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Feb 04, 2013, 04:13 PM
Gasbags & Gussets
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Notes By Bruce Matthews about FF glider and rubber model trimming


FINDING THERMALS FROM BMATTHEWS (rcgroups post on chuck gliders 12/2012)


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Well, you can simply keep tossing them up until you luck out and get a thermal. Or you can learn to sense the air movement changes and try to toss the glider right into some rising air.

Folks tend to wait for a calm spell and think that it's a thermal. But t'aint so. The calm you feel where the ambient wind drops is often the suction back towards the thermal which is still upwind of you. Instead of the calm wait for gust that comes after the calm. That gust is the addition of the amibient wind velocity of the day and the fact that the air is being drawn up and into the thermal which just passed overhead. When you toss your glider up into the launch and it transitions the first turn should be right into that thermal.

There's other signs as well but it would mean writing a pretty tidy book. But start with this idea of launching into the gust that comes AFTER the calm and be observant about where the lift is located and where the glider gets the most climb. Doing so will aid you in further learning about the qualities of thermals.

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Hey, it was still a thermal. You either missed the core by some amount or it was a small "pre lifter" that came along before the big one. I've seen that from time to time as well.

But you also now have a little more knowledge of what it feels like in the battle field instead of just the theory.

Things you can also do when it's warmer is to watch for insects coming up out of the grass. The warm air encourages them to fly and then they are lifted up. Also small groups of swallows or other small birds darting around in a "furball" is a sign of a thermal as they are eating the bugs that got lifted out of the grass.

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Finally there's the "cheater's way".... When they are in season find a few ripe cat tail flowers that are just shy of exploding into clouds of seed fluff. Wrap the "hot dogs" tightly in newspaper and crimp the ends for storage. To use them at the field simply peel back a bit of the paper and tease out a few seeds with your free hand and blow the loose seeds into the air. Watch them as they move around. Wait for the lull and give it a few seconds. If you blow seeds up into the air just a little before the "fill" blows in you'll see them begin to mix around like they are in a washing machine. THAT is your core and aiming point for a low altitude thermal that is still attached to the ground. When you find that sort of effect you don't even need to throw high and hard. You can actually just loft your model up into this maelstrom so it recovers at around 20 to 25 feet and watch it circle away to Never Never Land.

Since cat tails aren't really in season at the moment another option is fine wood sanding dust. Get a small bag full from a sander and you can just throw small pinches of it into the air and watch the particles for the same signs.

Mick, did you use the tricks along with feeling the wind and temperature or just the wind and temperature? If you managed to nail it unaided then GOOD ON YA! ! ! ! The one you felt was the sort of smaller one that peters out without breaking connection with the ground. Or perhaps you simply missed the core. In addition to watching the wind speed and temp watch for the DIRECTION as well. A sizeable shift in the wind direction along with a speed change can indicate a thermal over to one side of your location. If the winds are stronger then you can just adapt to the new direction and launch and hopefully it gets pulled over to the core that is anywhere up to 50 meters over to the side.

You're also well on your way to having the others consider you as a practitioner of the Dark Arts....

https://www.rcgroups.com/forums/show....php?t=1761799

************************** TRIMMING HLG's *****************************
They look great. Trim 'em as per the previous information and have fun.

A couple of hints for you in terms of chuckies or rubber band catapult launched gliders that I don't think where included in some of my previous writeups.

Because our models are trimmed to fly in circles it's common to "cross trim" a few things to encourage the circling flight path. The following all assumes a right hander launching the models into a left hand circling glide. The idea being that the model climbs from a right hand banked luanch upward with a slow left roll/S turn to slow down at the top in a nose high strong left bank which transitions smoothly to the glide circle by sort of slideslipping the nose down and gliding away. For the lefties out there reverse ALL of this and the following.
You want a little washIN on the left hand wing or wing tip. To get this skew the polyhedral joint so the cut line angles in at the front by around 3/32 to 1/8 inch worth of "toe in". Due to the dihedral this angle forms a higher angle of incidence in the outboard panel. In the event of approaching a stall this tip will mush first and the greater drag tightens the turn and avoids the stall or reduces the number of cycles needed to recover from a stall. If you think about it this also produces a right roll in the climb. But the rudder trim easily counteracts it so not to worry.
To encourage the left turn we typically mount the stabilizers with some tilt. For the left turn you want the left stabilizer tip raised about 1/4 inch. This results in less rudder tab being needed and makes it easier to get a nice 3/4 turn climb pattern. If the stab tilt isn't there the rudder tab amount needed for the left turn size produces too much turning/rolling action in the climb.
Mount the stabs with a 0-0 angle to the wing. You can get all the "decalage" you need for these models just by slightly warping some up trim into the wood by breathing on the wood and holding in some pressure to "steam" the trim adjustments into place.

