Does a tail lift, downlift, or what else?

[biber]

There seem to be many different opinions about that issue, so why not discuss it in this place? Or don't discuss, just vote, any way you want.

biber

[Tony Oliver]

'specify' - depends on whether pitching up or down - ie it's arranged to prevent pitch up (by having 'undercamber' to lift the tail) or vice versa (look at the tailplane section on a Phantom. It can also provide some lift in addition to that of the wing in a rearward CG/CP situation.

[BMatthews]

It depends. A model with a large tail area and/or longer tail length will support a relatively farther rearward CG. Such a model will have the tail lifing positively during normal flight. On the other hand a model with a small tail and/or short tail length will require the CG to be further forward and likely have a negative tail lift.

But even a model with a large stab and long tail will show negative tail lift if the CG is further forward than it needs to be.

[vintage1]

It can be either depending on the design of the model.

For stability, it doesn't matter whether it lifts or provides downforce. What matters is that as the model speed builds up, it produces LESS lift (or MORE downforce) overall..than the extra lift produced by the forward wing. So the model tends to noise up at high speeds...and put its nose down if it slows up.

For instance, if you take a model with 3 degrees of rigging incidence (angle of main wing relative to arbitrary datum) on teh main wing, and zero on teh tail, then if that model is flying fast, at say three degrees actual true incidence on th emain wing, then the tail is neutral. It neither lifts not 'downforces'

Slow the model down however, and the angle of incidence has to rise..so lets say its now +5 on the main and +2 on the tail. Guess what - the tail is now lifting.

Highly lifting tails - big ones on a long tail - were often used on duration models whose actual wing are was limited. By making all of it do something useful, you got a better overall sinkrate.

For speed though, where you want ultra low drag, a neutral tail is probably better.


And models that need a lot o stability - trainers etc - probably go still further.

[Ollie]

The tail lift is zero, up or down depending on:
1. Tail area
2. Tail arm
3. Tail airfoil
4. Tail aspect ratio
5. Wing chord
6. Wing area
7. Wing airfoil (including flap deflection, Cm change with Reynolds number)
8. Wing aspect ratio
9. Decalage (including twist)
10. CG position
11. Elevator deflection
12. Airspeed (including Reynolds number)
Etc., etc.
I am tired of listing them.

[biber]

I also think of multiple parametres coming into play, determining the state of Cl of the horizontal stabiliser. I'm just not really satisfied by the diffuse imagination of it as a very complex voodoo sort of thing, where it's hard to say what's actually going on for any given example. Several discussions in other threads finally got me to the point, where I tried to examine this issue for an example of a typical 60" model slope glider. Making some quite common simplifications, as e.g. replacing the real wing by a rectangular one of the same span and AR (and thus surface area), and assuming a quasi stationary situation, the whole thing pretty much boiled down to be depending on merely:

-currently flown lift coefficient Cl of the wing
-Cm of the wing at that moment (depending on Re, Cl respectively alpha (AoA), reflex/camber and the particular airfoil that is used)
-position of the CoG relative to the neutral point of the wing (c/4 on a rectangular one)

In the very most cases (models) the CoG's position is significantly behind the NP of the wing.
So for almost every actual model (with fixed flap/camber setting) there is one Cl where the stab does not lift nor downlift. Lower the Cl (speed up or decrease g load) and it will downlift and vice versa.

My example: http://www.rcgroups.com/forums/showpost.php?p=5098193&postcount=72

Any comments? Do you agree with that? Or did I go wrong at some point?

biber

[Tony Oliver]

You could, of course, not bother with a tailplane if it 's a cause of trouble. Tailless models fly blissfully unaware of such things, and they are quicker to build.

[biber]

Well, actually I like tailless designs quite a lot. :cool:
But things don't neccessarily get easier to design just because of obviously lacking some things. :rolleyes: ;)
Btw, as a matter of fact my next model is once more going to be a tailless one, ironic, isn't it? :rolleyes:

biber

[SaMx]

I agree with Vintage 100%

[biber]

Oh, yes, count me in as another vint supporter!.
As I would say, vint is an often underestimated contributer in this place. Though I don't read every single post of his (he seems to have way to much time on hands), I consider myself as a true fan of vintageism.

biber ;)

[hul]

So for almost every actual model (with fixed flap/camber setting) there is one Cl where the stab does not lift nor downlift. Lower the Cl (speed up or decrease g load) and it will downlift and vice versa.

My example: http://www.rcgroups.com/forums/showpost.php?p=5098193&postcount=72

Any comments? Do you agree with that? Or did I go wrong at some point?
I agree.
You may know this german site: http://www.fmsg-alling.de/Technik/GerFlug.htm
it confirms what you said and also has an Excel spreadsheet that calculates lift coefficient for the stabilizer for different wing lift coefficients, but not sure how accurate it is.

