Planes

A few months ago Zack and I decided to build an airplane out of one of those cheap styrofoam gliders. So we finally got one, a Cox .049 2-stroke engine, and a radio/servo kit leftover from a previous (totaled) model.

Last week was one of those weeks where you discover that a single charge on the nicads can last through 3 models, because the models are so crapulent that they get scrapped (or scrap themselves) almost immediately. The lessons learned:

• the plane can't be much shorter than the wings are wide or it may simply rotate around the wings as it hurtles through the air.
• the small wings (estimated .75sqft) are not enough lift for a 3oz battery pack (well, maybe at speed)
• the big wings (estimated 2sqft) are almost but not quite too heavy and draggy for the electric motor
• center of balance is important, even if you have an oversized tail (in fact, avoid the crazy lifting tail designs)
• ailerons are still a bitch to hook a servo to

We had a great set of flights today. While we were throwing around some of our more crazy experimental aircraft in the parking lot across from my apartment a while back, one of my neighbors walked up. His name is Matt and he is an airplane mechanic at the local airport. And he's building a really cool scale biplane. So today when we are burning in our new motor he shows up and decides to go out to the farm with us. He coached us and, following his example, we had about 5 successful flights, with very few amazing crashes.

Today Zack, Matt, and I went out to Steve's farm to fly the same basic plane we flew last weekend. It was a beautiful day, but a little windy (pretty consistent wind though). Our first flight we had Matt on the controls to test out our new ailerons. Zack had glued a strip of balsa onto a bamboo rod which went through some straws down the length of the wing. Alligator clips attached to control rods clipped onto the bamboo rods at the center of the plane, twisting the rods and moving the aileron surfaces at the end of the wings.

The design worked great on the ground, but in air, the wings had way too much flex. In the wind, the wings were bending almost imperceptibly but the ailerons were fluttering constantly. The effects averaged out, so they were still useful as control surfaces, but it made for a sketchy flight. The exciting part was when the plane got into a bit of a spiral battling the wind with the crazy control surfaces, one of the wings snapped just past our balsa reinforcement rod (nothing else broke in the landing though). We're going to have to do something to reinforce these previously-unbreakable foam wings.

We flew Steve's simple electric -- it is one of those two-channel (turning and throttle) V-tail trainer kits. Very simple, very light, not really meant for windy conditions. It actually held its own just fine though. I've been having a lot of trouble getting good performance out of my electric setup, but that tiny electric plane was plenty powerful. Once it was up there, our number one problem was that the control surfaces didn't work very well when the throttle was down, and when the throttle was up it was climbing like crazy. Would have been a very nice glider if it had had some sort of pitch control (elevator). I think I'm going to have to try making a plane just a little bit bigger than that and flying it like a glider.

Then we put the smaller (about 3 foot, instead of the 4 foot we'd been using) wing on the gas plane just to see if it'd work. As expected it went a lot faster and actually handled just fine. Maybe was even a little bit less a victim of the wind. The large wing on the electric plane, however, did not work. For some reason it made the plane roll like crazy, even though we had the wing in the center.

It's a real pity I forgot to bring my camera, because in the wind we had lots of great very-low-speed (or actually hovering with the electric) flights that would have made great photos.

We all had a bit of time at the controls and had some great flights. Only two tree landings and fairly minor damage to hardware.

Eventually had to stop flying because the tank started leaking like crazy (I think a seam was just being pulled open by a bent aluminum mounting bracket). Also some moron glued the engine upside down after it broke off.

Steve let us borrow an ancient .35 (we have been using .049) engine. It is extremely enormously fatally overpowered for the planes we've been flying, and it has absolutely no throttle control. I'm not sure there's any way to use it without killing ourselves, but I think the best bet is to put a huge piece of blue insulating foam on it -- something that (by all reason) shouldn't be able to fly. The only question is how to reinforce it (glue wood strips to it?) so it doesn't break up right away.

I've been putting a half-assed effort towards putting together a very light electric plane. I've got a joke of a motor with a small prop that looks like it was designed for rubber power, and a too heavy NiCad battery pack and some three and a half foot wings (about 1.3 sq ft), and (for now) rudder/elevator control. It flew level for a while, but rudder impulses were so powerful they rotated the entire body of the plane relative to the wing (which was held on by a single rubber band). I really like the idea of using a totally underpowered super light and tiny airplane. I built a primitive balance and it seems to weigh less than 9oz, making for a very light wing loading (about 7 oz per sq ft). Most sailplanes have a much wider wingspan (for a better "aspect ratio" -- thinner wings) but also weigh much more.

