My 250 build

[giiba]

I finished this over a year ago but never got around to flying it much until the past month. This meant all of my batteries (850mah, which I didn't store properly) had gone bad, which completed my setup funnily as I found 900mah ones that weigh 10g less (65g). I've now flown numerous times on the new batteries and I couldn't be happier.

I've a different setup than the usual around here. I've installed a 14t pinion and run at the lowest head speed the Talon 15 setup recommended with the 3900kv motor; I think 3400'ish rpm. In this way I'm getting 10min flight times doing moderate sport, draining 70-75% of the pack.

Given the lowered head speed I found the stock build to be rather pig-like and embarked on a weight loss mission. I tried to be smart not crazy; I trimmed a lot of fat but still kept things like the metal bearing and tail boom blocks for strength and the Chinese counter weights on the tail. My biggest savings were the servos (full set of ds76's), esc (Talon 15a), and canopy (plastic vs fiberglass makes a big difference).

My favourite touches are the Boca one-way-bearing which is so incredibly smooth in operation, and the Microheli head+grips which have a longer than stock arm on the blade grips and two positions to mount the swash links (stock and reduced movement). As I don't plan to 3d this bird the head reduces the leverage on the servos and increases the servo resolution of swash movements.

Specs:
- 250se afr w/belted tail
- Scorpion 2206 w/14t pinion
- Talon 15a esc
- DS76 cyclic servos
- DS76t on tail w/inline diode voltage reduction
- Tarot ZYX-S fbl w/spektrum satellite
- Microheli head and tail assemblies
- Wild Scorpion 900mah 30c batteries
- carbon 205 main blades and carbon tail blades

AFR weight: 221g
Flying weight: 285g

[dmorrow71]

That's a nice looking build! Awesome work

Sent from my HTCONE using Tapatalk 2

[Krager]

That's a light 250 you have. Mine started life as 250 Pro DFC. I've got it down to 329g all up and that's about best I think I can do. Yours is considerably lighter. I made some adapters and I'm using the Lynx 180 CFX servos, even on the tail. That and the plastic canopy were the biggest weight savings.

The Trex Pro DFC is a heavy 250. The torque tube adds weight over the belt and it has an all metal tail. It uses a different heavier landing gear set. I like all the metal bling, but costs in weight. I'm using metal bearing blocks, though the originals were plastic. The motor I'm using is bigger and heavier than the stock motor.

I'm actually running a higher speed on mine at 4600 rpm. I've tried lower head speeds but it just seems too mushy and under-powered to me. Probably because it's not light enough. I recently put a new FBL controller on it and haven't flow it yet, but I was running a BeastX before that and when I tried lower head speeds I had had trouble tuning it. The new controller which is a mini-Brain might do better, but I'm probably going to stick with the higher head speeds.

[Toocool4]

I think the 250 just likes high head speeds, i am running mine at 4700RPM. I did try 4900RPM but it just ate battery and i did not gain a huge amount.
Mine weighs in at around 330g. I am also using the plastic canopy, not in picture below.

[Wargamer]

Reading this, I was finding the weight and HS comparisons to the Oxy 2 interesting. The O2 in its stock (190mm) incarnation can get down to 325g flying weight if you really work on it. More normal is 350-360g. It generally likes 4500-5000+ depending on how you fly, although anything over 4000 is probably fine for basic sport. I haven't flown the stretch (210) yet, but that should come in maybe 10g heavier. Some of the guys are flying 210 at under 4000.

[giiba]

Thanks folks.

Reducing the weight was the key to the low headspeed, I even went so far as to shorten servo wires; the only one with some extra length is the esc plug so I can still access for programing.

There is some inverse relationship between size and headspeed for helis, and I suspect someone on the aerodynamics forum could tell us. Bigger birds use lower head speeds and micros need a relatively high headspeed. I'd guess it has to do with tip speed of the blades. I have a 'low' headspeed Nano CPX (3200rpm) and that's only possible because I use semi-symetrical blades on it it. I have gotten my 130x down below 3000, to 2600-2900'ish rpm that I'm still in testing with; and again with semi-symetrical blades.

Anyone know of semi-symetrical blades for this bird?

[Number]

Yup, belt FTW on the 250. Way more robust and lighter. My 250 was 270g. I didn't fly a canopy. Once I stripped the DS76s I put Spektrum 3050 and 3060 servos in it. Ended up at 286g RTF (no canopy). Flew fantastic.

Trex 250 DFC | Zeal NRG 210mm + KBDD EE 40mm (2 min 6 sec)

T-REX 250 DFC Before Crash (2 min 50 sec)

[Krager]

Originally Posted by Wargamer

Reading this, I was finding the weight and HS comparisons to the Oxy 2 interesting.