This is how I understand it:
A too small stabilizer can stall before the wing with total loss of stability. More forward CG improves things because the tail needs to lift less.
The stabilizer can also stall in fast flight, again causing loss of stability.

Hans

[BMatthews]

.....In the very most cases (models) the CoG's position is significantly behind the NP of the wing.....

If the CG is located behind the NP the airplane will not be stable in pitch. A good pilot may manage to not crash but at that point it is like flying an arrow backwards.

The CG does not need to be at or behind the NP for the stab lift to be of a positive nature. It happens well before that point in most model type planforms where the stabilizer is fairly generous in size. And it certainly happens with many of the old style free flight models and probably most of the new ones as well.


So for almost every actual model (with fixed flap/camber setting) there is one Cl where the stab does not lift nor downlift. Lower the Cl (speed up or decrease g load) and it will downlift and vice versa.....
Again, it depends on the CG location WRT the NP. Assuming that the CG is trimmed so as to provide a rear enough position that the model is still positively pitch stable but grossly so then yeah, there is a point where it transitions from one to the other as the model speeds up. But if you move the CG forward to achieve a higher degree of pitch stability as in an RC trainer then it's likely going to be all negative over the whole flight range.

[Tony Oliver]

To illustrate Bruce's comment 'And it certainly happens with many of the old style free flight models and probably most of the new ones as well.' here's a pic of my MiniWeaver.
On this model the centre of gravity is .75ins infront of the trailing edge of the wing, and I expect to move it further rearwards as trimming progresses. The tailplane serves two functions - under power it's positive to the blast from the prop which pitches the model down against the upwards rotation of the wing. The balance gives neutral stability in the climb (ie it goes straight up). On the glide, the tailplane physically lifts at the lower speed of the glide contributing to the wing lift and giving an over all decrease in model loading - it isn't just the wing lifting. Flying a model under radio with this sort of trim is difficult because it's a one speed glide setup. It gets very touchy and needs a different approach to wing and tailplane sections.

Fort the purposes of this thread, it's a up/up tailplane.

[biber]

.....In the very most cases (models) the CoG's position is significantly behind the NP of the wing.....If the CG is located behind the NP the airplane will not be stable in pitch. A good pilot may manage to not crash but at that point it is like flying an arrow backwards. That is only correct if you talk about the whole airplane's NP wich in fact is generally different from the NP of the wing that I was actually reffering to. The NP of the wing is some distance ahead of the NP of the whole plane if there is a tail at the rear end of the flyer. In case of my exemplary 60" slope glider the distance of NP of wing from NP the of the whole plane is 71mm (about 2 3/4"), not neglectable in any possible way, I think. Note, that in the exemplary calculation I assumed a stability margin of 5%, wich is positive and thus represents a stable state in pitch.

biber

[HELModels]

It is so rough and unfinished, I am almost embarrased, but it flies like a champ.
The CG is at 40% and when power is cut, it points the nose slightly downward. A little up and it flattens out - convenient for landing.

With the CG at 40% and acting like it is forward, then the tail is probably oversized. Or is that, the tail is correctly sized for the moment arm, mean aerodynamic chord and CG of This model. CG must be forward of the NP of the model, since it will pull out of a dive hands off - has proven helpful.

To stick to the topic, sometimes the tail must lift up and sometimes it lifts down to maintain equilibrium. It seems to vary with the dynamics of the situation. How well all factors like CG, tail volume, moment arm, decalage work together dtermines how quickly equilibrium is returned when upset. having RC control of the tail, allows flyer to influence where equilibrium is.
...something like that

[BMatthews]

That is only correct if you talk about the whole airplane's NP wich in fact is generally different from the NP of the wing that I was actually reffering to. The NP of the wing is some distance ahead of the NP of the whole plane if there is a tail at the rear end of the flyer. In case of my exemplary 60" slope glider the distance of NP of wing from NP the of the whole plane is 71mm (about 2 3/4"), not neglectable in any possible way, I think. Note, that in the exemplary calculation I assumed a stability margin of 5%, wich is positive and thus represents a stable state in pitch.

biber

Oh, I see why you said that now.

It's odd to see anyone refer to a wing alone as having a neutral point. I've always seen it referred to as the wing's aerodynamic center which is a whole different thing than a neutral point.

They used to talk about the wing's center of pressure and how that center of pressure moves about. It was an attempt to resolve the pitching moment of the wing to a single focal point where all the forces would balance and the wing would sit "poised" on a finger at a given angle of attack.

