Catapult glider

[Glen]

I love all that above help!
I'm excited to make the next couple gliders to test larger tails with less dihedral.  
I will make a couple planes and post on here.
  (I have work for a few days)
Thanks!

[OZPAF]

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 model

I'm afraid this is not correct. V tails have been around for quite a while and have been fully analysed - both in the full size and model worlds. V tails are and were originally designed to place the 2 rear stabilizing/control surfaces into one corresponding surface, which needs to be of the same total area. The V angle is necessary to obtain the separate functions of FIN/STABILIZER, with the V angle determining the relationship between these surfaces.

Both a conventional rudder and the rudder section of a V tail (no ailerons) produce a yaw in the required leading to a yaw induced roll. Neither roll the model into a bank directly - the yaw they generate produces the roll.The side force required  is  centred about 40% of the span of the fin or V tail from the root,   pushing the tail to the right for a required left turn  - generating a torque about the lengthwise axis(as the point of application is above the axis) rolling the model to the RIGHT .

Next surprise is that this counter turn torque is stronger in the case of a V tail as the required rudder deflection also induces a lift(vertical) force and an extra counter turn torque as well. Ok this can be used to provide the necessary pitch trim - it's just a bit harder to establish.

So how does the model roll in the required direction? This happens because the yaw moment about the CG is much greater - but the penalty due to the need to over ride the counter roll, is a small amount of extra drag. This extra drag as small as it is - is higher for the V tail.

Mike Thomas also mentioned the V tail and sub fin used on tip launched gliders.

It was found that with the RC Discus Launched gliders - that if the vertical fin area was not balanced top and bottom of the boom - the side forces on the fin generated while the model was swung around, would twist the boom. Eventually it became universal to use balanced fin areas (to avoid the twist) which allowed a lighter boom to be used.

Tip launched gliders followed this direction but split the fin area between a flat V tail on top of the boom and a under slung fin - which made adjustment a little easier. Successful TLG's have been made with conventional fins balanced above and below the boom.

John

[Rekitus]

Hi John,

I have changed my thinking based on cause and effect as described by Glen.
From just less angle of the tail getting more turn
rudder effect seems good enough there. 
I think I am confident enough in what I do and don't know
to have a lucid discussion so please share your thinking or understanding.
with any luck I will learn more and better. 

Glen, I ask you to avoid using the word dihedral when talking about v-tail angles
I think it adds confusion.
To be specific I may have not understood you and I may have said things that didn't make sense to you.

I think you are close to the goal you want.

victor

[Glen]

Sometimes I re use wings that I think have some "fly" in them.
   I made two Frankenstein models to try some larger stabs. I think what I saw in flight tests were a larger diameter flight circle, but not in a bad way.
     Difficult to isolate because the blue plane at times was willing to turn the wrong way. I accidentally broke it in the car so inconclusive results. The red plane made nice wider circles and seemed comfortable so the red plane might be a good one.
  I can imagine seeing a conflict with the tilted rudder trying to twist "right" while yaw tries to turn "left".
Also, I thought the "nose up" while it's turning was where it might see drag but in a floaty way?. I understand there may be more drag.  I don't see a way to build a model that's "straight" if it turns that's drag right,?

[Glen]

I made a new wing planform. I'm wanting to move the center of lift forward. I'm wanting to move the center of wing "area" forward. I feel these planes need to be able to just "sit" on the wing. A breeze can conspire to freeze the plane, suddenly zero airspeed. Presently the wing does a nice space-shuttle for a few feet then "starts up" again.
     These planes seem so small they benefit from floating on the wing sometimes. Is that a "thing" or am making things up again?
 I liked the new wing in its first test.
   Question: when the planes get lift or get into lift do they slightly increase speed? I can't tell if it's because I get excited but I think I see them pick up speed? Is it because "lift" is an additional force that actually does speed the plane?

[Glen]

Trying to decipher what you guys are saying:
   Less tilt of the v-tail will reduce the adverse roll effect, by turning the effect more towards lift.
   A longer moment arm to my tail will increase yaw effect without increasing adverse roll from v-tail?
   Is that what I hear? I will work it into my next glider

[Rekitus]

hi Glen,
The entire weight of the aircraft is balanced at the center of gravity.
so yep the plane sits on the main wing.
The "nose up" description has my attention.
I have read thermal riders do just that as they enter the thermal.

