Stab airfoiling--why and when?

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Rktman

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Under what situation would you put an airfoil on the topside of your stabilizer? I understand why it would make sense to put the airfoiled side on the bottom, but I came across a few old designs in the Model Rocketeer or Model Rocketry magazine that had the airfoiled side on on top. No explanation why.
 

frognbuff2.0

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It's the geometry of the top side of a cambered airfoil which creates lift. That's how flaps create more lift, the overall curvature and profile of the airfoil is increased with them deployed. Bird wings are also curved in such ways to generate lift. Supersonic aircraft rely on this principle since they must have symmetric airfoils to sustain supersonic flight, but the flaps can be used on takeoff and landing to simulate a cambered wing despite there being no bottom.
I hope this answers your question, if not I apologize.
 

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It's the geometry of the top side of a cambered airfoil which creates lift. That's how flaps create more lift, the overall curvature and profile of the airfoil is increased with them deployed. Bird wings are also curved in such ways to generate lift. Supersonic aircraft rely on this principle since they must have symmetric airfoils to sustain supersonic flight, but the flaps can be used on takeoff and landing to simulate a cambered wing despite there being no bottom.

It is actually the angle of attack (AoA) that creates the lift. The airfoil just means that the flow stays attached, particularly on the upper surface, at higher AoA thus giving less drag. Flaps create more lift because they actually perform the equivalent of twisting the wing so the chord of the wing is effectively tilted with respect to the airframe, as well as being made longer. Camber is also increased. Other things like leading and trailing edge slats help the flow stay attached to the upper surface at higher AoA, again to reduce drag, and also to keep the wing flying (producing lift). Google "high lift devices aircraft"

As for symmetric airfoils for supersonic flight the answer is that it depends. Most low-drag supersonic airfoils are very slim and can have very sharp leading and trailing edges. The slim section is to keep drag to a minimum. Some of them (NACA design of some sort I think) actually have a little hollow underneath towards the trailing edge which helps with performance. There is an adverse pressure gradient on the back of the wing at non-zero AoA and by shaping the airfoil appropriately the airflow can stay attached (laminar, not turbulent) to the wing.

Most rockets operate near zero AoA so airfoiling is a game of diminishing returns a lot of the time.

If you look at a lot of fin designs they are double diamond shape (eg Nike Smoke). One of the main reasons this is so prevalent is that in the early days of computing it was actually easy to analyse this shape mathematically. These days computers have so much power it is not a consideration. Anything can be simulated and made fly.

Fundamentals of Aerodynamics (Anderson) is a great book. It will take you from basics up to hypersonics if you want. Search online for a free pdf ;).
fundamentals-of-aerodynamics-6th-edition.jpg
 

watheyak

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I assume you mean in an old-timer free-flight model?

I have wondered the same thing. Usually horizontal stabilizers make lift pointed down, or tail downforce.

Why then, are there a bunch of old-timer free flight models the other way around? As if the horizontal stabilizer is making lift, not downforce.

There's gotta be some weird geometry going on. I have a feeling it has to do with powered flight, which is a vertical-ish climb and then the transition to glide.

satellite320-1.jpg
 
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frognbuff2.0

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It is actually the angle of attack (AoA) that creates the lift. The airfoil just means that the flow stays attached, particularly on the upper surface, at higher AoA thus giving less drag. Flaps create more lift because they actually perform the equivalent of twisting the wing so the chord of the wing is effectively tilted with respect to the airframe, as well as being made longer. Camber is also increased. Other things like leading and trailing edge slats help the flow stay attached to the upper surface at higher AoA, again to reduce drag, and also to keep the wing flying (producing lift). Google "high lift devices aircraft"

As for symmetric airfoils for supersonic flight the answer is that it depends. Most low-drag supersonic airfoils are very slim and can have very sharp leading and trailing edges. The slim section is to keep drag to a minimum. Some of them (NACA design of some sort I think) actually have a little hollow underneath towards the trailing edge which helps with performance. There is an adverse pressure gradient on the back of the wing at non-zero AoA and by shaping the airfoil appropriately the airflow can stay attached (laminar, not turbulent) to the wing.

Most rockets operate near zero AoA so airfoiling is a game of diminishing returns a lot of the time.

