Fully moving stabelizer
 

Would like to know the difference between the standard stabelizer and fully moving stabelizer.Ii know that it would be more difficult to build but taking that out of the discussion would fully moving stablizer not be more effective.

Most of them pivot about the hinge line of the elevator and raise and lower the nose to the horizontal stabilizer.

It depends on what sort of model you're using it on. With sailplanes the full flying stabilator has the advantage of offering ease of transport by being easily taken apart and at the same time it eliminates the small but still present source of drag from both the elevator hinge line as well as any trim offset.

If terms of being more effective? It really isn't. With a regular stabilizer and elevator combo you have a variable camber and angle surface. You ALWAYS have to consider both surfaces as being one as far as the air goes. So it doesn't see a separate elevator. Just a funny looking airfoil when the elevator is deflected.

So for sailplanes where you don't alter the angle much at all the full flying tail works out very nicely. For something like a 3D flyer you're actually better off with a fixed stab and separate elevator even if the fixed portion is small compared to the moving portion. That's because the fixed portion when operating at the big angle of attack during pitch changes looks to the air like a drooped leading edge. So the tail can actually generate MORE lift than the same size tail done as a fully flying surface.

The efficiency of each style suits different flight envelopes. So you could say that each would be more effective when it's slapped onto the appropriate airplane.

Notice that fighter jets nowadays usually have stabilators. You'll also notice those suckers move A LOT when those fighters are going slow. They may not be at their best at slow speeds, but since they are so good at fighting speeds (when somebody could be shooting back at you), the designers threw in a solution for making them better at slow speeds.

Notice also that an awful lot of planes have stab/elevator setups that include the ability to fly the front of that pair. So we've actually got 3 different setups, not just two. If you think about it, aren't cargo planes about the opposite of gliders? They certainly aren't very good at soaring. ;)

Cargo planes have a huge range in operating weight. Gliders have very little range. See why one can use a stabilator and the other needs not only stab/elevator but also "more" control than available from stab/elevator?

When your entire stabilizer moves it is actually referred to as a stabilator or full flying stab if they are fixed together, In the late 80s I was still working at Dryden Flight Research Center and was heavily involved with the F18 high alpha flight visualization program, one of the modifications to the aircraft was I had to remove both L/H and R/H stabilators, and modify them on the trailing edge root rib area so they would clear the thrust paddle actuator housings we built and installed on the fuselage sides. I never really fooled with stabilators or full flying stabs much after that until about 10 years ago or so when I purchased an ARF called the Diablotin-3D or something like that and it had stabilators. The effects of those things on that airplane was amazing, you could turn that airplane inside out with enough 3D rate of travel, the airplane would literally tumble around the C/G axis at the right airspeed. Just saying...

Bob

One reason for the all moving surfaces -on current fighters - is that the forces required to move em fast over a large throw - is easier.
An elevator/stabilizer arrangement is much easier to trim but the operating forces are higher especially under aerodynamic loads
said differently - the stabilators are "counterbalanced"

As I recall there's some other great reasons related to shock wave formation at transonic and supersonic speeds that produce pitch issues on conventional tailplanes with separate elevators. And if I'm remembering correctly that make the all flying stabs on jet fighters a requirement for that reason alone. The other things are just gravy.

This is the main reason, trans sonic and super sonic flight can cause the conventional elevator designs to reverse and/or lock.

Not eggszactly.
The reshaping of the horizontal stab - as the elevator is deflected can cause a problem- flutter /reversal -
The one piece setup allows the entire surface to be put at the angle needed for CONTROL. The entire surface -at speed - is closer to the actual line of flight, than a bent surface. (stab + ele trim)
The flapping required for low speed control is not just "gravy". .
These fighter types require a huge range of tailplane angles -impossible with a hinged surface setup
On point tho-- for our models- no such issues exist

a real issue on these ,is that the stabilator requires precise control and slop and inaccuracies in aerodynamic balance and poor linkages can make the setup dicey.

That makes sense, when I was pulling the stabs off the Hornet I had to disconnect the hydraulic/electric actuator output shafts from the fork on the stabilators, as I remember the output shafts were huge with ball ends large enough for .750"-1.00" titanium pins that went through the drive forks and 0 slop anyplace in the system. I also noted that the stab rotation yokes were located aft of the leading edges around 35 to 40% of the entire stabilators, so there was more surface area behind the center of the pivoting point instead of in front of it allowing the trailing portion of the surfaces to possess the loaded advantage.

