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Feb 26, 2015, 11:59 AM
that's gonna hurt...
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Free Flight - stab tilt, how does it work?


Playing around with some small catapult gliders, with some possible interest for an after school program our soaring club supports. It's a high school STEM program which teaches CAD / CAM and has a Laser cutter, mini-mills and 3D printers. So the idea is to get students some projects which can use the tools available to them. And playing with gliders in the gym is a pretty cool benefit too.

I built a couple of test models ( Simple Simon, 8" Sweepette ) which got me wondering about trim.

Specifically, how does stab tilt work to turn the glider? And should the vertical fin be aligned with the wing, or the horizontal stab? It would be good to have an intelligent answer should the question come up.

Thanks
Bruce
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Feb 26, 2015, 02:20 PM
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It depends on whether the stab is a "normal" one at a negative AoA [providing a down-pushing force on the rear of the fuselage to counter the normal forward pitching force of a normal lifting wing, and stably balanced by CG being ahead of the center of lift] .... or, as many old-time FF models and most soaring land-based birds, it uses its horizontal tail to add to the overall lift by having a positive AoA.

In the "normal" down-pushing neg. AoA case, tilting the stab tilts the stabs' neg--lift vector and add a sideways component to that vector. So if in the rear view such a stab is rotated clockwise, there is a left-pushing component to the down-vector, resulting in a right-yaw force.

Of course in the case of a lifting tail, the reverse is true and the lifting stab will pull the rear of the fuselage to the right, yielding a left-yaw force.

In a light "floater" FF plane, there is also a "helicopter/autogyro-blade" rotating effect when the plane is in a mushy almost stalled out descent. A clockwise tilted stab, as above, will tend to rotate the body counterclockwise in the top view, a left-yaw force, more of a rotating descent.

If the stab is in front [so-called, misnomered "canard"], it will have a positive AoA, and a rear-viewed clockwise twist will give a right turn.

[I used such tilting to control my toy RC "Airider" design: photo].
Last edited by xlcrlee; Jul 23, 2017 at 08:07 AM.
Feb 27, 2015, 04:17 AM
B for Bruce
BMatthews's Avatar
While what Lee posted is right you'll need to search far and wide to find the very few FF contest models that use a forward enough CG location to have stabilizers that lift negative compared to the wing. They generally have a more rearward CG location and thus the stabs are lifting upwards to balance with the wing's lift so the total lift is located at the balance point.

Since the stabs are flying with a fair degree of lift we can angle the stabilizer and make it lift somewhat to the side and use this for part or most of the turning effect.

FF power models and both hand launch and catapult launch models fly extremely fast in the climb. If we tried to use nothing but fin tab to "rudder" the glide turn there would be far too much turn in the climb. So we set the models up with 3 to 4 degrees of stab tilt to get most of the turn and then use the "rudder" (slight curling of the fin's trailing edge) to fine tune the climb and transition to the glide.

The bigger glow powered models that don't rely on auto tail functions also use stab tilt to generate much of the glide turn and again minor trailing edge tweaks of a trim tab to adjust the fine trim of the power climb.

You also asked about the fin being either vertical or perpendicular to the stab. It really doesn't matter one way or the other. I tend to make them vertical just so it doesn't look like the tail boom is twisted. But as far as flying the model goes the model simply doesn't care one way or the other. Any oddities of a tilted fin will be well buried by the other trimming.

I've also written up some design and trimming information that is over in the Free Flight sub forum. Instead of typing it all over again I'll just give you a couple of links.

Some good general information and some examples of results;
https://www.rcgroups.com/forums/show...apult+trimming

You'll find my notes on design features and trimming in post #6;
https://www.rcgroups.com/forums/show...lider+trimming
Feb 27, 2015, 11:48 AM
that's gonna hurt...
Thread OP
Thanks all for the responses. I have a lot of reading material to go over, which is good.

I took the "Simple Simon" and mini " Sweepette" into the school club meeting this week and was surprised at the positive reaction! I just have to keep myself from doing all the work - you know how hard it is to watch others "learn" how to build?

Bruce
Jul 22, 2017, 08:01 PM
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Stab tilt on xplorer 3.8 f5j ales


I recently broke the tail boom on my Xplorer sailplane.

