Ah-men!

The wing is what does the flying/lifting. The "tail feathers" are just that - guidance for the wing.

Minn Flyer

Welcome...to the church of the ballistic tail! LOL

Please give generously when the plate is passed!

Welcome? I'm one of the founding members!

I have planes with and without a airfoiled tail

With: Sig King Kobra, Goldberg Falcon 56

Without: Sig Kougar, and many others.

The one thing that I have noticed is at slower speed the planes with the airfoiled surfaces they have less of a tendendency to "Snap out."

To the original question. IMO the reason trainers don't have the airfoiled tail surfaces is cost. Especially in the "old days" Before the availability of "Arfs"

There is a big difference between an airfoiled tail and a lifting tail.

Would you please explain just what provides the FORCE to move that symmetrical wing into an AOA to produce the lift force needed to move and/or sustain the Sym. non-lifting airfoil into the AOA to obtain a desired result? IMO it is the horizontal stabilizer, be it a conventional stabilizer/elevator function or a slab. Pilot adjusts elevator/slab which loads wing into position to obtain desired result.
The stab lift force is changed to move and/or maintain the wing into the desired position of the pilot's request via the control system.

An airfoiled stab, if symmetrical, acts just like a wing. Something has to move it into a position to become zero-loaded or it will continue to guide/load the wing into a constantly varying load position. An airfoiled stab, such as a Clark-Y will provide lift, just like a wing, by the square of the airspeed. Only by moving that stab into its zero lift position will the force be neutralized, and that neutral position will change with a change of airspeed.

In 1:1 scale, some light airplanes, a flat slab can bare the structural load (except some of the 3D Arfs which cannot. BTDT [:@] ) In larger airplanes a symmetrical airfoiled stab is more structurally effective, plus the form drag is considerably less.

Still, except for racing events and Scale fidelity to outline, I can see no reason to worry about the need to try to maintain the one-trim-fits-all subject in toy airplanes. It doesn't happen in 1:1 scale and it ain't gonna happen in RC.

BTW, the FF model. Very overpowered. Lifting stab provided help in keeping the aircraft from looping as the lift on the stab provided same as down-elevator, during climb portion. Far aft CG kept things proper during glide. I won a number of trophies in '60s and early '70s in FF.
The late Hal deBolt was a model designer of the first degree. In the '60s, early '70s, there was a pylon event for Formula 2 Pylon Racing. Hal used a flat stab, but set at a positive incidence. Of course the stab actually sets the true incidence line so aerodynamically the flat stab was the real zero. It did work fairly well in reducing the trim problem when one really was going fast.
Back in those days, a pylon Racer actually got to fly straight and level between the pylons. Never happens now so all is purely academic.

And to all: The Beginner seldom goes to the advanced Aerodynamics forum. IMO, they need to learn that there is more to aviation than banging sticks like a computer game. This kind of discussion at least provides an opportunity to become better informed.

I agree with the ones that posted that tails in planes acheieve negative lift. I am also an aviation professional. Think about this in means of aerodynamic forces. A plane with a lifting tail (different than airfoiled tail) seems like it would be negatively stable in pitch. Say you are putting along in straight and level flight and the nose slightly falls. Airspeed would increase increasing the lifting force on the tail, which would pitch the nose over, which would cause an increase in airspeed, which would increase the lifting force on the tail, which would pitch the nose over....

This scenario is exactly the opposite of how a conventional airplane's aerodynamics works.

On a normal trainer, the tail is definitely not lifting during flight. That has been stated in several posts already. When it comes to lifting tails, I think one must imagine that the plane has two wings, one being the tailplane. The CG must be moved back, to allow the lifting tail to carry some of the plane's weight. I believe that I have seen pictures of airplanes where the tailplane was just a wing the same size as the main wing. On such an aircraft, the CG would be close to halfway between the wings (offset by the fore/aft weight difference of the fuselage). With this in mind, a lifting tailplane makes sense, provided that the CG is moved back. I don't have a degree in aviation/aerodynamics, I'm just a private pilot trying to understand how airplanes fly. So feel free to tell me that I'm wrong.

<div style="margin: 0in 0in 0pt"><span style="font-size: 6pt">

</span><span style="font-size: small"><span style="font-family: Times New Roman">If the down thrust on the engine is set correctly the tail will not have to make up the differenceby lifting up or down with changes in throttle and speed. With the correct down thrustthe engine will pull the plane forward and more down as power is applied. Ideallythe horizontal stabilizerprovides neutral lift unless the elevator is deflected to alter the planes path. If one puts a larger than required engine they may also need to increase the down thrust to overcome the tendency to climb.I have seen many folks suffer from constant climb at full throttle because they don't understand this.

