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Author Topic: How much vertical tail is too much?  (Read 352 times)
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Larry R.
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« on: August 23, 2019, 08:13:17 PM »

Typically, a free flight scale model will have a vertical tail somewhat larger than scale size in order to ensure sufficient flight stability.  Not long ago, however, I read an article which attempted to explain why too much vertical tail actually causes instability.  The explanation was fairly technical, at least for me, so I didn't really follow the logic.  It seems there is some rule of thumb about optimal tail size, both vertical and horizontal surfaces.  Is it based on wing area?
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Crabby
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« Reply #1 on: August 23, 2019, 08:57:17 PM »

Larry great question. No doubt guys will be chiming in. As the answers begin to trickle in you might want check this thread out. https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=24518.msg246517#msg246517  You may begin to get an inkling (if indirectly) of what role the vertical tail plays in stability.
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« Reply #2 on: August 24, 2019, 03:35:18 AM »

Hi Larry,

In the thread Crabby linked to I tried to address the aerodynamic mechanism of how the vertical tail (aka fin) affects Spiral Stability. It's a complex area and there is no one fixed number or rule of thumb that works - rather it's a judgement call based on several interacting effects.

Typically, a free flight scale model will have a vertical tail somewhat larger than scale size in order to ensure sufficient flight stability.

This is a common misconception. Horizontal tail area is often increased to ease trimming (not always necessary) but vertical tail area of a scale subject can be often be too large rather than too small. Examples include some of the post war powerful piston engine aircraft that had very large multiblade props and needed big fins to control the swing on take off. At the other end of the spectrum you might have something like an early Avro with a tiny circular rudder. The reality is that full size aircraft encompass such a wide variety of intended flying stability - or lack of it - that there is no rule of thumb for adjusting fin area to suit a FF scale model. Some will need enlarging in area, some reducing - many can be left as scale.

What we do have is a couple of measurements and calculations to assess the fin effectiveness - Vertical Tail Volume (Vv), Vertical Tail Moment Arm (lv), the Blaine Rawdon Parameter (B) and Equivalent Dihedral Angle (EDA). We can use these plus an overall understanding of the aerodynamic effects to help us make a much more accurate judgement call.

You can look more into those parameters here:
https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-01-unified-engineering-i-ii-iii-iv-fall-2005-spring-2006/systems-labs-06/spl8.pdf

And there are files that calculate them for you here:
http://tailwindgliders.com/Files.html
(Aimed at glider design but the calculations are the same.)

The idea is that you can use these calculations to assess successful (or unsuccessful) aircraft and pin down numbers for a new design. Tail Volume in particular can have quite a wide variation but it's a useful thing to know about a model.


Is it based on wing area?

Not so much on wing area but the following relationships are important:

1. Vertical tail moment arm relative to wing span
A long vertical tail moment arm on the fin is good for spiral stability. The importance of this is often missed, but a long tail moment allows the model to get the best out of whatever dihedral it has. Divide wing span by Tail Moment Arm and if it's less than 0.3 I worry a bit. 0.4 to 0.5 is 'normal' and 0.6 is really good. Interesting the Lacey M10 does very well in this area. The moment arm is actually much more important than fin area.

2. Dihedral
There no getting away from it but dihedral is critical to spiral stability. The 'fin too big' situation is often a case of marginal dihedral effect and a small increase in dihedral would help more than changing fin size. In the case of sport models with excessive dihedral tail volume must be quite large to avoid dutch roll. EDA and the 'B' parameter is a handy assessment tool here. If B is above 5 you have a decent chance of stability. Don't forget a few degrees of adjustment for low or high wing position when considering EDA.

3. Destabilizing stuff forward of the CG
When you have props, wheels, pylons etc ahead of the centre of mass a bigger fin is needed and the actual fin area becomes more significant. The relationship expressed in the 'B' parameter breaks down somewhat as it ignores these effects and so experimenting or a least a good TLAR judgement becomes necessary. That other thread goes into the effect of this on Yaw Damping.

