Can anyone simply explain mack tuck?

[sparkman]

If anyone would be so kind as to explain Mach tuck it would be greatly appreciated. I understand that a pressure wave builds up on the wing as some of the airflow reaches Mach crit but how does this cause a nose down result?

[Keith.Williams.]

If you do a search for the words mach tuck under you will find numerous explanations.

[Neptunus Rex]

It's all to do with the aft movement of the Centre of Pressure with increasing M. That is why Concorde shifted fuel into an aft trim tank, so that the C of G would keep pace with the movement of the C of P.

Draw a simple Lift / Weight Thrust / Drag vector diagram, then move the Lift vector aft, and you will see a resultant nose-down pitch.

[Machinbird]

Simplistically, subsonic, your lift is generated by the wing and control surfaces (tail) and effectively generates a constant moment about the wing ~25% MAC (mean aerodynamic chord.) Supersonic, everything in the breeze generates lift based on surface area and AOA (including the fuselage.) The wing now generates its lift around a 50% MAC point which is considerably further aft than 25% MAC and thus there isn't enough down force from the tail at its subsonic position to keep the nose from falling through. If you trim the whole horizontal stabilizer to bring the pitch moments back in balance and succeed in bringing up the nose and the aircraft then decelerates to subsonic, there will then be a pitch up effect.

[sparkman]

Many thanks.

[grounded27]

A real basic way of putting it is the wing quits flying as the center of lift moves behind it while the tail continues to fly.

[HarryMann]

A real basic way of putting it is the wing quits flying as the center of lift moves behind it while the tail continues to fly.

So basic its untrue!

[Willit Run]

Harry Mann,
That was the best laugh i've had in a long time!!

lets get a beer!

[411A]

Quote:
A real basic way of putting it is the wing quits flying as the center of lift moves behind it while the tail continues to fly.

So basic its untrue!

Not only untrue...it's total rubbish.

In the 'old days' of the NACA, much work was done with various wing designs/planforms, and although mach tuck was a bit of a mystery at that time, it was certainly recognised that all of these wings had a 'forward/downward turning moment' and the faster the wing was accelerated, the greater the effect became.
In fact, some piston transports had a 'mach number limitation', and...in the case of the DC-7, that limitation is M.585

[SinglePilotCaptain]

As the center of pressure moves aft.....there is a nose down tendency....


Mach tuck - Wikipedia, the free encyclopedia

[Newforest2]

I was looking for a story about Mack trucks.

[A37575]

When the single seat Vampire Mk 30 with a Goblin engine was upgraded to the Mk 31 with the more powerful RR Nene engine, a pair of additional air intakes were installed behind the canopy on top of the engine. They were known as "Elephants Ears" The wing root intakes were retained.

During Vampire conversion training at No 2 Fighter Operational Training Unit at Williamtown NSW, there were three fatal accidents I think in the very early Fifties when the Vampires concerned went into Mach tuck of sorts but known then as compressibility. The cause was shock waves forming over the air-intakes at around 0.78 mach number and seriously affecting airflow over the tail surfaces. One accident occurred during a practice quarter attack where one Vampire rolls in and attacks another Vampire flying straight and level as a camera gun target. I think the aircraft were around 15,000 ft.

In this case, the attacker dived towards the target aircraft but got into compressibility. The pilot reported on radio that his Vampire was vertical and the control column had no effect. Although the air was thicker at low altitude where the mach number is less, the pilot ran out of altitude to pull out of his dive and went in vertically still trying to pull out. The early Mk 31 Vampires did not have ejection seats and the speed was too great for the pilot to bale out.

Also before or just after this accident, two more Vampire Mk 31 crashed almost in formation while in compressibility. They had been conducting formation aerobatics where the technique was to extend the dive brakes and close the throttle as the aircraft passed through the top of the loop. This was to prevent a dangerous increase in speed during the recovery dive.

However, if I recall correctly, the lead Vampire did not extend its dive brakes and thus quickly accelerated during the recovery from the loop. The second Vampire held good formation but failed to realise the leader had left the dive brakes until too late. Both aircraft entered compressibility during the dive and were vertical at 10,000 ft with insufficient height to pull out.

In each case the control column becomes virtually useless until the aircraft reaches thicker air and the mach number drops off and so do the shock waves over the engine intakes. Again, with no ejection seats there was no hope of baling out. In all cases the pilots were able to transmit what was happening. All the pilots were inexperienced with around 400 hours total flying time each.

Whether these accidents can be put down to mach tuck I don't know because the cause was shock waves over the newly installed intakes but the effect was certainly the same. After these prangs, the Australian test pilot "Blackjack" Walker was tasked to explore the problem with the air intakes.

