Trex700 F3C

[choppertastic]

Does anyone know how this differs in spec / build when compared to an 700N?

[GravityKills]

pchristyuk converted one for F3C. I think the most important modification was one pound of lead in the nose. There were pictures in an article related to the European championships on Romania.

[5exi]

Like this 7th pic down

European F3C Championships 2010 - AllMYRC.com-A Radio Control Helicopter, Airplane and Car Community

[coolice]

Hey.

I'm not 100% sure on actual changes as yet but can speculate.

1. I would say head ratios are changed to provide more flybar feedback to the main blades to compensate for wind gusts etc. Probably even adding more adjustability for the professional flier able to fine tune the models response and feel.
2. Paddles might be heavier and even a different shape.
3. Main and Tail gear ratios might be changed to allow a slower rotor rpm, I know there is a lower Kv motor going to be available to compliment it.

Build wise it's going to be usual Align 700E, as it's exactly the same just with tweeks to make it calmer and less 3D chuckable.

I'm interested to know the actual changes as the kits start to filter through.
.

[Steve]

You can get a standard 700 to fly quite well for this style of flying by:

1. A load of lead in the front of the canopy - you want it slight nose heavy with a full tank of fuel.
2. A large area solid horizontal fin (the carbon Hirobo Freya one from Quick UK is ideal).
3. Turn down the cyclic throws on the swash mix quite a bit.
4. Increase the flybar ratio by drilling and tapping two new M3 mixing arm screw holes on the seesaw, in the gap between the original holes and the outside corner of the seesaw.
5. Adding dedicated FAI blades (i.e. heavy ones) like the Radix FAI 710mm at 230g each.
6. Adding flybar weights (K&S do some 4mm weights, as do JR - they were standard issue in the Vibe 90 3D kit).
7. Keeping the head speed down to 1500 or less in the hover.

I've got the heavy blades, the flybar weights and reduced cyclic throws on mine and it kinda works ok within the capabilities of my flying skills. I keep meaning to add a load of weight to the front but just never seem to get around to it.

[trillian]

Someone at one of the clubs I fly at has modded his 700n for F3C and he upped the flybar ratio which required drilling a new hole in the mixer arms.

But it seems Mark left the stock flybar ratio and is just compensating with different paddles, weights.

[Steve]

Someone at one of the clubs I fly at has modded his 700n for F3C and he upped the flybar ratio which required drilling a new hole in the mixer arms.

But it seems Mark left the stock flybar ratio and is just compensating with different paddles, weights.

Correct.

Mark overcame the low flybar ratio by using Stubz paddles with the weights about half way out along the flybar. He also had all the other stuff in my previous post, and his 700 was a lecky version. His model also came in significantly lighter than both Steve Robert's Sylphide and Dave Fisher's Avro in the European Champs scrutineering.

[Steve]

Here's a couple of pictures nicked from 'another' forum which show the mechanical alterations you can make to the head. Moving the mixer arm mounting location is what gives you an increased flybar ratio which should make the model better at auto-correcting to gusts of wind in the hover. They say this is mod makes a significant difference in this area.

Extending the blade grip mounting balls towards the centre of the head is reducing the delta offset - I've tried this and to be honest couldn't tell any difference so went back to the standard setup. I've also spoken to others who came to the same conclusion.

[trillian]

I've got the heavy blades, the flybar weights and reduced cyclic throws on mine and it kinda works ok within the capabilities of my flying skills. I keep meaning to add a load of weight to the front but just never seem to get around to it.

Yeah, I have yet to get serious enough about the C of G to add weights to any of my helis. I don't have a 700 but I am setting up two Robbe Futuras for F3C. I am curious to see how much difference getting the balance right using some weight (if needed) makes. In my experience it is quite common that helis tend to be slightly tail-heavy for some reason but for general flying a little elevator trim seems to compensate OK. I think the Raptors are the only exception among the helis I own, being slightly nose-heavy.

[GravityKills]

Like this 7th pic down

European F3C Championships 2010 - AllMYRC.com-A Radio Control Helicopter, Airplane and Car Community

yes... BTW metal in the nose is usually a tell-tale sign of prolonged coke consumption

[trillian]

yes... BTW metal in the nose is usually a tell-tale sign of prolonged coke consumption

You could in theory use less weight by putting a smaller weight on something like a carbon tube that extends in front of the heli. It wouldn't look as tidy but it is possible.

[jay916]

G-force adjustable mixing and washout arms, SG720 F3C blades, Rotortech F3C paddles. Also played with spacers on the blade grip ball links to change the delta. At zero delta I felt it hovered better but flew worse.

Made my 700N fly like a different machine. I think the blades where abit long but made it feel like flying with a forward CG. Great vertical penetration. Auto'd forever, well not quite.

I don't think the standard blade grips are very good at holding heavy blades. The inside of the grips have very little surface area contacting the blade root so getting the bolt tightness just right so you have abit of lead lag movement but not so loose you fold the blades back after a long auto is hard. Not so bad with the Radix blades because they have a narrower root than the SG's so you use washers.

Oh, don't think this post infers any flying ability but I do think I understand the theory.

Justin

[GravityKills]

You could in theory use less weight by putting a smaller weight on something like a carbon tube that extends in front of the heli. It wouldn't look as tidy but it is possible.

that's an interesting idea. But I think it would increase the moment of inertia proportionally:
The torque caused by the mass towards the COG is ~m*d. Scale mass and distance inversely, it doesn't change.
But the moment of inertia is, I think, ~m*d^2.

I'd do at least some flight testing with a cheap prototype before building something more expensive.

[trillian]

that's an interesting idea. But I think it would increase the moment of inertia proportionally:
The torque caused by the mass towards the COG is ~m*d. Scale mass and distance inversely, it doesn't change.
But the moment of inertia is, I think, ~m*d^2.

I'd do at least some flight testing with a cheap prototype before building something more expensive.

I don't think the net effect would be any different. To do otherwise would mean you're getting something for nothing, if you follow me.

If a weight balances it is essentially counteracting a constant state of inertia in the form of gravity.

It's all about the energy required to accelerate / decelerate a given mass, if the energy stays the same then the smaller mass will get moved farther and faster but in the end it will have the exact same effect on the C of G so long as the perportions are correct.

The moment of inertia will also be the same for the purposes here because the arc of the smaller weight will be milder but it's speed will be greater, but that's not really a parameter that matters in this case.

[Steve]

Helis (just like planes) fly straighter, truer and more smoothly with a forward centre of gravity, so for FAI competition, you want a more forward heavy machine for the 2nd half of the flight schedule than you do for the 1st (hovering) half.

When you've got a heli with the tank behind the engine (i.e. 700), you start with it slightly nose heavy with a full tank. The nose heaviness will make the heli want to pitch forwards in the hover, so you offset this with the big flat horizontal fin - the downwash from the main blades pushes down on the fin and counteracts the nose forward tendency from the extra weight.

As the flight continues and the fuel tanks empties, the heli becomes even more nose heavy, thereby improving its flying characteristics for the aerobatics part of the schedule. Also, with the elevator trims that are required to keep the heli level during hovering and flight due to the nose-heaviness, the heli should ideally appear to be flying flat and level when it is stonking along the flat at full chat. The trims allow this presentation in the air because the disc will be tilted forwards by an exaggerated amount due to the forward trimming caused by the flat horizontal fin.

All in theory anyway - everything has an effect on everything else.....