David Megginson said:

> Jim Wilson wrote:
> > You are right, that doesn't sound right.  At least if a positive value did
> > point down, it would be in conflict with the AOA parameter.  That said,  are
> > you sure the DC-3 is supposed to have a negative incidence?  I just looked up
> > the p51 and the diagram clearly shows a positive incidence.  The tail is +2.0
> > degrees (so that at level AoA=0 on main wing, the tail would have a AoA of 2.0
> > degrees).
> The role of the horizontal stabilizer is to produce negative lift to keep 
> the nose from dropping -- you balance the plane so that it is slightly 
> nose-heavy, then use the hstab (which is on a long lever arm) to apply just 
> enough down force to keep the nose balanced.  Flying with an aft CG is more 
> efficient, since you're not making as much (if any) downforce with the 
> hstab, but it can also result in pitch control problems and violent stalls.
> On typical non-aerobatic aircraft, the horizontal stabilizer has a lower 
> angle of attack than the main wings in any given flight regime, but there 
> are two ways to accomplish that:
> 1. give the hstab a lower physical incidence angle than the wings; and/or
> 2. take advantage of the downwash from the wings, which comes from above 
> rather than straight on (will not work for a t-tail, obviously).
> Since YASim does not model downwash, we have to adjust the incidence angle 
> to simulate it where the hstab should be according to its relative airflow 
> as well as its physical incidence angle.  This isn't an issue for nose-wheel 
> aircraft, since the front wheel keeps them from nosing over most of the 
> time, but it makes a significant difference for controlling taildraggers on 
> the ground.

Excellent,  thanks for the clarification.  Just looking at the cub you can see
down-wash is a major design feature.  The DC-3 has a high tail,  but I can see
the incidence in the main wing is pretty high.  I wonder what happens when you
increase the wing incidence and set the horizontal stabilizer to 0 or whatever
it is supposed to be.

As for the P51-D, here is a page out of the reference:

For some reason the designers seem to be going the opposite direction
(positive tail incidence).  I'd like to understand the reason in order to make
a decision on the p51d fdm config.  It did seem to handle better with the
negative incidence number.  But down-wash certainly isn't present on the hstab
and the diagram appears to show positive incidence.

The other related problem I'm not sure of is that with or without reducing the
incidence value, I can't seem to take off in a moderate cross-wind (> 12kts). 
 The tail always blows around and the rudder/brakes can't stop it.  Does
anyone know if this is normal behavior?



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