1
$\begingroup$

I'm looking for a high-pitching-moment symmetrical airfoil.

I understand the Zenith 701 uses a reverse NACA4412 as it has a high pitching moment. This allows for a smaller horizontal tail.

What about a symmetrical high pitching airfoil for the vertical tail? Wouldn't this reduce the size of the vertical tail?

Does a symmetrical airfoil with a pitching moment exist?

How about one with a high pitching moment?

$\endgroup$
  • 1
    $\begingroup$ By definition, the pitching moment of a symmetrical airfoil is zero. Cambered ones tend to pitch up as AOA is increased. These descriptions generally pertain to wings. Do you mean pitch authority? $\endgroup$ – Robert DiGiovanni Aug 24 '19 at 14:06
  • $\begingroup$ High Cm would be a better description. Just wondering if there is an airfoil series with a large Cm. Isn't this a free lunch? Higher Cm means smaller tail volume, no? $\endgroup$ – Fred Aug 26 '19 at 1:41
  • $\begingroup$ Cm = Pitching Moment/(Dynamic Pressure q ×Area×chord) no luck there, plus draggy. Not a big fan of tiny tails. You need to control that wing. Airfoil tails can stall. Larger flat plates as seen on the Piper Cub are tried and true, especially at cruise. Just doing their job holding pitch. You trade off needed pitch control (including elevator) vs drag in all cases. $\endgroup$ – Robert DiGiovanni Aug 26 '19 at 9:28
3
$\begingroup$

What you probably look for is a symmetrical tail airfoil which produces high control moments. Since those moments are the product of force and lever arm, placing those control surfaces far from the center of gravity produces high moments.

A symmetrical airfoil will have zero pitching moment around its quarter chord point in attached flow. This is understood as zero pitching moment, since the pitching moment of an isolated surface is per convention defined around its quarter chord point. For a thorough explanation please consult this answer.

If, however, you choose a different reference point for your moment, even the symmetrical airfoil can produce a pitching moment. That effect is used in tail surfaces and for a given geometry, a higher maximum lift coefficient will produce a higher maximum moment. Traditionally, the NACA four-digit range of airfoils starting with a double-zero have been used for tail surfaces, but more modern designs will produce a higher maximum lift and lower minimum drag. A good choice would be the Wortmann FX 71 range of airfoils, like the FX 71-120 or the FX 71-L-150 which can often be found on gliders.

$\endgroup$
1
$\begingroup$

I'm not aware why you need a large pitching moment, but if you want to have a strong self centering tendency, and more authority when displaced, design it with a trailing edge anti-servo tab, like this.

enter image description here

$\endgroup$
  • $\begingroup$ The anti-servo tab would increase the rudder effectiveness and the control hinge moments, but I don't see how it would increase the directional stability? $\endgroup$ – JZYL Aug 22 '19 at 21:41
  • $\begingroup$ If the plane yaws, the rudder will try to trail, but as soon as it starts to displace from neutral aligning itself with the airflow, the anti-servo tab is moved in the same direction, which drives the surface back to anti-servo-neutral, which is also rudder neutral. Stabilator all-flying tails depend on an anti-servo tab for static pitch stability. Otherwise, you'd have to physically hold the control all the time. Some gliders have all flying tails w/o anti-servo tabs, but they get what static stability they have from the trim bungee spring, and they are stick fixed all the time anyway. $\endgroup$ – John K Aug 23 '19 at 0:29
  • $\begingroup$ I see you're talking about a reversible control. And in the context of the ultralight it makes sense. $\endgroup$ – JZYL Aug 23 '19 at 1:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.