Who's feeling technical tonight?

[flystoolow]

We are told that a properly aerofoiled tail group is much better than a thin, flat tail group. Assuming the flat tail group is made well, with a rounded LE and a finely tapered TE...how much better, really, are the foiled surfaces?

[FrogChief]

Well...one would want to consider the lift and trim changes inherent in a lifting tail design.

With a lifting stab, as your airspeed increases the aircraft will want to nose down more and more. Constant upward trim changes are the result.



*Learned on my Father's Telemaster 40 back in the day.


I would think that on a *model aircraft with a wide speed range, that a non-lifting tail would be more appropriate.


*As model-sized aircraft behave differently than full scale in certain situations, as the air is ALWAYS 1:1 scale. ;)

[schrederman]

Froggie is right. However, funny that in full-scale sailplanes, there is a slight undercamber to the elevator as a safety measure against getting more nose down with greater airspeed. Seems that this slight lifting doesn't lower the nose, but acts as a servo tab which forces the elevator up slightly as airspeed increases, lowering both the tail and the airspeed. That's OK with a pilot that won't notice the very slightly more aft stick. However, our servo wouldn't like that situation at all and force the elevator to stay where it is, hence the increase in airspeed would be a tail lifting experience, hence increasing the airspeed more.... and so on. I am pretty sure that using something like the NACA-008 (fully symetrical) for tail surfaces is better than flat plate. We used a modified diamond on the Houston Hawk. I later built stabs with the NACA-008 and realy couldn't tell much difference. I do think the deadband around 0 was less with the NACA airfoil. Those assumptions are for the full flying stabilator. I honestly think the articulated stab/elevator setup would be better with the NACA as well, but you'd need it set up with either no decalage, or maybe only 1/2 degree... certainly not more than 1... for the way I fly, anyway. Some think I balance my models at 30% chord... of the stab...

One thing I find very interesting, though... we spend all this time making all our models so smooth, and then stick stabs on them that have very saggy covering over ribs that have no leading edge sheeting... Am I missing something? Sorry, but it just doesn't seem right...

Jack Womack

[seanpcola]

flystoolow beat me to the mark on this question. I was gonna ask a similar one myself . Just wondering what performance improvements might be gained on a model that was originally designed with a flat plate, built up tail (rudder/vert or elevator/hroizontal). I have built several kits that used a diamond configuration, usually a thin balsa "spar". I assume the only real noticeable change on a relatively slow woodie RES might be an improvement around center. I am sure there are several considerations here but would it be worth the possible weight gain in the back?

[flystoolow]

A nice fat symmetrical foil......or a razor thin sheet of carbon fibre laminate.......HMMMMM.

[FrogChief]

Froggie is right. However, funny that in full-scale sailplanes, there is a slight undercamber to the elevator as a safety measure against getting more nose down with greater airspeed. Seems that this slight lifting doesn't lower the nose, but acts as a servo tab which forces the elevator up slightly as airspeed increases, lowering both the tail and the airspeed. That's OK with a pilot that won't notice the very slightly more aft stick. However, our servo wouldn't like that situation at all and force the elevator to stay where it is, hence the increase in airspeed would be a tail lifting experience, hence increasing the airspeed more.... and so on. I am pretty sure that using something like the NACA-008 (fully symetrical) for tail surfaces is better than flat plate. We used a modified diamond on the Houston Hawk. I later built stabs with the NACA-008 and realy couldn't tell much difference. I do think the deadband around 0 was less with the NACA airfoil. Those assumptions are for the full flying stabilator. I honestly think the articulated stab/elevator setup would be better with the NACA as well, but you'd need it set up with either no decalage, or maybe only 1/2 degree... certainly not more than 1... for the way I fly, anyway. Some think I balance my models at 30% chord... of the stab...

One thing I find very interesting, though... we spend all this time making all our models so smooth, and then stick stabs on them that have very saggy covering over ribs that have no leading edge sheeting... Am I missing something? Sorry, but it just doesn't seem right...

Jack Womack

Right on Jack...

A symmetrical airfoil for the stab WOULD increase effectiveness due to increased lift vectors upon movement of the elevator. However it still would result in a more sensitive and difficult to trim model.

You touched upon that when you stated that the deadband around center would be reduced.

