Airfoiled stabs
 

What are the advantages of airfoiled stabs over flat stabs? :confused:

Airfoiled stabs
 

The horizontal tails of aircraft with articulated elevators change airfoil every time the position of the elevator changes. A horizontal tail with a well designed airfoil will have slightly lower drag than a flat airfoil in the undeflected elevator state. As soon as the deflection exceeds about 5 degrees or so the drag increases a lot due to flow seperation and there is little to choose between them.

For all moving horizontal tails, the well designed airfoil has an advantage over the flat plate because it can achieve slightly higher lift coefficients without stalling.

For most sports and aerobatic aircraft the small differences in drag won't be noticed.

Airfoiled stabs
 

The other thing you might want to mention is that a design with ribs and skin is much stronger. Therefor when you want the same strength you can make it much lighter.

Re: Airfoiled stabs
 

Hi 3-D guy. If you meant a symmetrical airfoil, as opposed to flat, for the horizontal stabilizer, there would be somewhat less drag, but not enough to notice at model speeds.

If you meant a lifting airfoil (non-symmetrical, as in flat bottom), as opposed to a flat stabilizer, you may be surprised to hear that it must be mounted "upside down," with the flat part on top. This arrangement can be seen on some free-flight models that were designed for high efficiency at low speeds.

The function of the horizontal stabilizer in conventional aircraft is to create a down-force on the tail to counteract the wing's tendency to pitch forward. This pitching moment is one of the by-products of generating lift, and it occurs whether the wing section is symmetrical or not.

The extreme example I like to mention is the Piper Vagabond, which required nine degrees negative on the stab, relative to the wing. Lotsa drag, but necessary for pitch stability with the short-coupled airfoils and relatively large, cambered wing. I've always loved the appearance of this 1947-era light plane, but I doubt if it's a good design for a model.

In a canard design, the "stabilizer" must create a lifting force to counteract the pitching moment of the wing, since it's forward of the wing. This is one of the reasons canards are more efficient.

The way that tail-less (flying wing) designs overcome the pitching moment is interesting, too, but that's another subject.

Re: Re: Airfoiled stabs
 

Small point here Ralph. Most free flight models with lifting stabs actually use the airfoil normally with the flat part on the bottom. There were just a couple of notable examples that used the inverted stab airfoil. Sal Taibi's Pacer being one of them if I'm remembering correctly. But these are quite rare. The vast majority of them use the stab airfoil in the upright position.

I'm guessing that you saw one of these exceptions at one time and the idea stuck?

Re: Airfoiled stabs
 

Oh, I almost forgot.

As you've seen from all the fine answers above usually the airfoiled stab is a better and cleaner way to design the model. But there is one advantage of the flat plate option that hasn't been brought up. The flat plate types have a soft neutral so if you're flying a fun fly sort of design then small corrections using only a degree or two of movement are softened and it's only when you deflect the control out into the un-turbulated air that the full control effectiveness kicks in. In effect the flat plate types give you a sort of built in exponential response.

Saw this in an article about pattern model design once and it stuck.

non-symmetrical stab
 

Hi Bruce;

Sal's inverted stab was just an example to explain the theory, much like the Vagabond reference, which is also an exception rather than the rule.

Nevertheless, in every conventional aircraft, whether the stab is symmetrical or not, it must generate a down force to balance the pitching moment of the wing, unless the wing has enough reflex to balance itself, in which case a horizontal stabilizer isn't necessary, as in flying wings.

That established, it would seem rather contradictory (or just plain dumb) to have an upward-lifting stabilizer, which would then have to operate at a negative AOA in order to produce the downforce necessary to balance the aircraft.

Airfoiled stabs
 

To me, a softer response around neutral for a flat type stab mean less precision. Is there even the slightest possibility I may be true here?

One thing for sure is that my next project is going to use airfoiled stabs. I have been told that airfoiled stabs do not "stall" when high deflections are used (30 degrees and more) so the airplane does not rock like if I were playing with elevator.

My Extra 300S from Great Planes (flat stab) tend to rock (like playing with elevator) when I try to make a small loop while being near stall speed (I am sure my wings are not stalled though).

I appreciate all your comments guys.

Re: non-symmetrical stab
 

Well, don't tell this to all the free flighters out there. I wouldn't want them to suddenly realize that their models shouldn't be able to fly....:D Competition free flight is a whole different enchilada. Check out some more free flight designs. They all have them this way other than a few sport and scale models.

