inverted Stab airfoil

[dusty IV]

The Telemaster and the full size Zenith CH 701 STOL use an upside down airfoil on the stab.

I was messing around to day and built a new foam/composite 40" wing just for fun. I have never used an airfoil stab in RC and inverting it sounds nuts. But it might be fun to try it to see what happens.

The Telemaster has a very good reputation. What's with the inverted airfoil? Would a simple Clark Y be a good choice?

[BMatthews]

As I understand it the Telemaster had a lifting airfoil. As in the curved surface was on the top side just like with many of my own free flight models.

Also the airfoil being curved means nothing. It's not if the airfoil is curved or not that determines if the tail is a lifting one or not. The lift load and direction at the tail is determined by where the center of gravity of the model is located. The further aft the more the tail works with a positive lift. But if you get the CG behind the aircraft's neutral point then it'll be totally unstable. The location of the Neutral point is set by the relationship between the wing area, wing chord, tail length and stabilizer area. If the tail length and stabilizer area are long and large enough the aircraft's neutral point will be well back behind what most folks consider normal. In extreme cases it may well be at or behind the wing's trailing edge. Almost a tandem wing in that case.

And since you can put the CG anywhere within reason and as long as it doesn't go behind the NP that means you can put the CG well back. In that case the stabilizer is operating with a strong upward lift coefficient. And at that point it makes sense to use a cambered airfoil shape so it can do its job more efficiently.

If you just try sticking a lifting airfoil on the tail for no real reason you'll just end up trimming the elevator until it's lifting the amount that the CG position says it has to have. You'll get no real fancy outcome from it other than the need for a lot of elevator trim.

[MarkusN]

That's a noo one by me. AFAIK the Telemaster did indeed have a Clark Y right side up stab. (It's possible that somebody "improved" the design by inverting it, though.)

Still, with the Telemaster usually being trimmed for rather low speeds it actually made sense to have a positively cambered stab. These concepts aren't too common these days, though. RC has improved, so designs of planes today are more about speed and control and less about inherent stability. Meaning that lower camber airfoils and more indifferent positions of CG are used.

To go into a littel bit more detail: With a higly cambered wing such as the Telemaster has, the center of lift in high lift configuration moves quite a bit forward of the CG. This has to be compensated with lift at the stab. Hence a cambered airfoil there has advantages. (And yes, this is true even though you actually pull for slow speed. The equilibrum situation that you get after the new speed has settled sees more lift at the stab, even though the flap is deflected upwards.)

[Tom Harper]

The only successful inverted airfoil I remember was on the Consolidated Pacer B by Sal Tabi (free flight). A friend of mine asked Tabi about it and his only comment was that it would thermal in light air. Actually he quiped "Yeah, that thing would thermal on a f__t".

I assume that the inverted stab compensated for the high Cm of the cambered wing.

I tried it once on a small RC .09 model. Maybe I over did it because the plane wouldn't do anything but loop.

[MarkusN]

I assume that the inverted stab compensated for the high Cm of the cambered wing.
I dunno. High Cm at high AoA (typical for free flight) usually means lift at the stab.

I can imagine that in free flight the effect is a deliberate (benign) stall at the stab when entering a thermal. When you enter a sudden updraft it first looks like increased AoA to the plane. With stable trim that means the plane wants to pitch down. Which is wrong as soon as it has accelerated upwards with the thermal, and must then be compensated by another change of attitude.
If upon entry of the updraft the stab does not incrase any more in lift because it stalls, the fuselage stays basically level, and the plane can accelerate upwards without changing pitch.

Just speculation, though.

[dusty IV]

Interesting comment on the Telemaster. I thought they had inverted the stab airfoil or something weird. See no reason for this design. I have seen stab airfoils using symetical sections and have plans for a Super Privateer that uses this this feature. I hope to build this plane someday for a water plane.

My only experience with a lifting stab was as a kid I built a self designed FF with matching Clark Y airfoils wing and stab. The plane climbed straight up at 90* under power. Beautiful to behold. When the motor stopped it it did a 180* and dove straight in. I never ever built another plane with that feature! :mad:

The Zenith CH701 is at www.zenithair.com/kit-data/sp7-95.html The details of the stab are shown. Very unique.

[kcaldwel]

Inverted airfoil stabs are very common on full size aircraft. Most jet airliners have them. They can reduce trim drag on airplanes that commonly fly with large positive static stability margins. An inverted airfoil stab will also provide more nose up elevator control at forward CG, for more flare power on landing.

Models with an inverted stab airfoil are likely designed to fly with the CG further forward than typical these days, for more positive static pitch stability. This would make the aircraft easier to fly, and a better trainer, by holding trim speeds better. It would fly through turbulence with less disturbance and return to trim quicker .

