DG800 – Life..

Oi, it’s too heavy!

DG800 Comments Off

Apr 112020

Yeah, sort of expected that to happen 😉

With the added mudguard and wheel bearings and other small bits, the Hitec 645MG is struggling to get the gear up. There is a point halfway between up and down, where max force is needed, it’s just how the design of the gear works. In theory I could redesign everything. (Yup, maybe sometime soon). Or I could add another voltage regulator to give it it’s own supply, 6V would work (most of the glider runs on old 5V gear), or solve it for now with slightly more powerful servo.

I found a SAVÖX 1256, which has probably twice the torque of the Hitec, so will strap that in for now.

Of course with the way I wired things up, that’s easier said then done. Since I installed a 9 channel Rx in the plane, and needed an extra channel for the Flashy Lights, I came up with the cunning plan to use a single channel for multiple things.

In short, gear up or down is only 2 positions. Light on/off same thing. So why not do a bit of mixing and send the resulting servo signal to an Arduino-Pro-Mini, and let it drive the LG servo and Lights. I cooked that software a year ago, knowing it was a quick fix for a self imposed problem. (That’s what this hobby is all about, if you don’t have a problem, make one!)

Anyway, long story short, the pulse that the servo needs for up or down is hard coded in the software. No biggy, but it means it’s not very convenient when you change servo’s.

Oh, it’s sooo much fun. With the aid of my little magic ToolkitRC.com M6 I measured the output pulses to the servo. 1000-2000mSec. Perfect, but the servo only rotates 90 degrees and a bit? (By the way, there is an software update for the M6, V1.31 dated 23 March 2020)

Oh dear. Now what. Time for a rethink, and do remember in future that you really want a 180 degree servo!

After some reflection and relaxing in the sun, I remembered that I used to convert servo’s to 180 degrees by adding a few resistors to the feedback pot. That triggered of course the thought ”did I maybe modify that servo?”.

*2 * 1.5k resistors make it a 180 degree servo.* (you call that soldering?? Sigh.. One gets older and the hands less steady…)

And the answer is yes, I did! Funny enough I posted that same fact on 3rd Dec 2016 on my Facebook account. Oh where would I be without my facebook memory. Scary..

Well that sure was a fun exercise for a locked-down Saturday. So next question is: Am I going to modify the Savox? The answer is below. Same trick, 2 * 1.5kOhm resisters either side of the potentiometer will do the trick. Funny this servo is rated to produce 4096 steps. It never stops to amaze me how you can get that resolution from a cheap 5k pot. But they work, so I’m not counting! by the way, there are no wires in this servo, which should make it more reliable (cheaper). But it does mean some sort of skill is required to remove the pcb!

*Luckily there’s enough space for 2 resistors, same 1.5kOhm as on the Hitec. Rotation is now just a tad under 180 degrees. That will do nicely!*

And while at it, might as well try to sort out the wiring mess a bit. I’ve got some space at the top of the fuselage, out of sight.

By the way, that stepdown converter acts as a current limiter as well. Max current the chip will produce is 4 Amps, stall current for the servo is 5 A. Should be safe. And since it is only ever used for up and down, it is not likely to get warm. And if it does, it will disconnect, saving the battery. In theory this is what is supposed to happen. Real life might work differently.

So, that’s enough for Corona-Easter-2020. Left’s hope and pray the world will not have to repeat this ever again!

What a difference a month makes.

Corona, DG800 Comments Off

Apr 072020

To say we have never experienced anything like this lock-down is an understatement. We are lucky to live in a nice place, with lots of space, so we should be ok. Even if this madness takes a lot longer then the official guestimates that our gov’ment now spreads, I will keep myself entertained.

Saying that, I managed to mess up my back (again) which sorta restricts my movements at the moment. Hence the lack of stories.

I have finished drawing the metal parts for the SA-750, and are waiting for the nice laser people to cut them for me. Obviously my parts are low on their list of things to do, they likely have other more immediate pressures on their business. Never mind, lots to do.

Last year I hardly flew the DG800, mostly because our field was dry, so unbelievable dry, that the stake I use for the bungee did not hold. I once had that metal spike flying in my general direction, and that was once too many times. I’ll try to think of a better solution for tying down the end of the rubber. I probably need at least a double stake, and most likely longer and heavier. And that’s the pita I want to avoid, I don’t want to drag all that stuff to the field. I’ll have a think about it.

At the moment the field looks fine, I could use the single stake, but you guessed it (or not if you read this in years to come) we are closed for business due to the Corona virus. Sad as it is, we are a sport, and all outdoor and indoor sports have been banned. It does not seem logical to us, but the gov’mnt does not have the time to look at each sport in detail to see if it makes sense. So, everybody stay at home, no exceptions.