Am gunna stroll out on a limb and state adverse yaw happens after
a yaw event happens.  think of it as an event that is the resistance to yaw.
It can/may offer a way to add stability. It also can give you a spiral of doom.
god knows I've seen that a lot...

and you said adverse roll caused by yaw...
this idea is not impossible. 
but I do wonder how to build and fly that aircraft.
 
the main wing dihedral and rudder are linked.
should the rudder shove the tail to the left (we are heading right...)
the left wing gets pushed a little faster and gives extra lift. lifting the left wing.
giving roll.
Should you stay on the rudder and the plane will spiral, corkscrew, crash... etc
so the rudder itself is not going be a way to circle flight the plane.
That job is done by the stabilizer.

and just to make your life difficult, the reason I would avoid
v-tails is from difficulties in making adjustments at this point in your efforts.

If it can fly straight, add a little weight to the inside wing-tip
is all you need to get a circle.

victor

[Glen]

I appreciate that. I'm sure you are correct, I can see that as the most reasonable way forward. Of course I am frustrated having seen the turning effect and nice glide first hand. Also I remember the one or two that flew right off the bench (seemingly straight and balanced) but I also had a few that I continued to work on the wing to improve.
   I understand the center of balance is the center of everything. I had a wing shape that I don't feel I could get my balance back far enough. I too am tired of failure on these planes.  I will probably try a few more.
   When I change to a new glider I will post it on here.
This is another "newest" wingplan model.
       Many contradictions with this concept I tried. Some of the best planes were not light and had stiff stabs but the opposite, one my best planes was light and had thinner, noticably flexible stab (when bending by hand) , but launches and flies similar. I have always had concerns that I was simply being lucky and can't recreate what I thought I had.
    

[Glen]

I had the gliders on the table for this picture, I was "flexing" wings when I discovered that my best two planes also had the stiffest wings. I'd never thought to test bend the wings and compare against each other.
    My favorite (green with teardrop fins) flies the prettiest. I almost don't even need to get to the field to realize the flexible ones will show more issues at launch. I read the book from Crabby so I've since learned the best balsa grade for wings, thanks Crabby!

[Rekitus]

I have built, lets say a bunch of,  model airplanes
and all have needed different trimming to fly and with one
exception that was a catapult type that went straight and far.
A white wings paper and balsa kit.

you can expect to adjust them with few exceptions.

I canna tell you what way is best.
I would play first with nose weight first.
if it acts like a dart... remove weight from the nose.
and frankly, I often over weight the nose and  remove weight until,
well in the case of the warbirds I like, until the plane starts scalloping.
then put a little weight back on the nose.  this is a hunt for the CG the wing and decalage like

While that trim point for a good CG is going, turns are common... for me.
and all sorts of things cause that.
if everything looks straight try wingtip weights.  go real slow here.

once it flys straight and level then weight a tip for a circle.

I have never been able to shoot 'em straight up.
and don't that look cool!?

you are very close.

victor

ETA:  in the Mad-Mac thread is an attachment for how to trim these catapult gliders.
read and try that.
 I read it after my typing the above... It seems better than my post.

[OZPAF]

I can imagine seeing a conflict with the tilted rudder trying to twist "right" while yaw tries to turn "left".

Less tilt of the v-tail will reduce the adverse roll effect, by turning the effect more towards lift.

Tilting the V tail to the right does generate a side force to the right , on the glide(negligible on the high speed launch) when the tail carries a small lift load, and this force has 2 effects -
- it pivots the glider around the CG in the yaw axis (looking down in a plan view) - pushing(yawing) the nose to the left and rolling the model into a left turn with left bank. The left wing needs a bit of wash in to balance the turn.
-  A much smaller torque in comparison is generated by the point of application of the lift (tilted tail) or rudder force which would be around 40-45% of the tail span from the root of the surfaces. The torque in the rolling plane is generated by this force x the point of application to the longitudinal or roll axis. The lever arm for this torque may only be 0.5" in your case while the yaw moment lever is likely to be 2-3 ". Thus this counter turn roll affect is small compared to that of the yaw and unlikely to be the cause of a lack of turn, I think you should stop worrying about it.

Lack of turn is more likely due to errors as mentioned before - skewed wings, different sized halves, slight incidence differences between the panels. All providing that you have enough dihedral.

  Question: when the planes get lift or get into lift do they slightly increase speed?

