Stabilator (Full Flying stab/elevator) Pros and cons?

[Fuegodeth]

Just like it sounds. What are the pros and cons and ins and outs of full flying stabilizers (stabilators). I am building a 3D style profile foamy, and my design lends itself to the use of these. I usually see them on jets, so I'm wondering if I'm barking up the wrong tree by considering them or not. Also, what would be the best pivot point for 3D style flight. This is not going to be a very low wing loaded plane as it is intended for flight outdoors in wind, and to fly in combat. It's 9mm EPP 32" wingspan, packing tape and tube spars. I've got the pivot tube all worked out and tried it on a couple of versions. On the first I think I had the pivot point too far back, as when you gave it elevator it really threw the plane around. I trimmed the LE of the stabilator, effectively moving the pivot forward a bit, and reducing the area, and I kind of like it. I just wanted to hear from some of the aerodynamics/flight physics experts what you thought of these on this style of aircraft. Thanks a lot in advance for your help.

[Brandano]

pro's:
They are less susceptible to transonic compression effects.
If stalled they can be brought back to a non-stalled state by aligning them with the airflow. (typical, in the Panavia Tornado to recover from a spin you PULL on the stick instead of pushing)
They have less parasitic drag, since there's no hinge line to mess with the airflow

con's:
They are mechanically harder to implement, all the aerodynamic forces are supported by the pivot.
They are easier to stall. (having no fixed portion they don't have an intrinsic stabilizing effect on the plane. Even restricting their movement can bring them outside their maximum AOA if the plane is pitching wildly)
You can't really take full advantage out of them without a fly-by-wire system.
The parasitic drag you get rid of will likely be replaced by interaction drag between the stab and the fuselage. Gliders with flying stabs take great care with the hinging arrangement.


On a full size plane there's really no reason to go for a flying stab unless you are approaching transonic regimes or you are trying to get the absolute minimum drag out of a design. The increased weight and quirkier flight behavior tend to offset any advantages. On a 3D style foamie that is flying stalled most of the time with the control surfaces immersed in the airflow from the prop then fully flying surfaces will work probably well, but you might want to reduce the throws when comparing them with those of a conventional tail. Any angle higher than 20 degrees will probably mean the tail is stalled and ineffective.

[nauga]

More pros:
You don't have to worry about decalage adjustments.
They have a lot more control power than a conventional elevator

Nauga,
who uses them because they're to scale ;)

[Aphorism's Dream]

pro's:
They are less susceptible to transonic compression effects.
If stalled they can be brought back to a non-stalled state by aligning them with the airflow. (typical, in the Panavia Tornado to recover from a spin you PULL on the stick instead of pushing)
They have less parasitic drag, since there's no hinge line to mess with the airflow

con's:
They are mechanically harder to implement, all the aerodynamic forces are supported by the pivot.
They are easier to stall. (having no fixed portion they don't have an intrinsic stabilizing effect on the plane. Even restricting their movement can bring them outside their maximum AOA if the plane is pitching wildly)
You can't really take full advantage out of them without a fly-by-wire system.
The parasitic drag you get rid of will likely be replaced by interaction drag between the stab and the fuselage. Gliders with flying stabs take great care with the hinging arrangement.


On a full size plane there's really no reason to go for a flying stab unless you are approaching transonic regimes or you are trying to get the absolute minimum drag out of a design. The increased weight and quirkier flight behavior tend to offset any advantages. On a 3D style foamie that is flying stalled most of the time with the control surfaces immersed in the airflow from the prop then fully flying surfaces will work probably well, but you might want to reduce the throws when comparing them with those of a conventional tail. Any angle higher than 20 degrees will probably mean the tail is stalled and ineffective.

Well, SORT OF an accurate summation.
DO NOT forget that the entire tail section is now one mass which imposes greater importance on proper balance (aerodynamic and mass) to control flutter or linkage distortion.