If you look at a lot of fin designs they are double diamond shape (eg Nike Smoke). One of the main reasons this is so prevalent is that in the early days of computing it was actually easy to analyse this shape mathematically. These days computers have so much power it is not a consideration. Anything can be simulated and made fly.

Fundamentals of Aerodynamics (Anderson) is a great book. It will take you from basics up to hypersonics if you want. Search online for a free pdf ;).
View attachment 423471
It seems I've forgotten just about everything from when I've taken an aerodynamics class at university which used a similar textbook. I was just trying to be concise and failed.
 
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watheyak

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It's the geometry of the top side of a cambered airfoil which creates lift. That's how flaps create more lift, the overall curvature and profile of the airfoil is increased with them deployed. Bird wings are also curved in such ways to generate lift. Supersonic aircraft rely on this principle since they must have symmetric airfoils to sustain supersonic flight, but the flaps can be used on takeoff and landing to simulate a cambered wing despite there being no bottom.
I hope this answers your question, if not I apologize.
I can't let this one pass without comment.

There isn't a single correct statement in your post. Over the Top's post, which you dismissed, is excellent and concise. I'd take his advice and read that book!

As someone who used to teach this stuff, it makes me cringe. I feel compelled to stop the spread of misinformation.

There. Got it off my chest. Thank you.
 
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frognbuff2.0

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I can't let this one pass without comment.

There isn't a single correct statement in your post.

As someone who used to teach this stuff, it makes me cringe. I feel compelled to stop the spread of misinformation.

There. Got it off my chest. Thank you.
I apologize for my gross misunderstanding then. I meant no harm
 

OverTheTop

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It seems I've forgotten just about everything from when I've taken an aerodynamics class at university which used a similar textbook. I was just trying to be concise and failed.
I apologize for my gross misunderstanding then. I meant no harm
Don't worry about it. All's good.

And I really would recommend that book by Anderson for anyone who wants to understand aerodynamics. If it can work for an electronic engineer (me!) it can work for anyone.
 

Rktman

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I assume you mean in an old-timer free-flight model?

I have wondered the same thing. Usually horizontal stabilizers make lift pointed down, or tail downforce.

Why then, are there a bunch of old-timer free flight models the other way around? As if the horizontal stabilizer is making lift, not downforce.

There's gotta be some weird geometry going on. I have a feeling it has to do with powered flight, which is a vertical-ish climb and then the transition to glide.

View attachment 423472
Your comment gets right to the heart of my question, although as it applies to rocket boosted gliders. Guess I wasn't too clear on that point.

As a clueless newbie I sanded an airfoil into the stab of my 1st glider. Needless to say, it promptly power-looped into the ground when launched.
I replaced the stab with a flat one and it performed perfectly.

So why would anyone airfoil the top of their RG's or BG's stab? (If I can find any of those plans I'll post it here).
 
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Rktman

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Here are 2 old plans that indicate stabs airfoiled on top. There are others that show it more clearly but these were the only ones I could find at the moment.

It's pretty easy to see in the Easy Rider IV.
Easy Rider IV D powered glider.jpg

In the Sparrowhawk plans, the text clearly states "sand airfoil into top of stab".
 

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watheyak

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Here are 2 old plans that indicate stabs airfoiled on top. There are others that show it more clearly but these were the only ones I could find at the moment.

It's pretty easy to see in the Easy Rider IV.
View attachment 423530

In the Sparrowhawk plans, the text clearly states "sand airfoil into top of stab".
There are a few unconventional things happening on that model. I starting to suspect that in free flight models with a powered phase and a glide phase these things are actually pretty common. You have to trick the plane into flying trimmed at two very different airspeeds.

First off, the CG on that glider is waaaaayyyy aft compared to other aircraft. Usually the CG is about 1/3 of the chord length aft of the leading edge. The angle between the chord of the wing and the horizontal stab also appears to be zero. Usually the stab has a bit of negative angle relative to the wing to give it some tail down force and positive dynamic stability.

Hopefully someone who is into FF models or boost gliders will chime in.
 

Rktman

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There are a few unconventional things happening on that model. I starting to suspect that in free flight models with a powered phase and a glide phase these things are actually pretty common. You have to trick the plane into flying trimmed at two very different airspeeds.