A model with an inverted stab airfoil and a bigger static margin is going to behave more like a full size airplane - climb under full power, descend under reduced power, level flight at cruise setting. Large elevator trim will be required to over come these tendencies at each power setting.

Kevin

[biber]

I'd say, on a airliner you want a stab to be as small as you can have it safely.
Thats because anything bigger, than necessary will be excess wetted surface producing drag and weight.
If you check the range of forces respectively Cl the stab has to deliver during all sorts of flight regimes,
you will see, how the stab should be airfoiled.
The airplanes setup for landings (a whole bunch of slotted flappery) requires
severe downforces from the stab, due to the wings enormous negative Cm in that config.
That may be much more force downwards, than you need upwards in any possibkle flight regime.
So what do you do?
You build a stab that is just capable of exactly what's necessary.
An airfoil that has a Cl range of, let's say, -0.8 and +0.35, or so.

biber

[Tom Harper]

biber,

On the Pacer B the chord alignment for wing/stab was near 0 - 0 (LE - TE). So, the incidence of the wing was the zero lift angle of the wing minus the zero lift angle of the stab (negative number).

I believe the advantage was that both the wing and the stab operated at the max L/D point but the inverted stab allowed 'decalage'.

I have a side view of it in an old magazine ad. Couldn't locate it on my first try. Maybe somebody else has it?

[kcaldwel]

Tom,

The wing will have to operate at the required Cl and resulting angle of attack to provide the necessary lift for a given speed. The stabilizer has to operate at the necessary Cl to balance the moments to make the wing operate at the required Cl. You cannot set them on the fuselage to make them work at the best L/D angle of attack.

The angles of the wing and stab on the fuselage only allow orienting the fuselage for minimum drag at the chosen speed (cruise or best L/D speed of the airplane sometimes). The stabilizer incidence is usually set so there is no elevator deflection at the chosen trim speed.

None of this has anything to do with the angle between the zero lift line and the chord line. The chord line is rather arbitrary to make the airfoil ordinates work out easily.

Kevin

A Clark Y airfoil will be rather thick for a low Re surface like a stabilizer. There are better, thinner non-symmetrical sections that might be lighter to build, and would have a bit lower drag at the cruise Cl of the stab. Something like the Drela HT22 might be good, and has a flat bottom for easy construction.

[vintage1]

Whether the stab is lifting or not depends on just two things really, the tail volume relative to the wing area, and the planes speed. For a sensible range of stability margins anyway.

The smaller/closer the tailpane is to the wing, the more likely it is to have a net downforce on it in cruse, and an even larger one under high speed conditions.

Its not a particularly efficient way to fly though. Downforce means drag as well as loss of total lift. That's why contest duration models have big tails and long fuselages and positive lifting tailplanes. Better overall lift to drag.

I thought modern airliners like the A300 used fly by wire in order to run nearer neutral stability and hence use more rearward CG on smaller tailplanes...to be more efficient. I have not noticed any markedly 'inverted' sections on airliners..

[BMatthews]

I have a P30 rubber model that I had to put a new flat bottomed stabilizer onto it. The original thin and undercambered stab with a higher camber value would actually overpower the wing and drive the model into a tuck and dive if the speed rose above a specific minimum. The new stab with a lower camber value fixed that nicely.

[Tom Harper]

Bruce,

I think that is what Tabi was doing. The model was rigged by inverting the stab section.

[Tom Harper]

kcaldwel,

A free flight model in gliding flight is a relatively constant speed aircraft. So, it can be rigged for optimum performance.

If the angle of incidence is not the difference between the zero lift lines of the wing and stab then what is it? On a Pacer B there were no movable surfaces. Rigging is all there was.

[macboffin]

The full-size Phantom jet has an inverted section tailplane.In addition, there are leading edge slots to further increase the down load possible.The wing had/has some sizeable leading edge droops also to enhance lift, the tailplane shape needed for pitch stability with max flaps and droops at low speeds.

[kcaldwel]

I'm not sure about the newer fly by wire jet liners, but all the older ones I've looked at have inverted airfoils on the tails. Given the similar geometry, same large flaps, and possible large CG ranges, I'd be surprised the newer airliners would be radically different, even if they've shifted the max aft limit back a bit.

Hard to find photos that show it well, but here is a couple.

Kevin

[kcaldwel]

I believe the advantage was that both the wing and the stab operated at the max L/D point but the inverted stab allowed 'decalage'.

Tom,

I never said anything about your description of incidence.