[Fuegodeth]

Thanks guys. That is great info. Fortunately what I am building is intended as a cross between a 3D trainer and a combat plane. Preferably good at both. Due to the combat (full contact, not streamer) large tube spars are used throughout the airframe, including the tail. I figured why have a separate spar for the stabilizer and one for joining and supporting the elevator halves. If I use slightly smaller diameter for the elevator halves and run it through the stab spar, you end up with a full flying stab. You can see pics of it at my blog, although what's up there is not the final version. From the few flights I've been able to try it out I've noticed that the elevator always seems to have good authority. It takes a lot of expo to make it controllable, and as was mentioned, surprisingly low throws for a 3D style plane. So far the hinge-spar, as I call it, has worked out well strength-wise, and is very smooth and fluid in comparison to a taped hinge. The flight loads on it certainly won't be a problem. the spar tubes are .298" OD/ .250" ID and .248" OD/.225" ID fiberglass tubes. Fiberglass is great. It's 1/3 the cost of carbon tubes, flexes farther than carbon before breaking, and has plenty of stiffness for a foamy style plane. It's actually quite easy to assemble the hinge arrangement, and on the first try took less than 10 minutes to cut, sand and install the hinge. Thanks again for the great info. I'll be paying close attention to that last post regarding the linkages. :D

[BMatthews]

If you're using a full flying stabilator for this sort of application then use a thick and stall resistant airfoil. As a bonus the extra material will make it less damage prone.

Look at using an Eppler 474 for the stabilator. Also install the spar tube so the hingline is set up with 25% of the area in front of the hinge and 75% behind so the aerodynamic loads balance out. For any symetrical airfoil this is the point where there's no load placed on the servo because the aerodynmic forces all balances out.

However to avoid flutter in combat flying you will want to add a weight to the leading edge such that the surface wants to flop leading edge down. In other words the surfaces must balance a hair in front of the hinge line to avoid flutter.

[eflightray]

One advantage I found for a largish electric glider, the all flying tail is easily removable for transport and storage, plus no push rod to disconnect. Mine just slide on to the two dowels, (pivot and control), with a tighter fit near the fuselage to retain them.

As for flying, it works, (like the bumble bee, if it doesn't know, don't tell it).

[vintage1]

Mu depron friends use thes all the time on scale-ish foamie jets..and there are no flutter problems at pretty high speeds.

The biggest downside is landing in long grass with full up elevator: the ruddy things tend to catch the leading edge and rip off..

For a 3D plane I'd say ts a very good idea. I dnt think you wil stall it either..if the speed and wingloading is low enough it will probaly flip the mainwing up and stall that..not much inertia in the pitch moment on a foamie acrobat!

Keep it down to 20 degrees to start with tho.

[Fuegodeth]

Cool. Thanks Vintage1. I didn't think flutter would really be an issue unless I was sloppy on the control linkages. I didn't see any flutter on the first one I tried. Can't wait for the last part to get here so I can try the new version out. I'm putting on a parkbec 6v to give my esc a break from the 4 servo's. The BEC has been getting warm on the esc.

[Texas Buzzard]

A few years ago I modified a Dodgeson designed sailplane so I had a full-flying stab. The problem was in how to build the pivot point and housing.

That was the weak point of my full-flying design. (Construction of the pivot is a problem )

I placed my pivot point at 30% and on that sailplane it worked O.K. I had some Pb Wts. at the L.E. to prevent flutter....seemed to work.

On a low wing loading 3-D electric I think the full-flying stab with a sorta' symetrical section would be as good as standard stab/elevator.Have fun.

[Fuegodeth]

Cool. It's flat plate 9mm foam, so it's as symmetrical as it gets. Very tough with strapping tape, gorilla glue and thick spars. Much tougher than the tape hinges that were used on earlier versions. "Have fun" - that is my primary goal. :D:D:D

[Brandano]

Vintage1 is quite right, I didn't think about it. A low AR flying stab with a reasonable sweep will probably still be effective even closer to 45 degrees AOA. In particular, wings with an AR close to 1 seem to maintain good lift even when fully stalled, and won't stall abruptly

[Fuegodeth]

So, something closer a square shaped flying stab rather than a higher aspect ratio for the stabilators. Interesting. That was going to be my next question was regarding the shape of them.