I did some reading online and it looks like the magic wing loading for small (1-meter) sailplanes is in the 3-5oz/sqft range. I've got these 43" wings I've been using that weigh by themselves about 2.5oz and are about 1.3sqft. So I put together the lightest flight pack out of the equipment I have.. 2x HiTec HS-55 servos (rudder / elevator), 4-channel HiTec feather receiver, and I built a battery pack out of 4x 180mAh nicads I had (4.8V) sitting around for a computer project.

Today Zack reapproached making a straight wing out of the foam gliders (which, in their natural state, have really severe dihedral and back sweep). The one we built in the past worked alright but was a little bit too flexible for the ailerons we had and dramatically ripped themselves apart in midair. The foam is very strong and the wood and epoxy that we used in the center part to hold the two halves together were also strong, but the joint where the inflexible wood ment the very flexible foam went the way of the dodo. So this time he ran a balsa spar almost the full length of the wing, and we used the less brittle hot melt glue instead of epoxy.

Today we took that same plane from yesterday and added servos inside of the wings (just cut a hole straight through the foam) to control ailerons. We also left off the rudder, figuring ailerons would do us. On the bench it worked great and trimmed out well, but when we put it to work in glide tests we ran into a few problems.

The first obvious problem is that I used the battery extension cable to make the aileron Y cable, so we had to put the battery under the wing, and almost an ounce of ballast in the front to get the center of gravity about right. This plus the extra servo added enough weight that the thing didn't glide long enough to get a satisfactory understanding of its flight characteristics.

The second obvious problem is a little more obvious in hindsight but was quite astonishing in person. The thing had a tendency towards extreme instability. Even moreso than our other no-dihedral projects. It would go into a flat spin at the slightest provocation. Duh, when we decided to leave off the rudder nobody ever bothered to reattach the fixed vertical stabilizer! We ran back inside and glued on some sheet balsa real quick, and it improved, but the whole thing was still a little unstable, and the wind, and etc., and I did a knife edge landing and managed to break off the last 9 inches of one of the wings.

A little discouraged that we'd almost demolished our plane during simple glide tests, we switched for the moment back to old faithful: walmart-style foam gliders (we actually buy ours from The Treasure Chest, a wonderful family-owned hobby shop downtown). I had one sitting around pretty much built (not that that takes much), so we just slapped some servos on it and it was ready to fly in about 15 minutes. Instead of the typical plastic U thing to hold the wing on, we used just two rubber bands, and that actually worked out wonderfully.

So we took it to the local park with the big hill and had a great time with it. The winds were unpredictable but mild enough that it mostly didn't matter. The only control issue was that the rudder didn't have quite the assertiveness that I wanted when messing with a strong wind. A couple times we caught the wind just right and actually rode it for maybe 5-10 feet of lift, but not enough to convert that into speed to do a 180 and ride it more. You sure get a lot of exercise running up and down a hill chasing a glider!

The process of learning is quite pleasant. Zack was throwing the glider up the hill towards me, and right before he tossed it I tried to work it out in my head "it's coming towards you, your turning is reversed" but it actually messed me up, already my idea in my head was naturally reversed because I was thinking "in the pilot seat." Now that I've spent some time with an r/c simulator, even after years of normal simulators, r/c perspective feels much more natural, especially when you are playing with ground effects and tight turns and slope drafts. Much easier to keep your relative-to-ground orientation from outside! Another great learning experience was knowing when we're in lift. I'd read from some thermaling pages that you very quickly gain an idea for whether or not your plane is in lift. Even from hours of sims I hadn't really gotten that reflexive feel, but watching the plane on the hill, you could really tell when you got uncommanded lift.

One thing that was a bit of a disappointment is the human eye. I always have a lot of depth perception difficulty in the sims, which is natural for a low quality 2-D projection of 3-D reality. But we were trying to fly the plane into a shelter at the park (some distance away from us) and both Zack and I thought that it had pretty much made it, so I stalled the airplane to avoid ramming the picnic tables. When the plane hit ground, we were a good ten feet away from the shelter. I guess when the plane is 190ft away and the shelter is 200ft away, it's hard to tell the difference.