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They look great. Trim 'em as per the previous information and have fun.

A couple of hints for you in terms of chuckies or rubber band catapult launched gliders that I don't think where included in some of my previous writeups.

Because our models are trimmed to fly in circles it's common to "cross trim" a few things to encourage the circling flight path. The following all assumes a right hander launching the models into a left hand circling glide. The idea being that the model climbs from a right hand banked luanch upward with a slow left roll/S turn to slow down at the top in a nose high strong left bank which transitions smoothly to the glide circle by sort of slideslipping the nose down and gliding away. For the lefties out there reverse ALL of this and the following.
You want a little washIN on the left hand wing or wing tip. To get this skew the polyhedral joint so the cut line angles in at the front by around 3/32 to 1/8 inch worth of "toe in". Due to the dihedral this angle forms a higher angle of incidence in the outboard panel. In the event of approaching a stall this tip will mush first and the greater drag tightens the turn and avoids the stall or reduces the number of cycles needed to recover from a stall. If you think about it this also produces a right roll in the climb. But the rudder trim easily counteracts it so not to worry.
To encourage the left turn we typically mount the stabilizers with some tilt. For the left turn you want the left stabilizer tip raised about 1/4 inch. This results in less rudder tab being needed and makes it easier to get a nice 3/4 turn climb pattern. If the stab tilt isn't there the rudder tab amount needed for the left turn size produces too much turning/rolling action in the climb.
Mount the stabs with a 0-0 angle to the wing. You can get all the "decalage" you need for these models just by slightly warping some up trim into the wood by breathing on the wood and holding in some pressure to "steam" the trim adjustments into place.

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Remember, about 1/4 inch high on the inside of the turn. It may be that if you're relying on simply warping the tail boom that it's not enough.

Yes, the washin or washout is trying to roll the model outwards. But NORMALLY the rudder should have more than enough authourity over the wing to counter the roll easily. Where this idea would break down is if you're still flying the model too fast. Can you try to slow down the glide speed a little more? Keep going by removing some nose weight until it shows signs of a repeating shallow stall. The last point before that was the slowest it'll glide smoothly.

Which brings up another trimming hint I never thought about. Once in the ball park the elevator trim is used for minor tweaking of the launch time and to set where the CG SHOULD be to achieve the desired MINIMAL pitch stability. The nose weight is then used to set the CG to agree with the tail and for speed control.

And when I say MINIMAL above I really mean it. You want to keep bending the stab TE down and retrimming for the glide until the model is VERY hesitant to recover even from a mild stall. Then move the CG back forward just a hair. And when I say VERY hesitant to recover I mean like it goes into a stall at 50 feet and darn near hits the ground before the nose rises thru level of the first recovery cycle. At that point it's so close that it's very easy to go a hair far back and see it stall, drop the nose and simply accelerate and stay in a straight line or even tuck tighter and finally "lawn dart" itself..... the "lawn dart" thing is a sign that you went a little TOO far But hey! NOT BY MUCH! ! ! ! !

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If you toss even a correctly trimmed glider up into a nose high situation like that with little or no bank it WILL stall and dive into the ground/floor. The key is to look at the flight path after the nose drops. Does it dive it at the same angle the whole way? Or does the nose lift up at least somewhat suggesting that if it had more open altitude that it would eventually return to level? No sign of the nose coming up says that you have the balance point too far back and the model is at the point of neutral stability. SOME amount of positive stability is a must or the model will not be able to self correct the flying speed. But you don't want too much positive stability either or it becomes difficult to obtain a good launch and recovery to a nice glide.

You mentioned that this is intended for a competition. Will the competition be for duration or distance? If it's for duration then you need to trim the model to fly in a normal low cieling handlaunch manner. If it's for straight line distance then you're doomed as there is simply no way to be able to launch the model so that it climbs up and in any manner achieves a SMOOTH transition to straight ahead. At least not with any degree of accuracy as to heading that I can think of.

Quote:
When I launched the model horizontally with half force, it travels nearly 10m straight down with a slight turn to right even though I've added weight at left wing. (maybe that I undercambered it more on the left, so lift on the left > on right.)
I suspect you intended to say "straight ahead" and not "straight down". Assuming this is the correct case then I'd say you are at the neutral stability point. And that is simply too much. A straight ahead push which starts the model out with more than normal flying speed SHOULD see the model nose up slightly to a gentle or moderate stall. That's where having some small amount of positive stability comes into play. I'd suggest adding a small bit of nose weight sufficient to move the balance about 3 mm ahead and then warp in a tiny bit of "up elevator" along the trailing edge of the stabilizer by exhaling through your mouth on the wood while holding a little tension with your fingers. This will warp the wood well enough to hold such a trim change. Re-test and look for evidence of some small amount of positive stability.

