Mr Spaceman
Topic Author
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What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 5:43 am

Hi guys.

I've read that pilots who fly business jets in the upper flight levels can experience a high altitude aerodynamic force called Mach Tuck. However I don't know what this term means. It sounds like it might have something to do with transonic flight and the formation of shock waves, but I'm not sure, and I don't know if this force is an advantage or disadvantage to the pilots.

Can someone please explain Mach Tuck?


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Chris  Smile
"Just a minute while I re-invent myself"
 
Ivo21
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 6:02 am

I don`t know for sure, but I think it has indeed something to do with transsonic flight. When a relatively small business jet reaches transsonic speed, the lift and thrust decreases. Therefor the aircraft will react by the bpilot in a nose up pitch. That`s your Mach Tuck, I think.
But of course I cannot say that for sure.

CUL8ER
 
Mr Spaceman
Topic Author
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 6:30 am

Hello Ivo21.

Thanks for your reply.

Well, just the word Tuck itself makes me think of something going down. In this case I believe it would be the pitch of the Biz Jet's nose. If a small jet's lift and thrust starts to decrease as it approaches Mach 1, the angle of attack will also decrease causing the pilots to react by raising the nose...as you mentioned.

I wonder if the concorde and fighter jets experience this at all? Or if they just punch right through Mach 1 without experiencing Mach Tuck?

Chris  Smile

"Just a minute while I re-invent myself"
 
seagull
Posts: 334
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 7:03 am

The center of lift moves aft as the aircraft approaches the speed of sound. This results in a pitch downwards. The reasons are not that complicated, but more than I feel like typing at the moment.
 
Guest

RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 7:23 am

You probably are talking about the TuckUnder effect. There are several factors causing it. The main one is that as airflow separation takes place, the downwash behind the wing is decreased. Prior to this, the horizontal stabilizer is trimmed to keep the airplane level. With this decrease in downwash, the stabilizer AOA, in effect, is increased thus pushing the tail up hence the pitch down.

There are two other factors. One is that as the shock waves move rearward, so is the center of pressure, adding to the tuckunder. The second is that, as Seagull said, is the aerodynamic center shifts rearward, adding to the Tuckunder effect. One interesting about the aerodynamic center shift is that in some aircraft, the aero center shift forward first (causing momentary pitch up) before eventually shifts rearward (pitch down). this was probably what CULBER was thinking about.

All aircraft flying supersonically suffer a nose down pitching moment.

Regards,
Nut
 
Mr Spaceman
Topic Author
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RE: Seagull

Sat Jan 26, 2002 7:29 am

Hello Seagull.

Thanks for your reply. I know what you mean by having to be in the mood for explaining aerodynamics. Perhaps if you notice that other members are not replying, you'll get back to me.

You mentioned that the center of lift moves aft, which I understand as being the center of pressure. It will be interesting to learn what the reasons for this are.

Take Care,

Chris  Smile
"Just a minute while I re-invent myself"
 
OldMan
Posts: 207
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Mach Tuck

Sat Jan 26, 2002 8:37 am

Capt. Seagull is correct, have you ever heard of a mach trim compensator? Regards, Oldman
 
seagull
Posts: 334
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 10:04 am

The problem with explaining this is that there is so much other information you really need to understand before you will understand the explanation.

I recommend you go to www.amazon.co.uk (note that the U.S. site will NOT work!) and purchase "Flightwise", both volumes, by Chris Carpenter, head of Aerodynamics at the Royal Air Force College.
 
411A
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 11:43 am

All airfoils have the tuck-under or turning moment effect, whether straight wing or swept, and this was discovered in the 1922-23 experiments by the NACA. It is however more pronounced on high speed sweep wing jets. The mach trim system on jets is certainly there for a purpose. If the system becomes inoperative enroute, many types require a speed reduction and/or manditory autopilot use.
 
SkyGuy11
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RE: What's "Mach Tuck" On Biz Jets?

Sat Jan 26, 2002 11:51 am

I don't understand this so I'm not going to try to answer, but I do have 1 tidbit of cool info:

During WWII, when some of the planes during combat and flight testing exceeded Mach 1 (yes it did happen, but not officially), the nosed picthed down uncontrollably. This unexplained phenomenon was later found to be the center of lift shifting backwards during the approach and breach of Mach 1.
.
 
TimT
Posts: 168
Joined: Sun Jun 03, 2001 1:38 pm

RE: What's "Mach Tuck" On Biz Jets?

Fri Feb 01, 2002 4:09 pm

The venerable DC-9 has a tendency toward 'mach-tuck" also. On the inboard side of the F/O's control column, there is an indicator that protrudes from it's housing when mach speeds are approched. As it does the column is pulled back to a nose up position. This prevents overspeeding the aircraft. As I see it, (and I'm not an engineer) the center of lift moves aft at higher speeds and could travel right off the trailing edge of the wing.
The DC-10 has an overspeed indicator too. It starts ticking at you. (Clicking sound from the master warning and annunciator)

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