Originally Posted by
PEI_3721
fdr, thank you for the extensive background.
Re 'angle grinder' fixes. One of the first western public conferences where TsAGI presented a paper on high alpha aerodynamics. - In answer to a question on their approach to strakes re nose slice, why some aircraft could perform 'cobra' manoeuvres (SU27) vs those requiring extensive theoretical development (F18); 'if a particular aircraft had a problem then change the nose cone until one was found which did not roll off'. A lesson in practicality.
EASA neatly concludes the certification need for MCAS:-
"MCAS has been established to play only a limited role in augmenting the stability and stall characteristics of the aircraft in certain conditions. … needed to ensure the stability margins that make the aircraft fully compliant to the applicable regulations on stall demonstration and pitch control characteristics. This explains its inclusion in the original 737 MAX design.
These stability margins are required by regulation in order to support the flight crew handling of the aircraft during certain manoeuvres such as approach to stall …
MCAS was needed to provide full compliance but also that the loss of this function does not preclude the safe flight and landing of the aircraft; i.e. the 737 MAX remains stable following the loss of the MCAS function."
Boeing 737 MAX Return to Service Report
https://www.easa.europa.eu/sites/def...ice_Report.pdf
AIAA's JoA had every 2nd article in the early 90s being CFD of the snout of the F/A-18, working on the flow instability at high alpha, about 1:10 of the remainder were dealing with the vortex interaction with the verticals tab... and the subsequent structural loads issues. Back to basics, fluids don't like having to make up their collective minds as to where to separate from a body. a crossflow over a pretty cigar nose is going to give instability in shedding, and shedding is bad on jumpers from cats and dogs, and from noses of aircraft at high alpha, as in the latter case, it generates random force vectors. In the former case, it affects allergies and makes for a messy wash.
The 16 was more stablerer, the nose is not as near to circular. The F-35 nose is much more stable, but even there, it could have done away with the short radius upper curve of the cross-section, which would have given more lift at the nose which could be an issue for the pitching moment but would be extremely stable at high alpha. Pyotr Ufimtsev's work may have led to a preference for roundy upper shoulders when run through better supercomputers, otherwise, the 117 style edge would work well, upside down. fluids don't care that much about the front bits they are very interested in the rear area. That point which is pretty basic seems to be lost in almost all car design, up through F-1, and to an extent in lots of light aircraft design,
[Funny thing is we have smart design of stealth structures and we use dumb designs of anechoics]
PS: F-15 nose slicing was also an issue identified in flight, similar deal to -18, 16 was a little better, flow control on the 16 was related to advanced manoeuvering more than stopping an instability, but they are similar in nature and the cures are similar too. jet blowing avoids the impact on target RCS but is a pain to maintain.
PPS: the fact that NASA has an F18 as the HARV probably added some emphasis to it being the poster child for CFD in the early 90s, and the cracking of the vertical stabs that started to become expensive, along with the tail bits from the vortex interaction from the LEX. neat fix for the classic F18s, cleaner with the E/Fs, but it took away the charm of the design.