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Old Aug 23, 2011, 02:25 AM
JustFlying
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Your basic ideas of aircraft stability are wrong.
No sir, in this thread I did not yet give any explanation about how stability works. I have demonstrated MANY times that even with a rearward CG of 33% of the MAC and sometimes more, some planes can be very stable.
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The reason the old Telemaster balloons, is they were always flown with the CG way forward of the aircraft neutral point, making them far too pitch stable.
If you take a Telemaster ARF and you change the tailplane to a flat plate, you can move the CG way back and it will still be stable.
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Any increase in speed would make a huge nose up moment.
Because of the way it is trimmed for the given CG, yes.
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It would react the same with symmetrical airfoils with the CG in the same place, and incidence angles to trim for the same speed.
No sir, you have it wrong. With a symmetrical airfoil the trim will be different than with a flat bottom wing. With both, if you move the CG, you must change the elevator trim, but the trim will not be the same for both. Take 2 planes, everything else the same but 1 with symmetrical and 1 with flat bottom wing. Both the CG forward and trimmed for level flight at the same speed, the flat bottom wing plane will balloon much more than the symmetrical wing plane when you increase speed. At the CG point where the plane with symmetrical wing is neutral stable, i.e. it will not pitch up or down with increase of speed, the flat bottom wing plane will still be positively stable, i.e. it will pitch up with increase of speed. Why do you think designers will choose a flat bottom wing for a trainer? Just to be different?

Why do you think a plane is considered stable? It is because when there is a disturbance causing its nose to drop, it will increase speed and because of increase of lift and trim it will pitch up again. Then the speed will decrease and less lift and trim will cause it to pitch down and this leveling action will slowly die out, because of trim which is influenced by CG. The amount of stability will cause the leveling out proses to be slower or faster.

You can even make a plane with a negative cambered wing stable. In fact, I once had a small plane I wanted to make faster than any big plane on the field. It had a slightly cambered wing. At high speeds it would keep on pitching up, even with the CG so far back and resulting trim that it is negative stable on low speeds, a bad thing. I turned the wing upside down to be negative cambered which worked OK because it had no dihedral. Now I could move the CG forward a bit and trimmed for S&L flight at low speed and it would keep S&L at high speed.

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If you won't go through the references I provided, ask Bruce Mathews to explain it to you, he has more patience than me
I have a small library of books on planes, many of it is technical work. I have some 40 years of experience, done courses on design and aerodynamics and read many technical books. Because of that I decided and am busy to do even further studies in aerodynamics. I have a very good knowledge about the subject, thank you.

If you take the time and read again what Bruce say here on this forum as well as other forums, you will find his explanation is basically the same as what I say, because that is the correct explanation. Whoís words is this?:
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Also, do not confuse a lifting stab with stab lift or a stab with a symetrical airfoil as being a lifting stab. A lifting stab has a cambered airfoil but any stab can lift if it has an angle of attack to the oncoming air. An airfoiled stab is not always a lifting stab.
And this?:
Quote:
Any aircraft with positive pitch stability is a single speed design. Any displacement from S&L will cause the speed to change and the balance between wing and stab lift components acting against the CG will change in such a way as to return the model to it's trim speed and S&L flight.
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The most recent one was a P30 rubber model design where I used a highly cambered airfoil for the wing and a fairly highly cambered one for the stab. If it was just climb and glide then all was well. But if the nose went down due to some turbulence it was a guess if it would pull out or if it was lawn dart time. I fixed this by making a new stabilizer that had a lower camber airfoil. But as a result I was able to operate with an even further back CG than with the higher cambered stab airfoil. Re-read this bold bit again if it isn't clear. It's a large part of the secret of understanding this whole issue.
Do yourself a favor and read more about this subject before you argue about it like this. I canít believe that with your years of involvement with planes you still donít understand the basics. A good book to start with will be Ė Model Aircraft Aerodynamics by Martin Simons.
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Old Aug 23, 2011, 02:49 AM
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I think the argument is getting muddled. The only major difference in flight behavior between a symmmetrical or cambered section and between a thin or a thick section, all other things being equal, is the angle of attack that they can reach before a stall (measured in relation to the zero lift line). They will have small differences in drag, but compared to the AOA excursion before a stall this is fairly minor.
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Old Aug 23, 2011, 08:38 AM
JustFlying
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huh...???... - So who said this? :
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Not all planes use a symmetrical profile on the horizontal stab either. Those designs that mostly fly at a fixed cruise speed will probably have an asymmetrical profile. A symmetrical profile is a good compromise when the plane is supposed to fly in a large range of speeds and attitudes, like in the case of acrobatic planes and indeed most RC models.
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Old Aug 23, 2011, 09:55 AM
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That's the small difference in drag I was talking about. On a liner, every little bit of fuel saved is a little gain on the competition. that said, the Pilatus probably would have a better cruise with a less cambered tail profile section.
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Old Aug 23, 2011, 11:46 AM
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rebell,