Your described settings will not mean the wing and stab are operating at best L/D, or that the overall aircraft would be operating at best L/D which is what usually matters. An inverted stab does not "allow" decalage. The required decalage is set by the stability margin and desired trim speed.

An inverted airfoil stab may have lower trim drag, if the geometry and CG position require stab down force. Most free flight models have enough tail volume, coupled with a CG near the aircraft neutral point, so that the stab lifts upward. In this case, an upright cambered airfoil would likely have lower drag for most trim conditions.

Kevin

[MarkusN]

The full-size Phantom jet has an inverted section tailplane.In addition, there are leading edge slots to further increase the down load possible.The wing had/has some sizeable leading edge droops also to enhance lift, the tailplane shape needed for pitch stability with max flaps and droops at low speeds.

The problem is also supersonic flight. Center of lift moves to 1/2 chord when going supersonic, which makes for some serious trim problems.

[Tom Harper]

Kevin,

I don't think that is the case with free flights. Some were trimmed with a neutral stab and the CG moved aft to compensate for Cm. I'm not sure of that was an unstable rig or bistable. Sometimes they got a steep climb and flat glide. Sometimes they got a steep climb to 100 ft or so followed by a vertical dive into the ground.


Actually my comments were trying to understand the success of models that used inverted stabs - not a general discussion of rigging.

Kalisch lists the Pacer B as Bay Ridge 1941 - I was looking in the wrong place. I found the ad in a Dec 41 Air Trails. Not much detail. I also found the Pinch Hitter in a 1943 Air Trails Annual. It also uses an inverted section on the stab. The 1946 Air Trails Annual has Shulmans Banshee and also a photo of Sal Tabi with the Pacer B. In this case the inverted stab is mentioned in the text. The Banshee had a full symmetrical stab.

[BMatthews]

There was a write up by Sal Taibi at one time about the use of the inverted airfoil on the Pacers. He originally tried it rightside up as per normal doctrine at the time and had no luck with trimming the model. Finally in desparation and out of frustration he turned it over since there was something odd about the pacer that indicated that it needed that. It worked and the rest is history. But it isn't the norm for aft loaded designs by any means. But when you look at the plans for the Pacer you'll see that the upside down stab has a lot of negative incidence compared to the wing.

[Texas Buzzard]

Dusty IV wrote, "The Telemaster has a very good reputation. What's with the inverted airfoil? Would a simple Clark Y be a good choice?"

Salvatore Taibi has been mentioned, I flew his Spacer sucessfully. His ideas work.

Dusty IV says his Telemaster has an inverted airfoiled stab. I have seen 3 or 4 eight foot versions and they had lifting stabs ( the curved part was on the top.)

If the airfoiled stab is mounted in an "inverted" manner then as the plane moves faster the downward force of that stab would cause the plane to nose up more. In the very conventional Telemaster we would want to prevet a loop when we increase the throttle - thus a normal stab is indicated. Adding throttle makes the wing move through the air faster thus the wing develops more lift causing a nose up attitude. But if the stab is also moving faster it would develop more lift too, thus balancing the wing's lift and maintaining normal flight.

I am sorry to say that if a stab is mounted on a Telemaster in the "inverted" manner, then the builder didn't follow the plans.

I have a 6-ft Telemaster here in my shop that I built as per plans. It has a normal stab.

[Texas Buzzard]

Dusty IV wrote, "The Telemaster has a very good reputation. What's with the inverted airfoil? Would a simple Clark Y be a good choice?"

Salvatore Taibi has been mentioned, I flew his Spacer sucessfully. His ideas work.

Dusty IV says his Telemaster has an inverted airfoiled stab. I have seen 3 or 4 eight foot versions and they had lifting stabs ( the curved part was on the top.)

If the airfoiled stab is mounted in an "inverted" manner then as the plane moves faster the downward force of that stab would cause the plane to nose up more. In the very conventional Telemaster we would want to prevet a loop when we increase the throttle - thus a normal stab is indicated. Adding throttle makes the wing move through the air faster thus the wing develops more lift causing a nose up attitude. But if the stab is also moving faster it would develop more lift too, thus balancing the wing's lift and maintaining normal flight.

** Yes the plans for the Pacer B show an inverted stab and by looking carefully the decalage between the stab and the wing is very great. I venture to say that Sal was wanting a very stable plane. That is, with such a great difference between the angular settings of the wing and the stab he would have had to have a very forward C of G. That would make his plane plow thru turbulence and/or wind. This is a guess.

I am sorry to say that if a stab is mounted on a Telemaster in the "inverted" manner, then the builder didn't follow the plans.

I have a 6-ft Telemaster here in my shop that I built as per plans. It has a normal stab.

