Full Flying Tail vs. seperate vertical and rudder.

[david holmes]

Is fixed vertical stab with rudder better than "full flying tall tail"?
Puff had fixed vertical stab and "full flying" rudder. I have to rebuild both and wondered if the aerodynamist out there could help me.

[Doug Riley]

No aerodynamicist answered, so you may be stuck with me.

A unit with separate fixed fin and hinged rudder has more control power per square foot than a one-piece job. The moveable rudder on the hinged unit acts like a wing flap. Flapped airfoils can make more lift per square foot than unflapped ones, without stalling. The "bonus" power can be on the order of 20-40%.

The 2-piece units tend to be more trouble to build and maybe a bit heavier. That's why you see so many one-piece versions.

[Mayfield]

When I conceived the SH I was phobic about weight. I knew that I was going to come out on the upper end of the acceptable empty weight envelope.

That being said:

I wish now that I had bit the bullet and designed a vertical stab/rudder. Being able to vary the camber of the vertical wing that we call a stab/rudder combo would have done a couple of things:

1. Increased yaw stability. If properly constructed and balanced, a vertical fin/rudder will always try to "undo" the camber change we put in with the pedals.

2. Increased control power a lot. Changing camber is a more efficient way to alter the sideways lift required than changing angle of attack.

I had planned to replace the aluminum tube spars in the HS and the vertical with "U" channel carbon fiber. This would have given me enough weight savings to be able to afford a separate stab/rudder and still fit within the weight budget.

Jim

[twistair]

Jim (hi!),

Could you tell something more about this separate stab/rudder. I'd try to make one, composite or whole-metal.

[Mayfield]

Hi Alex,

It's hard to believe that another winter is here. Seems like yesterday that I was trying to get your first kit out to you.

On the tail:

I wanted to maintain the same span and chord.

The vertical skins would be cut lengthwise about 6 inches aft of the leading edge. Fabricate two vertical "U" channels. 3 layers of 10 ounce carbon should be sufficient except for three 2" X 2" sections where the hinge would attach. I would probably use 8 layers there. The "U" channel would be the same width as the inside dimension of the vertical at max chord. inch bonding legs should be enough in the minor dimension. The "U" would face forward in the stab, and aft in the rudder. I had not settled on a hinge system, but small rod ends would work.

On the HS, Use the same "U" channel as a spar. Face opening aft. I would have to be bolted/bonded to the aft face of the vertical stab spar. Some H stab area would be lost because it would have to be cut to allow 25 degrees or so of rudder travel each side of neutral. The leading edge of the H stab would have to be reinforced to allow it's use as a forward H stab spar. The twisting loads would be too high without the equivilent of a forward and aft spar.

I no longer have my SH notes so I can't give you dimensions.

Regards Alex.

Jim

Jim

[twistair]

Jim,

Thanks for your suggestions. I have all SH tail dimensions so that's not a problem to draw the rest.
I have one concern and one further question to the moment:

1) is it possible that fixed stab will affect yaw control efficiency at engine out or idle?

2) in an original SH tail we have very good pivot points based on spherical bearings. With fixed vertical, what would be better way to pivot vertical to upper and lower tail supports? I try to understand if we still need some flexibility in this points to avoid vibration and/or rough landing damage?

BTW I feel we completely hijacked David's thread (sorry, David Probably we are to transfer this theme to another thread?

[C. Beaty]

Alex, because of the flexing of upper and lower supports for a tall tail, upper and lower mounting points should be soft with a drag link to maintain alignment.

Spherical bearings aren’t necessary but you should at least use plastic bushings.