Yes this does happen. The vertical speed component of the lift changes the direction of the mean airflow onto the wing,  momentarily tilting the wing total lift slightly forward - accelerating the model. It's a momentary effect until the model stabilizes to the new airflow direction. Good eyes to catch that.

I agree with Victor that the attached in the last post by Bob on the Mad-Mac is a good description of trimming a CLG.

Victor the only comment I would have re your last post is re "playing with the nose weight first" .

It's only necessary to have a safe - ie non lawn dart, glide by adjusting the nose weight and possibly the elevator slightly, adding a small amount of wash in on the left wing, before starting to trim the elevator and rudder for the launch. Trim the launch first then adjust the glide without changing the launch settings, by removing( or adding) small amounts of nose weight(pin head amounts of clay). This barely moves the CG enough to affect the stability but does alter the trim angle of attack of the wings to either slow the glider or stop it stalling.

I keep slowing the glider down - for a still air trim - until it starts rocking the wings gently - the onset of dutch roll, or until it starts to develop gentle stalls, which are killed by replacing a small amount of nose ballast.

to have a lucid discussion so please share your thinking or understanding.with any luck I will learn more and better. 

you're both welcome . As a more knowledgeable person than me quoted - "Knowledge is useless if not shared".

John

[OZPAF]

Glen, Victor,

Here is a very good introduction to understanding how Aerodynamics relates to models of all kinds. The basics are well covered and although some simple maths is included - it is not overbearing.

Here is a link to a free download of one of the early editions

https://archive.org/details/ModelAircraftAerodynamics

The first edition - 1978, of this book helped me to understand and particularly to cut through a lot of the misleading Modeller generated terminology such as "semi symmetrical airfoils".

happy flying

John

[Rekitus]

that book looks familiar...

hmmmmm....I have no idea or memory if I bought one or rented it from a library
of if I was the library and lent it out.

well, I've been packing things and unpacking things...
maybe it is in with the SF.

oh that was a long time ago...

I think it was in that book that gave me the idea that
model aircraft dynamics are not as well understood as the larger stuff.

I decided the grant money went into manned aircraft.

the discussion of Reynolds numbers not working at small and slow,  
I recall thinking something like you guys dropped it there?  who needs that knowledge?

well maybe computers figured it all out.
says the glorified file clerk.

victor

ETA:  there are parts that look very familiar and absolute blanks for other parts.
a diverting read for me.
thank you John

and I want credit for avoiding airfoils in this thread.

[Glen]

Hi guys. Thanks so much everyone! That's great. I love the link to more info , I so appreciate it and definitely will partake. I'm out of town for a couple days. Today i tried to fly the 4 or 5 gliders I've been working. So frustrating how I thought they "pretty good" but they weren't today. I'm wanting to build a really good one but unable.
In a couple days I can make new gliders. Thanks for the technical help, I really try to consider and incorporate everyone's advice.

[OZPAF]

As Victor mentioned previously - I would stick with one glider - the best of your efforts to this point(The green tipped one?) and work to improve it. Too many changes are not helping at this point.

the discussion of Reynolds numbers not working at small and slow, 
I recall thinking something like you guys dropped it there?  who needs that knowledge?

Not quite sure what you mean here Victor - but the following is a brief coverage of the lower Re numbers effects on CLG's

I have not said much about Reynold's number as the original problem was more about trimming the CLG, and Re does not affect that directly but does act to reduce the performance as it reduces.

One of the most critical effects is the lower max. lift and higher drag of the wing and tail airfoils. To maximise the performance at these lower Re numbers - the wing airfoil needs to be thin and of low camber to reduce separation and thus drag, enabling a reasonable max. lift before the drag increases dramatically.

Tail efficiency is also reduced but at least it is not required to produce much lift to balance the wing, as with an aft CG the wing is almost self balancing (moment from the lift almost balancing the nose down pitching moment) , with the resulting unbalance easily handled by the tail due to it's long tail moment.


John 

[Rekitus]

.... clip...

the discussion of Reynolds numbers not working at small and slow, 
I recall thinking something like you guys dropped it there?  who needs that knowledge?

Not quite sure what you mean here Victor -
.... clip...
John 

to overshare a bit...42 years ago...

Hi John,

To stay on the catapults and explain what I saw.

If possible, consider that I think a 2 inch by 8 inch flat airfoil balsa wing with
a little dihedral does not stall. These cats are about that size.

