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The F-104 Starfighter was launched in 1954, nine years after WWII. It had short stubby wings and a T-tail. According to wiki, the short stubby wings caused inertia coupling and the T-tail reduced this.

My question is: is that really the reason why the F-104 had a T-tail? How does that help reduce inertia coupling?

enter image description here

EDIT

@aeroalias answer has many of the reasons for the T-tail. Had edited the question with some additional info, it was suggested to put this in an answer instead.

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    $\begingroup$ @mins thx for that. The document mentions Since you do not need the maximum roll capability that the Starfighter has with full aileron deflection, the ailerons have been limited so that ample but not extreme rates of roll are available. The second preventive was the ventral fin with its increase in directional stability, especially at the higher Mach. No mention of the T-tail! $\endgroup$
    – Koyovis
    Commented Jul 27, 2017 at 6:24
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    $\begingroup$ Given the era, the fact that it looks great might have played a part in the decision :) But seriously, a T-tail design is usually selected to give cleaner air flow over the elevator when (it is suspected that) the air flow coming off of the trailing edge of the wing is rather turbulent. $\endgroup$
    – jwenting
    Commented Jul 27, 2017 at 7:01
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    $\begingroup$ @jwenting I agree to both things: a) looks great (but I'm Italian and therefore biased) b) Considering the Starfighter had a Boundary Layer Control System that, I presume, generated quite a lot turbulence behind the wings, the T-tail was probably the best choice. $\endgroup$
    – Marco Sanfilippo
    Commented Jul 27, 2017 at 13:02
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    $\begingroup$ It could be because of the placement of the engine, the exhaust is dead in the middle, it was more difficult to make a conventional tail? $\endgroup$
    – ROIMaison
    Commented Jul 27, 2017 at 14:18

3 Answers 3

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The initial preliminary designs had the horizontal tail in multiple locations (including in-line with the wing and cruciform), before settling on the T-tail by December 1952. The reason for selecting the T-Tail is given by Glenn L. "Snake" Reaves, Lockheed Production Flight Test Pilot in the F-104 Test Pilot notebook as

The high tail position was selected ... after extensive wind tunnel tests (which) proved that the location was necessary to obtain optimum stability and control about the pitch axis throughout the wide Mach range. The position also results in minimum transonic trim changes while accelerating to supersonic.

Another advantage was that the high tail on the swept vertical fin reduced interference drag. While the problem was inertial coupling was quite well known during 1950's, the design of the F-104 was frozen during the same period, so it is difficult to tell conclusively if that is the reason for F-104 having a T-tail.

The test pilot notebook also has some explanation about how the T-tail reduces the problem of inertial coupling:

If we now consider two facts about these rolling maneuvers, it will become clear how inertial coupling builds up:

  1. The sideslip build-up is controlled by the induced rolling moment.

  2. As sideslip builds up, the centrifugal or rolling forces increase and this tends to displace the aircraft fuselage perpendicular to the flight path.

The T-tail controls the the sideslip build-up by inducing a rolling moment. From the same document:

... the high tail position of the horizontal stabilizer raised the center of pressure.

Tail position and center of pressure

Image from F-104 Test Pilot notebook

... whenever we sideslip an aircraft, a rolling moment is induced that resists the sideslip. This induced rolling moment is generally termed dihedral effect.

... with any sideslip, the restoring force acting through the center of pressure does not act through the c.g of the aircraft. Therefore, a sideslip condition actuates the restoring force on the tail but at the same time, induces a roll. ... now that we have raised the tail and consequently the center of pressure and the positive dihedral effect has been increased considerably.

The T-tail reduced inertial coupling by preventing sideslip buildup.

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  • $\begingroup$ The last quote talks about dihedral effect, but the F-104 had a slight anhedral $\endgroup$
    – TomMcW
    Commented Jul 28, 2017 at 1:28
  • $\begingroup$ That clarifies much, but there is a lingering question. Inertia coupling takes place in space as well, with satelites that were longer than wide and started to wobble around their axis of rotation. As I understand the notes in your answer, the T-tail provides a dihedral effect which would reduce roll rate, which is what would be required. It also raises centre of pressure so that sideslip stability is increased. $\endgroup$
    – Koyovis
    Commented Jul 28, 2017 at 1:44
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    $\begingroup$ @TomMcW the wings had a slight anhedral. This was the result of the high dihedral effect due to the tail. $\endgroup$
    – aeroalias
    Commented Jul 28, 2017 at 10:28
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@aeroalias answer contains a link to the F-104 test pilots notes, which explains:

  • why the Starfighter has a T-tail;
  • what measures were taken to prevent inertial coupling from happening.

According to the document, the T-tail provided better stability, control, and trim characteristics throughout the speed range. It also provided the same aerodynamic coupling effect as wing dihedral does, that's why the wings have anhedral.

Inertial coupling is not an aerodynamic phenomenon. It has occurred in one of the first satellites put into orbit, which was longer than it was wide, and over time started to wobble around the axis of rotation, a phenomenon called nutation. Airplanes with a long wing span can maintain a high roll rate without pitch and yaw being affected - the Starfighter was the first plane with short wings, and with a high roll rate came nutation, with undesirable pitch and yaw effects. And also a challenge when turning, judging from the famous phrase of a Canadian pilot: "Banking with intent to turn."

The T-tail is not really a solution to inertial coupling, only a longer wing span around the CoG is. Inertial coupling was prevented by limiting the roll rate. The T-tail did increase directional stability of the vertical tail due to the winglet effect, as did the ventral fins.

So contrary to the wikipedia statement, the T-tail was not primarily implemented to reduce inertial coupling.

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    $\begingroup$ Inertial coupling is caused by a slanted axis of inertia with respect to the direction of flight. Shifting the horizontal tail up only increases this slant. Stubby wings have litte roll damping but do not cause inertial coupling. Also, the dihedral effect was more than enough and limiting it caused pronounced anhedral. It should be obvious that you are correct and the T-tail was only chosen for better effectiveness of the whole tail arrangement, mainly caused by the endplate effect and the backward shift of the horizontal tail on a swept vertical. You should accept your answer. $\endgroup$
    – Peter Kämpf
    Commented Aug 4, 2018 at 22:19
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The Wikipedia article lacks a reference. It seems it's to keep the fin short while being effective, which reduces weight and drag. A classic use of T-tails.

enter image description here

Source: General Aviation Aircraft Design: Applied Methods and Procedures, p. 973

The cited Whitford I believe refers to this book.

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