In subsonic flight, the center of lift is about 1/4 cord (wing width) aft of the leading edge. The center of mass must be forward of this point for static stability and the horizontal tail must "lift" down. In supersonic flight, the center of lift moves AFT to about 1/2 cord which requires a lot more "downward" lift on the horizontal tail to maintain level flight. If the horizontal tail cannot produce the required down force, the aircraft will pitch nose down. What happens next depends on several factors.
All older subsonic aircraft had fixed tails with movable elevators to control pitch. This worked because the pressure from the elevator propagated forward and acted on the fixed tail surface too. But supersonic, the airplane moves faster than the pressure can migrate forward causing major loss of elevator effectiveness. And if flow also separates, it can cause total loss of elevator effectiveness. Yeager's X-1 had a horizontal tail where the entire surface could be moved via the trim system, which is the only reason he could control the aircraft supersonic; he flew it using trim for pitch control If you look at every supersonic aircraft today, you'll see that the entire horizontal tail moves under hydraulic power, not just a small elevator at the trailing edge. -----Original Message----- From: mercedes-boun...@okiebenz.com [mailto:mercedes-boun...@okiebenz.com] On Behalf Of G Mann Sent: Thursday, February 16, 2012 4:03 PM To: Mercedes Discussion List Subject: Re: [MBZ] OT: airplane footage No worries... Just relaying what I learned in 40 + yrs of flying high and fast. At Mach, different set of aerodynamics come into play. You should be able to find other supporting reference material on what happens to Aerodynamic Center of Lift at Mach pressure wave. One reason the P51 could never break Mach was the straight wing... after the swept wing was put into use Mach threshold was "useful" ..instead of distructive.. Mach Tuck is a normal term for exceeding the altitude and speed design envelope because the aircraft will pitch nose down and tumble in a forward roll. The center of lift moves ahead of the wing and elevator forces can not counter act that with sufficent force to keep nose up attitude... The aircraft does a literal "tuck and roll" at that point.... recovery is rare and violent... if you can imagine doing an end for end roll at 700 mph.. the G force alone is deadly, plus structure design limits are quickly exceeded.. http://en.wikipedia.org/wiki/Mach_tuck You may find this rather simplified explanation helpful.. there are more involved ones available should you choose to pursue it at the doctoral level of flight physics. Grant... On Thu, Feb 16, 2012 at 10:52 AM, Rich Thomas < richthomas79td...@constructivity.net> wrote: > Quite novel explication. Not sure I would agree as it all depends on the > configuration of the airplane, wing shape, tail design, etc. Most simply, > when lift is lost on the wing (could be due to Mach effects -- you can > stall an airplane at any airspeed) the airplane generally stalls and breaks > away and will generally behave badly. Add into that poor control > responsiveness due to messed-up airflow over control surfaces (again, could > be Mach effects) and your day just got more interesting. All that other > stuff is just complicating the understanding of the basic physical problem > and I'm not sure it is generally correct (no disrespect intended). > > It's all about airflow! > > --R > > On 2/16/12 11:49 AM, G Mann wrote: > >> Approaching Mach speed the aerodynamic center of lift [the balance point >> of >> the flying aircraft] shifts forward. Almost all executive jets are speed >> limited to 0.89 Mach because of this. As ACL shifts forward the tail >> surfaces must respond to that change in center of lift balance point. At >> some speed point the aircraft will go in to what is known as a "Mach tuck" >> where the center of lift is so far ahead of the wing the elevator surface >> can not counter balance the aircraft and it goes into a 700 MPH forward >> tumble and as you may guess airplanes are not stressed to do that, so it >> breaks up in flight. >> > > ______________________________**_________ > http://www.okiebenz.com > For new and used parts go to www.okiebenz.com > To search list archives http://www.okiebenz.com/**archive/<http://www.okiebenz.com/archive/> > > To Unsubscribe or change delivery options go to: > http://mail.okiebenz.com/**mailman/listinfo/mercedes_**okiebenz.com<http://m ail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com> > _______________________________________ http://www.okiebenz.com For new and used parts go to www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com _______________________________________ http://www.okiebenz.com For new and used parts go to www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