So, do some maintenance. I knew I had a flat tire. Sorted that by inserting a length of silicone tubing inside the wheel. (I know, I could have bought a new one. No Fun). Then, since I have them, added some bearings to the wheel, that never hurts. I also wanted to make a mudguard, just to see if I can.

*wheel with bearings, the original plastic one did not survive my ‘homecomings’*

The plan is to print the form (positive) slap on some fiberglass and see if it looks like something I want to use. Right now the vacuum pump is doing it’s best to maintain -0.6Bar. I lost the manual for the little thingy that decides when to turn on and off, but it seems to work, so will look for that later.

I would be better off giving the walls a slight angle, so that it is easier to pop of the form. With this version I had to split it to get it to loosen. But in the end, no harm was done, and it looks good enough for now. If it survives, and this lockdown continues, and if I am getting bored, I’ll do another one. This one is approx. 8 layers of 25 gram and one of 225 gram. I have miles of the thin stuff, I use it to get a nice surface. (It was nice until I had to split it in half to release form the form 😉

DG800-4

DG800, Gliders Comments Off

Jan 042017

It has been some time, but today is as good a day as any other day for a wee update. A lot of time has gone into designing and building a retractable u/c. The final result is good, even though I say so myself. I still want to make a mudguard but that can wait.

Making the wheel doors work properly was alo a lot of work, where I had to made a negative from the fuse, and build the doors into that. (The old vacuum installation was a great help here! )

DG800-3

DG800, Gliders Comments Off

Aug 252016

Mounted the elevator servo, I found a one in the box that will do nicely. I wanted to use a beefier link to the elevator, but there was not enough space. So a 2 mm version will have to do for now. Maybe change it for 3 mm if I work out how to do it.

I also cut slots into the rudder for hinges, but in hindsight I want to redo them in such a way that there is less visible ‘hinge’ on the outside. The whole rudder needs to be removable, there is a plastic rod for a 1.5mm steel wire built into the rudder, so this is easy.

Next task will be to think of a way to make a quick disconnect link to the elevator. I don’t want to use a standard quick-link, they do fail with repeated opening/closing. It has to be simpler, and less fiddly in daytoday use.

Let the mind think about it for a bit, it will find a good solution!

Flying this week is a bit of torture to be honest. 30 C plus and clear blue skies, with a slight haze is not the best weather, simply because it is very hard to see the plane beyond 200 meters altitude. No doubt I will regret saying this soon.

DG-800-2

DG800, Gliders Comments Off

Aug 202016

Part 1 is a bit back, describing how I did a quick and dirty cockpit frame. Today was the start of more serious stuff, like drilling the holes in the fuse for the all important bit in the middle! First I created a jig that holds and aligns the fuse, and made very, very sure that everything is perfectly level and square. Lasers and camera’s don’t seem to like each other, hence the lack of quality in the pic. I will probably be able to do another one, once the first expoy has hardened. To tricky to touch anything at the moment.

Once the sleeve is in position, I can cut off the outer parts, which will go into the wings. The manufacturer states in the manual they kept the hole in the wing slightly oversized to allow for wiggle room. What slightly worries me, is there anything other than the sleeve in the wing holding things up? Do I need to add something? Glass the wingroot maybe? Time for more research!

DG800 from Rödel modell

DG800 Comments Off

Aug 072016

One minute you contemplate building yourself a nice big glider, next time you wake up you have a shed with a few big fish! So far it’s only a 5 meter ASK21 en a 4 meter DG800. Both in need of minor assembly. Since the DG800 is the smaller (and cheaper) one, I am going to try to get that one started first. The DG800 is a Rödel Modell kit, probably something that was made 10-15 years ago. (Haven’t been able to get details yet) However, the ”kit” is in pristine condition. The only giveaway of its age is the papertape holding various bits together.

DG 800 von Rödel

Spannweite: 4150 mm (mit Winglets 4400 mm)
Länge: 1790 mm
Gewicht: ca. 5700 g
Profil: E 207
Steuerung: Quer, Seite, Höhe, Störklappen, Einziehfahrwerk, Schleppkupplung erstanden.