Another interesting thing I noticed is how different flying personalities are. I mean there are some obvious differences from experiences, but I get the impression that I like the elevator trimmed substantially more "stick back" than most people (at least on gliders) -- definitely moreso than Zack, and moreso than the default on the sims I play. My reasoning is that I want it to go pretty much level even as it loses speed, so that the decision to trade altitude for speed is a conscious decision, so I get speed in bursts for maneuvering and stuff. I think the status quo opinion, though, is that a glider should be trimmed to have a glide path that pretty much maintains speed and slowly loses altitude. *shrug*

We had enough fun that the plan is to put together a second foam glider (we have all the parts here), slap some LEDs on it, and fly some duels tonight maybe with a camcorder.

Last night I was describing to Matt the problems that Zack and I were having with this foam glider. See, the hobby store now carries 3 varieties of foam gliders: swept back super-dihedral, straight mini-dihedral, and totally flat. On Monday we went up to the park with the hill with a swept back super-dihedral and a totally flat one. We found that the flat one had GREAT glide properties, but the slightest perturbation from exactly level flight yielded a dramatic instabilities. I'm talking about things like tail first dives. Zack finally got it trimmed out to where it was flyable, but even so (in the wind we were experiencing) there were frequent inverted landings and spectacular cartwheels. On the other hand it was the only glider to be able to do a full loop from a hand toss.

Now, I'm sure anyone who knows anything about planes, or perhaps anyone who was paying attention to what we were doing back in October could recognize right off what was going on here. And it became really obvious to me as I was explaining it to Matt. A quick check confirmed that indeed, the CG was maybe half an inch behind the center of the wing. Moving the battery forward to the very tip moved it about an inch ahead of the wing center. We took that out in the parking lot and gave it some tosses, and *boy* was that thing stable!

I guess that's just life, being reminded again and again of the lessons you apparently didn't really learn the first time.

Matt also suggested a direction for these foam gliders that I think will really make them into serious airplanes. All we have to do is coat one in fiberglass. 3 or 4 coats of some fiberglass and the foam planes will be stiff and probably just as resillient. Probably something to do once it gets warmer outside, because I don't really want curing resin inside and it prefers to cure warm.

Today we put together and flew the stick plane with the .15 motor we just got. One of the major improvements was a good quality stiff foam wing with ailerons. It has (among other features) a carbon fiber spar of some sort (arrow shaft?). This was loaned to the cause by Eric.

I received those li-poly batteries a few days ago and finally I had the time to play with them at the same time as the weather was accomodating (not raining *too hard* and not *too much* wind).

So I put the 1200mAh, 1.8oz, 2S1P (7.4V) battery on the foam-plus-fiberglass that we flew the first test flights with the brushless with. It was too tail-heavy from all the fiberglass I used to patch the tail boom that we had to add 3 lighters on the front (universal dead weight from the coffee table). We gave it a couple tosses and found that it went to the right almost uncontrollably and overall wasn't terribly side-to-side stable (it rolled easily, and didn't tend to right itself). When we put the motor on, we had it pointing to the right (to counteract prop twist), so we just ran new sticks in pointing dead ahead.

I flew it like that for a while and it was kind of fun, but it was totally roll-unstable.. Getting it upright was hard, but once it was there, you could pick which way you wanted to roll. But I noticed that getting out of left-hand turns was harder than getting out of right-hand ones and eventually got it doing about a 20 foot horizontal inside loop (knife edge), turning left. But I was giving it full right rudder. It wasn't losing any altitude, but I was pretty mystified by how stable it was in this one particular configuration. I eventually just pulled full back figuring it would destabilize the loop, which it did, but it turned it into more of a death spiral, so I cut the motor to save the prop. It immediately rolled back to level (right before hitting the ground *sigh*) once I cut the power. What a graphic demonstration of prop twist! In hindsight, if I'd cut the throttle or even probably tried to go left or down, it probably would have come out of it.

The fiberglass plane was a little heavy, and didn't have much dihedral, so we decided to use a stock super-dihedral swept-wing foam glider with the motor+elevator+rudder that we've gotten so good at slamming together. Those are always stable no matter what you do, we figured. Their wings just *flap* in vigorous maneuvers.

