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I am wondering about the direction in which horizontal stabilizer provide force. I know that it can provide either upforce or downforce. But I am wondering whether is it determined for a particular airplane, I mean it is established for example that B737 tail will provide only a downforce and CG limits are set up in a way that when correctly loaded the tailplane will provide every single time a downforce or it depends from the CG location that tailplane will have to provide either a downforce or an upforce?

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  • $\begingroup$ When I read the first sentence, I thought the rest of the question would be about horizontal component of the mentioned force. $\endgroup$ – Manu H May 28 at 18:26
  • $\begingroup$ You may find section 6.1.6 in "See How It Flies" to be instructive-- be sure and read the entire section to the end. av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium . See also section 6.1.1-- all the illustrations and explanations in section 6.1.1 are built around the idea of a LIFTING tail, further re-inforcing the idea that this is NOT incompatible with pitch stability -- av8n.com/how/htm/aoastab.html#sec-teeter $\endgroup$ – quiet flyer May 28 at 21:31
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The ideal cruise condition, which minimises induced drag, is for the tail to exert zero lift. This avoids any wasteful tip vortices from the tail.

However for most planes the CG shifts during flight, if only because fuel is being used up. Also, trim conditions change with speed, rate of climb, altitude and a host of other variables during flight. The tail spends most of its time a little away from the ideal zero-lift condition, and it can go either way.

The "lifting tail" was much discussed in the early years of aviation. It has a destabilising effect which limits the stable CG range and every plane has a maximum safe level of lift from the tail while remaining stable. Nevertheless, away from that limit it contributes lift which helps to offload the wing and allows it to be that bit smaller. According to Berriman; Aviation, Methuen, 1913.

"On some machines the tail planes are cambered like the wings and help to support the weight in flight. The angle of incidence of the tail is less than that of the wings, however, and the principle [of weathercock stability] apples equally to such cases." -- Page 69.

Berriman gives some supporting analysis in terms of CG and CP to explain what he summarises as weathercock stability. The subtleties of the lifting tail would be further elaborated in subsequent years.

The bigger the tail, the greater the CG range can be accommodated, and also the greater the temptation to offload the wing a little. The limit is the tandem-wing aircraft, where for stability the fore wing is loaded more heavily than the aft, but the aft still contributes substantially to the lift under all flight conditions. A tandem-wing modification of the Westland Lysander flew experimentally during WWII and test pilot Harald Penrose confirmed that its handling was more stable than the conventional version and had a greater CG range.

So ultimately, every plane has its own practical tail loading, which may be normally up, normally down, or variable around the zero point.

There is a myth that for a plane to be stable the tail must always exert a downforce. This seems to arise among students for two reasons. One is that, when the AoA increases, dynamically it must exert a larger relative change in downward moment about the aerodynamic center than the wing's upward moment. Diagrams showing this effect always have a large down-arrow at the tail and it is easy for the student to forget that this is a relative change not an absolute value. The other is that the wing works hardest at takeoff, when fully laden and at its slowest flying speed; when the tail pushes sharply down to rotate the nose up, the wing has to compensate for that as well, so this is the design condition for sizing the wing and everybody has to learn it. Put that first diagram alongside and you have a myth in the making. If you failed to understand any of this paragraph, then you too may fall for it!

