Why don't airplanes with high lift wings (like flat bottom trainers) have lifting/airfoil horizontal stabs. I would think ifthelift and tail momentwhere calculated correctly once the elevator is trimmed, the plane would fly level at any airspeed, above stall of course. I would think it would make a more stable plane which is idea behind a trianer.

I learned with a Sr. Telemaster. That was a good trainer and every surface on that plane lifts. I would guess money is the main reason?? Never gave it much thought. There are a lot of other planes on the market besides what the manufactures call trainers but they will cost you more. That brings up the question, what is a trainer?? I have seen everything from sticks , Kings and Masters used. For a long time the SIG Kadet was a big item, with and without ailerons.

No need for them. Much easier to produce a flat plan-form than airfoiled, especially with ARFs. Old Free-Flight models did so.

It does to a degree, However with any change in airspeed the lift of a lifting surface is increased by 1/2 of the square of the airspeed (airspeed being the actual flow of air molecules over the surface, known as Indicated Airspeed, NOT True Airspeed, Groundspeed, and/or Calibrated Airspeed) However the surfaces being different the change in lift-force produced with a change in airspeed is NOT directionally proportional.

The climb/descent of an airplane with a change in airspeed is simply an increase/decrease in the production of "lift" by the main wing in traditional convergent airflow. A lifting stab will not totally change that. It can help some but not adequately to rule out any change with aircraft velocity changes.

There is much more but not needed here.

The airfoil on the stab,if calculated properly it can become proportional, and lift the tailto change AOAcompensating for for added winglift at higher airspeeds. Thats fact and it is done on real aircraft.

The lift or negative lift is also there to compensate for center of lift changes (with airspeed) on swept back wings full size aircraft..

Yourformula has to be for a particular airfoil at a specific AOA. At 0 deg.AOA the lift on a symetrical airfoil wont change at all with air speed, and it will gain proportionally withincreased AOA ultill stall.

This is a valid conversation but I think it will just serve to really confuse beginners. Maybe it should be in the aerodynamics forum.

I think the answer is ease of construction/cost and they don't need a true lifting stab. Keep in mind that a flat surface with incidence will generate lift, so just because the H-stab does not have a traditional airfoil shape does not mean it is not generating lift.

The lift createdby a flat stabis <span style="font-family: Arial; font-size: 9pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA">insignificant. The point here is to generate enough lift toto pick up the tail more and more asthe planespeeds up changingthe planes pitch the correct amount,therefor not changing the neutral trim of the airplane with airspeed.

I agree it likely is cost. As Gray Beard mentioned its done onthe Telemaster, I have never seen one. I am curious how well it works.

</span>

In general, tails are not there to generate lift, as the wing is. It's a stability issue - think of feathers on an arrow. Unless balanced significantly tail heavy, the tail on a telemaster is not generating lift, even with a flat bottomed airfoil. And you wouldn't want it to - It would be like sitting both kids on the same side of a teeter-totter.

Kurt

Indeed. On most airplanes, the stabilizer holds the tail down, not up. If the tail feathers were to fall off your typical sport plane, trainer, or scale model in flight, the nose would drop and the plane would start to do an outside loop.

I will have to respectfully disagree. No one is saying the horizontals primary purpose is to produce lift, however I can assure you when real aircraft are built the airfoil on the stabilizer is designed to produce a certian amount of lift be it positive or negitive to react to the c<span style="font-family: Arial; font-size: 9pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA">haracteristics</span>of the airplane (usually the main wingsdesign/airfoil). I spent many years in aviation, have a degree in avation technology and was a DC-9 instructor for a major airline for 6 yearsand have a verygood understanding of what I'm talking about.

I think maybeChuckW is right, this topic would be better suited in the aerodynamics forum.

Seriously???

So the motor is there to keep the nose down cause if it fell off the plane would pitch up and do an inside loop? Im not sure where on the same subject.

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I will have to respectfully disagree. No one is saying the horizontals primary purpose is to produce lift, however I can assure you when real aircraft are built the airfoil on the stabilizer is designed to produce a certian amount of lift be it positive or negitive to react to the c<span style=''font-family: Arial; font-size: 9pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA''>haracteristics </span>of the airplane (usually the main wings design/airfoil). I spent many years in aviation, have a degree in avation technology and was a DC-9 instructor for a major airline for 6 years and have a very good understanding of what I'm talking about.

I think maybe ChuckW is right, this topic would be better suited in the aerodynamics forum.
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You are describing stability. I stand by my statement. There are many of us in RC who have an extensive aviation background. Aeronautical Engineering degree - USAFA 1988. USAF Instructor Pilot. F-16 Pilot.

Kurt

Maybe you could explain how this is possible. That statement alone tells me you dont undestand what I am talking about or the fundimentals of aerodynamics.

Back to the original question: Probably just cost. If you don't like the normal every day trainer then buy a high wing plane with a shaped airfoil stab, they are out there but tend to cost more then $79.00

This depends on the airfoil used and the balance of the airplane. But is generally correct. Most lifting airfoils used on aircraft have a negative pitching moment which means the airfoil wants to rotate nose down when creating lift. So in conventional designs without electronic stability systems the tail plane is pulling down when the aircraft is in normal 1g un-accelerated flight.

