Hello to you all.

I have had a doubt about correct sizing of fin/rudder-stabilizer/elevator sizing in scale RC model airplanes. In the old times of free flight we indeed increased those surfaces (fixed) but I don't know if there is a need of doing this in our RC planes. Is there some aerodynamic reason or is it just a try of increasing manoeuvrability of model airplanes? I've been in aeromodelling since I was a child (55 years old by now!) and I have built some WWII fighter stand-off scale models respecting the original sizes of their tail surfaces without problems. The only one I encountered was in an electric Me109G 48" WS that had very bad longitudinal stability, I tried to solve it by resizing the stab/elev area but had a violent stall in take-off and (sigh!) I recovered just a nice bunch of toothpicks and solartex amalgamated with batteries-ESC and motor.., so I never knew if that solved the problem. In our club this theme has become a classic and although I've found some articles that clearly state that there is no need of increase tail surfaces, there are also many commercial aircrafts and plans that actually increase them by 10% to 33%....
Apologies for my english, I'm spanish

Radios don't have interferences, pilot's fingers do have!

Aaaargh!
My first post here and the subject is mispelled. Excuse me![:@]

Tail size will play a roll in establishing the sensitivity to CG position. If your CG is spot on where it belongs, go with scale size.

Have fun!

Bedford

Basically, you're talking about horizontal tail volume. Dutch roll is seldom a problem with our models. But pitch sensitivity is.

The reason model's tails were enlarged years ago was because years ago the RC equipment wasn't very good. And a lot of the models were free flight designs originally. They needed the "extra" stability back then.

Nowadays we get by with less stable models. Our radio equipment helps us do that.

If you're laying out a scale model that you're going to compete with, it's important to try and keep the scale outlines. It can bite you, however. You can wind up with a really hard to fly model. But if you're not drawing up a pure scale model, what's the problem? Fudge the fuselage length a bit. Fudge the aspect ratio of the horizontal tail some, and make the chord a bit longer. Keep the proportions of the horizontal tail and nobody will notice.

Heck, almost all the "scale" arfs nowadays have flat surfaces anyway. Just depends on what bothers you and what doesn't, and what you can actually see and what you can't.

Well said! Someone who knows!

Increasing that span a little will have a better effect than increasing the chord.

I know someone who flew real Me109. He said it was a terrible airplane - the tail is too small, especially the rudder. So a scale model of it will just be worse, since Re are lower. Build the next one light, and increase the tail volume a bit (by increasing the span) if you can get away with it.

Thank for your quick responses!

darock

Yes, I agree with you in that with modern RC gear it is unnecessary to enlarge the tail "volume". So, why has been established almost as a rule to increase the real size (scaled down) of almost all scale model airplanes?. In fact my scratchbuilt Tempest Mk VI (electric, Mega 16/20/4 geared 3.2:1, LiPo 3s3p, balsa/solartex fuselage, extruded polystirene core/balsa wings 125 cm.(50")WS) has a stab exact to the original with a NACA 0010 airfoil and behaves quite correctly, never a problem and quite mild in basic acrobatics (aerobatics?).

mesae

That's an old "aeromodeller legend". My earlier (still flying!) 109E, all balsa, "cheated" washout in wings, is easy to fly and land, only troubles arise in taking-off from irregular fields mainly because of the narrow landing gear, maybe G model with its rounded wingtips, no washout, was more prone to abrupt stalls?

Radios don't have interferences, pilot's fingers do have!

I was referring to a person who flew a real FULL SCALE ME109 from WWII; not a model.

Even a scale-outline ME109 model can fly resonably well if built light, with possibly different (non-scale) airfoils.

And people tend to continue doing what they used to do. It becomes "what you're supposed to do".

I believe that on full scale GA aircraft (like a Cessna), the area of the elevators have little to no effect on stability, since they are just floating when the aircraft is trimmed. However on an RC model, the surface is in effect part of the horizontal tail area, since their position is tightly controlled by the servo (assuming enough torque to avoid blowback). So it really is unnecessary to enlarge surface area on scale models.

Well, from now on, no more headaches about sizing of tail feathers, just scale and up and away!.

Many thanks to you all

There is a measureable difference between stick-fixed and stick-free stability (stick-fixed is more stable), but stick-free stability is still much greater than "elevators-missing" stability. The elevators are free to move somewhat in stick-free, but not as much as you seem to assume. Much of the dynamic pressure on the elevators applies a centering force, which must be overcome by small changes in AOA. If the elevators were aerodynamically counterbalanced to provide no centering force, then what you wrote would be true. This is not usually done in practice, especially on light certificated GA aircraft.

OK, my point is still that an RC airplane operates in essence with a "stick-fixed" stability due to the servos holding position, which gives more stability than many of the light aircraft modeled that operate with a floating stick stability. I guess the question is, how much is gained.

As far as the ME-109, Charles Lindbergh flew it before the war and thought very highly of it. Of course he was used to aircraft of that era, as today even the Spirit of St. Louis is considered nearly unflyable.

Points taken and agreed with.