Other factors are at play as well. Correct CG, angle of incidence for the tail and wing(s). Model Aviation had a really good article explaining this a few months back.

Also... Don't assume that a built up airfoil is lifting. My old Carl Goldberg Falcon 56 has a built up tail but the airfoil is symetrical. For a slow plane a symetrical airfoil provides less turbulant air over the elevator surfaces resulting in better control. I beleive many of the pattern planeshave a symetricairfoilfor this reason. Built up is generallylighter than a slab as well. </span></span></div>

High lift wings produce a nose down pitch.
That is why a wing that gets deattached from a model in flight rotates rapidly while it free falls.
For that reason, the horizontal stabs must push down instead of lift, resisting the nose down tendency.
In my opinion, airfoiled stabs are stronger and less draggy that flat stabs; however, they are harder to build.
As foamies demostrate, a flat wing can fly very well; however, they are weaker and more draggy (good enough for 3D flying, but not for racers).

For a plane to fly level at any airspeed, the decalage must be zero; hence, the model would be less stable.
A model that is stable in pitch, needs to have certain amount of decalage.

A more maneuverable model must be less stable, and viceversa.

The attached pictures show us that decalage is there, even if the geometrical incidence angle is similar for wing and stab.
The downwash that the wing produces, makes the stab work as it would have negative incidence.
The AOA for the stab is negative in most of the cases, creating negative lift.

Symmetrical airfoils produce no nose down pitch moment; hence AOA for the tail should be zero for level flight.

By deflecting the elevator up or down, we are making the stab cambered in either direction, modifying the amount and direction of the lift that the stab generates.

Copied from http://www.americanflyers.net/aviati.../chapter_3.htm

"Most airplanes are designed so that the wing’s center of lift (CL) is to the rear of the center of gravity. This makes the airplane “nose heavy� and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward. Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight neg-ative angle of attack. The downward force thus produced, holds the tail down, counterbalancing the “heavy� nose. It is as if the line CG-CL-T was a lever with an upward force at CL and two down-ward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). Applying simple physics principles, it can be seen that if an iron bar were suspended at point CL with a heavy weight hanging on it at the CG, it would take some downward pressure at point T to keep the “lever� in balance."

I have seen quite a few Sig seniors with an airfoil built in the tail that flew very nice.
There are so many variables , and how the plane will be flown to take into account, so I don't know if thats true for every kit out there. One thing is for sure though..its quite a bit more work to build and cover one compared to a flat section...

Is it dejavu, or me, or post #19?

Kurt

There are a fair number of older free flight models which have the tail airfoil upside down. Also a fair number with symmetrical or flat tails, and many with the tail right upside up and highly undercambered. Let's see, what was my point?

"Ideally the horizontal stabilizer provides neutral lift unless the elevator is deflected." I disagree.

"Only by moving that stab into its zero lift position will the force be neutralized." I disagree.

Here is why: For the conventional set up with static stability, a tail generating a "zero force" or "zero lift" might as well not be there. On the contrary, we need it there to generate a downward force during steady state flight (straight and level) to counter the lift being generated by the wing, which is acting slightly behind the cg. Since the tail has a longer moment arm, the force can be significantly smaller, but must still act downward (opposite direction to what the wing is producing). Adjusting the amount of this downward force (via the elevator position) allows us to change the aoa of the wing, and increase or decrease lift.

I'm not talking about 3-d flight where the wing is rendered almost useless in favor of thrust.

Kurt


'