After careful alignment, in preparation of reattaching the tail boom, I installed the horizontal stabilizer and rudder and center wing to check the axial alignment. However, I noticed a few degrees stab tilt inherent to the model. The left side of the horizontal stabilizer was higher than the right side.

Is this normal for the Xplorer or is my alignment in error?

I double checked this a few times and kept verifying that the seam on the bottom of the fuselage was straight. I keep seeing the slight stab tilt with the fuselage seemingly in near perfect alignment.

Any comment would be appreciated.

Thanks!
Jul 22, 2017, 09:45 PM
Registered User
Quote:
Specifically, how does stab tilt work to turn the glider? And should the vertical fin be aligned with the wing, or the horizontal stab?
is it something like this one?
Sweepette 16 hand launched glider (0 min 22 sec)

I would assume the Tail's H-Stab is pushing down, The faster it goes the more it pushes down. if the H-Stab is not level with the wings. the line in which it pushes down will be off to one side, this side it is pushing down to is where the tail will go. causing the Glider to turn.
from the bottom of the HStab, imagine a line 90deg from it, if that line goes down and to the left of the plane the aircraft may tend to turn right.
Jul 22, 2017, 09:53 PM
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Quote:
Originally Posted by xlcrlee
It depends on whether the stab is a "normal" one at a negative AoA [providing a down-pushing force on the rear of the fuselage to counter the normal forward pitching force of a normal lifting wing, and stably balanced by CG being ahead of the center of lift] .... or, as many old-time FF models and most soaring land-based birds, it uses its horizontal tail to add to the overall lift by having a positive AoA.

In the "normal" down-pushing neg. AoA case, tilting the stab tilts the stabs' neg--lift vector and add a sideways component to that vector. So if in the rear view such a stab is rotated clockwise, there is a left-pushing component to the down-vector, resulting in a right-yaw force.

Of course in the case of a lifting tail, the reverse is true and the lifting stab will pull the rear of the fuselage to the right, yielding a left-yaw force.

In a light "floater" FF plane, there is also a "helicopter/autogyro-blade" rotating effect when the plane is in a mushy almost stalled out descent. A clockwise tilted stab, as above, will tend to rotate the body counterclockwise in the top view, a left-yaw force, more of a rotating descent.

If the stab is in front [so-called, misnomered "canard"], it will have a positive AoA, and a rear-viewed clockwise twist will give a right turn.

[I used such tilting to control my toy RC "Airider" design: photo].
This is what I was also thinking
Jul 22, 2017, 10:44 PM
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Quote:
Originally Posted by xlcrlee
If the stab is in front [so-called, misnomered "canard"], it will have a positive AoA, and a rear-viewed clockwise twist will give a right turn.
Why do you consider "canard" a misnomer? It's pretty consistently used terminology.

Is everybody else in the formation out of step? Or is it only you?
Jul 23, 2017, 12:52 AM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by HX3D014
is it something like this one?
https://www.youtube.com/watch?v=Em6n9Dtzin8
I would assume the Tail's H-Stab is pushing down, The faster it goes the more it pushes down. if the H-Stab is not level with the wings. the line in which it pushes down will be off to one side, this side it is pushing down to is where the tail will go. causing the Glider to turn.
from the bottom of the HStab, imagine a line 90deg from it, if that line goes down and to the left of the plane the aircraft may tend to turn right.

The problem with your thinking on this is that you think the stab is pushing down. But on models where the balance point is behind roughly the 27 to 27% point the stab is lifting upwards. Only when the balance point is strongly in front of the wing's 25% balance point is the stab "lifting" downwards.
Jul 23, 2017, 03:08 AM
Registered User
Yes Bruce, what you said is true . but I did question the model used and suggested that if it were that one......
Jul 23, 2017, 05:19 AM
Registered User
Bruce, I am new to the popularity of such a setup. I believe a great discussion on how Dynamic stability is achieved could ensue. Here is a video that is in line with what you are saying.
How to Build a Balsa Glider - 12" Sandpiper - Part 3 (11 min 14 sec)

