I think what he's trying to say is that you can believe whatever Ted and/or Chris tells you about flying high-performance passenger aircraft.
Everything else is B.S.
Ah, Mike and Bob. Not B.S., just discussion. By definition, the guys on SH are fascinated by flying things and have one degree or another of knowledge about the subject. Not all have the same degree but none of us have all the answers (as Mr. Rush is often pleased to point out). A forum is for discussion out of which, from time to time, wisdom will surface and other times not. I note that, generally, somebody will note that guys with a bit more experience might add more to the discussion which is a tacet acknowledgement that they're mostly aware that the discussion to date might not have provided all the answers.
I love it when these discussions take place and often sit back and see if somebody doesn't eventually hit the nail on the head. There were a few near misses in this thread, each of which added a bit of value to the mix.
Ted
p.s. Here's a bit of observable info on the subject of air carrier "pitch trim settings" which those of you living near airports can check out which will help explain the problem with Denzel's heroic efforts in the film.
If you can get to an observation spot near the end of a runway where jetliners are starting their takeoff roll take a close look at the horizontal stabilizers as they take the runway. For the most part Boeing airplane will have the stabilizer set with the leading edge well below the trailing edge, sort of like giving up elevator to a flying tail on a combat ship. (big jets trim pitch by changing the angle of incidence on the stab, not by using trim tabs on the elevators).
The exact angle at which the stabilizer is set is computed for each takeoff and, if correct, will result in an initial climb attitude that requires no elevator input and delivers an indicated airspeed of "V2+10 knots" (V2 being one of three speeds computed for each takeoff considering aircraft weight, density altitude, runway length, flap setting and so forth. The other two speeds are V1 (the "go"/"no go" speed to either safely reject the takeoff on the remaining runway or, if above that speed, continue the takeoff following an engine failure and cross the far end of the runway at a minimum of 35 feet and meet all the angle of climb requirements for terrain, etc. while climbing out at V2+10K. The third speed is Vr, the speed at which the flying pilot rotates the nose to break ground and establish the climb) Sorry, TMI and not exactly pertinent to the discussion.
This significant Nose Up trim is required by the Boeing jets because they are designed so as to require significant download on the tail to maintain an "in trim" condition for any given speed and/or vertical flight path--i.e. climb/level flight/descent. The download is required because the Center of Gravity is located forward of the Center of where the lift to support it is generated. (By the way if this is starting to sound familiar it's part and parcel of the discussion of aft CGs with stunt ships that has been a subject of great interest and discussion in the last 15 or 20 years.)
You can see the effects of this CG/CL relationship when watching Boeing jets on final approach (ideally inside five miles or so from the end of the runway when they are stabilized at approach speed with landing flap settings). Boeings will be in a noticeably nose high attitude and with high thrust settings in this configuration and the stab setting will be close to the maximum leading edge down (nose up trim) setting to maintain this attitude with no pilot elevator input.
To get a mental feel for what I'm talking about here, imagine adding more and more weight to the nose of your stunt ship. At some point in time the CG will be so far ahead of where the lift to support it is generated that it will take all the up elevator available just to fly level and maneuvering will not be an available option. The Boeing operates in that direction but, of course, not to that extreme.
By the way, if you look closely at the Boeings as they roll onto the runway you'll see the "track" in which the mechanism drives the leading edge of the stab up and down to adjust trim. You'll easily see that there is lots of "nose up" movement available but very little nose down (A great amount of "nose down" trim would be required to even pretend to fly inverted in the Boeing and is simply not available to the best of my guess!) The overall design concept of the Boeing is to minimize download required in cruise with the flaps up and at high speed to minimize SFC (Specific Fuel Consumption) while sacrificing any such fuel efficiency during slow speed regimes.
(A caveat, I'm uncertain about the CG/CL relationships on late model Boeings like the 777 and 787. They could well be more like the Airbusses which I'll touch on next)
Late model airbuses are substantially different in this regard. A large part of their popularity with airlines is the design concept that more approximates the aft CG stunt ship. Tail authority (and computerized control inputs for stabilitly and maneuvering) that allow aft CGs which even further minimize the need for tail download which reduces SFC to an even greater degree (the single biggest selling factor to airlines is, understandably, how much gas the jets will burn for each butt in the seat mile).
Airbuses on short final will tend to have "flatter" body angles and not require as much thrust (as a for instance, a Boeing 727 with full flaps at approach speed required on the order of 80% of maximum N1 (forward rotor speed) just to maintain a three degree glide slope. For another "recent stunt dialogue" comparison that high thrust is also a result of the great drag and negative pitching moment from the huge triple slotted flaps! They're all airplanes...some's just bigger than others!)
If you watch the Airbuses taxi out you'll notice a distinctly different stab trim setting situation. The stab angle of incidence (its angle with respect to the fuselage) will be much closer to zero/zero with respect to the wing (that relationship, wing angle of incidence in relation to stab angle of incidence is sometimes termed decalage although I understand that term is not universally embraced by engineers...not sure why).
The bottom line of all this with respect to Denzel's wonderful adventure is that Boeings (and probably DC9s, although I've no personal experience with them) are extremely ill suited to negative g (inverted) flight as the combined elevator and trim forces necessary to maintain the necessary high angle of attack inverted are almost certainly not available. The Airbus model might have a modestly better potential but I wouldn't want to be the test pilot.d\
Hope this didn't put you all to sleep.
Ted
Topic: Real Aircraft question (Read 11727 times)