I´m building a 122 inch wing span airplane, the wing is a copy of the G.P. Easy Sport. The tail will be airfoiled and I was wondering if there is a formula to know the size that the tail should have, the stab and elevators mainly. The fuse is 96 inches long without engine and rudder, so it will be about 108 inche with a Minarelli 125 and the rudder. Any help is welcome.
Steven

A basic rule of thumb would put the total horizontal surface area at about 20% of the wing area. The movable portion would be from 20% to 60% or so (or even more) of that. If you are planning extreme aerobatics, then the moveable surface (the elevator) would be larger. Most average aerobatic models run the elevator at about 30%-40% of the total horzontal area.

Thanks again Bill.... Back to the drawing board

Now, I forgot to ask the other question, with a fuse that is 9.75 inches wide and 12 inches high and 100+ inches long... the engine driving a 28x12 inch prop, what size fin and rudder should I install?

I recommend somewhere in the 20% to 25% of the wing area. I have flown kits with a 30% horizontal area. There is really no down side to a large tail. It gives you a wide lattitude in CG placement.

The vertical area is normally 1/3rd to 1/2 of the horizontal area. There is a down side here. Too small and you get into yaw instability. Too large and you get into spiral instability.

So these percentages are to either side, not only up but any direction as long as I keep the percentile in that range?
Say for example, if the stab and elevator area was 20%, and the size came out at 32" x 17.5" the fin and rudder could be 16 x 17.5 inches right? or I could modify it to be 20 x 14 inches and still have the same area without having control problems? Sorry to sound so ignorant about this, I have built a bunch of airplanes but this is sort of a scratch build own design so I need to be sure that the proportions are correct so that I don`t bring back home only pieces.
Thanks for the replies
Steven

Usually, a horizontal tail surface would have an aspect ratio of about 4:1 to 6:1 (chord is 1/4 or 1/6 of total span). Do what also looks good while the surface is in that range.

Please remember that airplanes (of any type) are collections of compromises. You design within "known-good" parameters, build it and then fly it and change as necessary.

Depending on what you want to with it, the numbers can be fudged. Go to the larger % for slow flying 3d type stuff where you need the larger size for the slow airflow over the surfaces. A pylon racer type can go for the lower end because of the higher speed and cutting down on drag for more speed.

how much lower can you go on the horizontal tail percentage for a pylon racer? 10% ?

Bringing back an oldie........ This thread started over two years ago......... kewl...............

What Bax said 2 years ago is still true. The part about a bunch of compromises especially.

There are a bunch of formulas that the aerospace industry uses every day. A bunch of them are put together in the www.geistware.com application that does CG placement. And the aerospace industry uses those same formulas to do redesigns as well as designs. And you can too. Just using the application itself but with a different purpose.

Work out your measurements just as if you were going to look for the CG of that model. And do the app twice to find the safe range for the CG locations. Now, change the numbers you started with. Change the tail numbers the way you think would make your pylon racer faster. Plug those numbers into the geistware app and see where the CG goes. Where it goes and what the new CG range is after the hack job will give you a really good idea if your new smaller tail will work or not.

It's exactly what designers do.

The CG application is actually at this link:
http://www.geistware.com/rcmodeling/cg_super_calc.htm

And it is dead simple to do the kind of modifications you're wanting with this software app.

Generally airfoils with zero or low values of camber or airfoils that have lower pitching moment values will make do with a smaller tail. But the downside of a small tail is that the CG ends up being further forward. All else being equal the wing has to work less hard (think while in a pylon turn) if the tail is a little larger and will support a more rearward CG location while still keeping the model stable.

Shooting for a 30% CG location while still retaining a small % of positive stability is likely not a bad location. Size the stabilizer area such that when you run the numbers you end up with a CG located at that point.

The size of the tail on a racer actually isn't nearly as important as the drag from it on the straightaways and sometimes more importantly the induced drag created when it has extra work to do. It'll be doing that work in the turns. Speed lost in the turns costs you every foot of the next straight. And the speed can be lost almost instantly compared to how long it takes to get back.

And what BMatthews just mentioned about the CG having to be carried farther forward for smaller tails applies twofold when that tail tries to pitch that forward CG around a turn.

When you move the CG forward from it's most favorable location, it begins to LITERALLY increase the weight the wing has to carry. It actually doesn't change the weight of the airplane, but it does create an increase in the moment (a moment is a force times a lever arm) the tail has to balance. And the tail does it by creating more negative lift. And the wing has to actually carry that extra lift and support that increased moment. So the airplane flies as if it were heavier, because the wing is truly having to carry more weight. And what happens in that situation? Even more drag is produced. Appreciably more. And what happens in the turns when centrifugal force multiplies the affect of the misplaced cg? More lift required, and more drag created.

You do not want to move the CG forward from it's most favorable position on a racing airplane. And if you reduce the size of the tail from it's optimum for carrying that sensibly placed CG, you do yet another thing that hurts a racer far more than it hurts your everyday model. For a smaller tail to produce the same pitching effects a larger one produces, the smaller one creates disproportionately more induced drag doing it.

How do pylon racers work out the best size tail?

They plan on the CG being as far back as possible. That doesn't mean so far back that the airplane is too hard to fly. It means they don't move it forward for any but the most important reasons. And there aren't any usually. And then they size to tail to match. What is a good way to do that? The CG locator application at geistware.com is an excellent tool to do just that.

When the CG is NOT forward, the tail is more efficient. The elevator is too. And that means the elevator deflection is less, not more. And less deflection means less induced drag, almost always.

The really hot pylon designers look up the airfoil's CP and consider that for their start point. And they usually try to figure that location based on the AOA of the wing in the turns. And then redraw and redraw as they compromise and fiddle and twiddle. And when the sucker starts flying they look very hard at how much their new killer slows in the turns. Because that's usually where the races are won.

You guys are awesome! thank you for the great explanation on the tail size and drag. Great information!

Look, you are designing a sport plane, not a pylon racer. All that stuff about small tails isn't for you. A larger horizontal area gives you a much wider latitude of CG location. With a small tail, you have to be more precise. For a big Easy Sport, I would not go below 22% of the wing area.

Ed, the guy who was building a sport plane started this thread back in 2005 and hasn't been around here for awhile.

The guy who resurrected this 2year old thread just a day or so ago asked specifically about a pylon racer. But you're absolutely right, larger tails work better on almost all models.

Ed,
You keep talking about bigger is better when it comes to a H Stab but half your planes don't have any. [X(]