As a pilot and flight instructor for some forty years, I can tell you that in full scale aircraft the elevator is seldom floating but is usually fixed by the pilots grip on the stick. Of course trim removes the stick force at a particular speed, but any deviation from trim condition results in immediate pressure to counter the deviation. The pilot not only provides a stick fixed condition but contributes positively to stability by resisting deviations from trim before the airplane has time to respond on it's on. Any other technique is simply sloppy flying.

Whether a particular model needs a larger tail is certainly a valid topic for discussion, but from the standpoint of stability, the pilot is a positive contributer and a remote control situation where the feedback is only sight at a distance cannot be as effective as an immediate feedback from the "seat of the pants".

You bring up valid points. But comparing a pilot's attempts at maintaining attitude/altitude to servos holding the control surface fixed is not necessarily a valid comparison. When test pilots test stick fixed stability, they hold the stick/yoke rigidly in place, to see how the airplane naturally responds to this condition. They are not applying corrections, as this would mask the stability of the airplane itself and make it impossible to isolate. You can't measure a phugoid if you are not allowing it to occur by damping it with out-of-phase inputs.

Conversely, when they test stick-free stability, it's hands-off, again to measure the airplane's natural stability response, unaided by the pilot.

I think you missed my point. I’m well aware that the test pilot holds the stick rigidly when testing for stick fixed stability. What I’m saying is that the normal pilot likewise generally holds the stick fixed. When he does move it, the motion usually is a corrective motion that contributes positively to the overall system stability. High plain’s statement that the pilot somehow just lets it flop around free is simply not true. There is little difference between the surfaces being held fixed by a servo or a pilot.

(In full-scale airplanes of conventional configuration, the phugoid period is so long that a pilot never lets it develop.)

The problem when scaling down a full size aircraft is that the tail moment arm scales down in a linear fashion, while the tail surface area scales down in an inverse square fashion. If you make a 1/4 scale model, the tail moment is 1/4 that of the original, while the tail surface area is 1/16 that of the original. Add to that the change in Reynolds number, and the effectiveness of the tail surface is further reduced. The scale model typically will not have near the GG range of the original, nor the stability.
Roger

LouW,

One of the points of having elevator trim in an GA airplane is not having to hold the stick to hold altitude during cruise. This frees the pilot to do some of the many chores that it takes to manage a flight. Some pilots even steer with slight pressure on the rudder and do very little hands on the stick while in cruise. Very common too are heading hold autopilots that only control the ailerons. None of that covers the original point of the effects of scaling tail surface area in models and why one may not need to increase the surface area.

Rog,
In your 1/4 scale example, you have 1/16 the area in the wing too. As far as CG range, in a model we don't typically have to worry about carrying two fat cousins in the rear seat with empty fuel tanks at one end of the loading spectrum or flying solo with full tanks, so the CG problem is minimized. Some of us even have the tank at the CG to avoid trim changes with burn, which allows very small tail surfaces (around 10-12%) in very special applications.

Using an airfoil that has a very low pitching moment will help a lot as well. For example, switching to a symetrical airfoil allows the use of very small horizontal tail areas in conjuction with a CG closer to the 25% mark and this will produce a decently stable model.

I don’t know where you get your information. I have flown thousands of hours over the last few decades and trained many pilots in the process, and when flying an airplane without an autopilot, there is seldom an occasion where a hand is not holding the stick. The phugoid mode mentioned by mesae causes the airplane to slowly climb and descend in cruise no matter how carefully it is trimmed, even in calm air. In addition, there are up and down drafts nearly all the time that require response in order to maintain a level altitude.

Unless the ailerons and rudder are interconnected, like on the Bananza, Tri-pacer, and some others, the practice of steering with the rudder in flight is just sloppy flying and still requires corrections in pitch to maintain altitude.

The single axis autopilot you describe is useful in tracking nav aids, but the pilot still has to maintain altitude with the stick and it is not a “hands free” device.

When you see that Cessna or Piper flying over, you can bet the pilot is holding the stick.

I must disagree. In measuring stick-fixed stability, (again) the conntrols are held rigidly in place. NO corrections are made, however small. If a pilot is making pitch corrections, we are not talking about stick-fixed stability, and no direct comparison can be made to servos holding the surfaces rigidly in place, as servos and receivers do not by themselves have the ability to make autonomous corrections to deviations from trim, as an on-board pilot (or autopilot) does.

Scaling down does not change the relative effects of the tail arm and the stab area on pitch stability. The formula for the tail volume, which measures the stabilizing power of the stab relative to the wing, is as follows:

TV = (stab area/wing area) x (tail arm/wing chord)

All the units, squared and linear, cancel out. The resulting TV can be used to calculate CG. For any given design, the formula gives exactly the same result for small planes as it does for large ones. I have used this formula on every plane I have built for over 20 years, from 2 oz CO2 powered models to .40 powered models and it works the same for both, as it should.

Increasing stab area allows one to move the CG back, and that can be very useful on scale models for two reasons:

1) Models land at much higher speeds relative to their size than full size planes and their wheels are smaller relative to the bumps on a typical grass field, and therefore models have a greater tendency to nose over on the ground. Moving the CG back decreases that tendency.

2) Scale models often have short noses, making them hard to balance. Moving the CG back obviously makes that easier.

I think that changes in stab area, within limits, are one of the least noticeable deviations from scale and also one of the most useful.

Jim