trex 600 stretch to 640

[quinn]

i have a trex 600 dfc with a kde 600xf. the heli is awesome the way it is, but i wouldnt mind it being a little bigger with lighter disc loading. i was thinking i would stretch the the tail 50mm by cutting down a 700 boom and splicing a torque tube to the right length. then i would throw some 640mm blades on it, or stretch tail 70mm and put 660mm blades on. the motor has plenty of head room to handle the extra power. pulling 90 amps max right now. i think after the stretch it should stay below 120 amps and its rated for 170 amps burst. the only thing im wondering is if the head and shafts can handle the extra load. i would think it should be fine since some guys are stretching to a full 700. i will be using 3800 45c thor lipos. disc loading should go down quite a bit even with weight increase of batts, blades and longer tail. what would be pros/cons to doing this? will cyclic be less snappy with bigger disc?

[ChadL]

I've been thinking of doing the same thing. The cons will be shorter flight times as the motor will have to work harder to spin the bigger blades. Elevator performance will also be degraded due the boom being longer. With that said though, you can probably compensate for some of the degraded cyclic performance with roll rates and flip rates in your FBL controller.
My primary concern is the decreased tail performance due to the decreased head speed (bigger blades remember). If I go this route I think I will do the 4.5:1 gear ratio mod so I can turn slower head speeds and not have tail issues. The longer boom should help with tail authority, but given the stock tail rotor gearing I think 4.5:1 is still a must. The bigger blades will also exhibit greater torque so the tail is going to need as much help as possible.

[quinn]

Maybe 110 mm tail blades? If main blades are 660mm with full 700 tail, there should be plenty of room. Also like you said, the 700 tail will give more leverage and better authority, especially since main blades won't be a full 700mm, so less torque. Also should be able to have higher head speed than 700 with 660mm blades. I'm thinking like 2300. The cyclic should at least be as fast as a normal 700 right? Also im switching from 3300 to 3800 batteries so hoping for at least the same run time as before. Should be a sweet setup. Thrust to weight should go up about 10 percent and disc loading gets lower too.

[ChadL]

110 blades will certainly help. If you go that route don't forget to get the 700 landing struts and tail fin. Since I've not stretched I can only presume the cyclic response will be similar to a 700.

[quinn]

actually i already have a 700 gear on the back of my 600 and a tail fin thats 2cm longer than stock 700, so no problem there. i still cant decide if i want to go 660mm or just 640mm.

[martin_]

I have done the 640mm mod but in the other way in shortening a 700DFC, I have also put a 600 tail case to save weight. Other than the heavier 700 rotor head, final weight is only a few gram more than a 600.

I spin the head close to 2600rpm and have the benefit of the 4.7:1 tail ratio, I can use 95mm T-balde, even 90mm T-blade is manageable with the 4.9:1 Nitro tail ratio.

Other benefit is the rotor head is up to the task, I would never risk a 600 head at that speed with 640mm blade.

You can use an Outrage Velocity 50 torque tube for 640mm mod, it's a direct fit, of course you need to cut a 700 booom to the right lenght.

Have a set of 3800mah but the flight time is not very long, somewhere in the 3 to 3.30 min mark, so I use 4400mah on that bird.

[quinn]

wow! 640mm blades at 2600 head speed? i thought i was pushing it with 600mm blades at 2400. what motor are you running on that thing? how many amps you pull? i bet its a beast! i was under the impression that the 600 airframe was quite a bit lighter than the 700. i guess if you think about it, its just a little more carbon. most difference would be in the head

[martin_]

I said close to 2600, but not all the time, I moslty get that at the beginning of the flight when gov is overspeeding a little, last time I made adjustment I tached it somewhere in the 2530rpm, I'm running around 90% gov on yge esc, not much head room I know.... but it's only for iu-2, most of the time I fly in iu-1 at 2350rpm

Motor is earlier scorpion HK4035-560, measured at 595kv and comfirmed with a tach on the head (common with scorpion motor to have higher KV)

Both the 600pro and 700dfc have 2.0mm thick frame, so not much difference here, and I don't have put the bottom plate to compensate, only frame tube spacer to keep the rigidity.

