The DT-760 ARF And a Farwell to Hobby People

By Carl Murphy

Along the way I was declared a “Bad Example” by the Harbor Soaring Society. Along with every other place I’ve ever flown, you either have to be a fool to fly like that, or really good. The latest “regrettable incident” was year 2015 in Hidden Valley (on the Santa Ana River wash) where I flew a Reinforced Fun Cub in twenty to thirty mile an hour wind. During my over half an hour of mostly slope soaring two others launched their RC airplanes (they still fly fuel burners there) to be reduced to wreckage on landing after five minute flights. It’s not that I’m dangerous or rude, far from it. But the; If he’s flying I can too has resulted in reckless behavior by others before they determined my airplanes are improved and I have excellent flying skills. My personal record was four (you read that right, (4)) standard (30) watts-out Easy Star (I)s ruined while I was flying a (300) watts-out Easy Star (I) (bigger rudder too) right there at the SEFSD flying field in Mission Bay back, with gusting on shore wind, in the 2000s. Well, I was at the down wind end of the runway, my first notice was as their airplanes cartwheeled past me.

 

Back then the HSS offered that either I find a different club or agree to limit myself to not more than a lifetime total of (25) pusher propeller RC airplanes along with cease and desist instructing others in their operation or assembly. Recent years when I find myself at Estancia in Costa Mesa nobody remembers me, with the flying area moved back from the cliff you are just in a field and Costa Mesa now requires a license from them to fly there, so I don’t. I’m ok making this public. That having the propeller in back and being some kind of modern impact absorbing foam (it looks like the wing was extruded, the tail surfaces cut from sheets) though makes for landing anywhere an RC airplane can be flown in Southern California (SC). Rhein-Main has plants to soften impacts without landing gear, folding propellers in front are enough for there. That whine of a pusher propeller gets irritating after a while.

 

This will be my first pusher propeller RC airplane in five years or six years. Why, they can be landed most anywhere. I also have a left lieing around completed, decade old, Hobby People flying wing almost ready for it’s first flight.

 

Farwell to a Supplier

 

For all the changes over the years including relocating to San Diego and then back and forth to Rhein-Main (where I once again have a fine woman to go home to, I’m even a husband now) I continued to shop at Hobby People. But they hadn’t been doing well since the Recession (turned to a Depression our government refused to admit to) since back about 2006 and neither had I. The worst of it was all the companies had to down the quality and Hobby People was always the leader in what was latest. I’m neither happy nor satisfied with the current ARF twenty flights and replace phase. Few of them are worth the effort to improve them other then replacing the often junk parts they come with even if the airframes fly ok. I’d rather pay more up front.

 

The first of four RC hobby shops on Convoy Street in San Diego to pack it in was Sureflite, then Diamond. When the San Diego Convoy Street Hobby People store closed year 2016, I figured that the El Cajon one was all they needed in the area. Too much Internet buying, too many customers like me too broke to buy. But when I stopped by the Corporate Headquarters in Fountain Valley on my latest back to SoCal trip in February year 2016, they as a company were selling off, after forty-five years closing forever. What was left wasn’t all that interesting.

 

Just the same, the second week of February year 2017, I went back in and bought some small stuff and a little RC airplane, ailerons and elevator, high wing pusher propeller, I’d seen. They had been laying around the store for a couple of years. The original price was $80-, which with tax would have been $86.00- net. As I stood in the check out line I thought I was paying $40- plus sales taxes. Later on examining the receipt I paid $21.50- net for my DT-760. Just the disappointment of a supplier you’d been buying from for twenty five years disappearing, and the concentration of focusing on every item knowing that either hold it in your hand and buy it, or forget it, it wasn’t until I opened the package I realized what I’d bought. When I left at 4 pm that Sunday they reportedly had decided to stay open one week more before ending (45) years as a company. I’d have bought two at that price.

 

It was a whole little airplane, mostly foam with just a little reinforcement, complete except for receiver and battery, in a box 2/3s the size of the Wanderer’s. And nothing much to build except find a way to mount the receiver and a battery. As marked on the box it was manufactured in China, as directed by a German company.

 

Carl 2

 

Initial Check

 

The instruction manual leaves out a lot of details, despite the many pictures. It does show a void in the nose where the receiver and battery could go. As it turns out the canopy is held by a magnet. This airplane is mostly push the horizontal stabilizer in use a single screw to secure the wing (screw and tiny allen wrench included) however, neglected are the information for Center of Gravity (CG) and power draw. The propeller is listed as a 6X3, which given the small size of the electric motor (looks like about fifteen grams, typical you can’t feel the magnets of this low price range) will turn slowly. For reference, I often use Hacker Brand electric motors, one this size (about fifteen grams) from Hacker costs $60- retail.

