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737 Max Solution, Please Explain

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737 Max Solution, Please Explain

Old 21st Sep 2022, 14:11
  #21 (permalink)  
 
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I'd expect the opposite. Some hardware fix is faster than software. AFAIK the MAX software is done externally (Rockwell Collins?), one of the reasons the modifications finally took so long. Unusual strakes at certain places might point to aero issues that some might prefer to not put on display if possible?
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Old 21st Sep 2022, 15:31
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fish

The charts above do not indicate the nature of the longitudinal stability issue - seen by the pilot as change of stick force with speed.

The MAX problem was twofold.
First identify the nature of the stability problem, as required for certification, which might be fixed with a relatively easy ‘bent tin’ mod.
Second was to make the MAX handling (stick force vs speed) as close to that of previous variants so that there was minimum type difference training - no additional simulator time.

The optimisation task was further complicated by the -800 certification being marginal for these aspects with respect to the latest certification standards, avoided by ‘grandfather rights.’

So where a simple ‘bent tin’ mod could be certificated, new, variable - ‘auto reconfiguring tin’ with speed would be required for type similarity. As I recall there was previous discussion on this re tests of LE ‘flaps’, strakes, etc (#13 and previous threads)
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Old 22nd Sep 2022, 08:25
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Originally Posted by safetypee View Post
The charts above do not indicate the nature of the longitudinal stability issue - seen by the pilot as change of stick force with speed.

The MAX problem was twofold.
First identify the nature of the stability problem, as required for certification, which might be fixed with a relatively easy ‘bent tin’ mod.
Second was to make the MAX handling (stick force vs speed) as close to that of previous variants so that there was minimum type difference training - no additional simulator time.

The optimisation task was further complicated by the -800 certification being marginal for these aspects with respect to the latest certification standards, avoided by ‘grandfather rights.’

So where a simple ‘bent tin’ mod could be certificated, new, variable - ‘auto reconfiguring tin’ with speed would be required for type similarity. As I recall there was previous discussion on this re tests of LE ‘flaps’, strakes, etc (#13 and previous threads)
Actually it does.

The chart refers to CL, and of less importance to Cd. The CL chart is everything of interest. From the CL, and reading the thesis alone, the pressure distribution on the wing can be determined, and that gives the Cm, and that is the point of interest. Strakes/Vanes/Chines whatever their name, act to mitigate the interference effect that occurs from the.... NACELLE. The effect is a high AOA effect, and the interference suppresses CL. So remove tghe chine and CL in that area of the wing reduces at high AOA, which can be tuned to match the effect of the component from the nacelle of the MAX that causes the longitudinal stability issue, which was Subpart B to Part 25, §25.173, arguably most subparas.

Why would the trimming of the chine affect static stability? It suppresses the peak of the CL/AoA (a-slope) for that part of the wing. As the wing is a swept wing, that area is forward and therefore the Cm of the whole wing will become more negative than previously, which is what is needed for the stability, along with of course a beneficial reduction in downwash to that tail that occurs due to the local high CL peak at high AOA that. otherwise exists, which happens to increase the tail downforce for a given stab trim (THS for bus drivers) which otherwise exacerbates the problem at the higher AOA's.

As a reduction in the chine will suppress the peak CL for that section, the total lift available at the Vs1g condition, (well, Vsr, §25.103) will be slightly reduced, and so the V will increase slightly, which affects:
  • Vr
  • V2
  • Vref
and those in turn affect slightly
  • TODR
  • SSL (speed is marginally higher, therefore to meet §25.115 the ROC is slightly higher, as in... less than the thinkness of the ASI needle...)
The TODR and SSL changes may affect the RTOW on some occasions, but are basically negligible.

The tested performance that was certified was presumably inadvertantly (please tell me it was inadvertent.... ) predicated on the additional "benefit" of the lift from the nacelle, which consequently gave the static stability problem and that then gave the rat cunning repurpose of a high speed system that required 2 trigger events, altered to give both a high speed (wind up condition event) and the low speed (the discovered surprise package in the middle of the testing) surprise, low speed triggering being achieved by removing the speed logic, making the system open to a single trigger event, from a single AOA probe sensor, and that gave us the carnival rides that ended in tears.