As to the original question, many old free flight designs did use lifting tail with the CG farther back than what RC designs normally fly with. There were also some early RC designs that used lifting tails, but one of the problems with the approach was that sometimes when the airplane increased it's speed in a dive or steep turn, it could not recover. Flat sections, symmetrical sections on the tail did not have this problem and took over. Most RC design has been monkey see, monkey do.

tripower222,

Perhaps you should google some of the following concepts and how they relate to each other: Longitudinal stability, static stability, force diagrams, and tail moments. You will see which way the horizontal tail vector points in a conventional, statically stable design. Enjoy.

Kurt

Results from a quick search:

"For a statically stable aircraft of conventional (tail in rear) configuration, the tailplaneTailplane...
A tailplane, also known as horizontal stabilizer, is a small lifting surface located behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes...
lift typically acts downward. In canard aircraft, both fore and aft planes are lifting surfaces. The fundamental requirement for static stability is that the coefficient of lift of the fore surface be greater than that of the aft surface; but even this general statement obviously does not apply to tailless aircraft. Violations of this basic principle are exploited in some high performance combat aircraft to enhance agility; artificial stability is supplied by electronic means."

Kurt

tripower222 - per your request. Results of another quick search (but overly simplified):


"Figure 4-20 shows an aircraft in straight-and-level flight. The line CG-CL-T represents the aircraft’s longitudinal axis from the CG to a point T on the horizontal stabilizer.

see attached

Most aircraft are designed so that the wing’s CL is to the rear of the CG. This makes the aircraft “nose heavy� and requires that there be a slight downward force on the horizontal stabilizer in order to balance the aircraft and keep the nose from continually pitching downward. Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative AOA. The downward force thus produced holds the tail down, counterbalancing the “heavy� nose. "

Here is the best diagram yet showing tail forces.

Kurt

This discussion really has not addressed the OP's concern or goal heighlighted in the above quote. A trainers tendancy to climb under accelleration or higher air speed. This is a product of the main wing's and horizontal stabalizer's reletive incedence and the engine's thrust angle. I feel it has nothing to do with the airfoil, or lack of airfoil, in the horizontal stab. My Funtana X100 has flat stab tail feathers (epanage SP?) and it does not care what speed I fly it at, it goes where it is pointed.

If you, tripower222, are comfortable with your trainer as it is and want to play with it, Try shimming the back of the wing up a little at a time reducing it's angle of attack reletive to the thrust line a little at a time and see what results that produces.

Who says the conventional non airfoiled tail surface isn't exerting a lifting force? Kinda depends on what it's angle of attack is doesn't it? As an example I'll use the Mojo 60 that I'm currently enjoying with the CG set so far back that at low airspeeds (5 MPH?), the plane will fly around literally dragging it's tail. Increase the airspeed, or give it a burst of power, and it will level out into normal flight.

Extreme example I know, but I'm able to see what the tail is doing with my own eyeballs everytime I fly the dang thing. Low speed coupled with rear biased CG equals high angle of attack and low lift on the stab. Increase airflow (airspeed) equals increased lift on the stab until it becomes neutral.....

Something like that anyway.

It even seems to me that having an airfoiled (lifting) tail surface would be detrimental. Everytime you'd change the speed, you'd have to retrim the aircraft. Fine in full size aircraft, not so practical in models.

If you took an ordinary trainer and gave it a lifting stabilizer, changing nothing else, you'd get a plane that would fly badly, if at all. The Telemaster, which has a lifting stab, also has a large amount of negative incidence in the stab, to keep the total lift of the stab negative, as it should be on this type of plane. The OP, despite his degree in "aviation technology," seems to think that the tail of a trainer is supposed to provide positive lift. Since the original question was based on that mistake, it's not going to be possible to give him a satisfactory answer without somehow disabusing him of that notion. His basic intuition, that a lifting stab can be part of a design meant to keep the angle of attack about the same despite speed changes, is right: that's what the Telemaster design was meant to do. But you can't get there just by adding an airfoil to the stab.

The Carl Goldberg Jr. and Sr. falcons both have airfoils for the tail surface. I think it was more common back in the day. I do not now of any current kits that have airfoil shaped stabs. I have a jr. falcon Had it for years flies great. But the one thing I have noticed is all airfoil shaped Stabs are symmetrical. so I am sure they are set up at 0 degrees incidence.
If I remember right Goldberg plans said it was to enhance the performance of the elevator. I am not an engineer by no means but I could see where a flat bottom airfoil on a horizontal stab would make for uncontrollable flight, the only exception I can see is a free flight model that would fly straight and level, even then inducing a stall on the tail would result in the nose going up and stalling the wing. Maybe

OP I think you're comparing apples to oranges here anyway.

As a instructor on DC 9's, when was the last time you you put that thing into a hover, or used a "bank and yank" to turn around, or (gasp) fly it inverted? Or for that matter, put a Cessna 172 into sustained inverted flight? I may be wrong, but most full size aircraft are designed to fly upright and at a designed constant cruising speed. Once up to altitude, trim it out, and put the thing on auto pilot, go get a cup of coffee, sit back and relax. All fine and dandy to have a lifting tail to help git this bucket of bolts into the sky under these conditions.

I don't see any kind of lifting tail being beneficial on a model aircraft though. We're not going up, reaching designed cruising speed and drinking coffee going to Minneapolis. (pick the city of your choice or 150 mi beyond if you're busy...LOL)

Give me a neutral tail for ballistic guidance and nothing else.

Everyone say Ahmen!