Tailplane area ratio? How big should I make it?

[Meneer]

I'm finishing the fuse plug for a SP400 sized pylon racer and will soon be making the mold.

I've cut the foam cores for the wings and will be vacuum bagging them with kevlar and carbon fibre.

My question regards the tail surfaces. I'm building standard and V-tail versions to compare cornering characteristics, but don't know the area percentages I should be using.

Anyone know what area I should be making the V-tail and standard tail surfaces compared to the main wing?

Thanks
Dan Kitching

[Ollie]

Shoot for a horizontal tail volume coefficient of between 0.4 and 0.5. Find the mean aerodynamic chord (MAC) of the wing and the tail. Measure the distance between 25% of the MAC of each. That is the tail moment arm length. Divide the tail moment arm length by the MAC of the wing to experss it in multiples of the MAC. The horizontal tail volume coefficient is the tail moment arm (in multiples of MAC) times the ratio of horizontal tail area to wing area.

The vertical tail volume coefficient should be in the range of 0.025 to 0.035. The vertical tail volume coefficient is the (vertical tail area divided by the wing area) times the (tail moment arm length divided by the wing span).

For V-tail design see:
http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm

[Meneer]

Thanks Ollie, I appreciate the help.

Now, control surface area and throws are the next thing to address. Maybe I'll just the percentages similar to my Sokol, except the trailing edge on my wing will be very sharp (basically two layers of 1.7oz kevlar).

Thanks again,
Dan Kitching

[Ollie]

Control surface width is usually about 25% of the local chord.

[WoFlyer]

Use this if your model has a V-tail, you take the total area of the two surfaces and multiply by the square of the cosine of their dihedral angle, and use that as the tail area when working out Tail Volume Raio (V-bar) in the CG formula.
Suppose the included angle of the Vee is 120 degrees, each side has a dihedral angle of (180 - 120)/2 or 30 degrees. Cos 30 = 0.866, so cos^2 = 0.75 so the effective area of horizontal stab is 3/4 of the total area.
By the way, the effective area of the vertical stab is the area times the square of the sine of dihedral angle. sin30 = 0.5, so sin^2 = 0.25 so the vertical stab area is 25% of the total area. (Notice that effective fin area + effective tail area = total area because from mathematics sin^2 + cos^2 = 1) I just stay with a v-tail at 120 degrees. Some people like 110 degrees. Obvioulsy it's a trade off between verticle stabalizer area and horizontal stabalizer area. If you have lots of side area, maybe you can get away with less vert.stab. area. If it's a pod type fuselage then you will need to make up for the lack of side area. You can do this by only haveing a 90 degree spred. But then you may find the tail getting larger to make up for the loss of hor. area. It is a trade off. And useing any type of" average size for this" attitude may get you in trouble, then it may not.

[WoFlyer]

So now you know that as in the example you will be using 75% of the v-tail area to figure out how much horizontal area to figure on. I don't know the area of your plane so we will use 25% of the main wings area. Wich I also don't know but lets say about 250 sq.inches. 25% of that would be 62.5 sq. inches. So If you planed on a tail area of about 62.5 sq. inches, you would be using 46.875 sq. inches as your actual horizontal tail area. Now we can figure out the CG of the aircraft. First we need to figure out the Tail volume ratio mentioned above.
Tail Volume = V-bar

(tail area/wing area)X(tail arm/wing cord)= V-bar

[WoFlyer]

To follow our example. tail area =46.875 sq, inches, wing area = 250 sq. inches. So 46.875\250=.1875.

The tail arm of your plane I will guess on to. It is important to know that this lenge will detemine the CG if all else were to be left as is. A longer arm length alows a smaller tail area to do the work of a larger area on a shorter arm length. Less effective tail area means you need to put the CG further forward to stabalize the forward pitch moment of the wing, ect. Ollies description of the tail arm length is good so use that. Lets say 20" just for giggles. I have no clue what that will give use. I also need to guess on wing cord so lets say 8.334". That make the wing 30" in span.
So 20"/8.334=2.399. Now we multiply the two to get the V-bar.

.1875x2.399=.449, .449 is our V-bar

[WoFlyer]

OK now we can figure out the CG location best to start with. I am no mathamagician. And I have no clue as how to type this formula in so I will describe it useing the exmaple we have so far.

