Originally Posted by
mustafagander
Sounds good fdr,
BUT MCAS does not operate with flaps extended as I understand things. So then, do what you like with the flappery, it matters not.
A simple old engineer like me sees the easiest and best fix as small strakes on the aft fuselage set to zero incidence in the CRZ case. Now they cost a bit of wetted area and maybe a bit of extra drag on CLB coz the strakes do not align with the streamlines. However it is a simple once for all fix with no failure case. Never mind this software stuff, KISS.
Noted re flaps, however, the underlying issue is this. The flow from the engine nacelles of fans under almost all wings results in a loss of CL in the area of the wing behind the nacelle. The addition of the low aspect delta from VG's control the flow off the nacelle and result in coherent vortex structure streaming over the wing LE, and that happens to recover the CL for the section. Along comes the new blender, with "biggly" nacelles sticking out further, and the same VG is placed on the nacelle, and, oops, we have a surplus of goodness. So out with the angle grinder. As CL correlates to V1Sg which correlates to Vr, and V2, and Vref, The beancounters and hawkers of wares at the bazaar don't want higher V speeds, they want higher $$$$ per pound of plain. To get the Veezes back in harmony with the multiverse, just add. ... tabs. tabs increase CL, "biggly" but they move Cp rearwards on the section, which is what y'all driving Boeing's overly huffed n puffed toy need for happiness. The surprise of doing that is... you happen to also aft load the wing in transonic flight, which happens to put the shock and lambda foot of the SBLI back where the CFD models would predict they would be for a clever wing such as Bob Liebeck envisaged with SC-1 and similar foil types, the basic shapes which oddly go back to Herr
Kawalki back in '44. The neat thing about your common variety supercritical foil is that they are not acting as advertised. (the 737 is aft loaded, and the LE radius makes it, and most TBC designs a hard ask for being supercritical in practice due to excessive LE suction. Airbus makes a really neat wing, since the 330/340 and the 380... I say through gritted teeth, cuz, damn! ) Take a DES, LES, RANS 2D or 3D preferably model of the wing and you get a different location of the shock to what you see when you look out the window. A schlieren image of the shock foot is observable as a shadow on the wing surface. The neat thing about that it is is in the wrong place, but it is the real world, and it is what the as-built vs the generic monolithic sections used in CFD are employed. as opposed to detail). The effect of a DTE is to aft load a section and the flow effect is the same in detail as a LET, so you get the chance to stop the wear n tear of the 737s odd flap tracks by stopping unstable separation points, reduce inflight vibration, (it aint from the engines... its from the flow over the TE of the wings being unstable, and making the shock location unstable... ) you reduce drag, and increase lift, and... for the MAX machine, you shift the CP rearwards in that section, and that mitigates the nacelle lift Cm effect. tail trim force is increased slightly but it is a fraction of the total forces. A Max runs out at about 70+ tons, more or less, with a L/D around... say 20:1, so the drag in cruise is around 3.5T, and the thrust is the same. 1% drag change is... a big deal, but it is 350kg, 2 x "400lb hackers" on their beds.
The SFC is around 0:35 for the older blenders at SL, going to about 0.65 at cruise, and the new biggly built versions of the TFE731, the GENX etc are about 20% better. but, still, drop off at altitude. Why? cuz, the fan is a fixed pitch prop. yes, it changes the inflow velocity, but it is fixed pitch, and so the faster y'all go, the more turbojet you end up relying on (residual thrust) you still get the pleasure of the blade drag. If only there was a way of altering Cl vs Cd at static and high-speed conditions. And indeed there is. How that can be done is in a dumpster load of papers at ARC, NTRS, DLR etc, but as far as I am aware, it has only been tested once, on a really daggy JT15D-1A which was less than enjoyable to work on, and we only got around to doing the static runs. The prop mod stuff though we have flown since 1994, and that was fun in its own rights. turns out there is a philosophical question on the simple calculation of efficiency, otherwise, it is hard to get a 20% improvement in thrust output from a prop that is P=po/Pi= 85%. The static test of the JT15 got a marginal increase in efficiency for the thrust output, but it did put out 30% more thrust at 100% N1, while remarkably remaining within T5 limits. It put out the normal thrust at 94% so that wasn't a bad thing. I digress.
Miller time...
An angle grinder and some velcro would have removed the requirement for MCAS.