Stab airfoiling--why and when?

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Rktman

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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:
YES! THANK YOU @Mugs914!!
Finally a definitive answer and it makes total sense! You can ramble anytime you want, this was eye-opening for me, and clear and logical. Thanks for putting it in layman's terms that I could digest. 👍
 
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Rktman

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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:
With the "how it works" part now answered, will using this method (CG aft of center of lift) allow the glider to launch straight, or does something else structurally need to be done to prevent it from looping into the ground?
 

OKTurbo

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With the two very different flight envelopes, boost vs glide, you need to consider these physical points of the design. All objects is free space will rotate about their center of gravity...aka center of mass. The forces you have will be the center of pressure and the center of lift.

Center of pressure is the geometric center of all your surfaces. It stays constant. The CG actually changes a bit as the propellant burns up, but it should stay fairly close to the same position. Center of lift, which is the balance of all your aerodynamic forces perpendicular to the lifting surfaces change dramatically with velocity...and angle of attack to the forward motion.

The canard design works well on airplanes to soften, or better control stall characteristics. The lifting stab of a canard will stall before the main wing since the plane rotates/pitches about the CG. With the canard out front it’s angle of attack increases faster about the CG and stalls first....but the main wing still provides the majority of lift so the stall is less severe.

The reason to use a lifting stab is because it has lift! In free flight you want to fly as long as possible so more,lifting surfaces is good....but it will only really want to fly at one speed. The faster is flies, the more lift you get in the tail and it change the whole planes angle of attack...etc.

Kinda rambling at this point, but hopefully I gave you some things to think about.

Hey...at least it’s complicated. 🙂
 

Rktman

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With the two very different flight envelopes, boost vs glide, you need to consider these physical points of the design. All objects is free space will rotate about their center of gravity...aka center of mass. The forces you have will be the center of pressure and the center of lift.

Center of pressure is the geometric center of all your surfaces. It stays constant. The CG actually changes a bit as the propellant burns up, but it should stay fairly close to the same position. Center of lift, which is the balance of all your aerodynamic forces perpendicular to the lifting surfaces change dramatically with velocity...and angle of attack to the forward motion.

The canard design works well on airplanes to soften, or better control stall characteristics. The lifting stab of a canard will stall before the main wing since the plane rotates/pitches about the CG. With the canard out front it’s angle of attack increases faster about the CG and stalls first....but the main wing still provides the majority of lift so the stall is less severe.

The reason to use a lifting stab is because it has lift! In free flight you want to fly as long as possible so more,lifting surfaces is good....but it will only really want to fly at one speed. The faster is flies, the more lift you get in the tail and it change the whole planes angle of attack...etc.

Kinda rambling at this point, but hopefully I gave you some things to think about.

Hey...at least it’s complicated. 🙂
Yes, but the question remains does a BG/RG need to be constructed differently to use a lifting stab to prevent looping into the ground when launched?
 

Mugs914

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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
You can take your helmet off Pat, that is just what it is. :p

Canards are easier to set up because the minor lifting surface is in front and, like others have said here, it is usually designed, through airfoil selection, angle of incidence or both, to stall (lose lift) before the main wing even approaches a stall. When it does, the CG being ahead of the center of lift of the main wing (which is still lifting) naturally causes the nose to drop, reducing the angle of attack of both surfaces and restoring lift to the (no longer stalled) canard. The main lifting surface maintains lift throughout the event. Canard bad drawing:

--- RF^-----------------------CGv------CL^.

The lifting tail is trickier for the same reason the canard is simpler, but kind of just the opposite...:rolleyes:

If we try to use the same logic with a lifting tail (minor lifting surface stalls first), then loss of lift when the tail stalls causes the tail to drop, increasing the angle of attack of the main lifting surface which then stalls itself. Compounding the problem is that the CG is behind the CL so there is no restoritive force; in fact, it makes the matter worse. If we set it up so the main wing stalls first, the nose still drops, but as is typical with the lifting tail arrangement, there isn't much of a resoritive force coming from the tail because the typical set up is to trim for equilibrium in a nice level glide. Another reason the lifting tail is tricky...

The problem most folks have with canards is yaw stability. Typically with a canard design the GC is pretty far aft. That leaves a pretty short moment when it comes to rudder/fin placement. This means that a rather large fin is required, or, as most seem to have, smaller fins out on the (usually swept) wings. Others resort to very high dihedral angle on the canard, but this is less effective in the overall scheme of things.

Again, HTH...