The dimentions you gave for the hall suggest that it is a very compact hall. So I'm guessing that the flights will be for duration instead of distance. And that makes things a lot easier.

The optimum low cieling flight pattern for a compact hall of this sort is to have the model turn towards the hand direction of the launch. So if you are right handed then you want the model to climb up and transition into a right hand circular flight path for the glide back down. This is the opposite of the typical outdoor pattern where the model would "S" from a right hand bank at the launch up and around to a steep left bank just as the model runs out of energy at the top of the climb and sideslips into a left circle glide.

First thing you need to do is adjust the balance location and get the model so that when you push it into a mildly nose up stall that you see the model recover and lift the nose so it doesn't just dive into the floor from a gentle stall. Once you've got that then you can move on to trimming for the launch and transition to glide.

The way to work to this is to set some washIN on the right wing. That seems like it would roll/turn the model to the left and you're right. But then we use right rudder warping to counteract this and force the model to turn to the right. The yaw/roll couple force from all that dihedral ensures that the rudder is a lot more powerful than the simple rolling force from the washin. I'd suggest you warp the right wing (assuming a right hand launch, so the lower surface at the outer dihedral break is about 1.5 to 2mm high at the leading edge of the wing. Then warp in enough rudder trim to achieve a circular glide that is about 9 meters in diameter. The more open the more efficient it will be. But it's got to be tight enough to fit within the hall comfortably as well or you'll just end up with it riding the walls down all the time.

To begin testing for the transition bank the model about 30 degrees to the right and push it up at about a 45 climb angle with a moderate power sufficient to get it up to around 5 meters. Ideally it'll climb in a bit of a spiral shaped path then pause at the top with the nose high and lots of bank in the wings. At that point the wings will sideslip down until the nose of the model is level and it'll glide on with little or no stall or dive. If it looks promising but isn't quite there then alter the launch angle and bank angle as needed to achieve this. The other option is that it trys to arc over and dive instead of climbing. If so then reduce the bank angle you're holding at the launch. Or it may be that you are trying to set it up for too tight a turn.

Hopefully you can see from this description that the setup you put into the model to direct it into a circular glide path is also what is helping the model to climb in a spiral pattern in a manner that allows it to smoothly transition into the glide.

We free flighters have to do it this way because there's nothing in or on the model that will allow it to climb straight away with no bank and perform a transition. So we have to trick the model into using the built in stability factors to be in the right place and attitude to let the stabilty factors perform the recovery to the glide for us.

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*********************** RUBBER POWERED FF MODELS *********************

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OK, before we get into worrying about rubber lube or other nice to haves it's time to start back at the basics. And the Javelin you built was actually a pretty good model for the "basics". Perhaps dig up the pieces and have another go at it. Or try something that is similar again.

Trimming a rubber free flight model is a multi step thing which you need to do in proper order. No skipping ahead allowed.

First comes test gliding and setting the CG and stabilizer trim for a nice floaty glide. The CG isn't a static thing. You will want to take note of how the model stalls and recovers while in the glide when you force it into a stall with a slightly energetic launch. You want the model to recover by lifting the nose as the flying speed comes back up. But you don't want it to recover to quickly or in a short distance. Instead you want a model the size of the Javelin to recover from a gentle forced stall just as it is almost pulled out and the landing gear skims the ground and pulls it to a halt. Yet when you push it out at the correct flying speed it has a lovely slow glide all the way to a rolling landing. The way to set this is to shift the balance point, or CG, around and compensate for the shift by altering the wing to stabilizer trim angles. Oddly enough when it wants to pull up too quickly and in a short distance it means that the balance is too far ahead. So you shift it back and flatten out the angle between the wing and stab a little to restore the glide. As you move the balance back and re-trim the glide eventually it won't pull up properly from a glide. It's almost at neutral stability. That's a little TOO far towards neutral. You want enough but no more than that. During this glide testing you also want to set in a little rudder trim to start to show a noticable but not sharp turn to the right. Eventually you'll trim the turn to fly the model in 60 to 70 foot circles when flying.

Once the glide testing and CG locations are set it's time to start moving on to the powerered flight trim. You used the rudder stabilizer and wing angles to set the glide. So you don't want to adjust those for the power portion at all. You will do ALL your power portion adjustments by chaning the amount of down and side thrust at the prop block ONLY. You'll start with low winds and get a feel for how it's flying. You'll adjust the side thrust so that it keeps the same size circle during power as it has when gliding. Stalls that occur with only a few hand turns are notice that you need to also put in some downthrust. With some hand winds the model may climb or it may simply cruise at level for a moment before the power runs out. But all during these low powered flights you want to see the model flying under power at the same airspeed that it has in the glide.

As you move from the low power of a few hand turns to the medium number of winds adjust the side and downthrust to again hold it in the same size circle and direction of circle under power that it has in the glide. But as the nose comes up a little it's OK to let the model slow down a LITTLE from the glide speed. But ideally you still want it to fly at the same speed as the glide.