First off, the CG on that glider is waaaaayyyy aft compared to other aircraft. Usually the CG is about 1/3 of the chord length aft of the leading edge. The angle between the chord of the wing and the horizontal stab also appears to be zero. Usually the stab has a bit of negative angle relative to the wing to give it some tail down force and positive dynamic stability.

Hopefully someone who is into FF models or boost gliders will chime in.
It looks like there is a very slight angle to the bottom of the fuselage as it narrows toward the aft end. That would put it at a slight incidence to the wing. If so, would that counteract the airfoil on the stab's topside, which begs the question: why put the airfoil in in the first place? Hmmm...Curiouser and curiouser.
 

Rktman

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It is actually the angle of attack (AoA) that creates the lift. The airfoil just means that the flow stays attached, particularly on the upper surface, at higher AoA thus giving less drag. Flaps create more lift because they actually perform the equivalent of twisting the wing so the chord of the wing is effectively tilted with respect to the airframe, as well as being made longer. Camber is also increased. Other things like leading and trailing edge slats help the flow stay attached to the upper surface at higher AoA, again to reduce drag, and also to keep the wing flying (producing lift). Google "high lift devices aircraft"

As for symmetric airfoils for supersonic flight the answer is that it depends. Most low-drag supersonic airfoils are very slim and can have very sharp leading and trailing edges. The slim section is to keep drag to a minimum. Some of them (NACA design of some sort I think) actually have a little hollow underneath towards the trailing edge which helps with performance. There is an adverse pressure gradient on the back of the wing at non-zero AoA and by shaping the airfoil appropriately the airflow can stay attached (laminar, not turbulent) to the wing.

Most rockets operate near zero AoA so airfoiling is a game of diminishing returns a lot of the time.

If you look at a lot of fin designs they are double diamond shape (eg Nike Smoke). One of the main reasons this is so prevalent is that in the early days of computing it was actually easy to analyse this shape mathematically. These days computers have so much power it is not a consideration. Anything can be simulated and made fly.

Fundamentals of Aerodynamics (Anderson) is a great book. It will take you from basics up to hypersonics if you want. Search online for a free pdf ;).
View attachment 423471
Would that book be applicable to the low speeds and small scale of our free flight rocket gliders and boost gliders? I've read the Frank Zaic series, but it deals with gliders that will never see the kind of launch speeds of a rocket boosted glider. Would you say Anderson's book would be more helpful? Would like to acquire it if so.
 

OverTheTop

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Would that book be applicable to the low speeds and small scale of our free flight rocket gliders and boost gliders? I've read the Frank Zaic series, but it deals with gliders that will never see the kind of launch speeds of a rocket boosted glider. Would you say Anderson's book would be more helpful? Would like to acquire it if so.
IIRC it goes from basic aerodynamic theory starting at subsonic and working up through transonic to supersonic and even hypersonic. There are different considerations in each speed regime of course, and they are discussed. There is also discussion on the Reynolds number and how it applies. This is a measure of speed relative to a "reference length" and can be used as a measure of how well aerodynamics can scale. Commercial aircraft have Reynolds numbers in the hundreds of thousands. You might be surprised to find those numbers are not entirely different to the Reynolds numbers for your gliders, due to the smaller reference lengths at your smaller scale. They will be a little lower, of course.

From a theoretical side I found this book to be very well written and immensely readable (FYI I am an electronic engineer). YMMV, so I suggest you search for a free pdf to see what it is like and if it suits your needs. I liked it so much I have the hardcover now.

There are quite a few books written by Anderson and all are well-written and understandable. . Check out some of the other titles. You may fine one that is less mathematical and more descriptive that suits you better. In any case you can usually get lots of useful theory by ignoring the mathematical stuff if needed, in most cases. I seem to remember "Introduction to Flight" is a good read too. Just search "John Anderson books" on google.
 

Rktman

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IIRC it goes from basic aerodynamic theory starting at subsonic and working up through transonic to supersonic and even hypersonic. There are different considerations in each speed regime of course, and they are discussed. There is also discussion on the Reynolds number and how it applies. This is a measure of speed relative to a "reference length" and can be used as a measure of how well aerodynamics can scale. Commercial aircraft have Reynolds numbers in the hundreds of thousands. You might be surprised to find those numbers are not entirely different to the Reynolds numbers for your gliders, due to the smaller reference lengths at your smaller scale. They will be a little lower, of course.

