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If an airplane is traveling through a uniform environment and its main wing is parallel to its horizontal stabilizer, how does its tailplane generate downforce? Only explanation I can think of is because it is submerged in the downwash of the main wing as shown in this picture

enter image description here

But if this is the case how is it possible for aircraft which use outboard horizontal stabilizers which extend out of the downwash to be stable?

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  • $\begingroup$ aviation.stackexchange.com/questions/36934/… $\endgroup$
    – abelenky
    Jul 21, 2022 at 15:01
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    $\begingroup$ Do you know for a fact that any designs w/ outboard stabilizers as you've illustrated, have zero difference in incidence between stabilizers and wings? $\endgroup$
    – quiet flyer
    Jul 21, 2022 at 17:15
  • $\begingroup$ Don’t forget that they have elevators. $\endgroup$
    – Jim
    Jul 22, 2022 at 2:13

4 Answers 4

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For a stable configuration the incidence of the wing is normally higher than that of the tail. Also, the wing has more camber, so its zero-lift angle of attack is lower. Even at the same incidence the wing therefore will create more lift per area than the tail, the basic condition for static longitudinal stability.

Flying in the downwash of the wing actually has advantages:

  • increasing the angle of attack also increases the downwash, so the airflow over the tail will see less variation in angle of attack. Once the wing comes close to stall, the tail is still comfortably in the linear range.
  • Download on the tail in combination with the downwash means that a tail might even produce a bit of thrust.
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  • $\begingroup$ How does the wing has more lift per area when it has the same aoa as the tailplane, because of the downwash? if they're parallel and also same aoa then OHS config has some downwash on the tailplane too (maybe air out of the wingspan is affected too?) edit: thanks for the useful links. $\endgroup$
    – Kozakov
    Jul 21, 2022 at 21:21
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    $\begingroup$ @Kozakov Please do not confuse incidence (angle versus a reference line defined by the airplane's structure) with AoA (angle versus direction of flow). At the same incidence, both wing camber and downwash will make sure that the tail has less lift per area than the wing. At the same AOA, lift per area will also be the same (actually still a bit less on the tail due to its lower aspect ratio, but that influence should be small). $\endgroup$
    – Peter Kämpf
    Jul 22, 2022 at 6:59
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    $\begingroup$ @Peter, Just a quibble, but to be accurate, won't the airfoil shape, (as well as aspect ratio), affect the lift per unit area? At the same AOA, a highly cambered wing will generate more lift than a symmetrical wing of the same area. I mean, camber shifts the C$\sf{_{L}}$ curve to the left. Unless you redefine AOA, a cambered airfoil generates lift at zero AOA, a Symmetrical airfoil does not. $\endgroup$
    – Charles Bretana
    Jul 23, 2022 at 12:42
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    $\begingroup$ @CharlesBretana Stability depends on the lift derivative over AoA, and camber has no influence here. Not the geometrical AoA is important but the difference between geometrical AoA and zero-lift AoA. Camber only shifts the zero-lift AoA to more negative values but leaves lift curve slope unaffected. Camber does, however, make a wing more unstable because of the shift of the center of pressure with angle of attack. $\endgroup$
    – Peter Kämpf
    Jul 23, 2022 at 12:47
  • $\begingroup$ @Peter, Yes, I agree, but you said in prior comment, "At the same AOA, lift per area will also be the same." $\endgroup$
    – Charles Bretana
    Jul 23, 2022 at 12:48
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Many tailplanes are inverted airfoils, such that their "lift-vector" is pointed downwards.

tailplanes also frequently have a negative AoA (eg, aimed slightly downward a few degrees).

The downward force is created by both of these: the upside-down airfoil, and the downward AoA.

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I did some more research myself and wanted to post what I have found.

Short answer: Static longitudinal stability does not require download on the tail.

After reading quiet flyer's comment, I wasn't sure if there was zero difference in incidence between stabilizers and wings of a plane which uses outboard horizontal stabilizers and the drawings in the paper which I've found the ohs image I added to the question looked like they had parallel flight surfaces but I wasn't sure if they were original drawings which they made the tests with.

So I designed and built this small guy which has parallel flight surfaces to be sure.

I started testing by placing CoG somewhere around here.

cg 1

which caused it to immediately pitch down after hand launching it

I removed weight from the tip until none was left and CoG was shifted all the way to the back of the trailing edge like this, and finally it flew very good at this CoG location.

cg 2

After searching for "center of gravity" in the paper which I took the OHS image in the question from I found this;

enter image description here

After some more research I came across this which answers my question.

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    $\begingroup$ Static stability doesn't req downforce since the tail is just contributing to the overall weathervane effect where the horizontal aerodynamic center wants to trail the C of G in the airstream. Downforce is req'd to TRIM the body to weathervane at a specific angle to the flow away from zero lift. If the angle of attack is to be positive (nose up) 5 deg, the tailplane must be configured to create static equilibrium at that angle, by creating a 'force fight" of opposing pitching moments that achieves balance at the desired +5 deg off the flow.For that case, the tail must make downforce for trim. $\endgroup$
    – John K
    Jul 22, 2022 at 4:11
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    $\begingroup$ To add to @John's comment, for aircraft without a tailplane (delta wings) the distribution of lift on the wing creates the same effect. We use the concept of a Center of Pressure, or Aerodynamic Center, but in fact, that is an aggregate simplification that is incorrect when analyzing stability. The large upwards lift generated by the forward part of a delta wing is closer to the CoG, and exerts a nose down pitching moment that is balanced by the smaller downwards lift at the trailing edge (From elevons),, which is further from the CoG, $\endgroup$
    – Charles Bretana
    Jul 24, 2022 at 13:11
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How does a tailplane provide downforce if it has the same AoA as the main wing?

Simply by deflecting the elevator. For horizontal stabilisers that can be trimmed: by adjusting the angle of incidence, and then it won't have the same AoA as the wing.

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