[kcaldwel]

double post

[kcaldwel]

The lift curve slope of the stabilizer will be the same whether the airfoil is right side up, upside down, symmetrical, or a slab. The lift curve slope of a wing or stabilizer is determined by it's aspect ratio. If the aircraft has positive stability (CG ahead of the neutral point) if the aircraft flies faster than trim, there will be a nose up moment, regardless of the stabilizer airfoil.

The only thing a cambered stabilizer does, is potentially reduce the trim drag. If the tail has to generate a down load to trim the airplane at normal cruise, then an inverted airfoil stabilizer might be the lowest drag option to generate the required negative Cl. If the stabilizer has to produce an upwards load to balance the aircraft moments, then an upright airfoil may be the lowest drag option to produce the required positive Cl. A slab stabilizer with the correct incidence, or some elevator deflection will generate the up or down force. So will a cambered airfoil facing either way. The likely only difference will be some trim drag, some differences in final flare authority, and possibly less elevator dead band with the airfoiled stabs.

There is a very small effect from the stabilizer airfoil moment coefficient, but this will be tiny on anything approaching a normally proportioned tailed airplane.

The direction of the airfoil normally will not make any difference to the aircraft responses. There are exceptions with laminar separation bubbles on the wing or stabilizer that cause discontinuities in the lift curve slope of one or the other surface. Or if the aircraft CG is very strange, and causing the stabilizer to get near stall positive or negatively. But these are very unusual circumstances.

The aircraft stability is only effected by the aircraft geometry and CG position. Incidence or "decalage" only effects the trim speed, and the fuselage attitude at a given trim speed.

Please see this excellent web based calculator for determining the neutral point, CG location for a given static stability margin:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Kevin

[BMatthews]

.....The only thing a cambered stabilizer does, is potentially reduce the trim drag.....

Yep, that's the whole thing in a nutshell...

[MarkusN]

Yep, that's the whole thing in a nutshell...
Well, there's one more aspect: having the stab cambered the right way will shift the range of flyable trims.

[Mike the Snake]

The tail's function is to counter the pitching moment of the wing, which wants to push the nose down. The tail counters this. On high lifting foils, the faster you go, the more they want to pitch over.
I THINK the use of inverted airfoils on the stab when applied properly results in not needing trim changes at different speeds.

Airliner jets, as far as I've seen, have Both full flying stabilizers, AND articulated elevators, this is so the entire stab can be trimmed neutral, at whatever CG the planes might have at the time for efficiency. The inverted foils on the stabs simply keep the trim neutral as speeds and pitching moments increase.

[biber]

Mike, please reread #24 and #26.

biber

[mnowell129]

The only thing a cambered stabilizer does, is potentially reduce the trim drag.
Kevin
Good summary. I agree. I have some tangential observations to go along with the discussion. No disagreement, just some other things to think about.

Consider that putting on a cambered stab incorrectly can be worse than a non-cambered stab. For example the telemaster I had, had a CG position (25% of the wing MAC) that clearly didn't require an upload on the tail, hence the airfoil in the up direction was a waste. Also the downwash from the wing affects the effective AOA of the tail, so generally the tail is flying at much more negative an AOA than it would appear just by rigging angles. And since highly cambered airfoils don't behave very well at high negative AOA, you can create a situation where the tail stalls before the wing causing some very funky maneuvers.

Also we should keep in mind that a non-cambered stab is only a non-cambered stab if you use a full flying stab (stabilator), or fixed as in FF model, for once you deflect the elevator the non-cambered stab becomes cambered. The choice of airfoil may change depending on whether or not the design point is cruise or aerobatics. In an aerobatic aircraft you are probably looking for an airfoil that works well with flap deflection, which may not be the best airfoil with no flap deflection.

regards to all.
mick

[Tom Harper]

Kevin,

"If the aircraft has positive stability (CG ahead of the neutral point) if the aircraft flies faster than trim, there will be a nose up moment, regardless of the stabilizer airfoil."

Is that correct? Increasing force due to Cm would cause a nose down moment.

For an aircraft with fixed rigging I believe the normal case is always a down load on the stabilizer.

An experimenter friend once built a free flight with a hinged rear fuselage. Any up load on the stab would have folded the hinge. The model flew fine and there was never any positive force generated by the stab.

[biber]

Tom, Kevins statement is absolutly correct.
For an aircraft with fixed rigging I believe the normal case is always a down load on the stabilizer.That's not generally correct.
You can, however, always have that, if you only have the CG far enough forward.
That is often much to far forward for optimum performance, though.

biber

[Tom Harper]

biber,

What about Cm?