From a theoretical side I found this book to be very well written and immensely readable (FYI I am an electronic engineer). YMMV, so I suggest you search for a free pdf to see what it is like and if it suits your needs. I liked it so much I have the hardcover now.

There are quite a few books written by Anderson and all are well-written and understandable. . Check out some of the other titles. You may fine one that is less mathematical and more descriptive that suits you better. In any case you can usually get lots of useful theory by ignoring the mathematical stuff if needed, in most cases. I seem to remember "Introduction to Flight" is a good read too. Just search "John Anderson books" on google.
Thanks for the feedback! Just located a pdf copy online, downloading right now.
 

Rktman

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Found another plan with a stab airfoiled on top (see build instructions in pdf). Apparently it wasn't all that uncommon a thing to do. Still can't see structurally how these differ from the majority of plans that use stab incidence or have the airfoiling on the bottom.
Seagull-2.jpg
 

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BABAR

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How hard would it be to either build two birds, identical except for the airfoil, and compare flight characteristics? Alternatively, one bird with a swappable horizontal stab?

IIRC white glue and maybe wood glue is softened by heat, so swapping them out may not be completely impractical.

First compare hand toss performance (I suspect they may trim a bit differently.)
 

GuyNoir

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Found another plan with a stab airfoiled on top (see build instructions in pdf).
Well, that certainly was the way I designed and built the model back then, but it's not what I do today. The stab and rudder now use symmetrical airfoils. The stab is glued on the top of the boom with a small shim (not more than 3/32") under the trailing edge. The rudder should be attached with jusssst a hint of offset to induce a turn during glide. Too much rudder trim will result in a much lower boost and increase the chance for a spiral dive.

(Aside: I used the same planiform on my A BG at NARAM-61 to win the event in C Division.)
 

OKTurbo

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This warms my heart. I’m into stick and tissue free flight models and when the topic comes up, which is rare, I explain how much skill it takes to build and fly one of these old birds. Most people think the big rc planes are difficult...lol.

I don’t have any boost gliders, but trimming for two very different flight envelopes is what it comes down to. If you don’t have a way to adjust the flight surfaces between high speed launch and the slow glide you’ve got to build it into the design.

In free flight we build specific warps into the wings, we angle the propeller thrust line to help with motor torque forces, etc. The high tech FF planes have built in timers that adjust the wing and tail incidence....VIT variable incidence tail.

So when you see an old bloke flying and old stick and tissue realize it took quite a bit of engineering science to get it to spiral up in one direction, transition into a glide at the top, circle in the opposite direction and the hopefully DT before a thermal takes it away.

Not much help, just an old Luddite wanted to share his thoughts.
 

Rktman

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Well, that certainly was the way I designed and built the model back then, but it's not what I do today. The stab and rudder now use symmetrical airfoils. The stab is glued on the top of the boom with a small shim (not more than 3/32") under the trailing edge. The rudder should be attached with jusssst a hint of offset to induce a turn during glide. Too much rudder trim will result in a much lower boost and increase the chance for a spiral dive.

(Aside: I used the same planiform on my A BG at NARAM-61 to win the event in C Division.)
Do you recall why you airfoiled the top of your stab? My understanding is that a stab with negative incidence or a bottom airfoil functions to help it transition to glide and "keep the nose end up", and that doing the reverse would...well...have negative consequences (like the way it looped my glider into the ground when launched). I think I may have read in one of the model plane forums that with hand launched gliders with long fuselages, it may somehow work to increase glide time (I think they call it a "lifting stab")? Not clear why though. But I can see where it wouldn't be catastrophic at the low speeds they're dealing with versus a rocket launched glider.
 
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Rktman

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This warms my heart. I’m into stick and tissue free flight models and when the topic comes up, which is rare, I explain how much skill it takes to build and fly one of these old birds. Most people think the big rc planes are difficult...lol.

I don’t have any boost gliders, but trimming for two very different flight envelopes is what it comes down to. If you don’t have a way to adjust the flight surfaces between high speed launch and the slow glide you’ve got to build it into the design.

In free flight we build specific warps into the wings, we angle the propeller thrust line to help with motor torque forces, etc. The high tech FF planes have built in timers that adjust the wing and tail incidence....VIT variable incidence tail.

