550 DFC with 510kv motor ?

[npomeroy]

I've just bought a near-new stock Trex 550: DFC head, 1220 motor, 112T main gear, 600 boom, Talon 90 ESC.

I happen to have a spare 600MX motor of 510kv, and was thinking of putting a 16T pinion on it for 1200 - 1600 HS. Elsewhere it was suggested this is very inefficient and it's best to use small pinion/higher kv motor (890 would be good). But I wonder how much difference it makes. At least the 510 motor would be running near 100% which is good for the ESC.
I've ordered the swash driver, grip arms (and already have the older swash plate) to remove the DFC stuff as people say they are not so good at low HS (but is it that bad?).
I'm running 6S - have 5000 and 2x2600 parallel mAh lipos.

The stock blades are 520mm which I expect is a bit short for this application.

I don't do 3D. In fact this project is to create a platform for me to learn sport flying better - I mainly fly scale but am not that skilled and decided a pod-n-boom would allow me to be more adventurous with less at stake in crashes!

[Dennis P]

I did not calculate your headspeed but I am sure you have that figured out. Sounds like a good setup and will be fine on 6s at the lower speeds. As you have mentioned the 520s will not be great. I would go 560s or bigger as I assume it has the 600 boom? If so 600s would be fine at the lower speeds plus some longer tailblades.

[npomeroy]

Anyone know WHY the DFC heads are said to be less smooth (or whatever) at low HS?

[trillian]

Originally Posted by npomeroy

Anyone know WHY the DFC heads are said to be less smooth (or whatever) at low HS?

The only logical reason I can think of is simply because they often use quite hard dampers to keep teetering to a minimum. I am building up a heli that uses the 800 DFC head (this is with 780mm blades) and was also told it's not good at low headspeeds but I think I'll try it before changing it out.

I think are ways to limit the amount the feathering shaft can teeter and yet still have some damping because my 2009 version TDR has a very rigid head and yet it is perfectly fine at low headspeeds.

Regarding the 600mx/510, it's a great motor on 12s, has bags of power. Obviously you'll run less watts at low headspeeds so it's worth a try especially if you're not flying aggressively. I recently converted two 600 size helis to 6S but there was just no pinion options that would have allowed me to keep the 510KV motors so I swapped them for the 730MX / 890kv and run them both at about 1700 which is working very well. One of them (Beam Avantgarde) is quite small for a 600, more 550 size so I have flown it with Align 550 mains as well but upped the headspeed a bit to get enough tailrotor authority. Not sure how low you can go with 550s but at anything under 1800 ish you can definitely get plenty of punch on 6S.

[npomeroy]

Thanks for the comments trillian. Yestrday I looked further into the DFC business (especially here http://www.rchelicopterfun.com/DFC.html). It seems:
- When the head teeters it strains the swash-to-arm connection, therefore the head needs to be fairly rigid, hence hard dampeners. (But I don't see why this in itself limits low rpm flight. I have a rigid scale head on an AS350 that flies smooth at low rpm.)
- DFC heads are built low, so boom/tail strikes are more likely. Running high rpm tends to hold the blades "out" with more force so the down-flap may not be as great.

W.r.t. the 730MX 890kv motor: Isn't it wider than the 600MX, and intended for the Dominator? It may not fit between the frames of a V2 or early DFC Trex 550.

[trillian]

Originally Posted by npomeroy

W.r.t. the 730MX 890kv motor: Isn't it wider than the 600MX, and intended for the Dominator? It may not fit between the frames of a V2 or early DFC Trex 550.

Yeah the 730MX is wider. In my case clearance wasn't a problem and it was just a matter of it being an easy motor to get hold of and test whether these helis would work well enough on 6s. I actually have three 600 size helis on 6s, two have the 730mx and the other has a Hyperion 4025 / 740.

[npomeroy]

It's had a some flights now. I swapped the Talon for a CC ICE75 that I had, to get logging.

1200 rpm
510 kv
16T / 112T gearing
560mm Rotor Tech mains
Draws about 13 amps average in hover and FF flight

[Smoggie]

If you want to try sports aerobatics then this setup will be horrible. Very low headspeed gives little margin of power to get out of trouble, plus the tail ratio gearing on the Pro is optimised for RPM well over double what you are running, tail hold will be diabolically poor. Hovering and gentle flight may be ok but anything more aggressive will almost certainly blow the tail out.