These days they don't do this anymore as there was this little aspect about infinity showing up in the calculations around the zero lift angle. Instead they consider this center to be constantly at the 25% wing chord point but add in a pitching moment that is a twisting force around that point caused by the airfoil's camber value and overall shape. As I understand it this is based on the fact that symetrical airfoil consistently have a lift center at the 25% chord point and show no torque effects as the angle of attack changes. Adding camber however does cause a change in the torque that the airfoil generates.

[biber]

Yes, it's because calculation is far more easy :cool: if one chooses to look at it as a wing's NP plus a constant torque/pitching moment coefficient rather, than if a (shifting) CoP is imagined for that.

If you by chance get to do the 'shoving around figures' by yourself, I bet you will quickly get accommodated to that NP thing and end up happily :) throughing away good ol' CoP for that. :rolleyes:

biber

[biber]

It's odd to see anyone refer to a wing alone as having a neutral point. I've always seen it referred to as the wing's aerodynamic center which is a whole different thing than a neutral point.Quite interesting, what exactly is the meaning of the AC then? I mean, in german language it is quite clear to me what NP and CoP are. And any of both can apply to one single wing alone or to the whole of a multiple wing system, while a hor. tail can always be considered as a little wing (and why the helmet not). And there doesn't seem to be another term like AC neccessary to describe all that stuff.

So once again:
NP is a (pretty much fixed) point for wich the Cm is constant.
CoP is a (not at all fixed) point where the summ of all forces can be imagined acting on without any torque / pitching moment being left.

So how does AC fit in all that?
I'm seriously curious.

biber

[Tom Harper]

biber et al,

The function of the horizontal stab is to set the AOA of the wing.

Within the limits of engineering approximations the stab always flys neutral. The elevator varies the 'camber' of the stab to be positive or negative. Due to the mechanical advantage of the stab, the AOA of the wing must rotate until the stab is flying at neutral. The error in this statement is that some force must be maintained to hold the angle and some is required to counter the Cm. But, for all practical purposes the stab must fly neutral.

The exception is where the stab is loaded by moving the CG back. Some FF competition models balanced on the TE. Some of these were also prone to prong into the ground with dramatic results. The same rules apply. The stab just has an added lift vector.

[Tom Harper]

Biber,

I don't find neutral as a choice in your poll.

[biber]

Hm, Tom, I have to confess I just forgot to implement a neutral option in the poll.

biber

[Tom Harper]

Biber,

Neutral's the answer. In steady flight any vectors on the stab must be very small.

Once built a free flight with an inverted airfoil on the stab ( like Pacer B/Sal Tabai ). All it would do was loop.

[biber]

But that just doesn't compute. I don't see any reason why a stab couldn't be operating at Cl's different from zero. On the contrary, it is possible to calculate the tail's state of lift for any steady flight regimes with quite acceptable accuracy.

biber

[Tom Harper]

True, one can calculate the vector for any Cl. But, in steady flight, the stab can only exert the force necessary to hold the wing at a desired AOA and to compensate for the Cm. This must be small because of the mechanical advantage of the moment arm.

If the stab does not fly neutral then what is your reference for setting the wing angle of incidence?

[peterangus]

Why poll ?

Scientific facts are not determined by democratic vote.

Better answers in www.rcgroups.com/forums/showthread.php?t=415163 posts #7 and #19.

[biber]

Scientific facts are not determined by democratic vote.That's true, but OTOH I wanted to get all the different attitudes towards that issue pictured.

One thing still bothering me a bit is, that even if one figures out and calculates what's going on,
most folks will insist on their diffuse feeling on what they believe is happening.
But this is the scientific area of the board, right?
So figures should be appreciated at least in this place.
I saw some comments in other forum areas recently where there was ranted against all the figuring and calculating and all that and everybody should get out flying instead.
To be honest, I have much more fun flying when I know, why it is flying and why it does fly the way it does.
And even better, if I know, what to change, when I want to improve the design towards a certain goal.

The linked thread is quite valuable, thanks for digging out.
I thought there was once one on that, but am truely guilty of not searching it.

biber

[biber]

If the stab does not fly neutral then what is your reference for setting the wing angle of incidence?The wing incidence angle (normally reffering to the fuselage) is chosen according to a design Cl and the minimum possible drag of the fuselage for that Cl.
Then the decalage (difference of AoI if wing and tail) is chosen to match that Cl with e.g. zero elevator deflection.
The particular amount of decalage again depends on the stability margin / state margin, in other words position of CG.

biber

[Tom Harper]

biber,

Problem - The air does not recognize arbitrary references like the fuselage center line.

The stab flys at or very near zero. Therefore the zero lift line of it's section is the proper reference line. Decalage is the angular difference between the zero lift lines of the stab and wing sections. It is independent of any other datum line.

You are correct. If the CG is forward the stab will, of course, have to compensate and that wil produce a small downward vector on the stab without changing the AOA of the wing.