# Propellers in yaw

#### Jean Claude

##### Junior Member
Title: Propellers in yaw
Author: Herbert S. Ribner
NACA report n° 820
Comment: A propeller does not only create the axial force that is asked of it. It also creates a radial thrust as it moves in airflow with a pitch angle. Contrary to general belief, it then causes a destabilizing moment when it is placed forward of aircraft GC.
This report helped me to quantify this effect, which fortunately was quite low, for my tractor project.

Last edited:

#### kolibri282

##### Active Member
It's a bit sad, JC, that you never publish the complete sequence of calculations which you followed to arrive at your results. This makes it impossible for the members of the forum to learn from your insight or your errors. This is, in my opinion, one of the reasons why an increasing number of well trained members feel that this is a folklore, rather than a science forum.

Last edited:

#### Jean Claude

##### Junior Member
Juergen,
Is not here the section where to put technical publications interesting our autogires?
Of course, I'm happy to answer at your questions about applying this NACA report to my tractor project.

As described in this report, a tractive propeller produces a radial force when its shaft is not parallel to the general flow.
When the propeller is placed in front of the aircraft gravity center, then this force produces a torque that tends to increase the initial fault of parallelism (unstability)

Quantifying this effect seemed to me necessary to possibly enlarge the surface of the tail.
This fig. shows Cy’ /ψ = 0.125 for my propeller at takeoff with nose up 10 degrees (or 0.175 rad)

And since Y = ½ .ρ. V². Sprop .Cy’ , then Y = ½ .ρ .V². Sprop . (Cy’ /ψ) . ψ
Hence, replacing the letters by their value and ψ by 0.175 rad it comes Y = 9.5 N
Too weak to significantly destabilize my gyro.

Last edited:

#### kolibri282

##### Active Member
Quote: Is not here the section where to put technical publications interesting our autogires? /Quote
Thank you for your answer, JC, and yes of course you could have posted it to a different section of the forum, eg. "Theory of Flight"
I must admit though, that I have been fairly sloppy in the past when it came to applying the rules I had set in the beginning...;-)