Tail feather shapes

[Ted Eggleston]

Another of the issues on the Aussie forum which has come off the rails.

Has anyone ever flown, designed or thought of a "T" type tail? I have envisioned a tail where the vertical fin rises to carry a horizontal stab just above the centre line of the prop.

It was suggested to me (including that I shouldn't be thinking about this matter) that this is not an original concept. Notwithstanding this point, I'd appreciate any thoughts on this issue please.

Ted

[Dean_Dolph]

.....Has anyone ever flown, designed or thought of a "T" type tail? I have envisioned a tail where the vertical fin rises to carry a horizontal stab just above the centre line of the prop.TedTed, if you do a search on this Forum I suspect you will find that Ron Herron's T-tail pops up. Ron's T-tail (vertically located close to the prob thrust line) has been used, and praised, by enough people that have used it on Bensen type machines that I would think that is has become an acceptable design. However, I don't believe the horz stab is above the prop thrust line. Do you see an advantage in doing that other than providing more vert stab area?

Since you have raised this issue on the tail, I have another tail question for our experienced designers/pilots that I'll start another thread for.

[Timchick]

Here's a couple T-tails. Picture one is Ron Herron's and picture 2 is on the Monarch Butterfly.

[Ted Eggleston]

Tim, is there a way of calculating the strength requirements of attaching such a "T" tail?

Ted

[Ted Eggleston]

Dean, I've been looking at lots of gyro's trying to work out why the designer thought they needed the tail that they have.

While the moment potential of the tail is governed by the distance from the centre of gravity, and is considered with the desired usage of the gyro, I am often left wondering why the size.

Take the UFO Helithruster for example. A vertical fin the size of the proverbial dunny door, yet references are often made that rudders are not the primary turning method when flying. Is the airflow around the pod so badly disturbed that the tall tail is required to improve the action of the tail feathers? Why does it need to be so big? Why couldn't you get away with something only half that size?

If the primary function of the horizontal stab is to assist in maintaning an attitude particularily when altering power settings, why shouldn't they be bigger? We could keep them the same size, but move them further back to increase their moment potential, but then how far back would you go?

Those who don't believe in CLT will probably ignore this posting anyways I guess, but its an interesting design area for me.

Ted

[MikeBoyette]

Ted,
The tall tail is designed to straighten out the vortex coming off the prop aka P Factor. Chuck Beaty has posted many times about this if you do a search under "Tall Tails". Another thing a tall tail does, because it is on both sides of the prop, is it all but eliminates torque roll. It also enables you basically not touch the rudder once your in the air. The virtues of this design outweigh the uglyness. As for the horizontal stab, imagine an arrow with only a vertical set of feathers. It probably would go completly out of control. I hope this helps.

[pwendell]

Ted,

I'm no expert, but the way I understand it, the vertical stabilizer has to exert enough force to counter the drag of everything forward of the HORIZONTAL CoG. In an enclosed, 2 seat gyro like the UFO, Sparrowhawk, RAF, J2, etc, the cabin creates a lot of drag, especially with the doors are on. If the vertical stabilizer is not adequate, the gyro will unstable in yaw and can have a tendancy 'swap ends' in forward flight. Not Good. Since pusher gyros are generally close coupled, i.e., they have short tail booms, they often require a lot of vertical surface. This is why all the certified machines had several vertical stabilizers on their tails.

[Ted Eggleston]

Peter, This then would explain why Magni's have three vertical fins. Yet others have just a single fin. Would this then simply indicate an ineffeciency in this area?

Ted

[pwendell]

The effective power of the vertical stabilizer is a product of size, distance from the CoG, and the speed of the airflow. A vertical stabilizer directly in the prop stream will be very powerful as long as the engine is producing good power, but much less so at idle, for example on a power-off approach. I suspect that the three stabilizer design is to take advantage of both the prop stream and airspeed to provide adequate power under all speed/power combinations. Machine with no cabin require much less stabilizer power. Machines with tall tails have a single, very large surface.

[Timchick]

Ted,
I have no idea how to calculate strength requirements for attaching a t-tail. Per your reference to the magnis 3 vertical fins, I've heard the RAF owners talk about liking the stabs with the fins out on the ends better than the stabs without. I'm assuming the magnis are compensating for the long body pod up front. I have seen some t-tails on single place machines with fins out on the ends of the stabs also.

[Ted Eggleston]

Tim, This is a kind of how long is a piece of string question but is there a way of calculating the speed and volume of the air being generated by the Prop for a given speed? Its not such a simple calculation since the incoming airflow is interupted by the build of the machine in front of the prop. Could you use a figure of say 80% of the possible volume being generated as the general figure and then work out the moment potential of the proposed tail?

Ted

[Timchick]

Ted,
Sorry but I can't help you on that one. Your speaking Greek now. There are some guys on the forum that can figure that stuff out but my thinking is limited to "when the engine is on, giving it more throttle makes the prop push more air".

[Doug Riley]

Ted: Estimating the lift that an airfoil (like a tail) will generate isn't complicated. However, airspeed is basic to the calculation. You need to know what airspeed the tail actually "sees." This airspeed will usually vary with throttle setting, prop pitch and other variables. It's likely to vary over the span of the tail, too.

The most accurate way to find out what airspeed a given tail experiences would be to place on or more pitot tube openings at the leading edge of the tail. Hook these up to an extra airspeed indicator (with selector valves if you have more than one) and put the aircraft through it various flight paces. You then would capture the effects of both prop slipstream and the freestream.

For structural purposes, take the worst case (highest tail-surface airspeed and largest angle of attack) to determine the highest expected load. Add a generous reserve and you should then have a load to design around. Engineering reference books (such as Marks), and aerodynamics texts, will give you the formulas for lift, drag and airfoil moment as functions of airspeed and angle of attack.

[Ted Eggleston]

Yes I once had the opportunity to purchase the Marks book but thought it too expensive. Perhaps I shal have to revisit the opportunity.

Doug, One of the more knowlegeable guys on the Aussie forum (Murray Barker) suggested that by extending the tail feathers further back the more positive a responce you will get. While I agree with this suggestion, I'm concerned that the further back you go, the smaller (height wise) the vertical fin due to the normal angle of the rotor plus an allowance for flap.

So I'm guessinghere that by extending the keel back say a foot would make a significant difference. True??

Ted

[Timchick]

Ted,
One of the upgrades to the Air Commands was an extended keel which moved the tail back some. I'm not sure how far back it moved it. Someone with an Air Command may need to jump in and let us know.

[scottessex]

12" We just put the extension on Phil Ruffin's A/C, we thought it would be a good idea with the rotax 618.

[Timchick]

Thanks Scott. DVDs are on the way.

[Doug Riley]

Yes, moving the tail back adds both control power and (especially) the ability to damp oscillations. As you move back, though, tail size gets squeezed in both directions. You must leave some ground clearance to allow proper rotation on takeoff and landing, as well as rotor clearance.