kalvado wrote:OldAeroGuy wrote:kalvado wrote:What you're showing in this example is the rate of force decrease, d^2F/da^2.
I am referring to fig. 7.2 in AC 25-7D where "acceptable" curve (vs speed) shows that exact behavior - decrease of derivative to zero at the edge. You interpret such behavior (vs AoA) as unacceptable.
So where does that come from?
It comes from this:
"126.96.36.199.3 During the approach to the stall, the longitudinal control pull force should
increase continuously as speed is reduced from the trimmed speed to the
onset of stall warning. Below that speed some reduction in longitudinal
control force is acceptable, provided it is not sudden or excessive."
Fig 7.2 addresses an overall airplane speed stability requirement. For small portions of overall speed stability, small slope reversals are acceptable. This is distinct from the stall handling requirements of FAR 25.203 that are quoted above. For stall handling, a reduction in stick force gradient (dV/dF or dAoA/dF) is not acceptable prior to stall warning. As an aside, stall warning is based on AoA, not speed due to stall speed variation with airplane weight.
As a point of reference, AC25-7C was applicable for 737 MAX flight test. AC25-7D was not adopted until 05/04/2018, after 737 MAX certification.
This particular graph is the same in -7C and -7D, so we're good.
I understand what you say, but I fail to see reversal in your numbers. POssibly just a typo?
Again, here is the dataset:OldAeroGuy wrote:Case 2 - Fails 25.203
AoA - Stick Force - lbs
4 - 0 (trimmed)
5 - 5
6 - 10
7 - 15
8 - 20
9 - 25
10 - 28
11 - 30 (stall warning)
12 - 32 (stall)
A stall of this type would be indicative of stall starting on the outboard wing. The configuration is stable throughout the stall as stick force must be increased to stall the airplane. The change in gradient after 9 deg AoA causes the airplane to fail 25.203. This is the type of stall handling is typically called "stick lightening". The 737 MAX probably has this type of stall approach behavior and is why MCAS was added.
Force numbers increase across the range of AoA values. What is decreasing, is the slope of change, but not the sign of slope which FAA interprets as acceptable.
Or my eyes are not doing a good job, can you point specific range where you see the problem? 9-10-11-12 is still increase of force situation.
Nothing wrong with your eyes or your analysis of the stick force situation in my example. As I said above: The change in gradient after 9 deg AoA causes the airplane to fail 25.203. The FAA wants a continuous (or increasing) force gradient. The force gradient cannot decrease as my example shows.
The kicker is in this Section:
188.8.131.52.1 The airplane should be trimmed for hands-off flight at a speed 13 percent
to 30 percent above the reference stall speed, with the appropriate power
or thrust setting and configuration. Then, using only the primary
longitudinal control, establish and maintain a deceleration (stall entry rate)
consistent with that specified in § 25.201(c)(1) or (c)(2), as appropriate,
until the airplane is stalled. Both power/thrust and pilot selectable trim
should remain constant throughout the stall and recovery (to where the
angle-of-attack has decreased to the point of no stall warning).
To maintain a steady deceleration rate, dAoA/dF must remain relatively constant. Otherwise, as the stick force gradient changes, the deceleration rate will not be steady.