How do I tell if my tail surfaces are too small? - RC Groups
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Jun 24, 2005, 03:40 PM
Sussex, UK
RobinBennett's Avatar

How do I tell if my tail surfaces are too small?

Here's my latest own-design plane (made from EPP from - someone at my club commented that they thought the tail surfaces were a bit small for 3D aerobatics and it does fly 'differently' to my other planes but I don't have enough experience to know what's right or what tests I should try.

The tail was roughly copied from a mini-3DX and scaled up a bit, the wing ended up quite a bit bigger.

It's flown several times and will loop and roll OK (although slow rolls aren't axial) I've moved the CG back until it'll fly almost hands off either-way up.

Does the tail look small to you?
Are there any rules of thumb for tail area vs wing area?
Are there any manovers I can try to test it?

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Jun 24, 2005, 03:49 PM
Registered User
Jun 24, 2005, 04:31 PM
Sussex, UK
RobinBennett's Avatar
If I've got this right the values it calculated for the distance from wing to tail is about right for this size tail, but it's predicted a pitch stability and damping factor rather lower than the 'prefered' amount.

Here's the results: Does it look as if I've got the dimensions right? Does it say I need a bigger tail?

File: da_templ.des

Effective_Stab_Area_(in^2)................ 54
Stab_Aerodynamic_Center_(AC)_coordinate... 21.5
Effective_Fin_Area_(in^2)................. 50
Fin_AC_coordinate......................... 25
Inside_wing_panel_1_root_LE_coordinate.... 0
Inside_wing_panel_1_root_TE_coordinate.... 13.5
Inside_wing_panel_1_span.................. 19
Inside_wing_panel_1_dihedral_angle_1...... 0
Inside_wing_panel_1_tip_LE_coordinate..... 0
Inside_wing_panel_1_tip_TE_coordinate..... 11.5
Next_wing_panel_2_span.................... 0
Next_wing_panel_2_dihedral_angle_2........ 0
Next_wing_panel_2_tip_LE_coordinate....... 0
Next_wing_panel_2_tip_TE_coordinate....... 0
Next_wing_panel_3_span.................... 0
Next_wing_panel_3_dihedral_angle_3........ 0
Next_wing_panel_3_tip_LE_coordinate....... 0
Next_wing_panel_3_tip_TE_coordinate....... 0
Next_wing_panel_4_span.................... 0
Next_wing_panel_4_dihedral_angle_3........ 0
Next_wing_panel_4_tip_LE_coordinate....... 0
Next_wing_panel_4_tip_TE_coordinate....... 0
Next_wing_panel_5_span.................... 0
Next_wing_panel_5_dihedral_angle_3........ 0
Next_wing_panel_5_tip_LE_coordinate....... 0
Next_wing_panel_5_tip_TE_coordinate....... 0

File:  ctrl-z.des

Design Analysis Results:

W_Area =  475.076 = Projected Wing Area
W_Span =   38.000 = Projected Wing Span
Cavg   =   12.502 = Average Wing Chord
AR     =    3.040 = Wing Aspect Ratio
C_Mom  =    0.013 = Datum to Leading Edge of Cavg
P_Mom  =   17.736 = 30% Cavg to Stab AC
Y_Mom  =   21.236 = 30% Cavg to Fin AC
Psf    =    0.161 = Pitch Stability Factor (0.4 preferrred)
Ysf    =    0.059 = Yaw Stability Factor        
                    (0.020 for polydih. wing, 0.030 for aileron wing)
Rmf    =    0.000 = Yaw to Roll Coupling Factor 
                    (0.20 for polydih. wing, 0.052 for aileron wing)
Pdf    =    0.672 = Pitch Damping Factor ( 1.000 preferred)
Jun 24, 2005, 05:24 PM
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Dan Leonard's Avatar
This is one question I will never hear from my wife.
Jun 24, 2005, 05:28 PM
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Larry3215's Avatar
The easy way to tell is if you have trouble controlling it in pitch and yaw

Jun 24, 2005, 05:34 PM
Registered User
gone fishing
Last edited by FRAMEDNLVS; Aug 04, 2006 at 04:51 PM.
Jun 24, 2005, 06:25 PM
Registered User
Tails on airplanes, are kind of like the tail end of a woman. Some people prefer them to be larger, and others like smaller ones. If it feels good to you than it's right.
Jun 24, 2005, 07:28 PM
Permanently Banned
Larry3215's Avatar
Ok, I was a little short

The rule of thumb, as I remember it, was that the stab/elevator should be roughly 15% of the wing area for a single wing and 20% for a bipe.

Fin/Rudder should be in the range of 10%/15% of wing area.

But every plane is different.

Hi speed patern types or hi efficiency gliders like smaller tail feathers.

Slow flyers or 3D planes like larger tails.

I personally like the higher percentages/larger surfaces on my planes and especially on the 3D models.

Jun 24, 2005, 07:29 PM
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Larry3215's Avatar
Oh yea - one other detail. Smaller tails are more sensitive to balance.

Jun 25, 2005, 04:15 PM
Sussex, UK
RobinBennett's Avatar
> stab/elevator should be roughly 15% of the wing area

Both horizontal stab and fin are about 11% of wing area, so I'll make some a bigger stab and try it. It's a fully moving stab so it should be easy enough to change.

Flying today it had enough authority to hover but always seemed unstable in level flight, even with the CG at the point where it flew inverted much the same as RWU.
Jun 25, 2005, 06:00 PM
Permanently Banned
Larry3215's Avatar
Suposedly, you can use a smaller stab if it is full flying. My experience is that they tend to be more sensitive tho.

Flying today it had enough authority to hover but always seemed unstable in level flight, even with the CG at the point where it flew inverted much the same as RWU.
For most planes, flying equally up-rite and inverted is still on the tail heavy side of pitch stablity. Smaller stabs will make the tail heavyness more noticable.

Jun 26, 2005, 09:24 PM
Crash Master
Gene Bond's Avatar
Pitch stability at higher speeds is something I've given up on with a 3D plane, if the cg's set to do everything else easily. I've resigned to moving the cg further forward for windy days, and leaving the hovering and other extreem alpha stuff alone. When the wind is down, I slide the cg back and can fly very slow as well as do all the goodies.
Jun 27, 2005, 06:53 AM
“There’s no place like Foam”
gpw's Avatar
What GeneBond said .....
I've noticed planes with smaller tails (especially stabs ) will do strange things in the air , like sudden changes in attitude .... the smaller the model gets , the more apparent this effect .... I believe that in our scale air , as we know , it takes a bigger wing /lighter weight , bigger ailerons , etc. to effect control at our lower speeds .... the slower the plane the more deflection/surface you need ....
Fer' instance ..... Look at the P-nut scale designs , ample tail surfaces and thay are smooth, stable flyers .... when I scale UP a design , I try to use those parameters for a better (easy ) flying experience .... on these park size Foamies....
If you like smooth , easy flying small planes , make the stab about 20-25% of the wing area and the rudder 18-20% ... that works fine for me (for many years ) ... all flying surfaces are no different as far as stability is concerned , just way more sensitive and less draggy since you're moving the whole surface ....

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