 

Mostly some kind of foam, the wing is flat, except for some slight bow of the right and left wings (already continuous) with no washout. That may make for difficult stalls and it may require more attention to level flight. A single servo embedded in the wing with a thin cable serves both ailerons, the hinges of which are already free enough.

I haven’t had that one servo to actuate two ailerons with a cable since the Author marketed his Mad Dog kit two decades ago in a time when servos were more expensive and 8 gram servos almost unknown.

 

Among the small parts I purchased my last time ever from Hobby People (without which you don’t have an RC airplane) were a package of four Brand APC 6X4 propellers and a 5.25X4.75. Along with some Brand “X” servos. And I have a Hobby People simple four channel radio that is reportedly compatible with Airtronics systems.

 

Required to finish the DT-760 were a battery and receiver in addition to a transmitter and the usual RC stuff. In addition, although not strictly necessary I purchased additional male and female leads as I wanted to measure power draw. My NeuMotors charger already included a suitable charge lead.

 

Carl 3

 

Assembly

 

Included in the box were basic instructions on how to assemble the DT-760 from parts. Unnecessary as it comes assembled.

 

After looking over the parts, I later discovered the “barrel” connection from the already installed elevator control line to the elevator was laying on the floor. The connection screw was missing. I used a spare. It should be glued in place, for now I just pushed the horizontal stabilizer in.

 

The Electronic Speed Control was used to run up the elevator and get it centered.

 

The ailerons were trial checked. One aileron the allen screw wasn’t tight enough, both were centered as much as the curvature of the warped wing will allow. At least the warp is symmetrical. Bad, a crash already prepared, the servo control horn isn’t strong enough, it bends way too much. The allen screws are difficult to tightened as the supplied allen hardly fits. Another bad, the aileron horns aren’t strong enough.

 

Something right, the differential at the ailerons is about twice as much up as down, both sides were the same. That often isn’t the case! Travel wasn’t much.

 

The simple, but decently made, propeller was mounted and the motor was run up, it seemed ok. Plenty of static thrust.

 

Final assembly went without issue, about half an hour of fitting things together. Until it was run up as a complete airplane on the bench. At first I thought the connectors were defective, the one to the battery from the power meter melted, it almost burned up a new LiPo battery. So, another battery with different connectors were tried. Motor performance was erratic, in short order the motor controller failed. The motor controller burned up!

 

As luck would have it I had an old (18) amp speed controller with the correct (1)mm connectors. The motor was run up again, current draw went to (14) amps. At first I blamed the supplied propeller, not the burned up speed controller. But usually a motor controller can be slightly over amped for a few seconds, this one quit at a count of five to ten on what should have been a matched system. There was no identification of any kind on the motor controller, the expected amp draw was from the label on the box it came in. This was a complete system, the LiPo I used the voltage drop was way too much and even at that it quickly burned up. P.O.Expletive Deleted.

 

The output connector of the (new) battery was damaged (which took a while to find, unlike the there side which had come apart) so, I spent most of two hours resoldering connections to get everything to work together. In retrospect I was going to need those connections for the power meter for another project. After getting it connectable again the LiPo may be damaged as in just half a minute half it’s charge was gone. Still, it wasn’t more than a little warm and evenly so.

 

Instillation of an APC 5.25X4.75 was the wrong direction. No, not the propeller on backwards (seen way too often) the amp draw flashed to (15) amps.

 

After adding in a couple of washers an APC 4.75X4.25 was mounted. With the possibly damaged 2S800 mAh LiPo current draw was now

 

(10.5) amps.

Worrisome, no load voltage was

(7.8) volts, at full load that fell on a freshly charged LiPo to

(6.6) Volts. Wattage-in seemed ok at

(70) watts-in. I wouldn’t expect much out of a motor at this price. And the older motor controller is one set for in-line lead at (5) degrees, outrunners need more like (15-20) degrees. Figure net efficiency, after the washers that hold the grease in and most of the dirt out in the bearings wear in, and the twenty to as many as fifty flights before the round balls in the bearings wear to gravel, as a maximum of (65)% watts-out to watts-in. Well, when buying stuff new you get what you pay for and this was cheap…

 

I would expect a smaller Hacker Brand motor to put out as much power at a lower current draw. But then just that motor costs almost as much as this whole RC airplane system. The difference between (80)% efficiency and (65)% efficiency isn’t (15)%, it is (23)%. In practice the difference is double the flight duration.