Back then, I was puzzled by that observation.
You see to learn, I was trialing wing planforms for stall characterizations.

I got no real confirmations for a hersheybar type planform stall or with tapers or long triangles.
it was pretty clear that putting trailing edge tapers onto any wing form flew further.

So I had a sense of well, what did I learn?
that a wing with no dihedral will roll to 90 and fall.

I went back to the books and found no explanation.
that was when I understood I was looking at a hole in human knowledge.
... convince some one you know it...
My honorific was the same then as now, Mr.
Tell me how that would look when the person stating it
is standing in a pair of rubber canvas athletic shoes (chucks)
and does not yet understand Reynolds numbers.

so I built bigger planes and waited for the answer.

victor

[Mike Thomas]

This DVD from the NFFS is well worth getting.     https://www.freeflight.org/product/how-to-build-and-fly-catapult-and-tip-launch-gliders/         

One comment that I would like to add to the discussion relates to launching. Ozpaf has correctly stressed the importance of launch bank and angle (elevation). I would add that after initial trimming with a few hand glides (the DVD is very useful on this), all catapult launches should be at close to maximum stretch. You want to get as high as possible on each launch in order to allow the model enough height to settle into its flight pattern. Randomly half hearted launches of the type that characterised my early attempts at trimming tell you little.

Unless you are tall with a long armspan, you cannot stretch a 9" loop of rubber to anywhere near its maximum, which is at least 8 times with Tan SS. I have to shorten the loop to around 7.5" to get anywhere close. As with winding rubber motors, e.g. in F1B, F1G, and P30, the power is in the last few turns, the last few inches of stretch. The rest is the cruise phase. With the launch of a CLG, the rubber is in contact with the hook for a fraction of a second; it is only that last little bit of stretch that counts.

[Rekitus]

that snap at the end of  the stretch is that valuable?

victor

[stupid]

  Unless you're flying at a contest, there is no limit to what type of rubber for launching that you use. Getting a stock of 1/4" FAI rubber isn't cheap.
   If you are in the USA Safeway stores secure their lettuce stocks with the purple rubber bands, (Signature brands). Much stronger than FAI rubber, and it's free. No one minds if you slipped the rubber bands from the celery stalks in the store, that's how I get mine.
  Use 3 looped together and if you cannot stretch it fully, wrap the end around the top of the launch stick, until you get the rubber to its maximum stretch.   
       Have Maximum Fun

[OZPAF]

I was given some old unused FAI(black) 1/4" rubber which I use to launch my CLG's. The most I can pull this is around 20-27"? ie around a ratio of 3. This was enough to get my 12"WS CLG very high.

I could possibly stretch it further if I used a more modern approach - placing the hold point midway along the fuselage - however as these were originally designed for beginners/kids - I haven't bothered.

As an alternative to the FAI  rubber I have used 3 joined No 64 office rubber bands - similar to Bob, for the kids and they give a good launch. not as cheap as Bob's - but 5$ for a bag weighing 100g  is pretty good.

Victor - we are wandering far off Glen's original questions - so perhaps if you started a thread in the aerodynamics section it could be discussed in more detail.

John

[Mike Thomas]

Paul Lagan wrote a very good article  that appeared in Free Flight Downunder in 2017.  Yes Victor, the "snap" is important.

I agree that having maximum fun is important, and it is true that not everyone derives their enjoyment in the same way. But from what I have read on your current thread, and the thorough and skilfull way you have gone about building the elliptical dihedral model, I cannot imagine that that wouldn't lead you to want to take advantage of what the experts in the field ( and emphatically I do not include myself in that category) have to say about trimming. You do not have to be interested in contests to take advantage of what competitive flyers have learned.
I can't comment on the quality of Safeway rubber bands, but the cost of 18" of FAI 1/4", or 36" of 1/8th is pretty small in the scheme of things, and for CLG it will last for a lot of flights.

[Mike Thomas]

John, I think that the aerodynamics and the test flying are inseparable. The flying is the test of the hypothesis.
 As an aside, since you mention it, the black FAI did reach maximum energy at a lower stretch, which meant you could get the model high without having long arms. It was a consequence of the recipe. But as its primary use was for rubber models, not CLG, it was superceded by FAI Tan SS which was stretchier and reached a comparable maximum energy at higher turns/greater stretch and therefore a much longer cruise, irrelevant for CLG.
I apologise for not expressing myself clearly when I said that "you cannot stretch a 9" loop of rubber to anywhere near its maximum". I was thinking in the context of currently available high quality model aircraft rubber.

