Hi all you aerodynamic whiz kids,

I would appreciate some help in sizing the horizontal and vertical tail feathers for a sailplane. The reason for the question arises from the following:- I have a pair of 100" polyhedral sailplane wings that were attached to a conventional balsa fuselage, until I smashed it. I have now been given a very nice fibreglass fuselage with a carbon boom which I intend to use the wings on. However the seat for the horizontal stab is approximately 20% further aft than on my previous model. What I'm after is a method for calculating the reduced area for both the horizontal and vertical stabs. Can I just calculate the reduction by assuming the force acting on the horizontal stab. is directly proportional to the area and then calculating this new area from the equivalent moment from my earlier model? Or is this assumption too crude?

Thanks in anticipation!!

Tony

Try it
actually if the finished result weighs the same - you COULD get by with slightly smaller surfaces
If'n it were me - I would let well enough alone- n fly it .
Mebbe the guy who did th first model made the surfaces too small .
Most of these designs are by guess and by God.

A rough rule of thumb is to have the tail moment arm around 2.5 or 3 times the MeanAerodynamicChord. And the HorizontalTailArea about 20% of the WingArea.

Since you plan to use an existing TMA and your WA already exists, you could simply plug your existing measurements into a formula like the one I used last time I drew up a glider.

HTA = (3 x MAC x .20 x WA) / TailMomentArm

That would tell you what area would work with about the same authority as a 20% area works in the average layout.

I have to suggest that most of the designs I knew that others did, and that I did, weren't by guess, and certainly weren't by God. At least none of us would ever have even been mistaken for Him.

We often did start with a known and work from there. For example, I did a 2M thermal glider that went through a number of versions. And I used the formulas to give me some numbers to keep in mind while I flew the suckers and found out what they did in practice. The numbers then sort of helped when the next version was drawn up.

One problem you're going to possibly brush against with gliders is your tail size. They wind up being rather small the farther back you place them. The small that matters is the Reynolds "small". That is, the chords we're playing with can actually affect the way the glider handles if you get it too short. And the formulas help you deal with stuff like that.

If you're just looking for a weekend warrior, not a competitive bird, don't stress over any of it. Go with what looks right to you.

What I'd do in your situation is run the numbers and see if the length of the boom is a bother. If it is, it would take me about 15seconds to shorten the sucker. Carbon fibre is massively rigid and splicing it with epoxy and another section of tube changes nothing that matters. OK, it'd take me 20 minutes.

well the model here, is a polyhedral setup -which assumes a slow type glider and more tail moment won't make any real difference -actually likely improve slow approaches
I am guessing , by God.

Thanks guys, most helpful. Will have a look at your various options and make an inspired decision i.e. "guess", the story of my life. Flying is nearly upon us, have to get out my 52 yaer old Custom Bipe from the attic once more.

Tony

In truth, the formulas cut out the guesswork.

You want to know how much Horizontal Tail Area you need, right. You know your MAC. Lets say it's 11" for example. You can easily figure out your WingArea. Let's say it's 1100sq.in. And for the example, let's say the TailMomentArm is 36".

(3 x MAC x .20 x WA) / TailMomentArm = HTA

so..........

3 x 11 = 33
33 x .20 = 6.6
6.6 x 1100 = 7260
7260 / 36 = 202

If you wish to have an average size horizontal tail, then it'll need to be about 202sq.in.

Now all the numbers used in the example were guesses, because your parts weren't available to measure. You've got the old airplane size to work from, and most of the parts. No guessing there other than what your old TailMomentArm was if the fuselage (or parts) aren't still around. In fact, if you have all the dimensions from the original, you can plug them into software like geistware, and it'll give you some figures to start with that make it easy to recreate the performance of the original wreck with just changing the TMA length. That does take a touch of understanding of the formulas, but it's not really hard at all. Get a couple of measurements from the old bird and you can easily work out what changes would be needed with the new fuselage.

Tell ya what..... if you'll provide the numbers, I'll show you how to use something like the above formula and geistware to give you reliable numbers for the new bird.

What

BTW, the slower a glider flies, the more important it is to have it sized right, not the less important.

Unless I red the original query incorrectly, he was looking to reduce th area
I simply said leave it alone as the "oversize "
typical tail/horiz volume for a decent setup such as the casual glider is about 20% Iuse more on some les on others here is a nice sporty setup from a CZECH RTF gereat flyer
It is greater than 20% becuse it is really a slow ,small model.