Some of the smaller (40-60 ) planes have shaped horiz stab & rudder surfaces ,most seem to have flat plates . .Is there a real advantage in the shaped ( aerodynamic ??)surfaces & is it worth the extra work . If they are worth using is there a place to get the appropriate aerfoil sections .

Our models are so over powered that the improvements in less drag, more efficeincy etc. of shaped surfaces means little to us. When you get into the larger 1/4 and 1/3 scale models, it is somewhat more meaningful. It also helps structurally, i.e. with the thicker mid section, the spars can be stronger by using shear webs between them and seperating them for best strength to weight ratios.

Aerodynamically, flat plate airfoils on horizontal tail surfaces seem to work just fine. Although a flat plate stalls at about half the angle of attack of a normal section, the downwash over the tail prevents it from developing a very high effective angle of attack, for normal layouts. The main problem with flat plate tails is lack of stiffness, which tends to make them prone to flutter, often requiring draggy bracing wires. I like to use about 9% symmetrical sections on horizontal tails to stiffen them up while keeping them light, building them up with ribs and spars, and sheet balsa covering if necessary.

This topic historically gets into a pi$$in' contest between those that say that anything other than a flat plate stab (for small models) is a waste of time and those (like me) that advocate built up, airfoiled stabs. Everything I design & build lately is for 1/2A power and I automatically try to use a symmetrical built-up stab whenever possible. Personally, I think I can "feel" the difference in the flight characteristics of the 2 types and enjoy the smoothness and predictability in pitch I think I get. Can I prove it with bags of numbers and equations? Nope.

One thing though, isn't a stab just a small wing? Can one tell the difference betwen an airfoiled wing and a flat plate wing? Methinks you can. And yes, before someone jumps in and shouts that flat plate foamy planes are a superior concept, I have flown them and they fly pretty good!

As far as what airfoil sections to go with; conventional wisdom calls for an NACA0009 or 0010 but one of my favorite, best-flying planes has an Eppler 168 stab which is 12.44% thick. Go figure. The thickness was needed to fully enclose the twin rudder bellcrank & pushrods.

I haven't done a built-up, airfoiled vert/stab yet but I think they should be superior in the yaw axis as well. Just my opinion, YMMV.

I remember an airplane called the Stark Shark a few years back that had a very thick horizontal tail. According to the designer it had a nice soft pitch feel. As I was working for a living at the time I did some looking in the available literature and did discover that there was a benefit to effectiveness if everything else is kept the same, just thickness changed.

When you throw in varying area, elevator size, etc. the difference can get lost. Control deflections of 40 degrees when the elevator is half the horizontal size pretty well negates any differences. However I have always felt that the airfoiled tail is worth while on anything bigger than a .15 size airplane where precision is desired. I believe it is going to be the same on the vertical tail as well.

We use flat surfaces on the small foamys because they weight nothing and are overpowered (at least the fun good ones are) so arn't a good example of what an ideal aerodynamic machine should be. At the lower Reynolds numbers the flat plates arn't that bad. Look at a butterfly. But for anything that has a reasonable Reynolds number the drag, etc. of the airfoiled surface is better.

You can always point to some full scale machines and models also that have flat tails that work fine, however it doesn't mean that it is the best way to go.

And as has been mentioned, the structural strength for weight is better with a thick tail.

Thank you all for the comments. It's going to be a long winter ,I like building so I'm going for some TLAR and aerfoil .They look better anyway.

My 2 cents is that the designs I have flown with sectioned tails - typically naca 0009, naca 0012 have more powerful control responses for a given control size and deflection. This very anecdotal of course - it's the flat plates which seem "softer" to me.

Torsionally there is a benefit to a curved leading edge up to a spar, also the thickness aids strength and stiffness.

If you are doing a carbon boom glider, you can even sneak a couple of nano servos (e.g. Saturns) on their side into a 9-10% sectioned tail.

My guess is that if you are a really good designer you could probably use a smaller area of sectioned tail against a flat tail - so you could save wetted area.

Totally agree that for well-powered models, these are all moot points... I guess I just like the way they look


I have seen some even small 3D planes start to have the option of a sectioned tail...

I agree with the "contest" part part.
I also think that in the very low Reynolds numbers where models operate that the "true Difference" is harder to define than we would think.
Everything on the airframe including the mechanics exerts influence on pitch behavior. Moments, Surface proportions, decalage, control surface ratios,weight and balance, etc all have input on pitch behavior.
I fly both plate and airfoil designs from 1/2 A to 150 CC and find that this particular element is hard to quantify as a stand alone item.
I do find that it is a interesting exercise for me though, jumping back and forth across the aerodynamic fence.
Hope to see you again next year at SMALL STEPS I will be the one holding your car keys.[8D]

Very interesting comment, Dickeybird. I too, figured that a thicker fin would develop higher lift. I built a small scale fuselage and fin/rudder assembly, and wind tunnel tested it with both a thick, and a thin fin. The test was simple. I measured only the angle to which the rudder would deflect the fuselage, relative to the wind. Tried a relatively thick fin, carved from foam, and then, a thin, sheet balsa fin. Result: The thin fin produced identical yaw angle, within the limits of experimental error. Rudder area was equal to fin area, and deflection 45 degrees. I have used relatively thin fins since then, although I usually build them up with ribs and spars, to give them an airfoil thickness, including the rudder, of 5% or so - makes them stiffer, and thus more resistant to flutter.

