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Here is a polar for a piston single-engine aircraft:

Yak-52 polar

(This is lifted from a Russian manual for Yak-52, so $C_x$ is drag, $C_D$, and $C_y$ is lift, $C_L$).

1 is the 'normal' case without propwash. 2 is nominal, and 3 is the max takeoff power.

It is evident that the whole lift curve slope $C_L^{\alpha}$ is increased significantly (by some 30%) as the engine revs up, with $C_{L_{max}}$ reaching 2.

But what immediately follows, amongst other things, is that the AoA (pitch) stability should increase accordingly: it is proportional to $C_L^{\alpha}$ (as well as to the distance from NP to CG).

However, somehow I've never seen this fact mentioned explicitly, despite it being quite significant. Can anyone confirm it, either from literature of experience flying higher-powered prop aircraft?

(It may not be very obvious to feel the difference because other conditions are rarely the same between idle and full power, but the closest approximation I can think of is glide descent vs full power climb at the same speed: the aircraft should be 'stiffer' and possibly more oscillatory in pitch at full power. However, propwash over tail may mask this effect; perhaps a twin is a better testbed for this...)

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  • $\begingroup$ At first glance, this seems to not take into account P-factor and the need to apply rudder to compensate? $\endgroup$ – Juan Jimenez Jun 26 '19 at 10:02
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    $\begingroup$ @Juan, this is a totally different effect. Here I'm trying to isolate specifically the lift/pitch effect of propwash over wings. $\endgroup$ – Zeus Jun 27 '19 at 0:26
  • $\begingroup$ @Zeus "stiffer" is consistent with higher speed effects, the plane becomes so stable and deflection of elevator so difficult the plane cannot pull out (of the dive). Oscillatory would indicate directionally unstable, but it would be interesting to see (if ever) the entire plane developed a feedback "flutter". If there was some bending in the fuse, maybe, but at that point it would probably be coming apart. $\endgroup$ – Robert DiGiovanni Jun 29 '19 at 2:15
  • $\begingroup$ If the plane becomes somehow more stable at the same speed (e.g., in the trivial case, by moving CG forward), it will feel 'stiffer': that is, greater effort will be needed to manoeuvre it. Maybe that's not the best word because it may imply less movement, but that's how people describe it: movement is much less relevant than the force for us. Oscillations may arise if the system is overly stable but damping is not sufficient; this again may happen if you increase stability without changing the aerodynamic configuration, like when moving CG. It can happen in pitch just as well as in yaw. $\endgroup$ – Zeus Jun 30 '19 at 6:47
  • $\begingroup$ And in roll too! An oscillation in pitch probably would require large amounts of pitch torque from mass away from CG. Fortunately, in aircraft design, this is rare (although you can create a phugoid with poor matching of CG and elevator trim). "Stiffness" definitely was an issue when aircraft with manually controlled surfaces progessed past 300, then 400 mph. An interesting match when combined with the unstabilizing effect of the puller prop Peter K pointed out. $\endgroup$ – Robert DiGiovanni Jun 30 '19 at 18:15
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There is not an obvious answer to this. I'll outline three effects (among others):

  1. Placement of thrust line. If the thrust line is placed below the CG, it will have a destabilizing effect; the converse is true. That's why wing mounted engines tend to destabilize aircraft as throttles are increased. But that's not the title of your question.

  2. The local increase in flow from prop wash increases the wing lift slope. This tends to decrease the pitch stability. An increase in lift has an associated increase in downwash as a function of AOA. This also tends to decrease stability.

  3. The local increase in flow from prop wash increases the tail lift slope. This tends to increase the pitch stability.

The neutral point contribution is as follows (cited from Etkins, Dynamics of Flight):

$$h_n=h_{n_{wb}}+\frac{a_t}{a}\overline{V_H}(1-\frac{\partial\epsilon}{\partial\alpha})-\frac{1}{a}\frac{\partial C_{m_p}}{\partial\alpha}$$

where $h_n$ is the location of neutral point, $h_{n_{wb}}$ is the wingbody aerodynamic center (AC), $\overline{V_H}$ is the tail volume with respect to the wingbody AC, $a_t$ is tail lift slope, $a$ is the total aircraft lift slope, $\epsilon$ is downwash on the tail, $C_{m_p}$ is the pitch moment contribution from thrust.

Forget about thrust line for a moment, then we have:

$$h_n=h_{n_{wb}}+\frac{a_t}{a}\overline{V_H}(1-\frac{\partial\epsilon}{\partial\alpha})$$

Therefore, increasing the wing lift slope decreases the neutral point, as will an increase in downwash. Increasing the tail lift slope has the opposite effect.

So there is no blanket statement.