Yes that is. I haven't looked into the specs on the Oxy 2. So the rotor disk on my Trex 250 is about 30mm bigger, yet it's about 30 grams lighter than a typical weight for the Oxy 2. Guess my little Trex is not so bad after all.

[Number]

Well, the Oxy2 is VERY heavy for its size. People are using bigger electronics on it than on the T-REX (other than servos).

[giiba]

I've been pondering how to get a 250 down below the new 250g restrictions. Stepping down to a 130x is too far for my taste.

However cutting a further 35g seems impossible without some serious compromises.

[Wargamer]

It's tough to cut that much. You might save 3g per servo if you're willing to live with 1/3 of the torque. (Probably fine if you just fly scale and light sport.) You could save a few grams if you de-case the FBL. The canopy is worth another 15g or so if you fly without. (The Oxy 2 is 18g...) Throw in a smaller battery and you probably don't need to worry about de-casing the FBL.

But I'm not sure I totally understand the Canadian constraints. I've seen what I think are the restrictions, and it sounds like the one that could be challenging is the 75m distance to buildings. Although if it's your own house on private property, I can't imagine local police caring much.

On a different note, it was kind of funny to hear the T250 described as heavy for its class, and the Oxy 2 as very heavy for its size. That makes it sounds like there are a bunch of lighter helis that are serious competitors at 200 mm.

[Number]

Not really, it's just about disc loading. It's quite high on the 250 and now even higher on the Oxy. And because the RC world for some reason seems stuck in the high headspeed stupidly overpowered phase, no one is coming out with solutions (a lightweight ~200mm machine)

[Krager]

Yes the 250 class is not exactly rife with options. In fact the Oxy 2 is the only serious competitor to the Trex 250 we've seen for some time. Though it doesn't matter since it's about disk loading. It's high for the 250 Pro DFC in stock form with an all up weight around 350g. My Protos 380 has almost four times the rotor disc area, but only three times the weight. Even compared to my mCPX the Trex 250 Pro DFC in stock form has only three times more rotor disc area, but over four times more weight.

I think it's a bit tricky for makers to produce a light 250. It's a size where they need to start using design concepts of a bigger heli like CF frames and such, but the rotor disk is still a bit small to make that jump. If they were to use a modular style design out of plastic like they do on smaller helis there could be durability issues, such as frames that flex too much or crack. They could utilize a lot of plastic in the head and tail, but I actually like the metal bling even though it adds weight. It's possible to formulate plastics as strong as metal, but metal provides a known quantity. Plastics can vary a lot.

With higher disk loading it takes higher head speed to make the heli fly like it should. I guess that's the preferred solution for makers instead of coming up with a lighter design.

[giiba]

I definitely see a market for a new machine just under the new restrictions.

[gimbleguy]

Originally Posted by CraigHB

Yes the 250 class is not exactly rife with options. In fact the Oxy 2 is the only serious competitor to the Trex 250 we've seen for some time. Though it doesn't matter since it's about disk loading. It's high for the 250 Pro DFC in stock form with an all up weight around 350g. My Protos 380 has almost four times the rotor disc area, but only three times the weight. Even compared to my mCPX the Trex 250 Pro DFC in stock form has only three times more rotor disc area, but over four times more weight.

I think it's a bit tricky for makers to produce a light 250. It's a size where they need to start using design concepts of a bigger heli like CF frames and such, but the rotor disk is still a bit small to make that jump. If they were to use a modular style design out of plastic like they do on smaller helis there could be durability issues, such as frames that flex too much or crack. They could utilize a lot of plastic in the head and tail, but I actually like the metal bling even though it adds weight. It's possible to formulate plastics as strong as metal, but metal provides a known quantity. Plastics can vary a lot.

With higher disk loading it takes higher head speed to make the heli fly like it should. I guess that's the preferred solution for makers instead of coming up with a lighter design.

the thing with plastic, the UV rays here is ultra harsh, and in time it becomes brittle

[bazsound]

Getting under 300g is really something!


Mine last time i measured was around 360g AUW with an 850mah battery.

Although mine starte as copter x and got the usual align upgrades. so maybe the frames heavier.

Id love to go with the lighter servos but i crash too much and so far the haevy turnigy 213c;s have not only survioved many crashes on my 240, they survived 2 crashes on my 450 the recent one almost totallying the heli

[Murph_d]

The CopterX frame is lighter than the stock Align frame. It has been awhile but if my memory serves, the difference is around 4 or 5 grams.