hs piviot

[Iron]

Hi
I haven't asked any question for a while my building came to a halt because of my real job. I was wondering if some one could tell me what they use for the piviot for the hs? A drawing would be helpful along with pics. Thanks in advance Roy

[Doug Riley]

What kind of HS? Most are fixed and lack a pivot. Some have ground-adjustable incidence. Designs vary widely.

[C. Beaty]

I gather, Doug, Birdy is using a hinged horizontal stab, referred to as a dunny lid on the Australian forum. Evidently a fixed horizontal stab interfered with his toss and catch routine.

Hinged so that it is free floating for upward gusts but against a stop for downward gusts.

Although dunny lid is an item of Australian slang that escaped me, the context in which it was used gave me at least a notion of its meaning.

[birdy]

Reckon he'd be think'n along the lines of a controlable one, linked to the stick.
Not a good idea IMHO.

[Aussie_Paul]

The upside down extruded rotor blade blade stabs possibly do what Birdy wants. They only work really efficiently one way, but what would I know, :eek: according to Birdy!!!! :rolleyes: LOL ;)

Aussie Paul. :)

[Iron]

Doug
I plan on building a fiberglass hs similar to the air command. I think it was on this forum i read that some one was able to adjust the hs a degree or two as needed for better stability. I was just curios how they did it or if I should worry about this? Thanks

[Adam H]

I'm wondering the same thing as iron, does pitch in the hs matter?

[Doug Riley]

Yes, guys, the incidence (equivalent of pitch when you talk about non-rotating airfoils) does matter. If your gyro is truly CLT and the airframe doesn't itself have a tendency to nose up or down as a result of airflow over its shape, then the HS should be set so that its chord is parallel to the airflow. This way, the HS does not impose a continuous load (either up or down) on the frame as you fly. The HS then acts only when there's a disturbance in the flow reaching it (such as an updraft or downdraft). This HS setting is known as zero incidence.

If, OTOH, your prop thrustline is above or below the CG, and/or your airframe is shaped so that the airflow produces a nose-up or nose-down pitching tendency, the HS can be used to compensate. Mounting the HS with its leading edge lower than its trailing edge (taking the airflow as your horizontal reference*) will apply a tail-down and nose-up torque to the frame. This is negative incidence. The opposite setting --leading edge high -- is positive incidence.

Most gyros** have nose-down tendencies arising from both the airframe effects and more or less high thrustlines. Hence most gyros can use some negative incidence. How MUCH varies tremendously from HS to HS and gyro to gyro. Therefore, it's not possible to give a general rule. As an example only, the Gyrobee has a thrustline that's an inch or two higher than the CG, and the Watson HS that's shown with the 'Bee plans works best with the HS set at a negative incidence of between 2.5 and 3 degrees.

Many HS designs don't incorporate an incidence adjustment feature as such. It's not too hard to change incidence, however, using spacers in the mounting bracketry. Even if the HS and vertical tail are a solid unit (as in the Watson-Gyrobee and Dominator tails), you can rotate the whole unit by lengthening or shortening the mounting plates or struts. It's a little more challenging with an Air Command 1-place HS -- you'll have to drill new holes to change HS incidence. I'd be wary of putting much of a load on this model of HS anyway; that cantilever tail pivot can only take so much force.

* Note that "horizontal" doesn't necessarily equal "parallel to the tail tube," but rather "parallel to the airflow." Whether the tail tube rides parallel to the airflow is one of the variables from gyro to gyro. This tube rides a little nose-up on the Gyrobee and noticeably nose-down on the older models of 1-place Air Commands, for example.

**Dominator and similar "high rider" gyros often have their prop thrustlines BELOW the CG -- the opposite of the more typical HTL gyro. I would not suggest putting much, if any, positive incidence in their HS's, however. The underslung thrustline is there for a reason, and setting the HS to fight it amounts to messing with a good design. Leave the HS at zero or +1 deg. on these machines.

[Iron]

Thanks Doug. I will mount this so I can change the angle 1 or 2 degrees in either direction.