One cannot in general swap out an airfoil for another, without changing the angle of attack to match the produced lift. That may be a catching point in your reasoning for things like the Telemaster horizontal tail.

If one took the Telemaster's "lifting" horizontal tail (positive cambered flat bottomed, IIRC - it's been years since I've seen one) and swapped in an otherwise identical symmetrical foiled horizontal tail, one would have to alter the mounting angle. The original Telemater horizontal tail has a zero lift line that is some negative angle. One can approximate it using a very crude formula - that for each percent of camber, the zero lift line shifts by one degree. This is not super accurate, but lacking anything else, and for conventional shaped airfoils, it gets one in the ballpark. The Telemaster's tail probably had about 2% camber, so its zero lift line would be shifted negative 2 degrees (roughly) compared to a symmetric foil. So, when one put a symmetric foil in its place, one would have to angle the new tailplane up roughly two degrees to match the aerodynamic behavior. One would need more specifics to be more precise.

Now, what other things would this change?

By thin airfoil theory, the lift produced by a surface is going to be proportional to its angle of attack, relative to its zero lift line. Since both the old and the new surface are of the same size and in the same place, they will see the same angle of attack change, and therefore the same change in lift. That is, up until flow separates or something else non-linear happens to the flow characteristics. That is of course the caveat with thin airfoil theory - it is only an approximation of what works with real airfoils. But for many purposes it is a pretty good approximation!

Since both the old and new surface would then have the same response behavior to pitch changes, they produce the same response for stability, in first approximation.

This does neglect the change in Cm of the horizontal tail, but if you run the numbers I think you will find it can be ignored in first approximation. The difference would be a slight shift in the neutral point.

Please take a little bit of time studying up on airfoil behavior and static/dynamic stability. In my overly brief reading here, your mind seems locked on a set of concepts, but those concepts are not improving your understanding of what is happening with planes.

IMHO, of course. And of course, I apologize in advance if I have been misunderstanding what you have typed.

As the saying goes, over and out.

Gerald
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Old Aug 23, 2011, 01:23 PM
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Originally Posted by BillzillaAus View Post
Apologies for the thread hijack, but I have seen this 'neutral point' thing written here several times and it's still a mystery to me.
Is it another name for Centre of Pressure?
No. The centre of pressure isn't a very good concept for understanding aircraft stability and response.

The neutral point of an airplane is set by it's geometry, and the airfoils and incidence angles do not have an influence on it, at least at higher Re where airfoils behave pretty linearly. At model Re, particularly with some older airfoils with poor separation bubble control, there are some caveats to that. For most situations, we can say the neutral point is fixed by the wing and stab geometry and positions, and to a lesser extent the fuselage shape and heights of the surfaces relative to the CG. Power and propellor factors can also influence it.

The neutral point is where the change in aircraft moments about the CG with a change in lift coefficient sum to zero (dM/dCl =0). Practically, this means if the aircraft CG was at this position, the airplane would be neutrally stable, and would have no tendency to return to it's trim lift coefficient, or diverge from any lift coefficient that it is adjusted to even after the control input is removed.

