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Nov 27, 2019, 11:10 PM
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Mini-Review

OMPHobby T720 4-channel gyro-stabilised Trainer RTF


There's a new breed of inexpensive park flyers and micro planes emerging that uses gyro stabilisation in interesting ways. Each manufacturer seems to have a slightly different take on how they use the automation so you can't assume anything really. And this is just the beginning. I expect ever more sophistication and innovation in this class.

As well as the gyro, the T720 has another innovation - It has no exposed control horns, push rods or clevises. You just push the wings on, they lock in with a satisfying "click", and the servo horns mate with recesses in the ends of the ailerons. A similar hidden mechanism drives the elevator and the pre-assembled rudder.

The plane is a powered glider, pusher style, reminiscent of the famous Bixler, but scaled down to 718mm wingspan. It's aimed at the beginner and intermediate flier, but experienced fliers will also have fun with it in the Sport mode. I think it will be popular as a slope soarer, but its small size might limit it in that role to flying close in. It is available from Banggood here and you can get it in both Mode 1 and Mode 2 versions.

Assembly is dead simple. Just click the wings in, fit the prop and spinner with a single screw, and you're ready to go. Some people have had trouble because they have fitted the prop the wrong way round. So here's a simple rule that will work for ANY prop, even if it's on a boat. The convex side of the prop blades always points towards the direction of travel.

If you do fit it wrong, the symptom will be an awful lack of power, because a prop fitted the wrong way round is extremely inefficient.

You have the choice of two flying modes, Normal (beginner) and Sport (advanced). Both of them use gyro stabilisation, but unlike with some others such as the XK A800, you have full aerobatic capabilities in Sport mode, with no limitation on pitch or roll angles. In the Sport mode, the gyro stabilisation seems to be limited to restoring level flight if you give no stick input, but otherwise doesn't interfere with you. So rolls, loops, Immelmanns and inverted flight are no problem.

In Normal mode, there are restrictions on both roll and pitch, and if you throttle off and let go of the sticks, it will glide straight and level, so this mode is ideal for a beginner. And if you are in Sport mode and get into trouble, a click of an easily accessible switch will put you into Normal mode.

There is also a Trick button, but I haven’t tried it yet. The manual doesn’t say exactly what it does, just “A set of aerobatics can be automatically completed by long pressing the trick button for 2 seconds. (Make sure the airplane is in the air and at a decent height to accommodate)”.

When I first assembled it, I checked where the centre of gravity is, as I always do. To my surprise, it was far further to the rear than I expected. So I set about testing the glide with no power. To make it glide to my satisfaction I had to add almost 20g of weight to the nose. That's a lot in a plane that started out at 139g, including battery! Now I suppose the gyro stabilisation will be able to compensate to some extent for a serious case of tail heaviness, but I just can't believe that very tail heavy is ever a good thing. It's probably because of the old saying "A nose heavy plane may fly badly, but a tail heavy plane only flies once!"

So in the end I put about 15g of weight in the nose, in the form of a lead-headed roofing nail. The flight video below is done with that in place.

I did a check by turning off the transmitter with the motor running, to see how it would handle flying out of range or a flat transmitter battery. Not only did the motor stop almost immediately, but the rudder flicked to full throw on one side. The plane would just glide down in tight circles if it lost the signal. (NOT like the Volantex/Eachine Mini Mustang!).

I also noted that where the wings join the fuselage, the servo horn on the left side side sticks out just under 2mm more than it does on the right, so there is a slight gap between the wing and the fuselage near the servo horn. It doesn't seem to make any difference in flight though.

Other than those points, I think this plane is very well done, innovative, a bit different from anything else I have seen, and fun to fly.

Pros:

- Extremely well packaged. Three separate sturdy boxes inside the main box.
- Clever use of stabilisation that helps the beginner but doesn't hamstring the expert
- Robust construction and easy assembly.
- Nose piece made of highly energy absorbing foam, different from the foam in the rest of it.
- THERE IS A MODE 1 VERSION!
- Innovative hidden control surface operation that seems to work very well.
- Handy charger that charges the 2S 300mAh battery through the balance port, using any computer or powerbank USB port. The label says it charges at 1000mA, but I measured it at only 600mA, which is a little high, but shouldn’t have much impact on the life of the battery.
- Battery arrived at the correct storage charge of close to 3.8V per cell
- Battery form factor similar to many other 2S 300mAh batteries, so three other different ones I have could all be easily accommodated in the battery bay
- Spare prop connector piece provided
- tiny screwdriver provided for the one screw to fit the prop
- Very good manual, clear, comprehensive and in good English, with excellent illustrations. Very few errors, but Mode 1 and Mode 2 are referred to as “Model 1” and “Model 2”.