Mike
 

Mugs914

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Yes, but the question remains does a BG/RG need to be constructed differently to use a lifting stab to prevent looping into the ground when launched?
Yeah, they do. Again the clue is on the plans that were posted. One is a pop-pod design, the other is a slide wing, indicating that a rather significant shift in CG or CP is required in these designs to achieve a stable boost (said Captain Obvious).

(I hope you guys can put up with a few more of these bad drawings because I'm having fun trying to figure out how to make 'em! :p)

First off, is a conventional Astron Falcon configuration that spits the motor. This is what it looks like when gliding:

<=====
\\\\\
-----CGv--CL^------------------------BFv

Under boost, two things change; the CG and the fact that it is under thrust. Now it looks like this:

<===== T<
\\\\\
D> -CGv----CL^-------------------------BFv TM^

Where we have added a few things: This time < and > indicate directions of forces, T = thrust, D = drag and TM is what we'll call thrust moment.

First off, the motor being way out front shifts the CG a bit forward (CG further ahead of CP). Second, the thrust of the motor is significantly offset from the drag axis of the model, generating a torque (our TM) along the fuselage that cancels out, to some degree, the BF of the tail. When it spits to motor, the CG is restored to it's glide position and the TM goes away completely, restoring the proper balance of forces required for gliding.

The CG doesn't need to shift very much because the CG is already (a little) ahead of the CL/CP in the glide mode. Also, the TM is essentially a nose down force countering the downforce of the tail. Put 'em together (properly) and you get a pretty straight boost.

If we try a lifting tail arrangement with the same configuration (motor spitter), we end up with the forces all ganging together to cause balsa splinters:

<===== T<
\\\\\
D> --CGvCL^---------------------------BF^ TM^

:oops:

First problem is that moving the CG forward enough just with motor weight is going to be tough because the CG is so far aft to begin with. It gets worse from there; The CL, BF and TM are all working together to create the Outside Death Loop from Hell. It gets worse than THAT when the wing's angle of attack goes negative due to the tail BF and thrust moment rotating the entire thing around the CG. Now we have this:

<===== T<
\\\\\
D> --CGvCLv----------------------------BF^ TM^

Yipes...

If we make it a slide wing it gets a whole lot easier. Our boost now looks like this:

<=====T<
D> -----CGv------------------------------CL^--BF^

First we can reduce the height of the thrust line relative to the drag axis because we don't need the TM to overcome the downforce on the tail. Second, while the wing and tail will still have some lift force (less from the tail, since it is directly blocked by the wing) there are no longer any surfaces anywhere near the CG to provide any significant "counterforce" to facilitate a change in direction. Think of a sailboat with no keel; turning the rudder just makes it go sideways. A swing wing with a lifting tail arrangement will be tougher to trim for boost because there is still significant surface area forward.

Pop pod designs are mechanically simpler and kind of do the same thing. Their boost mode will look like this:

<============== T<
D> ---CGv----------CL^------------------------BF^

Again, lower thrust line but now it is easier to move the CG forward because we can use a really long pod to stick everything way out front. It is kind of like the slide wing in that we have moved the surfaces away from the CG (vice versa, actually), but they are still close enough to make boost trim a bit tricky. Other than that, a pop pod design with a conventional design (downforce tail) is essentially the same.

As an aside, this occurred to me while I was typing this up:

D> ---GCv-CL^----------------------BF^ TMv
/////
<======== T<


Hmmmmmm... 🤔🤔🤔🤔

Thanks for letting me fiddle with all the bad drawings! (even though they don't look exactly like what I typed)

Hope I haven't added to the confusion...

Mike
 
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OKTurbo

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I’ve got a GoPro so might as well use it.


For your consideration....
 

PatD

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So hang two of them on a boost pod and pop them off at apogee. If the gliders have been built very similar they should cancel out looping tendencies on boost. Probably take a fair bit of nose weight for stability on the way up tho. :oops:
 

Mugs914

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So hang two of them on a boost pod and pop them off at apogee. If the gliders have been built very similar they should cancel out looping tendencies on boost. Probably take a fair bit of nose weight for stability on the way up tho. :oops:
Seems to me that Cox used to sell an all plastic rocket with two gliders on either side that worked like that. It was cool looking!
 

modeltrains

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It's the geometry of the top side of a cambered airfoil which creates lift.
It is actually the angle of attack (AoA) that creates the lift....
Those bring this to mind,

https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

HOW IS LIFT GENERATED?
There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.
And this, though from a less rocket-centric source than the NASA quote above,

No One Can Explain Why Planes Stay in the Air
Do recent explanations solve the mysteries of aerodynamic lift?