It worked out pretty much as we wanted, it was totally flyable. The plane was about 10oz. I was expecting a little bit more power out of my motor+prop, but it turned out to be a totally tame climber and we had to keep it at or near full throttle most of the time to not lose altitude. I suspect I should be using a bigger prop (I was using an 8x4, and then switched to a 7x3.5). But a tame climber is really what I wanted to fly, just something that would comfortably keep the thing in the air without going crazy, so I was happy with it. We didn't totally drain the battery (it's still at 8.1V...peak is 8.4V, cutoff is at 6V, and I think it has a relatively linear discharge curve), so all of the high-throttle stuff doesn't seem to have cost too much. I'm going to have to put an ammeter on the battery with various props to see if I'm drawing anywhere close to the 6A that lensrc says this motor can draw. I think I may need to get a 3-cell li-po pack.

No damage to the plane or the motor, but we are going through props like crazy (the 8x4 and 7x3.5 from GWS are very light-weight brittle delicate props). We threw on a Cox 6x3 that we'd used with a .049 for a while, and it actually worked pretty alright. It wasn't a real climber, but it held altitude almost.

I think the big lesson is that it is time to invest the time into some ailerons. Turning with the rudder basically sucks, especially if you're trying to stay in a small field like this neighborhood park.

I have my fiberglass plane sitting behind my chair, with a wing poking into my arm space. And I accidentally bumped my arm into it, and it didn't budge, it didn't bend, and it hurt my arm. Fiberglass rules!

I tried to get the rubber-power-oriented props on my LensRC motor again. I think I managed to balance two of them well enough, but they still wouldn't go over half throttle. They seem to work smoothly up to about half throttle, but then the motor starts missing and the motor controller turns off. Maybe I should fiddle with the advance settings on my controller. Probably they're just way too pitchy for high-RPM. They don't seem to generate a tremendous amount of pull at low RPMs, though, so I think I may abandon that idea. I should try the middle-sized props I guess.

I flew the rudder-elevator foam plane for a while at the park with a Cox nylon 6x3 (because it's the only prop the motor seems to like that I happen to have not broken yet). I glued on a prop saver and it seemed to mostly do its job (though in the end I let a kid fly it into some concrete and that broke the prop saver...the prop is fine though). With the 6x3 it is pretty much at the bare minimum to be able to gain altitude at all, but it does work. Really hard to recover from a near miss with the ground or a tree and regain altitude, especially because I was flying in such a small park on such a steep hill, I usually would have to turn and start gaining altitude right away. Again, this rudder-elevator thing sucks for tooling around in tight spaces.

The prodigal plane! There were a series of storms over the weekend, and I went up to check that the plane was still in the tree in the middle of the storms. But it apparently fell out at the end of the storms, and the next day someone found it in the street and put a sign on the nearby telephone pole. Thanks Kyle!

So I got it home and checked the damage. The voltmeter says the Li-poly is toast -- 1.5 V. But, being a little adventuresome, I just took my spare battery and threw it in place. Everything works just as before! This thing had a LensRC motor, Castle Creations Phoenix-10, Hitec Feather rx, and Hitec HS-55 servos. Kudos! Surviving the rain with the battery still plugged in, with no damage at all (knock on wood)! I sure wish the Phoenix-10 had a second low-voltage cutoff that triggered like .5V below the first one, to save the battery in the not-rare-enough event of long-term treeing. Or maybe the Kokam batteries should have the cut-off, but then they'd have to worry about how to bypass it when recharging.

The really surprising part is that the balsa pieces (the rudder and elevator) are both still fine. I think the wood feels a little different, and the elevator has some non-critical cracks in it, but considering how much water they probably absorbed, it is nothing. I guess the bamboo cross-grain supports that Zack added really did their job.

I can't find any information about recovery of li-poly from deep discharge, so I think I will recharge it at a slow rate and see if it recovers at all.