Cons:
- Tail heavy out of the box
- Rather heavy, doesn’t glide as well as others such as the WLToys F959. On the other hand, the extra weight would work well if you’re slope soaring
- Slightly inaccurate manufacturing in the area of the join of the wings to the fuselage
- No spare prop, but you’re unlikely to need one, as it’s a pusher
- I don’t think it uses a common protocol, so you have to fly it with the very basic transmitter supplied. I would LOVE to be proved wrong, so I can fly it with my Taranis
Last edited by 1auriec; Nov 29, 2019 at 02:29 AM. Reason: More on tail heavy
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Nov 27, 2019, 11:15 PM
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Flight Video - Normal and Sport mode flights


The first flight is in Normal mode. Towards the end of that flight I tried an experiment flying very close and very low to see if I thought I could fly it inside on a double basketball court. I think I could. Not much point though, not enough head room to do any aerobatics .

The second flight is in Sport mode.

OMPHobby T720 RC Trainer Flying in Normal and Sport Modes (5 min 37 sec)
Last edited by 1auriec; Nov 28, 2019 at 06:27 AM.
Dec 03, 2019, 10:02 PM
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Beware of crack in joiner


Just been flying, and had a landing where the wings fell off, which has happened before and isn't a problem. However, when I picked them up, I noticed one of the black plastic joiners had cracked where it enters the wing (see photo).

I've repaired it with CA glue, but I'll be watching to make sure that's adequate. If worst comes to worst, I'll attach the wings permanently.
Dec 04, 2019, 02:01 AM
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Uh-oh!


Well, the CA fix didn't work as expected. I reattached the wings, intending to fly later today, but the wing with the CAd part didn't want to fit properly.

I had assumed that when I put some CA in the crack, closed the crack and applied kicker, the joiner would automatically assume the right angle. WRONG! When the wing wouldn't fit well, I checked it against the other one, and found the joiner was not at the right angle. It should be exactly parallel to the carbon fibre spar in the wing.

I tried to straighten it, but couldn't. It eventually broke at the same spot, although only after I had applied a lot of force, so I did prove the CA does stick this material pretty well.

Rather than try again, I took another tack. I decided to put in a new spar, and fix the wing in place semi-permanently. I thought of using a carbon fibre rod, but then changed my mind and used a bamboo barbecue skewer, 3mm in diameter. That way it will break in a bad crash instead of the wing.

I ground the remains of the joiner back flush with the rest of the wing root. I drilled 3mm holes in both wings just forward of the joiner. To make the hole positions accurate I first used the tip of my hot soldering iron to mark on the plastic where the hole has to go. There's really no way I could have done it with a centre punch. Then I took my skewer and poked a hole through the fuselage with it to align with the holes in the wings. I cut the point off the skewer to get a piece long enough to go through the fuselage and 65mm into each wing. I fitted the wings and put a small piece of fibreglass tape under each wing root joining the wing to the fuselage to stop it coming off too easily.

If there had been parts available, I probably would have ordered the relevant wing and flown something else until it arrived.
Dec 04, 2019, 04:13 AM
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Repair seems to be very successful. I went to the park this evening with four batteries and exhausted them all, mostly in Sport mode. What fun. No sign of any wings falling off.
Dec 06, 2019, 12:22 AM
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Beware the outside loop!


Flew it again today, and with a rush of blood to the head decided to attempt an outside loop. I THOUGHT I had plenty of altitude. WRONG! It became apparent it wouldn't complete the outside loop without hitting the ground first. I contemplated up elevator to pull out, but was just too late. It went into the ground nose first at speed.

One wing and both tail pieces came off. The bamboo skewer was broken of course, and with it the remaining joiner which just happened to be on the wing that came off.

However, amazingly, no real damage. Replace the skewer, new piece of tape, reattach tail pieces, ready to go again. The foam in this plane is indestructible, it seems.

While I had the tail pieces off I inspected the elevator linkage. I noticed the control horn had two holes, and the stock position was with the push rod on the outer hole. By removing a tiny screw that holds the linkage together, I was able to release the mechanism enough to move the push rod to the inner hole, thus giving more throw (see photo).

