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aircraft design? - 5/9/2008 7:35:47 PM   
grant-RCU


 

Posts: 384
Joined: 1/11/2003
From: Plains, GA, USA
Status: offline 		I am trying to understand how tail surfaces are sized for aircraft. I have always used a basic design guide of 18 to 20% of the wing area fot eh H stab and 1/2 that for the V stab and this has always yielded good stable flying airplanes.

I know tail sizes seem to vary a bit on full scale aircraft.

I have been doing some searching on the net a I came across a few terms that are used in determining the h stab size and the CG range.

Neutral point
Static Margin
Aerodynamic Center
Center of Gravity
Tail Volume Coefficient.

From the things I read the CG should never fall behind the NP or the plane becomes unstable.
Static margin is generally 5% to 15% for a nice and stable plane.

Aerodynamic center is located about 25% of the wing chord of a square wing.

CG is ideally located slighty behind this point but ahead of the NP.

I dint quite get the Tail Volume Coefficient.

The way I see it is it doesn’t make any difference what size the H stab is as long as the cg is ahead of the NP. However ideally you want the CG around the AC so the tail size and length would be tailored to align the CG and AC.

Please someone with knowledge in this go into more detail.

Grant







       Post #: 1

RE: aircraft design? - 5/9/2008 8:33:54 PM   
da Rock



Posts: 6085
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		I'm not sure what you might have missed, but you've asked for about one semester's worth of Aero101.

You can get most of those terms depicted in the application linked here:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

I've included an attachment that shows what you see at that link. The arrows show one detail.

The sizes of things like the H-stab and Elevator (they are the horizontal part of the empennage) aren't the only consideration. Size matters, but equally important is the distance a structure like that is from what it's supposed to stabilize.

There is a good book that covers what you've asked about. BASICS OF R/C MODEL AIRCRAFT DESIGN by Andy Lennon. It's often found in LHSs, but is also available off the internet. It's probably $20 in the LHSs.

Attachments
Click to see fullsize image.
Click for fullsize

(in reply to grant-RCU)
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RE: aircraft design? - 5/9/2008 8:48:48 PM   
da Rock



Posts: 6085
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: grant-RCU

I am trying to understand how tail surfaces are sized for aircraft. I have always used a basic design guide of 18 to 20% of the wing area fot eh H stab and 1/2 that for the V stab and this has always yielded good stable flying airplanes.

I know tail sizes seem to vary a bit on full scale aircraft.


Horizontal tails are sized by area and how far back they are. There is more to it than that, but thats the simple take on them.

quote:

I have been doing some searching on the net a I came across a few terms that are used in determining the h stab size and the CG range.

Neutral point
Static Margin
Aerodynamic Center
Center of Gravity
Tail Volume Coefficient.


Those definitions are all in wikipedia. Can be found easily on the internet with Google.


quote:

From the things I read the CG should never fall behind the NP or the plane becomes unstable.


That is true and not really very good as a rule of thumb. Your airplane becomes almost impossible for you to fly when the CG hasn't even made it back to the NP.

quote:

Static margin is generally 5% to 15% for a nice and stable plane.

Yup.

quote:

Aerodynamic center is located about 25% of the wing chord of a square wing.

CG is ideally located slighty behind this point but ahead of the NP.

Aero center is located a bit more scientifically. And is of almost no value knowing for most of us.


quote:

I dint quite get the Tail Volume Coefficient.

It's basically a formula that considers how far back the horizontal tail is, how big it is, and compares that to the wing. It's used to get an idea how effective that tail will be.

quote:

The way I see it is it doesn’t make any difference what size the H stab is as long as the cg is ahead of the NP. However ideally you want the CG around the AC so the tail size and length would be tailored to align the CG and AC.


Not actually.
It makes a difference. And you usually move the CG to suit a design that is appropriate to the task. Tail size and moment will be designed to deal with controlling the pitch of the wing and provide pitch stability of the airplane. You'll move the CG to establish decent pitch stability when you're building the sucker and later to tailor your elevator response for more pleasing flying.

(in reply to grant-RCU)
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RE: aircraft design? - 5/9/2008 11:13:56 PM   
Mike Connor



Posts: 1333
Joined: 1/10/2002
From: Tulsa, OK, USA
Status: offline 		Da Rock knows his stuff
Us layman could look at the Tail Volume Coefficient like weight and balance. The further back the weight the less it has to weigh to have the same effect. The further back the tail is from the CG (longer moment) the smaller it can be to have the same effect.

_____________________________

Mike
http://www.mikesrc.us/

(in reply to da Rock)
       Post #: 4

RE: aircraft design? - 5/10/2008 12:24:31 AM   
crasherboy


 

Posts: 97
Joined: 10/21/2006
From: Bryant Pond, ME, USA
Status: offline 		It is all a matter of relativity!

_____________________________

Do unto others as you would be done unto,well most of the time anyway!

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RE: aircraft design? - 5/12/2008 10:54:40 PM   
AERORICH


 

Posts: 283
Joined: 4/21/2006
From: Scappoose, OR, USA
Status: offline 		RIGHT ON da Rock!!!!!

AERORICH73@NETZERO.NET

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       Post #: 6

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