Tom

[biber]

The role the Cm plays gets bigger relatively at higher speeds.
It does count only little at slow speeds.
For most models you have a certain speed (that depends on the actual stability margin, speak CG) where the stab does zero lift.
Go faster (by elevator trim) and it hast to downlift, to counter the increased negative wing pitching moment (not the Cm!) due to the higher dynamic pressure.
Go slower and it will have to lift, due to a lack of negative wing pitching moment.

I've googled a bit and found this article that seems to hit the nail on the head.
If you are too deterred by the maths, even the text is worth a reading.
http://adg.stanford.edu/aa241/stability/staticstability.html

biber

[kcaldwel]

Let's look at a simple airplane design as an example. We'll make a 60" span, 10" constant chord wing with a symmetrical airfoil. We'll stick the wing on the fuselage centreline, which will also be the thrust line. A 6" constant chord stab with a 20" span will be mounted so that the stab 1/4 chord is 20.6" behind the wing 1/4 chord, also on the fuselage CL. Let's say the airplane weighs 5 lbs, or 80 oz, and that we put the wing and stab on at the correct incidence angles so the fuselage is horizontal at cruise trim.

This gives a simple layout with the aircraft neutral point at 40% of the mean Aerodynamic Chord behind the wing LE, or 4" behind the wing LE in this case of a 10" constant chord wing.

Let's put the CG at a nice stable 20% static margin for good static stability for a trainer. This puts the CG 2" ahead of our neutral point, or 2" back from the wing LE in our simple case.

Now we hang this airplane from the ceiling at a point on the fuselage centre line, 25% MAC behind the wing LE. This is the point where the wing lift will act through. Our symmetrical wing won't have a moment coefficient, so we can ignore that. With our CG at 5% ahead of the 25% point, we'll have a nose down moment from the aircraft CG of (5% x 10" chord) * 80 oz = -40 oz-inches (nose down is negative by convention). The tail will have to supply a downward force force of 40 oz-in/20.6" = 1.94 oz at all time for level flight. If we attach a spring scale at the stabilizer 1/4 chord point, we would have to pull down with the 1.94 oz force to bring the fuselage to the horizontal

Now let's say we want to make the airplane more responsive to the elevator. We move the CG back to a 15% static margin, or 40% - 15% = 25% behind the LE (2.5" behind the LE in our simple case). Now the CG is directly on the wing 1/4C point it is hanging from, so the stabilizer doesn't have to provide any trim force in level flight.

Now let's move the CG back to a 10% static margin, or 30% (3") behind the wing LE in our simple case. This is a nice low static margin that might be used for an aerobatic aircraft or maybe a sailplane or free flight glider in order to indicate lift better. With the airplane hanging at the wing 25% point, the stabilizer to provide a moment to counteract the torque of the CG of 80 oz * 5%* 10" = 40 oz-in. This requires a lift force of 40 oz-in/20.6" tail moment = 1.94 oz upwards at all times for level flight.

Now we have the same airplane that needs different stabilizer forces to fly at a level trim with our 3 different CG positions. In our stable trainer first case, since the stab is usually going to be producing a downward force, and inverted airfoil would do this with the least drag. It would also provide more Cl margin for lifting the nose at slow speeds for flaring. So an inverted airfoil stab might be appropriate.

In our 15% static margin case, the stab is usually not providing a force one way or the other. A symmetrical airfoil stab might be the least drag option.

In the third case, the stab must usually provide an upwards lifting force. An upright airfoil stab will likely be the lowest drag.

Of course there are many complications, such as wing down wash, airfoil moment coefficients, wings and stab and thrust not all on the centerline, stall angles of the stabilizer airfoil, CG height below the wing, dihedral, etc. But this gives an idea why an inverted, symmetrical, or upright airfoil stab may be appropriate on even a single airplane design.

Hope that helps a bit, and that I didn't make too many glaring errors.

Kevin

[Tom Harper]

Biber,

The link evaluates an entire aircraft - rigging and all. The Cm shown includes the stabilizer. Since the Cm of the aircraft is positive for AOA<10 deg there is a download on the stabilizer.

That supports my point.


Tom

[Tom Harper]

Kevin,

"Now the CG is directly on the wing 1/4C point it is hanging from, so the stabilizer doesn't have to provide any trim force in level flight"

In order for the symmetrical section to produce lift it will have to have a positive angle of attack. no matter how slight. At a positive angle of attack it will have a negative Cm (check symmetrical section polars in Abbot & D). The stabilizer will have to provide a downforce to compensate for the Cm.

As you move the cg aft of the neutral point the downforce can be decreased until it becomes zero. This has been done in FF but it is not for the faint hearted. A steep climb angle can move the cg even further back causing disastrous instability.

