confusion with the MACHcrit and Mach Trimmer

[aslan1982]

Do jets fly above MACHcrit?

Whats the relationship between MMO and MACHcrit?

using the 737 800 as an example

What effects the MACHcrit number? is it AOA and weight...

I know on the 737 800 the mach trimmer comes into effect automatically above
mach number .615
Is this because the aircraft will be cruising at, near or above the MACHcrit number.

If the MACH trimmer is U/S does this effect our cruising speed? Are we limited to a certain value below MACHcrit?

Im preparing for an interview and just wanna make sure its clear in my head so I can talk about it if asked.

[Microburst2002]

Mcrit is just that MN at which a part of the the airflow reaches speed of sound. Cruising Mach usually are above this Mcrit.
The adverse effects of shock waves, tuck under, aileron reversal, buffet etc... occur at faster speeds, after "divergence MN". At this MN, drag increases sharply because the shock waves are very intense.

I guess that if the Mach trim compensator is U/S, MMO will probably be reduced accordingly.

[aslan1982]

if an aircraft operates above MACHcrtit aren't the operating close to if not at coffin corner??

[flyjet787]

Mcrit/Critical Mach No. is the free stream mach number at which the local velocity (velocity of air over any part of the aircraft, mostly the wing) reaches sonic speed.

Critical Mach No. is affected by Thichness:chord ratio and by AOA. Therefore if your weight is increased you will be flying at a higher AOA which reduces the Critical Mach No.

I don't think aircrafts operate at or above M.crit because the drag rise with the formation of the shock wave at M.crit is huge. So I think a/c's fly close to the M.crit.

Coffin Corner is the altitude at which your lowspeed buffet (stall) and high speed buffet (M.crit) speeds are same. At this alt you can fly only at one speed if you decelerate you stall and if you accelerate you exceed M.crit.

[FE Hoppy]

Aircraft most certainly do fly around above Mcrit although some may be restricted to speeds at or below Mcrit when is Mach Trim is inop.

Why would a Mach trim system operate at speeds below Mcrit? It is fitted to pre-trim to compensate for the aft movement of CP due to the formation of the shockwave.

It's a pretty safe bet that Mcrit is at or close to the Mach number at which the Mach Trim becomes active.

Of course some aircraft happily fly around above Mcrit without a Mach trim system.

[aslan1982]

MACH TRIM is automatically applied above M0.615 (classics & NG's), M0.715 (-1/200) to the elevators to counteract this and to provide speed stability.

source: The Boeing 737 Technical Site (http://www.b737.org.uk/flightcontrols.htm#Rudder_Trim)

[aslan1982]

i found a post from smokey in 2004 - seems to know what he's on about

http://www.pprune.org/archive/index.php/t-131271.html

"
Jet aircraft have been routinely flying above Mcrit long before the super critical wing came along.

A large exceedance of Mcrit will yield a host of aerodynamic / control problems, including the onset of 1G 'Mach buffet'. The manufacturer obligingly places Mmo just below the threshold of aerodynamic problems, leaving a portion of the flight envelope between Mcrit and Mmo for normal operations - The problem then becomes one of additional drag due to the shock waves - wave drag.

Wave drag increases very slowly at first, up to about M0.05 above Mcrit, and then rises exponentially. The new drag curves (High speed polars) attributable to wave drag can be superimposed over the total drag curve (Low speed polars) and show some interesting properties.

Maximum Range Cruise (Cost Index zero for all you FMC users) is found by at the point where a straight line originating at 0/0 is tangential to the 'revised' drag curve. Due to the very shallow curve of the High Speed Polars for the first .04 or .05 above Mcrit, this 'Maximum Range Plot' invariable leads to Maximum Range Cruise just above Mcrit.

Consider Long Range Cruise, which by definition offers 99% of the economies of Maximum Range Cruise or 1% additional fuel burn per mile. The line is drawn from 0/0 at a 1% steeper gradient than that for the Maximum Range case, and has a low speed and a high speed intersection with the Total Drag curve. The higher plot (Long range Cruise) is at a point even further beyond Mcrit.

Econ cruise, where the total cost of airframe time (best at high speed) and fuel cost (best at Maximum Range Cruise) is at a minimum. In byegone times, when lower fuel costs prevailed, higher fuel useage was tolerable against relatively expensive airframe time. In the 'fuel expensive' modern era, Econ cruise usually falls between MRC and LRC.

All 3 of these cases indicate that normal cruise operations, even when aiming for maximum possible range, occurred at speeds above Mcrit. The only exception would be low altitude operations when Mcrit is not reached, but this is not the usual habitat for the jet aircraft.

The discussion above spoke of tolerable levels of drag in the cruise phase. Even small drag increases introduce penaltys in climb performance, and here, flight above Mcrit is avoided, or minimally tolerated. If you don't know thr 'generic' Mcrit for your aircraft, the climbing Mach Number will be a pretty good indicator.

Descent employs much the same tolerance of small amounts of Wave Drag as does cruise. Indeed, the rapid onset of drag for a small mach number increase above normal cruise speed will be most beneficial (and demonstrable in the simulator) when maximum possible rate of descent is desired during an emergency descent.

I hope that this helps to fill any remaining gaps in your understanding."