Some quite serious full-scale airplanes, such as the Extras and early Edges flew quite well with flat-plate stabs. A flat plate develops a steeper slope of lift coefficient versus angle of attack than does a thicker section, so that in theory, a flat plate stab is a slightly more effective stabilizer. I use flat plate tail feathers only on rather small, slow models, since they are far more prone to flutter, requiring bracing at higher airspeeds. Even with bracing, their thin, flexible elevators are more prone to flutter than the thicker elevators used with a thicker stab. It might be interesting to ask Edge 540 pilots whether they noticed any improvement in pitch control when the airfoiled, thicker, unbraced stab was adopted.
I like NACA 009 or 0010 sections for horizontal stabs.

FWIW- a diamond shaped stab/elevator setup works as well as anything - at least as far as I can tell-
When we did some Staudachers, we did not want to rig the tail -so the hinge line became 1.5" thick at the root and tapered to 3/8" thick at all edges - then I cap stripped all of it to make it look like ribs.
very rigid and strong , smooth responses and looked like a flat setup to the casual observer.
One thing that always puzzled me.
why woulf a thick foil have more lift than a thin one
or does it?
Why?

.

if a flat airfoil was inferior then how do the foamie airplanes work so well. answer: flat airfoil is fine

.
For the purpose.
Lots of flipping around, little speed needed.

Just for info, the Grumman Agcat, a bi-wing cropduster, (see my avatar for those who have never seen one) I flew has the flat plate for both horizontal and vertical stabilizers. That plane is highly maneuverable due to the job is was designed to do. Lots of snap turns and pull ups!

I remembered reading that Phil Kraft changed the Kwik Fli II to the III version with some cosemetic changes to make it more streamlined and an airfoiled horizontal tail which was a change from the flat plate on the II version. He said it stopped the Kwik Fli Dance (a noticable tail wobble under certain conditions - it flew fine, just wobbled). He made enough of the airplanes and flew enough to notice the difference. After all he won the Nats with it!

Somebody should take a really big aerobatic machine with a flat horizontal and add an airfoiled shape with just a few pieces of shaped foam taped in place. Make the tail 20 percent thick to really make a change! That way the difference could be readily determined. The foam's weight would have no influence so the only effective variable would be the thickness.

real change - better response by
Making the tailas stiff as possible
stiffer is better
just ask yer "live in"

We always assume the fuselage and attached tail both have the proper stiffness and dynamic response.

That just leaves you with the thickness of the tail, which, as you know can effect your response a lot.

I try to never assume---simply because the stiffness of the flat plate stuff - especially on models IS the major problem on all of the set ups I have seen

you can dull any control surface response by making it too thick
for example -when it becomes as thick as it is wide -

I've messed around with both built-up airfoil & flat plate tails for years, & in the end I've decided that (other than in scale applications) built-ups aren't worth the trouble. I have never really been able to actually measure a meaningfull difference, nor observe one in flight. At times I managed to convince myself that the built-ups were marginally better, but it was really only wishfull thinking, to provide mental solace for all of the extra work.

Back in the "old days" Jim Kirkland published the best , most comprehensive
procedures ever for trimming an airplane before the computer radios made this
a lost art. He believed , and so do I , that a flat stab has a dead spot around neutral
and an airfoiled stab has more positive control in all its movement. This dead spot ,
in my opinion , is why some people say the flat stab feels "softer". Back in the 70's
and 80's I built several versions of the same pattern airplane , one with a flat stab
and the rest with airfoiled stabs and I could tell the difference. Kirkland believed ,
like Dick Hanson stated , that a diamond shaped stab was just as effective as a
regular airfoiled stab. This is what he used on his Intruder that won the Nats.
If you ever saw him fly you would believe it also.
If you are taking the time to build an airplane a little extra time for the stab , I think ,
is well worth it.

tommy s

The problem is that we as active builders always change more than the tail thickness. A different motor, fuselage shape, wing taper, tail length, etc. Someone with more time and money than me could make a model like the big acrobats that use carbon fiber tubes to make removable horizontal tails and simply make a flat plate, "normal" and thick airfoiled tail. In a competition where everyone is looking for an advantage I would think the experiment would be worth it.

I spoke with a fellow aero engineer who said his research of several years ago found wind tunnel data that indicated the airfoiled tail would be better but he declined to share it with me. Company propriatry I guess.