Why not just fly it with the stock motor? For sports flying what advantage do you hope to get from very LHS?

[npomeroy]

Originally Posted by Grumpy_Old_Man

If you want to try sports aerobatics then this setup isn't going to be good. very low headspeed gives little margin of power to get out of trouble plus the tail ratio gearing on the Pro is optimised for RPM double what you are running, tail hold will be terrible, hovering and gentle flight may be ok but anything more aggressive will almost certainly blow the tail out.

Why not just fly it with the stock motor?

PS.. is this the version of the Pro that uses the 600 boom?.. if it's the one with the 550 boom then running 560mm main blades is a disaster waiting to happen, and it probably wont wait very long!

I do gentle sports. It's really a practice airframe to improve my scale flying. I don't think this is a "Pro" at all. Its a 550 DFC frame with 600 boom, converted to the 550 V2 head.
Low HS gives high aerodynamic efficiency - low current - long flights. Why does anyone on this sub-forum want low head speeds?

[Smoggie]

Originally Posted by npomeroy

Low HS gives high aerodynamic efficiency - low current - long flights. Why does anyone on this sub-forum want low head speeds?

If long flights are your one and only objective then fair enough, but that's not consistent with what you said you were looking for in the first post? Running 1200RPM on a Trex 550 will give very marginal performance, it should just about struggle into the air but that's about it, plus it will have extremely poor tail hold which will further limit the helis capabilities and potentially be pretty dangerous.

IMHO These things don't really fit with building a heli for you "to be more adventurous" in your flying?

[ic-racer]

DFC feathering shaft can't pivot up and down on the mainshaft spinning axis. It has to wobble side-to-side to pivot due the the fixed dfc shafts.

[Smoggie]

PS.. you cant fit the flybarless swash driver to the DFC head/shaft. There isn't enough clearance between the bottom of the head block and the swash, the swash would hit the head when you applied positive pitch. You would at least need a longer main shaft, ideally the non-DFC head block too. You would also need the non-DFC grip control arms and (obviously) ball links.

If you wanted to realise any advantage from the conversion then softer dampers would be needed too.

[Dogdipstick]

They make a 3.85 ratio tail gear set for high headspeed and a 4.5 ratio for more conservative headspeeds, it was the version 1 vs the version 3 that had the differences, there is a autogear availible for either one way bearings, for the higher ratio the part numbers are

http://www.ebay.com/itm/181299567843...%3AMEBIDX%3AIT

and

http://www.ebay.com/itm/182106725704...%3AMEBIDX%3AIT

These along with larger tail blades might restore tail authority with the low headspeed. Example parts shown are for the new style one way. You also want to research the blade choice and clearance, if you have a 550 boom not the 600 boom. Requires a small frame mod for best gear engagement, kde also makes a reinforcement bracket for the higher ratio.

[npomeroy]

A few points:
- I do understand the features of the DFC head. I've retained the head block, but inserted softer dampers and changed the shaft, grip arms, swash, swash driver etc to the EFL parts.
- A have used the higher tail gearing on another heli and so may convert if I need it.
- I am using larger than standard tail blades (92mm).
- Let me define "more adventurous" please.
- This is an experiment, and I may want to change, but so far am simply reporting how it goes. I'm happy with the flying so far.

[Smoggie]

fair enough.. if you have flown it any you are happy then that's fine.

[Keyrigger]

Nelson:

One of the drawbacks to the DFC system is that when you have any teetering of the spindle, the two links swing in concert the same direction so if the spindle teeters to the left (viewed from behind), both DFC arms swing to the right. This causes one or both DFC arms to bend and that, on high head speed systems, has caused all sorts of breakages ranging from bent links to broken swash balls. A few manufacturers have taken steps to have one solid link to be the driver and the other to be a standard two plastic ball end pitch link. My Heli-Baby is an example of the single hard, single soft, pitch link system which was developed from the Diabolo 550. Works like a charm. Take care.

Don

[npomeroy]

Hi Don:
I hadn't heard of the hybrid system with just one "solid" link. But as said above, mine is totally converted to the older system except for the outer housing of the head hub.