So when you see an old bloke flying and old stick and tissue realize it took quite a bit of engineering science to get it to spiral up in one direction, transition into a glide at the top, circle in the opposite direction and the hopefully DT before a thermal takes it away.

Not much help, just an old Luddite wanted to share his thoughts.
I have a lot of respect for you guys. I'm in the process of building a small built-up wing delta glider BG. Since it's just for fun there's no airfoil, it's just flat. Even so, cutting and precisely fitting all those balsa strips that make up the internal wing structure is difficult and time-consuming and takes meticulousness. And with motorized planes like yours I realize there's a lot more to take into consideration, like motor torque and small mechanisms to adjust things. So hats off to you.
 
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Rktman

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How hard would it be to either build two birds, identical except for the airfoil, and compare flight characteristics? Alternatively, one bird with a swappable horizontal stab?

IIRC white glue and maybe wood glue is softened by heat, so swapping them out may not be completely impractical.

First compare hand toss performance (I suspect they may trim a bit differently.)
Thanks but no thanks, since I already know the negative results of having that kind of stab. But if there's some advantage to airfoiling the stab topside and how the glider needs to be structurally different to leverage that kind of stab, then I wouldn't need to experiment. I'd know how to build the glider different from the start, yes?
 

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So why would anyone airfoil the top of their RG's or BG's stab?
Because they have a less-than-complete understanding of low-speed aerodynamics?

The lift vector acting on the horizontal stabilizer of a subsonic aircraft with a traditional planform in steady state, unaccelerated flight should be down, not up.
 

Rktman

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Because they have a less-than-complete understanding of low-speed aerodynamics?

The lift vector acting on the horizontal stabilizer of a subsonic aircraft with a traditional planform in steady state, unaccelerated flight should be down, not up.
The question is why did some designs work using airfoiled stab tops? How did they make them work successfully? (From the sample of plans above they somehow did successfully). Maybe I'm overlooking something obvious, but for the life of me I can't see any difference in these gliders structurally from other designs with "normal" stabs.
 

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This warms my heart. I’m into stick and tissue free flight models and when the topic comes up, which is rare, I explain how much skill it takes to build and fly one of these old birds. Most people think the big rc planes are difficult...lol.

I don’t have any boost gliders, but trimming for two very different flight envelopes is what it comes down to. If you don’t have a way to adjust the flight surfaces between high speed launch and the slow glide you’ve got to build it into the design.

In free flight we build specific warps into the wings, we angle the propeller thrust line to help with motor torque forces, etc. The high tech FF planes have built in timers that adjust the wing and tail incidence....VIT variable incidence tail.

So when you see an old bloke flying and old stick and tissue realize it took quite a bit of engineering science to get it to spiral up in one direction, transition into a glide at the top, circle in the opposite direction and the hopefully DT before a thermal takes it away.

Not much help, just an old Luddite wanted to share his thoughts.
AMA Free Flight is how I got into rocketry. Was the NFFS Membership chairman for a couple of years in the late 1980’s.

I visited a rocket launch, saw birds being boosted into the air, chased by the fliers...sometimes on dirt bikes, and I said to myself “These rocket folks are my people, too! ...But with more Shock and Awe!”.

Lotta fun building something on my bench that can break the sound barrier.
 

James Duffy

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The question is why did some designs work using airfoiled stab tops?
Probably because the stab installed with a massive amount of negative incidence or a bunch of really draggy trim. A flat horizontal stabilizer would probably improve any of the designs you encountered.
 

Rktman

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Probably because the stab installed with a massive amount of negative incidence or a bunch of really draggy trim. A flat horizontal stabilizer would probably improve any of the designs you encountered.
The Easy Rider IV was designed and flown by Dr. Gregorek, so I have to assume he knew what he was doing and airfoiled the stab top intentionally and for a reason--whatever it was. The Sparrowhawk instructions say to "bend the stab trailing edge up 3/32" (which is the same as incidence) but wouldn't that negate the topside airfoiling of the stab? Which then begs the question: why airfoil it in the first place)?
 
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The clue as to why these designs use a lifting tail plane (and are designed to) is in the CG location that you noted on the plans.

Imagine it this way:

In a conventional situation (downforce on the tail plane), the wing center of lift is the fulcrum of a beam (the fusalage), like the pivot point on a see saw. The CG is slightly to one side of the fulcrum, causing that side of the beam to drop. Now we add a downward force (tailplane) to the high end of the beam that brings the beam back to level. This of course is massively simplified, but you get the idea.