February 1, 2020
https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/


In Brief
On a strictly mathematical level, engineers know how to design planes that will stay aloft.
But equations don't explain why aerodynamic lift occurs.
There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.



In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What Does Keep Them Up There?” The point of the piece was a simple question: What keeps planes in the air? To answer it, the Times turned to John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum and author of several textbooks in the field.

What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders. At this point in the history of flight, this situation is slightly puzzling. After all, the natural processes of evolution, working mindlessly, at random and without any understanding of physics, solved the mechanical problem of aerodynamic lift for soaring birds eons ago. Why should it be so hard for scientists to explain what keeps birds, and airliners, up in the air?

Adding to the confusion is the fact that accounts of lift exist on two separate levels of abstraction: the technical and the nontechnical. They are complementary rather than contradictory, but they differ in their aims.
 

Rktman

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You can take your helmet off Pat, that is just what it is. :p

Canards are easier to set up because the minor lifting surface is in front and, like others have said here, it is usually designed, through airfoil selection, angle of incidence or both, to stall (lose lift) before the main wing even approaches a stall. When it does, the CG being ahead of the center of lift of the main wing (which is still lifting) naturally causes the nose to drop, reducing the angle of attack of both surfaces and restoring lift to the (no longer stalled) canard. The main lifting surface maintains lift throughout the event. Canard bad drawing:

--- RF^-----------------------CGv------CL^.

The lifting tail is trickier for the same reason the canard is simpler, but kind of just the opposite...:rolleyes:

If we try to use the same logic with a lifting tail (minor lifting surface stalls first), then loss of lift when the tail stalls causes the tail to drop, increasing the angle of attack of the main lifting surface which then stalls itself. Compounding the problem is that the CG is behind the CL so there is no restoritive force; in fact, it makes the matter worse. If we set it up so the main wing stalls first, the nose still drops, but as is typical with the lifting tail arrangement, there isn't much of a resoritive force coming from the tail because the typical set up is to trim for equilibrium in a nice level glide. Another reason the lifting tail is tricky...

The problem most folks have with canards is yaw stability. Typically with a canard design the GC is pretty far aft. That leaves a pretty short moment when it comes to rudder/fin placement. This means that a rather large fin is required, or, as most seem to have, smaller fins out on the (usually swept) wings. Others resort to very high dihedral angle on the canard, but this is less effective in the overall scheme of things.

Again, HTH...

Mike
Once again, thank you @Mugs914! That totally lit up the lightbulb in my head re: how top airfoiled stabs work in RGs and BGs. As a “show me” more than a “tell me” kind of learner, your text illustrations instantly fit the jigsaw pieces together for me. (BTW, very cleverly devised, and cross-platform and cross-browser too). Thanks for the patience and sharing your knowledge on this one, I'm sure there are a lot of fellow glider fans that will benefit from this. (Hope you don’t mind me posting a link to this from the “Glider Design and Trimming” thread).
 

Rktman

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I’ve got a GoPro so might as well use it.


For your consideration....
Appreciate the video (I'm totally a visual learner as well) so this really cemented the concepts for me). And while "a picture is worth a thousand words", a video is worth many times that.
 

Rktman

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o1d_dude

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I’ve got a GoPro so might as well use it.


For your consideration....
Well done.

I have a collection of balsa and tissue similar to yours also but many more HLG and Catapult gliders. They were my thing.

Noticed you’re a fan of Scott Binder Rockets. My Thor will ship next week. 😃
 

Mugs914

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Once again, thank you @Mugs914!
Glad to be able to help! (I try to be good for SOMETHING at least once a day, but sometimes it's a struggle :rolleyes: )

And thank you for your very kind comments! I am flattered to be included on the trimming page!
 

dosco

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You should probably always apply an airfoil of some sort to your tailfeathers.

Back in the day, before there was a good understanding of dynamic stability, the theory (which is completely wrong) of the "lifting stab" was used quite frequently.

As to why it's incorrect, that is the subject of collegiate stability and control (upper-classmen) classes. It's not terribly complex if you're steeped in technical education (much of which has leaked out of my brains over the last 30 years), but trying to quickly explain this stuff in text, online, is basically impossible.

Just do it. If the curvature is on the top or bottom, it doesn't matter all that much, trim the plane's CG accordingly. Find lift, launch, repeat.

-Dave
 

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