Tail Volume Coefficient
 

Is the use of the TVo a good way to find the cg on a short coupled funfly? How about a long pattern plane?

http://www.aalmps.com/tv.htm

RE: Tail Volume Coefficient
 

I like to use Lidberg's formulas for initial testing on a new model with unfamiliar force arrangements; then final tune the CG during flight testing. His formulas were put together for rubber powered scale models but work amazingly well in most other applications.

RE: Tail Volume Coefficient
 

Get data for the planes you're interested in, and see how their tail volumes compare with the averages suggested.
You might see where the end points for the limits are also, in terms of lengths and areas.

RE: Tail Volume Coefficient
 

ptulmer,
If you try to calculate a CG position WITHOUT involving the Tail Volume you are wasting your time.
Alasdair

RE: Tail Volume Coefficient
 

I don't doubt that.:D It was the particular formula behind the link. What made me think twice about it was I checked a couple of sport/pattern planes and the TVo came out low. For instance a Bridi Chaos .60 comes out to .7. That seems kinda low. I mean it's designed for aerobatics and should have a strong TVo right? I don't doubt the formula works. I wanted to know how it applies to sport r/c planes.

Thanks
Patrick

RE: Tail Volume Coefficient
 

Most CG formulae are designed to produce a model with a fixed amount of Stability from whatever size of tail they use. I can't speak for the one on the site you quoted, but the formula I use tries to give a Stability Factor (Kn or dCm/d-alpha) of 0.15, that's 15% of mean chord.

My simplified formula, expressed like the one on that website is
CG position = 10 + 40*Vbar as a % of mean chord
where Vbar is the Tail Volume Ratio (the standard symbol written in text books is a Capital V with a bar over it, but that's hard to type so I write V-bar, as it is said).

I said that's the simplified form, which works for your average sort of power model with wing Aspect Ratio (ARw = 5 to 8).

To allow for a wide range of Aspect ratio my full formula is
CG = 0.1 + 0.25*(SQRT(SQRT.ARw))Vbar as a fraction of mean chord (x100 for %)
You take the wing AR, take its square root, hit square root again (to get the fourth root), multiply by 0.25 and by Vbar and add 0.1

All these formulae work the same way. They calculate how far the Neutral Point of the whole aircraft is behind the wing's Aerodynamic Centre (that's the second part of the formula, the bit with Vbar).
Then they add the distance of the AC behind the mean chord LE (0.25 or quarter chord or 25%) and subtract the Stability Margin you want (0.15 or 15% chord in my case).

That website seems to cater for Vintage or Free Flight models?
For regular powered Sport or pattern models Vbar is usually in the range 0.5 to 0.8
For scale models it is often from 0.35 to 0.6
Gliders tails are very often very small, but the long tail arm gives Vbar of 0.3 to 0.5

AS Tall paul suggested, measure up some successful models and see what Tail Volume works on the models you like.
Alasdair

RE: Tail Volume Coefficient
 

Quick question, when figuring the tail area, do you use the area of the horizontal stabilizer and the elevator? Or just the area of the stabilizer?

RE: Tail Volume Coefficient
 

Use the total area of the complete surface, including the elevator.

RE: Tail Volume Coefficient
 

Thank you much.

RE: Tail Volume Coefficient
 

So a larger V-bar would make a plane more stable?
Does .3 to .45 sound about right for an aerobatic plane?

RE: Tail Volume Coefficient
 

Sort of, but it is like saying carts are for pushing horses.

The amount of Stability REALLY depends on how far the CG is ahead of the Neutral Point (NP). The NP is the CG position which would give zero, or neutral, stability, and that's really what we are trying to calculate.

The distance of the CG ahead of the NP is usually given as a fraction, or percentage, of the wing mean chord. A figure of 5% to 25 % (0.05 to 0.25 as a fraction) is the range we aim for. 5% gives a twitchy, lively and responsive model. 25% gives a slow to react over stable model that may not loop and probably won't spin.

The size of the tail volume determines where the NP will be. A big tail volume pushes the NP further aft. A small tail volume brings the NP forward. A zero tail volume puts the NP at the wing's 25% chord point (a flying wing). A foreplane moves the NP in front of the wing's quarter chord point.

So if you take an existing model and enlarge its tail volume but keep the same CG, YES, you make it more stable because you shift the NP aft, further from the CG. But really you should re-calculate a new CG. You should calculate the NP, and you should CHOOSE the CG, because you CHOOSE how much stability you want, not just let it happen.

Aerobatic aeroplanes usually have tail volume coefficients of 0.5 to 0.9
Alasdair