Catapult glider

[OZPAF]

Victor I suspect the fuselage past the stab is where Glen holds the model for the launch as I did the same thing.

Excess stability hurts performance at both the high and low speed ends of the speed range. For a FF model with fixed surfaces it is a challenge to find the optimum stability level - it is very close to just enough - which in many cases needs to be increased for windy weather.

I do need to find and settle on a planform. I would say my glider does the immelman launch.

This is usually an indication of too much angular difference between the wing and tail. Varying the elevation of the launch - reducing it can help, but I would suggest in this case bending the V Tail tips down - a small amount over a number of attempts until the glider travels almost straight up finally ending in a stall and dropping back down almost vertically to the ground. At this point take a little of the "down elevator" out and it should start to pull out into it's glide circle. Once this happens you can then trim the glide -without changing the tail setting(very important) - by removing small amounts (pin head amounts of modelling clay) of nose ballast to slow it down or adding small amounts to remove slight stalls.

Without going too deep into the theory of stability - what does help significantly in operating at these low levels of stability(actually static stability) is to have light tails at a relatively long tail moment - the long tail moment providing good damping(dynamic stability) at a low inertia level due to the low weight.

Our hobby includes craftsman, builders, and those who enjoy the mystique of flight and wish to try and understand it as well as they can so they can produce their own designs with a good idea of how they will perform. I do not knock any of these areas, but will admit I belong to the last group and did actually intend to be an aeronautical engineer but didn't try hard enough. I enjoy reading, and  studying aerodynamics which can be understood and applied without the excessive use of complicated mathematics, and has definitely helped me to understand and analyse model behavior.


John

[Crabby]

Glen one other thing then I will shut up for a while. You can learn to trust your TLAR sensors. (That Looks About Right). My Olde Man was a nationally well known watercolorist, and he taught for decades in colleges and privately. He began by teaching perspective drawing which was hell for some students. After learning the basics in drawing they just had to learn to rely on their TLAR. One can just sense when something is off, or down right see it if it’s obvious. After lots of practice and loving care in your practice, you will gradually enter a world where great practitioners exist. You can chum about with guys like Segovia, YoYo Ma, etc, and will thoroughly enjoy the process. It’s not work. Sermon concluded.

[Glen]

Preaching to the choir. I love it. You guys are so right about this stuff. A boat designer I liked said something like "don't underestimate your eye. Form follows function yes,  but still retains a beauty. Think of a duck in flight or a penguin underwater." Something like that.
      I want to get to the stability element because of course you guys are right. But,  I had a moment where I finally "got it"! I'm almost certain I figured out what's been causing me problems.  (center of lift vs center of balance)
 Yay! Bear with me,

[Glen]

A plane that wishes to turn using a tilted stab only (either v-tail or tilted stab and straight rudder combo) must fly "tail heavy"
..they must have "constant" air pressure on the bottom of the stab in order to affect any turn.
A plane wishing to turn via "tilted stab" only,  must have its center of wing lift be equal to or in front of the overall center of balance.
If the center of lift is behind the center of balance the tail will be "lifted" up from its angle of attack...it won't be "tail dragging" it won't turn.
This results in a glider that mysteriously won't turn in spite of its tilted stab. They fly beautifully...but they won't turn!

[Glen]

I've been mistakenly chasing stab sizes and fuse length and dihedral degrees and wondering why most of them "want to fly" but wouldn't turn.
     And when trying to trim a glider that has (center of lift issues) the addition and removal of nose weight will make you even more confused.
     A "tail heavy" airplane is a bad thing I understand but a slightly tail heavy glider is ready for business! They are very responsive to lift.
    On another subject I tried to film these gliders with an inexpensive drone. It wasn't easy but I feel I can get some video that might be fun. I got 5 seconds where you see the top of a launch transition. In another video the glider comes into frame, seems to lift up over a mogul turns and flies off, really cute. The quality is not super but the viewpoint is fun.
   My son bought me the drone for fairly cheap ($120) I can't believe what the thing does to be honest. For example I fly it to an elevation and direction I like, I start recording and then  just set the controls (phone) on the ground and walk away! The damned thing keeps its position and orientation using gps!
It is the very "bottom end" of drone technology and it shows but still.

[Rekitus]

ah third try to write this post...