 

I’d have mounted a folding propeller assembly, but had none with me that would function. After flying the DT-760 I don’t think it would change things much.

 

With no other information, this simple straight wing likely should balance on the single wing mounting screw. Actual balance in the only position the battery will fit is a little forward of that. I often fly a first flight with the balance a little forward, time to see what this little investment can do.  

 

Costs

 

$3-      replacement propeller For most people that would be more as I did not include the cost of propellers I tried that didn’t function correctly. But then that kind of information is exactly why I organize my notes into this form, and why anybody might benefit from reading this.

$?/15-            Old stock Castle Creations (18) amp electronic speed controller changed to a smaller battery connecter Slightly larger, it can only be just pushed into the “tunnel” into the airframe.  

$24-                A single new battery in size 2S800 mAh (as specified by the manufacturer) brand E-Flite and one each male and female connectors already on leads.

$21.50-          initial purchase on close out of original $86-

 

Carl 4

 

Flying, a First Flight

 

On a perfect spring afternoon, the wind blowing as into the slope as ever, knowingly nose heavy, my new little airplane was pushed into the wind. That forty feet of fall leaves room for some response before the airplane hits the ground. As expected elevator trim up was required to offset the too far forward CG.

 

Flight is ok at one third amps. As expected it had neutral stability along the roll axis, the pitch and yaw are both fine. Neutral response to power on and off. With it nose heavy the power had to stay on most of the time. Not that I wrung it out, a first impression is that the control throws are about right as it came. This is, as designed with that flat bottom airfoil, just a flyer, no more acrobatics then the minimum.

 

Fun to fly, no. I didn’t add any recognition paint, so not if but when, it falls off over one wing or the other several times a minute, you had better already know which one. Out beyond a hundred yards its too small for my six decade old eyes to make constant roll corrections.

 

I fly RC airplanes, well, further out then the AMA recommends or almost anybody else, but they have to be stable, guide them through airplanes. I checked on the map, I fly my six times as big Twin Star II on the Point Loma bluffs in San Diego up to half a mile away. That would be an intended place to fly on windy days with a DT-760 as the excess weight then becomes an advantage. Except in less time and cost then it would take to get a DT-760 “right” you could assemble a flying wing that way outperforms it… I fly the very stable Easy Star IIs at similar distances.

 

This was unusual, I always add recognition paint before first flights even, but it was almost sun down… Although I had it do a loop, no rolls or anything else was tried. With it nose heavy and some parts of limited duration (break that aileron control horn and it’s over) I made little use of full amps.

 

Stalls were no problem, just that with this zero roll stability, what ever direction it is going at the time of the stall continues. That kills airplanes with beginners as the combination of stall, pointed down and reaction to getting back to flying speed meets the ground first. The issue is that roll and general lack of stability.

 

The whole airframe is heavy compared with most flying machines. Even pointed straight down that flat Clark Y airfoil won’t allow much speed and strictly limits any aerobatics to the minimum. I avoided any high speeds as the elevator wasn’t glued in, yet.

 

Knowing that the brand knew twenty dollar battery may already have been ruined when the (possibly overloaded by the supplied propeller) speed controller burned up, I landed (perfectly) long before any signs that the charge was down. I was not impressed. Kind of irritated actually. On recharging that rather short flight (about five minutes) the battery was ¾ down.

 

The closest I have in inventory is a flying wing combination from Hacker. A little less wing span with a little less area, I use the same size battery with. Flights in similar conditions go on for at least twenty minutes. But then I am notorious for staying up two or three times as long as anybody else with similar combinations.

 

It is a blast to fly, except that after thousands and thousands of elavon flights I’m bored with the restrictions. Assembling the kit (takes about two hours as part of that is verifying your equipment) required gluing the wing halves together (the best cut EPP I ever laid hands on, the printed on contrast is perfect) selecting and mounting the servos (your choice at what to purchase) and attaching the motor and supplied speed control. That motor is half the size of the one in the DT-760- and you can feel the magnets cog past the windings. Net cost (they are still available) about half again more than the DT-760-. Time to assemble, if the DT-760 had gone together easy (not hours of finding and fixing melted connectors plus then finding a suitable propeller, missing screws etc.), is about an hour. It is slightly fragile to transport (compared to a Wanderer that would be a joke, but the light tail of the DT-760 could easily be bent) and not convenient to separate the wing from the fuselage every time you want to fly. That is important when you carry it everywhere in your camper with you, for the rest of you likely irrelevant.