I agree that if you intentionally load the stabilizer it must carry the load with a positive force. That is a rare condition on aircraft that make it through the test phase.

Interestingly the DC-9 was the example aircraft in biber's link. Stab loading was a problem on DC-9s. I have been on DC-9 flights where all passengers were required to sit in forward seats.

So, I modify my statement. On stable models, trimmed for level flight, at subsonic velocities, there is a download on the stabilizer. I think that is the normal condition for flight in model aircraft.

[kcaldwel]

Tom,

No. There are lots of stable aircraft with and up load from the stabilizer. Many free flight models are. And no free flight model will fly for more than 3 seconds with the CG behind the neutral point. Even an RC one will be virtually impossible to fly for even the best pilot without electronic stability augmentation.

I'll admit I played a bit fast and loose with the zero Cm of a symmetrical airfoil. It is not quite zero, and it does vary slightly with alpha (see attached). But it doesn't change enough from zero to change the results of my three examples. The third one would still have the stabilizer lifting. It is far from an uncommon condition in free flight or even RC sailplanes, or a lot of old timer models with big stabs and long tail moments.

There is a continuum from rear tail through tandem wing to canard aircraft, The rear wing an tandem wings and canards definitely lifts, wouldn't you agree? Well, you can keep making the rear wing smaller, and it just keeps lifting less until some cross over of CG position and tail volume where it starts to require a down load.

We never seem to understand each other Tom. So I'm done on this.

Kevin

[Brandano]

All tandem wing design have positive lift on the stabilizer. Just call it a rear wing.

[mnowell129]

On stable models, trimmed for level flight, at subsonic velocities, there is a download on the stabilizer. I think that is the normal condition for flight in model aircraft.
This isn't true. Kevin's example is spot on. I CG adjust all my aerobatic models so there is no load on the tail so inverted takes no trim. I make sure the aircraft has enough tail volume to do this without getting twitchy or I modify the tail volume to make it so.

Download on the tail is a normal condition for full sized aircraft because the FAA requires that airplanes exhibit a nose down recovery from a stall. It's hard to assure that a lifting tail will stall after the wing to provide the nose down stall recovery (you need the tail to keep flying while the wing stalls), so full sized aircraft have a down load on the tail. Once you have the certification requirement that leads you to a download on the tail it leads you to the design objective of minimizing the trim drag, ergo inverted airfoil section on full sized aircraft. It also leads you to a smaller stab area to minimize profile and parasite drag, thus full sized aircraft don't have a lot of tolerance for tail heavy conditions.

If you examine tandem/canard designs like Rutan's you will find that the front surface has a higher angle/loading to make sure it stalls first. This is a good reason for not seeing canard sailplanes as the highly loaded draggy canard is less efficient than a lightly loaded horizontal surface behind the wing.

Not having the FAA required nose down stall recovery allows for lots more freedom of design in models. :)

mickey

[biber]

Heck, Martin Heide, the designer who is the 'H' in Schleichers ASH 25,
did even measure the loads on the elevator in flight, with built in strain gauges.
There is not much of a mystery there.
It's just, you have to learn to deal with the numbers.
You have to know, what exactly NP, MAC, CG mean in this regards and have to be able to do the math for the rather simple mechanics.

Brandano's suggestion to stop call it stabilizer and say little wing instead, helps a lot, simply because it's 100% true.
The 'stabilizer' (little wing!) does nothing more to the stability, than the (big) wing does.
You need no 'stab' for stability, anyway.

biber

[biber]

This isn't true. Kevin's example is spot on. Yes, he is!I CG adjust all my aerobatic models so there is no load on the tail so inverted takes no trim. I make sure the aircraft has enough tail volume to do this without getting twitchy or I modify the tail volume to make it so.You will have to CG your aerobat very close to the airplane's NP (that's behind the big wing's NP!) in order to have a neutral behaviour with no trim needed for inverted.
That means the tail has to uplift for upside up flight and downlift (speaking for the airplane frame of reference) for inverted flight.

You can easily have a nose down stalling behaviour even with the CG behind the wing's NP.
Gliders do that all the time.
The reason, why there is lots of downlifting stabs powered planes, I suspect, is that you must not go behind the airplane's NP, but want to be able to fly a wide CG range for convenience.
To get that feature you can only extend the CG range forward, because it is possible to do it safely.
For performance it is not so good, but there is still the engine in front of you...

biber

[mnowell129]

You will have to CG your aerobat very close to the airplane's NP (that's behind the big wing's NP!) in order to have a neutral behaviour with no trim needed for inverted.
biber
Not the airplanes NP but the NP of the wing/body. When the CG is on the wing/body NP, the tail load is zero (ignoring Cm). You still have a static margin because the NP of the wing/body/tail is behind the NP of the wing/body and CG.
You can get as much static margin as you want with zero load on the tail by putting the CG on the NP of the wing/body and increasing the size of the tail. This just keeps moving the NP of the wing/body/tail further from the CG, increasing the static margin.
I've bought/built and then gotten rid of quite a few aerobatic airplanes where the designer didn't understand this and I couldn't get the plane neutral upright/inverted because the tail was too small to have any SM with the CG in the right place for zero tail load. Now I measure before I buy and modify before I build.

