# How to choose the tapered ratio for AR=4 horizontal tail?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Anna Tian, Oct 16, 2014.

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1. Oct 16, 2014

### Anna Tian

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[FONT=宋体] [/FONT]Hi,

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I use conventional tail. The horizontal tail has AR=4. Since it stalls too early (AoA for CLmax is too small), I’d like to taper it so that its root has a lower AR thus could stall at a higher AoA. I’m wondering how to calculate the tapered ratio I need to put on it?

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What’s the drawback of tapering it?

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I'm also wondering, after I taper it, shall I swept it? What are the advantages and disadvantages if I make it forward swept (to have a straight leading edge) or backward swept it (to have a straight trailing edge), or not swept at all? What are the considerations?

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Last edited: Oct 16, 2014
2. Oct 16, 2014

### cheapracer

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I generally use the Microsoft calculator that comes with Windows software but there are engineering calculators available for download from the internet for free.

Sometimes I do use my iPhone calculator as well when it's not convenient to use my PC.

If you wanted to go nostalgic, you might find a battery powered desk calculator in a junk store maybe.

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3. Oct 16, 2014

### John Newton

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Hi Anna, As a general rule of thumb, if a surface has no twist (i.e. no washin or washout) a taper ratio of 0.5 or higher will help ensure that the root stalls before the tip to avoid tip stalling, Reducing your taper ratio from 1 to something like 0.6 whilst keeping an AR of 4 may make the tail stall later IF the horizontal stabiliser is stalling at the root first.

4. Oct 16, 2014

### John Newton

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As for sweepback, this "pushes" the air outboard loading the tips more and increasing the lift distribution at the tips, therefore it can cause the tips to stall sooner, similarly sweeping the wing forward will reduce the chances of tip stall, a word of caution, swept forwards wings need to be very torsionally rigid to prevent structural failure as the wing tips are wanting to twist up and back in this scenario, someone can probably explain this better than me! John

5. Oct 16, 2014

#### Well-Known MemberHBA Supporter

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It is usually advantageous to make the horizontal stabilizer's hinge line straight, and let the sweep fall where it may. This normally result in the lightest, simplest structure and control linkage.

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6. Oct 17, 2014

### Anna Tian

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I meant I'd like to know about the estimation method (formula). Is there any reference giving an estimation method on: what taper ratio does AR=4 correspond to in order to be close to elliptical lift distribution?

7. Oct 17, 2014

### Anna Tian

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Thanks for the rule of thumb. My Re is only 200000. Would that rule work as well? How do GA company usually design this ratio? Try different taper ratio by CFD to see which one stalls latest?

8. Oct 17, 2014

### John Newton

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A taper ratio of 0.5 will approximate an elliptical lift distribution pretty well for that AR I would think provided you have no washout/washin, at that Re I would have thought you would not see a tip first stall at a taper of 0.5 or greater as this rule of thumb applies to model aircraft with low Reynold numbers (the lower the nRe number the higher the chance of a tip stall occurring for a given taper ratio, generally speaking). I have dabbled in CFD but this sounds excessive for you application, try the free to download software XFLR5 XFLR5. John

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9. Oct 17, 2014

### John Newton

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Forgot to mention XFLR5 will give you a plot of lift distribution across a wing so you can see how close to elliptical it is.

10. Jan 25, 2015

### flyoz

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Trying to start to understand XFLR5
I searched the Homebuilt data base - not too much in the way of tutorial material
Found some older Tutorial PDF's
I have dug around the the XFLR5 forum
I can open the Dat file and see the section but when i try to run Batch Analysis it basically shows an error code
I have tried to change most of the parameters
The wing i want to check is basically a glider wing and has 1.0 m chord
What am i doing wrong ?
Flyoz

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