Aerodynamic Center

[CloudyIFR]

I'm trying to calculate the AC for a V-tail. I'm using Martin Simons formula in the back of his book on Model Aircraft Aerodynamics.

I've successfully calculated this for a Cruciform Tail and am now trying to do this for a V-tail. To do this the formula called for Aspect ratio, 1/4 chord point and stab area.

To do this I have formulas to convert a V-tail to a conventional tail which gives the required area for the horizontal, however, what aspect ratio do I use? Would I use the aspect ratio of the V-tail? From what I understand the aspect ratio for a V-tail is higher than it is for a Cruciform tail, at least that seems correct to me. If that statement is correct than that means the the balance location for a properly converted cruciform tail would change when converted back to a V-tail. That doesn't make sense.

I hope I'm clear about what I'm asking.

Thanks for the help.
Curtis

[Sparky Paul]

Go here... at the bottom of the page are links to v-tail design.
http://www.charlesriverrc.org/articles_modeldesign.htm

[CloudyIFR]

Go here... at the bottom of the page are links to v-tail design.
http://www.charlesriverrc.org/articles_modeldesign.htm

Sparky Paul,

I see you're from Palmdale, I used to fly out on 110th and G street with Howard, Paul and Bud. Had a great time flying TD ships off the winch. It happens to be the same place the Free Flight guys went. This was in the early 90's.

Those are excellent articles and I've put those formulas into a spreadsheet, however, they don't answer the question on which aspect ratio to use for the V-tail.

Thanks
Curtis
Montana

[HELModels]

Using whatever V formula you feel comfortable with, maintain the tip and root chords and reduce the Span according to that formula. This will reduce your AR in a logical way, not arbitrarily.

[CloudyIFR]

Please allow me to rephrase my question:

I programmed the Mark Drela article into a spreadsheet to convert a conventional tail to a V-tail and vice versa.

It seems that when converting a conventional tail to a V-tail that the aspect ratio of the V-tail will be greater than when it was a conventional tail, even though the overall areas will be the same.

Is this correct?

If so, then it seems that when computing the Neutral Point, that point will be slightly further aft on the wing than it would be with a conventional tail.

Is this correct also?

Thanks
Curtis

[raptor22]

Please allow me to rephrase my question:

I programmed the Mark Drela article into a spreadsheet to convert a conventional tail to a V-tail and vice versa.

It seems that when converting a conventional tail to a V-tail that the aspect ratio of the V-tail will be greater than when it was a conventional tail, even though the overall areas will be the same.

Is this correct?

If so, then it seems that when computing the Neutral Point, that point will be slightly further aft on the wing than it would be with a conventional tail.

Is this correct also?

Thanks
Curtis


No. the opposite is true.

--Alex

[HELModels]

Using whatever V formula you feel comfortable with, maintain the tip and root chords and reduce the Span according to that formula. This will reduce your AR in a logical way, not arbitrarily.

What I was trying to say is this:
To go from + tail to V, take the horizontal stabilizer and increase each semi span according to the formula you like - by keeping the same root chord and tip chord and stretching the span, you increase the AR.

To go from V to +, you still maintain the tip and root chords and shrink the semi spans - this decreases the AR.

There is no aerodynamic difference between a V and cruciform, until you consider interference effects - the V has less.

[Sparky Paul]

The v-tails I've made from cruciforms using the Charles River equations all have physically longer spans, keeping the root and tip chords... but when bent up, the AR drops in the plan view.
If you know the area you want, then compute the v-tail stuff, but consider the AR to be that of the cruciform configuration.

[CloudyIFR]

Sparyk Paul and ElectroStorch thanks so much, I fully understand and I think my spreadsheet is complete and accurate.

Still not sure what Raptor22 is trying to say. My first statement has to be correct and if it is then the second statement is correct. Could you please elaborate?

The way the formla works for finding the Aerodynamic Center if the AR goes up thus the adjusted slope lift of the stab is more efficient, thus the AC can go further aft.

Here's the formula:
As = (Horiz AR * .095) / (Horiz AR + 18.25 * .095)

Curtis
Montana

PS I'm not very good with math but that's how I see the formula.

[Sparky Paul]

I can't find an equation relating AR to anything, in Simons or Lennon..
Where'd yours come from?

[CloudyIFR]

I can't find an equation relating AR to anything, in Simons or Lennon..
Where'd yours come from?

I have the second edition 1993 reprint of Martins book and it's on page 243, Calculation of the Neutral Point and Static Margin. It's in Appendix 1.

Would you like a copy of the spreadsheet I've compiled? send me your email to suterc@msn.com and I'll email it to you. It's about 200k unzipped.

Curtis