We (I'm very much a free flighter) kind of compensate for this apparent anomaly by using a very rearward center of gravity along with a very high tail volume coefficient. This lets us load up the stab yet still allows the model to operate at a slight but positive stability margin. It's not unusual to set our models up with a balance point in the rear half of the wing chord with this sort of setup. In fact a couple of designs from the 50's that took this to extreme and had the balance point either on or behind the trailing edge. At that point there is still a need for a degree or two of proper decalage but not anywhere near as much as you would think. In some cases I wouldn't be surprised if it turned out that the decalage is actually a true 0-0 or even a little positive. The stability then coming from the very high center of drag resistance from the wing being up on a pylon and having the polyhedral along with the high lift of the often highly cambered wing compared to the lower cambered stab.

And Indoor flying is another area that uses nothing but airfoiled stabs on upright. When I was flying indoor EZB, Pennyplane and even a couple of microfilm models the hot setup was to calculate a chart for each design to allow the wing to be positioned for a true zero margin of stability. The positive recovery stability then being strictly provided by the high center of drag and low thrust line. Here again many models would be balanced closer to the trailing edge to achieve this.

I've been flying a Henry Struck Record Hound using electric and RC for about 12 years. I've got it balanced at about 55% and if I didn't have to perform major surgery or add unwanted lead to the tail I'd move it back to about 65% to perk it up. It fails the dive test miserably as being too stable just now. And it too has the lifting stab on upright and with only a slight amount of negative stab incidence, perhaps 2 to 3 degrees but with about 1/8 inch of down elevator trim to boot.

But to get this to work you first have to let go of the "balance at 30%" rule. Otherwise your assumption that it would need a crazy amount of negative decalage would be quite right.

Airfoiled stabs
 

It's not a case of less precision but rather a chance to sneek in a very light correction without the model visibly jumping in response. That's valuable in pattern from what I got in the article. They used airfoiled stabs but the article showed how to make the elevators thinner than the stabilizer in an effort to dupicate the inherent softness of the flat plate types. And the current 3D guys are using a lot of exponential anyways to a little extra neutral softness is just icing on the cake.


Go back and read Ollie's bit again. Angle the elevator much past 5 degrees and you get a big separation bubble on the low pressure side of the elevator and probaly a smaller one in the hollow of the high pressure side also at higher angles. At 30 degrees any kind of stab is stalled fully but is supplying lots of control force as well. That rocking or pulsing is more than likely from some other source.

Nope, your wing is alternately stalling and reattaching. I've had a number of models that perform like you're describing and it has nothing to do with the elevator other than it's just deflected too far for the model at that airspeed. Perhaps some changes to your Extra like adding sealing strips to the aileron hinges or tape type turbulator strips about 1 inch behind the leading edges will help cut out this pulsing AT THAT ELEVATOR SETTING. Give it more elevator deflection though and the pulsing will be right back because you're just trying to get more out of the wing than it can give.

But it wouldn't take much to make that flat stab into an airfoiled type for a test. Just strip the covering off the stab and add a set of rib strips on both sides and recover. Voila, instant airfoiled stab and elevator.

Hope this helps.

Tail Force Direction
 

Ralph the tail force isn't always down. I used to have this wrong and it is worth taking a look at

http://naca.larc.nasa.gov/reports/19...-792/index.cgi

to see an old report on actual flight test loads of a P-40. They summarize by saying the horizontal loads are basically always up loads.

Why? Assume a symmetrically wing airfoiled airplane. The airfoil has no pitching moment around its 25% chord point. If you take moments around the CG of the airplane (the only point that you can take moments about) then the lift of the wing (located at 25% of wing chord) times its moment arm has to be equal to the lift of the tail (located at 25% of tail chord) times its moment arm. If the wing is lifting up in front of the CG then the tail is lifting up to balance.

If the CG is at 20% of the chord of the wing then the tail load is down.
The sign of the tail load switches at a CG location of 25% of the chord of the wing.
If the CG is at 30% of the chord of the wing then the tail load is up.

Most airplanes balance around the 30% point.

When the elevators are deflected (or all moving tail is deflected nose down) it produces less up lift and the airplane pitches nose up. Until the all moving tail deflects more negative than the wing angle of attack is positive the tail will lift.

If there is a large negative pitching moment coming from a fuselage, nonsymmetrical airfoil or with flaps deflected (or all three) then that moment must be included in the moment equation and might lead to a down load also. Certainly the F-4 had slats put on the horizontal tail to increase the effectivity of down force from the tail at the large leading edge down tail deflections encountered during landing.

Another way to visualize the whole tail load direction thing is to get a small balsa airplane (0-0 wing tail incidence set up), balance it with the CG at 30% wing chord point, then hold it with pins at the 30% chord location stuck in at the wing tips toward the fuselage. Rotate the airplane to about a 20 degree pitch angle. Keeping that angle move the airplane horizontally. Both wings and tail are now at 20 degrees angle of attack, the tail load is up and is trying to restore the model to level flight.

