Does propwash increase pitch stability?

Question

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

(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...)

Answer 1

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.

Answer 2

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.

Answer 3

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.