B uses 1625’ and the 172 uses 1650’. (I doubt the average pilot…even the exceptional pilot… could tell the difference…..and they’d BOTH have their seat-cushions clinched between their cheeks at that obstacle.)
Those are both at zero flaps. (and BTW… do NOT retract the flaps until AFTER clearing the obstacle.)

C-170B Performance

‘56 C-172 Performance

Here’s the Owners Manual for the 56 172:

1956 C-172A Owners Manual.pdf

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