[Rekitus]

And I get a whole bunch of truly good info for the CLGs.

I have some balsa here... Am likely to try this.


Hi John,
My post was pretty much about the gliders Glen is building.
I did try to explain how the stalling looked and edited it out.
That really distracted from expanding on the thoughts I wanted to share.

The part I hoped was not missed is that you said reducing drag is a clear benefit.
I did confirm that. the wings got a coat of dope as weather proofing and I sanded it
with a 400 grit to unbump the high spots. if any of that reduces drag.  (ETA: past the trailing edge tapers)

I have little doubt Glen will get his v-tails circling high above us.
His descriptions have me thinking his CGs might be involved in his variations
and I am not sure enough to comment because just the different weights can look similar.

reports from the other side of the clouds are welcome.

victor

[stupid]

I cannot imagine that wouldn't lead you to want to take advantage of what the experts in the field ( and emphatically I do not include myself in that category) have to say about trimming.

   Mike,

     I don't usually get into conversations about the hobby. Everyone's entitled to their opinion. But I have to make a comment about what makes a expert in the field of catapult launched gliders. Being that there isn't a institution offering formal training, and having a license from AMA is not a qualification. I find it hard to determine who is an expert on trimming catapult launched gliders.

  By "trimming"   I'm assuming you mean the procedure we use to determine what adjustments are necessary for flight without looping and crashing behind us or going straight up and straight down, known as a figure 1. 

   Once you have achieved flight without catastrophic failure, further trimming is just a matter of degrees, getting your glider to do what you think it should. If you cannot get your glider design to perform as you wish, then you need to look at improving your glider design. Then the question becomes, how do you improve your glider design to fit your venue, or flying conditions. Which then would be a very interesting topic for discussion.

    Further, Joe Mekina's STRAIGHT-UP article. The last 2 paragraphs gives you an indication that there are different types of thermals, and  that gliders need to be designed and trimmed in a different way to fly in a specific type of thermal. (Reading between the lines)In the same respect you wouldn't expect an indoor glider to fly successfully in a brick lifting thermal.

   There is a wide range of thermal activity, from evaporating morning dew on a sunlit grass lawn, to a fast-moving brick lifter on a windy day.. You wouldn't expect one glider design to fly equally well in these varying conditions. But you could possibly get 3 different designs to fill in the complete range. What would be the examples of the 3 designs and how would you go about trimming them to fly at optimum performance? Can you find any experts that have documented their observations?
                                Bob

[Mike Thomas]

I cannot imagine that wouldn't lead you to want to take advantage of what the experts in the field ( and emphatically I do not include myself in that category) have to say about trimming.

   Mike,

     I don't usually get into conversations about the hobby. Everyone's entitled to their opinion. But I have to make a comment about what makes a expert in the field of catapult launched gliders. Being that there isn't a institution offering formal training, and having a license from AMA is not a qualification. I find it hard to determine who is an expert on trimming catapult launched gliders

As far as I know, the definition of expert in any field does not depend upon having formal training from an institution and a licence.  Experts are people, generally, who are, because of their proficiency, highly regarded by their peers. This does not mean that they have nothing more to learn. On the contrary, experts are generally people who are acutely aware of their own limitations. The link I gave earlier to the NFFS DVD, "How to Build  and Fly Catapult and Hand Launch Gliders" names Bob Stalick, Tim Batiuk, and Stan Buddenbohm as contributors. They, I think, may legitimately be regarded as experts. Tmat, who is a member of this forum, I think deserves to be regarded as an expert in many of the aeromodelling classes, including CLG.
Joe Mekina, whose "Straight-Up" article you reference is, I think, an expert, and deserves your reference.

In your last paragraph you end by asking "can you find any experts that have documented their observations?" My answer to that is "not yet". But it could turn out to be you.

P.S. Coincidentally, MarkM has just posted about trimming his Mosquito on the "what did you do  aeroplane wise today " thread, and thanks Ivan Taylor for his "expertise" in the trimming process  (replies 341, 342. and 343). Ivan Taylor certainly qualifies as an expert, both as a scale modeller, and as an F1B/ F1G flyer. He represented the U.K. in the 1983 Free Flight World Championships held in Australia. I happened to be one of the two timekeepers  on his fatefull first flight, and very privileged to watch an expert in action.