So far so good -- I am charging it with a fixed-current charger I use to trickle charge NiCads -- something like 25mA (this is a 1200mAh pack). I remember reading somewhere that if you ever get a Li-poly deep discharged, you should charge it at no more than .5C until it reaches a sane voltage, so this is kind of following the same plan. :) It is increasing the voltage at about .01V/6 seconds (1V every 10 minutes). From about 1.5V unplugged, 2.6V when I first put it on the charger, now it's at 3.4V and it has slowed considerably. I am sure it won't have a very linear charge curve, but when it reaches a more sane voltage (say, 6V), I'll put it on the li-poly charger at like .2C and see how long it takes.

I suppose I really should be charging both cells in parallel -- they may not have sustained equivalent damage from the deep-discharge and may need levelling. It is not difficult to level a pack, but i do not really want to cut this pack open. So I guess I will only do it if it doesn't hold a very good charge after this.

It has slowed down its charging substantially, so far averaging about 1 volt an hour. It has reached 5.3V and so far I have only put about 75mAh into it. My hypothesis is that it stores much more energy (exponentially so) in the higher voltage part of the charge than the low-voltage part.

(the next day) It seems to have charged, after experiencing about 9 hours of 25mA to bring it up to 7.2V, then about 2 hours of 110mA to bring it up to 7.6V, then about 3 hours of 250mA, for a total of about 1300mAh, of which about 1000mAh was delivered after it reached 7.4V. It was originally supposed to be a 1200mAh pack, so if I can draw a sizable fraction of that 1000mAh in real use, it will be an "acceptable loss" and I will keep on using the pack, with pleasure. Straight off the charger it was at 8.45V, and I put it on a plane and just ran the motor for about 20 seconds and it was down to 8.38V after that. Not bad considering how badly it was discharged.

In sadder news, when I actually tossed around the plane, it turns out that the motor has seen better days. The repeated pounding into concrete has definitely jarred some of the magnets loose, and sometimes it won't even spin (but once you get it spinning it is happy). My dad speculated that the magnets were being pulled towards the stator, and I was like "that's crazy", but then I looked really closely, and that's what's happening. Once it's up to speed they stay out of the way. But I'm going to have to add some glue to the system at some point. Considering the nose-first approach to landing on concrete, I am not terribly surprised.

Also, my tard ass dropped the little shaft clip in the grass. *sigh*

So I've been building this computerized mixing thing to add to my radio. The idea being that then I'll get easy access to features such as mixing (like for elevons), exponential control throws, and aileron differential. I'm really satisfied with the design, but the implementation isn't there yet, so it is kind of frustrating waiting for it.

We built an aileron ship a while back, and I actually have a lot of video of it behaving weirdly. The trouble is that we put little ailerons at the tips of the wings with a tiny vertical stabilizer and no rudder and were experiencing strong adverse yaw tendencies that, coupled with a slightly back center of gravity, resulted in a bizarre frisbee-like aircraft. We actually had all of the clues necessary to figure it out, including a couple long (for a craft with one aileron) flights with -- essentially the perfect experiment ground for playing with simulated differential by isolating the control to only one wing. Anyways, we never did figure it out. I had one crazy idea, but I didn't really put any faith in it and it really didn't seem like it could be causing this dramatic an effect.

But then I did a little bit of websearching and there's this adverse yaw phenomenon that is quite common and well understood and particularly affects craft like this one. It's caused by an effect that my intuition says should be minor, but turns out to be huge in some situations. What happens when you turn left with ailerons is the aileron on the left side of the ship goes up (producing less lift) and the aileron on the right side of the ship goes down (producing more lift). When the aileron is down, it is acting kind of like flaps -- it is increasing the amount of lift but also dramatically increasing the amount of drag. The opposite effect is occuring on the other side of the plane -- the upward-ailerons allow the wing to fall through the air in a reduced drag fashion and gain speed. The resulting yaw effect goes in the opposite direction of your turn. You can solve this problem to some extent with coordinated rudder usage (or mixing), or some variation on aileron differential.

The idea of aileron differential is to do something to make the up aileron have about as much drag as the down aileron. The typical approach is to make the control have much less travel in the lower half of its movement, so that the down aileron does not produce as much lift or as much drag. There are other cool approaches but I figured this differential idea fit well in my head and is something I would enjoy playing with. It also fit with my experimental data: flying with only the right aileron worked quite well so long as I only made right turns (thus using only up aileron control).