In his book Black Jack, he explained (edited for brevity): "After we had delivered half a dozen Vampires to the Air Force and they appeared to be functioning quite well from the RAAF station at Williamtown, two Vampires after flying in formation went into the ground in an almost vertical dive. Apparently they never even looked like recovering.. So De Havilland, to whom the matter had been referred, told me. "You, Mr Test Pilot, had better take the next Vampire up and see what happened to those two aeroplanes"

Blackjack Walker continued his story: "Well do I remember that flight. I knew it must have something to do with compressibility. I took the Vampire to well over 40,000 feet and put it into a steep dive. Once the aircraft went over Mach 0.80, the nose got heavier and heavier and the aeroplane kept endeavouring to go past the vertical and the controls were largely ineffective.

So I closed the throttle and put on the dive brakes immediately and this would have been somewhere around 27,000 ft. The aeroplane obviously had to be got out of a very sticky situation. As it got into the lower denser atmosphere, the dive brakes started to slow it up. I was even thinking of throwing the undercarriage out which would probably have destroyed the fairings - but anything to slow it up. Otherwise I would not come back with any answers.

We eventually came out of that dive at about 13,000 ft. It was pretty close and as the characteristics were so different to the English Vampire at high mach numbers it seemed to me it must have something to do with those wretched Elephants Ears on the upper surface of the fuselage.

We redesigned the intakes and placed them on the bottom of the fuselage. I took the aircraft to beyond 40,000 ft an put it into a dive of about 75 degrees. This time instead of ducking its nose down, to my relief once I reached Mach 0.8 the nose began to rise. This was in 1951. After that ejection seats were fitted"

............................................................ ................................

As a matter of personal interest this scribe flew the Mk 31 Vampires during fighter training. Session No 2 was to take the Vampire to high altitude and carry out what was called a Mach Run, in order to experience the onset of compressibility or mach tuck. Of course we didn't quite know what to expect although we had been briefed about the previous accidents. At this stage the modifications to the intakes had not reached our squadron, so the risk of mishandling was still there. Since we wouldn't have a clue if the first nibble of compressibility was there, an instructor in another Vampire would fly in close formation with us. He would stick on our wing tip as we commenced the dive towards compressibility. As the instructor felt the onset of compressibility and the control column losing effectiveness in his own aircraft he would order the other Vampire to close the throttle and extend the dive brakes before things got too dangerous.

Later, after the modded Vampires arrived on the scene we would delight in climbing to height then rolling inverted and pulling through past the vertical and passing mach 0.75 or thereabouts, the Vampire would pull sharply out of the dive on its own accord without action by the pilot instead of the old problem of bunting over into a lethal situation.

The modded Vampires were fitted with Martin-Baker ejection seats, which was a relief. The only problem with these ejection seats was that tall long legged pilots were in danger of losing their knees during the ejection sequence because of the lack of space in the cockpit. Being vertically challenged (as I was), was thus considered a Good Thing instead of being a normal object of amused contempt

[Microburst2002]

Thank you very much for that!


I understand it was a case of compressibility effects such that rendered control surfaces ineffective?

I wonder if in that situation (a steeper and steeper dive with nose heavy air
plane) you would be able to roll the wings and achieve inverted flight so that the nose heavy tendency would then tend to reduce the pitch. Not much time to do that from 15,000 at Mach .80, anyway...

Many phenomena have been discovered after a few good pilots died, specially in the old times. Maybe there are still a few traps set by nature awaiting us. Not many, I deem. At least not as many as the traps set by computers and software, of which we still have many to find... By fallin in them.

[gunshy67]

Mach Tuck

Everything A375....stated is correct. And also the Wikipedia has the aerodynamics.

The shock wave desensitizing the tailplane and eliminating the download on the tailplane it was inevitable causing a dive to become steeper.

This characteristic gave rise to the high set tail planes ...out of the turbulent (stalled) flow and produced the design described as "Mach Trim".

Mach Trim was a built in program that altered the tailplane automatically to provide greater tailplane (Tail plane plus elevator) power and provide conventional reaction to control inputs.

These days it is all done by computers sensing speed, altitude, temperature control input, pilot alcoholic content, colour of eyes, bald or haired.........LOL.

[Microburst2002]

I think it is a change in center of pressure, backwards, due to the shockwave when flying at high transonic speeds. This creates a pitch down moment which is unstable, since more pitch down, more speed, more mach, more shockwave, more pitch down moment... You could quickly go well into very dangerous airspeed leading to any number of undesirable compressibility effects, aileron reversal, fluttering, etc... and most likely structural failure.