 

The Hacker Flying Wing a far better flyer, the net cost per flight vastly better. Hacker didn’t get established in the USA, if they were available in SoCal I’d buy the next size up and quit looking for an RC airplane for my camper.

 

I should have taken out AMA membership as preparation for re-joining the San Diego Electric Flyers instead. I wanted a smaller airplane for my camper I could use wherever I was, this isn’t it.

 

Well, it was what I had, so I continued to fly the DT-760 as it came from the manufacturer with the replacement propeller and motor controller. Those little “barrel” connectors are a liability. I have excellent tactical feel, even I didn’t get the set screw for one aileron tight enough. The wire isn’t enough to get tension, you have to use a light pliers to hold the barrel as you tighten the set screw.

 

Flown with the recommended size battery it is nose heavy, and so flights are shorter then they could be. About five minutes. A smaller battery in the 2S 600 mAh size along with a smaller propeller to reduce current draw to what the limiting factor, the battery, would improve that. I took it as an omen when I went by NeuMotors in early March 2017 that they didn’t have any batteries in that size. Discount Hobby warehouse had the necessary battery and possible an APC 5X3 propeller, but this project wasn’t worth the additional twenty dollars to me.

 

Improvements(?)

 

Although I have been openly critical of reports on just one flight that everything was wonderful and perfect, my (new) standard is fifty flights, this one won’t hold my interest for that long. I doubt any changes would make the DT-760 a satisfying RC flying machine. I made a dozen flights with it. Well, it does fly, but has been a long time since I was so disappointed with an RC airplane.

 

The main problem is the flat bottom airfoil (already warped on both sides) wing along with too much weight. It is never going to be stable to steer it around and never going to be agile. This is a slightly difficult to fly RC airplane that is neither relaxing nor exciting to fly.   Too heavy to fly slow and not fast enough or agile enough to be interesting. Stalls aren’t an issue.

 

My past experience with cheap servos has been bad. At least these ones center ok. Well, since I plan to not fly it much more… Maybe move the currently installed as the single aileron servo in the fuselage next to the elevator servo and add a rudder. And while I’m in there replace the unknown elevator servo with a decent one. Is it worth it to anybody to save thirty bucks on a pair of HiTek HS-55s that last for hundreds of flights verses hitting something when a cheap servo quits?

 

I have thousands and thousands of flights with just aileron, elevator and motor, somehow I kept reaching for a rudder on the DT-760. Not that it didn’t go straight, it does, and using the rudder won’t do anything for the lack of stability in roll, make it worse actually. In the case of the Twin Star II and Mini Mag (ha, back in Rhein-Main I found an unbuilt Mentor) after initiating a roll with the ailerons I use the rudder to make it go straight. But the DT-760 just isn’t, by the standards of this LiPo battery and brushless motor era, just not a fun RC airplane at all.

 

More motor wouldn’t help either as what it has, after fitting it with a correct propeller, a good match as it. Should somebody use this article as a basis for their own attempts the target is a static draw of ten amps on a 2S LiPo. Note that even using a decent quality LiPo that the voltage drop was way high. A better, higher “C” battery might draw too much power for my combination. That is why I fitted out my power meter for measurements. The bearings in such cheap motors often last about as long as the current expectation of twenty flights of an ARF. Better check the power draw regularly as when it goes up, as the bearings create drag, they burn out. Were I to continue with the DT-760 I’d fit it with a smaller propeller. There is so minimal satisfaction with the airframe though that it isn’t worth the six bucks to buy two alternatives to try out.

 

Years ago I built from plans out of a magazine (FMT about six years ago) a modified European Gump racing class RC airplane with a flat wing, by changing materials made to function much lighter with a small electric motor instead of the class racing fuel burner. The magazine one was built out of Depron and I added quite a bit of fiberglass. So mine weren’t racers, more of a general fun to fly RC airplane that could tolerate landing on Rhein-Main farm fields.   Just the same with it’s one meter wing spar, the wing exactly flat, I made hundreds of enjoyable flights with them. I twice added rudder to the aileron, elevator and motor combination, which demonstrated that sometimes less is more, I took the rudders back out. I estimate they had 2/3s the wing loading of this junker, flights went four times as long. The DT-760 just doesn’t do it for me. I doubt anybody else would like it any better.