If the CG is behind the neutral point, the aircraft will diverge in pitch, and will be very difficult to fly. If the CG is ahead of the neutral point the aircraft will be pitch stable to varying degrees. I've re-attached a diagram showing aircraft responses with the CG in various positions relative to the neutral point.

The neutral point is a very useful concept. If you find a static margin percentage you like to fly with (percentage of the mean aero chord the CG is ahead of the aircraft neutral point), you can set any new aircraft to the same static margin, and the static pitch behaviour will be the same.

This is a very good spreadsheet for calculating the aircraft neutral point, as well as for positioning all the components to end up with the CG on the airplane where it should be:

http://www.rcgroups.com/forums/showthread.php?t=1106300

This spreadsheet takes into account the fuselage shape and the CG and surface heights, as well as the wing downwash and tail effectiveness. There are many simpler on-line neutral point and static margin calculators, but they ignore these factors. They also just use a fixed tail effectiveness factor that is only a guesstimate for conventional designs.

Kevin
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Old Aug 23, 2011, 02:13 PM
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Originally Posted by rebell View Post

Do yourself a favor and read more about this subject before you argue about it like this. I can’t believe that with your years of involvement with planes you still don’t understand the basics. A good book to start with will be – Model Aircraft Aerodynamics by Martin Simons.
I think Bruce had a moment of epiphany on stability fairly recently, and his understanding may be a bit different than it was previously, due to a discussion we had on this forum. That is one reason I suggested he might be able to help you, as well as his patience. Our discussion is buried in this thread:

http://www.rcgroups.com/forums/showthread.php?t=1467909

I don't have a copy of Martin Simmons' book, but a lot of the ideas of people that have seem to be misguided. I would suggest looking at perhaps "Aircraft Performance Stability and Control" by Perkins and Hage, or "Dynamic of Flight" by Bernard Etkin:

http://www.amazon.com/Airplane-Perfo...4126549&sr=1-1

http://www.amazon.com/Dynamics-Fligh.../dp/0471034185

I again post a pdf of the static stability section from Perkins and Hage. If you have a technical background, perhaps you could review it and see what you think. Those two books have been the standards for aircraft stability for 60 years, and are perhaps a bit more authoritartive than Martin Simmons' model airplane book.

Kevin
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Old Aug 23, 2011, 05:42 PM
JustFlying
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Gerald, I want to jump right to the end of your post:
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IMHO, of course. And of course, I apologize in advance if I have been misunderstanding what you have typed.
The difference is you talk about AoA changes and I about speed changes which influence different cambered airfoil differently. When I say increase in speed, I do not mean the increase that will happen when a plane is disturbed by a gust from level flight, I talk about when you open engine / motor power and because of propeller thrust the plane speed-up. Increase of speed = increase of lift (unless the plane is negative stable which does not happen much, and if it happens the possibility exist that the pilot will pick up the pieces. ), and of course this is influenced by the way the aircraft setup is done, the type of airfoil, wing incidence, trimming including CG and for which speed it is done so. Same as a wing with deployed flaps (higher CL) can carry the same aircraft at lower speed, so does the more cambered wing. Increase the speed on both and the lift increase. If you donít change the trim, the plane will climb. With a nice and big tailplane and a rearward CG you trim the elevator so the tailplane is actually a lifting surface and the plane can still be positive stable. Same with the Telemaster with itís huge tailplane. (You can move the CG way forward to change the situation, but that is only good for novice model pilots, scale models or full size planes.) Instead of using the elevator to trim, you use a lifting airfoil, which is a positive cambered airfoil for the tailplane. Increase the speed and its lift increase. Wow, I really donít know what is so difficult to understand about that.