I'll be trying the outside loop again as soon as I can, but this time with more altitude.
Dec 06, 2019, 05:50 AM
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Pixelpeter's Avatar
Still waiting to maiden my T720 on a calm day (when it isn't freezing cold!). Think I'll fly it from day one with a small camera for extra nose-weight.
Dec 06, 2019, 04:50 PM
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Quote:
Originally Posted by Pixelpeter
Still waiting to maiden my T720 on a calm day (when it isn't freezing cold!). Think I'll fly it from day one with a small camera for extra nose-weight.
I suspect "freezing cold" is colder than most Australians can even imagine. I hope you do get a good day. More nose weight is definitely a good idea.

I did get a chance to fly this morning, and did succeed with the outside loop, although it doesn't do too well on the second half of the loop. Needs more power, and possibly more elevator authority.

Unfortunately, my rudder servo seems to have failed. My rudder would only go to one side at the start of the flight, and by the end I had no rudder at all. Not looking forward to opening it up to try and replace it.
Dec 06, 2019, 05:47 PM
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Quote:
Originally Posted by 1auriec
I suspect "freezing cold" is colder than most Australians can even imagine. I hope you do get a good day. More nose weight is definitely a good idea.

I did get a chance to fly this morning, and did succeed with the outside loop, although it doesn't do too well on the second half of the loop. Needs more power, and possibly more elevator authority.

Unfortunately, my rudder servo seems to have failed. My rudder would only go to one side at the start of the flight, and by the end I had no rudder at all. Not looking forward to opening it up to try and replace it.
Temperature goes up the coming days, but it will be rainy and windy for a change

Good luck with the surgery
Dec 07, 2019, 05:37 PM
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I think I'll hold fire on the surgery. It's possible it is the Tx that is at fault and not the servo. I have a S720 arriving in the next few days, so I'll bind my T720 to that Tx and see if the servo works.
Dec 11, 2019, 09:00 PM
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It looks as though the surgery is going to be necessary. Now the elevator servo has stopped also. My S720 has arrived, and I have been able to bind the transmitter that came with it with the T720, but the servos still didn't work. So I am thinking that either the servos have failed, or else the controller/receiver board ports for those servos have a problem. It does look as though I can get enough access to the servo ports to plug in another servo and see if that makes any difference. I had to cut away the foam where the battery sits to get the access.
Dec 12, 2019, 08:56 AM
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Pixelpeter's Avatar
Hope this turns out okay for you
Dec 17, 2019, 08:02 PM
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siriusflier's Avatar
1auriec, I have read all of the RCGroups posted reviews on the T720 and found the best information in your review. I just received mine today and put it together, also tried a brief flight.

Like you after some difficulty in getting the left wing installed(the round hole does not line up quite right), I noticed that the left wing would not push in completely, leaving a small gap near the back. You nicely explained that in one of your posts.

Also, after checking the COG, you and some others posted that it is tail heavy, so I added a weight to the front before the maiden. I used a strong magnet that fits nicely in the indentation where the metal ball is in the canopy/battery cover. Unfortunately, that magnet only weighs about 6 grams, and it looked like it might need more nose weight added.

It was only about 45'F(7.2'C) and getting dark when the wind died down, so it was not that long a flight, but it did fly well.
The only problem I had was when I landed. It landed level on dry grass and was not a hard landing. but when I picked it up, the main wings had come loose. I know they had snapped together properly when I installed them. I am wondering if maybe the left wing being kept from being pushed in flush could have contributed to it coming loose? Will just have to tinker with it and see. I seldom have a need for disassembly so I will probably add tape to hold the wings in place better.
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Dec 17, 2019, 10:56 PM
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Siriusflyer - Thanks for your kind comments. I have found the wings did fall off on landing very frequently, but never in flight, but I took that as a good thing because it stopped them from breaking. However, if the wing gets stopped too suddenly on landing, the bending forces can cause one of the joining parts to split as I describe above, and I think that's a weakness of the plane. My repair with a skewer has worked just fine, but now I have a different problem, as two servos have stopped working.

The S720 has exactly the same arrangements for holding the wings on, and so far in quite a few flights I have not broken them. Fingers crossed.
Dec 18, 2019, 11:06 AM
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siriusflier's Avatar
"I also noted that where the wings join the fuselage, the servo horn on the left side side sticks out just under 2mm more than it does on the right, so there is a slight gap between the wing and the fuselage near the servo horn. It doesn't seem to make any difference in flight though."

On mine, the round tube that the wing dowel goes into was sticking out around 2mm. I filed that flush with the fuselage, and now the wing fits flush with no gap.
Latest blog entry: Beginner RC Plane Pilot


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