 

One side piece (the forward fuselage is a “sandwich” of four flat parts) could be removed to make necessary clearance for the battery (to make balance it has to go under the elevator servo) and reinstalled. It would likely take me most of two hours as I’d add some fiberglass here and there while I was at it.  As is the recommended battery will not fit under the elevator servo nor can balance be made with it all the way forward. Given the high voltage drop under load a smaller battery seems pointless although were the DT-760 interesting I’d try a 2S600 mAh Lipo. Power as used was sufficient, if any thing less would be fine.

 

The vertical stabilizator was a little warped, I’d add a stiffener. A flat carbon fiber strip would likely suffice, at five minutes for the instillation. As long as I had it open to clearance for the battery I might add a rudder with the servo required too. That would take an hour or two. The material is so thin you can’t just laminate on fiberglass as when it shrinks it would warp. Glue on carbon fiber would be reasonable.

 

The single aileron servo could be replaced with two (8) gram servos (lighter down to four grams ok too), one in each wing, before the control arm breaks and a cable to connect them to a single channel. While I was at it replace the aileron horns or improve them with fiberglass overlay. Required for drilling them out to use Multiplex “barrel” connectors as the originals are marginal. Or use a more elaborate radio and put each of the two aileron servos on it’s own channel. That makes balancing the servo centers easy and allows tuning the differential between up and down. That would take about two to three hours and using the $2- Brand X servos and $1- Y connector, bought at the same Hobby People was closing, costing about five dollars. I made four hundred flights on a Mini Mag, about a hundred more than I recommend for the HiTek HS-55 used as the elevator servo. Two HiTek HS-55s and a Y connector at full price is more like thirty-five dollars for the aileron conversion when you add in the rods from the servo to the control horns (bought some at going out of business too) and improving the control horns.

 

Fortunately, Multiplex “barrel” connectors that outlasted their airframes fit, I have a small quantity on hand.

 

Recognition paint would take minutes and cost some minor percentage of the value of the can of spray paint.

 

But I just don’t see any reasonable way to correct that flat wing and it is too heavy. Most airplanes (from peanut sized to 747s) use some washout. That way when a stall happens it is to the outside of the wing, the inside keeps flying. The lack of washout of the DT-760 didn’t seem to matter. Although some of the EPP (expanded poly propylene) foams can be heat shaped (steam, dangerous, or boiling water, still some burn risk) this one doesn’t seem a likely one to mess with.

 

As would be expected for something intended for short races, neutral stability is fine for a standard Gump. But I often stay aloft for twenty minutes and more (with a 3S 5000 mAh LiPo flight duration of my current Twin Star II, cheap motors with already going batteries even, is over an hour) constant corrections to roll become irritating, so, I added angled up wing tips to a Gump. But that required either lengthening or increasing the size of the whole tail. For the DT-760 I’d have to model the wing tips from Depron and fiberglass and make a bigger tail. That is too much effort for the DT-760. You would be better off with a whole different wing. Or cut the existing at the inside of the ailerons and reattach angled up for some dihedral. Except that although that downward bow of each side of the wing, would turn into a mess trying to cut and reattach it.

 

In theory, a slit could be made and a flat spar added in. Getting a different wing would be a better solution.

 

I made hundreds of flights with various Mini Mags, only slightly more expensive they are far better fliers just as they come. With slight modifications I already know how to make them fly so well even Multiplex was surprised. Or, you can get a Multiplex Easy Star II. Both cost more initially, perform great right out of the box and have expected life spans of hundreds of flights. Still around (I could see them from the Point Loma Bluffs, slope soarers at the College on the ridge above) are flying wings (Zaggie, if it looks like a duck, and quacks like a duck) easily assembled, durable and a blast to fly that are a better use of time and money then attempting to fix the DT-760.

 

I won’t fly with servos that cheap longer then just enough to try an RC airplane, may I recommend not reusing the DT-760 servos in anything else including the original airplane. I’ve had the servos in this price class quit in as little as two hours of flight. But then how many people, at five to maximum ten minutes a flight, are going to get a dozen flights on a DT-760?

Where I test flew (not that a single flight is a sufficient test) the wind isn’t enough to slope soar. With it’s high wing loading about the only place I would use the DT-760, after making it reliable, is at Point Loma. I wanted a general purpose RC airplane, not a indifferent flyer for exactly one location and wind direction.

 

Conclusion

 

Probably the easiest way to make the DT-760 really flyable is;

Get a different airplane and start over.

 

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