# Why don't commercial aircraft use all-movable tail surfaces like my RC plane? [duplicate]

Many conventional RC planes use control surfaces at the tail just like the image below:

You can see there is a part in front of each surface control that never moves and only the surface control itself moves.

So, I built an rc plane without that part in front of the surface control. Take a look below:

This plane I built flies really well (no aileron). So I was wondering: why commercial aircrafts or even rc planes dont use this type of rudder/elevator without the part in front of it? See: making the control surface the way I did reduces drag since you dont have a fix part at the tail that does nothing, it reduces weight cause you just need the control surface and nothing more and it is also way easier to build. I cant understand why commercial aircrafts and rc planes dont make use of this exactly same type of rudder/elevator!

Could you please tell me the reason?

• Some aircraft do (for the horizontal stab). The tail in my Cessna 177 is an "all flying tail", also called a stabilator. I haven't seen this for a vertical stabilizer, which I believe is because if it ever got stuck (which sometimes happens), a full-moving vertical stab would be uncontrollable. Most aircraft don't need that much rudder/elevator authority anyway, and the structure required to do that would be significant. – Ron Beyer Oct 23 '18 at 19:43
• @Ron Beyer thanks. I checked the CESSNA 177 and the tail does not look like mine, it looks like a normal tail -> mms.tiger.tw/upload/2009-5/20095154522.jpg . Despite most aircrafts dont really need that much controls surface, on my case I realized I could easily cut my control surface in half and I would still have plenty of control. So that's another good point on my question, why most airfrats dont use the type of tail I built? I still cant understand cause this is so effective and I see no downside. – amandanovaes Oct 23 '18 at 19:52
• That picture is of a model airplane and isn't accurate (at least not for the 177's that have it, some did not), here is what one looks like: goo.gl/images/gRUxDW . The downside is that there is still a lot of force there, and there has to be structure to hold it. It isn't much on an RC plane, but you are talking literally tons of force that have to be transferred to the fuselage (for an airliner). – Ron Beyer Oct 23 '18 at 20:06
• The Lockheed TriStar L-1011 also has an "all flying tail" or stabilator if you want to see something commercial sized. Most airliners have a tail trim system that can move the entire tail some number of degrees, and the elevator takes care of the rest. – Ron Beyer Oct 23 '18 at 20:34
• Do you realize that your model is capable of maneuvers that (compared to it's size and speed) would tear apart not only a placid airliner but a high-performance fighter as well? – Agent_L Oct 24 '18 at 10:29

What is better and easier for small-scale models is not necessarily better for larger aircraft.

First, you can't say that the fixed part "does nothing". The tail is primarily a stabiliser; without it, a normal airplane will not fly at all. Only then it is a control surface, which allows it to fly well and how you want it.

Consequently, the size of the tail is primarily driven by stability considerations. For control, you need a certain amount of it and not too much more. If you have an all-moving tail, you may find that you need to move it only very slightly, or else control will become too sensitive. This is not necessarily easier to do.

From design point of view, the all-moving tail requires that the whole stabiliser was attached to a single shaft. On a small RC plane you can afford to make it of a simple wire. On a large aircraft, this translates to something bigger than your suburb water supply pipe: stabilisers of planes like A380 or An-124 are larger than wings of some airliners. Apart from the scale problem, this makes it a single point of failure, which makes it still heavier to be reliable.

Another design problem is nice aerodynamic coupling between the moving tail and the fuselage. Often designers have to invent something like this (A330):

Overall, it is usually easier and lighter to make a conventional tail and attach an elevator (and rudder) to it.

Nevertheless, quite many airplanes have an all-moving stabiliser. There are several different reasons for this.

1. To accommodate a large range of centre of gravity (CG) locations, i.e. load distribution, and/or different aerodynamic configurations (e.g. flaps). This requires a large amount of control. But this control is typically 'slow': the load or configuration doesn't change in fractions of a second. So the tail is split between the slow all-moving part providing the so-called trim, and the fast conventional elevator. Most airliners have such arrangement, like the A330 from the above picture. But the largest ones, the An-124 and -225, don't use it: too hard. They entirely rely on the elevator.

There is rarely such a demand for the vertical tail: airplanes are more or less symmetrical, so rarely will you find an all-moving vertical stabiliser like this.

2. To provide easier load balance for small airplanes with reversible control, i.e. those where the control surfaces are moved by the pilot's muscle force. If you make an elevator on normal hinges, it may be quite hard to move it at high speed. Various tricks are used to make the force comfortable (which should be 'just right', not too much, not too small). But one of the straightforward solutions is to make the whole tail move about a carefully chosen axis. For smaller GA aircraft, the scale problem is not too bad yet, so many designs use this approach, for example, Piper PA28

(The thing you see at the trailing edge is not a traditional elevator, it is the anti-servo tab that provides the 'just right' force).

