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ACRO E-mail Archive Thread: aerodynamics on upline


Thread: aerodynamics on upline

Message: Re: aerodynamics on upline

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From: Herman Dierks <dierks at>

Date: Thu, 09 Jan 1997 23:55:54 UTC


   Tim, I was thinking the same thing.
 However, the engine thrust line is may not be exactly parallel
 to the wing zero lift line.  I know in most aircraft the engine
 is canted off to the right and (I think) down. 
 Therefor, there would still be a little asymmetric thrust,
 but it should be minor.  This would vary from aircraft to aircraft
 based on the angle of incedence of the wing attach to the fuselage, etc.

 (Not sure I want to step into the middle of this discussion. :-)
 dierks at
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>  Scott,
>  Here is where your theory is flawed ( please don't take this as being
> flip). You are assuming the there is an asymmetric thrust being produced by
> the propeller. In vertical flight this is not the case. The reason that
> there is asymmetric thrust in a climb of less than vertical is due to the
> direction of the relative wind acting on the propeller. This is an easy fact
> to show but I don't know if my writing skill will be sufficient explain it
> properly but here goes. 
>  With the airplane in horizontal up right flight with the propeller turning
> clockwise as viewed from the cockpit an increase in pitch will produce a
> yawl to the left. This yawl is caused by the increased angle of attack on
> the downward swinging blade and the decreased angle of attack of the upward
> swinging blade. This change in angle of attack is caused by difference in
> the aircraft's pitch relative to its flight path. The angle of the flight
> path will be less than it's climb angle in a stabilized climb less than
> vertical. If knowing this you look at the prop disk in a climb you will see
> that it is at a slightly greater angle than the flight path. Looking at the
> propeller its self you will see that the blade on the left is at a lower
> angle relative to the flight path than the right. You can demonstrate this
> to your self by going to the airport walking up to a tail dragger placing
> the propeller horizontal, holding a plumb line at the prop tip and noting
> the angle. Then go to the other side and do the same. You will see that the
> blade on the right will have a greater angle than the left. If you think of
> the ramp as your flight path and the angle of the aircraft as the climb
> angle you can easily see what's happening. Now walk up to a tricycle gear
> airplane and do the same. Now the flight path and aircraft angle are the
> same and there wont be any difference in the blade angles. This is what you
> have in vertical flight.
>   Wheuuu that was a mouth full.
>      Timothy S. Bastian
> >Greg-
> >
> >I'll give it a shot... I might be chasing a gluon here, and I really had no 
> >intension of developing a rigorous proof... but here goes...
> >
> >Given near vertical flight, disregard the relatively minor pitch/yaw
> moments coming 
> >from the wings and tail... also disregard the slipstream effect of prop
> wash on the 
> >fuselage and tail (although the more I think about that one the more merit
> I give 
> >to the idea--- slipstream is pushing 'down' on the top of the stab, causing a 
> >pitchup,,, humm, someone else mentioned the effect of thrust line
> moments,,, could 
> >be a good paper for a TPS stud to work on- but way above my level of 
> >commitment<g>)... what we have left is a spining disc, a gyroscope...
> >
> >-This is my train of reasoning (all directions are relative to the
> aircraft, pilot 
> >perspective):
> >Now that gyro is not a perfect one operating in a vaccuum... I contend that
> at high 
> >power settings it is assymetricaly loaded... that P-factor (caused by the old 
> >standby of the descending blade at a higher angle of attack than the other
> blade) 
> >is the cause of that assymetric load which induces a moment on the gyro
> inducing a 
> >precession... I'm assuming the blade at a higher aoa is the right blade...
> if that 
> >is so there is a larger amount of thrust imposed on the right side of that
> gyro and 
> >the reaction will be felt at the bottom of the gyro ('bottom' of the
> propellor arc) 
> >inducing an upward pitching moment on the aircraft... ie. the tail tends to be 
> >forced 'down'... to control the pitching moment you've got to push a little. I 
> >believe there will always be some degree of relative wind (theoretically of
> course 
> >you might be able to induce a true prependicular wind, where the disc loading 
> >will symetric... but I think it highly unlikely for anything other than
> very short 
> >periods).
> >
> >If this is so... then if you fly a Sukhoi/Yak the opposite control
> reactions ought 
> >to be observed due to the propellor rotating in the opposite direction (I
> flew a 
> >Su-29 once but was not concerned with this idea at the time<g>)... I figure
> that 
> >there are probably several processes influencing the pitching moment and it
> would 
> >be a complex endeavor to really sort out what is happening, but my
> intuition tells 
> >me (admitedly not stellar) that if the larger forces are taken out of the
> equation, 
> >as they are on an upline, then the resulting phenomena must be induced
> byproducts 
> >of producing thrust (and why the slipstream idea may have merit imo)
> >
> >That's as good as I can get without a whiteboard and pens... how's that?
> >Scott
> >


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