In all honesty, the planes stall just fine, and the average driver would not be getting into any real difficulty, however, while the -800 met the rules, the Max did not. Flying a plane with this static stability is actually quite nice, not for everyday IFR, but darned if it isn't what makes planes like the Pitts fun to fly, it does exactly what you ask it to do. Pilots have problems with stalls as we are not exposed to the extent that we should be, and that is not the problem of the plane, that is a training and competency matter. The response to an inadvertent stall by the average (is there such a thing?) crew range from the wrong input (confusion, instrument failures...etc) to nothing, sit back and watch as the plane does a wifferdill, to wild over reaction. All that is necessary is to ease off the g load at that time, by lowering the AOA, that's normally about 1-2 pounds of pressure on the stick, but we are white knuckled and freaked out as we are not trained correctly. Sims today are getting better aeromodels, they are variable, some are still shockers, but many give a good training point for drivers to become less freaked out by stalls. To get to a stall at low level, the crew have to be out to lunch, lights on, no one home. Recovery is simple, and prompt, and that plane talks to the driver. High altitude, the stall is very benign, but there will be a height loss in most cases, the excess thrust in minimal in most cases, and the plane is often on the backside of the drag curve. Still not a big deal, enjoy the scenery while speed builds up, and wave at the planes at the lower level as you pass them by.

One jet I fly has no stab trim change from takeoff, through 450Kts Indicated, to stall. (above 420 indicted there is a slight, noticeable push force, but nothing to bother grabbing the trim wheel for). That plane is beautiful to fly form, aerobatics or low fly in. (rather different to say the T-6 or T-28 where there is a fair amount of trim change with anything you do with power or speed, or any control).
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Last edited by fdr; 22nd Sep 2022 at 08:40.
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Old 22nd Sep 2022, 09:47
  #24 (permalink)  
 
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Originally Posted by DaveReidUK View Post
What are these "other aircraft" that exhibit the characteristics of the MAX that MCAS was intended to mitigate?
The Dash 8 Q400 has long fuselage strakes on the lower rear sides; below and forward of the empennage. Not sure what causes the issue they solve, however. The long thin fuselage perhaps?
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Old 22nd Sep 2022, 14:26
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I understand the ventral strakes on the Q400 are to straighten prop wash swirling around the fuselage. Pratt & Witney didn’t want to make a counter-rotating version of the engine for cost/parts reasons so the props turn the same direction. As a result, every change in thrust requires a rudder trim change.
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Old 23rd Sep 2022, 00:47
  #26 (permalink)  
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strakes

Strakes can be applied in varying means for varying outcomes.
  • On a tail boom, a longitudinal strake on the opposite side to the anti-torque direction of thrust provides a local flow that gives lateral force. (vertical flow oriented VGs on the opposite side improve that outcome considerably)
  • On a chipmunk or tigermoth, they alter the spin characteristics when applied as extensions to the horizontal stab.
  • On the KingAir, Learjet etc they are applied so as to improve lateral directional stability. They can also reduce total drag, as they can cause reorientation of the flow that has considerable separation otherwise at the rear of the fuselage on most aircraft.
  • The Q-400 implementation gives flow cahnges around the body of the aircraft otherwise influenced by the wake of the propellers.
  • Strakes were added to the belly of the Harrier and reduced the thrust required to hover.
  • A series of strakes were trialled on the C-17 to reduce drag.
  • Strakes on various cargo aircraft are applied to control flow particularly when airdrop operations are being conducted.
  • Strakes were added to the DC-9-51 and other variants of the long beach cable companies toy. They are in an area of upwash around the lower forward fuselage, which increases the upwash at higher AOAs.
  • Strakes have been evaluated on various "slender bodies at high AOA" that happen to end up with asymmetric shedding of vortex structures coming off the nose cone, that cause nose slice and consequent roll off at very high AOA.
The downside with strakes is they can be overly effective in lateral directional stability, and that gets to be a problem with normal control authority. While they can reduce drag, particularly the lower angled types, for longitudinal stability they will mostly interact with the stabiliser when it is a variable stab, and can have an adverse drag count change.
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