So, in this case our objects are arranged, front to rear, like this:

-----CGv---CL^---------------------BFv

(Where v and ^ indicate force/load direction, CG = Center of gravity, CL = Center of lift (fulcrum), BF = Balancing force.)

In a "lifting tail" arrangement the wing center of lift is still the fulcrum for our beam, but the GC is placed on the opposite side of the fulcrum, again causing that end of the beam to drop. This time we add lift to the low end of the beam to bring it back to level.

In this case our objects are arranged like this:

-----CL^----CGv-------------------BF^

Both achieve equilibrium by balancing masses and dynamic forces, but in different ways.

If you look at the first (bad) drawing, you'll notice that the wing not only has to lift the entire mass of the glider, but overcome the downforce of the tail as well.

In the second bad drawing the mass of the glider is divided between two lifting surfaces, and there is no extra downforce working counter to the wing's lift. Imagine it like a reverse canard arrangement.

Both are trimmed by varying relative incidence between the wing and tail. In the case of the conventional tail, raising the trailing edge increases downforce. In a lifting tail raising the trailing edge decreases lift. Net effect is the same...

The lifting tail set-up has the potential to be more efficient, but can be more difficult to trim. Like O1d Dude said, the free flight guys have had it figured out for quite a while, but it still tough to get right over two very different flight regimes.

Sorry to ramble so. I hope this made some kind sense... :rolleyes:
 
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I think you have beaten me to it Mugs :). I was pondering this last night. As to being a more efficient way to lift the tail weight, I am still not convinced on that although it may be a factor. It may be something to do with the CP/CG relationship on the wing and where the weight is taken there.

I am wondering if it is a quick way of damping any phugoid motion in the flight. This would make for a more flat and energy-efficient flight for the glider. So the glider is a bit nosedown and picking up speed. The additional negative lift will quickly drop the tail and level the flight out. On the other side of the phugoid, with nose up, it would act less quickly thus allowing the kinetic energy to be better turned into potential energy, then nicely leveling the flight. This might likely damp the phugoid motion quicker. Maybe I am barking up the wrong eucalypt. Thoughts? My guess is that it might make the model easier to get an optimum trim by using this method.
 

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The clue as to why these designs use a lifting tail plane (and are designed to) is in the CG location that you noted on the plans.

Imagine it this way:

In a conventional situation (downforce on the tail plane), the wing center of lift is the fulcrum of a beam (the fusalage), like the pivot point on a see saw. The CG is slightly to one side of the fulcrum, causing that side of the beam to drop. Now we add a downward force (tailplane) to the high end of the beam that brings the beam back to level. This of course is massively simplified, but you get the idea.

So, in this case our objects are arranged, front to rear, like this:

-----CGv---CL^---------------------BFv

(Where v and ^ indicate force/load direction, CG = Center of gravity, CL = Center of lift (fulcrum), BF = Balancing force.)

In a "lifting tail" arrangement the wing center of lift is still the fulcrum for our beam, but the GC is placed on the opposite side of the fulcrum, again causing that end of the beam to drop. This time we add lift to the low end of the beam to bring it back to level.

In this case our objects are arranged like this:

-----CL^----CGv-------------------BF^

Both achieve equilibrium by balancing masses and dynamic forces, but in different ways.

If you look at the first (bad) drawing, you'll notice that the wing not only has to lift the entire mass of the glider, but overcome the downforce of the tail as well.

In the second bad drawing the mass of the glider is divided between two lifting surfaces, and there is no extra downforce working counter to the wing's lift. Imagine it like a reverse canard arrangement.

Both are trimmed by varying relative incidence between the wing and tail. In the case of the conventional tail, raising the trailing edge increases downforce. In a lifting tail raising the trailing edge decreases lift. Net effect is the same...

The lifting tail set-up has the potential to be more efficient, but can be more difficult to trim. Like O1d Dude said, the free flight guys have had it figured out for quite a while, but it still tough to get right over two very different flight regimes.

Sorry to ramble so. I hope this made some kind sense... :rolleyes:
OK, so I will say this so somebody can beat me over the head with it. It's just a reverse canard setup with two lifting surfaces, but a canard is lots easier to get to work reliably. :D
 
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