News:
2019----><---- T Shirt



Advertise Here

  • July 20, 2019, 12:49:43 PM

Login with username, password and session length

Author Topic: Stabilizer area?  (Read 714 times)

Offline FLOYD CARTER

  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3668
    • owner
Stabilizer area?
« on: April 04, 2019, 07:24:03 PM »
I searched for "stabilizer area" without results.  Modern stunt models show larger stab areas.  I believe a larger stab makes for a more tolerant CG, and I also think it would help control authority.

Without getting into actual numbers, or ratios, let's just say a stab "looks" a bit larger than "normal".   Does this actually improve things?

For instance, a glo engine having a 6 ounce tank ahead of the CG at takeoff will shift the CG toward the end of the flight.  It would help if a larger stab would lessen this effect.

What else could I expect?
"Growing old is easy.
 Staying old is hard"
AMA #796  SAM #188  LSF #020

Offline Howard Rush

  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6589
Re: Stabilizer area?
« Reply #1 on: April 04, 2019, 08:51:04 PM »
Bigger tails enable a greater CG range.  This is the case with full-scale aeroplanes as well as models. 
The Jive Combat Team
Making combat and stunt great again

Offline GerryG

  • 2018 Supporter
  • Trade Count: (0)
  • New Pilot
  • *
  • Posts: 15
Re: Stabilizer area?
« Reply #2 on: April 05, 2019, 12:29:36 PM »
Yesterday I did a search using "stab area" and was able to find some good info from one of the Masters, Ted Fancher.
Do the search and go down to item #19 At the Handle for Ted's comments.

Offline FLOYD CARTER

  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3668
    • owner
Re: Stabilizer area?
« Reply #3 on: April 06, 2019, 11:28:11 AM »
Howard.  We agree.  I once flew a V-tail Bonanza cross country.  It surprised me that I only needed to lean forward and the nose dropped below the horizon.  Certainly a candidate for a larger tail.
"Growing old is easy.
 Staying old is hard"
AMA #796  SAM #188  LSF #020

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 9855
Re: Stabilizer area?
« Reply #4 on: April 06, 2019, 12:56:41 PM »
I searched for "stabilizer area" without results.  Modern stunt models show larger stab areas.  I believe a larger stab makes for a more tolerant CG, and I also think it would help control authority.

Without getting into actual numbers, or ratios, let's just say a stab "looks" a bit larger than "normal".   Does this actually improve things?

   The most extensively discussed/argued topic in stunt in the last ~60 years, and you couldn't find a reference?   

http://library.modelaviation.com/ma/1979/10

   Brett

Offline FLOYD CARTER

  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3668
    • owner
Re: Stabilizer area?
« Reply #5 on: April 06, 2019, 02:46:07 PM »
Most discussed topic has been BOM.
"Growing old is easy.
 Staying old is hard"
AMA #796  SAM #188  LSF #020

Offline Matt Spencer

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 3513
Re: Stabilizer area?
« Reply #6 on: April 06, 2019, 08:27:27 PM »
Quote from Al Rabe .

As far as I know, the 1973 Mustang V was the first stunt ship design taking tail volume calculations into consideration for determining moments and areas.  First, I measured all of my airplane’s moments and areas to determine their tail volume and related this to their handling characteristics.  This gave me an approximate design goal for the Mustang V.  To calculate a tail volume, I use the entire width of the wing including flaps, and emphanage (stab plus elevators) as shown in the plan view to determine the mean aerodynamic chord (MAC) of these surfaces.  The MAC is the wing chord location where the areas of the wing, both inboard and outboard of that chord, are equal.  The calculation takes into consideration the width of the wing and emphanage MACs, the areas of the wing and emphanage and the distance from the center of gravity to the emphanage, quarter chord   This formula tells us that for a given value of tail volume we can vary the area of the tail, or length of the tail moment in our calculations.  Specific TVCs can be calculated by varying the chords, moments or areas within the formula.  The engineering need for using MAC lies in stability calculations for airplanes with swept wings where the MAC may be a long way from the root chord where the wing enters the fuselage.
 