Die Maschine fliegt an sich sehr gut. Ich bin sie einmal in den Alpen am Hang geflogen und hatte viel Spaß!
Leider habe ich sie bei diesem Erstflug fast verschrottet.
Das Problem ist, dass die DG, wenn du schon sehr langsam bist (z.B. Landeanflug) und dann die Störklappen voll ausfährst, schlagartig abreißt. Ich habe meinen Absturz zufällig auf Video aufgezeichnet und ihn x-mal angesehen. Kollegen von mir haben alle bestätigt, dass die Maschine nicht zu langsam war. Da scheint es wirklich ein kleines Problem zu geben, da der Abriss exakt mit dem Ausfahren der Klappen kommt.
Sobald die Klappen drin sind besteht keinerlei Gefahr und man kann die DG herrlich langsam fliegen.
Auch wenn sie es nicht zugeben, ich glaube das Problem ist bei Rödel bekannt, da ich nach dem Absturz von Rödel folgenden Rat bekam:
“Die Klappen nur halb ausfahren und gut Tiefenruder beimischen”

Kurzum wenn du die Störklappen voll ausfährst mußt du immer auf deine Geschwindigkeit achten.

Die Serie E-205/207/209 hat bei den typischen Modellflug-Re-Zahlen starke laminare Ablöseblasen bei mittleren Geschwindigkeiten, d.h. deutlichen Zusatzwiderstand beim beschleunigten Gleiten.

Die Ablöseblase kann man sichtbar machen, wenn man an einem ruhigen kühlen Abend, bei dem Tau entsteht, einen Flug mit gleichmäßiger Geschwindigkeit macht; man sieht dann nach der Landung einen “Tau-Streifen” auf dem ansosnten trockenen Flügel. Das ist das sog. “Totwasser-Gebiet” der Ablöseblase, in dem sich der Tau am Flügel absetzen kann, obwohl der Flügel eigentlich umströmt ist.

Warum erzähle ich das? Weil es sich gerade beim E-205/207 lohnen wird zu versuchen, die Ablöseblase mit einem Turbulator zu zerstören. Es genügt ein gerader Tape-Streifen (Zierstreifen aus dem Autozubehör), 2 mm breit, 2-3 Lagen übereinander, bei ca. 25 – 30 % der Profiltiefe (kurz vor dem “höchsten Punkt” der Profiloberseite). Der Leistungsgewinn kann spürbar sein, erst recht die (gerade bei Vorbildgetreuen zu beobachtende) Verbesserung des Überziehverhaltens.

Wenn man das Ablöseblasen-Problem löst, sind die Profile E-205/207 gar nicht schlecht; sie haben recht ordentliche Allround-Leistungen.

hab folgendes gefunden:
Das Profil ist für den gleichen Einsatzzweck gedacht wie das E 205, jedoch sollte wegen seiner größeren Dicke die Re-Zahl sicher über 100000 liegen. Das bedeutet Flügeltiefen möglichst über 240 mm. Das E 205 wurde für RC-Segelflugmodelle berechnet, die sowohl langsam in der Thermik kreisen sollen als auch schnell fliegen müssen, ohne daß die Sinkgeschwindigkeit zu groß wird. Es war daher bei seinem Erscheinen ausgezeichnet für die Klasse F3B geeignet. Inzwischen gibt es leistungsfähigere Profile. Auch für den Hangflug ist das Profil gut geeignet, da es bis hinunter zu ca = 0,1 recht niedrigen Widerstand aufweist. Die Polaren wurden im Stuttgarter Windkanal gemessen. Zum Vergleich wurden die theoretischen Polaren mittels des Eppler-Programms errechnet. Weiterhin sind die Ergebnisse der Messungen im Delfter Windkanal und die der Messungen in Princeton von zwei von verschiedenen Erbauern hergestellten Meßmodellen aufgeführt. Die theoretischen Polaren für den Einsatzbereich von HLG im Re-Zahlbereich von 50000 bis 300000 wurden mittels des Winprof-Programms errechnet.

Entwurf: Prof. Eppler
Quelle: MTB 1 und MTB 1/2; Model Builder 5/1982; Soartech Nr. 8 “Airfoils at Low Speeds”; H. Eder “Mehr Leistung mit dem Hand-Launch-Glider”, Verlag für Technik und Handwerk GmbH, Baden-Baden 1996, S. 106.

Clear enough: It’s good enough for a Flatlander!

Anyway, since a challenge is only a challenge if you create on, I decided to try and make a 2016 scale canopy. The original blow-molded plastic afair will be kept for nostalgic reasons). First I wanted to see if my idea sort of works. (My idea: likely something I read somewhere on ye interweb. )

I want to create a well fitting carbon frame. Easiest way to make it well fitting, is to build it on the existing fuse. I stuffed some foam in the inside, and some foam on the outside to create a channel in which to stuff the carbon/epoxy/microballoons. Since it is a trial, it does not yet have to be perfect.

First impressions are: This works! Now I need to order some fresh epoxy, and do it for real. This time I will try to make the frame an even thickness, and offset it a bit to the inside, leaving enough space for the hood.