Kevin, I have both those books and read it many times. While it is very good works with the full size plane in mind, it does not address all situations and setups modelers are using on models, keep this in mind when talking to modelers. Many modelers donít want the extreme positive stability that is required for full size man carrying aircraft for safety. One thing I saw over the years is that many times full size pilots and engineers find it very difficult to understand the different way modelers are doing things, it is two different worlds. It is not recommendable to do these different model setups with full size planes as it will put human life in danger and a definite suicide attempt if it is setup like an extreme 3D model. Models have different dynamics of which some is the different power to weight ratio, low wing loading, different wing/tail ratios and many times it use huge control surfaces with extreme strong and fast servos for its size. This together with a total different CG and trim setup put is in another class. One of my 3D planes has the CG at 50% of MAC. That makes it a bit different than a full size, doesnít it? I for one will not get into a full size plane if I know it is setup like my models. Kevin, I think this is where the difference in opinion originates from. Modelers are a different spesies than full size pilots/engineers which sometimes can intermix.
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Old Aug 23, 2011, 05:56 PM
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I'm very aware of all the low Re effects, and have been designing and building models for almost 50 years now. I even linked a very good paper previously in this thread that takes into account all the strange low Re effects that can occur with models. But, the basic stability equations are the same for full size and models, and generally they apply quite well to models even at low Re

I have never argued about rearward CGs. I have airplanes that quite happily fly with positive static margins with the CG at 85% of the wing MAC. I have built and flown many canards and tandem wings with the CG behind the front wing entirely. I fly discus launch gliders with the CG every near the neutral point (2 to 5% static margin), at very rearward MAC positions. I don't know why you keep bring that up, I understand it quite well.

The problem is, you are still telling people that airfoiled stabs cause ballooning characteristics. It is large static margins that cause that, and the airfoils have nothing to do with it. A lot of modellers do not understand zero lift lines, and the common practice of putting a Clark Y type airfoil with the flat bottom at zero relative to a slab stab makes and airplane that needs a very forward CG to trim above stall. This results in modellers thinking flat bottom airfoils balloon, because they will nose up strongly with any speed increase because of the large static margin.

Go through the static stability equations, and the you will see the airfoils have nothing to do with it. The airfoil moments are at best a second order effect, and reasonably stable for most airfoils, so they have only a small effect on trim, and none on whether the airplane balloons or not.

You are making incorrect statements, as I originally said. This is not correct. The airfoils have nothing to do with it:

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Some planes with asymmetrical / flat bottom main wing use the same asymmetrical / flat bottom tailplane to prevent ballooning at higher speeds. As the speed increase, the main wing produces more lift and the plane wants to pith up. The tailplane then also produce more positive lift and prevent this pitching moment.
Kevin
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Old Aug 23, 2011, 06:02 PM
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as G_T also remarked, once you have accounted for the zero lift line when rigging the stab, it won't make a difference how it will behave within a very large range of attitudes whether it is symmetrical or not. Where it will make a difference is at extreme attitudes. a cambered profile only produces "more lift" than a symmetrical profile because it can reach a higher AOA before stalling.
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Old Aug 23, 2011, 06:54 PM
JustFlying
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I don't know why you keep bring that up, I understand it quite well.
Because you keep on referring to stable setups with forward CG positions.

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The problem is, you are still telling people that airfoiled stabs cause ballooning characteristics.
Where did I say that? I said it counters ballooning as trimming with the elevator will, that is where rearward CG has a influence.
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The airfoils have nothing to do with it
You have it totally wrong. If airfoils have nothing to do with it, there is nop reason to use it for anything at all. Get real.

To keep on urging with a person like you who thinks he knows everything, is totally fruitless.
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Old Aug 23, 2011, 08:35 PM
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Because you keep on referring to stable setups with forward CG positions.


Where did I say that? I said it counters ballooning as trimming with the elevator will, that is where rearward CG has a influence.

You have it totally wrong. If airfoils have nothing to do with it, there is nop reason to use it for anything at all. Get real.

To keep on urging with a person like you who thinks he knows everything, is totally fruitless.
Yep.

I sure don't know everything, but I have a pretty good handle on aircraft static stability and trim.

You will notice that Gerald tried to correct you as well.