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    $\begingroup$ There is a myth that for a plane to be stable the tail must always exert a downforce – finally, I'm not alone with that opinion here! $\endgroup$ – Peter Kämpf May 28 at 17:53
  • $\begingroup$ @PeterKämpf Don't worry, I'll continue sharing a link to the section about pitchwise equilibrium of "how it flies" in comments when needed. $\endgroup$ – Manu H May 28 at 18:34
  • $\begingroup$ @ManuH you have to do better than that av8n article written by who knows who. Link me to a NASA or Boeing article or a university paper that describes what you guys are talking about. I have had no luck. $\endgroup$ – John K May 28 at 18:49
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    $\begingroup$ Boeing are certainly wrong. The guy who wrote that never put their name to it. No peer review, no academic credentials, nada. It's sadly typical of the half-baked guff people often dish out to pilots who only need to fly it, not to understand its design. Never rely on company web sites! (and remember what Boeing never told the MAX pilots) $\endgroup$ – Guy Inchbald May 28 at 19:19
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    $\begingroup$ @PeterKämpf -- re "There is a myth that for a plane to be stable the tail must always exert a downforce – finally, I'm not alone with that opinion here!" -- you've never been alone; during the few years that I've participated on ASE I've made multiple posts and offered multiple links in support of this-- $\endgroup$ – quiet flyer May 28 at 20:49
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In stabilized steady state flight, if the tail is providing lift UP, it means it is opposing a net pitch up moment (it's lifting to keep the nose from rising). To have a net pitch up moment, the CG must be aft of the "mean center of lift" or Neutral Point, in order to create a nose up torque resultant from mass acting about NP. This is a dynamically unstable condition and only FBW fighters can operate this way.

In any normal airplane, the CG must always be forward of the Neutral Point, this distance being called "positive static margin". A positive static margin means there is always a net nose down pitching moment present and the tail has to provide down force to balance it out. The only time the tail will lift up is during maneuvering cases where a stick forward control input is sufficient to pitch the airplane over faster than the natural pitching moment can do it. Then the tail will be lifting, briefly.

If the tail was lifting in any normal steady state case, it would mean that that Static Margin is negative, and that to slow down, the trimable tail's incidence would have to move leading edge up and the elevator move down. I am not aware of any aircraft in existence in this universe with a conventional tail where the elevator has to move down, or the stab has to move LE up, to slow down.

From Boeing:

1 Positive and Relaxed Longitudinal Static Stability In flight, the wings of a conventional airplane generate a nose down pitching moment. To balance this, a download is required on the tail. Airplanes loaded with an aft forward CG require less download on the tail.

Since download on the tail is negative lift, effectively increasing the weight of the airplane, the location of the CG affects the cruise performance of any airplane. Flying at an aft CG will reduce the download on the tail and improve cruise performance.

When airplanes are operated near the aft CG limit, download on the tail is minimized and angles of attack and drag are reduced. However, moving the CG aft reduces the longitudinal static stability of the airplane, something that all flight crews should be aware of (see figure 1 and "Static Longitudinal Stability and Speed Stability" below).

As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point. The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and requires some form of augmentation to be flown with an acceptable workload.<

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    $\begingroup$ That old chestnut. It is true that the tail must provide a dynamic relative downforce when the AoA changes, but it can perfectly well be trimmed for a small net up-force in normal flight. $\endgroup$ – Guy Inchbald May 28 at 17:07
  • $\begingroup$ Sorry not buying it. You are saying airplanes will fly with negative static margin. Again, find me an AUTHORITATIVE source for this, NOT some other posting on here. I have found nothing that says airplanes will fly with negative static margin, neither on line or in the books I have. They all refer to a requirement for positive static margin, and to have positive static margin there has to be a ND pitching moment for the tail to oppose. If the tail is lifting there is negative static margin. If the tail was lifting, and speed decreased suddenly, the nose would pitch up. $\endgroup$ – John K May 28 at 18:14
  • $\begingroup$ Nope. Static margin and absolute tail downforce are not directly related, that is the cheap fallacy you have fallen for. Static margin is related to the change in up/downforce moment with AoA. If you cannot understand that, then you will not understand why you have been misled. $\endgroup$ – Guy Inchbald May 28 at 18:28
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    $\begingroup$ @JohnK I'm afraid Guy is right on this one $\endgroup$ – JZYL May 28 at 20:39
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    $\begingroup$ No, static margin is not related to tail lift direction. Your confusion stems from your belief that an aircraft pitches about the NP. That's not accurate: it pitches about the CG, always. NP is a stability point, not the point of rotation. $\endgroup$ – JZYL May 28 at 21:36

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