B Mathews
Well, actually down thrust counters the effect of too much pitch stability brought on by the CG being too far in front of the neutral point. It's got nothing to do with P factor effects.
Please read my statement again. I agree the PRIMARY purpose of down thrust is to help with pitch problems due to power changes. I said the down thrust AIDS in minimizing P factor by placing the propeller more perpendicular to the local airflow thus minimizing the source of the P factor in the first place
In tail draggers a strong P factor results in a bad yaw at takeoff. But there's also going to be an associated torque effect at the same time. You're certainly right though that some side thrust will deal with the P factor effect.
In ANY single engine propeller aircraft P factor will cause a yaw at ANY AOA higher than cruise configuration. It will be a bad yaw during the demonstration of slow flight.
But many of our models will also roll into a yaw as well since many of them use some dihedral. With such models adding a bit of yaw with some right thrust will produce enough roll couple to cancel out much of the torque induced roll. So due to the design of the craft the right thrust CAN do both.
For the torque of the propeller to affect our models it has to have a mass that is relatively high when compared to the mass of the airplane. For most “sport” and Pattern models this is not the case. A typical trainer weighs 6-8 pounds the prop weighs around ONE OUNCE.
I offer the mass-induced or air load-induced torque on a TYPICAL Glow powered RC plane is insignificant. There isn’t enough torque to induce the roll coupling you refer to. This can easily be tested by placing a scale under each MLG of a model, restrain it and run it at full throttle. The difference in weight on the scales will give us the actual torque. HOWEVER in small light 3D airplanes the mass of the prop is high enough to affect the plane, resulting in TORQUE ROLLS not YAWING.
DaRock
There are a couple of regulars at one field who haven't taken off straight down the runway in years. Most takeoffs are about 10' worth of roll, and a LEFT TURN at right angles to the runway.
And that turn is due to P factor more thrust running down the right side of the fuselage. You can demonstrate this in flight. Take a trainer and transition to slow flight; nose high, just enough airspeed and power to keep it in the air just above a stall. Now, without adding any other control input (more difficult than it sounds) punch full throttle. The airplane will YAW left not roll left, until it accelerates away from the stall speed.
Fledermaus
One thought I have, however, is that these helical patterns do not necessarily mirror the airflow. The axial velocity of the air in the prop wash is much greater than the tangential velocity (if it wasn't, it would be a pretty crappy propeller) and there is not that much momentum transferred tangentially as a result. This suggests that the prop wash effects might not be that substantial, but I'm not sure about that conclusion.
Maybe I'm just blowing smoke...
Actually sir, you’re confusing cavitations vortices with the spiraling airflow theory. The tip vortices on the C-130 are supposedly perpendicular to the supposed spiral in the downstream airflow.
Jet Plane
So explain why the Hawker Hurricane, among many other aircraft, has the vertical stabilizer offset a degree or two to the right in order to counteract spiral slipstream? The Hurricane by the way was designed about 10 years before the publication of Stick and Rudder, when according to you spiral slipstream hadn't been invented
And the F-4U Corsair’s fin cranks over 6 degrees when the flaps are lowered. In both cases, the fin is offset to counter a yawing tendency. No one has attributed the cause of the yaw to “spiraling slipstream”. Again, I said the spiral theory did not catch on until after 1944. At my work we have one of the largest aviation libraries in the country. I’ve researched over two dozen stability and control text books. Spiraling theory is not mentioned prior to 1944, and after that it is only stated as accepted factual explanation of why the plane yaws. The books never go into an actual analysis of whether or not you can mathematically predict its existence, t and therefore design out its effects.
Back to the original intent of my original post; It is quite evident from just these few posts that the hobby is full of a lot of theories and preconceptions that have nothing to do with the actual engineering or science involve. I guess I’m just beating my head against the wall hoping for some improvement in the accuracy of the sport.
Sorry if I've caused this to become a tempest in a teapot.