700 size blades on T rex 600 ?

[guyken]

Is it possible to have 700 size blades on T rex 600 (with stretched tail boom 700 size) . I would fly it on 6s with align 1220 kv motor ? Main gear 188T and pinion 15 or 16

[Smoggie]

There is a long running thread on stretching the Trex 600, here: https://www.helifreak.com/showthread...hlight=stretch

It's certainly possible but you will have to reduce headspeed either by using a lower kV motor and/or different gearing. I'm sure there will be setups covered in the thread.

[guyken]

Nice , THX .

[Gladius]

I would highly recommend converting to 12S if you're going to fly 700 blades. You will almost certainly cut short your battery life if you try to run that on 6S, unless you're running really low headspeed and not doing anything more than hovering.

[RC/DC_5000]

Not to mention you will smoke your ESC and packs with anything more than a hover. (assuming you have the 80 CC ESC). The components kind of go to together with a little bit of leeway in between sometimes.

Your case would not be a little bit of leeway.. It would be a ton. The 700 blade disc is basically 8 inches larger. Thats a ton more current that's going to go into spinning them.

[guyken]

TYHX guys i will stick with my 600 size blades

[Smoggie]

No reason that spinning 700 size blades should take more power than 600 PROVIDING you drop the RPM to compensate. A bigger blade disk area is actually more efficient so you should be able to produce the same thrust for less power consumption. To compensate for increasing to 700 size blades you would need to reduce RPM to about 80% of it's old value, so for example if you were running 2200RPM go down to approx 1800RPM and power would be the same or less.

This of course assumes that you could do the 'stretch' without adding much to the weight of the heli, more weight will always tend to mean more power required to fly.

As an example to illustrate that a big slow spinning rotor actually needs less power to produce thrust, what about something like 150KG of thrust from only around 600Watts?:

Atlas Human-Powered Helicopter - AHS Sikorsky Prize Flight (1 min 45 sec)

[RC/DC_5000]

Yea. To hover. Running 700 blades on a 6s 600 isn't gonna work on a 3D helicopter. Thats 8 inches bigger... thats freaking huge. The Trex 550 on 6s is the breaking point of disc size to voltage for modern 3D.

So yea. You can hover around your 700 disc on a 600 power system. But the fun will stop there. I agree that lowering the headspeed a LOT will help... but even a lower headspeed in 3D is going to over amp the ESC and possibly the battery in 3D flight. It takes a lot more current (or torque) to keep that big disc turning under pitch loads and aerodynamic forces. And the super low headpeed on the big disc is going to be a dog for 3D. Super floppy blades, no pop... low performance.

Take your low headspeed oversized disc 700 at high speed and make the blades bark and bog... watch your ESC start on fire. Getting that big disc spinning is not the problem... keeping it spinning under load is the problem.

On top of it all.. The OP has 6s... No way on earth that would work. If he had 12s, there'd be a better chance... but the ESC would still be over taxed.

If you're going to fly a 700 disc.. you need a 700 power system. That should make good common sense.

[Smoggie]

It depends on the weight of the heli. the power needed to fly is mainly based on weight, not size. There are scores of examples in aviation of very large aircraft that fly on very little power because they are very light and have very lightly loaded flying surfaces (the man powered heli is just one of them)

'If' the OP can stretch his 600 to 700 and not have a big impact on weight (should be possible) then he would be able to fly on the same, or less, power as before. So his 6s power system would cope just fine.

You simply trade off rotor RPM for rotor diameter and the power required to spin the rotor stays the same.

But I agree that for hard 3D it wont work very well, not because of power, but just because the stretched 600 wont be strong enough and rigid enough to cope with 3D flying with those longer blades and spindly boom. You would need to physically 'beef it up', which adds weight and then you do need more power.

[RC/DC_5000]

This is a screenshot of another thread posing the question- Can I use 700mm blades on a 6s 600. The reply, was that there's no need to try it, its already been done. This is what happens. ESC on fire. https://www.helifreak.com/showthread.php?t=294834

All the guys that stretch that far with success are just hovering or doing aerial photography. Never catching that big disc in the air at high speed like a parachute. Go ahead and try it. And watch your ESC start on fire.

If you had a boat and made the propeller larger... you really think that it would not take more power to turn it?... no matter what RPM it was spinning?

You're saying the bigger disc is more efficient at lifting... that's not a power efficiency. Thats a surface area and aerodynamic efficiency. Windmills with larger blades make more electricity. What does that tell you about having to power the windmill on our helicopters?

"Rotors generate lift by accelerating air downward. The lift is proportional to rate of change of inertia of the air. This, however, also changes kinetic energy of the air and therefore requires power. This is called induced drag." - Aviation Beta

So a larger disc has more induced drag. This really should be common sense. Bigger air scooper requires more power to scoop air. Unless you're sitting at hover with minimal load on the disc. Then its all roses.

[RC/DC_5000]

"So a larger diameter prop reduces the engine's RPM at any given power setting, because there is more for the engine to turn over and hence more work to do."




Copied and Pasted from RC Plane World
http://www.rc-airplane-world.com/propeller-size.html

[Luvmyhelis]

Originally Posted by RC/DC_5000

"So a larger diameter prop reduces the engine's RPM at any given power setting, because there is more for the engine to turn over and hence more work to do."




Copied and Pasted from RC Plane World
http://www.rc-airplane-world.com/propeller-size.html

Helis use gearing, no direct drive.

I have stretched up past the 800 level years and years ago. But on 12s. It was a wonderful flying heli. Till I crashed it.....