I haven't explained this too well but I think the basics of it are there.

Airfoiled stabs
 

Ben, that's quite interesting. It also explains what I saw in a sailplane analysis program that I have. At certain balance points and tail volume coefficients the charted data of the model showed the elevator loads going from slightly negative to zero and then slightly positive as the speed climbed (I think, this was a few years back). I realized that this was due to the migrating center of pressure but it was still quite surprising to see actual numbers. I just assumed that it meant that I was getting greedy with my balance point. I took this with a grain of salt as I was thinking that I'd be doing the dive test to optimize the balance point in the real world anyway. But perhaps the program was telling me that is WAS optimized when I was hovering around the zero load.

Thanks for the info.

Airfoiled stabs
 

Wind tunnel testing of some symmetrical airfoils often used for tail surfaces shoe a kink in the coefficient of lift vs, angle of attack characteristic around zero degrees AOA. this is caused by the laminar seperation bubble shifting from one side of the airfoil to the other as the airfoil goes from a slightly negative to a slightly positive AOA. The effect has the symptoms of a dead band of 1/2 to one degree or so. This effect may be disrupted by the turbulence in prop, wing or fuselage wake.

Small differences in the surface gap of an articulated stab can have appreciable effects on the flow pattern and control response. These differences may, in some cases, even be larger than the difference between a flat and airfoiled stab. Its risky to make generalized conclusions without the particular comparison's shapes being closely defined and actual polar measurements taken.

Airfoiled stabs
 

Ollie, this sort of behaviour was also described by Selig in my Soartech 8 book in the section about the 8020. He designed that airfoil to particularly be very linear around the zero AoA just for use on all flying stabilators and to try and eliminate that deadband or, I believe he called it a hysterisis, at that point.

Thank you all
 

I BMatthews, sounds like I did not understand what Ollie said earlier. I think now you are right on the money and I was wrong. My ailerons are not sealed and have a somewhat large gap (more than 1/16"). Where can I find those tape type turbulator strips?

Airfoiled stabs
 

You make them yourself using 2 layers of automotive pinstripe tape. This is thicker than monokote so it works well to trip the air. You can find it at Canadian Tire or Lordco or any other place that has cheap car trim stuff. You wnat the 1/8 wide stuff. I found that 2 layers was thick enough but if you don't notice much effect try 3 layers or pull it up and move it closer to the leading edge until you do notice an improvement. Start with it at the 15% chord point.

Airfoiled stabs
 

Ben, in all of this discussion I have prefaced my remarks with "in a conventional aircraft." When you talk about specialized models with CG near the trailing edge, you are talking about tandem wing designs, not conventional aircraft.

In "conventional aircraft," the Center of Gravity, upon which all external forces operate, is located slightly forward of the center of aerodynamic effort, for the purpose of achieving positive pitch stability. The cost of this stability is a forward pitching moment, which must be counteracted by a downforce produced by the horizontal stabilizer. Aero 101.

The lesson in this discussion is that when the CG is set up so that the force required from the stabilizer is zero, the aircraft would be in perfect balance in the pitch axis. The result of this undesirable condition is that the aircraft may demonstrate a pitch-axis oscillation, from slightly nose-up to slightly nose-down.

The solution is to move the CG forward so that some downforce is required from the stabilizer. This is known as positive pitch stability.

When the CG is moved aft of the center of aerodynamic effort, the aircraft will tend to pitch up so that an upward force is needed from the stabiizer to fly straight and level. This is called negative pitch stability, and is not desirable in conventional aircraft. Certainly not man-carrying ones.

Definitions
 

-------- Ben, in all of this discussion I have prefaced my remarks with "in a conventional aircraft." When you talk about specialized models with CG near the trailing edge, you are talking about tandem wing designs, not conventional aircraft.

In "conventional aircraft," the Center of Gravity, upon which all external forces operate, is located slightly forward of the center of aerodynamic effort, for the purpose of achieving positive pitch stability. The cost of this stability is a forward pitching moment, which must be counteracted by a downforce produced by the horizontal stabilizer. Aero 101. --------

Back to Aero 101 then. It wasn't me who said it but my remarks are valid for all airplanes. Pitch stability is the same whether it is a pattern ship, free flight with aft CG, indoor ship, canard, conventional, or tandem wing airplane. If you want an airplane to be stable in pitch then the aerodynamic center of the TOTAL AIRPLANE has to be located aft of the CG.