I am gunna state what I know and if any questions stand, I'll add to answer.
OZPAF, we are in complete agreement and I learned areo-stuff much the same way.

Stability is easiest to see in flight by watching how dihedral functions.
by putting some angle in the main wing a balancing act created as the plane flies.
left and rt sides lift and may have a small difference.
Roll is stabilized, should one side angle up that side will create less lift.
and the other side will create more lift. gravity is involved :--D
the wings will find a level they like.
a theory is a flat wing will lift more than an angled wing...
if that is true, that difference is a real price to be paid for the roll stabilized wing.

Stability for the other two, pitch and yaw, operate a little differently.
pitch is similar in that the stabilizer gets balanced against
the nose weight and center of gravity and perhaps the center of lift.
For most planes to keep the main wing at a good angle of attack,
the two angled flying surfaces are often 2 ish degrees apart and
otherwise balance much the same as dihedral.
For my thinking the angle of attack of the main wing is always zero.
The stabilizer is in degrees relative to the main wing.
The 2 degrees is by most conventions is declared as negative.
I think of it as stabilizer downforce and "more" would be negative 4 degrees.
I you have a plane that flies at neg 2 and change it to neg 4,
with no other change, the plane will fly slower.
that is a price paid for the increase in pitch stability.

Yaw is hard to show as a balancing act.
the rudder is the only item on the aircraft doing the job
and it simply hunts for the least drag by pushing the tail of the plane
until the two sides of the rudder have equal aero drag.
The rudder is also affecting the main wing if there is dihedral
or the dihedral of the main wing can require a bigger rudder area
to be able to stabilize the airframe in yaw.
the price paid is the weight and drag of carrying a rudder.  
... in exactly the place you'd like to not have extra weight...

one last item... swept back wings create dihedral.
thus a flat mainwing can have some effective dihedral
the rudder is still going to be as large as if you had the same dihedral
by angling the wings up.

that is why the advice to use as little stability (some is needed) as possible.
it costs in weight and drag.

victor

[Rekitus]

won't turn.

too much dihedral
or
weight center left to right
or
aileron effect in the wings
in contention with the above
or
more rudder area

turns are often done with stabilizer and aileron in flight
the rudder not so much.

[OZPAF]

ve been mistakenly chasing stab sizes and fuse length and dihedral degrees and wondering why most of them "want to fly" but wouldn't turn.

My first check in a situation like this - providing you do have some tail tilt / and rudder offset(as set for the launch) for a turn - would be to check that the wings are not skewed in plan(to the right, looking from above, for example),  that both panels are exactly the same size and weight and that both wing panels are not twisted and that there is only a small amount of wash in on the left inside panel.

Actually all these errors will be smaller and more critical on a small glider - I would also advise building slightly larger - minimum of 12" wing span.

John

[Crabby]

So how is the center of lift adjusted? I may be building my models in conflict. I am thinking about my Mini-Maxer which is designed with a v-tail and pronounced neg incidence. It has been a strange flying plane. Only climbing so high and with a county-wide turn radius. Glen you might be onto some higher level aeronautical thinking that might be the magic ingredient that made George Perryman’s planes fly so magnificently. How is this adjustment made? My tail heavy models all commit suicide.

[Rekitus]

ah crabby, I think  you tease...
I can tell you what I have read
center of lift is the average point of the lift of the wing.
often given as the percentage of chord...

I may have more than an intuitive sense of what that means
and I've never been able to test it with my model aircraft...

I may have some time soon to try out an answer...
and thanks.

victor


oops   forgot to say how to.
It looks to me like you can try a different airfoil.
the peak of the top arch is close to center of lift...
if this or if that...

for this size, wing planform is about the best access
I can think would work

[Glen]

I agree wing planform might be the best way to get what I need. Crabby, my theory may be wrong but I'm trying to understand why 2 identical gliders to the eye, both fly great, one turns great, one doesn't.  
 I have had models with good success. This isn't a proof of concept thread. I am perplexed by my model and my struggles. I am building my model, not a traditional tail.
Its my fault for not laying out my design parameter:

I want to build a v-tail glider that has zero incidence. I want zero offset of the stab. I want zero "balsa bending". (all that is for launching)
I want the glider to build fast, because I want the glider to fly well enough that I'm gonna lose it.
   I have built a handful of models that surpass the above design goal. I'm struggling to "repeat' I don't yet understand why the good planes are so good. I feel you are dismissing my questions too quickly. You assume slight wing differences or weight imbalance or any man made mistake. But consider that I've made about 20 of these things. At that number one can start to question what's going on? I feel very few people understand how a tilted stab is supposed to work. (please bear with me) One good way to prove my point is simply ask you to look at my models and tell me how they turn?
I want to address stuff you guys said: I need to take a break for a minute:

[Glen]

You are the only folks trying to help me and I love you for it!. Please don't let anything I say alienate you.
But I think you're driving down the wrong road. A traditional stab and a v-tail could not be more different. A traditional rudder turns the craft on its center of balance (near the center of the wing) by pushing the tail to one side, which increases angle of attack on the dihedral wing being pushed forward. The secondary effect is the increased air pressure lifts that wing thereby turning the airplane. (Of course you knew that)
My glider is completely different. A v-tail does not turn the model on its center of balance like a rudder does. It does not swing the tail out to one side lke I suspect you are thinking. Remember the v-stab is pointed "straight ahead". It has some of the effect of a "straight" rudder
   A v-tail rolls the craft on it axis. (I hope that's the right word) Rolling a plane has a secondary effect of turning. Both methods result in a turn but in a completely different way. I believe it's called a "secondary effect"
..."turning the airplane causes the wings to bank in your models, banking the wings causes my airplane to turn in my models.
(I still want to touch on the great comments and opinions you made above. I will try later to catch up with stuff you said or asked)

[Rekitus]

hi Glen,
I understand your frustrations in seeking flight goodness.
so I will try to answer your questions in a more direct way
and keep in mind that I see you've done some excellent results already.

I think you need more rudder. but...
add some weight to the inboard wing.
this will force a bank.
too much weight will end in a cartwheel so go easy.

we are back to 'just enough'

other advice... a fellow named Konrad has a thread or two about
F3F gliders that sport V-tails.  you might find it interesting reading.
I think Konrad does know the subject.
I find he tends to say things in different ways than I would.

victor

[Glen]

I believe your intuition is correct. I'm finding info on powered freeflight regarding v-tail turn only. They say I must have 25%. For a tilted v-tail to work. I remember you said that before!

[Glen]

Catching up on comments: I'm surprised that too much stability affects both low and high speeds, high speed surprises me.
Yes, in order to fly these are "close to the edge." There is no incedence to help, only a little dihedral. Two dihedrals.
  not surprising stability needs to be changed in a breeze. I can't believe how great they fly one day and look out of trim if they are getting blown around the next.
Light tails, long moment. My best plane is light. It's also my shortest, but I thought that kept its tail light.
(I definitely think I am messing up with my stab being too small)
One of you said more dihedral was more responsive. I'm interested in that?
When I go from 12 degrees to 13.5. my plane banks really steep. I want to experiment more but the 13.5 degree glider flies really good but super tight, fast
 turns.
So, I agree it turns more, but i find it turns too far too steep a turn, maybe a 15'-20' max circle.

[Glen]

Size matters, the smallness may be killing me. I have no doubt you are right.
I try putting weight on a wing sometimes  but  shouldn't  it work without much or any of that?
Maybe it's me but it seems to fly like a glider with one heavy wing. It seems like a fix. I spend a lot of the time with the bad gliders trying to figure them out. I continue to inspect and sand and add camber since they aren't working. Sometimes I will make them better. Most I will ruin by trying to much. (They weren't working anyway) Yes, I've tried bending balsa. Try it on these planes when you launch and it will loop around and hit you in the ass before you know it. Seriously, I know you guys do it very successfully but I can't seem to touch these.

[Glen]

And from way back, Crabby I was trying to go back through the successful gliders. Originally I made a 25% airfoil. More like an airplane airfoil with a 1/3 - 2/3rds leading edge.
 Some gliders had 1/16" stabs, some 1/32".
Anyway, I later fell for an old fashioned glider airfoil which had the camber placed aft (inadvertently moving my center of lift back. Then, I decided I needed to try the 1/3 high point that many guys use successfully (again I moved it farther back!)
 Now seems to me I moved the center of lift., not an insignificant amount  because I'm an idiot.