Bob

This was interesting reading and i must thank all of you for your input. Have learned a lot. The conlusion from all this that replacing a standard stadelizer with fully moving atabelizer is not worth the work and will most liky not get any improvement in a sport plane or a war bird.

Robert

yeh -other than making a scale setup -or a glider - it is really has nothing to offer for our model aircraft.
if you are in doubt about this - consider the pitch difference you typically get with just a click or a few clicks of elevator trim--once basic trim is established.

Stabilators are of no real use to sub-sonic convergent airflow. They do have a place in the realm of trans-sonic and super-sonic divergent airflow.

If I remember my full-size sailplane with stab it was a "bit" less stable in pitch! :)

Pitch stability should not be a function of whether the horizontal tail has a hinged stab-elevator or a stabilator. There are a number of details that are responsible for an airplane's pitch stability. The big 4 are wing area and chord, tail volume and tail area.

I also think that length of the tail moment comes into play. trailing edge of wing to the leading edge of the horizontal stab.

Bob

That's a rather broad statement that dismisses some of the points made in this thread. It may well be true for the (lack of) transonic issues on a model but there are some other reasons that make the option desirable for a model. First off there's the matter of scale fidelity in the case of scale models. Then there's the issue of slightly lower trim and parasitic drag of a neatly executed method on a model sailplane. Not to mention the ease of breaking down the sailplane more easily for transport.

It does bot only insomuch as the tail length to stabilizer area needs to produce a suitable tail volume coefficient. It doesn't matter if the tail is fully flying or uses a hinge line with separate surfaces. And in fact for simply flying around you can use either method on any given design simply by altering how the tail moves. Stability isn't going to care about how the tail alters its angle for pitch control.

da Rock: I call Center-of-Gravity a strong contender for pitch stability. If you don't believe it, try flying a standard airplane with a CG aft of 40% MAC.
Watch these scale guys try to get a scale model off with that CG back of 36% MAC. When I was young I got convinced of CG importance while I was flying the B-47.
CG aft of 36% was almost a sure one "partial take-off" ride. Transport aircraft have a definite limit. Hand flying an airliner (I did a lot of that) a REAL piot could feel the control pressure when a Stew walked just a few feet back and forth, especially if the load was mostly back.

Yes, the tail is much more subtle to pitch change with length, considering...

Bob

?????

It most certainly will affect the way the plane flies. That's why full scale manufacturers publish the safe range for CG so that operators of their design will know where to place the cargo. Cargo location is an operational variable the designers have no way of controlling after the plane is delivered. Changing the tail volume (tail moment and area) isn't something the operator is going to do on the ramp before takeoff.

When the B47 designers were working out the numbers, they used tried and true formulas to give them guidance on where the bomb load would best be located, for example. They indirectly supplied the CG (indirectly through the static margin number they supplied) location to see how their design suited the specifications etc.

This discussion is about the design characteristics, not so much about the operating ones.

I’ll have to side with those who say a fixed stab plus a hinged control surface produces more control power. If you look at the NACA airfoil databases you will see that many of them include the effect of a flap, and many times the lift generated at zero angle of attack with the flap deflected is greater than the un-flapped airfoil at stall. So, for a hovering 3-D machine where the stab is pretty much at zero angle of attack an elevator will provide more control power than a full flying stab. When you have a vehicle flying at higher angles of attack it get a bit more complex, depends on the downwash off the wing and what the effective angle of attack of the stab is. At subsonic speeds the elevator still normally wins out.

For a high performance sailplane where drag reduction is job one, a flying stab has an advantage, especially if you are trying to design for a wide speed range. Since the stab is in the downwash of the wing it can be designed so that the down force it has to generate can be pointed slightly forward, thus actually generating a small thrust. If you have to trim out over range of speeds the angle where this sweet spot occurs moves. I worked on the S-3 Viking program a long time ago, and it did this.