<div style="margin: 0in 0in 0pt"><span style="font-size: medium">I will attempt to explain my original post to make more sense it&rsquo;s becoming obvious many don&rsquo;t understand what I was saying. The issues came to mind while reading someone&rsquo;s post, which was about having elevator trim change at different speeds. The idea is simple. Take a trainer plane of any kind that has exhibits these characteristics regardless of it&rsquo;s horizontals angle of incidence main wing style all that is irrelevant at this point.</span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium"></span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium">Now give the horizontal stab a small lifting airfoil. Small enough to be almost insignificant at slow speed (as I mentioned it would need to be calculated). </span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium"></span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium">Now as the aircraft speeds up it starts to gain altitude, the faster the plane goes the quicker it gains altitude. This is whatwill happen. The tail will start to produce lift the faster the plane goes the more lift the tail produces calculated to be proportional to the altitude gain. The lift produced by the tail will pitch the nose down keeping the plane from gaining altitude.</span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium"></span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium">It no different than you forcing the tail up (lifting) with your elevator to keep the plane level. It just lifts automatically due to the airfoil rather than elevator input.</span></div><div style="margin: 0in 0in 0pt"><span style="font-size: medium"></span></div><span style="font-size: medium">Now tell me why that will not work with facts. Not by just saying or quoting internet to say the tail supposed to push down. Of course it is in some cases and in those cases, now the airfoil will allow it to push down less keeping the airplane level.</span>

OK. Try it yourself. Put in a few clicks of down trim. Now your horizontal tail surfaces have a slight lifting (indeed, undercambered) airfoil. Will this solve the problem? Of course not, because it will be way out of trim for your normal cruise speed. You can't just add a lifting airfoil without changing some of the other things you don't want us to talk about.

You do realize lift changes with airspeed. If I click my trim proportional to airspeed(as the airfoil would react)of course it will work and not be out of trim.

You can talk about whatever you want I just ask that it be relevent.

tripower222,

If I understand correctly, your goal is a model insensitive to airsped changes, regarding pitch.[sm=confused.gif]

Now you're talking about an airfoil that changes with your speed; you can get that by mixing elevator and throttle. Of course lift changes with airspeed: this is as true of the airfoil you get by putting in some down trim as of any other lifting airfoil. The problem is, as an earlier poster noted, it doesn't change enough to do the job, and the wing's lift changes with speed, too. The wing's bigger. So the reason adding a lifting airfoil wouldn't do what you want (without other changes) is exactly the same as the reason flying with a couple of clicks of down trim wouldn't do what you want. Adding down trim to what was a flat surface is exactly the same thing as adding a lifting airfoil.

People are quoting textbooks. You are not taking into account that our conventional designs are set up for static stability. Stability tells us (both in theory and practical) where the cg needs to be. So we do that. That drives what kind of trim (elevator force) is needed for steady state flight at a given speed. So we trim for that. Now we fly faster and the plane climbs. So we change the trim. At no time will the force required from the stab point up. Less "down" but not "up." Your flatbottomed stab (wasn't that a Queen song?) would be more efficient mounted upside down, with the flat side on the top. Disclaimer: Aerobatics, unstable configerations (aft cgs), canards, etc.

Kurt

In response to this thread in general....

Instead of a bunch of chest beating, full scale mumbo-jumbo examples, textbook theory, etc....
..just outfit a model trainer with interchanging stabs and see for yourself what the difference is.

Can't argue with that logic.This thread is wearing me out(Itype sooo slow)I am going to keep my eye out for an old used Telemaster apperently they have a flat bottom airfoil stab. First I will see if it will even fly, dosent sound like it will. then I'll put a flat stab on it and see if it has a tendancy to gain more altitude with speed. Wonder why they did it on the Telemaster?

If I'm not mistaken, the C130 uses a lifting section on the horizontal stabilizer. The interesting thing is that the airfoil is upside down.

Kurt, reading other people's posts and comprehending them is not a requirement to post anything. That is why you can read what you posted again and again. Why just last week I discovered water, next week will be air.

The decalage is easy to measure on the bench. What is harder to see is the effects of down-wash on the horizontal stabilizer. Because the amount of down-wash varies with the speed of the aircraft, but you have a greater effective decalage than actual measured decalage.

I've seen a couple Telemasters out at the field, but never noticed if they had an airfoiled tail or not. The only reason I can think of for having a lifting tail on one of these that generally they are running around with very little power and a (relatively) long tail moment. It might need a little help getting the tail up to fly level?

I think the original use of the Telemaster was to string lightweight cables across valleys in Germany, I remember reading about that 30 or 40 years ago. Perhaps that is why they decided a lifting tail was of some value.

Kurt has flown full size aircraft with a lifting tail if I remember correctly. Or more correctly, he controlled the computer(s) that controlled the airplane. Four computers that voted to decide what the correct solution was to maintain flight. On a later variant, the tailplane was enlarged to enhance the load the airplane could carry. I think that was the C/D version. It's been such a long time since I've read Aviation Leak.