[mnowell129]

You can easily have a nose down stalling behaviour even with the CG behind the wing's NP.
biber
Sure with the elevator deflected. But when you are trimmed and just get too slow, it's hard to guarantee that the tail won't stall first, especially since it's smaller, operating at a lower Re and with lower aspect ratio.
You have to design carefully to make sure that the tail doesn't stall first, and is probably worth the effort in a sailplane, but probably too risky for a GA or commercial aircraft.

[Brandano]

I think that the intention on planes where the tail is rigged to have a down load in cruise may be for the tail to stall first, though in the opposite direction to the one you'd imagine. Especially if the plane has a flying tail, any excessive up input will stall it and reduce its effectiveness, causing the nose to drop or at least to "mush" and not increase the plane's AOA any further. And if the wing stalls the tail automatically becomes a lifting surface restoring the flight condition. Flying tails and tailerons are a bit weird, in this respect. For example, I remember from an old but accurate sim game, that on the Panavia tornado (high wing loading, slats, spoilers and tailerons for control) in order to recover from a spin you needed opposite rudder (ok, fair enough), and you had apply roll into the spin and pull back on the control column. This is because you need the taileron inside the spin to reduce its AOA to exit from the stall and regain lift. Pretty academical, since the Tornado is meant to fly low level missions, and the standard recovery from a spin usually means pulling the eject handles.

[Tom Harper]

(Mikado)

"And I am right,"
"And you are right,"
"And everything is quite correct!"


You can rig the stab loading for various purposes. And the consequences of doing so will vary.

Some of the fine points are open to discussion. As biber points out moving the cg aft until a model requires minimum trim change for upright and inverted flight will place it on the model's neutral point not the neutral point of the wing. This is well covered in biber's link.

There will still be a diference in trim. Will that produce an upload or download on the stab?

[mnowell129]

As biber points out moving the cg aft until a model requires minimum trim change for upright and inverted flight will place it on the model's neutral point not the neutral point of the wing. This is well covered in biber's link.

Yea, I had this class as an undergrad too.

The governing equation is
Cm = Cmac + Clw * (Xw/cbar) - Clh(Vt) + fuselage effects = 0.
Vt = (Sh (tail area) * Lh (cg to tail ac distance)/(Sw (wing area) * cbar (average wing chord)) = tail volume which is fixed by geometry.
Cm is total moment,
Clw = Cl for the wing,
Cmac = Cm at the AC of the wing, not the aircraft
Xw = distance the CG is from the wing(not total aircraft) AC (as clearly stated in the link)

If you ignore the fuselage effects, and assume a symmetrical section (Cmac = 0) then this becomes
0 = Clw * (Xw/cbar) - Clh(Vt)
If the CG is on the MAC of the wing then Xw = 0
then
0 = -Clh(Vt)
Vt is some constant based on wing/tail geometry.
So for the CG on the AC of the WING (Xw = 0)
the only trim condition where Cm = 0 is where Clh (lift of the tail) = 0!
Thus when the CG is on the AC of the wing (or wing-body if you want to consider fuselage effects) load on the tail at trim = 0.

Same equation Kevin used earlier in his example.

[MarkusN]

I think that the intention on planes where the tail is rigged to have a down load in cruise may be for the tail to stall first, though in the opposite direction to the one you'd imagine. Especially if the plane has a flying tail, any excessive up input will stall it and reduce its effectiveness, causing the nose to drop or at least to "mush" and not increase the plane's AOA any further.
Not so, I'm afraid. Again: contrary to control input, lift (in positive, up direction) on the stab rises as you approach slower trim. So if the tail stalls in this condition you get a sharp nose UP.

[biber]

Kevin, if you CG the plane (lets assume symmetric airfoils for an aerobat)
to go neutrally stable, so you don't have to trim for upright nor for inverted,
then you will have the CG pretty close to or even right on the total plane's NP.
The loadings on the stab will only be zero, when the wing's is too,
or in all other cases be signed just like the load on the wing.