There are other considerations such as inertia but these are three important ones.

All of this is fairly technical and most people are not going to run a bunch of numbers when designing unless there is some reason for concern ie an unconventional subject. Knowing the Vertical Tail Volume and Tail Moment Arm/Wing Span is always helpful though. My default position is too leave fin area as scale if there is a reasonable amount of dihedral and if the tail moment arm is good (at least 0.4 times the span) and if Vv is within reasonable bounds.


Jon
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Larry R.
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« Reply #3 on: August 24, 2019, 12:35:57 PM »

Thanks, Crabby and Yak.  I've been under the impression that some model designers enlarge the vertical tail simply as a matter of course for scale subjects.  And this may be the case.  I do not recall any model plan or its associated construction article explaining why the tail area was enlarged a bit from scale, other than the assumption that this was needed, or at least couldn't hurt.  Then, there is the "eyeball" approach to design, i.e., if it looks right, it probably is right.  Or close enough. 

For Bill Hannan's Flying Funtique model (can be found here: http://www.airplanesandrockets.com/airplanes/flying-funtique-article-plans-apr-1969-aam.htm), although I was careful in the construction, in test flights the model spiraled to the left rather badly.  I was able to correct this by adding modelling clay to the starboard wind tip.  With the prop hub shimmed to produce down and right thrust line, I got in some good flights.  I'm now building a second Funtique, making a change in the landing gear mounting, and enlarging the vertical fin as a shot in the dark at increasing spiral stability.  Now I'll be applying the Basic Aircraft Design Rules to help determine the best vertical fin volume for this model, then see how it flies.

The Basic Aircraft Design Rules brings another question to mind regarding the Funtique and other nonscale models which have a wing that can be moved forward and aft on the fuselage as a means to trim flight. I have seen, as is the case with the Funtique, the instruction to move the wing aft if the model tends to climb too steeply.  Since moving the wing aft shortens the tail movement arms (both horizontal and vertical), would it be better to leave the wing where it is and add a bit of weight to the model's nose to move the CG forward?
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lincoln
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« Reply #4 on: August 24, 2019, 09:50:04 PM »

Making the vertical stab larger probably won't help with spiral stability. If the model dives off to one side or the other, no matter how you trim it, that may mean the vertical stab is too large and/or the model needs more dihedral.

Moving the wing aft may make sense if the tail surfaces are oversized. However, you might try down elevator first if the cg seems to be in a reasonable place.
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charlieman
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« Reply #5 on: August 25, 2019, 01:12:36 PM »

I concur. Vertical enlargement seems more a rote practice than a bonfide aerodynamic need. I suspect that some scale designs might need a visual proportional enhancement to go with the larger Horiz or even to make room for a stab enlargement (McHard's Me 109E  for example)!

I see  big verticals on post war piston engine types as more of an outgrowth of what started before WWII. Nearly all the frontline developments kept getting larger and larger Fins/rudders. Spitfire, Me 109 are shinning examples. Focke Wulf 190D go ta lengthened fuse, as did TA152 ( and more fin area!). It would probab ly be easier to name those that didn't. (A6M, P-38. Mosquito...?)  Engines were getting bigger more powerful ,BUT air combat was increasingly taking place at higher altitudes and speeds. Other factors of stability were making themselves known, as typical operations were getting ever closer to upper limits of sub-sonic flight.