[Rekitus]

Glen,

I want to be direct. I think you are waaaay out in front of my skills in plane building
so I am trying to stay with what I think I know about aeronautics and NOT how to build 'em.

I think a vee tail was created by some one with a calculator.
so it will tend away from the more conventional trimming methods.

Perhaps the idea is to lower weight as a main goal.
The tail surfaces have to grab as much air as a three feather tail and weigh less.

how big do they need to be? I can't tell you. 
I do know you have some indication that
maybe a little bigger is a consideration, because some turn and some don't.
The other option is to lower the dihedral some to see if you get turn.
I think this option is the easiest and it should get a turn,
if the aeronautics I was given are correctly understood.

For this test plane go with half your usual dihedral.
well, it is what I'd try.

victor

[Glen]

And the hard truth is I may be proving my idea wrong. I may not be able to build this plane.
    Here's a thought on my troubles. What if the tilted stab on the low side resides in wing down-wash while the other fin sticks out like a sore thumb and that is something?

 If a v-tail turns a plane by rolling the wing...then what if I sanded my little balsa fuselage so skinny that  when the stab sends torque down the fuse (In order to roll the wing) what if my fuse loses half of the intended effect because the fuse twists?

[Rekitus]

Hi Glen,
I think perhaps you are at the point in your trimming where
you should pick on one plane for a little while.

Get to know what it does. put alu trim tabs on it and adjust them.

one of the reasons I have tended away from catapults is that they are hard to adjust.

to turn an airplane, you need to coordinate two forces
roll and pitch. (aileron and elevator)
and you can't change either in flight.
one way to do the above is to get a glider to run perfectly straight.
(not easy for my stick and tissue planes)  then add a little weight to a wing tip
until the plane turns.  it is often a small weight.  A bb sized ball of plasticene
is about all you will need... just get it off a level wing flight. it will turn.

[Glen]

I love it Victor. I like what you say about needing to grab as much air as a three feather, but weigh less.
And I'm not "advanced" of anybody. In fact, I'm trying to only focus on one simple glider in hopes I stumble onto something. I have so many contradictions. Three planes identical except dihedral. The one with the most dihedral turned the most, what? Doesn't make sense but this thing could core a squirrel fart. Needless to say I have no idea what's going on.
    I love your thinking re the stab. I want to make larger (you all told me) stabs and I want less stab dihedral for the next go. Hi Retikus. 'just seeing your post, thanks I want to get back to that it's late, thanks guys!

[Rekitus]

Hi Glen,
you ask hard to answer questions.  I guess you can make a fuse of floppy woods
buuuuuut it won't fly...  if it does level, it can turn.
the stabilizer is the power for driving the turn.
the roll you see is from the dihedral balancing lift.
and the rudder is keeping the nose into the airstream of the plane.

the shading of the tail surfaces with turbulence is possible.
I do doubt it is your problem.
the thinking goes like this...if the tail is not functioning the plane will stall
pitch up or down and then dive for the deck.  it may recover on the way down.
planes your size do not generate much turbulence.

you are likely close to your v-tail satisfaction.

victor

[Rekitus]

Hi Glen,
oooof your observations have to be the tail feathers?
with less dihedral being the ruddervators are closer to flat?

so the tail feathers are flatter and that bird turned better?

victor

[OZPAF]

Three planes identical except dihedral. The one with the most dihedral turned the most, what? Doesn't make sense but this thing could core a squirrel fart. Needless to say I have no idea what's going on.

Actually this does make sense. The glider needs to roll into a bank to turn and this is achieved when there are no ailerons by the rudder yawing the model which causes differential lift across the wing due to the dihedral. The roll and turn for a given rudder generated yaw is directly proportional to the dihedral angle.

As the model rolls - it will also start to side slip - inwards towards the centre of the turn and this increases until the stabilizing effect of the dihedral balances the fixed rudder. It's a fine balancing act and that is why with FF models in general - a large degree of roll stability is needed which usually requires a fair amount of dihedral and a rudder which is not too large. That is why I advised you to flatten your V tail to reduce the effective rudder area.

John

[Mike Thomas]

I seem to remember that V tails are good for tip launch gliders; for CLG/HLG the fin/conventional tailplane work much better. What I don't remember is why.