# Why does a T-tail produce a pitch-down moment in sideslip?

Here is something which I did not find in any book, but confirmed in several wind tunnel and flight tests: A T-tail causes a strong nose-down moment in sideslip. This can even be observed in a potential flow analysis, so no fancy viscous effects should be required for an explanation. But I have never understood why it happens. I would even have an explanation for a pitch-up, but the pitch-down really has me puzzled. Does someone here know more?

• any correlation with the airfoil selected / incidence of the horizontal surface? – Federico Jul 7 '14 at 22:02
• Fairings in the intersection area do modify the magnitude of the effect, but it shows up also in simple potential flow codes which use a flat surface for a wing (vortex at 1/4, control point at 3/4, two trailing vortices per panel). As for the incidence - my data is only for a trimmed configuration, so I have no data on different incidence angles. – Peter Kämpf Jul 8 '14 at 21:26
• FWIW I can't say I've experienced any kind of strong pitch-down moment until slipping very aggressively in the T-tail Katana. There was one time where there was a sudden pronounced pitch-down, but to be honest, that felt more like flow separation from the stabilizer. I suppose it's possible that humans simply tend to compensate without noticing, there is a lot of moving going on when entering a slip after all. Perhaps it's only noticeable in a wind tunnel where you can keep everything else constant? – falstro Jul 10 '14 at 7:30
• @falstro: The pitch-down moment should build up slowly with sideslip, so you will just pull a little with moderate sideslip angles. Depending on the static stability, if you use a tape measure or ruler to measure stick position in flight, the effect should be detectable. – Peter Kämpf Jul 10 '14 at 14:53
• @PeterKämpf Does it also happen for a normal tail, just less strong? Or doesn't it happen at all with normal tails? – ROIMaison Aug 26 '15 at 15:46

The image shows the situation for high angles of $\beta$:

At large angles of sideslip with swept horizontal tails, the loading will probably not be antisymmetrical mainly because of the difference in lift effectiveness of the leading and trailing portions of the horizontal tail caused by the difference in their effective sweep angles. This would result in a net lift induced on the horizontal tail which is a function of sideslip and tail height. This possible effect of tail height is illustrated in sketch 9 for large positive sideslip angles:

Interesting to see is that the model has no wings, so we can rule out any causes related to wings, in case people were thinking in this direction.

I putsome more thought into it, and I drew these diagrams, which helped me understand the things being said in the paper.

The blue pluses and minuses are the resulting velocities caused by the horizontal velocity as a function of the sideslip ($V_{\beta}$). The red distributions are the forces on the elevator as consequence of this $V_{\beta}$.

This component creates an increased velocity over the left side of the rudder, and a decreased velocity over the right side of the rudder. If there's no sweep, the effects are equally strong, and no effect on the lift is present (denoted by the top two images)

However, if there's sweep, the effect on the left side is less strong, whereas it is stronger on the right side (indicated by the two tails in the bottom row).

In a T-tail, this stronger influence is acting on the lower side of the tail. As it is a minus (meaning a relative reduction in velocity), it will lead to a reduction in suction on the lower side, or an upward force, causing a pitch down.

In a normal tail, the minus will act over the upper surface. It causes a deceleration of the flow over the elevator, thereby reducing the lift it generates. This causes a pitch up movement.

• Polhamus only has an explanation for swept horizontal tails (which is simple and convincing). However, I found the effect even in unswept tails, so I am still not sure why it happens. OTOH, your answer is clearly the best so far, so I accept it. – Peter Kämpf Mar 5 '16 at 17:06

Loss of aerodynamic lift of the horizontal tailplane (downward force) causes the aircraft to pitch down.

A disruption of airflow over the low pressure side of an airfoil has a greater effect on the airfoil's ability to create lift than a similar air flow disruption to the high pressure side of an airfoil.

On a conventional tail, the wind shadow of the vertical stab affects the high pressure side of the horizontal tailplane (the top surface) which is not as aerodynamically sensitive.

On a t-tail, the wind shadow of the vertical stab affects the low pressure side (lower surface) of the horizontal tailplane which is the aerodynamically sensitive side causing a greater loss of effectivity of the tailplane.

• What disruption? If the effect shows up in potential flow codes, there is no separation involved. The effects on the windward side should compensate for those on the leeward side of the vertical, so in total the effects should cancel each other. And the whole thing happens also at higher AoA when the tail creates positive lift. I'm not convinced. – Peter Kämpf Jul 8 '14 at 21:21
• What disruption?!? you slip the aircraft, block the flow over the tailplane with the vertical stab and you say "what disruption?" – Steve H Jul 9 '14 at 3:41
• This is a rather drastical description for sideslip. The effect is linear, growing with sideslip angle well before any separation occurs. Again, it shows up in fully linear potential flow. If you still think this involves "disruptions", please reconsider. – Peter Kämpf Jul 9 '14 at 19:20
• Basically you are having a vertical fin that is lifting so one side of the fin has low pressure and the other has high pressure. So, there is compression in one side of the fin affecting expansion on the horizontal plane and you have expansion of the vertical fin affecting also expansion on the horizontal fin. The "distortion" or the "effect" of compresion over expansion is bigger of the effect of expansion over expansion. Netly there is lower downforce on the horizontal plain leading to pitch down. – Trebia Project. Feb 3 '15 at 23:18

From the test pilot notes of the F-104: the centre of pressure changes with vertical position of the tail. One of the reasons for implementing the T-tail on the F-104 was to create a more desirable sideslip-roll coupling, however the upward shift in centre of pressure also creates a nose-down pitching moment with sideslip.

• Wouldn't the pressure on one side be balanced by an equal amount of suction on the leeward side? The pressure and suction on the vertical add up, resulting in a strong sideslip-induced rolling moment, but both should cancel on the horizontal tail, so the pitching moment is still a mystery to me. Also, the pictures imply a nose-up pitching moment change in case of a low tail in sideslip, but that does not occur. For a low tail, suction and pressure do balance, after all. – Peter Kämpf Jul 29 '17 at 8:46