[RC/DC_5000]

The prop reference was to show that a bigger disc is not more "efficient" with power. Its more efficient with lift. IE, for every turn of the disc, you get maximum surface area with minimal aerodynamic disturbance. The bigger disc requires more work to be done and more power to do it.

A gearing reduction will help increase leverage... but a couple teeth is not going to cut it. The current is still going to be too high for the ESC. There again.. reducing the RPM of the bigger disc would be good to hover and cruise around- fluttering those big blades. But when you 3D that disc and catch it like a parachute in the wind... Poof goes your ESC.

Think about when you pedal your bike up a hill. To make it as easy to go uphill as it was on flat ground... you drop the drive gear in half and double the cog gear. That makes the power input more similar. Change the drive gear but a couple teeth? Yea right. Its still pretty hard to pedal.

I searched and found quite a few threads about this. And the greater majority of people said that their logs, temperatures and in some cases burnt up ESCs say that the current goes up. I agree you could get some nice stability and float with an oversized disc. But your current problems are going to be largely related to how you fly it.

[guyken]

Well the main idea is to put the 600esp in a scale heli fuselage . Meanwhile i changed the main gear to 170 T and pinion to 10 or 11 T . According to headspeed calc i should get about 1400 or 1600 rpm
For the moment i have a 100A esc (HK stuff) but when i see the 700 blades next to the 600ones
Please some more reply's

[Smoggie]

A bigger diameter is more efficient in terms of lift produced vs power consumed. Note that efficiency can never be defined in terms of a single variable. By definition efficiency is the ratio of one variable vs another. So there is no such thing as 'lift efficiency' or 'power efficiency'.. Efficiency must by definition be a ratio of one against against another, i.e 'lift to power efficiency'.

That's why the heli in the previous video use huge diameter props because it gives the most efficient conversion of power to lift. .. Or put another way it needs less power to produce the required amount of lift.

Induced drag reduces with greater diameter because the downwash velocity reduces. This is part of the reason that all helis use large diameter rotors rather than small fast spinning propellers (small fast spinning props would after all be much more convenient, if they worked it would do away with big cumbersome rotors and complex reduction gearboxes and replace with a simple direct drive prop.)

Don't believe me? Then maybe believe Prof Martin Hepperle (world renown expert on propeller/rotor dynamics):

For a given power P, it is always desirable to use the largest possible propeller diameter D, which may be limited by mechanical restrictions (landing gear height) or aerodynamic constraints (tip Mach number). That's why most man or solar powered airplanes use large, slowly turning props. These catch a large volume of air and accelerate it only slightly to achieve the maximum efficiency.
[clip]
Using the quite simple momentum theory, we can already deduct important information about the performance of propellers. We can study the influence of the propeller diameter on efficiency as well as how it depends on flight speed or the density of the air (corresponding to a certain altitude). We learn, that an efficient propeller should have a small power loading per disk area, i.e. a large diameter is required.

[RC/DC_5000]

Guyken- With that information.. then yea I'd say you're going to have success. Advantageous gearing, decent current ESC, and very light duty flight speeds and pitch loads. Should be floating around like the bicycle video above. My point of view was based on flying it for performance as it was built.

Grumpy.. All your reference said.. was that a big disc catches more air. Its a no duh that its more desirable to use the largest disc. With restrictions on the mechanical capabilities like the aerodynamic constraints of a 3D helicopter popping full speed into the wind. (His words) You're talking about simple lift. Hover lift. I already agreed that it works. But thats it... no performance 3D helicopter. Just a simple lifting machine like your bike video.

Hovering is like pedaling your bike on flat ground. Now add aerodynamic forces on the huge disc, collective pitch forces, G forces.... all of a sudden.. you're pedaling up a hill that your poor legs can not withstand. We aren't talking about the same thing. But then again. This subforum is about a performance built 3D heli. So it wasn't wrong of me to assume thats what we were talking about flying.

I could attach a 20 pound weight to my helicopter and lift it no problem. But I'm not gonna make it long banking turns at speed and doing big air loops. Things change at 80 mph. And that goes especially for that big disc.

[Smoggie]

RC/DC... i already stated in my first reply to you that for a 3D heli a big stretch was not good because the heli wouldn't have the structural strength to cope. It would need structurally 'beefing up' at which point it would become heavier then yes, it would need more power to cope with the extra weight... But Guyken never mentioned flying 3D (not everyone want to fly 3D you know)

But that's a different issue. In terms of how many amps the motor would pull the stretch (if geared down appropriately and if roughly the same weight) would actually require less power and less amps to make the same lift.

Bigger rotors ARE more efficient, that's just simple physics, but there are practical limits on how big you can go.

[RC/DC_5000]

And there are practical limits to how you can fly it. Again. You are only talking about simple lift. Helicopters do a lot more than push air straight down at hover. Thats a pretty easy task.

But we mostly agree when talking about apples and apples.

[Smoggie]

The same efficiency advantage is just as true in forward or turning flight as it is in hover.
The practical limits really come in if you want to push the airframe in hard 3D, at which point the (relatively) lightweight 600 frame would struggle to cope with the big rotor and big stretched boom.

But the OP hasn't even said if he's interested in 3D?... If he is then yes, he would probably be better leaving it 'as is'.

The other practical problem is getting the heli to balance, that can be a real issue as the long boom makes it tail heavy, but that's a different problem.

[RC/DC_5000]

You're too much... So the FRAME couldn't handle the disc... but the ESC could.

That's too illogical for another response. Happy flying.