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What's "Mach Tuck" On Biz Jets?  
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Posted (12 years 9 months 3 days 5 hours ago) and read 9648 times:

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"
10 replies: All unread, jump to last
 
User currently offlineIvo21 From Netherlands, joined Jan 2002, 25 posts, RR: 0
Reply 1, posted (12 years 9 months 3 days 5 hours ago) and read 9597 times:

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


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 2, posted (12 years 9 months 3 days 4 hours ago) and read 9574 times:

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"
User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 3, posted (12 years 9 months 3 days 4 hours ago) and read 9569 times:

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.

User currently offlineMD11nut From , joined Dec 1969, posts, RR:
Reply 4, posted (12 years 9 months 3 days 3 hours ago) and read 9555 times:

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


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 5, posted (12 years 9 months 3 days 3 hours ago) and read 9538 times:

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"
User currently offlineOldman From United States of America, joined Sep 2006, 0 posts, RR: 0
Reply 6, posted (12 years 9 months 3 days 2 hours ago) and read 9532 times:

Capt. Seagull is correct, have you ever heard of a mach trim compensator? Regards, Oldman

User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 7, posted (12 years 9 months 3 days 1 hour ago) and read 9509 times:

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.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 8, posted (12 years 9 months 2 days 23 hours ago) and read 9501 times:

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.

User currently offlineSkyguy11 From , joined Dec 1969, posts, RR:
Reply 9, posted (12 years 9 months 2 days 23 hours ago) and read 9496 times:

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.


User currently offlineTimT From United States of America, joined Jun 2001, 168 posts, RR: 0
Reply 10, posted (12 years 8 months 3 weeks 5 days 19 hours ago) and read 9429 times:

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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