GAHorn wrote:JOe, it is my view that thrust from the engine/prop results in a forward “pull” at the front of the crankshaft.
I “get” that prop-blast might be “felt” at the flaps. This prop-blast would result in one thing as far as flap is concerned: relative wind against the flaps...which would result in Lift and drag.
Since ALL prop-blast is felt by ANY portion of the airframe ...then it is UN-avoidable. But a benefit of that relative wind created by the prop would also be LIFT... over by the flaps as well as the wings. To imagine flap deployment as a detriment to acceleration (and therefore to obtaining flying speed) would be equivalent to seeing the much larger and blunter airframe as being such an impediment. The greater resistance to acceleration (and therefore the generation of lift by the wings) would be the drag caused by rolling-resistance, especially on a rough-field/soft-field situaiton.... in-which the lift simultaneously created by deployed-flaps would reduce said drag. That is why soft-field/rough-field techniques recommend take-off flap deployment on rough/soft fields.
If your suggestion (that deployed flaps inhibited acceleration as compared to retracted flaps) then the Cessna performance charts would not reflect shorter takeoff distances with flaps deployed versus undeployed.
You would be 100% accurate if the chord line was perfectly aligned with the thrust line, but since the chord line is a few feet above the thrust line, not all of the prop blast goes across the wing generating lift and much of it strikes only the lower half of the deployed flap. This is pretty normal for any high wing, fowler-type flap setup where the thrust-line is not aligned with the chord of the wing. Unlike the C-130, where the thrust line is almost aligned with the chord line and virtually all of the thrust from the prop is divvied up evenly between both the top and bottom of the wing generating effective "blown lift", the Cessna and most other GA high-wing STOL aircraft do not benefit from this phenomenon because of the misalignment of the thrust line and the chord line. Most of the lift during takeoff is generated by forward speed, not blown thrust across the wing.
Further, since the thrust line is lower than the chord line by a few feet, much of the thrust is indeed only striking the lower portion of the deployed flaps and also being directed downward by the deployed flaps instead of directly rearward. Consequently, it is more detrimental to acceleration vs flaps up.
I think the confusion lies in the conflating of acceleration with takeoff distance. In any fowler-type flap setup, there is a point of diminishing returns - low flap settings will tend to generate more lift than drag and high flap settings will tend to generate more drag than lift and the trick is to find the "sweet spot" - this is often where the flaps begin to track downward more than rearward due to camber changing more than chord. Regardless, this is why we don't take off with 40* of flaps - too much drag. It's also why takeoff with flaps 10-20 is shorter than flaps up, but not necessarily quicker from an acceleration point of view. And you become airborne at a lower speed with flaps deployed than with flaps up. Because acceleration and takeoff distance are not necessarily linked and distance is more a function of climb and how quickly you get off the ground vs how quickly you get there.
So, if you put two identical 170Bs side-by-side and had one deploy flaps to 20 and another leave the flaps up and they both took off - you might find that the flaps-up will accelerate quicker, but take longer to get off the ground distance-wise, whereby the exact opposite would be true with the aircraft with flaps set at 20*.
So, Joe's observations in no way invalidate or contradict the Cessna performance charts.