Mach Tuck
 

Hello everyone!

Can anybody enlighten me as to the exact meaning of "mach tuck". I heard it mentioned in conversation and cant seem to find a reference to it in my AT + G books.

Thanks in advance to anyone who takes the time to reply.


:confused:

A nose down trim change as speed is increased, caused by Mach number effects.

The magnitude of the effect will depend on the design.

It may be quite sharp edged.

Mach tuck or mach tuck under is the phenomenon whereby some aircraft tend to dive suddenly as they accelerate through the transonic speed range.

When airflow over the wings reaches the local speed of sound, shock waves form. Air flowing through these shock waves experiences a sudden increase in static pressure. This sudden pressure increase causes and equally sudden deceleration which tends to cause the airflow to separate from the wing just behind the shockwave. The overall effect is a loss of lift and an increase in drag, similar to that caused by ordinary low speed aerodynamic stall. For this reason the effect is called shock stall.

The shock waves form wherever the airflow reaches the local speed of sound, so they form first at the wing roots where the thicker aerofoil section produces the greatest acceleration. This means that the shock stall affects the wing root area before it affects the wing tips. In swept back wings, the wing roots are ahead of the wing tips, so the loss of lift due to shock stall at the wing roots causes the centre of pressure to move rearwards. this causes the aircraft to pitch nose down in the mach tuck under.

With straight wings the effect is slightly different. Shock stall reduces lift behind the shock waves so the rear area of the wing is most affected. This means that the C of P initially moves forward, causing a tendency to pitch nose up. But as speed increases further, the shock waves move rearwards causing the the C of P of the wings to move rearwards to about the 50% chord point. It is this rearward movement of the C of P which causes straight wings to tuck under.

The overall effect in both cases is that the aircraft tends to pitch nose down as it accelerates through the transonic speed range.
The mach trim system counteracts this tendency by adjusting the longitudinal trim of the aircraft.

Possibly the simplest way to picture the story is

(a) in subsonic flow the CP typically is somewhere near 0.25MAC

(b) for supersonic flow read 0.50MAC

(c) transonically the situation with shockwaves is rather dynamic.

Overall, if the CG remains constant, as the CP moves aft .. albeit not necessarily steadily, there will be an increasing nosedown couple leading to a nosedown pitching moment.

Depending on the design of the aircraft, what is happening on the tailplane so far as shocks, and pressures are concerned might be having a significant effect as well ...

I can add that FAR25 says you need 1 pund of yoke pressure (minimum) for a 6 knot speed change (I think it is). On aircraft with less force required, a Mach Trim is required. Hand flying at high altitude is not easy, but it doesn't make it easier with the mach trim trimming all the time.

As has been stipulated earlier ,as the aircraft acc'the nose has a tendency to pitch slowly down.Two types in mind which demonstrated these tendencie- the DC8 and the Learjet.
On the DC8 above the speed regime of .825 the Pitch Trim Compenstor used to extend(an electric motor would drive the elevator up to offset the down pitch).The aircraft was unstable slippery) in these conditions,and the PTC was to prclude upsets.
Once one had the PTC activate one just reduced thrust until the aircraft had stabalized.This is described indetail in the NTSB reports of DC8 upsets(Eastern,AirCanada)..
The Lear was limited to M.78,but operators pulled the restrictive C/breaker and cruised faster(82?).This led to some upsets,as the Auto pilot fought the trim.Once the auto pilot let go the a/craft was out of trim for an immediate recovery.Again covered by the NTSB reports:rolleyes:

As an aside to this topic. Before Chuck Yeager broke the sound barrier in the Bell X-1. In fact just after WW11, tests were done in the UK and US with Piston Fighters ie. Spits and alike.

These tests were usually controlled high speed dives. During some of these flights Mach Tuck was experienced and unfortunately some of the test Airmen and Women lost their lives..
The centre of pressure as John Tullamrine says, moved rearwood on the Tail Plane rendering it useless due the shock obstruction of airflow over the elevators. Saw this on the box one night. I stand to be corrected ofcourse.

So, when the B737 Mach trim compensator kicks in at just above M .60, there are actually shock waves forming over the wings already or..? I thought civilian aircraft's wings were designed for subsonic operation only?

Extracts from JAR 25 - what is described here is what I have heard is the main reason for mach trims - there must be at least one pound of control wheel force to change the speed by 6 knots, i.e. pitch up to reduce speed 6 knots takes one pound.

If it takes LESS, one needs a mach trim to compensate for it.




STABILITY
JAR 25.171 General
The aeroplane must be longitudinally,
directionally k d laterally stable in accordance
with the provisions of JAR 25.173 to 25.177. In
addition, suitable stability and control feel (static
stability) is required in any condition normally
encountered in service, if flight tests show it is
necessary for safe operation.
JAR 25.173 Static longitudinal stability
Under the conditions specified in JAR 25.175,
the characteristics of the elevator control forces
(including friction) must be as follows:
A pull must be required to obtain and
maintain speeds below the specified trim speed,
and a push must be required to obtain and
maintain speeds above the specified trim speed.
(a)

This must be shown at any speed that can be
obtained except speeds higher than the landing
gear or wing flap operating limit speeds or
V„/MFc, whichever is appropriate, or lower than
the minimum speed for steady unstalled flight.
The airspeed must return to within 10%
of the original trim speed for the climb, approach
and landing conditions specified in JAR 25.175
(a), (c) and (d), and must return to within 75% of
the original trim speed for the cruising condition
specified in JAR 25.175 (b), when the control
force is slowly released from any speed within the
range specified in sub-paragraph (a) of this
paragraph.
The average gradient of the stable slope
of the stick force versus speed curve may not be
less than 1 pound for each 6 knots. (See ACJ
25.1 73 (c).)
(d) Within the free return speed range
specified in sub-paragraph (b) of this paragraph,
it is permissible for the aeroplane, without control
forces, to stabilise on speeds above or below the
desired trim speeds if exceptional attention on the
part of the pilot is not required to return to and
maintain the desired trim speed and altitude.




ACJ 25.173(c)
Static Longitudinal Stability (Interpretative Material)
See JAR 25.173(c)
The average gradient is taken over each half of the speed range between 0.85 and 1.15 Vtrim.





When manoeuvring at a constant airspeed
or Mach number (up to V„/M„), the stick forces
and the gradient of the stick force versus
manoeuvring load factor must lie within
satisfactory limits. The stick forces must not be so
great as to make excessive demands on the pilot's
strength when manoeuvring the aeroplane (see
ACJ No. 1 to JAR 25.143 (f)), and must not be so
low that the aeroplane can easily be overstressed
inadvertently. Changes of gradient that occur with
changes of load factor must not cause undue
difficulty in maintaining control of the aeroplane,
and local gradients must not be so low as to result
( f )
in a danger of over-controlling. (See ACJ No. 2 to
JAR 25.143 (0). 1
'[(g) (See ACJ 25.143(g)). The manoeuvring
capabilities in a constant speed coordinated turn
at forward centre of gravity, as specified in the
following table, must be free of stall warning or
other characteristics that might interfere with
normal manoeuvring1