Aircraft from WWI that got  larger vert fin and rudder was Sopwith Snipe. Early form, as flown by Barker, had propensity to "dutch roll" ( a tail down waggle , constantly threatening to develop into a vicious snap and spin). Larger more rounded fin and rudder seems to have cured those pilot concerns, but the mfger also added balanced ailerons with additional area. Snipe has a rather high dihedral angle. Did larger fin/rudder cause a reduction in roll response?
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piecost
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« Reply #6 on: August 25, 2019, 05:17:21 PM »

My Nolan Aero Bristol Scout ended up with extra dihredral due to the tension in the covering. The wing spar being rather insubstantial and on the lower surface. The model would not consistently trim for circling left flight but would stall turn randomly to the left or right. I enlarged the rudder (there being no fixed fin) with foam stuck on with tape to find the size needed then made a larger stick and tissue rudder. It is very consistent now.

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Yak 52
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« Reply #7 on: August 26, 2019, 04:21:48 AM »

I've been under the impression that some model designers enlarge the vertical tail simply as a matter of course for scale subjects.  And this may be the case.

I suspect this happens because when you add non-scale dihedral, you generally need more fin area to balance it too.


...although I was careful in the construction, in test flights the model spiraled to the left rather badly.  I was able to correct this by adding modelling clay to the starboard wind tip.  With the prop hub shimmed to produce down and right thrust line, I got in some good flights. 

If the model consistently diverges to the same side you may have a warp rather than a spiral problem.


I'm now building a second Funtique, making a change in the landing gear mounting, and enlarging the vertical fin as a shot in the dark at increasing spiral stability. 

The Funtique appears to have various fin options of different sizes. If you have a spiral problem then increasing the fin area will make it worse.

Which brings me to an important question: are you experiencing a spiral problem or is it a snap and spin? The two are different behaviours and you need to correctly identify which is which.

A spiral stability issue is due to excessive yaw stability with insufficient yaw damping (too big a fin) and is evident when a model slowly diverges from level flight and doesn't roll back upright. Rather it turns towards the roll, the nose drops a little and the model begins to accelerate into a spiral dive.

A snap and spin is due to insufficient yaw stability (small fin) and looks different. The model will yaw excessively, causing one wing to stall and will flick into a spin much more quickly. If a full spin doesn't develop and you just get a wing drop caused by asymmetric stall this may look like a spiral dive or even initiate one, but the cause is quite different.

You need to correctly identify which problem you have. If it is a spiral issue the fin area increase will make it worse. If it is a snap-spin issue the fin area increase should help.


The model would not consistently trim for circling left flight but would stall turn randomly to the left or right. I enlarged the rudder (there being no fixed fin) with foam stuck on with tape to find the size needed then made a larger stick and tissue rudder. It is very consistent now.

I presume you mean flick roll or snap roll rather than stall turn? But this is classic behaviour from a model with insufficient yaw stability and fin enlargement was the correct action.


Dihedral requires the model to be yawed in order to work. If you put a wing flat on a desk and look at it from the front you can see that yawing causes an increased angle of attack on the advancing wing. This means more lift and a roll reaction. However if this is taken to the extreme and you reach the stall angle of the advancing wing it will lose lift and roll towards that side. An intentional aerobatic flick roll is initiated in the same way.


Jon
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Crabby
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« Reply #8 on: August 26, 2019, 10:26:00 AM »

A long vertical tail moment arm on the fin is good for spiral stability.


Jon, what is vertical tail moment arm when referring to the fin? Could you be referring to the east-west dimension of the fin? asking not guessing!
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« Reply #9 on: August 27, 2019, 07:00:09 AM »

Hi Crabby,

The vertical tail moment arm (lv) is the distance between the CG of the aircraft and the 'aerodynamic centre' (AC) of the vertical stabilizer.

The CG is the 'pivot point' around which a plane rotates in flight. The AC is the point at which the lift of the fin acts (sideways lift) and the moment arm is the lever length for that lift force.

The AC of any flying surface is considered to be at 25% of the mean chord. In the example of the PC-6 shown below with a rectangular fin this is easy to find. With a tapered or rounded fin it must be calculated geometrically. Spin strakes can be mostly ignored but in some cases the rear fuselage blends into the fin and some judgement/guesswork is required anyway.


Jon
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