So anyways, that all happened a few weeks ago. We entered into this night of experimentation in the context of poor aileron behaviour and an inclination to wait for the computer to get differential right. So we were not going to bother with ailerons. But we're also recently enamoured of flat wings, I guess, so rudders also seemed out. So we came up with a lot of not-aileron ideas for roll control.

The most ridiculous was the shark idea. We put a huge vertical control surface over the center of gravity of the aircraft -- basically an oversized rudder in the middle of the plane. The hope was that by providing this side-to-side force with no fore-aft moment arm we would neutralize the rudder effect and instead it would roll the aircraft. It didn't work. Are you surprised? It had just a very very weak rudder effect. Forget we ever mentioned it. Really it would have been alright if we had put another fin on the bottom to provide a counter. Since they weren't generally lifting surfaces, I think they would have acted like ailerons without any need for differential.

Then we kind of had this idea that started out as big dramatic airbrakes on the wingtips but in reality turned out to be undersized funny-shaped ailerons with only up movement (i.e., a pull-only string system... down aileron movement just introduced slack into the line). They provided good predictable and uselessly tiny roll control. I guess they qualify as a proof of some concept, but the church parking lot was simply not ammenable to their turning radius.

I noticed a kind of neurotic flying habit I have. I try constantly to keep the plane within a relatively narrow range of altitude and pitch. It gets worse when I'm in limited urban spaces, where I wind up doing silly stuff like keeping the plane below 20 feet constantly and trying to do tight turns with unpredictable control surfaces within 10 feet of the ground. This habit has not made our planes into terribly long-lived instruments of mayhem. I think I picked it up due to my overwhelming awareness of the urban environment I'm flying in and my inexpertise in climbing rooves. So I guess I'll hold onto it until next time I'm at the farm or something.

The little LensRC motor I got has been one of the really outstanding aspects of these planes. The first thing I notice when I throw this motor on one of our thrown-together stick-foam constructions is that it is really easy to mount. Then I notice that Zack seems to have a little difficulty hanging onto the plane if I throttle it up when he's not expecting it. Then I get the plane into some crazy attitude in midair and it starts climbing violently, or hovering, or drifting slowly towards the ground. And never am I stuck in level flight with insufficient forward velocity. Finally, an electric power system that simply provides an easy to use harness thrust that hardly weighs down the plane!

But the real thing I'm learning is how durable it is. A long time ago I broke some of the magnets loose with some concrete. They would sometimes jam when the motor was first starting, so I decided to reglue them with some CA. It should surprise no one that I did a very poor job of this, and before you know it, the magnets were all loose again. I was watching a video from a couple weeks ago showing the LensRC motor making some bizarre rattling noise and you can see two guys standing around it poking at it, and in the next clip the plane is in the air. I mean pretty much nothing will make the motor unflightworthy. Tonight we finally cracked a magnet, and magnet dust completely jammed the thing. It took a few tries, but I finally got all the magnet dust out, and it runs just fine, even missing about a quarter of one of the magnets.

"Now, Greg," you might say, "why not simply stop running the motor into stationary objects?" And basically, I am starting to see the wisdom of our haphazard approach. We're learning basic lessons of aerodynamics, construction, and piloting all through the basic experience of trial and error. Certainly, we read a lot and talk to some people I'd even consider experts. But ultimately we just do what seems like a good idea at the time, and then later we learn why it wasn't such a good idea. I can think of a few lessons and the planes that taught them to me (some lessons had several planes...I'm a slow learner I guess). The fact that I can get motor technology that supports this approach is sweet, sweet icing on the cake!

Shortly after my last entry, I bought a house and I've been too broke and overwhelmed to play much since. But I *do* have a huge workbench in this house, so...

A few months ago (Feb 2005 Model Aviation) I saw plans for Bob Aberle's Scratch-65 (a 65% scaled down extra light version of the Scratch-One, published there before I subscribed to MA). It is designed to introduce us new timers to the art of balsa stick construction, so it features an interesting shortcut. Instead of cutting out a zillion little ribs, there were just bent sticks of balsa over the top of the spars. I decided I would build one.

The Scratch-65 was too small (Bob was using super-micro servos and receiver that I do not have), and I couldn't conveniently get ahold of Scratch-One plans, so I went ahead and built one a little bit smaller, say a Scratch-90. Hopefully its flight characteristics will not suffer too much from the number of decisions I had to make in this approach.