Some airplanes are more prone to this effect, and have to augment their stability by means of a Mach trim compensator. I think the MD 80 family have them. I don't remember if that tendency has something to do with the tailplane, which is the cause of another unstable effect, the pitch up effect with can lead to the "superstall" and has a stability augmenting device of its own, the stick pusher.

[Keith.Williams.]

As an aircraft accelerates through the transonic speed range, shock waves form on the upper and lower surfaces of its wings.

A shock wave is a sudden increase in static pressure. As the air flows through the shock waves, this higher pressure compresses it. This sudden pressure increase causes an equally sudden decrease in the velocity of the air. This in turn causes the boundary layer to separate and become turbulent. The separation of the boundary layer causes a large reduction in lift on the area of wing directly behind the shock waves. The turbulence caused by this separated airflow also cause buffeting.

The separation of the boundary layer causes a large reduction in lift and a large increase in drag on the area of wing directly behind the shock waves.

The loss of lift over the rear area of the wing initially causes the Centre of Pressure of that section of the wing to move forward. In the case of straight winged aircraft this will tend to cause the aircraft to pitch nose up.

If the aircraft continues to accelerate, a second shock wave forms under the wing and both of these shock waves gradually move aft to the trailing edge. This causes the Centre of Pressure of that section of the wing to move aft. This aft movement of the Centre of Pressure tends to cause the aircraft to pitch nose down.

The shock waves form first at the points at which airflow velocity is greatest. This occurs at the wing roots because this is the area where the thickness of the wing and hence acceleration over it is greatest. This means that the shock-induced separation and loss of lift will occur on the upper surface at the wing roots.

In the case of swept-back wings, the roots are ahead of the outer wing and wing tips, so the initial loss of lift is on the forward part of the wing area. This causes the overall centre of pressure of the whole wing to suddenly move aft, causing the aircraft to pitch nose down.

Prior to the onset of shock waves, the lift generated at the wing roots will produce downwash, some of which will flow over the tailplane. This increases the down force generated by the tailplane and in so doing contributes to the correct trimming of the aircraft. But the sudden loss of lift caused by the formation of shockwaves on the wing roots greatly reduces this downwash. This in turn reduces the down force generated by the tailplane, thereby upsetting the trim of the aircraft. The combined effects of the rearward movement of the C of P and the loss of downwash over the tailplane cause the aircraft to pitch nose down.

[A37575]

We eventually came out of that dive at about 13,000 ft. It was pretty close and as the characteristics were so different to the English Vampire at high mach numbers it seemed to me it must have something to do with those wretched Elephants Ears on the upper surface of the fuselage.

We redesigned the intakes and placed them on the bottom of the fuselage. I took the aircraft to beyond 40,000 ft an put it into a dive of about 75 degrees. This time instead of ducking its nose down, to my relief once I reached Mach 0.8 the nose began to rise. This was in 1951.

Apologies for using my own quote but could someone explain why the Vampire "Elephant Ears" being now placed beneath the fuselage instead of on top, would cause the aircraft to pitch up quite strongly at critical mach without any apparent action on the part of the pilot? It was never explained to us at the time. Mind you, memories fade somewhat after sixty years..

[Mad (Flt) Scientist]

Apologies for using my own quote but could someone explain why the Vampire "Elephant Ears" being now placed beneath the fuselage instead of on top, would cause the aircraft to pitch up quite strongly at critical mach without any apparent action on the part of the pilot? It was never explained to us at the time. Mind you, memories fade somewhat after sixty years..

Purely speculation here ...

The characteristics described for the "ears on top" scenario sounds a lot like the tailplane being blanked by a large amount of separated flow off the upper fuselage, that separateion being triggered by the ears location at a position where the cross-sectional area was high and thus susceptible to early formation of shocks. So rather than a direct effect due to changing the lift distribution on the fuselage, it's a secondary effect through action on the flow around the tailplane. (If it was a change to the fuselage lift distribution, you might have expected a pitch UP, as described in Keith's paras 2 through 4 for the first shock on the upper surface of a wing (with the fuselage considered a crude aerofoil).

I'm going to speculate that the ears-under configuration still had a shock forming, but this time of course with separation below the fuselage, and perhaps this iduced a bit of additional downwash on the tail (the upper surface flow maybe being 'pulled' down into the low presuure in the seperated flow?) If so, that additional downwash would cause a pitch up.

Pure speculation - it'd take access to test data to provide the correct explanation, assuming it ever existed...

[Escape Path]

Keith.Williams.

Great post, sir. It helped me to illustrate the whole thing in my head with a language that every pilot can easily understand.

[HarryMann]

Keith

Enjoyed that wee treatise immensely