Friday, September 21st, 2012 11:54 am GMT -6 Friday, September 21st, 2012 11:54 am GMT -6Friday, September 21st, 2012 11:54 am GMT -6
 
Old timer model airplane

Why did old timer free flight models use lifting horizontal stabilizers? Why did they go out of favor?


 

 

 

Definition

All the time I come across two entirely different definitions for the term “lifting stabilizer”. One is totally unrelated to what I want to talk about here.

A very common definition is a horizontal stabilizer that has an airfoil shape to it. That is a good thing to have, but it has nothing to do with the definition that I am interested in right now.

The other definition is about a horizontal stabilizer that contributes to the lift of the airplane as a whole. Tails on airplanes are normally pushing down. That is a key requirement in order for them to have pitch stability. A lifting tail is actually pushing up, just like the main wing.

Reading online discussion threads on the subject, I see the participants of these threads being very confused about what they are actually talking about. Someone will make a comment referring to one definition, and the very next comment in the same thread the poster assumes that they are talking about the other definition. Read very carefully what others are saying on this subject, otherwise you are risking becoming very confused, too.

Old Timer Free Flight Models

Sixty years ago, radio control systems did not exist and control line was in its infancy. Free flight was king. Compared to modern power plants, the engines that these models used were unbelievably heavy. I have calculated that a modern engine of the same weight produces seven times more power. Put another way, the engines weighed seven times more than they do now.

I do not know where the idea originated, but somebody somewhere along the way decided that they would rather have the tail contribute to the airplane’s lift rather than push down as they normally do. This is understandable, given the very heavy power plants.

Many years ago I read in a model airplane book how a lifting tail could not possibly be stable. This was in Martin Simons’ Model Aircraft Aerodynamics. Even then, I knew enough about model airplane design to see that the logic just did not add up. It was misinformation and misguided statements like these that led me to write my own model airplane design book, so I suppose something good came out of it.

Static Pitch Stability

For an airplane to be stable in pitch, its design must meet two requirements. I am talking about static pitch stability, which is what keeps the airplane pointing in the same direction. These requirements apply to all airplanes, regardless of how their wings are configured.

First, there must be an increasing force that pushes the nose down as the airplane is pitched up from a wind gust. Similarly, you must have an increasing force that pushes the nose of the airplane up after a gust has pushed it down. The most common way to get both of these is by putting the center of lift behind the center of gravity of the airplane.

Second, the tail must be pushing down when the airplane is in a dive and the wing is not producing any lift. Otherwise, the airplane will not be able to pull out of a dive on its own. This is called positive decalage. In other words, the angle of attack of the tail in back must be less than the angle of attack of the wing in front. If the airplane is in a climb, gravity will tend to pull the nose down, so decalage is not needed in that case.

Canards Are Not a Joke!

The French word canard means duck or hoax. It originated from the trick used by parent ducks to make believe they have a broken wing as a tactic to lure away a predator from their ducklings.

A canard airplane has a large wing in the back and a small wing in the front. Both contribute to the airplane’s lift. These airplanes have been proven to be perfectly stable.

Do you see where I am going with this? A conventional airplane with a lifting tail is aerodynamically very similar to a canard. For that matter, it is also very similar to a tandem wing airplane.

Lifting Tail Pros and Cons

To be stable, a canard has to have a higher wing loading on the front wing. Just like a regular airplane, the center of gravity has to be forward of the center of lift. Similarly, an airplane with a lifting tail has to have the center of gravity relatively far forward. A natural consequence of this is that their horizontal stabilizers can contribute very little to the total airplane’s lift.

I do not know the reasoning behind this, but the horizontal stabilizers on these old timer models are huge. Instead of a normal area of about 20% of the wing, they have an area more like 30 or even 40%. This pushes the overall center of lift of the airplane back towards the tail. An unintended consequence is that lowering the flaps (if it has them) will make the model pitch up. This is the opposite problem of a canard, where deploying flaps on the main wing pitches the airplane down. A modern computerized radio transmitter can be used to compensate for this.

One of the main advantages of a canard design, stall resistance, does not apply to a lifting tail design. The front wing must always stall first, but in these designs it is responsible for most of the lift. The stall is just like a stall on a conventional airplane.

On the flip side, a disadvantage of canards is their higher stall speeds. The highly loaded front wing must stall first, keeping the rear wing from reaching its maximum lift generating potential. The stall speed is not a problem with lifting tail designs.

I hear that the old free flight models also used a lifting tail for increased stability. There is a big transition between the climbing phase and the gliding phase of their flights. Both the attitude and airspeed change dramatically.

I can see a lifting tail having an edge over a conventional tail in this scenario. Since the tail lifts up, when the airplane is flying faster both the main wing and tail are contributing more to the airplane’s lift. Maybe that is the reason for the extra large tails on these models?

By the way, I have read over and over online that the reason why these models have better pitch stability is because their center of gravity is closer to their center of lift. That is just bonkers.

Conclusion

I think the main disadvantage of a lifting tail airplane design is that they are quite a bit different than a conventional airplane design. It is clear to me that the average modeler has very wrong ideas of how they really work. Even so-called experts manage to get this one wrong. The tail design and the center of gravity have to be just right, otherwise the model might not have very good pitch stability.

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