T Tail aircraft???

[MikeSF340]

What are some of the advantages and disadvantages of a T Tail in high speed jet aircraft? Seems wierd that some planes have one (727,MD80,CRJ, ERJ, ETC.) and some don't (777, 747, 757, ETC.) Thanks for the help.

Mike

 

[dav8or]

Think about where the engines are on those planes.

 

[MikeSF340]

Good point, but.......

Definitely a great point, but why was the aircraft designed like that? There has to be an aerodynamic reason for it. Thanks for all the replies.

Mike

 

[sqwkvfr]

Handling characteristics in a thrust vs. no thrust situation would be more constant.

It'd be more difficult to control an aircraft if it's jet/turbopropwash were blasting across the horizontal components of it's empennage.

 

[minitour]

Okay, I have no clue if I'm even talking about the right concept here so just ignore me if I'm way off base:

If you had engines on the wings and a T-Tail aircraft (KingAir 350 comes to mind) wouldn't you have less elevator authority than say an aircraft with wing mounted engines (KingAir 90, 737, 777) and no T-Tail?

Again...I could be way off base, but I'm guessing that the airflow from the props/jets over the horizontal stabilizer/elevator would give you more control rather than less?

-mini

 

[avbug]

Minitour,

The placement of the engines below the wings or on the wings really makes little difference, especially when considering a T-tail configuration. One reason, a big one, that the T-tail came about is sex appeal; it looked classier, and that's why you see it on a lot of smaller airplanes.

Placing the tail high above the airframe puts it in relatively undisturbed air from the powerplants and airframe. It also provides a greater aerodynamic range, with a greater moment potential against the center of lift, and center of gravity. It tends to permit, generally speaking, a wider CG range for a given design than a tail placed lower, and it means that the inboard sections of the horizontal stab are less affected by the aerodynamic influence of the empennage.

A t-tail puts the horizontal stab, spar,and associated mechanisms away from the engines, and allows for a thinner back end, with a reduced cross section. This is frequently reduced adjacent to the engine pods with aft mounted engines in an area rule configuration (empennage gets thinner, or concave, adjacent to the engine nacelles) to reduce interference drag.

A horizontal stabilizer that's out of the prop wash and out of the jet blast means less potential for damage to the stab, and less drastic changes in pitch trim with changes in power.

 

[dougal28]

I believe another reason for t-tails is to avoid some of the problems of Mach Tuck. As an aircraft reaches speeds close to Mach 1, some of the air that passes over the wing is actually traveling at speeds greater than mach 1. The shock waves disrupt the air and making the horizontal stab/elevator ineffective. By placing the elevator higher up and out of the disturbed air from the wing, it maintains effective control.

 

[dougal28]

I have got to sign out from other peoples login as well.

 

[mattpilot]

YOu can find a lot of information about T-tail on this forum if you do a search. But to summarize it, everything in designing an airplane is a tradeoff. Everything has advantages and disadvantages. Ask the engineers of the aircraft why they choose that particular configuration. To get a detailed answer, use the search function.

 

[mar]

Load it up

One of my former instructors was a retired Douglas engineer from the Long Beach plant. He told me that he was never fond of the T-tail design simply because of the load it imparts on the vertical stabilizer.

Obviously you'd need a pretty beefy vert. stab. with a T-tail compared to the empennage mounted horiz. stab.

For what's it worth...

 

[SuperFLUF]

The original 757 design concept had a T tail.

Looking at the drawing now it seems strange but what a weird alternate universe that would be.....T tail 75's.

 

[LearLove]

Minitour,

The placement of the engines below the wings or on the wings really makes little difference, especially when considering a T-tail configuration. One reason, a big one, that the T-tail came about is sex appeal; it looked classier, and that's why you see it on a lot of smaller airplanes.

Placing the tail high above the airframe puts it in relatively undisturbed air from the powerplants and airframe. It also provides a greater aerodynamic range, with a greater moment potential against the center of lift, and center of gravity. It tends to permit, generally speaking, a wider CG range for a given design than a tail placed lower, and it means that the inboard sections of the horizontal stab are less affected by the aerodynamic influence of the empennage.

A t-tail puts the horizontal stab, spar,and associated mechanisms away from the engines, and allows for a thinner back end, with a reduced cross section. This is frequently reduced adjacent to the engine pods with aft mounted engines in an area rule configuration (empennage gets thinner, or concave, adjacent to the engine nacelles) to reduce interference drag.

A horizontal stabilizer that's out of the prop wash and out of the jet blast means less potential for damage to the stab, and less drastic changes in pitch trim with changes in power.

Yes what bug said but also:

another "advantage" is that the T-tail acts like a winglet or tip-sail. In all respect putting the Horz tail way out there you can make the vert tail smaller because it becomes more eff. due to winglet effect. However putting the horz. tail way out there also make more bending/twisting moment in the vert. tail thus the vert tail structure must be beefed up inturn increasing weight. Sometimes this pays off sometimes this doesn't.

In the end is all just a trade off with the said advantages and disadvantages.

When I was an engineer (I have designed/built/tested electrical spindle motors, heat sinks, airfoils and UAV's) 99.99% of what you do is just a compromise between cost/perf or cost/manufacturing or some other combo.

Something else to consider: Sometimes a design feature such as the T-tail becomes a "fad" among designers/manufactures. (I think someone touched upon this above) In this case the T-tail did become a fad and was incorporated into some aircraft when it gave little design advantage. Case in point: piper did this with the chyane 3 or 4 (whichever one or both) as it made the aircraft look bigger with that bis T-tail way up there and thus sales appeal.

I would suggest reading "the illustrated Guide to Aerodyanmics", by Hubert C. Smith. Its the best non-technical book out there that explains aerodyanmics and aircraft design.

Also "aircraft design, a conceptual approach" is good but basic Algebra and calc are needed to grasp his reading.

 

[Flying Illini]

When I was an engineer 99.99% of what you do is just a compromise between cost/perf or cost/manufacturing or some other combo.

Think about Falcon's...they have a mid-tail. Maybe they had all of the employees vote. Then they tallied the votes for the T-tail and the votes for the low-tail and it came out even. So they held a recount and it was still dead even. The lawyers from both sides finally agreed to a compromise and that is how the mid-tail was born.

Or not, but the story sounds good!