3. Normal elevator control efficiency. Sometimes, airplanes just need more control than the normally-sized tail can provide. This primarily applies to agile supersonic aircraft. There are several reasons for it:

• Agile manoeuvring obviously needs more control;
• At supersonic speeds, pitch stability (and thus control effort) increases significantly;
• Elevator efficiency falls. The thing is, at subsonic speeds, the normal elevator redistributes the airflow over the tail such that more air is affected than just the area of the elevator assumes. The relation is $$\sqrt \frac{S_{elevator}}{S_{tail}}$$, so if you have elevator occupying half the tail, its efficiency will not be just $$0.5$$, but $$\sqrt 0.5 \approx 0.7$$ compared to the all-moving stabilator deflected to the same angle. But at supersonic speeds this doesn't apply, and the relation is just linear. So all-moving surfaces make more sense. Occasionally, you can even find an all-moving vertical tail, like on Tu-160 (note it has all-moving horizontal tail as well):
• The anti-servo tab on the Piper is also used to trim the airplane. – JScarry Oct 24 '18 at 15:33
• Thanks, it took me a while to understand everything you said and I am pretty sure I totally agree with it :) One point that you didnt mention and I think is the greatest advantage of full control surface is that if you trim it you will always have the minimum drag as possible. If, for some reason, your tail is not well aligned with the main wing, I can easily trim the elevator and I will have the most efficiency as possible. On the other hand, with a wing that has a fixed stabilator if you trim the elevator you will still have a misalisgment cause of the fixed part (you cant move it to trim it – amandanovaes Oct 24 '18 at 16:38
• @amandanovaes, you are right, although for very small angles this is not necessarily true (it may cause less drag to change the curvature of the surface slightly than to tilt it). Also, the compromises you have to make with aerodynamic coupling of the tail to fuselage will likely offset much of the gain. But there are other advantages too, I just didn't try to list them all. For example, if you have a combination control like on airliners, you are not using up the elevator capacity for trim. And this creates control redundancy to some extent. – Zeus Oct 24 '18 at 23:48

For a small RC plane that travels at slow speeds your design would work great. However these type of designs have structural limitations. For an all movable vertical and horizontal stabilizer setup all the forces exerted on the control surfaces would be transferred to a movable bearing. This would be a single point of failure if the bearing were to fail. Normally the vertical stabilizer or horizontal stabilizer would use several bolts to attach it to the airframe. In some airplanes these control surfaces are welded on. This distributes the stresses more equally. I fly a Piper Cherokee with an all movable horizontal stabilizer (stabilator) and one of our pre-checks is to make sure there is no give in the bearing. If the bearing was to give out we would have a bad day.

• Structural limitation may be the only reason why this design I am using is not widelly used on commercial airplanes cause in every other point it's more efficient, less drag, more maneuvability... Thanks for your answer! – amandanovaes Oct 24 '18 at 16:39

Well they do to a degree.

Large commercial aircraft have considerably larger CG ranges than their smaller counterparts do in order to accommodate a wide range of loadings as well as shifts in the Cp due to compresssibility effects at high Mach numbers and changes of AoA. To accommodate this, most airliners make use of a full moving tailplane for pitch trim, thence an additional elevator control surface is added for pitch authority by the pilot. This offers a compromise between excellent pitch and minimal drag and weight as opposed to actuating the entire tailplane. Now fully movable tailplanes for pitch control have been implemented before, as Ron Beyer points out in the L-1011. But it’s not very common.

• Yeap, I think trim is a good point in using the full moving tailplane cause you can trim it as you need and you will reduce the amount of drag as you with (if you have a fixed part in front of the control surface it will always be that same position and if it's not well aligned with the main wing, triming the elevator will just cause loss of efficiency). Thanks for your help! – amandanovaes Oct 24 '18 at 16:40

The fixed tail surfaces are to provide a passive weathervaning effect for stability. If you have an all-flying tail, it tends to just want to trail into the airflow, so you need one of two things; hydraulic control, where the control surface is rigidly positioned by actuators and the control feel is by springs (The F-86 had an all flying hydraulically operated horizontal tail), or if non hydraulic, the surface needs an anti-servo tab (like the C-177 and just about every cable operated stabilator tail out there) to provide a speed sensitive restorative force that drives the surface to a certain position hand holds it there, and to provide a desirable stick force gradient (increasing force with displacement from trim speed). The anti-servo tab also provides the pitch trim function by changing its neutral point when you adjust the trim, changing the airplane's trim speed.

There are some gliders that have all flying tails with no servo tabs, and these depend on a bungee spring in the elevator control circuit for all of their stick free pitch stability. I used to fly a glider called an LS-1 that was like this, and the bungee wasn't particularly stiff, and the airplane was divergent in pitch if I let the stick go, but it was fine if I was holding it.

And airplane like the Cardinal or Cherokee, with a manually operated stabilator tail, would be pretty difficult to control without the anti-servo tab. The tab's control rod is a very critical inspection item on a pre-flight. The principal advantage of flying tails is more control authority for a given surface area (the Cardinal has MASSIVE tail power compared to similar aircraft), and the ability to fine tune the pitch feel gradient by how the anti-servo tab is geared.

Your RC airplane gets away with no anti-servo tab because the electric servos hold the surface at a given position. Basically the same as hydraulic controls.

The Zenith family of homebuilts use an all-flying rudder with no fixed vertical fin. This wouldn't normally work so well without a fixed fin to provide a passive weathervaning force, but the Zenith gets away with it by having a very large fuselage vertical surface area aft of the wings. Otherwise it would have to have an anti-servo tab for the rudder in the absence of a vertical fin.

• Thanks I reallly appreciate your answer, I never thought about the anti-servo you said, very clear! – amandanovaes Oct 24 '18 at 16:42

It mostly comes down to force moving the entire control surface would require a massively beefed up connection point that would detract from the primary want of a commercial aircraft, carrying cargo, whether that be people or things. Military aircraft use it for the massive gain in maneuverability but most passengers get a might bit distressed if their 737 where to suddenly stand on its tail at the slight flick of a pilots wrist.

• It may come down to force but since you have a much bigger control surface you dont need to move it that much so you can use gear reduction on your motor to move the control surface. But thanks for your contribution! – amandanovaes Oct 24 '18 at 16:42