Old 01-02-2019, 03:43 PM
  #4  
JamesNoBrakes
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Corkscrew Effect
The high-speed rotation of an aircraft propeller gives a
corkscrew or spiraling rotation to the slipstream. At high
propeller speeds and low forward speed (as in the takeoffs
and approaches to power-on stalls), this spiraling rotation
is very compact and exerts a strong sideward force on the
aircraft’s vertical tail surface. [Figure 5-48]
When this spiraling slipstream strikes the vertical fin, it
causes a yawing moment about the aircraft’s vertical axis.
The more compact the spiral, the more prominent this force
is. As the forward speed increases, however, the spiral
elongates and becomes less effective. The corkscrew flow
of the slipstream also causes a rolling moment around the
longitudinal axis.
Note that this rolling moment caused by the corkscrew flow
of the slipstream is to the right, while the yawing moment
caused by torque reaction is to the left—in effect one may
be counteracting the other. However, these forces vary
greatly and it is the pilot’s responsibility to apply proper
corrective action by use of the flight controls at all times.
These forces must be counteracted regardless of which is
the most prominent at the time.
This is from the PHoAK.

A conventional gear airplane is inherently unstable during part of the takeoff run due to the gear configuration, push a tricycle backwards and try to keep it straight, same principle. Airplanes are stable about the vertical axis in flight and will weathervane into the relative wind if displaced, except a conventional gear airplane will further displace when an outside force acts on it until it gets fast enough for this effect to take place. There is a critical speed range with a conventional gear airplane that you have to accelerate through to get to that weathervane-range, if something happens at the lower speed, you may not be able to control the aircraft or keep it from going further in the direction it was displaced.

The exact effect of the "corkscrew" depends specifically on that airplane's configuration and the effect, it's not a very "consistent" force in that idea, between different types of airplanes and configurations. Such as, the aircraft may be trying to roll right, but that puts more weight on the right tire, which creates more drag to the right, counteracting the yawing moment. Or you may have the tail more blanked out, or more exposed, etc...

I guess it's worth a mention in the PHoAK, but with it being so variable in the effects, it's not worth a lot of time spent on it. You won't be able to discern it from the more primary effect, p-factor. Kind of like torque, it's not like when you push the throttle up the plane rolls inverted to the left and in steady RPM state there is no "acceleration" in the opposite direction. The plane is way way heavier and is going to "win" during RPM change, except in some extreme situations you will definitely get this effect, such as a very high HP single with a massive prop, like a WWII fighter. The most this is usually going to affect anyone is a little more weight on one of the tires during the initial RPM spool-up. Spiraling slipstream/corkscrew really gets "into the weeds" IMO and unless you are an aeronautical engineer, you can't go very far with it.

Last edited by JamesNoBrakes; 01-02-2019 at 04:09 PM.
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