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  • $\begingroup$ I'm trying to concentrate purely on the effect of the polar change, i.e. on (2), and specifically on the AoA stability. (1) is clear and is a very different effect. For (3) it would be interesting to learn in what proportion the tail lift slope changes vs the wing's, but for the moment let's remove the tail from the propwash (on a high T, for example). So, for (2), why does it decrease the pitch stability? As I noted in commens to the @Peter's answer, in my mind it shouldn't necessarily... On the other hand, the effect of the increased downwash must be significant, thanks for pointing out. $\endgroup$ – Zeus Jul 2 '19 at 4:01
  • $\begingroup$ @Zeus I've added some math in my answer to show the rationale $\endgroup$ – JZYL Jul 2 '19 at 4:15
  • $\begingroup$ One way of looking at it are helicopter rotors, where forward motion will increase the Clift graph the same way. Another thought is to consider the prop blast perhaps as a local source of air velocity, maybe having more effect on the front of the wing? But management of thrust line, and impingement effects on the entire aircraft, may be were we want to go with these new, incredibly powerful fans. Older designs successfully mounted props smack in front of the wings, or on the nose, for a long time. $\endgroup$ – Robert DiGiovanni Jul 2 '19 at 5:42
  • $\begingroup$ @Jimmy, thanks. It's much clearer now. (I would dare to suggest to explain $V_H$ as well just for completeness; I know it's the 'tail volume', and btw the way Etkin uses it it should have overscore meaning it's related to AC rather than CG). It would be interesting to know though in what proportion the tail slope $a_t$ changes vs $a$ in practice when the tail is in the same wash... But that's another question. $\endgroup$ – Zeus Jul 2 '19 at 7:05
  • $\begingroup$ Good catch. I've modified the answer to show meaning of VH. $\endgroup$ – JZYL Jul 2 '19 at 14:31
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No. Only for pusher types it does.

Pitch stability is the change in pitching moment over angle of attack. The polar doesn't mention this and I would expect that pitch stability decreases with power.

Pitch stability is $$\frac{X_N - X_S}{l_{\mu}} = -\frac{c_{M\alpha}}{c_{L\alpha}}$$ and with an increase in $c_{L\alpha}$ the absolute value of the whole term should become smaller. That would be normal for tractor propeller aircraft, see this answer. Note especially the reference to an old NACA report (NACA TN 2586) on this by John L. Crigler and Jean Gilman, called Propellers in Pitch and Yaw.

In order to measure pitch stability in flight just measure the stick travel needed to trim different airspeeds for the fixed-stick stability or the stick force needed to trim different airspeeds (without changing the trim setting, of course!) for the stability with free-floating elevator. Both should decrease with power on.

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  • $\begingroup$ This would be absolutely correct when comparing a glide and powered flight at the same speed for tractor (puller) prop. Is it possible that increased airflow over the wing trailing edges and tail somewhat counteracts the prop effect? (Getting glide and powered handling close would be a plus for design). $\endgroup$ – Robert DiGiovanni Jun 29 '19 at 14:49
  • $\begingroup$ @Peter, that's what I generally knew but such a large change confused me. I made a childish mistake of mixing the wing AC and overall AC (NP). Still, if it's fair to say that the whole effect is caused by the local increase of airspeed on a part of the wing (assuming perfect axial propwash), the AC won't move, and both $c_{M\alpha}$ and $c_{L\alpha}$ of the wing will change proportionally, causing no change in stability - if we remove the tail from propwash for a moment. There will be destabilising effect due to increase of downwash-per-alpha, which we can't exclude, but is it the main cause? $\endgroup$ – Zeus Jul 2 '19 at 3:01
  • $\begingroup$ ...Interestingly, $c_{M0}$ must change: the absolute moment on the affected part of the wing will increase, so if we still want to relate the overall moment to the whole wing, we need to change the coefficient as a function of propwash/power. But this is not a consideration for pitch stability: it doesn't depend on $\alpha$. It may shift the trim with power, but in reality the propwash effects on the tail will probably override this change. $\endgroup$ – Zeus Jul 2 '19 at 3:09
  • $\begingroup$ As for stability increase for pusher props, the cause still eludes me. The NACA report essentially talks about the cross moment: yawing for a pitched prop etc. - the cause of the famous 'P-factor'. But it doesn't show any significant pitch moment. If so, the effect must be caused purely by the aerodynamic effects of the propwash (or suction before the prop?) That's worth a separate question though... $\endgroup$ – Zeus Jul 2 '19 at 3:19
  • $\begingroup$ @Zeus: Propwash is only part of the answer. The thrust vector of a propeller shifts with angle of attack and sideslip changes and also is only parallel to the propeller's axis of rotation for zero angle of attack and sideslip. The NACA report contains more than only the p-factor: A propeller acts similar to another tail (or canard) surface; it contributes its own c$_{L\alpha}$ so a tractor propeller's change in thrust with angle of attack destabilizes the aircraft all by itself. $\endgroup$ – Peter Kämpf Jul 2 '19 at 4:55
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Thanks for including the identity of the aircraft. The Yak 52 is a low wing single engined tractor design. The graph indicates prop blast over WING increases its lift at a given angle of attack.

It does not seem possible to infer anything else from that graph and I would not wish to guess. The prop blast could affect the horizontal stabilizer trim and improve the directional stability, which includes pitch (and yaw). This would be a reasonable generic assumption for all aircraft of this design, with the caveat that prop flow swirl could also have effects on the empennage as well.

However, I doubt the Yak designers would leave it with borderline stability without prop blast. The test would be to glide it. Under power, trim may have to be adjusted, as is normal for aircraft of this type.

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