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  #21  
Old 06-19-2012, 08:20 PM
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Alan Carroll Alan Carroll is offline
 
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Default

Quote:
Originally Posted by hevansrv7a View Post
The bold-red is mine.

If the air is being deflected downward then normal lift would be produced. Negative lift would require air deflected upward. Right?
I may not be explaining this very clearly. What I mean is that the force generated by the tail is directed downward. The horizontal stabilizer has 0 degree incidence (its parallel to the longerons), and the local airflow as indicated by oil streaks is directed slightly downward relative to the longerons.
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  #22  
Old 06-19-2012, 08:38 PM
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hevansrv7a hevansrv7a is offline
 
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Default I guess it is not so simple..

Quote:
Originally Posted by Alan Carroll View Post
I may not be explaining this very clearly. What I mean is that the force generated by the tail is directed downward. The horizontal stabilizer has 0 degree incidence (its parallel to the longerons), and the local airflow as indicated by oil streaks is directed slightly downward relative to the longerons.
Yep, I got that. So far, so good.

The theory as explained by Roncz at OSH11 was that the lift of a wing (up) is the result of deflection of a quantity of air down at whatever velocity thus producing a F=MA Newtonian action-reaction situation.

Since I expected the tail to be pushing down, I expected that the air under and behind it would be deflected in the opposite or up direction.

However, since my recent post I have been looking at old textbooks and thinking about it. In climb, the tail of an RV pushes down to raise the nose thus increasing AOA of the wing and so on. However, in high speed cruise, we need to lower the AOA which requires a neutral or even upward pushing tail plane. The nose, being heavier, wants to be lower on its own. But, don't we all trim the nose down as we get to cruise speed? The CG forward of the lift point, alone, may or may not be enough?

I know that my airplane's "natural" speed with neutral or zero trim is about 110 KIAS. If I want to go faster, I have to push the nose down which requires that the elevators be pointed down which pushes the HS up. The deflected air from the elevators would be downward and so would the air from the HS, but this is not negative "lift" at this point. Or so it seems to me as I scratch my head and re-think all my primary instruction from 35 years ago.

Your oil experiment is a wonderful example of practical curiosity. I commend you!
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  #23  
Old 06-19-2012, 09:03 PM
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Alan Carroll Alan Carroll is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
Yep, I got that. So far, so good.

The theory as explained by Roncz at OSH11 was that the lift of a wing (up) is the result of deflection of a quantity of air down at whatever velocity thus producing a F=MA Newtonian action-reaction situation.

Since I expected the tail to be pushing down, I expected that the air under and behind it would be deflected in the opposite or up direction.

However, since my recent post I have been looking at old textbooks and thinking about it. In climb, the tail of an RV pushes down to raise the nose thus increasing AOA of the wing and so on. However, in high speed cruise, we need to lower the AOA which requires a neutral or even upward pushing tail plane. The nose, being heavier, wants to be lower on its own. But, don't we all trim the nose down as we get to cruise speed? The CG forward of the lift point, alone, may or may not be enough?

I know that my airplane's "natural" speed with neutral or zero trim is about 110 KIAS. If I want to go faster, I have to push the nose down which requires that the elevators be pointed down which pushes the HS up. The deflected air from the elevators would be downward and so would the air from the HS, but this is not negative "lift" at this point. Or so it seems to me as I scratch my head and re-think all my primary instruction from 35 years ago.

Your oil experiment is a wonderful example of practical curiosity. I commend you!
Thanks for the further thoughts on this. Sounds like "negative lift" was probably the wrong term for what I was trying to say.

I think the oil streaks may not provide very good evidence for how the tail deflects the air, because there is no more fuselage behind the tail to record what happens in that area. Could well be deflected upward?

The figure below from David Lenicer's article in the April 1997 SA shows a pretty fair approximation of what the oil streaks were doing. This is based on computer modeling of an RV-6, ignoring propeller affects.



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  #24  
Old 06-19-2012, 10:35 PM
AeroDog AeroDog is offline
 
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Default Neglible swirl

Quote:
Originally Posted by hevansrv7a View Post
The tip vortexes (vortices?), assuming the airplane is moving in those shots, suggest something other than a spiral airstream hitting the rudder and empennage.

I am not saying that the spiral of accelerated air is not there, only that the tip trails don't actually show it.

In a way this reminds me of the argument about "slowdown" of air as it approaches the plane of the prop because of the fuselage or cowl behind it. Most agree there is higher pressure. So how can there be both slowdown and higher pressure? All the videos of smoke in wind tunnels, mostly of motorcycle fairings, suggest that there is deflection, but no slowdown.

I know that the P factor is evidence, but that gets mixed up with gyroscopic forces. Do we have direct evidence of air moving somewhat sideways?
There is, unavoidably, a swirl motion (as we called it in the jet engine business) imparted to the air. Its magnitude, however, is small compared to the other forces at work.

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  #25  
Old 06-19-2012, 10:37 PM
johnny stick johnny stick is offline
 
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Default tail incidence

Hi, I think the elevator trim being neutral or negative during high speed cruise has more to do with the stab incidence than whether the tail is pushing the nose down or not JMO.
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  #26  
Old 06-21-2012, 05:39 AM
Firstlight Firstlight is offline
 
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Default Downward deflected air...

Downward deflected air relative to a fuselage line does not mean anything about the direction of lift production, unless you also know the relationship to the horizontal stab and the zero lift angle of attack of the section.

I suspect that the original poster was discussing the fact that dirty oil applied to the fuselage forward of the empennage showed that the local flow was slightly inclined below parallel to the fuselage centerline. This is probably due to downwash produced by the wing, and is not uncommon.

The tail on an airplane pushes down in all cases where the CG is ahead of the center of lift.

Cheers,

WBK
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