From there it's time to go for high power. Again sneak up on this and be ready for some additional tweaks of the side and downthrust. But usually the models that passed the medium power trim will do fine or at most need some minor adjustments for high power.

You won't see it with a model like the Javelin. But rubber scale models can have designs that will show up some other troubles as the amount of power is increased. If they don't have much dihedral or if the wing is a low mounted one and if the vertical tail is big or placed well back you can run into spiral stability issues when you try to climb at a higher angle. The model can become very tricky to trim with a nudge one way or the other causing the turn to become very unstable. If this happens it suggests that the model is not spirally stable at the higher climb angles. If you run into this there is really no option but to go for a lower power motor that has more turns in it and let the model have a longer and more gentle climb.

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AHRRRRG! I forgot about the washout or in.

For Eratix and any other new free flighters reading this;

Wash IN is where the wing is twisted to make the leading edge higher than the trailing edge at the wing tip compared to the center area.

Wash OUT is the opposite where the wing is twisted from the center out to the tip so that the leading edge at the wing tip is lower than the trailing edge.

Wash IN tends to promote the tip to stall earlier than the root of the wing. Wash OUT causes the wing tip to stall later than the root of the wing.

Actually the best option is to twist the wings in a specific way. Our models turn in one direction all the time. And like oval track race cars where they "weight jack" the suspensions to deliberately make the wheels press on the ground unevenly to handle turns in the one direction well it's worth twisting our model wings to do the same.

The way that has worked out best for me is to warp the inside wing of the turn so it has about 3 degrees of washIN or leading edge high and make the outside wing on the turn dead flat or very slightly washed OUT by just a skosh. Then the rudder forces the turn against the rolling action of the wing.

What this does is produce a model that tends to resist spiral diving into the turn under power. And a side benefit is that it tends to tighten the turn a little when the model is in lift. But mostly if you've ever had a model that gets into a series of stalls that becomes increasingly deeper until it dorks it's nose into the ground setting up the wing with a twist of this sort will damp out the stall and recover to level flight where washing the wings out evenly often will not.

I know it sounds odd to make the wing twisted like this but it's not that odd when you stop to consider that the model is constantly flying in a turn in one direction only.

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Great video Gnu'. Is that your model or just a YT video you found from a search?

Let's look at the trim we see in the video starting with the glide since that's where the trimming should start.

The glide circle is about half the size it should be for best performance. Also it's got a trace of a stall that shows up too often. Opening up the circle and then adjusting the tail angle for a floating but non stalling glide would be job #1.

Next up is the initial power pattern which flies straight away and has a hit of left turn during the higher power portion. That hints that it needs some more right thrust to get it to climb with about the same size circle as the glide... which needs to be opened up anyway.

At least that's about what I see.....

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I always found that tabs only seem to work out up to around a 15 to 20 degree angle. After that they seem to be "stalled" or something of the sort and increasing the angle just doesn't do anything. So I gave up on tabs many years ago and I prefer to make the fin itself adjustable in some manner. With sheet style fins like on the TL I simply leave the rear unglued and use wedges to camber the whole rear 1/4 to 1/3 of the fin over to the side. In your case it might mean having to cut away part of the joint. Once the proper turn is set up and you're sure it won't change a little glue added to the wedging will again secure the fin fully.

With a tighter turn in the glide the left power portion will open up and possibly lead to a stall under power. I'm a big fan of making my models go in the same direction with and without power. I don't get a sloppy transition that way. I'll bet that with the tighter glide trim you would not need much right thrust to pull the power portion around to the right as well.

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For low powered sport flying of this sort there's nothing wrong with left-left either. It's not what I'd do but it won't produce any sudden "arrivals" as long as you watch your trim.

I'm still a big fan of a little washIN on the inside wing. It tends to ensure that any stall cycles will damp out rather than becoming repetitive or even escalate in depth.

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For many years I was under the misapprehension that free flight flying meant fitting into contest classes. Or maybe it was the sort of situation where I felt that I had to fit all my projects into valid free flight classifications.

Scrubs, I don't suggest the preceeding to in any way minimize the worth of your post but rather to point out that we don't always need to build to suit contest events. Especially if we're simply looking for something to fly regularly at some local field that we stand a better than even chance of bringing it back home in one piece.

Frankly at this point if I wanted a good flying model design that fit decently well within a small field and under light to moderate at most wind conditions I'd likely look at some sort of 24 to 30 inch span sport model or simpler scale model and include a light, simple as possible but most of all accurate DT system so that I could control the flight duration and pick a landing spot.

I'd also control the motor run via whatever means that applies to further limit the climb and thus much of the higher altitude increase in drift speed that comes from climbing too far up on a breezy day.

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