[tim_mellor]

From a high performance standpoint my F3B Tragi's (NACA 9% thick) flown at the world champs handled much nicer than the Cobra's (6% thick) at thermal speeds but above that speed it was of less importance and may have been a benifit at speed due to reduced lift available. Personally 8-9% for thermal planes is nice :)

Tim

[Ollie]

"--how much better, really, are the foiled surfaces?"

What do you mean "better?" Better tail than low drag? Better tail than high speed?
Better tail than low speed? Better tail than large? Better tail than small? Better tail than elevator and rudder control power in degrees? Better tail than L/D? Better than a tailless wing? Etc, etc.

What do have in mind? All of the above? Which mix of the many "better than?"

Do you want a simple minded answer to a complex question? Is that have meaning?

[Andy W]

I think the question is simply:
if I have a kit that supplies flat-sheet tail surfaces, and instructions to "round the leading edge and slightly taper the trailing edges of these surfaces",
is it worth the effort to actually sand a more defined airfoil-like shape into those surfaces.

For sailplane, at various speeds, I'd think there would be a slight reduction in drag, and the higher the speeds, the more significant this would be..?
I can't say, but I've gone to the trouble on several occasions! :)
..a

[mhodgson]

Personally I find a thicker tail section gives a much smoother control with less chance of having to put in corrections. They might be a little more draggy but in the overall picture I reckon I gain by being able to control things more smoothly.

[schrederman]

Perhaps I should explain the "modified diamond" tail. It actually comes out as a smoothly curved, 3/8" thick airfoil if done correctly. You build the whole thing from 1/4" stock, sand it tapered to almost nothing from the spar back, sand it curved from the spar forward. Add 1/16 X 1/4 spruce top and bottom for the spar, with 1/16 vertical grain shear webs. Then sheet the leading edge area and sand to a nice symetrical airfoil. The covering material doesn't touch the ribs from the spar back to the trailing edge. If you want to make it thicker, use 3/8 stuff and you end up with a 1/2" thick stab. If you want to keep it light, SAND..... You can even taper the spruce and diminish the shear webs to 1/32 as you go out. Another thing you can do is use .007 X 1/4 cf under the back edge of the sheeting, and eliminate the spruce. Don't eliminate the shear webs, though... At the root end of th espar, you can box that like the wing for the axle tube.

While a little less elegant than plotting ribs and so on, it does work well, and the stab's as bullet proof as anything I've seen. Some guys seem to have trouble sanding enough and the stabs come out heavy. The trailing edge and the leading edge need to be sanded almost to a knife edge. The trailing edge section needs to be flat tapered, and the leading edge needs some curve... the same curve for both sides, please... we don't want to create camber. If the leading edge is sanded sharp, when the surface is sheeted, there's 1/8" there for rounding into a leading edge radius.

I'm with our British friend, I like a thicker tail... don't get me started...

Jack Womack

[histarter]

Personally I find a thicker tail section gives a much smoother control with less chance of having to put in corrections. They might be a little more draggy but in the overall picture I reckon I gain by being able to control things more smoothly.

As I look through my sailplane collection for the last 15 years, every sailplane I built has a diamond profile employed on my tailsections. The added stringers improved torsion and overall strength, appearance, smoothness of the surface, and handling. The small inverted MCL on the stab seems to make the machine run on rails when racing off to new lifting areas. At high speeds adverse pitch increases, so a bit more (rationed) counter-lift should improve overall LD. ;)

You could say that I am spoiled rotten flying short coupled sailplanes with strong tailfeathers that can survive a carwheel (while having a full tummy of ballast)! :eek: :D

[ChuckA]

Flat plate airfoils are weak structurally and very inefficient aerodynamically. Otherwise I can't think of a reason to not use flat plate stabs.

[schrederman]

Don't sugar coat it, Chuck...

[Peytr]

I think a flat plate has very low resistance at low angles. At higher angles the flat plate will be dragier than a decent airfoil BUT less sensitive to low Renolds numbers as well.

So I'd say:

Flying a small stab at low speeds and/or low angles might be the domain of the flat plate stab;

Flying a large stab at high speeds and/or high angles to my idea would be better with a foiled stab.

[histarter]

I think a flat plate has very low resistance at low angles. At higher angles the flat plate will be dragier than a decent airfoil BUT less sensitive to low Renolds numbers as well.

So I'd say:

Flying a small stab at low speeds and/or low angles might be the domain of the flat plate stab;

Flying a large stab at high speeds and/or high angles to my idea would be better with a foiled stab.

Excellant. Your heavy message is very succinct - and accurate. [PS High angles would have a short time base.] :D