fdr
26th Feb 2018, 21:34
Back in the day, from the AP3456A and Aerodynamics for Naval Aviators, we would be briefed on the cause of Mach Tuck, and then go up to altitude and push the blender though to the experience of mach tuck. Was fun, and we didn't have much better to do, beyond drinking beer and getting into mischief that would end up with duty officer duty.
The cause of the tuck was attributed to a rapid movement forward of the shockwave at some point, which resulted in the trim change, and the consequent nose dropping. That worked for some 40 years in my day to day life.
While doing a workup for a flight test for clearance on FAR25.251(b), I had modelled for me a wing section in 3D of the mid outboard section of the design in question. We stepped the operation from relatively low Mach numbers through to Mdive, with an AoA altering to compensate for dV^2, so as to have a relatively constant total lift case (hey, its an approximation, good enough for a start...). The findings were interesting to me at least.
CL, CD and CM behaved more or less normally, until around limit Mach, at which point things got interesting. This point is well short of Mdive, and still short of the Mach for demonstration of compliance. At Mmo+0.02, Cl started to collapse, CD increased and CM started to march. This became more and more pronounced as Mach increased, resulting in approaching a CL=0 condition at M dive. looking at the pressure, temperature and mach plots of the section in 3D, there was nothing of interest on the upper surface, it was all happening on the lower surface. At Mmo +0.02, a strong shock formed on the lower side of the section, and increased in intensity as the mach increased.
For the purposes of the exercise, this observation did alter the manner we did the high speed tests, I elected to ensure that we did them in a manner that substantial thrust was resulting in the overspeed condition, so that in the event of reduction in CL, we were able to reduce speed by more than any pitch down would increase speed. In the end, the tests were not aggressive but were interesting as 251 is always.
The CFD modelling we undertook in DNS was repeated prior to test, and the AOA was modified to account for the non linear reduction in CL at higher speeds, and this mitigated to an extent the effect of the shock development and the reduction in CL/AOA. Ended up, that increasing AOA to maintain constant lift reduced the rate that the lower shockwave developed, and how that collapsed CL/AOA. indicated that a case where an open loop control occurred, where the nose was permitted to lower, and flight path angle lower, would result in a more marked mach tuck event.
I must have been asleep in aero 40 years ago, I cannot recall any comment on the change in CM being attributed to the loss of CL from the establishment of a strong normal shock on the lower surface of the foil. Is there a reference out there that covers the lower normal shock as a causation of tuck that I studiously avoided?
The cause of the tuck was attributed to a rapid movement forward of the shockwave at some point, which resulted in the trim change, and the consequent nose dropping. That worked for some 40 years in my day to day life.
While doing a workup for a flight test for clearance on FAR25.251(b), I had modelled for me a wing section in 3D of the mid outboard section of the design in question. We stepped the operation from relatively low Mach numbers through to Mdive, with an AoA altering to compensate for dV^2, so as to have a relatively constant total lift case (hey, its an approximation, good enough for a start...). The findings were interesting to me at least.
CL, CD and CM behaved more or less normally, until around limit Mach, at which point things got interesting. This point is well short of Mdive, and still short of the Mach for demonstration of compliance. At Mmo+0.02, Cl started to collapse, CD increased and CM started to march. This became more and more pronounced as Mach increased, resulting in approaching a CL=0 condition at M dive. looking at the pressure, temperature and mach plots of the section in 3D, there was nothing of interest on the upper surface, it was all happening on the lower surface. At Mmo +0.02, a strong shock formed on the lower side of the section, and increased in intensity as the mach increased.
For the purposes of the exercise, this observation did alter the manner we did the high speed tests, I elected to ensure that we did them in a manner that substantial thrust was resulting in the overspeed condition, so that in the event of reduction in CL, we were able to reduce speed by more than any pitch down would increase speed. In the end, the tests were not aggressive but were interesting as 251 is always.
The CFD modelling we undertook in DNS was repeated prior to test, and the AOA was modified to account for the non linear reduction in CL at higher speeds, and this mitigated to an extent the effect of the shock development and the reduction in CL/AOA. Ended up, that increasing AOA to maintain constant lift reduced the rate that the lower shockwave developed, and how that collapsed CL/AOA. indicated that a case where an open loop control occurred, where the nose was permitted to lower, and flight path angle lower, would result in a more marked mach tuck event.
I must have been asleep in aero 40 years ago, I cannot recall any comment on the change in CM being attributed to the loss of CL from the establishment of a strong normal shock on the lower surface of the foil. Is there a reference out there that covers the lower normal shock as a causation of tuck that I studiously avoided?