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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.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
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I apologize for my gross misunderstanding then. I meant no harm
Don't worry about it. All's good.
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.
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.
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.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
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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.
Thanks for the feedback! Just located a pdf copy online, downloading right now.
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.
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.
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.
AMA Free Flight is how I got into rocketry. Was the NFFS Membership chairman for a couple of years in the late 1980’s.
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)?
. 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.
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.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...