Airfoils are selected for lift, drag, and stall properties, to tailor lift distributions across a wing, and sometimes for airfoil moment coefficients, The airfoils do not cause or stop ballooning whether used on the wing or stab in whatever combination. Ballooning is caused by too large a static margin, totally independent of the wing or stab airfoils.

Old FF airplanes used to use lifting stabs because of weird rules that limited wing area and span, but not stab area. People made big lifting stabs combined with aft CGs to try and reduce the sink rate. All the arcane trimming requirements of FF, and primitive airfoils with big separation bubbles at low Re, developed some strange theories. A lot of that got carried over into RC.

So if I understand your ballooning theory, a Telemaster with a symmetrical stab airfoil would balloon more than one with a cambered stab airfoil? Why?

Since the lift curve slope for a surface (stab) is set by the aspect ratio of the stab, not the airfoil, can you tell me why the lifting airfoil would behave any differently than the symmetrical airfoiled stab?

The aircraft neutral point is only dependent on the aircraft geometry, not the wing or stab airfoils (check your Perkins and Hage), so changing the stab airfoil does not change the static margin as long as the CG stays in the same place. If the airplane is trimmed for the same speed with either stab airfoil, the pitch characteristics will be exactly the same, as long as the stab airfoil does not introduce elevator deadband or some other secondary effect (unless the stab is getting near stall - unlikely).

Kevin

Edit: I've attached a Profili plot of a NACA 0010 and a ClarkY airfoil at Re = 150k. The lift curve slopes aren't perfectly straight like they would be at higher Re, but they are very similar except the Clark Y slope is shifted upwards due to it's camber.

So let's say we have two identical Telemasters except for the stabs, which have these two different airfoils. Let's say that the stabs have to develop a Cl = 0.2 for our desired trim speed. The Clark Y stab has to be at an incidence angle of -1.5 degrees, and the NACA 0010 stab would have to be at an incidence angle of about 1 degree. If either of these airplanes has it's angle of attack disturbed, the tails will respond basically identically, because the lift curve slopes are the same except for some very minor low Re effects. Of course the lift curve slopes will be less due to the finite aspect ratio of the stab, but they will be the same.

If the airplanes have large static margins, they will both balloon the same. If they have small static margins, changes in speed will not produce large pitch forces, as in the diagram I have posted twice previously.
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Old Aug 24, 2011, 01:59 AM
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Originally Posted by kcaldwel View Post
stuff
Thanks for that.
Sounds like a way to make simple things complex.
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Old Aug 24, 2011, 09:02 AM
JustFlying
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Say the plane has a symmetrical or flat plate tailplane and for starters is setup to be stable. When moving the CG back / reducing the static margin, you have to re-trim the plane by deflecting the elevator down otherwise the plane will pitch up and more so when more power is given. That whole tailplane surface than has an effective change in camber. If you move the CG back far enough and re-trim the plane for level flight, you might actually find that the elevator can be in the down position. I have flown ARF planes like that with the elevator in the down position when trimmed. The plane can then still be stable by pulling slowly out of a dive when pointed downwards with no pilot interference but the initial trim, like your nice drawing of stability show. If you want, you can cancel this stability out completely, you move the CG even further back and re-trim again, until the plane does not pull out of a dive anymore, nor steepen its dive. Now the elevator is deflecting the lowest without pithing the plane up or down at different speeds when flying S&L. The tailplane has the aerodynamic appearance of a cambered airfoil. The same as when you lower the flaps on the main wing, you actually alter the effective camber. If you now straiten the elevator so that there is no deflection, what will the plane do? It will pitch up. But you donít do it, as you want the plane to fly level.

If this or the little more stable way is the way you intend the plane to fly, you can remove the tailplane and careful replace it with a more effective cambered airfoil tailplane. (I know it will not be an easy task to do it precise, but keep it theoretical.) Now you still have a plane which is flying straight and level at different speeds, and has less drag than with the elevator deflected down.
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Old Aug 24, 2011, 10:07 AM
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Or you can keep the same flat tailplane and increase its incidence slightly. it does not matter that the tail in this case is cambered or not, what you are changing with this "camber increase" is the zero lift line.
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