Steven

HA HA! The airplane was changed over a number of times, such as JATO from inside to outside, Added aditional fuel tank (Forward Aux) which if NOT FULL on take-Off, well, last attempt for that airplane and crew. I saw where one pitched up and rolled over and burned, scorched the earth. Man, 90,000 lb.+/- of JP-4 can roast a lot of marshmellows and hot dogs. I have also flown very slow machines, the C-123 being one and was great fun to fly. I know about cargo and such and PX folks milling around in an airplane. BTDT. I have flown dual afterburners and done the super-sonic thing, more than once.

One interesting CG item was a day when leaving ORD for Indianapolis. B-727, Stretch. Clear skies and low wind. Less than half a passenger load. Had a brand new co-pilot, right out of school, and I was going to let him fly the Indy leg. For some reason I changed my mind, and said I will fly first leg. We were turning around and coming back to ORD then to SEA. He could fly both of those legs, Maybe the man upstairs gave me that decision because we might have had a problem. The 727 started to lift up some 20 knots below rotate speed. I held her down and started rolling in nose-down trim. Made it OK. (727 trim is slab. Also has elevators for minuscule flight control) CP would have had a difficult time because not expecting such. ( ME -- Old B-47 experience)
Funny but not so funny !!!! Load Planning screwed up the load and had 165 bags supposedly in the forward baggage compartment. There were 6 bags there. Our trim was set by load planning numbers.
Ramp guy came into dispatch wondering where all the bags were. HA HA. Good lessons when everything works.

To ALL: CG changes when liquid-fuel is burned off. One way or another except for certain light airplanes, Pilots have to know about load plannig and FUEL MANAGEMENT. They are supposed to do so. I did,
There have been several close calls in commercial aviation when pilots leave it to computers. Elevators make sub-sonic flight much
easier to .manage. Stabilators make it easier to fly when going through the Trans-Sonic range.
I have built several RC (original self design) models using slabs. I find them to be very much squirrley. In the WW II era, a number of fighter pilots found out "reverse control: when in the trans-sonic dive. Story is that one could not pull out of a trans- sonic dive so he just pushed the stick forward for a final "Glory" and the machine pulled out. With throttle back and slowing of High G pull, everything returned to normal. When I was in USAF pilot training, we got lots of classroom on the trans-sonic range and very good filming done by the British, showing the magnitudes of Sonic, especially trans sonic airplanes of those days. In those days the supersonic stuff was still a mostly a theory.
Keep your slabs until you go past Mach .9. A week ago I think I saw some jets just about there at the Bomber Field (about 60 miles west of Houston) War Bird Fly-In. WOW!!!!

Ok, guys I have Byron F-18 which I am converting to EDF. I want to upgrade the stabs using the Jet Legend servo and pivot mechanism. Byron used a separate linkage to actuate the stab forward of the pivot. I want to know if I can still use the stock pivot location on the stabs , of if they need to be relocated and then balance the stabs. The stock pivot location is at 43% of the cord, measuring it the only way I know how. From the front of swept stab at the root, to rear most point of the outer tip. HELP?????

Roger,
Tghe AMA's guidelines for Large Model Aircraft (LMA) require the pivot point to be a few % forward of the Mean Aerodynamic Chord (MAC) of the flight surface. Typically the MAC for symmetrical airfoils is about 25% and that places the pivot in the 22-23% range. Recommend that you read the LMA requirements on the AMA website and caculate the MAC of the stab surface. You also might consider static balancing the stabs also, although the jury is out on this point.
The 43% pivot point is too far rearward leading to flutter and divergence. Some other posters may chime in here with advice also.

Art ARRO

Art, I'm not sure I understand the guideline. It seems to not take dynamic balancing into consideration. Considering the amount of sweep an F-18 stab has I would consider dynamic balancing is something that needs to be considered. Unfortunately with models the only way to do that is through testing. I'm not too sure I would change the pivot location or the mechanism for that matter. Reason being that when that kit was developed we did not have many servos that were over 100 oz of torque so I would have to assume that Byron did a fairly good job at R&D as I had never heard of stab issues on either the F-15, F-16 or F-18 models. I suspect the Byron mechanism has a lot to do with that as well. Being that the stab is actuated by a rod forward of the pivot gives the servo good mechanical advantage. Trying to drive it off the pivot will give it poor mechanical advantage. I'm going to guess that the reason for the change is to get more deflection and/or to be able to use the stabs as tailerons?