But remember the aero center of the TOTAL AIRPLANE is a combination of the aero center (ac) of the WING (25% chord wing) , the ac of the FUSELAGE and the ac of the TAIL (25% of the tail) with the various interactions of wing body, etc. This is easily calculated - a document called Datcom gave a graphical method to do the wing body ac. Then you could add the tail effects, etc.

Ralph what you have done is take the TOTAL AIRPLANE ac (wing ac plus tail ac) which must be aft of the CG and is up and then also use the TAIL ac again (which is already a part of the TOTAL AIRPLANE ac) to come up with the down force on the tail. You can only use the TAIL component one time in the moment equation. Sorry.

Alasidair Sutherland writing in Model World (an English publication) has covered this in detail. I know because I wrote him about what I thought was bad aero work and he politely differed with me and led me through my misunderstanding. Verrrry embarrrrassssing. After that I found the NACA document that I referenced that, not surprising, had real data to show the same thing. Sparky Paul also helped me along the path to enlightnment. Both tend to know what they are talking about.

In the NACA document it shows a time history of the tail force developed in a pullup as actually measured with no hand waving. As the plane is flying level there is an initial blip of down tail load when the tail initially moves. As the airplane responds to the down tail load with nose up pitch acceleration the measured loads go from down on the tail to up until in a steady state maneuver is achieved where the load is up.

Again the key and a summary is that if the CG is forward of 25% wing chord you will end up with a down load on the tail. If the CG is aft of 25% wing chord you will end up with an up load on the tail.

-------The lesson in this discussion is that when the CG is set up so that the force required from the stabilizer is zero, the aircraft would be in perfect balance in the pitch axis. The result of this undesirable condition is that the aircraft may demonstrate a pitch-axis oscillation, from slightly nose-up to slightly nose-down. ------

Actually a zero load on the tail is achieved when the CG of the airplane (assume a symmetrical wing) is at the 25% chord of the wing. Since there is no wing input to the moment equation the moments are entirely due to the tail which can be near or to zero.

----------The solution is to move the CG forward so that some downforce is required from the stabilizer. This is known as positive pitch stability.

When the CG is moved aft of the center of aerodynamic effort, the aircraft will tend to pitch up so that an upward force is needed from the stabiizer to fly straight and level. This is called negative pitch stability, and is not desirable in conventional aircraft. Certainly not man-carrying ones. -----------

Ralph, Don't mix up the fact that elevators seem to put in a negative load in maneuvers. If you mount the fuse in a wind tunnel, put it at a fixed level and then go through the elevator variations. The loads are in the direction that you would think. Trailing edge up elevator gives down loads. When the fuselage is free to pitch as in flight it is not the same.

Pitch stability requires that the CG be in front of the TOTAL AIRPLANE aero center. The airplane may go down or up or whatever but it will be stable and not diverge in pitch when disturbed by a control input. This says nothing about the trimmed condition or lack of it of the airplane.

If the CG is aft of the TOTAL AIRPLANE aero center then it will diverge when disturbed. This is handy if you have a good stability augmentation control system working. For instance, you give just a little back stick. The horizontal tail moves leading edge down, load is down. The plane diverges up. The tail load then changes to up and the control system has to move the tail fast enough leading edge up to literally beat the airplane to the upper trim point desired, stop the pitch and stabilize the airplane. It makes for a very maneuverable airplane but does require a lot of horizontal tail control power and speed. The unstable airplanes usually have a larger horizontal tail area than a stable airplane. I worked in this area for awhile so am a little familiar with it.

Be sure and don't mix up pitch stability and angle of attack stability.

Angle of attack stability is achieved when the wing in front has a higher angle than the wing in back. This is desired for almost anything flying that isn't a pattern ship. If you have ever looked at a plot of lift coefficient verses pitching moment for different horizontal tail deflections (an all moving horizontal tail) it will show only one trimmed angle of attack for a given tail deflection (assume a constant speed/altitude condition for now). Using the all moving horizontal tail is the same thing as incidence angles with a conventional tail.

Angle of attack stability is the same whether it is a pattern ship, free flight with aft CG, indoor ship, canard, conventional, or tandem wing airplane. If you want it to trim out at a given angle of attack the wing in front has to be at a higher angle than the wing in back.

I had a rudder only contest airplane called the Charger (designed by Milt Boone, I built mine in 1964) that had very large angular difference between the wing and tail. It had a forward CG, I think forward of the 25% point. This gave a down load on the taii, a large pitch stability and rock solid angle of attack stability. It also had a large variation of airplane pitching moment with airspeed due to the down tail load. It would trim to level flight at one speed, pitch down at a lower speed and pitch up at a higher speed. This allowed pitch aerobatics with rudder only.