We need the aspect ratio. Span/chord. 30/8.334=3.599
Using a calculator enter in the Aspect ratio, press the square root, press the squar root again. you get 1.377. multiply that by .25 and the V-bar of .449 and add .1. Or .1+(0.25x.449x1.377)=.254. Multiply by onehundred to get the percentage of MAC to use for the CG. 25.4% MAC. Now we now that the example models CG should be set at a point .254x8.334=2.11 inches back from the leading edge at the MAC.

[WoFlyer]

As you can see the CG is very far forward in our example. My experince is Pylon plane are like Stock cars. Loose is fast, tight is slow. For airplanes Loose whould be the smalest tail plane area you could get with a CG about say 28-29%. I bet someone out there dissagries with that but this is what I use for my own designs that fit into this area. Most club meatings will have rules as to size and area, ect. But for unlimeted pylon racing this is the cg I shoot for. The formulas above can all be adjusted and recalculated to see the differance changes will make. Adjusting the tail arm length in the v-bar calculation will make the most drastic changes. For example a tail arm length of 24" would bring the CG to 28.55% of the MAC.Perfect for my tastes. By making the arm lenght more I made the horizontal tail act larger. But save the weight and drag of a bigger actual surface. But now we are getting beyond the point of all this. Hope this helps, if you need help post the esential numbers and I will help. Wo

[Meneer]

Wow, thanks WoFlyer. You've spent some time writing this up and I do appreciate it. The new plug is progressing well, and I should begin laying up the mold this coming weekend. Foam cores for the wings are cut (trying out two different thickness airfoils), and I'll soon be muddling through the design of the tailfeathers. I understood that tail surface design is much more important than it seems, but now I have the reference material (above) to design with some intelligence.

Thanks to you and Ollie,
Dan

[WoFlyer]

No problem, but in reading what I wrote I forgot to mention the what to do if you have an excesivly long nose.( On your airplane) The formula I gave you works heavily on the volume of the hor. stab.. This volume can be offset by a very long nose. So, if you have any area which is more than one MAC length ahead of the MAC you basicly need to treat it like a forplane. This my sound like a really long nose but it depends on the MAC of the wing. A narrow wing has a narrow MAC. So this my not be very long at all. Take the area ahead of this line in planform in square inches, say 5 sq. inches. Multiply it by the average lenght it is in front of the wing's quarter cord point (1/4 of MAC) say 10 inches and divide by the wing area, 250 sq. inches then the wing's mean cord 8.33. ((5x10)/250)/8.33=.024. This gives use a nose de-stabalising volume .024. subtract that from our original V-bar, leving us with a V-bar of .425. Now redoing the original equation for the CG with the new V-bar number, brings the CG forward to 24.6% MAC. The original CG of 25.4 % would have left the example model tail heavy!! oppps, Watch that nose area. Wo

P.S I grew up in Vernon B.C. so I understand. ;)

[Ollie]

There are even more considerations beyond getting the CG properly placed and the tail volume coefficient right. There is the matter of damping. When there is an abrupt control input or gust, a stable aircraft will return to its previous flight condition. If there is insufficient damping, the correction will overshoot and a damped oscillation will ensue. When the model is critically damped, there will be no overshoot and the aircraft will return to the previous attitude quickly. If the aircraft is over damped it will take longer to recover. The damping can be controlled by adjusting the tail moment arm and area while maintaining a constant tail volume coefficient. The damping factor is proportional to the square of the moment arm length and to the first power of the tail area. By making the arm longer and the area smaller the damping can be increased while keeping the TVC constant. Long tail moment arms make for groovy handling characteristics. Care must be taken to keep the tail end light, stiff and strong so that the CG can be maintained without adding weight. It's a clasic design problem involving various conflicting objectives.

[WoFlyer]

Right on Ollie, could you put that in english please. Sounds like an interesting equation to consider in any design. But you need to actually put the equation in the post. And maybe explain the equation, and where the numbers used will come from. And of course we will need to be able to interpet the results, so some examples would be nice for the sake of some referance. Maybe you could figure out some numbers using the example plane I used above? Beyond that it seems rather like a quote, from a book, the one us laymen can't understand!! Wo

Added 7/4/03
I know now that this would take a whole tread and a lot of typing to explian in any way that would lead us to anything we could use in models. It's one of those things that is great when it happens. Wo

[WoFlyer]

You made me think about this one Ollie thanks. I asked someone about this and he sent me a good reply. But it is a word document with a graph. I don't know how to post it in here. Basicly he said the same thing just with a graph. He did ask if I had found an intellagent person in a forum though. I would take that as a compliment from this person. If you have a chart you could show, it is an interesting topic. Wo