Timeline for Why do trijets (3 rear engines) usually have a T-tail instead of a normal tail?
Current License: CC BY-SA 3.0
11 events
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| Apr 13, 2017 at 12:59 | history | edited | CommunityBot |
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| Aug 15, 2015 at 15:27 | comment | added | Jan Hudec | @DrZ214: Due to the engines getting in the way a conventional elevator would have to be placed on them and that is quite a bit further forward from the green line you drew. | |
| Aug 15, 2015 at 6:42 | history | edited | Peter Kämpf | CC BY-SA 3.0 |
added 4 characters in body
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| Jul 26, 2015 at 6:49 | comment | added | DrZ214 | I checked out the Boeing 747 here: en.wikipedia.org/wiki/File:B747FAMILYv1.0.png and the A380 here: en.wikipedia.org/wiki/… Looks to me like it's about right. The back end of my green line doesn't have to be attached to fuselage; it just has to be the outer tip of the swept stabilizer. | |
| Jul 26, 2015 at 6:34 | comment | added | Peter Kämpf | @DrZ214: The wing is swept and produces lift over most of its span. The center of mass looks right. The green tail is really way back - look where Airbus and Boeing place the horizontal. There needs to be some fuselage structure left to attach it to. That was the reason I asked for a realistic horizontal tail. It will be sufficient to look at the horizontal component of the distance - only the part orthogonal to lift counts. | |
| Jul 26, 2015 at 6:16 | comment | added | DrZ214 | I did the calculation for Tu-154 and it looks like its T-tail provides a lever arm 1.383 times longer. i.sstatic.net/k98CB.png Also, are you sure the Center of Mass is that far back in the Tu-154? I thought CoMs had to be ahead of the Center of Pressure to be stable, and it looks like the last 1/3rd of the wing chord is pretty far back there. | |
| Jul 26, 2015 at 5:50 | comment | added | Peter Kämpf | @DrZ214: How did I imply that you measure in the side view? I did not. Look at the top view. The center of mass is at the last third of the wing root chord. Draw a realistic horizontal tail at the rear fuselage, and repeat at the tip of the swept vertical tail. Again, measure the difference. It might not be exactly double, but the change in lever arm should be significant. | |
| Jul 26, 2015 at 5:22 | comment | added | DrZ214 | No because again, you are not measuring between the control surface and center of mass, which is somewhere very near the center of the fuselage. While flying, this is the lever arm for elevators. Instead you are picking some other distance, like top of the tail to the bottom. But the pivot point is not the aft of the plane. The pivot point (while flying) is the center of mass, and your lever arm must be measured from there. | |
| Jul 26, 2015 at 5:09 | comment | added | Peter Kämpf | @DrZ214: 1) The middle engine of the Tu-154 is at the very end of the fuselage. Don't confuse the intake with the engine! 2) It's about vertical tail sweep. Put the horizontal tail at the bottom of the vertical. Now put it on the top. Measure the difference. Can you now agree? | |
| Jul 25, 2015 at 21:23 | comment | added | DrZ214 | I like your comment about engine failure possibly damaging a nearby conventional tail, but the T-154 would seem to shoot itself in the foot by this reasoning, since an uncontained engine failure in its middle engine could disable the entire tail. Also, I can't agree with "its lever arm is doubled". Lever arm is measured from the elevators to the center of mass, or if you're talking about takeoff V1, from the elevators to the wheels. Sweeping the horizontal stabilizer back such that the elevators are a little more rearward does not double the distance to either one. | |
| Jul 25, 2015 at 21:12 | history | answered | Peter Kämpf | CC BY-SA 3.0 |
