Originally Posted by
MechEngr
Quite a few of the astonishing items in that article are false. But it is written as a Gish Gallop, so an equally lengthy response is required to debunk them.
Example: That the 737 MAX was unstable and MCAS corrects that instability. So far, all facts point to False. The 737 Max is stable and MCAS is not needed to correct instability. There are many others, but since it starts off with misinformation, it is not a good sign.
Pity that the conversation loses it's force in loose facts.
For the record, the strakes on the engine cowl are not fitted as an aid to stability, they make up for the decrement in CL at the higher AOA that arises from the interaction of the cowl flow and the wing with a close coupled cowl/wing design. The AOA that occurs at is high but all speeds for TO and landing are related to the stall speed, and therefore the decrement at stall impacts TO and landing requirements.
Indeed, as far as stability goes, they actually are slightly destabilising on a swept wing.... Why? Because... as AOA increases, the normal cowl will start to degrade flow conditions in the wake of the nacelle, (and a bit inboard...) and that lowers local CL. For the section it has a modest effect on Cm, but the overall geometry of losing CL inboard means that there is a shift of lift distribution span-wise towards the tip, and as the tips are swept... rearwards (makes the flutter stability nicerer) then there is an increase in nose down pitching moment as AOA increases. Yaay. That's nice. So. with the MAX-facktor, we get a biggerer effect from the interaction of the cowl, and the strakes were added... and that means we got a betterer CLmax, yaay, but it is destabilising. Fix MCAS, reset the strakes... add water and mix. Thats the aero fix, and you get a bit of a loss of performance for TO and LDG, which can be picked up with fun stuff on the TE of the flap, which also makes the wing work better in the cruise. Yet, here we are, 16 months into this debacle, awaiting a fix of a lousy software code and system architecture that could be removed completely with a hacksaw and some bondo. Now where did I put my bottle... its Miller time.
Simply put: for the restoration of CL that the strake give, you get rid of the natural negative Cm that occurs without them,... yet the program keeps them in place. Loopy $h[#.
[b]Happy pickky time:
Zhang, W., Chen, H., Zhang, Y., Fu, S., Chen, Y., Li, Y., and Zhou, T., “Numerical Research of the Nacelle Strake on a Civil Jet”, ICAS2012, 2012.
References cited in the above paper. The understanding on the effect of the strake goes back further than these to work in the mid 80's and early 90's. The improvement in CFD gives prettier pictures though. BTW, the same effect does occur on TP aircraft dependent on their cowl design, and a strake may improve CL at modest AOA, DLR has nice work on that recently giving a 10% increase in CL.
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