I will only watch from now because I don't know what the op intends with his indoor sling launch glider is.
Jul 23, 2017, 05:47 AM
Registered User
Eagles and similar broad-winged raptors typically sweep their wings forward when circularly soaring in thermals, moving the CL forward and enabling their tails to also create lift at a positive AoA. Note how she uses her lifting tail as a rudder: at first shown giving an "outside"-resulting yaw force (rear of body pushed towards inside of circle) to compensate for too much drag on the inside wing which is normally kept at a higher AoA than the outside wing (very important when circle radius approaches the size of the wingspan, resulting in the outside wing actually flying faster than the inside wing). Then you if you look very carefully can see the yaw action of the tilted lifting tail act in conjunction with the separate "aileron"-twisted wings >


Bald Eagle Soaring (0 min 27 sec)




action much more obvious with the Redtail


Red-tailed Hawk Soaring, 7/11/2014 (6 min 7 sec)



Note that when flapping, birds will normally push down and depress their tails in synchronized co÷rdination with the powerful downstroke (to avoid the energy-wasting body-tilting "push-up" effect) -- on the upstroke the wing and flight feathers bend downward like a collapsing umbrella, an excellent design-solution -- and thereby use all 3 members (2 wings + tail) similar to a squid to mutually push air downward
Last edited by xlcrlee; Jul 23, 2017 at 08:00 AM.
Jul 23, 2017, 06:10 AM
Registered User
Such amazing Beauty
Jul 23, 2017, 06:19 AM
Registered User
Quote:
Originally Posted by jruley
Why do you consider "canard" a misnomer? It's pretty consistently used terminology.

Is everybody else in the formation out of step? Or is it only you?
Your first guess is correct

When my engineer dad was working at Curtiss-Wright they developed the https://wikipedia.org/wiki/Curtiss-W...XP-55_Ascender (they then referred to it with a slight alteration in pronunciation, changing the "c" to a second "s"). Years later I asked him what they had then called "the thing in front". He told me "the STABILIZER"!!!!

Having grown up reading his many aeronautical engineering books, I well-knew that "Canard" referred to a design configuration. The French word used to describe the Santos-Dumont 14-bis's duck-like appearance was "Canard", French for "duck" >



However the American culture being what is is, with practical Americans preferring simplicity, just as a TV remote-controller winding up being called "the remote", so have mostly NON-engineering types and those new engineers with minimal interest in historical Etymology come to shorten the correct nomenclature of "the thing in front" from the "Canard stabilizer" to simply "canard".

However it is today on some subsonic aircraft STILL often the canard-configuration's horizontal stabilizer, though MUCH more often instead today used on supersonic jet fighters as an important flow-control device for the actual wing
Jul 23, 2017, 07:05 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by HX3D014
Bruce, I am new to the popularity of such a setup. I believe a great discussion on how Dynamic stability is achieved could ensue. Here is a video that is in line with what you are saying.
.....I will only watch from now because I don't know what the op intends with his indoor sling launch glider is.
In one of the test glides he prints over the video that the model turned due to some tilt in the tail. I would suggest that this is bushwa and the turn that appears so soon after release is due to some skid angle in the launch. Tail tilt DOES turn the model. And can do so quite well. But it simply does not occur that radically and instantly at the launch point..

When I build my hand and catapult launched chuckie gliders I start by deliberately building in some serious non symmetrical features. These models are intended to turn in one direction only so we want to twist and tweak this way and that so we get a good stable and self correcting trim setup for that turn.

Stab tilt is one feature of this. I launch right handed so I set up my hand or elastic launched gliders to patter over to the left and after the transition to circle to the left in the glide. This means the left side of the stabilizer is often 5 to 7░ higher to induce a good left hand turn without the need for a lot of left rudder tab. Too much left rudder tab would prove to be an issue during the climb.

Another feature is that I angle one dihedral joint to induce some washin on the left hand (inside of the glide turn) wing tip. Or if the wing is a V dihedral wing like the Sweepette I angle the center joint and glue the wing on "square" so the dihedral joint is actually running slightly diagonal to the center line. Again this induces a little washIN on the left wing and a little washOUT on the right wing. This fights a turn to the left but it's easy to use the stab tilt and ruder to force the turn. The upside is that a "twisted" wing like this tends to self recover from any stalling that occurs.

I also set the CG back a ways so the model is only slightly positively pitch stable. This encourages a good climb pattern and good transition vs a model with a too strongly forward CG location.

After that it's a little tweak or three this way and that to get a good climb and low loss transition.


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