If you CG it on the wing/body's NP, you'll have a non loaded stab,
but each flown Cl needs another elevator trim.
Depending on the size of stab the plane will be pretty stable
and not at all neutrally stable.

biber

[mnowell129]

Kevin, if you CG the plane (lets assume symmetric airfoils for an aerobat)
to go neutrally stable, so you don't have to trim for upright nor for inverted,
Neutral stability doesn't mean that you don't have to retrim for upright to inverted. This is confusing trim and stability.
Neutral stability is the case where, when the aircraft is disturbed, is doesn't return from the disturbance. This occurs when the CG is at the NP of the aircraft.

It's time for me to walk away as well.
:)

[biber]

Here you go contradictory.
Disturb you plane a bit more, so that it find itself upside down and it won't return,
but just keep on going that way, and that with no retrimming involved.
Et viola! You have neutral stability, x_CG = x_NP_total.

biber

[vintage1]

All the confusion would be removed if people just understood calculus.

Stability is not ultimately about this position or that position, its about the differential of the overall center of lift with respect to speed. If the center of lift move backwards as speed increase, a dive will tuck under and a climb will increase to a stall.

If you have a perfectly symmetrical wing and tail, and its so neutral that a climb or dive results in no movement of the center of lift, it will simply stick in whatever position you put it, including upside down. And the wing and tail may well stall together..

As far as the requirements for the main wing to stall first in an airliner, yes, I see that, but its no barrier to a lifting tail either. The only requirement is that the main wing is at MORE incidence to the airflow than the tail. Assuming similar sections. Then it will stall first.

One of the more dangerous conditions is a tail with net downforce on it, that stalls in the opposite way in a high speed pull out..at that point control reversal may take place...

Most aircraft WILL have net downforce on the tail at very high speeds. And little or no downforce at cruising speeds and may be upforce. Who cares?

The key is that the CHANGE in combined centers of lift moves in the right direction with changes in airspeed and angle of attack. If it does, its stable. If it doesn't, Deep Trouble.

A single normal wing without a tail has the Wrong Change. To make it RIGHT you have to reflex it, put a wing with more incidence in front of it, or a wing with less incidence behind it. Then stick your CG at whatever the combined center of lift for all the wings is.

The key being that wing with more incidence, produces more unit change in lift per unit increase in speed than a wing with less incidence. Its easy to se that a tailplane with zero incidence will produce no lift, so the main wing if at positive incidence will produce more lift, pulling the plane out of the dive. Its even easier to see that a tailplane at negative incidence and a wing at positiuve, will produce more lift but in opposite directions, achieving the same result. A net forwad shift in the overall center of lift. What is less intuitive, is that a tail at a small incidence, but still positive, will produce less extra lift than a wing with a larger incidence, as speed increases in a dive.

It does though. Simply because to match the planes weight, the incidence on the main wing drops, and that of the tail drops even further..as it approaches zero incidence, or even negative.

The way to achieve a dead drop stall, and still have flying stability, is to have the CG well below the wing and tail..here stability is totally neutral, with wing and tail maybe at the same incidence, and stability is achieved by the fact that as the nose rises, the CG moves forward with respect to the combined center of lift..which doesn't itself move. A lot of contest free flight planes were built like that, and yes, they do tend to stall, stay stalled and come down in a flat stall. I never liked them for that. But hey got the absolute MOST lift out of the allowed wing and tail areas.

[macboffin]

Sure with the elevator deflected. But when you are trimmed and just get too slow, it's hard to guarantee that the tail won't stall first, especially since it's smaller, operating at a lower Re and with lower aspect ratio.
You have to design carefully to make sure that the tail doesn't stall first, and is probably worth the effort in a sailplane, but probably too risky for a GA or commercial aircraft. LOW aspect ratio surface will stall after HIGH aspect ratio surface. One of the reasons that tailplanes are usually lower AR than wings.

[macboffin]

Of course, when we talk about incidence.......there is the actual difference in degrees of the mounted surfaces reference each other or a specified datum line ; but there is also the consideration of flow on a rear surface induced by the surface in front ; in which case there may well be a difference in physical static incidence and aerodynamically "experienced" incidence. Hence stick shakers etc for t-tailed aircraft capable of "Deep Stall" and other phenomena.
Got to think about the whole deal, bits of airplanes don't fly on their own.

[mnowell129]

LOW aspect ratio surface will stall after HIGH aspect ratio surface. One of the reasons that tailplanes are usually lower AR than wings.
Yes I agree, you are correct my mistake for forgetting this. But still a little beside the point which is that it takes more design effort to assure a nose down stall when the tail is loaded positive.

[biber]

More design effort, certainly, but it can pay off to buy it.

biber