Here is a vid I've made the last summer in his first few flight:

Align T-rex 700 DFC new servo test. (6 min 3 sec)

[quinn]

Are you running the yge 120hv? I tried the yep 120 and was having trouble with gov gain and cut outs. What is your p and I gain?

[quinn]

just ordered the parts for my stretch. i ended up getting some rotortech 640mm. it was either those or sab 640's that are 150 grams heavier per pair. have any of you guys tried these blades? i dont really like the ugly yellow, but hopefully they fly well. going to use a velocity 90 torque tube.could have just used the 50 tube, but wanted to be able to go long enough in case i need 110 or 115 tail blades. the ends are pinned on with screws, so i should be able to cut it to the length i want and drill a hole for the screw.

[338federal]

I have 690mm blades and 105 in tail. Headspeed is about 1700-1800. The maximum headspeed with these blades on 600 head??? I take it careful. I don´t fly hard 3D yet so little rookie flight.

[quinn]

the stretch is complete! everything went together nicely. the velocity torque tube worked perfectly, i just cut it to length and drilled a new hole. i did have to grind of the teeth that stick out sideways from to torque tube end because they were rubbing the inside of the boom block. i decided to go 75mm longer in the tail so i can still put on 660mm blades if i ever want to, or i can cut it down further if i decide elevator is too slow from the extra length. now im just waiting for the jb weld to dry. i cut the control rod and lengthened it by inserting both ends into an aluminum tube with jb weld. after it dries, i will drill holes through it and pin with screws just in case. i did spool it up without the blades and the tail actually had less vibration with the velocity torque tube. the new edge 120hv will be here saturday. should easily be able to pull 110 or 120 amps with the 640 blades. i was already pulling 95 with the stock 600. should have tons of power now. cant wait to maiden it!

[martin_]

If using the align 600 head, you still have room for 660mm main and 115mm tail with a velocity 50 T-Tube, a tight fit but it clear. With an align 700 head, 640mm blade is about the max since the head is larger by 35 + mm.

Out of curiosity, how much distance you came with between the center of the main shaft and the center of the tail output shaft? Mine measure 872mm.

[quinn]

Ill get a measurement when I get off work. It seems like the 50 would be too short, but I guess if it worked for you, then it should work. Are you running a dfc head? I only cut about 2 inches off the 90 tube, but I made sure I had plenty of room. It would be nice to know if the 50 was long enough for anyone else doing this mod. For me it was no big deal to resize it since I have a drill press, but if you didn't, it might be difficult to get that hole just right.

[quinn]

just did the first few flights. flies awesome! huge difference in the way it flies. since i still have the hv 80 on it, i flew at 2250 headspeed and 12 degrees collective, but it felt like 2350 and 13.5 did with the old blades. i expected the cyclic to be a little slower, but its actually a lot snappier than before. tomorrow the edge 120 esc shows up. then ill crank up the head speed and pitch. should be pretty insane. the rotortech blades seem to be very efficient. i had 10 percent more capacity left than i normally would with the old setup at same headspeed and pitch.seems odd, i expected the other way around. im sure ill be getting much less flight time once i crank up the head speed to 2400 and pitch to 14

[quinn]

well, i put a whopping 6 flights on my stretch setup before i stripped a tail gear and smacked the ground hard. really not to bad, but obviously the new blades are done. new esc came today, but didnt get the chance to try it out. i liked the rotortech blades but dont like the color at all.the disc pretty much disappears in the sky. looking for some different ones to try out after i rebuild. ill probably go back to the stock 600 setup for a while since i already have blades and a whole tail to put on. just need servo gears and landing skids.

[quinn]

the stretch is rebuilt and im a happy camper. went with the rotortech 640's again. they are awesome. anyone thinking of doing a stretch should do it. the difference is amazing. just make sure you have a good motor. kde 600xf is perfect.