Is there a builder who hasn’t considered the possibility that if enough is good, then might not more be better?  The problem with adding both tail area and tail moment, or a lot of either, is that it creates very powerful pitching moments, which is another way of saying very sensitive and responsive controls.  Strong pitching moments and sensitive controls make airplanes turn easily with small control deflections.  Large tail volumes though, don’t work well for semi-scale airplanes because powerful controls limit control deflections.  Semi-scale airplanes with reduced wing area depend on airfoils and flaps designed to produce the maximum possible lift, and this only occurs with relatively large control deflections.  Practically speaking, I can get the same corner radius and control sensitivity with a different configuration, but need a carefully considered tail volume and larger line spacing at the handle.  Tail volume coefficients must be big enough for longitudinal stability and small enough to allow large control deflections for maximum lift.  My design TVC is .4.
 
What about moving the CG back to reduce longitudinal stability a bit for sharp corners with only a twitch of the wrist?  This is an example of how builders can make a seemingly minor and logical change which has dramatic effect.  Moving the CG back 1/2” has a negligible effect on the tail volume.  This would typically be less than .01 or, for example, from .42 to .41.  That same 1/2” happens to be a very large reduction in the distance between the center of gravity and the center of lift for a substantial loss of stability.   We could reduce elevator sensitivity by making the elevator area or deflection smaller.  I can only guess what, if any, desirable effect this reduction in stability and less effective elevators might have. 
 
Area limited airplanes must have bellcrank/flap horn ratios large enough to deflect the flaps.  Flap/elevator ratios, elevator area, CG location and tail volume can all be varied, but with results difficult to predict.  I stick with relatively forward locations of the CG, and target TVCs, for adequate stability and use approximately 50-50 elevator/stab areas.  This puts me in the ball park with enough pitch for a competitive corner, and enough flap deflection for the necessary lift.  Performance is fine tuned with trim adjustments.
 
Personally, I consider tail volume calculations necessary only for the design of stunt ships which depart from conventional values.  My goal is to make a grooving airplane with a competitive corner without over sizing moments or areas.  The calculated value is useful, but at best, it is only a rough estimate in determining the distance between the wing and tail or an appropriate area for the tail.  In any case, when designing the Mustang, I drew the fuselage using the desired outlines and tail moment, and used the formula to determine the area of the tail.
 
If you are interested, the formula for tail volume coefficient is: area of the tail divided by the area of the wing, times, length of the tail divided by the wing MAC.  A value between .4 and .45 is usually enough.

Might pay to skip to the last paragraph .  VD~

A few of mine with inadequate LOOKING tailplanes , are inside the formula , and work fine .

Just to throw a spanner in the works , Im thinking Trailing Edge Deflection is more relevant than elevator area .
I like equal or larger elevators , which makes em toey .
Conversly , narrower elevators requier more deflction for the same ' offset ' dimension ;
( Distance above centerline Vs angle of deflection of control surface )
and would be less jittery .

Theres a ton of stuff here ,
https://stunthanger.com/smf/open-forum/m-a-c-t-v-c-aerodynamic-center-and-'tail-moment'-formulas/msg482235/#msg482235

and elsewhere , Search :"  Tail Volume  " .

Online Dennis Toth

  • 2014 Supporters
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2285
Re: Stabilizer area?
« Reply #7 on: April 07, 2019, 05:20:53 PM »
Floyd,
The simple answer is that today's ships have stab/elevator area of 22 - 25% of the wing area. The tail moment is around 12 - 15% longer when compared to classic designs. This does allow the CG to be a bit further back (old Fancher rule of thumb is CG should be back from the MAC the same % as the tail is to the wing area). Works well for all ships as a starting point.

Best,   DennisT
« Last Edit: May 16, 2019, 10:01:10 AM by Dennis Toth »

Offline FLOYD CARTER

  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3668
    • owner
Re: Stabilizer area?
« Reply #8 on: April 11, 2019, 03:30:13 PM »
I appreciate all the comments.  At this late date, even after a dozen flights, I went to the plan and calculated stab/elev area and wing area (with flaps).  Turns out exactly 1/3.  It looks big, but the thing really flies great.
"Growing old is easy.
 Staying old is hard"
AMA #796  SAM #188  LSF #020


Advertise Here
Tags:
 


Advertise Here