When I put a elevator on the airplane I could give it a stick rap and it would return to the trimmed condition immediately. I took out most of the angular difference between the wing and tail, moved the CG aft and flew it that way for a long time. It would trim for level flight and was stable but the tail load was now up and it wouldn't do aerobatics in pitch again unless the elevator moved.

I found some interesting things when I did a search for tail loads, lifting tail, etc. One document for pilots insisted on using the terminology of moments about the aero center of the wing. There are a lot of other misprints along the way. A search of this forum leads to some interesting reading also.

Airfoiled stabs
 

You make them yourself using 2 layers of automotive pinstripe tape. This is thicker than monokote so it works well to trip the air. You can find it at Canadian Tire or Lordco or any other place that has cheap car trim stuff. You want the 1/8 wide stuff. I found that 2 layers was thick enough but if you don't notice much effect try 3 layers or pull it up and move it closer to the leading edge until you do notice an improvement. Start with it at the 15% chord point.

This is the part I understood :D . What I find in this thread is REALLY interesting but I must have missed aero 101 since I have a hard time following you guys :o :o

Is there somewhere I should go to learn all this stuff so I can read this thread again (about sixth time) and not just say : "oh, that makes sense !!".

Airfoiled stabs
 

Hey, don't feel bad. I had to read Ben's epistle twice to soak it all up too.

Nicely layed out and explained Ben. I had a couple of new synaptic links clang shut upstairs myself reading that. Thanks. Ralph, I think you're in the same boat I'm in. I have a lot of bits that I know are right but they don't always tie in properley until I see something like this. I just went through this with the duration thread a couple of weeks ago. Learned a whole heap on that one too.

So Ben, with the stuff in that Datcom article I could calculate the CG that would give a zero stability margin for a specific narrow speed range? That could be handy....

3D Joy, the Martin Simmons book on model aerodynamics is a great place to start. Also whenever there's an article in a magazine that's about any type of model setup be it pattern, racing, 3D acro or glider it's worth reading. They may look different but they all follow the same laws. I've been working on this for about 25 years of indoor and outdoor free flight, control line and radio control. It's not easy to catch it all but it's rewarding as blazes when you can nod your head at a sudden flash of light.

Airfoiled stabs
 

3D-Joy; Don't pay any attention to the guy behind the curtain. His name is Theo. R. E. Offlight, and he would like to confuse you.

Your user name says it all; Joy of Flight.

Actually, you adjust the pitch stability of your airplane often, by varying the load on your stabilizer. It works when you tweak the right-hand stick, up or down.

That's all you need to know about pitch stability, unless you wish to design your own airplane. Even then, if you don't make it too different from another successful design, it will probably be successful, too.

Here's a link that explains some principles of aerodynamics, with graphics and no formulas.

http://142.26.194.131/aerodynamics1/

Now, how about a Quickie?

Airfoiled stabs
 

Hey, Ben, there's a great discussion of flight with CG aft of the envelope, at the link I posted above. See it under "Stability and Control."

I wish my old friend and mentor Jim Kelley could be here for this discussion of pitch stability. He was a consultant to Hughes and others, back in the 60s. His specialty was Spiral Stability. Our team won the California R/C Marathon, twice, with Jim's designs, when the event was first established back in the 80s.

http://camarathon.com/index.htm

Airfoiled stabs
 

I Ralph,

that thing flies for real?? I have seen this airplane many times but it was always on the ground.

Your link about aerodynamics has been added to my favorites. Thanks a lot to you all.

Airfoiled stabs
 

Thanks for that link Ralph, I've added it to my fav's too.

Stab 'n Liza
 

Ben, you and those other FF designers better quit ignoring the laws of physics, or Mother Nature will cause your airplanes to fall out of the sky.

The NACA report you cited examined high loading of the horizontal stabilizer during accelerated stalls, but I found a definition of the "normal" downforce on page 5.

http://naca.larc.nasa.gov/reports/19...i?page0005.gif

Item (a), the normal down-force required to counteract the forward pitching moment of the wing-fuselage-propeller combination, in level flight at 200 mph, was found to be 560 lb. (That's pushing down on the stabilizer to prevent the nose dropping).

By the way, the aircraft appears to be a P-47, not a P-40.

Item (b) is the force required to balance the effect of the location of COG relative to the aerodynamic center of the wing-fuselage-propeller combination, was found to be positive. This reinforces Dick's rule that it doesn't matter where you put the CG.

Finally, Item (c) is the force resulting from elevator deflection.