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Old Jun 24, 2005, 03:40 PM
Sussex, UK
RobinBennett's Avatar
Crawley, West Sussex, UK
Joined Jun 2004
7,372 Posts
How do I tell if my tail surfaces are too small?

Here's my latest own-design plane (made from EPP from foamcraft.co.uk) - 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?

Thanks
Robin
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Old Jun 24, 2005, 03:49 PM
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http://www.charlesriverrc.org/downlo...sis/da_web.doc
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Old Jun 24, 2005, 04:31 PM
Sussex, UK
RobinBennett's Avatar
Crawley, West Sussex, UK
Joined Jun 2004
7,372 Posts
Thanks.
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?

Code:
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)
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Old Jun 24, 2005, 05:24 PM
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Dan Leonard's Avatar
Lenexa KS USA
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"HOW DO I TELL IF MY TAIL SURFACES ARE TOO SMALL"
This is one question I will never hear from my wife.
Dan
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Old Jun 24, 2005, 05:28 PM
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Larry3215's Avatar
Gig Harbor, Washington, United States
Joined Sep 2003
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The easy way to tell is if you have trouble controlling it in pitch and yaw

Larry
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Old Jun 24, 2005, 05:34 PM
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gone fishing
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Old Jun 24, 2005, 06:25 PM
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Dallas Texas
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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.
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Old Jun 24, 2005, 07:28 PM
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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.

Larry
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Old Jun 24, 2005, 07:29 PM
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Gig Harbor, Washington, United States
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Oh yea - one other detail. Smaller tails are more sensitive to balance.

Larry
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Old Jun 25, 2005, 04:15 PM
Sussex, UK
RobinBennett's Avatar
Crawley, West Sussex, UK
Joined Jun 2004
7,372 Posts
> 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.
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Old Jun 25, 2005, 06:00 PM
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Larry3215's Avatar
Gig Harbor, Washington, United States
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Suposedly, you can use a smaller stab if it is full flying. My experience is that they tend to be more sensitive tho.

Quote:
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.

Larry
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Old Jun 26, 2005, 09:24 PM
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Indianapolis, IN
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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.
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Old Jun 27, 2005, 06:53 AM
gpw
“There’s no place like Foam”
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Joined Jul 2002
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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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