empannage size and stability?

[Spiteful]

I have a GWS Spitfire which has a humongous empannage for its size. I know that as a plane i scaled down it needs larger empannage to maintain stability, but can anyone please explain why?

Many GWS warbirds are hypersensitive to elevator input and 3mm up/down is recommended on these forums. Wouldn't it be possible to just make the empannage smaller and use longer trow? It would make the Spitfire look a lot better.

[Sparky Paul]

The latest versions of the Spitfire needed enormous verticals to offset the longer nose and additional area caused by the Griffin engine.
What your plane has is scale.
I wouldn't change it.
The sensitivity in pitch is due to the aft c.g.
Move it forward and the plane will become easier to fly.
Making the horizontal smaller would make it more difficult to fly.

[Spiteful]

I agree about the late mark Spitfires, but the GWS spit is clearly a MK V or early. It has a short nose, so I wouldn't call it scale.

Moving the CG forward will probably help.

What I was wondering though was what are the aerodynamical reasons for the enlargement of the empannage as the model shrinks?

[Sparky Paul]

Ask GWS.. only they would know.

[kf2qd]

Enlarging the empange some percentage adds some stability to the model. The full size plane was designed with the pilot seat inside the plane and much better feedback to the pilot. Our models have a slightly long feedback loop so enlarging the tail slightly helps us to have a bit better control over the plane. Try building a model that is absolutely scale and see how well you can fly it. I think you will spend a lot more time chasing the plane while it is flying because of the time it takes you to process the visual only feedback you get from the plane. If you fly a lot you will have better success, but a slightly larger tail will make it much easier.

[Spiteful]

I'm building a .4O size Spit with absolute scale empannage. Maybe not such a good idea.
I didn't like the look of the kit the way it was and decided to change it to make it look right. How bad do you think it will be, should I stop now before it's too late?

[Ollie]

The full size Spitfire was designed to maximum speed and maximum maneuvability. That's minimum tail area for speed and ragged edge of stability for maneuvability.

The smaller the model size the needed a higher tail area and more stability for R/C pilot handliness.

[BMatthews]

Horizontal area and tail length as it relates to stability is often given as the Horizontal Tail Volume Coefficient. This is a mathematical way of describing the planform of a design so that it can be compared to vairous other designs.

But one aspect that is not factored into the Vh is the pitching moment of the airfoils. Or perhaps it is in some other equation that is not common knowledge in the model building world.

However it is fair to say that wings with airfoils that have a lower pitching moment will get away with tail volumes that are on the lower side of the acceptable range. WIngs with higher pitching moment airfoils will require a larger horizontal tail to achieve the same stability. CG placement plays a part in this as well but it needs to be combined with a suitable tail volume.

So.... let's look at the airfoils. The stock full sized Spit uses a low camber "semi symetrical" airfoil. The GWS likely uses a far higher camber airfoil with a stronger pitching moment or one of those horrible arched single surface wings (sorry, I've never seen the GWS Spit up close to know). So to get the level of stability needed for flying it requires a larger than scale tail area.

If it used a full on symetrical airfoil you could get a way with a smaller than scale tail area up to a point since symetrical airfoils have a zero value pitching moment and require far less control to maintain stable flight.

[Spiteful]

This is all rather interesting, how do you calculate the coefficient? Is there a given number that's needed for a given airfoil? The gws Spit uses a somewhat unconventional combination of a semi symmetrical airfoil and an undercambered one. My .40 size Spit from Cambrium uses a convetional semi symmetrical airfoil.

When buildin a close to scale kit for sale do you think the designers choses the minium empannage size possible to keep it close to scale or go for a bigger one to make it easier to fly and thus more accessible?

[BMatthews]

GWS definetly chose a larger one to enhance the stability issue since they know that some of their customers believe that a CG is something you add to the model with a felt pen.

Here's just one site that has a method for calculating the Tail Volume Coefficient. There's lots of others. Try a Google search on "tail volume coefficient".

http://www.eaa62.org/technotes/tail.htm

All the sites I've ever seen do not have a different range of values for higher camber airfoils compared to lower camber ones. However a Vht that is on the higher side of "normal" is wise for a highly cambered airfoil like a Clark Y or any of the higher cambered ones that are typically callled "undercambered". For airfoils of 0 to 3% camber you can get away with a Vht on the lower side of the acceptable range. That seems to be all that anyone bothers with in the model world. I'm sure the full size designers get a little more fussy when they are planning on carrying real people.

[davidfee]

I'm sure the full size designers get a little more fussy when they are planning on carrying real people.

Oh yes, they certainly get more fussy. They consider things like what happens if a control surface flutters and departs... can the aircraft still fly and be controlled?

[biber]

Well for the most common designs, the smallest recommendable tail volume is given by the case of a steep dive.
If the wing has a negative pitching moment, the tail wil have to counter that with a nose up / tail down torque in that situation.
That can be generated by a big tail volume and a slightly negative lift coefficient of the tail surface,
or by a more little tail volume and accordingly more negative lift coefficient,
since both contribute as factors to the product (the needed torque).
You should never make a tail so small,
that the minimum lift coefficient of the tail's airfoil (whatever is chosen) can likely be exceeded by such a dive.
The tails bottom surface would stall, and thus to pull up would be impossible.
Some airplanes before WWI incorporated that problem as they had heaps of camber in the wing and the tail,
so that the tail lacked the ability to develop the needed downlift in a dive.
That caused unrecoverable dives.

biber

[KenSt]

A.G."Andy" Lennon writes in his book, R/C model airplane Design, "To determine your tailplane area, the following simple formula may be used". Sht=K x MAC x Sw/Dh
Where: Sht= tailplane area MAC= Mean aerodynamic chord (inches) Sw= Wing area (inches) Dh= Tail moment arm (inches, MAC TO MAC of wing and stab) K= A constant .52 for symetrical, .57 for flat bottom, and .60 for undercamber.

[vintage1]

Tail area can vary from nil, in a flying wing, to the whole wing area, in a flying wing, with various strange CG positions and relative sizes of foreplane/mainplane (canard) and even equal area planes in some weird designs, as well as the CG aft of the trailing edge of the wing if the tail is big enough and far enough away.

ALL of these can be made stable in pitch.


There is NO rule at all, other than that the final configuratuion has to be stable.

Now when it comes to YAW you have to do the same calcualatins but with the model flipped on its side so to speak.

The bigger problem comes when the CG that's right in pitch, is not right in yaw..model's that won't turn into banked turns..or turn in too much..or wag their tail's. Or go into death spirals..then yiuy need to adkust te fin area or position or CG..